171

Chapter 4

Synthesis, X-ray crystal structure analysis, spectral & magnetic studies

and catalytic activity of Cu(II), Ni(II) and Zn(II) complexes with di- and

tri-podal ligands

Catechol oxidase is a copper containing, type III active site protein which shows

catecholase activity i.e. oxidation of a broad range of catechols to quinones through the

four-electron reduction of molecular oxygen to water.1, 2

According to the crystal

structure, the active site of the enzyme in its native state has a strongly

antiferromagnetically coupled, dinuclear copper(II) core where each Cu(II) is coordinated

by three histidine nitrogens and a bridging OH- ion.

3 In nature this metalloprotein acts as

a very efficient catalyst to catalyze a kinetically unfavoured reaction of triplet O2 with

organic substrates. Thus studies on the model compounds mimicking the catecholase

activity, are very useful and promising for the development of new, more efficient,

bioinspired and environment friendly catalysts, for in vitro oxidation reactions. The

catecholase activity of various mononuclear and binuclear complexes of copper(II) have

been investigated in the past twenty five years.4 According to Reedijk et al.

5 four

approaches have been used in the mechanistic studies on these model compounds:

namely substrate-binding studies, structure-activity relationship, kinetic studies on

catalytic reactions and stoichiometric oxidation of catechol substrates by the peroxo- and

oxo-dicopper complexes. Out of these, the second approach has been more frequently

employed by various groups. Under this approach the relationships between metal-metal

distance; electrochemical properties; exogenous bridging ligand; ligand structure and pH,

with the catecholase activity have been exploited. Non-planar mononuclear as well as

binuclear Cu(II) complexes with Cu···Cu distance in the range 2.9-3.2 Å have been found

to show good catecholase activity. The binuclear complexes have generally been found to

be more reactive than the former, pointing towards the possibility of a dicopper-

catecholate adduct as an intermediate in the process.

From the reports in literature on various copper(II) model compounds and also

from the known crystal structure of catechol oxidase, it is clear that the functional models

172

must have free coordination sites on the metal atom where the substrate can bind.

Therefore the ligands which would have lesser number of donor atoms are more

promising. In this work synthesis of copper(II) complexes which may act as functional

models for the active site of catechol oxidase have been reported. The ligands HL1-H3L5

(Scheme I) contain a phenolic group in a correct chelating position with an imine and an

amine nitrogen or with two amine nitrogens which can offer a N2O2 donor set and a

central Cu2O2 core, in the binuclear complexes. The designs of their copper acetate

complexes offer following structural features known to enhance the activity.

1. Tridentate ligands to support lower coordination number around the metal.

2. An endogenous -OH group capable of bridging between to metal centers, similar to the

active site.

3. Presence of an exogenous acetate ligand facilitating the deprotonation of phenolic

group and also the formation of binuclear complexes with short Cu···Cu distance.

4. Easily replaceable acetate ligand to augment the binding of substrate to the metal.

To analyze the structure-activity relationship for such ligands we have varied the

flexibility of the ligands, nature of nitrogen donor and length and number of podand

arms. Although some complexes of HL1 and HL2 are known, for Cu(II) salt of azide,

perchlorate, chloride, oxalate and triflate anions,4h, 6

our complexes are new because the

counter ion in all our complexes is acetate, either coordinating or non-coordinating and

they have different molecular and crystal structures than the reported ones.

At the same time hydrolysis of phosphodiester bond is also of considerable

importance.7 DNA phosphodiester backbone is essential requirement for the survival of

life8

but in genetic engineering including specific cleavage and ligation, to achieve

desired functions, the remarkable stability of DNA phosphodiester7b

backbone is a

challenging task for scientists.9

Hydrolytic enzymes such as polymerases, recombinases,

topoisomerases and reverse transcriptases, that cleave phosphate ester bonds efficiently

often have in their active site two transition metal ions such as Zn(II), Mg(II), Mn(II),

Ni(II), or Fe(III) that act as Lewis acid sites in the catalysis and generally facilitated by

cooperative action of two metal ions.10

The metal ions activate the phosphate group and

generate an active nucleophile. This stabilizes the pentacoordinated phosphorus transition

state and the leaving group by cooperative action. The phosphate ester hydrolysis has

173

been studied with various synthetic model compounds with two metal ions.11

There are

many reports on the metal complexes with two metal centers, such as Zn(II),12,13

Cu(II),14

Co-(III),15

Ni(II) 16

and lanthanides(III),17

which are separated by particular

distance with the help of ligands. Therefore the dinuclear complexes of Cu(II), Ni(II) and

Zn(II) being reported here have been used to find their role in the hydrolytic cleavage of

the phosphodiester bond, if any as explained below.

Nine complexes of Cu(II), Ni(II) and Zn(II) acetate with Schiff base ligands based

on salicylaldehyde and N, N-dimethylamino)ethyl/propyl amine and their reduced

products, HL1-H3L5 (Scheme I) have been synthesized and characterized by various

spectroscopic methods. The solid state structures of 1, 2, 3, 6, 7, 8 and 9 have been

determined using single crystal X-ray diffraction method. The structures of the other two

compounds have been proposed on the basis of spectroscopic and physical methods. The

compounds 1, 3 and 4 are dinuclear complexes of the tridentate ligands, where the two

Cu(II) centers have square pyramidal geometry with bridging acetate or phenoxo

groups. Each arm of the tripodand ligand forms a mononuclear, magnetically dilute

complex 5 having five coordinated Cu(II) ions. Complex 2 is mononuclear with a square

pyramidal stereochemistry. The compound 6 and 7 are mononuclear and dinuclear

complexes of Ni(II) respectively, where all the Ni(II) centers have octahedral geometry.

Compound 8 is a mononuclear Zn(II) complex where metal center is five coordinated via

two nitrogens and one oxygen from the ligand and the acetate ion, whereas compound 9

is a dinuclear complex having two Zn(II) ions in two different coordination

environments. One of the Zn(II) ions is pentacoordinated whereas the other one is four

coordinated.

The catalytic performance i.e. oxidation of 3,5-di-tert-butylcatechol to quinone

only for the copper complexes 1-5 and catalytic hydrolysis of bis(4-nitrophenyl)

phosphate (BNPP) for 1, 3, 4, 7 and 9 were studied using UV-vis absorption spectral

methods. Complex 4 exhibits the highest catecholase activity with a turnover number of

41 h-1

whereas other copper containing complexes showed lower rates of oxidation. For

the catalytic hydrolysis of BNPP, only dimeric nickel complex 7 exhibited rate

acceleration in the hydrolysis of BNPP. Pseudo-first order rate constants were determined

174

for all the active complexes and a kinetic treatment on the basis of Michaelis-Menten

model was also applied.

OH

N

NOH

N

N

OH

NH

NOH

NH

N

S

SN

N

S

N

OH

HO

OH

HL1 HL2

HL3 HL4

H3L5

Scheme 1

4.1 Experimental:

4.1.1 General information

All solvents were dried by standard methods. Unless otherwise specified,

chemicals were purchased from commercial suppliers and used without further

purification. TLC was performed on glass sheets pre-coated with silica gel. The elemental

analyses were performed on a Flash EA 1112 elemental analyzer. 1H and

13C NMR were

performed on a JEOL 300 MHz instrument with TMS as reference. FTIR spectra were

recorded on a Shimadzu 8400S IR spectrometer for the compounds in the solid state as

KBr discs or as neat samples, in the range 400–4000 cm-1

. The electronic absorption

spectra were recorded on a Shimadzu Phramaspec UV-1700 UV-vis spectrophotometer.

Room temperature magnetic measurements were carried on a Johnson-Matthey magnetic

susceptibility balance and Sherwood Scientific magnetic susceptibility balance with

Hg[Co(SCN)4] as standard. For 1 & 3 complexes the variable temperature magnetic

measurements were carried out, at the Unitat de Mesures Magnètiques (Universitat de

Barcelona), on polycrystalline samples (30 mg) with a Quantum Design SQUID MPMS-

XL magnetometer working in the 2-300 K range. The magnetic fields were 0.5 and 1.0 T.

The diamagnetic corrections were evaluated from Pascal´s constants. R is the agreement

factor defined as Σ[(M)exptl

– (M)calcd

]2/Σ[(M)

exptl]

2. ESI Mass spectra were taken on a

175

Bruker Esquire3000_00037 instrument in methanol. Solution electric conductivity

measurements were carried out with a Hioki 3532- 50 LCR Hi Tester conductivity bridge

with a solute concentration of 1mM.

X-ray Crystallography

Data for single crystal X-ray structure analyses of complexes 1, 2, 3, 6, 7, 8 and 9

were collected at 295(2) K on a Siemens P4 diffractometer. The -2 technique was used

to measure the intensities, up to a maximum of 2 ~ 50° with graphite monochromatised

Mo-K radiator ( = 0.71073 Å). Cell parameters were refined in the range, 10-12.5o,

the data were corrected for Lorentz and polarization factors and an empirical scan

absorption correction was applied,18

using XSCANS. The structures were solved by the

direct methods using SIR9719

and refined by full matrix least squares methods based on

F2

using SHELX.97.20

All non-hydrogen atoms were refined anisotropically and hydrogen

atoms were assigned at calculated positions. In the complex 6 the hydrogens of the

lattice water (disordered) could not be located from the difference Fourier synthesis

despite the best efforts. All calculations were performed using WinGX.21

The relevant

crystallographic and refinement parameters are given in Table 1.

4.1.2 Ligand and Complex syntheses

Ligands HL1- HL4 were prepared by methods similar to those reported in the

literature,22

H3L5 was prepared as reported earlier in our lab.23

[{Cu(L1)}2(-CH3COO)2] (1) A methanolic solution of copper (II) acetate (1.99 g, 10

mmol) was added to a solution of HL1 (1.92 g, 10 mmol) in CHCl3. The solution was

refluxed for two hours. After the completion of reaction, the solution was filtered. After

the evaporation of the solvent, a dark–green compound was obtained which was washed

with methanol and dried under vacuum. Dark-green colored crystals were obtained by

vapor diffusion method of crystallization using CHCl3 as solvent and PET (petroleum

ether) as a precipitant. Yield 62 %. mp = 192 0C

. Anal. Calcd. C26H36Cu2N4O6, C 49.76,

H 5.78, N 8.93; found C 49.84, H 5.83, N 9.01. UV-vis max. (nm), (M-1

cm-1

) in

CH3OH: 647(100), 370 (3140), 269 (11670). FTIR (KBr, cm-1

): 3475 (m) νO-H, 3001 (w)

νaromatic C-H, 2924, 2888 (w) νaliphatic C-H, 1625 (s) νC=N, 1598 (s), 1523 (s) νasym OCO, 1450(s)

C-H, 1379(s), 1327 (m) νsym OCO, 1195(m) νC-N, 1149 (m) νC-O. μB = 2.6 BM. Conductivity

176

(nitrobenzene, 1 mM solution at 298 K): Λ = 2.0 Ω-1

cm2

mol-1

(the range for 1:1

electrolytes in nitrobenzene is 20-30). ESI-MS (CH3OH, m/z): Obsd. 314.3 [M+H+

where M= (L1+Cu+Ac)] Calcd. 314.5 , Obds. 253.8 [(L1 + Cu)-H] Calcd. 253.5, Obsd.

188.8 [L1-H] Calcd. 189.0.

[(CuL2)(CH3COO)] (2) A methanolic solution of copper (II) acetate (1.99 g, 10 mmol)

was added to a solution of HL2 (2.06 g, 10 mmol) in CHCl3. The solution was refluxed

for two hours. After the completion of reaction, the solution was filtered. After the

evaporation of the solvent, a dark–green compound was obtained, washed with methanol

and dried under vacuum. Dark-green colored crystals were obtained on recrystallization

from methanol. Yield 70 %. mp = 168 0C. Anal. Calcd. C14H20CuN2O3, C 51.29, H 6.10,

N 8.54; found C 51.35, H 6.09, N 8.61. UV-vis max. (nm), (M-1

cm-1

) in CH3OH: 666

(110), 369 (4530), 302 (4550), 271 (13130). Selected IR (KBr, cm -1

): 3002 (w) νaromatic C-

H, 2925 (w) νaliphatic C-H, 1633 (s) νC=N , 1550(m), 1537 (m) νasym OCO, 1444 (m), 1414(s)

νsym OCO, 1341(m), 1325 (s) C-H, 1236(m), 1197(m) νC-N, 1147(m) νC-O. μB = 1.99 BM.

Conductivity (nitrobenzene, 1 mM solution at 298 K): Λ = 2.54 Ω-1

cm2

mol.-1

ESI-MS

(CH3OH, m/z): Obsd. 328.1 [M+H+ where M= (L2+Cu+Ac)] Calcd. 328.5, Obsd. 267.9 [

(L2 + Cu) -H] Calcd. 267.5.

[{Cu(CH3COO)}2(-L3)2] (3). A methanolic solution of copper (II) acetate (1.99 g, 10

mmol) was added to a solution of HL3 (1.94 g, 10 mmol) in CHCl3. The solution was

stirred for half an hour, then, a light–green solid was separated, filtered, washed with

methanol and dried under vacuum. Dark-green colored crystals were obtained by vapor

diffusion method of crystallization using CHCl3 as solvent and Petroleum ether as a

precipitant agent. Yield 75 %. mp = 198 0C. Anal. Calcd. C26H40Cu2N4O6, C 49.44, H

6.38, N 8.87; found C 49.37, H 6.48, N 7.97. UV-vis max. (nm), (M-1

cm-1

) in CH3OH:

661(100), 423 (630), 281 (4540), 235 (5970). Selected IR (KBr, cm -1

): 3156(w, br) νN-H,

3120(w) νaromatic C-H, 2926, 2867 (w) νaliphatic C-H, 1594 (s), 1570 (s) νasym OCO, 1481

(m),1451(m) C-H, 1397(m), νsym OCO, 1290(m) νC-N, 1021(w) νC-O. μB = 2.5 BM.

Conductivity (nitrobenzene, 1 mM solution at 298 K): Λ = 7.8 Ω-1

cm2

mol-1

ESI-MS

(CH3OH, m/z): Obsd. 315.1 [M+ where M= (L3 + Cu+Ac)] Calcd. 315.5.

177

[{Cu(L4)}2(-(CH3COO)2] (4). A methanolic solution of copper (II) acetate (1.99 g, 10

mmol) was added to a solution of HL4 (2.08 g, 10 mmol) in CHCl3. The solution was

refluxed for two hours. After the completion of reaction, the solution was filtered. After

the evaporation of the solvent, a dark–green compound was obtained, washed with

methanol and dried under vacuum. Dark-green colored powder was obtained on

recrystallization from methanol. The compound is hygroscopic when exposed to air for

some time. Yield 68 %. mp = 162 0C. Anal. Calcd. C28H44Cu2N4O6, C 50.98, H 6.72, N

8.49; found C 51.06, H 6.79, N 7.62. UV-vis max. (nm), (M-1

cm-1

) in CH3OH:

690(109), 383 (1010), 277 (4920). IR (KBr, cm -1

): Selected IR (KBr, cm -1

): 3439 (s) νN-

H, 3121(w, br) νaromatic C-H, 2908 (m, br) νaliphatic C-H, 1581 (s) νasym OCO, 1435(s) νsym OCO,

1331 (m) C-H, 1279(m), νC-N, 1054 νC-O. μB 1.96 BM. Conductivity (nitrobenzene, 1 mM

solution at 298 K): Λ = 3.8 Ω-1

cm2

mol.-1

ESI-MS (CH3OH, m/z): Obsd. 659.1 [M+

where M = (L4+Cu+Ac)2] Calc. 659, Obsd. 540.1 [(L4+Cu)2 -H ] Calcd. 540, Obsd.

269.9 [(L4 + Cu) -H] Calc. 269.5.

[Cu3(L5)(CH3COO) 3]. 3H2O (5)

A methanolic solution of copper (II) acetate (0.597 g, 3 mmol) was added to a solution

of H3L5 (0.843 g, 1 mmol) in CHCl3. The solution was refluxed for two hours. Once the

completion of the reaction, the solution was filtered and the solvent evaporated, a dark–

green compound was obtained which was washed with methanol and dried under

vacuum. Dark-green colored powder was obtained on recrystallization from methanol.

Yield 68%. mp = 205 0C. Anal. Calcd. C57H57Cu3N3O12S3, C 54.21, H 4.55, N 3.33, S

7.62; found C 53.87, H 4.63, N 3.01, S 7.83. UV-vis max. (nm), (M-1

cm-1

) in CH3OH :

CHCl3 (29:1) : 659 (177), 392 (7220). Selected IR (KBr, cm -1

): 3429 (br) νO-H, 3052 (w,

br) νaromatic C-H, 2917 (w, br) νaliphatic C-H, 1608 (s) νC=N , 1575 (s), 1526 (s) νasym OCO, 1458

(s),1440(s) νsym OCO, 1379(m), 1325 (m) C-H, 1181(m) νC-N, 1147(m) νC-O. μB = 2.3 BM.

Conductivity (nitrobenzene, 1 mM solution at 298 K): Λ = 3.78 Ω-1

cm2 mol

-1. ESI-MS

(CH3OH, m/z): Obsd. 1262.2 [M+H+, where

M= L5(Cu)3(Ac)3+3H2O] Calcd. 1262.5,

Obsd. 1228.6 [M+ where M= H3L5(Cu)3(Ac)3+H2O] Calcd. 1228.5, Obsd. 983.7 [

H3L5+Cu+Ac+H2O] Calc. 983.5.

[(Ni L1)(CH3COO) (H2O)2]) 0.25 H2O (6) A methanolic solution of nickel (II) acetate

(2.48 g, 10 mmol) was added to a solution of HL1 (1.92 g, 10 mmol) in CHCl3. The

178

solution was stirred for two hours. After the completion of reaction, the solution was

filtered. After the evaporation of the solvent, a green colored compound was obtained

which was washed with methanol and dried under vacuum. Green colored crystals were

obtained on recrystallization from methanol. Yield 66 %. mp = 186 0C.

Found: C 42.57,

H 6.41, N 7.58 % Calc. for C13H24N2NiO6 : C 43.01, H 6.66, N 7.72 %. Selected IR

(KBr, cm-1

): 3503 νO-H, 3383 νaromatic C-H, 2879 νaliphatic C-H, 1652 (s) νC=N, 1582 , 1452 νasym

OCO, 1411, 1333 νsym OCO, 1246 νC-N, 1121 (m) νC-O. UV-vis max. (nm), (M-1

cm-1

) in

CH3OH: 886(154), 600 (54), 368 (13170). μB 3.39 BM. Conductivity (nitrobenzene, 1

mM solution at 298 K): Λ = 4.8 Ω-1

cm2

mol.-1

[{(Ni( µ-L2) (CH3COO)}2 (µ-H2O)] (7) A methanolic solution of nickel (II) acetate

(2.48 g, 10 mmol) was added to a solution of HL2 (2.06 g, 10 mmol) in CHCl3. The

solution was refluxed for two hours. After the completion of reaction, the solution was

filtered. After the evaporation of the solvent, a dark–green compound was obtained which

was washed with methanol and dried under vacuum. Dark-green colored crystals were

obtained by vapor diffusion method of crystallization using CHCl3 as solvent and

petroleum ether as a precipitant. Yield 68 %. mp = 198 0C. Found: C 50.99, H 6.91, N

8.70 % Calc. for C28H42N4Ni2O7 : C 50.64, H 6.38, N 8.44 %. Selected IR (KBr, cm -1

):

3473 νO-H, 3146 νaromatic C-H, 2864 νaliphatic C-H, 1542 νC=N , 1411 νsym OCO, 1196 νC-N, 1026

(m) νC-O. UV-vis max. (nm), (M-1

cm-1

) in CH3OH: 1032 (31), 634 (72), 364 (11200). μB

2.90 BM. Conductivity (nitrobenzene, 1 mM solution at 298 K): Λ = 5.2 Ω-1

cm2

mol.-1

[(ZnL2)(CH3COO)] (8) A methanolic solution of zinc (II) acetate (2.19 g, 10 mmol)

was added to a solution of HL2 (2.06 g, 10 mmol) in CH3OH. The solution was stirred for

two hours. After the completion of reaction, the solution was filtered. After the

evaporation of the solvent, an off–white colored compound was obtained which was

washed with methanol and dried under vacuum. Pale-white colored crystals were

obtained on recrystallization from methanol. Yield 65 %. mp = 174 0C. Found: C 51.16,

H 6.18, N 8.66 % Calc. for C14H20N2O3Zn : C 51.00, H 6.11, N 8.50%. Selected IR

(KBr, cm -1

): 2899 νaromatic C-H, 2837 νaliphatic C-H, 1639 (s) νC=N , 1556 νasym OCO, 1442,

1419 νsym OCO, 1325 C-H, 1242, 1207 νC-N, 1157νC-O. 1

H NMR (300 MHz, CDCl3) : 1.89-

1.96 (m, -CH2, 2H); 2.07 (s, -CH3COO-, 3H); 2.46 (s, -NCH3, 6H); 2.97 (broad s, -NCH2,

179

2H); 3.89 (broad s, -NCH2, 2H); 6.65 (t, Ar, 1H, J=7.2 Hz); 6.68 (d, Ar, 1H, J=8.2 Hz);

7.07 (broad s, Ar, 1H); 7.21- 7.27 (m, Ar, 1H), 8.10 (s, -CH=N, 1H). UV-vis max. (nm),

(M-1

cm-1

) in CH3OH: 357 (4820), 268 (7930), 232 (17440). Conductivity

(nitrobenzene, 1 mM solution at 298 K): Λ = 4.5 Ω-1

cm2

mol.-1

[Zn2(L4)(-CH3COO)2, (CH3COO)] (9) A methanolic solution of zinc (II) acetate

(2.19 g, 10 mmol) was added to a solution of HL4 (1.04 g, 5 mmol) in CH3OH. The

solution was stirred for two hours. After the completion of reaction, the solution was

filtered. After the slow evaporation of the solvent, off–white colored crystals were

obtained. Yield 72 %. mp = 204 0C. Found: C 41.92, H 5.34, N 5.62 % Calc. for

C18H27N2O7Zn2 : C 42.04, H 5.29, N 5.45 %. Selected IR (KBr, cm -1

): 3236 νN-H, 3018

νaromatic C-H, 2917, 2846 νaliphatic C-H, 1591 (s) νasym OCO, 1444, 1403 (s) νsym OCO, 1277 νC-N,

1033 νC-O. 1

H NMR (300 MHz, CDCl3) : 1.25 (broad, -NCH2, 2H); 1.73 (broad s, -CH2,

2H); 1.84 (s, -CH3COO-, 3H); 2.05 (s, -NCH3, 6H); 2.37-2.40 (broad s, -CH2, 2H); 3.08

(broad s, -NCH2, 2H); 3,48-4.17 (broad s, -NH, 1H); 6.65 (t, Ar, 1H, J=7.2 Hz); 6.90 (d,

Ar, 1H, J=7.5 Hz); 6.99 (d, Ar, 2H, J=6.3Hz); 7.17 (m, Ar, 2H). UV-vis max. (nm), (M-

1cm

-1) in CH3OH: 286 (3020) 236 (8150) 211 (7340). Conductivity (nitrobenzene, 1 mM

solution at 298 K): Λ = 3.6 Ω-1

cm2

mol.-1

4.1.2 Kinetics of 3, 5-Di-tert-butylcatechol Oxidation

To determine the catecholase activities of the complexes, 10-4

M solutions of

copper complexes in methanol (except complex 5 [in CH3OH: CHCl3 (29:1)]) were

treated with 100 equivalents of 3,5-di-tert-butylcatechol (3,5-DTBC) in methanol under

aerobic condition. The UV spectra of solutions were recorded directly after the addition

and subsequently after regular intervals of 2 minutes and absorption values at 390-410

nm were measured as a function of time over a period of 2-3 hours. To determine the

dependence of the rates on the substrate concentration and various kinetic parameters,10-4

M solutions of copper complexes were treated with 10 to 100 equivalents of 3,5-DTBC in

methanol under aerobic condition.

180

4.1.3 Kinetic Measurements of phosphate ester hydrolysis

All the studies were performed in spectroscopic grade DMSO. Double distilled

deionized water was used for the buffers preparation. Sistronics digtal pH meter was used

to measure the pH of different solutions. Various buffer solutions* HEPES, pH < 8.0;

EPPS, 8.0 < pH < 8.9; CHES, 8.9 < pH < 11.0; CAPS, pH > 11.0 were titrated to the

desired pH with NaOH (1 M) and NaNO3 (0.2 M). All absorbance spectra were recorded

with a Shimadzu Phramaspec UV-1700 UV-vis spectrophotometer fitted with a

thermostated cuvette holder accessory. Samples were prepared in 1 cm path length glass

cuvettes. Deionized water (480 μL), buffer (0.1 M, 1500 μL), DMSO (660 μL), the

substrate BNPP (50 mM in DMSO, 120 μL), ligand solution (10.0 mM in DMSO, 120

μL) and aqueous solution of metal salt (10.0 mM in DMSO, 120 μL) were added into the

cuvette to constitute a 30% DMSO solution. Ligand would form complex with metal ion

in situ. Efficient metal binding under these conditions was expected, and stable metal

complex formation was confirmed by the absence of precipitation of any metal hydroxide

at high pH. The UV spectra of solutions were recorded directly after 15 min equilibration

time at 75 0C. The increase in concentration of p-nitrophenolate (λmax 400-410 nm) was

measured after every 3 min. To determine the dependence of the rates on the substrate

concentration and various kinetic parameters, 0.2 mM solutions of metal complexes were

treated with 2, 4, 6, 8 and 10 mM solution of BNPP in 30% DMSO solution.

* 0.1 M solution of each buffer salt (HEPES, pH < 8.0; EPPS, 8.0 < pH < 8.9; CHES, 8.9 < pH <

11.0; CAPS, pH > 11.0) were prepared and mixed with equal volume (10 ml) 0.2 M NaNO3

solution. These mixture were titrated to the desired pH with NaOH (1 M)

181

Table 1. Unit cell and refinement parameters

Identification code 1 2 3 Empirical formula C26H36Cu2N4O6 C14H20CuN2O3 C13H20CuN2O3

Formula weight 627.69 327.87 315.85

Temperature 295(2) K 296(2)K 293(2)K

Wavelength 0.71069 Å 0.71069 Å 0.71069 Å

Crystal system Monoclinic Orthorhombic Monoclinic

Space group P21/n Pbca C2/c

Unit cell dimensions a = 11.176(5) Å a = 9.8500(10) Å a = 13.057(5) Å

b = 8.262(5) Å b = 11.2110(10) Å b = 11.992(6) Å

c = 15.569(5) Å c = 27.1090(10) Å c = 19.238(8) Å

β= 103.630(5)° β = 107.96(3)°

Volume 1397.1(11) Å3 2993.6(4) Å

3 2865(2) Å

3

Z 2 8 8

Density calculated 1.492 Mg/m3 1.455 Mg/m

3 1.464 Mg/m

3

Absorption coefficient 1.568 mm-1

1.467 mm-1

1.530 mm-1

F(000) 652 1368 1320

Crystal size 0.28 x 0.22 x 0.19 mm3 0.20 x 0.20 x 0.18 mm

3 0.20 x 0.18 x 0.17 mm

3

Theta range for data collection 2.03 to 27.54°. 1.50 to 25.50°. 2.23 to 25.52°.

Index ranges 0<=h<=12, 0<=h<=11, 0<=h<=15,

0<=k<=10, 0<=k<=13, 0<=k<=14,

-20<=l<=19 0<=l<=32 -23<=l<=22

Reflections collected 3181 2781 2798

Independent reflections 3021 2781 2674

(TMax. and Tmin.) 0.7459 and 0.6789 0.783 and 0.743 0.7810 and 0.7496

Data / restraints / parameters 3021/0/173 2781/0/181 2674/0/172

Goodness-of-fit on F2 1.243 0.926 1.060

Final R indices [I>2sigma(I)] R1 = 0.0804, R1 = 0.0374 R1 = 0.0836,

wR2 = 0.2013, wR2 = 0.0965 wR2 = 0.1862

R indices (all data) R1 = 0.1159, R1 = 0.0662, R1 = 0.1843,

wR2 = 0.2342 wR2 = 0.1209 wR2 = 0.2374

Largest diff. peak and hole 0.865 and -0.673 e.Å-3

0.336 and -0.253 e.Å-3

0.681 and -0.911 e.Å-3

CCDC No. 638641 630907 630906

182

Identification code 6 7 8 9

Empirical formula C52 H88 N8 Ni4 O21 C28 H42 N4 Ni2 O7 C14 H20 N2 O3 Zn C18 H28 N2 O7 Zn2

Formula weight 1396.14 664.08 329.72 515.16

Temperature 295(2) K 298(2) K 571(2) K 293(2) K

Wavelength 0.71069 Å 0.71069 Å 0.71073 Å 0.71069 Å

Crystal system Tetragonal Monoclinic Orthorhombic Monoclinic

Space group P4/n P21/c P bca P21

Unit cell dimensions a = 21.049(4) Å a = 9.452(5) Å a = 9.9976(9) Å a = 9.285(5) Å

b = 21.049(4) Å b = 16.230(5) Å b = 10.9932(11) Å b = 9.248(5) Å

c = 7.178(5) Å c = 20.254(4) Å c = 27.464(4) Å c = 12.897(5) Å

= 90° = 90 = 90° = 90.000(5)°

= 90° = 95.720 = 90° = 99.670(5)°.

= 90° = 90°. = 90° = 90.000(5)°.

Volume 3180(2) Å3 3092(2) Å3 3018.5(6) Å3 1091.7(9) Å3

Z 2 4 8 2

Density (calculated) 1.458 Mg/m3 1.427 Mg/m3 1.451 Mg/m3 1.567 Mg/m3

Absorption coefficient 1.243 mm-1 1.267 mm-1 1.635 mm-1 2.237 mm-1

F(000) 1472 1400 1376 532

Crystal size 0.20 x 0.20 x 0.18 mm3 0.20 x 0.10 x 0.10 mm3 0.18 x 0.15 x 0.15 mm3 0.2 x 0.2 x 0.2 mm3

Theta range for data collection 1.37 to 25.00°. 1.61 to 24.99°. 2.52 to 25.50°. 1.60 to 24.99°.

Index ranges 0<=h<=19, 0<=k<=25,0<=l<=8 0<=h<=9, 0<=k<=19, -24<=l<=23 0<=h<=12, 0<=k<=13,0<=l<=33 0<=h<=11,0<=k<=10,-15<=l5

Reflections collected 2783 5673 2800 2179

Independent reflections 2639 [R(int) = 0.0411] 5129 [R(int) = 0.0607] 2800 [R(int) = 0.0000] 2047 [R(int) = 0.0480]

Completeness to theta = 25.00° 93.4 % 94.1 % 100.0 % 100.0 %

Absorption correction Psi scan Psi-scans Psi-scans None

Max. and min. transmission 0.8072 and 0.7891 0.879 and 0.781 0.7915 and 0.7573 -

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 2639 / 0 / 194 5129 / 0 / 370 2800 / 0 / 181 2047 / 1 / 262

Goodness-of-fit on F2 0.879 1.157 1.025 1.096

Final R indices [I>2sigma(I)] R1 = 0.0591, wR2 = 0.1333 R1 = 0.0530, wR2 = 0.1342 R1 = 0.0557, wR2 = 0.1152 R1 = 0.0512, wR2 = 0.1239

R indices (all data) R1 = 0.1389, wR2 = 0.1715 R1 = 0.0890, wR2 = 0.1624 R1 = 0.1170, wR2 = 0.1454 R1 = 0.0561, wR2 = 0.1310

Largest diff. peak and hole 0.872 and -0.562 e.Å-3 0.463 and -0.428 e.Å-3 0.407 and -0.409 e.Å-3 0.712 and -1.376 e.Å-3

CCDC No. 638642 630909 630908 290776

183

4.2 Synthesis, spectroscopic and structural characterization

4.2.1 Synthesis of receptors

All the complexes have been characterized by elemental analysis, IR and UV-vis

spectroscopy. The stoichiometry has been verified by measuring molar conductivity

values. Whenever possible the NMR (for 8 and 9) spectra have also been determined.

The single crystal X-ray structures of 1, 2, 3, 6, 7, 8 and 9 have been determined.

4.2.2 Spectral Characterization

The IR spectra24

of 1, 2, 5, 6, 7 and 8 show –C=N characteristic bands around

~1649- 1608 cm-1

, which are absent in the reduced products 3, 4 and 9 (Figure 1).

1

2

3

Figure 1a. Showing I.R. Spectra for the complexes 1 to 3

The latter three complexes show medium to weak broad bands due to N-H

stretching frequency. The νC=N stretching band shifts to a lower frequency by 10, 26, 58,

28, 31 and to a higher frequency by 14 cm-1

in 1, 2, 5, 7, 8 and 6 clearly showing its

184

participation in coordination. In complexes 1, 3, 6 and 7 medium to sharp bands in the

region 1599-1523 cm-1

and 1411-1323 cm-1

have been assigned to asymmetric and

symmetric vibrations of coordinated acetate groups. A large difference ν between νasym

OCO and νsym OCO ~ 200 cm-1

or more is indicative of a monodentate coordination through

carboxylate groups25

Complexes 2, 5 and 8 show strong, symmetrical bands in the range

1526-1575 cm-1

(νasym OCO) and 1414-1458 cm-1

(νsym OCO) with ν lying in the range 113-

123 cm-1

which indicates bidentate chelating coordination through acetate group.24b,26

Complex 9 shows strong, symmetrical bands in the range 1573 cm-1

(νasym OCO) and

1554-1365 cm-1

(νsym OCO) with ν lying in the both ranges i.e. less than and more than

200 cm-1

indicates the presence of monodentate and bidentate chelating coordination

through acetate group. In the complexes 1, 6 and 7 bands at 3132-3431 cm-1

show the

presence of water molecule.

4

5

3783.001

3503.793

3383.606

2879.227

2708.1382577.708

2189.993

2073.814

1904.120

1652.578

1582.203

1452.0751411.730

1333.157

1246.7861121.971

1010.590

943.129

852.734

761.733

654.431

608.807

450.572

VSI-Ni acetate

3500 3000 2500 2000 1500 1000 500

180

160

140

120

100

80

60

40

20

Wavenumber

%Tr

ansm

ittan

ce

6

Figure 1b. Showing I.R. Spectra for the complexes 4 to 6

185

3473.7203146.208

2301.402

1542.793

1418.240

1196.856

1153.428

1026.522

967.437

904.971

818.175755.290

678.638

629.378

555.255

VS-2-Ni(ac)2

3500 3000 2500 2000 1500 1000 500

30

25

20

15

10

5

0

Wavenumber

%Tr

ansm

ittan

ce

3525.661

3421.659

3236.806

3018.370

2981.312

2917.666

2846.812

2546.780

2362.683

2134.9362050.914

1895.147

1591.164

1444.816

1403.629

1338.967

1277.114

1173.532

1103.751

1033.538

940.033

867.111

766.439

665.588

606.751

479.793

V2R-Z

3500 3000 2500 2000 1500 1000 500

220

200

180

160

140

120

100

80

60

40

20

0

-20

Wavenumber

%T

rans

mitt

ance

7

9

Figure 1c. Showing I.R. Spectra for the complexes 7 and 9

The electronic absorption spectra of complexes 1-5 show strong red

shifted (cf. spectra of ligands) charge transfer bands in the range 27100- 23640 cm-1

which may all be assigned to ligand to metal transitions. Compounds 1 to 5 show weak d-

d bands in the range λmax 15456- 14493 cm-1

in the visible region (Figure 2). The

broadness of the absorption maxima and low intensity are suggestive of d – d transitions

and a high energy shift of the bands, falling above 14000 cm-1

(or below 700 nm),

indicate distortion of the octahedral geometry to square-pyramidal.27, 28

Complex 5 has a

tripodal ligand with three metal centers. A single d-d band indicates that all three of them

are in a similar five coordinated mode. It could be achieved by coordination of each

Cu(II) with a N of the imine, an O of the phenol, a molecule of water and a chelating

acetate group.

The electronic spectra of Ni(II) complexes 6-7 show features typical for an

octahedral environment around the metal ion. Usually the octahedral Ni(II) complexes

exhibit three energy bands at 7000-13000 cm-1

, 11000-20000 cm-1

and 19000-27000 cm-1

186

corresponding to 3A2g →

3T1g (F) (ν1),

3A2g →

3T2g (F) (ν2) and

3A2g →

3T1g (P)

(ν3)transitions.27a

Both 6 &7 show ν1 bands at 11286 and 9689 cm-1

and ν2 bands at

16863 and 15772 cm-1

(Figure 2). The third band being observed at 27173 and 27472 cm-

1, respectively is having very high intensity 1.3x10

4 and 1.3x10

4 M

-1cm

-1, respectively

hence cannot be termed as a d-d transition. It has been tentatively assigned to a L→M

charge transfer transition. However, using the ratio of ν1/ν2 bands, the ν3 bands are

expected to lie at 26144 cm-1

and 26490 cm-1

respectively.27b

That means that the ν3 bands

are lying very close to the CT band and thus are being overlapped by them. The 10Dq

and B values calculated from these data are 11200 cm-1

, 622 cm-1

for 6 and 9690 cm-1

,

881 cm-1

for 7, respectively. In the spectrum of 6 a spin forbidden transition 3A2g →

1Eg is

also seen on the high energy side of ν1 band at 13292 cm-1

. In these cases when Dq/B is

close to unity the ν2 transition is often seen as clear doublet which may be a consequence

of the transition to the 1Eg level gaining intensity because of the interaction with the

3T1g

(F) level.29

The spectra of Zn(II) show strong CT bands in the region 42000 - 43000 cm-1

.

187

0

0.005

0.01

0.015

450 650 850 1050

Ab

s.

nm

1

2

3

0

0.05

0.1

0.15

0.2

450 650 850 1050

Ab

s.

nm

4

5

0

0.02

0.04

0.06

0.08

0.1

450 650 850 1050

Ab

s.

nm

6

7

0

0.2

0.4

0.6

0.8

300 400 500 600

Ab

s.

nm

1

2

5

0

0.03

0.06

0.09

0.12

300 400 500 600

Ab

s.

nm

3

4

0

0.5

1

1.5

280 330 380 430 480

Ab

s.

nm

6

7

a b

c d

e f

Figure 2. (a-c) Showing d-d bands for the complexes 1 to 7 (d-f) showing CT bands for the

complexes 1 to 7.

In the 1H NMR spectrum of HL2 there are different signals at 1.59-1.91, 2.23,

2.73, 3.63, 8.43 and 13.5 corresponding to multiplet of -CH2, singlet of -NCH3, two

triplets of -NCH2, singlet of CH=N and singlet of -OH proton respectively. On

complexation of HL2 with Zn2+

(complex 8) the signal of -CH2 has shifted from 1.59-

1.91 in free ligand to 1.89-1.96 giving a downfield shift of 0.03-0.05. The signal of -

NCH3 has moved from 2.23 to 2.46 giving a downfield shift of 0.23. The signal

188

of –NCH2 has shifted from 2.73, 3.63, to 2.97, 3.89 giving a downfield shift of

0.24 and 0.26 respectively. Similarly the signal of – CH=N has moved from 8.34 to

8.10 giving an upfield shift of -0.24.

Figure 3. Showing changes in 1H NMR of HL4 on Complexation with Zn

2+

In the 1H NMR spectrum of ligand HL4 there are different signals at 1.66-1.75,

2.24, 2.35, 2.73, 3.98 and 5.46 corresponding to multiplet of -CH2, singlet of -NCH3, two

triplets of -NCH2, singlet of -CH2, and -NH proton respectively. The signal corresponding

to -OH proton is not visible due to broadness in the signal. Similarly as in complex 8 the

1H NMR of complex 9 showed upward shift in the -NCH3 signals of the ligand from

2.24 to 2.05 with -0.19 (Figure 3). The signal of –NCH2 has shifted from 3.98 to

3.08 giving an up field shift of -0.90. Signals are corresponding to -NH showed

189

upfield shift from 5.46 to 3.83 with -1.63 with simultaneous appearance of signals

corresponding to acetate group at 1.84 on complexation. All these evidences confirm

coordination of the ligands with metal salt.

Table 2 showing spectral and other data for 1 to 9

Cpx UV-vis max, nm, cm

-1 (M

-1cm

-1)

μB

(BM)

Λ ( Ω-1

cm2

mol.-1

) CT band d-d band

1 370, 27027 (3140) 647, 15456 (100) 2.6 2.0

2 369, 27100 (4530) 666, 15015 (110) 1.99 2.54

3 423, 23640 (630) 661, 15128 (100) 2.5 7.8

4 383, 26109 (1010) 690, 14492 (109) 1.96 3.8

5 392, 25510 (7220) 659, 15174 (177) 2.3 3.7

6 368, 27173 (13170) 886, 11286 (154),

593, 16863 (54)

3.39 4.8

7 364, 27472 (11200) 1032, 9689 (31),

634, 15772 (72)

2.9 5.2

8 232, 43103 (17440) - - 4.5

9 236, 42372 (8150) - - 3.6

4.2.3 Description of the X-Ray molecular and Crystal Structures-

[{Cu(L1)}2(µ-CH3COO)2] (1) - Figure 4 shows the final structure of compound 1. It

shows a dinuclear structure due to bridging through O2 of the acetate group. The second

oxygen, O3 of the acetate group remains uncoordinated and is involved only in H-

bonding interactions. The bond lengths C12-O3 1.231(8) and C12-O2 1.298(8) Å (Table

3) also indicate coordination through single oxygen only. The bond distances Cu-O2 and

Cu-O2i

are 1.961(4) and 2.399(4) Å, respectively which is that the bridging is highly

unsymmetrical through a single oxygen O2 of the acetate group (i = 1-x+2,-y+1,-z).

Mean Cu-N and Cu-O distances are 2.032(6) and 1.942(4) Å, respectively. Coordination

geometry around Cu may be considered as square pyramidal with amine nitrogen N1,

imine nitrogen N2, phenolic oxygen O1 and acetate oxygen O2 at the equatorial

positions whereas the axial coordination position is occupied by the bridging O2i atom in

190

the dimer. The geometry around the copper(II) ion is best described as an ideal square–

pyramidal with a very small trigonal–bipyramidal distortion parameter () of 0.06 [ =

(β–α)/60, with α and β being the two largest coordination angles, 174.6° and 171.0°,

respectively] (Table 3). In a perfect square-pyramidal geometry equals 0, while a value

of 1 would indicate perfect trigonal–bipyramidal geometry.30

The four coordinating atoms

O1, O2, N1 and N2 define a plane with maximum deviation of 0.02 Å. The Cu(II) ion is

0.1 Å out of this basal plane towards the apical O2i atom. Such a displacement is quite

common in square–pyramidal copper(II) complexes.31

In the dimer the two square

pyramids are sharing one base-to-apex edge with parallel basal planes. The N1–Cu–N2

angle (83.8(2)°) is ~ 7° smaller than the N1–Cu–O1 (91.0(2)°) because the former is a

part of a tight 5-membered chelate ring and the latter is involved in a 6-membered ring.

The 5–membered chelate ring (defined by Cu, N2, C9, C8 and N1) makes a dihedral of

11.5(1)° with the 6–membered chelate ring (defined by Cu, N1, C7, C6, C1, and O1).

Both chelate rings are non-planar with the former adopting an ‘open envelope’

conformation due to the disposition of C9 and N2, being 0.4 and 0.2 Å above and below

its plane, respectively. The five non-metallic members of the six membered chelate ring

are almost in a plane with a maximum deviation of 0.03 Å for C6 and the Cu(II) lies 0.3

Å above this plane, again forming the open end of an ‘envelope like’ conformation. The

six membered chelate ring is almost in plane with the phenyl ring (dihedral angle

6.2(2)). The Cu···Cui non-bonding distance is 3.405(3) Å which is much too large to

suggest any significant interaction.

Figure 4. ORTEP diagram of complex 1 and the labeling scheme used

191

The crystal structure shows (Figure 5) a H-bonded 2D network. Intermolecular H-

bonding between free acetate oxygen O3 and methyl group of the acetate ion gives rise

to a 1D polymeric chain parallel to the b axis. This chain formation is further facilitated

by H-bonding between methylene carbons C10 and C11 with phenolic oxygen O1 and

between methyl C13 and N1 ( Table 4). Free acetate oxygen O3 is also acting as a H-

bond acceptor from imine C7, which cross links these polymeric chains, forming a ‘brick

wall shaped’ 2D extended H-bonded network in the ab plane. C8···O3 and C8···N1

interactions also take part in strengthening this cross linkage.

Figure 5. Representation of the H-bonded 2D network in (1). C13-H13B···O3 (horizontal green

dotted bonds) form 1D polymeric chains parallel to b axis. O3 is also acting as a H-bond

acceptor from imine C7, (vertical green dotted lines) which cross links these polymeric chains,

forming a ‘brick wall shaped’ 2D extended H-bonded network in the bc plane, C (grey), H

(white), O (red), N (blue), Cu (cyan).

192

[(CuL2)(µ-CH3COO)] (2) Figure 6 shows the monomeric structure of the compound 2.

The Cu(II) ion is five coordinated by an imine through N1, N2 and O1 and acetate

oxygens O2 and O3. The acetate anion is behaving as an unsymmetrical bidentate

chelating ligand with Cu-O3 1.993(3) and Cu-O2 2.331(3) Å. The bond lengths C13-O3

of 1.257(5) Å and C13-O2 of 1.244(4) Å being equal indicate chelation through the

acetate group. Cu-N (imine) 1.929(3) Å is significantly shorter than Cu-N(amine)

2.142(3) Å. The stereochemistry around Cu (II) may be considered as highly distorted

square pyramidal with O1, O3, N1 and N2 at the equatorial positions and O2 at the axial

position. A high value of the trigonal-bipyramidal distortion parameter τ 0.43 indicates

towards a distorted structure. The angles N1-Cu-N2 93.47(12) and N1-Cu-O1 95.04(11)°

are similar (Table 3). The two six-membered chelate rings are having a dihedral angle of

40.7° and are twisted significantly with respect to the each other. The chelate ring which

contains three methylene groups and a flexible amine nitrogen N2 is in a chair

conformation whereas the other six membered chelate ring is rigid and almost planar with

maximum deviation of 0.04 Å for the imine nitrogen N1 and is planar to the phenyl ring

as well (dihedral angle 2.3(1)).

Figure 6. Showing the ORTEP diagram for complex 2 and the labeling scheme used

The crystal packing of 2 shows an interesting network of weak C-H···O H-bonds.

Imine carbon C7 exhibits a double H-bond donor to hydroxyl O1 and acetate oxygen O3,

193

respectively, whereas methylene C8 displays a H-bond with hydroxyl O1. Thus O1 is a

double H-bond acceptor and C7 is a double H-bond donor. These interactions give rise to

the formation of zig-zag chains parallel to b axis. These chains are then crosslinked

among each others due to C9···O2 H-bonding interactions, forming a 2D structure in the

bc plane and connected by methylene C10···O3 and methyl C14···O2 in the ac plane

(Figure 7).

Figure 7. Showing crystal packing for complex 2 with zig-zag chains parallel to b axis due to

C7…O1 H-bonding interactions. These chains are interconnected by C9···O2 interactions forming

a 2D structure, , C (grey), H (white), O (red), N (blue), Cu (cyan).

[(Cu-µ-L3)(CH3COO)]2 (3) Figure 8 Showing the ORTEP diagram of complex 3 with

µ–phenoxo group. This compound forms a centrosymmetric dimer due to bridging

bidentate coordination through phenolic oxygen O1. The stereochemistry around each Cu

(II) is best described as a distorted square pyramidal with a value of τ being 0.31. The

equatorial positions are occupied by the amine nitrogen atoms N1 and N2, oxygen O2 of

the monodentate coordinating acetate group and the phenoxo oxygen O1. The axial

position is occupied by the bridging phenoxo oxygen O1 from the symmetry related

molecule. The monodentate coordination through acetate ion is also ratified by unequal

C12-O2 1.284(12) and C12-O3 1.236(12) Å bond distances. The two square pyramids in

194

the dimer are sharing one base-to-apex edge with parallel basal planes. Average Cu-N

and Cu-O distances are 2.044 and 1.976 Å, respectively (Table 3). The Cu-O(phenoxo)

distances Cu-O1i

and Cu-O1 are 1.953(6) and 2.222(7) Å, respectively which shows that

the bridging by phenolic oxygens is highly unsymmetrical (where i = -x+1,-y+2,-z+2 )

and the symmetry related phenoxo oxygen O1 is closer to the Cu(II) ion than the one

belonging to the original molecule. The Cu···Cui non-bonding distance between the two

metal ions is 3.185 Å, shorter than that found in compound 1. The N1–Cu–N2 bite angle

(84.0 °) is ~ 5° smaller than the N1–Cu–O1 angle (89.3°) as explained above (Table 3).

The six membered chelate ring is neither planar nor it is in plane with the phenyl ring

unlike that in (2). It is forming a dihedral angle of 25.8(3) with the phenyl ring. The five

membered and six membered rings form a dihedral angle of 70(3) ° which is much larger

than found in 1 giving rise to an ‘open book- like conformation around the Cu-N1 bond.

This is due to the change from a sp2 C and N of (1) to sp

3 C and N in the reduced

compound 3 making it more flexible and also accounting for the distortion of

stereochemistry towards trigonal bipyramid. This tilting of the five membered ring brings

methylene C8 close to the phenyl ring resulting in a C-H··· interaction with centroid···C8

distance as 3.94 Å.

Figure 8. Showing the ORTEP diagram of complex 3 with µ–phenoxo group.

195

Crystal structure of 3 shows that each dinuclear unit has weak C7-H7B···O2 and

N1-H1N···O3 kind of intramolecular H-bonds within it which further connect the

dinuclear species with each other. Packing diagram (Figure 9) shows intermolecular H

bonds between amine nitrogen N1 to the free oxygen O3 from the acetate group,

forming linear undulating chains running parallel to c axis (Table 4)

Figure 9. Showing N1-H1N···O3 intermolecular H-bonds in 3, forming linear, undulating chains

parallel to c axis (a) down the b axis (b) down the a axis, , C (grey), H (white), O (red), N (blue),

Cu (cyan).

[(Ni L1)(CH3COO) (H2O)2]) 0.25 H2O (6) The molecular structure of the complex 6

(Figure 10) shows that the Ni(II) is being coordinated by phenolate oxygen O1, imine

nitrogen N1, amine nitrogen N2 from the deprotonated ligand L1. A monodentate acetate

group coordinating through oxygen O2, and two water molecules (O4 and O5) occupy

the remaining positions in an octahedral environment around Ni(II) ion. The Ni-

N1(imine), Ni-N2(amine), Ni-O1(phenolate), Ni-O2(acetate) and Ni-O4/O5(water) bond

lengths are 2.029(6), 2.174(6), 2.048(5), 2.058(5), 2.123(5) and 2.120(5) Å, respectively

(Table 3). The bond angles around Ni(II) show that there is small deviation from the

octahedral geometry around the metal ion. Apart from this there is a disordered lattice

water molecule (O6) lying on the four fold axis thus contributing 1/4th

towards each

molecule. The six membered chelate ring is almost planar to the phenyl ring with

dihedral angle 5.99(2) between them similar to that seen in (1). Similarly the five

membered chelate ring adopts an ‘open envelope’ conformation with C9 forming the

a

b

196

open end. The dihedral angle between the five and six membered chelate rings is

8.75(1).

Figure 10. ORTEP diagram of (6) at 50% probability showing the final structure and the

labeling scheme used. Lattice water has been removed.

Crystal structure of the mononuclear complex is rather interesting showing self

assembled molecules resulting in the formation of parallel layers with four fold symmetry

down the c axis. The self assembly pattern within each layer may be understood by

considering the formation of a dimer of the complex molecule due to bifurcating H-

bonding interactions between water molecule O4 with phenolic oxygen O1 and acetate

oxygen O2. Four such dimers in turn, interact in the same plane (ab plane) via bifurcated

O5(water)···O3(acetate) and O4(water)···O2(acetate) interactions forming 24 membered

octagonal shaped cages encompassing 16 oxygen atoms about the centre of inversion

(Figure 11).

Each such cage is sharing walls with four other similar cages which generates a

smaller octagonal cavity consisting of eight oxygen atoms (four O5 and four O3 atoms)

with a four fold axis of rotation passing through the centre. A regular repetition of the

pattern generates a highly symmetric 2D structure having alternate cages and cavities

running perpendicular to c axis (Figure 12). While the interior of each cage is occupied

197

by the inwardly pointing phenyl groups the octagonal cavities are occupied by the water

molecule(O6) lying slightly above the plane formed by eight oxygen atoms. This water

molecule is weakly hydrogen bonded to all the eight oxygens with O6···O5 3.8 and

O6···O3 3.7 Å. These 2D parallel sheets have hydrophilic interiors with channels of water

molecules with hydrophobic edges, running parallel to the c axis.

Figure 11. Showing the formation of dimers and octamer due to H-bonding interactions in

complex (6). All oxygens in contact are given as red balls and Ni(II) ions are as green balls.

Hydrogens have been removed for clarity and the interactions are shown as O···O.

198

Figure 12. Crystal structure of (6) showing ab plane consisting of alternate arrangement of

octagonal cages and cavities. The cavities are filled with lattice water molecule (big red balls) (a)

hydrogens have been removed (b) hydrogens and carbons have been removed for clarity.

[{(Ni( µ-L2) (CH3COO)}2 (µ-H2O)] (7) Figure 13 shows an ORTEP view of the

complex which is dinuclear with each Ni(II) ion being six coordinated by one imine

nitrogen (N1/N3), one amine nitrogen (N2/N4), two bridging phenolate oxygens (O1,O2)

of the two deprotonated L2 ligands, a bridging water molecule(O3) and a monodentate

carboxylate group(O4/O6). The stereochemistry around each Ni(II) ion may be

considered as distorted octahedral with a N2O4 coordination sphere consisting of two six

membered chelate rings. The geometry around two nickel ions in this dinuclear complex

is akin to two face sharing octahedra as shown in Figure 14, with Ni···Ni distance being

2.857(2) Å (Table 3).

a b

199

Figure 13. Showing the ORTEP diagram of (7) and the labeling scheme being used. Hydrogens

have been removed for clarity.

The two Ni-N(imine) bond distances 2.004(5) and 1.988(5) Å are shorter than the

two Ni-N(amine) distance 2.175(5) and 2.143(5) Å as expected for the sp2 and sp

3

hybridised nitrogen atoms. The phenolate oxygens are unsymmetrically bridging between

the two Ni(II) ions with Ni-O distances being 2.024(4), 2.185(4) and 2.020(4), 2.174(4)

Å, respectively. The water oxygen O3 however, is symmetrically bridging between the

two ions with Ni-O distances being 2.077(4) and 2.078(4) Å, respectively. Similarly the

two Ni-O(acetate) distances are also equal and comparable to the Ni-O(water) distances.

The two phenyl rings in the dinuclear compound are almost planar with an interplanar

angle of 26.7(2) but the minimum distance between their centroids is 4.42(1) Å which

deludes any possibility of ··· interactions between them. The six membered chelate

rings adjacent to the phenyl rings are almost planar with interplanar angles as 5.7(2) and

10.3(2) with the corresponding phenyl rings. However the other six membered chelate

rings are in chair conformation. The dihedral angles between the two six membered

chelate rings are 65.4(1) and 66.5(1), again giving an ‘open book’ kind of conformation

around the metal ion. The coordinated water molecule O3 forms strong intramolecular H-

200

bonding with the free acetate oxygens O5 and O7 (O3-H1W···O7, 2.528(6) Å, 1.66 Å,

167 and O3-H2W···O5, 2.510(6) Å, 1.65 Å, 147) as well as the coordinated acetate

oxygens O4 and O6 ( O3-H2W···O4, 2.977(5) Å, 2.77 Å, 93 and O3-H1W···O6,

2.961(6) Å, 2.56 Å, 108).

Figure 14. Showing the two face sharing octahedra in the dinuclear complex (7).

[Zn(L2)(CH3COO)] (8) Zn(II) ion is five coordinated via two nitrogens and one oxygen

from the ligand and a bidentate chelating the acetate ion. The ligand is thus forming two

six membered chelate rings with the metal ion whereas the acetate is forming a four

membered ring. The stereochemistry around Zn(II) may be described as a distorted

square pyramidal with a large contribution from the trigonal-bipyramidal distortion

constant () 0.21 in it ( = (β-)/60 where β and are the two largest coordination

angles 149.64 and 137.10 around the metal ion). The Zn- N distances are 2.009(4) and

2.104(5) Å and Zn-O are 1.932(4) (phenolate) and 2.098(5) and 2.299(5) Å for the

acetate group (Table 3). Two mean planes were passed through the two six membered

chelate rings about the metal which show a dihedral angle of 45.9(1) between them

forming ‘an open book’ type conformation around the metal ion. The chelate ring

containing three methylene groups and a flexible amine nitrogen N2 is in a chair

conformation whereas the other six membered chelate ring is almost planar. The latter is

almost planar to the phenyl ring as well (dihedral angle 4.2(1) ).

201

Figure 15. Showing the ORTEP diagram of the complex (8) and the labeling scheme being used.

Hydrogens have been removed for clarity.

The crystal structure shows that there are C···O type H-bonding interactions between the

imine C7 and phenolate oxygen O1 and acetate oxygen O2 and methylene C10. The C7-

H7···O1 (3.317(7), 2.53Å, 142 ) and C10-H10B…

O2 (3.588(8) Å, 2.68 Å, 157 )

interactions give rise to a 2D structure in the ab plane (Figure 16).

Figure 16. Showing the weak H-bonding interactions forming a 2D network in the ab plane, for

complex (8).

202

[Zn2(L4)(-CH3COO)2, (CH3COO)] (9) The complex is a dinuclear (Figure 17)

compound having two Zn(II) ions in two different coordination environments. Zn1 is

pentacoordinated by two amine nitrogens N1, N2, one bridging phenolate oxygen O1 and

two bridging bidentate acetate ions. Whereas Zn(II) is four coordinated by the above

mentioned bridging atoms and a third monodentate acetate group. These two polyhedra

share a corner between them as shown in Figure 18. The coordination geometry around

Zn(II) is distorted tetrahedral but that around Zn(II) may be called as a trigonal bipyamid

because of a large contribution from the trigonal-bipyramidal distortion constant () 0.90

in it ( = (β-)/60 where β and are the two largest coordination angles 177.2 and

123.1 around the metal ion). M-O phenolic and carboxylic distances with the tetrahedral

Zn(II) are significantly smaller (1.957(6),1.951(7), 1.988(7)Å) than with the five

coordinated Zn(II) (2.126(6), 2.011(6), 2.002(6)Å), respectively thus pointing towards

unsymmetrical bridging by the phenolate and acetate groups. (Table 4). Zn-N distances

are 2.058(6) and 2.190(7) Å.

Figure 17. Showing the ORTEP diagram of (9) along with the labeling scheme being used.

Hydrogens have been removed for clarity

203

Both the six membered chelate rings around Zn1 are non-planar unlike the

structure of its corresponding Schiff base complex (8) where one was found almost

planar and also in plane with the phenyl ring. This may be due to the reduction of imine

N and also due to the bridging nature of the phenolate group. The dihedral angle between

the two mean planes is now reduced only to 17.6(1) as compared to (40.7(1) ),

(65.4(1),66.5(1) ) and (45.9(1)) in 2, 7 and 8 (Table 3). Here the phenyl ring is making

a dihedral angle of 34.1(1) with respect to the adjacent six membered chelate ring

instead of being in plane as found in 2, 7 and 8 complexes. The latter is due to the fact

that Zn1 is in trigonal bipyramidal geometry and does not remain in plane with the

phenyl ring. The Zn(II)···Zn(II) non-bonding distance is found to be 3.169(6) Å. The M-

O distance between the monocoordinated acetate group is 1.901(7) Å whereas its other

oxygen O7 is at a much larger distance 2.936(6) Å away from Zn2. The latter is also

ratified by its unsymmetrical C-O distances (C17-O6 1.249(12), C17-O7 1.190(11) Å).

Interestingly there are present agnostic interactions32

between phenyl proton H6 and Zn2

with Zn2···H6 distance being 2.82 Å.

Figure 18. Showing a corner sharing between the two polyhedra around two Zn(II) ions, in the

complex (9)

The crystal packing shows (Figure 19) the formation of linear chains running

parallel to the b axis due to H-bonding interactions between the free oxygen O7 of the

monodentate acetate group and the amine nitrogen N1, O7 and the methyl C11 (Table 4)

and also due to C-H··· interactions between the methyl C18 and the phenyl ring

204

(C18···centroid distance of 3.681 Å). Two such centrosymmetric chains are in turn joined

to each other due to methylene C7···O7 and methylene C8···O1(phenolate) H-bonding

interactions as well as methylene C-H··· interactions between methylene C8 and the

phenyl ring ( C8···centroid distance being 3.955 Å ) forming linear tapes.

Figure 19. Showing H-bonding interactions leading to the formation of centrosymmetric tapes

along the b axis.

Table 3 –Showing selected bond lengths [Å] and angles [°]

Complex 1

N(1)-Cu 1.955(6) N(2)-Cu 2.109(6)

O(1)-Cu 1.923(5) O(2)-Cu 1.961(4)

O(2)-Cu#1 2.399(4) Cu-O(2)#1 2.399(4)

O(1)-Cu-N(1) 91.0(2) O(1)-Cu-O(2) 88.55(19)

N(1)-Cu-O(2) 174.6(2) O(1)-Cu-N(2) 171.0(2)

N(1)-Cu-N(2) 83.8(2) O(2)-Cu-N(2) 96.0(2)

O(1)-Cu-O(2)#1 89.51(19) N(1)-Cu-O(2)#1 107.6(2)

O(2)-Cu-O(2)#1 77.79(18) N(2)-Cu-O(2)#1 99.0(2)

Complex 2

N(1)-Cu 1.929(3) N(2)-Cu 2.142(3)

O(1)-Cu 1.901(2) O(2)-Cu 2.331(3)

O(3)-Cu 1.993(3)

O(1)-Cu-N(1) 95.04(11) O(1)-Cu-O(3) 94.23(11)

N(1)-Cu-O(3) 156.14(12) O(1)-Cu-N(2) 130.28(12)

N(1)-Cu-N(2) 93.47(12) O(3)-Cu-N(2) 97.19(11)

O(1)-Cu-O(2) 131.38(11) N(1)-Cu-O(2) 98.03(11)

O(3)-Cu-O(2) 59.82(10) N(2)-Cu-O(2) 95.44(12)

O(1)-Cu-C(13) 116.29(12) N(1)-Cu-C(13) 127.53(13)

O(3)-Cu-C(13) 30.15(12) N(2)-Cu-C(13) 95.85(12)

O(2)-Cu-C(13) 29.74(11)

205

Complex 3

N(1)-Cu 2.008(8) N(2)-Cu 2.081(9)

O(1)-Cu#1 1.953(6) O(1)-Cu 2.222(7)

O(2)-Cu 1.993(7) Cu-O(1)#1 1.953(6)

O(1)#1-Cu-O(2) 98.3(3) O(1)#1-Cu-N(1) 167.5(3)

O(2)-Cu-N(1) 90.6(3) O(1)#1-Cu-N(2) 92.6(3)

O(2)-Cu-N(2) 148.9(3) N(1)-Cu-N(2) 84.0(3)

O(1)#1-Cu-O(1) 80.7(3) O(2)-Cu-O(1) 99.8(3)

N(1)-Cu-O(1) 89.1(3) N(2)-Cu-O(1) 110.6(3)

Complex 6

Ni-N(1) 2.029(6) Ni-O(1) 2.048(5)

Ni-O(2) 2.058(5) Ni-O(5) 2.120(5)

Ni-O(4) 2.123(5) Ni-N(2) 2.174(6)

N(1)-Ni-O(1) 89.0(2) N(1)-Ni-O(2) 177.8(2)

O(1)-Ni-O(2) 89.7(2) N(1)-Ni-O(5) 94.1(2)

O(1)-Ni-O(5) 91.4(2) O(2)-Ni-O(5) 87.7(2)

N(1)-Ni-O(4) 91.4(2) O(1)-Ni-O(4) 85.8(2)

O(2)-Ni-O(4) 86.8(2) O(5)-Ni-O(4) 173.85(19)

N(1)-Ni-N(2) 82.6(2) O(1)-Ni-N(2) 169.7(2)

O(2)-Ni-N(2) 98.6(2) O(5)-Ni-N(2) 95.1(2)

O(4)-Ni-N(2) 88.5(2)

Complex 7

N(2)-Ni(1) 2.175(5) N(3)-Ni(2) 1.988(5)

N(4)-Ni(2) 2.143(5) O(1)-Ni(1) 2.024(4)

O(1)-Ni(2) 2.185(4) O(2)-Ni(2) 2.020(4)

O(2)-Ni(1) 2.174(4) O(3)-Ni(1) 2.077(4)

O(3)-Ni(2) 2.078(4) O(3)-H(1W) 0.8871

O(3)-H(2W) 0.9564 O(4)-Ni(1) 2.049(4)

O(6)-Ni(2) 2.064(4) Ni(1)-Ni(2) 2.8572(12)

N(1)-Ni(1)-O(1) 90.31(18) N(1)-Ni(1)-O(4) 97.55(18)

O(1)-Ni(1)-O(4) 165.92(17) N(1)-Ni(1)-O(3) 169.58(18)

O(1)-Ni(1)-O(3) 79.34(15) O(4)-Ni(1)-O(3) 92.34(16)

N(1)-Ni(1)-O(2) 99.23(17) O(1)-Ni(1)-O(2) 78.80(16)

O(4)-Ni(1)-O(2) 88.38(16) O(3)-Ni(1)-O(2) 77.70(15)

N(1)-Ni(1)-N(2) 86.98(19) O(1)-Ni(1)-N(2) 102.74(17)

O(4)-Ni(1)-N(2) 89.38(18) O(3)-Ni(1)-N(2) 96.43(17)

O(2)-Ni(1)-N(2) 173.63(17) N(1)-Ni(1)-Ni(2) 124.77(14)

O(1)-Ni(1)-Ni(2) 49.66(11) O(4)-Ni(1)-Ni(2) 116.61(12)

O(3)-Ni(1)-Ni(2) 46.57(11) O(2)-Ni(1)-Ni(2) 44.83(10)

N(2)-Ni(1)-Ni(2) 131.93(13) N(3)-Ni(2)-O(2) 90.53(19)

N(3)-Ni(2)-O(6) 96.2(2) O(2)-Ni(2)-O(6) 165.08(16)

N(3)-Ni(2)-O(3) 171.39(18) O(2)-Ni(2)-O(3) 81.22(16)

O(6)-Ni(2)-O(3) 91.27(16) N(3)-Ni(2)-N(4) 87.9(2)

O(2)-Ni(2)-N(4) 100.76(19) O(6)-Ni(2)-N(4) 92.80(19)

O(3)-Ni(2)-N(4) 96.1(2) N(3)-Ni(2)-O(1) 100.28(19)

O(2)-Ni(2)-O(1) 78.64(15) O(6)-Ni(2)-O(1) 87.05(16)

O(3)-Ni(2)-O(1) 75.75(15) N(4)-Ni(2)-O(1) 171.79(19)

206

N(3)-Ni(2)-Ni(1) 125.47(15) O(2)-Ni(2)-Ni(1) 49.36(11)

O(6)-Ni(2)-Ni(1) 116.65(11) O(3)-Ni(2)-Ni(1) 46.55(10)

N(4)-Ni(2)-Ni(1) 128.95(16) O(1)-Ni(2)-Ni(1) 44.92(10)

Complex 8

N(1)-Zn 2.009(4) N(2)-Zn 2.104(5)

O(1)-Zn 1.932(4) O(2)-Zn 2.098(5)

O(3)-Zn 2.299(5)

O(1)-Zn-N(1) 94.66(18) O(1)-Zn-O(2) 98.05(19)

N(1)-Zn-O(2) 149.64(19) O(1)-Zn-N(2) 121.46(19)

N(1)-Zn-N(2) 94.50(19) O(2)-Zn-N(2) 102.03(19)

O(1)-Zn-O(3) 137.10(19) N(1)-Zn-O(3) 94.13(18)

O(2)-Zn-O(3) 58.33(19) N(2)-Zn-O(3) 99.5(2)

Complex 9

N(1)-Zn(1) 2.058(6) N(2)-Zn(1) 2.190(7)

O(1)-Zn(2) 1.957(6) O(1)-Zn(1) 2.126(6)

O(2)-Zn(1) 2.011(6) O(3)-Zn(1) 2.002(6)

O(4)-Zn(2) 1.951(7) O(5)-Zn(2) 1.988(7)

O(6)-Zn(2) 1.901(7)

O(3)-Zn(1)-O(2) 121.8(3) O(3)-Zn(1)-N(1) 123.1(3)

O(2)-Zn(1)-N(1) 115.2(3) O(3)-Zn(1)-O(1) 90.2(2)

O(2)-Zn(1)-O(1) 89.3(3) N(1)-Zn(1)-O(1) 90.2(2)

O(3)-Zn(1)-N(2) 88.1(3) O(2)-Zn(1)-N(2) 89.7(3)

N(1)-Zn(1)-N(2) 92.6(3) O(1)-Zn(1)-N(2) 177.2(3)

O(6)-Zn(2)-O(4) 118.4(3) O(6)-Zn(2)-O(1) 124.7(3)

O(4)-Zn(2)-O(1) 104.2(3) O(6)-Zn(2)-O(5) 97.9(3)

O(4)-Zn(2)-O(5) 108.2(4) O(1)-Zn(2)-O(5) 100.6(3)

Table 4 –Showing important H-bonds for the different complexes.

For complex (1)

C8-H8A...O3i 3.692(9) 2.95 134

C8-H8B...N1i 3.647(10) 2.77 150

C7-H7...O3i 3.252(8) 2.40 152

C10-H10B...O1ii 3.268(8) 2.35 160

C11-H11A...O1ii 3.714(11) 2.93 139

C13-H13A...N1ii 3.804(9) 2.89 159

C13-H13B...O3iii

3.567(10) 2.69 152

(i) -x+1/2+2,+y+1/2,-z+1/2 (ii) -x+2,-y+1,-z (iii) -x+2,-y,-z

For complex (2)

C8-H8A...O1i 3.661(5) 2.92 134

C7-H7...O1i 3.236(4) 2.48 138

C7-H7...O3i 3.851(4) 2.97 159

207

C9-H9B...O2ii 3.348(5) 2.65 129

C10-H10B...O3iii

3.659(5) 2.73 159

C14-H14B...O2iv

3.701(6) 2.91 141

(i) -x+1/2,+y-1/2,+z (ii) -x,-y+1,-z (iii) -x-1/2,+y-1/2,+z

(iv) x-1/2,-y+1/2+1,-z

For complex (3)

N1-H1N...O3 3.259(10) 2.79 131

C2-H2...O3i 3.415(15) 2.50 169

N1-H1N...O3ii 2.911(11) 2.32 150

(i) -x+1,-y+2,-z+2 (ii) -x+1,+y,-z+1/2+2

For complex (6)

X-H…Y X…Y H…Y X-H…Y

O5-H51...O3 2.846(7) 1.89 145

O5-H52...O3i 2.807(7) 1.83 159

O4-H42 ...O2ii 2.822(7) 2.07 150

O4-H41...O1ii

2.726(7) 2.01 149

( i) -y+1/2+1,+x+1,+z ( ii) -x,-y+2,-z+1

For complex (9)

X-H…Y X…Y H…Y X-H…Y

C7- H7A...O7i 3.387(1) 2.63 135

C8-H8A ...O1i 3.683(1) 2.71 177

C11-H11A...O7ii

3.549(1) 2.61 165

N1- H1...O7ii

3.016(1) 2.11 174

(i) -x+1,+y-1/2,-z+2 (ii) x,+y-1,+z

4.2.4 Magnetic Studies

Solid-state, variable-temperature (2-300 K) magnetic susceptibility data using 0.5

and 1.0 T fields were collected on polycrystalline samples of compounds 1 and 3,

respectively. The resulting data are plotted in Figures 20-21. The data for compound 1

show a behavior characteristic of very weak, antiferromagnetic coupled Cu2+

ions. The

MT value at 300 K is 0.86 cm3mol

-1 K, higher than that expected for two uncoupled S =

208

1/2 spins assuming g = 2 (0.75 cm3mol

-1K) and it does not vary upon cooling until

approximately 14 K, dropping faster then and arriving finally to 0.78 cm3mol

-1K at 2 K

(Figure 20) The experimental magnetic data were fitted using the equation of Bleaney-

Bowers33

for dinuclear copper(II) complexes with the Hamiltonian in the form Η = -

JS1S2. The best fit parameters were found for J = -0.37 ± 0.05 cm-1

, g = 2.14 ± 0.01 and

R= 2.0710-4

. Temperature-independent paramagnetism (TIP) was considered equal to

12010-6

cm3mol

-1.

0 50 100 150 200 250 300

0.78

0.80

0.82

0.84

0.86

0.88

MT

/ c

m3m

ol-1

K

T / K

Figure 20. Fitting of the MT vs T of compound (1) between 2.0 and 300.0 K. The experimental

data are shown as black spheres and the red line corresponds to the theoretical values.

The MT for compound 3 is depicted in Figure 21. The magnetic susceptibility

value MT at 300.0 K is 0.78 cm3mol

-1K varying smoothly and finally dropping to nearly

zero at low temperatures (0.02 cm3mol

-1K at 12 K). As before, this dinuclear system

exhibits an antiferromagnetic behavior although this time, there is a stronger interaction

between the copper centers. Indeed, the M vs T data also show a maximum at

approximately 50 K which is indicative of a significant coupling between the two Cu2+

ions (Inset Figure 21). Using derived Hamiltonian than the described above and taking

into account impurities in the sample, the best fit were J = -54.2 ± 0.4 cm-1

, g = 2.06 ±

0.01, = 0.046, TIP = 12010-6

cm3mol

-1 and R = 3.6710

-3.

209

Figure 21. Fitting of the MT vs T of compound (3) between 12.0 and 300.0 K. The inset shows

the MT vs T. The experimental data are shown as black spheres and the red line corresponds to

the theoretical values.

Compound 1 and 3 exhibit a similar global arrangement, all copper atoms are

pentacoordinated and in each compound, the two square pyramids are sharing one base-

to-apex edge with parallel basal planes.34

The single electron on each copper atom resides

mostly of the dx2

-y2

type (basal) and consequently, the interaction between the magnetic

orbitals of the two copper atoms is expected to be negligible.35

This is the case of

compound 1, in which local geometry for each copper center is an almost perfect square-

pyramid ( = 0.06)36

showing a very small magnetic coupling constant. Instead, the

environment of the copper ions in compound 3 shows a higher distortion towards trigonal

bipyramid ( = 0.31). This variation is enough to partially avoid the orthogonality of the

magnetic orbitals and the final coupling is antiferromagnetic (J = -54 cm-1

) and stronger

than for compound 1.

In principle, a coupling between two Cu(II) ions through two phenolate ligands is

expected to be stronger than one due to methoxides or like in the case of 1, via acetates.37

Such a difference in the linkers leads to a Cu-O-Cu bridge angle of 102.22° for 1 and

99.29° for 3, the latter being lower than the former. Both compounds show a rectangular

Cu2O2 core, with a Cu-Obasal distance shorter than the Cu-Oapical distance, however, the

difference between both distances is more significant for compound 1 than for 3. The Cu-

210

O bridging distances are 1.961 and 2.399 Å for 1 and 1.951 and 2.221 Å for 3, as well as

the Cu···Cu non-bonding distance being 3.405 and 3.188 Å for compounds 1 and 3,

respectively. Nevertheless, the J value found for compound 3 is relatively small

comparing with what one would expect for a coupling through a double phenoxo group

bridge.37

Once again, the square-pyramidal nature of the copper centers and their

interaction in a base-to-apex fashion (like for 1) would explain this value perfectly.

Analogous results were obtained by Rodriguez et al. studying a similar dinuclear cluster

to compound 3 using an amine bis (phenolate) derivative as a ligand.38

The most relevant

crystallographic parameters of this core are of the range of 3 (Table 5) with a final J of -

45 cm-1

. Earlier work performed by Saimiya et al. and Cros et al. also showed an

antiferromagnetic coupling between the two copper centers with a J value of ~ -20 cm-1

.39

Table 5. Structural and magnetic parameters of compounds with a [Cu2O2]2-

core sharing a base-

to-apex edge.

Compound

(deg)

d(Cu-Ob)

Å

d(Cu-Oa) Å Cu-O-Cu (deg) Cu··Cu Å J (cm-1

) ref

1 0.06

1.961

2.399

102.22

3.405

-0.39

*

3 0.31

1.953

2.221

99.29

3.185

-54.2 *

[(Cu(L1)2Cu)

(MeCN)2]

0.45

1.959

2.182

99.95

3.174

-45

33

[Cu(L2)2Cu]

[(CuL(L3)Cu

)] (ClO4)

0.40

0.43

0.43

1.971

1.974

1.936

1.917

1.980

2.199

1.998

2.093

1.989

2.106

99.65

103.27

101.18

101.76

103.84

3.19

3.114

3.124

-19.9

-277

34

[(Cu(L4)(L5)

Cu)]ClO4

0.14

0.29

1.917

1.884

1.961

2.524

88.9

107.2

2.742

-22

35

* This work. Ob= O basal, Oa= O axial. J values are referred to H=-JS1S2. L1 = N,N-bis(3,4-

dimethyl-2-hydroxybenzyl)-N,N-dimethylethylenediamine deprotonated, L2 = N,N-bis(2-

hydroxybenzyl)-N,N-dimethylethylenediamine deprotonated , L3 = N,N-bis(2-hydroxybenzyl)-

N,N-dimethylethylenediamine, L4 = 2-((2-((o-Hydroxy--

methylbenzylidene)amino)ethyl)amino)ethanol deprotonated, L5 = 2-((2-((o-Hydroxy--

methylbenzylidene)amino)ethyl)amino)ethanol. Cu3 (ref 69) shows two subunits in the cell

which distances are different and depicted separately; however, the value is identical in both.

211

4.3 Catecholase studies and kinetics

OH

OH

O

O

O2

Catalyst

Scheme 2 – Showing basic reaction involved in oxidation of catechol.

The complexes 1-5 were subjected to catecholase-mimitic activities to find their

capability to act as catalysts for the oxidation of alcohols to quinones, scheme 2 like

catechol oxidase. The oxidation of 3,5-DTBC to corresponding product 3,5-di-tert-

butylquinone (3,5- DTBQ) was followed by the development of a considerably stable and

strong absorption band at 390 nm in methanol.40

All the complexes 1-5 showed activity

towards the oxidation of catechols (Figure 22).

Immediately after addition of substrate 3,5-DTBC to the solutions of the catalysts 1

to 5 various LMCT bands of the complexes disappeared and a new band corresponding to

3,5-DTBQ appeared at 390-410 nm, as noted by Zipple et al.6a

They suggested that this

could only be due to the formation of Cu(I) and not due to the destruction of the complex

because the chelating ligands used by them and here as well, are much stronger than the

substrate, solvent or quinone. The subsequent increase in absorption of this band is linear

for all the copper(II) complexes. Thus all of these complexes are showing catecholase

activity.

212

1 2

35

4

Figure 22. showing catecholase activity for complex 1, 2, 3, 4 and 5 in methanol. An increase in

the band at ~390 nm with time for a fixed concentration of catalyst has been shown.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 5 10 15 20

T ime (min.)

Lo

g [

A∞

/ A

∞-

At]

[1]

[2]

[3]

[4]

[5]

Figure 23. Showing the course of absorption maxima at 390 nm with time for 100 equiv. of 3,5-

DTBC in solutions of 10-4

M complexes in methanol.

213

The kinetics of the oxidation of 3,5-DTBC was determined by the method of

initial rates by monitoring the growth of the 390-410 nm band of 3,5-DTBQ as a function

of time. A plot of log [Aα / (Aα -At)] versus time the catecholase activity of the complex

was obtained (Figure 23) and the rate constants for the catalytic oxidation were calculated

and given in Table 6. The rate constant values show that the reactivity of complexes is

differing significantly from each other. Compounds 4 and 1 show significant catecholase

activity. To determine the dependence of the rates on the substrate concentration and

various kinetic parameters, solutions of copper complexes were treated with different

concentrations of 3,5-DTBC (from 10 to 100 equivalents) under aerobic conditions. At

low concentrations of 3,5- DTBC a first-order dependence of the substrate concentration

was observed. At higher concentrations, saturation kinetics was found for all the

compounds. The effect is more pronounced for 4 and 1 whereas for the remaining three

complexes the rates are almost independent of the substrate even at lower concentrations.

The dependence on the substrate concentration indicates a catalyst-substrate binding to be

an initial step in the catalytic mechanism. The rates of reactions obtained for various 3,5–

DTBC concentrations were fitted to the Michaelis–Menten equation (Figure 24) and

linearized by means of Lineweaver-Burk plot to calculate various kinetic parameters

(Table 6) for these compounds.

0

1

2

3

4

5

6

7

8

9

10

0 0.002 0.004 0.006 0.008 0.01 0.012

V0

(M/s

)x 1

07

[ 3-5, DTBC ] (M)

[1]

[2]

[3]

[4]

[5]

Figure 24. Dependence of the initial rates on substrate concentrations for the oxidation process

promoted by various copper complexes.

214

Table 6. - Kinetics parameters for the oxidation of 3, 5-DTBC catalyzed by various Cu(II)

complexes.

Complex Rate Cons.

(m-1

)

Vmax (Ms-1

) Km (M) R* kcat (h

-1)

Turnover

rate

(1) 4.63 x 10-2

6.44 x10-7

33.12 x10-4

0.9593 23.2

(2) 3.65 x10-2

1.14 x10-7

21.28 x10-4

0.9960 4.1

(3) 4.17 x10-2

9.02 x10-8

12.25 x10-4

0.9874 3.2

(4) 8.12 x10-2

1.148 x10-6

20.01 x10-4

0.9946 41.3

(5) 4.07 x10-2

2.089 x10-7

15.68 x10-4

0.9976 7.5

* Discrepancy value of the Lineweaver-Burk plot.

Turnover rates ranging from 3.2 to 41.3 h-1

have been found which are

comparable to those reported by Krebs et al. and Neves et al.41

and to an order of two or

three times more than those reported by Neves et al.42

but are significantly lower than

those reported by Krebs et al., Monzani et al., and Vittal et al.22c, 43

Keeping in mind the

complicated mechanism of the reaction involved the data obtained from Lineweaver-

Burk plot are suffice for comparing the catecholase activity of the complexes which

follow an order 4 > 1 > 5 > 2 ~ 3. The differences in the relative performance of these

catalysts may be reasoned in the light of following known facts from the literature.

Rationale for the observed relative performance of catalysts

Various factors affecting the structure activity relationship, have been recognized

as Cu···Cu distance, flexibility of the ligand, type of exogenous ligand and coordination

geometry around the metal ion.5Although dimeric complexes are considered to be more

relevant for mimicking the catalytic activity owing to the active site of the naturally

occurring enzyme, both mononuclear and dinuclear copper complexes have been found to

show significant catecholase activity. Among the mononuclear complexes non-planar

complexes with intermediate coordination geometry between trigonal bipyramid and

square pyramidal are potential cases while square-planar complexes show little or no

activity.4a

In the dinuclear complexes a Cu···Cu distance 2.9 -3.2 Å has been suggested to

give the maximum activity44

owing to the requirement of a steric match between the

215

substrate and the catalyst. However, many dinuclear complexes having longer

metal···metal distance (up to 7.5 Å ) are known to be good catalysts.45

At the same time,

the nature of the exogenous bridging ligand in the dimeric complexes also shows a

remarkable influence on the activity, owing to the fact that a weakly coordinating ligand

may easily be displaced by the incoming catechol thus enhancing the activity.5 Neves et

al. have reported that in general acetate groups are competing with the substrate for the

binding site, decreasing the activity.41c

A single hydroxo- or phenoxo- bridged complex

on the other hand has been reported to augment the activity by enforcing a very strained

geometry to the complex and also by helping to deprotonate the substrate.41a

In the present case since all five complexes 1 to 5 have acetate ions and phenoxo

groups their activities may be compared. Complex 4 and 1 are both bridged by acetate

groups but 4, being a reduced product, with one extra –CH2 group is a more flexible

system and hence favors the catalysis phenomenon more than 1. Complex 3 shows less

activity than 4 and 1 though phenoxo group offers a minor competition to the incoming

substrate than the acetates. The inhibition of the activity may be due to the presence of

two endogeneous phenoxo bridges in 3 as reported earlier.46

This observation is also

supported by the fact that the previously reported [Cu2(L4)2](PF6)2.2CH2Cl2 double

phenoxo-bridged complex22c

shows less activity (5.13 h-1

) than our compound 4 with

bridging acetate groups. The activity of 3 is even less than the monomeric complex (2).

The Cu···Cu distance in the acetate bridged complex (1) is 3.405(8) Å and is expected to

be ~ 3.40 Å for 4 (cf. the distances reported for other bidentate bridged complexes42

both

of which are larger than 3.185 Å found in 3. All these distances are however comparable

with 3.25 Å observed in the only o-catecholate bridged complex known in the literature.44

The mononuclear complex 2 is less efficient than 5 having a mononuclear complex in

each of its three arms for the obvious reason that there is a labile water molecule which is

easily replaceable by the catechol. Thus dinuclear complexes with acetate bridges have

shown more activity than mononuclear complexes. The presence of two endogenous

bridging phenoxo groups reduces the activity of the dinuclear species up to the extent of a

mononuclear one.

216

Mechanistic inferences

Krebs and coworkers47

have proposed a monodentate asymmetric coordination of

the substrate to the enzyme whereas Solomon et al.48

have suggested a simultaneous

coordination to both the copper centers, as depicted above. Though the former

mechanism is largely accepted there is much contest on the initial monodentate or

bidentate coordination of catechol to the enzyme. Later on many groups (Lindtvedt &

Thuruya,49

Oishi et al.,4a

Karlin,44

Demmim)4b

have stressed upon the bidentate bridging

coordination of the catechol to the active site owing to the higher activity of binuclear

catalysts. Another point of concern among various workers is regarding the production of

water or hydrogen peroxide as a dioxygen reduction product in the catalytic oxidation of

3,5-DTBCH2 by copper(II) complexes, as reported by Chyn and Urbach,33

Casella et al.50

and by Reedijk et al.51

The latter have shown a dinuclear complex catalyzing the

oxidation by two different pathways; one proceeding via the formation of semiquinone

species with the subsequent production of dihydrogen peroxide as a byproduct and

another proceeding via the two-electron reduction of the dicopper(II) center by the

substrate giving two molecules of quinone and one molecule of water. They also showed

that the product quinone itself behaves as an inhibitor at later stages of the catalytic

oxidation.

More recently Das and coworkers6d

have shown for the first time that in

acetonitrile the catalytic reaction proceeds via the formation of two enzyme-substrate

adducts ES1 and ES2, which were detected spectroscopically. Although present work is

not aimed towards the mechanistic studies of the catalytic process nevertheless the effect

of change of solvent on these results was undertaken due to curiosity, taking a cue from

the above. The catalytic studies were repeated in acetonitrile for complexes 4 and 2,

hoping to find out information regarding two enzyme-substrate intermediates. The UV-

vis spectra for both are shown in Figure 25 & 26. These, do not show any drastic changes

immediately after addition of DTBC in the 200-500 nm region except for the expected

gradual increase in the intensity of band at ~400 nm. Thus unfortunately there is no

revelation of any intermediate adducts. This of course, does not rule out their formation

but merely suggests that these adducts, in our case are too unstable to be detected even in

acetonitrile unlike that reported by Das et al. However one definite difference between

217

the results in acetonitrile and methanol was observed i.e. the reaction rates are much less

in the former. Using the method of initial rates the rate constants have been found to be

1.2 X 10-2

and 1.1 x 10-2

/min. for 4 and 2, respectively. This influence of the solvent on

the rates of reaction indicates that acetonitrile is either competing with the substrate for

binding at the copper (II) centre or it is stabilizing the intermediate reduced deoxy Cu(I)

state thus slowing the reaction rate. This indirectly hints at the essential requirement of

coordination of the substrate to the catalysts for the activity. This is also corroborated by

the observation that the rates of oxidation for 4 and 1 are dependent on the concentration

of the substrate at lower concentrations (Figure 24). As 4 and 1 are both dinuclear in

nature and are also the most active catalysts among the five complexes hence bidentate

coordination mode of substrate to the catalysts is evident.

0

0.05

0.1

0.15

0.2

300 350 400 450 500 550

Ab

s.

nm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100

Log(

A∞

/A∞

-At)

Time (Min.)

Figure 25. Showing increase in absorption due to the quinone band at ~ 400 nm after addition of

100 equivalents of (3,5-DTBC) to a solution containing complex (4) (10-4

M) in acetonitrile at 22

°C.

218

0.1

0.12

0.14

0.16

0.18

0.2

300 400 500 600

Ab

s.

nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 10 20 30 40 50 60

Log(

A∞

/A∞

-At)

Time (Min.)

Figure 26. Increase in absorption due to the quinone band at ~ 400 nm after addition of 100

equivalents of (3,5-DTBC) to a solution containing complex (2) (10-4

M) in acetonitrile at 22 °C.

4.4 Phosphodiester cleavage studies and kinetics

There are a many reports in literature in which vacant labile coordination site

transition metal and hydroxo bridging assist the hydrolysis of phosphate esters. The H2O

coordinated catalytically active site of these complexes decrease the pKa of a coordinated

water molecule, and as a consequence, provides a hydroxide nucleophile coordinated to

the metal center. This nucleophilic attack activates the substrate and stabilizes the

transition state by releasing the product.

O2N O P O

O

O

NO2

O

NO2

Catalyst

BNPP

Scheme 3 – Showing basic reaction involved in hydrolysis of BNPP

The kinetic studies of bis (4-nitrophenyl) phosphate (BNPP) hydrolysis for the

catalytic activity of the synthetic ligand-metal complexes were performed for the

complexes 1, 2, 3, 6, 7, 8, 9. Solution of substrate BNPP (50 mM in DMSO), ligand (10.0

mM in DMSO), metal salts (10.0 mM in H2O) and buffer (0.1 M in H2O in the presence

of 0.2 M NaNO3 for constant ionic strength) were added with H2O, DMSO, to give a

30% DMSO solution. In situ complex formation occurred for the different ligands and

219

respective metal salts, which was inferred from the absence of precipitation of metal

hydroxide at high pH. The hydrolysis of BNPP to corresponding hydrolyzed product p-

nitrophenolate was followed by the development of a considerably stable absorption band

at 400-410 nm in 30% DMSO solution.

The kinetics of the BNPP hydrolysis was determined by the method of pseudo-

first-order rate constants (kobs) by monitoring the growth of the 400-410 nm band of p-

nitrophenolate ( = 1.65 x 104 M

-1cm

-1) as a function of time. The slope of a plot of log

[Aα / (Aα -At)] vs. time was determined and the rate constants for the hydrolysis were

calculated. Only 7 exhibited rate acceleration in the hydrolysis of BNPP. A 7.18 x 107

times greater rate enhancement was observed for the complex 7 than that of uncatalyzed

reaction (1.3 x 10-11

).15b, 52

The rate constant values show that the rate acceleration in the

hydrolysis of BNPP also depends upon the pH of the solution. The reaction rate increases

with increase in pH, and finally gets saturated at higher pH 11. The complex shows very

low activity at pH 7 and 8 which slightly increases at pH 9 and maximum at pH 10. The

rate constants for this complex were calculated to be 2.9 x 10 -2

and 5.1 x 10 -2

at pH 9 &

10 respectively Figure 27. This pH dependent rate constant suggests that deprotonation of

a coordinated water molecule is necessary to generate the catalytically active Ni(II)-

coordinated hydroxo species.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40

Log[

A∞

/ A

∞-

At]

Time (min.)

pH 7

pH 8

pH9

pH 10

Figure 27. Showing the course of absorption maxima at 406 nm with time for BNPP (50 mM) in

solutions for complex 7(10 mM) in 30% DMSO solution at different pH

The incoming substrate first binds effectively to the catalyst, for any complex to

act as an efficient catalyst for hydrolysis of the substrate. The substrate further undergoes

nucleophilc attack by the metal-coordinated hydroxide nucleophile. The dimeric complex

220

7 provides a coordinated water molecule which acts as the nucleophile to catalyze the

hydrolysis of a phosphodiester bond. Complexes 8 and 9 show no phosphatase-like

activity due to the lack of coordinated water molecules. It is also observed that

mononuclear Ni(II) complex 6 has no hydrolysis activity as compared to those of

complex 7 due to the cooperative hybrid catalysts in case of dinuclear Ni(II) complexes

7.53

Hence the highest activity of 7 among all the complexes is due to the presence of

coordinated water and dinuclear environment.

Further to determine the dependence of the rates on the substrate concentration

and various kinetic parameters, solution of complex 7 was treated with different

concentrations of BNPP (from 2 to 10 mM ) at pH 10. At low concentrations of BNPP a

first-order dependence of the substrate concentration was observed. An enhancement in

the rate was observed with the increase of substrate concentration (Figure 28). The

dependence on the substrate concentration indicates a catalyst-substrate binding to be an

initial step in the catalytic mechanism. The rates of reactions obtained for various

concentrations of BNPP were fitted to the Michaelis–Menten equation and linearized by

means of Lineweaver-Burk plot to calculate various kinetic parameters, Km, Vmax and the

turn over number kcat (Table 7). For the BNPP hydrolysis with complex 7 as a catalyst

Km, Vmax and kcat were found to be 6.6 x10-3

M, 2.9 x10-6

M min.-1

and 5.8 x10-4

min-1

,

respectively.

0

5

10

15

20

25

0 2 4 6 8 10 12

rate

(M/s

) x

10

-7

[S] (mM)

Figure 28. Substrate concentrations versus kobs for the cleavage of BNPP in the presence of

complex 7

221

Table 7 - Kinetics parameters for the hydrolysis of BNPP catalyzed by complex 7.

Complex Rate Cons.

(min-1

)

Vmax (M min-1

) Km (M) kcat (min-1

)

Turnover rate

7 5.1 x 10

-2 2.9 x10

-6 6.6 x10

-3 5.8 x10

-4

Following mechanism has been largely accepted by the researchers in this field

11b, 54 where the metal ion as a Lewis acid is supposed to activate the phosphate group and

generate an active nucleophile. This stabilizes the pentacoordinated phosphorus transition

state and the leaving group by cooperative action. The phosphoryl oxygen of the substrate

binds to one of the metal ions and displaces the hydroxide ion from the metal. The

hydroxide ion attacks the electrophillic phosphorus of the substrate and causes the

inversion in the stereochemistry to expel the leaving group.

M

NO2OPO

O

O-

O2N

M

HO

NO2

O

PO

O

-O

O2N

M M

:OH

NO2

O

PO-

O

-OO2N

M M

+

OH

Scheme 4- Representation of the phosphate ester hydrolysis mechanism

(Adapted from Holden et al. Biochemistry 1996, 35, 6020)

4.5 Conclusions-

Nine Cu(II), Ni(II) and Zn(II) based complexes have been synthesized and

characterized. These copper(II) complexes are found to act as the synthetic catalysts for

the oxidation of catechols to quinones and the dimeric nickel(II) complex 7 has been

222

found to catalyze the hydrolysis of a phosphodiester bond. The catalytic activities have

been measured by the rate of aerial oxidation of 3,5-DTBC to 3,5-DTBQ in the presence

of Cu(II) complexes and by the hydrolysis of BNPP to corresponding product p-

nitrophenolate for the Cu(II), Ni(II) and Zn(II) based complexes. The oxidation of 3, 5-

DTBC and hydrolysis of BNPP was followed by the development of a considerably

stable absorption band at 400-410 nm corresponding to 3,5-DTBQ and p-nitrophenolate,

respectively. The systems have been designed to imitate the naturally occurring enzymes

in providing the N and O as donors. Complex 4 exhibits the highest catecholase activity

with a turnover number of 41 h-1

whereas only dimeric, aqua bridging nickel complex 7

exhibited rate acceleration in the hydrolysis of BNPP with a turnover number of 5.8 x10-4

min-1

. The results obtained are in accordance with earlier observation that a preliminary

binding of the substrate with the complex is mandatory for the catalytic cycles to take

place. The latter in turn depends upon the availability of a vacant site on the metal which

may be realized by the removal of some labile group. Ease of removal of the exogenous

acetate ligands and easy access to the metal ions has been seen to affect the activity in the

complexes. The presence of two endogenous phenoxo bridges in the dinuclear complexes

reduces the catecholase activity whereas the catalytic activity for the hydrolysis of BNPP

is enhanced by the presence of a bridging water molecule in it.

223

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