Chapter IV

Coordination behaviour of 2,2'-dipyridylamine (L3) towards

cadmium(II) and mercury(II) in combination with dicyanamide

Abstract: A mononuclear [Cd(L3)2(dca)2] (7) (L3 = 2,2´-dipyridylamine and

dca = dicyanamide) and a tetranuclear based 2D coordination polymer

[Hg4(L3-H)4(dca)4]n (8) (L3-H = anion of 2,2´-dipyridylamine) have been synthesized

through self-assembly of the corresponding metal(II) acetates, L3 and dca in 1:2:2 and

1:1:1 molar ratios, respectively. IR spectra show characteristic ν(dca) bands at 2287,

2226, 2160 cm-1 for 7 and 2291, 2245, 2180 cm-1 for 8. Absorption spectra show

bands at 312, 274 nm for 7 and 313, 275 nm for 8. Some pertinent crystal data are: 7,

monoclinic, space group C2/c, a = 8.5640(1) Ǻ, b = 13.3161(1) Ǻ, c = 22.3754(2) Ǻ,

β = 94.31°, Z = 4; 8, triclinic, space group P-1, a = 12.0261(2) Ǻ, b = 12.4997(2) Ǻ,

c = 16.3737(3) Ǻ, α = 89.571(1)°, β = 88.673(1)°, γ = 88.576(1)°, Z = 2. X-ray

structural analyses reveal that cadmium(II) center in 7 has a distorted octahedral

geometry with a CdN6 chromophore ligated by two bidentate neutral L3 units along

with two nitrile N atoms of two terminally bound dca units in mutual cis-orientation.

Each of the four independent mercury(II) centers in 8 adopts a distorted trigonal

bipyramidal environment coordinated by two pyridine N atoms of two different

anionic L3-H ligands, two nitrile N atoms of two µ1,5 bridged dca units and the fifth

position is occupied by the amide N of one L3-H moiety. Cooperative intermolecular

N-H···N and C-H···N hydrogen bondings result in a 3D supramolecular architecture in

7. Thermogravimetric analyses exhibit that the compounds 7 and 8 are quite stable

within the temperature range 40-750 °C. The compounds display intraligand 1(π-π*)

fluorescence in DMF solutions at room temperature.

71

IV.1. Introduction

The last two chapters (Chapter II and Chapter III) cover the syntheses and

X-ray structures of a variety of coordination compounds using different 3d/4d metal

ions as templates, polyamines as organic blockers and dicyanamide as

terminals/bridges. This chapter includes the preparations and X-ray structural

characterizations of mononuclear cadmium(II) and polynuclear mercury(II)

complexes with 2,2'-dipyridylamine (L3) and dca. 2,2'-dipyridylamine (L3) shows the

great versatility as a ligand, which can act not only as a bidentate ligand through its

two pyridyl nitrogen atoms but also as a bridging ligand after removal of the amine

hydrogen atom [1-3]. Interestingly, the free L3 ligand has a dimeric structure in which

two L3 molecules are linked by N-H···N hydrogen bonds [3(c)] and is also able to

form hydrogen bonded network through active amine (-NH-) hydrogen which plays

an important role to direct different properties of the target molecules [4] and to

luminescence through π-π* transition [5]. Dca [6,7] has been increasingly used as a

component of coordination polymers because its large variety of bonding modes

enable the formation of a wide range of structural types. Cadmium(II) and

mercury(II) [8,9] are regarded suitable for this study, due to the variety of

coordination numbers and geometries provided by the metal centers because of their

symmetrical d10 configurations. Recently this group [10-12] and other groups [13,14]

have investigated the coordination behaviour of various dn ions towards L3 in

presence of different pseudohalides like azide, thiocyanate, cyanate, carboxylates and

even with dca. But the chemistry of 2,2'-dipyridylamine (L3) with Group 12 metal(II)-

dca systems have been still unexplored.

(L3)

As an extension of the earlier work, in our present endeavor, we have tried to

examine the ligational motif of this tailored diimine (L3) towards three Group 12

metal ions viz. zinc(II), cadmium(II) and mercury(II) in combination with dca.

Successfully, we have synthesized one mononuclear compound [Cd(L3)2(dca)2] (7)

N

N

N

H

72

and a tetranuclear based 2D coordination polymer [Hg4(L3-H)4(dca)4]n (8). With

zinc(II), compound of the composition [Zn(L3)2(dca)2] was obtained but single

crystals of it could not be grown; so chemistry of it is not discussed. The synthetic

details, X-ray structural characterizations, spectral patterns and other physicochemical

properties of the compounds of cadmium(II) and mercury(II) are described below.

IV.2. Results and discussion

A. Synthesis and formulation

The hexacoordinated mononuclear complex [Cd(L3)2(dca)2] (7) was initially

formed from a methanolic solution containing a 1:2:1 mixture of Cd(OAc)2.2H2O, L3

and dca with an extraneous addition of NaClO4-a reactant ratio expected to yield

either a double dicyanamido bridged dicationic dinuclear species of composition

[(L3)CdII(dca)2CdII(L3)](ClO4)2 or a polycationic polynuclear single dicyanamido

bridged species of type [Cd(L3)2(dca)]n(ClO4)n. However, microanalyses showed a

1:2:2 ratio of metal, tailored diimine and pseudohalide and in IR spectrum, presence

of perchlorate bands was not noticed. Reactant ratio corresponds to the product

stoichiometry afforded better yield of 7. An effort to get either dinuclear or

polynuclear species, in which starting material was changed to Cd(NO3)2.4H2O and a

1:2:1 molar ratio of the nitrate salt, ligand and pseudohalide followed by addition of

KPF6 (1 equv.) produced 7 in lower yield. Similarly, using 1:2:1 molar ratio of

cadmium(II) chloride, L3 and dca followed by addition of NaClO4/KPF6 afforded 7.

Also, in an attempt to prepare a double dicyanamido bridged neutral polymer of the

type [Cd(L3)(dca)2]n using 1:1:2 molar ratio of cadmium(II) chloride/nitrate/acetate,

L3 and dca once again resulted in mononuclear 7 with lower yield. 8 was

serendipitously formed from a methanolic solution containing a 1:1:2 molar ratio of

mercury(II) acetate, L3 and dca. Under reaction condition the ligand, L3 got

deprotonated and to our surprise, a tetranuclear based coordination polymer of

composition [Hg4(L3-H)4(dca)4]n (8) was the result corresponding to a 1:1:1 molar

ratio of mercury(II) acetate, L3 and dca. Reactant ratio pertaining to the product

stoichiometry afforded better yield of 8. Compound 8 was also the result even on

external addition of NaClO4/KPF6 (1 equv.) to the reaction mixture containing 1:1:1

molar ratio of mercury(II) acetate, L3 and dca. The synthetic results reflect that there

73

is an inherent tendency for the formation of 7 and 8 which may be related to their

special stabilities. The different syntheses were reproducible as was evident from

repetitive microanalytical results, spectral behaviours and other physicochemical

properties. The synthetic procedures are summarized in Equations (IV.1) and (IV.2):

The air-stable, moisture insensitive complexes are insoluble in common

organic solvents like methanol, ethanol, acetonitrile but are moderately soluble in

DMF on warming.

B. Infrared spectra

The infrared spectra of both the complexes were recorded in 4000-400 cm-1

range. Characteristic stretching frequencies are tabulated in Table IV.1 and the

spectral patterns of the respective compounds are displayed in Figures IV.1 and IV.2.

The primary concern is the bands due to dicyanamide group. Three strong bands at

2287, 2226 and 2160 cm-1 in 7 close to the free dca stretching frequencies [15] [νas +

νs(C≡N) 2286 cm-1, νas(C≡N) 2232 cm-1 and νs(C≡N) 2179 cm-1] are indicative of

monodentate nitrile nitrogen binding of dca [16]. In the polynuclear 8, the bands are

shifted towards higher frequencies [2291, 2245 and 2180 cm-1] than those in

mononuclear 7 presumably due to bridging coordination of dca [17]. The results are

consistent with the X-ray structures. Bands corresponding to νas(C-N) and νs(C-N)

stretches are found in the range 1370-1310 cm-1 and at ~900 cm-1, respectively in both

the complexes. The ν(C=N) stretch of L3 and L3-H units are observed at 1596 and

1602 cm-1 in 7 and 8, respectively. Additionally, the ν(N-H) stretching frequencies of

L3 are observed at 3295 and 3290 cm-1 in 7.

Cd(OAc)2.2H2O + 2 L3 + 2 NadcaMeOH298 K

[Cd(L3)2(dca)2] + 2 NaOAc + 2H2O ...(IV.1)(7)

4n Hg(OAc)2 + 4n L3 + 4n Nadca MeOH298 K

[Hg4(L3-H)4(dca)4]n + 4n NaOAc + 4n AcOH ...(IV.2)(8)

74

Table IV.1

Infrared spectral (in cm-1) dataa

Compound ν(N-H) νas +

νs(C≡N)

νas(C≡N) νs(C≡N) νas(C-N) νs(C-N) ν(C=N)

[Cd(L3)2(dca)2]

(7)

3295,

3290

2287 2226 2160 1365,

1315

902 1596

[Hg4(L3-H)4(dca)4]n

(8)

- 2291 2245 2180 1360,

1320

905 1602

aKBr discs

Figure IV.1. Infrared spectrum of 7

Figure IV.2. Infrared spectrum of 8

75

C. Electronic spectra

The complexes display absorption bands in 200-900 nm range. Results in

DMF solutions are given in Table IV.2 and Figures IV.3 and IV.4 exhibit their

spectral patterns. Colourless solutions of 7 and 8 in DMF show two intense absorption

bands at ~315 nm and ~275 nm. The transition may correspond to intraligand π-π*

transitions [18].

Table IV.2

Electronic spectrab

Compound λmax, nm (DMF)

7 312, 274

8 313, 275

bAt room temperature (298 K)

Figure IV.3. Electronic spectrum of 7 in DMF solution

76

Figure IV.4. Electronic spectrum of 8 in DMF solution

D. Crystal data collection and refinement

Single crystals of 7 and 8 suitable for X-ray analyses were selected from those

obtained by open evaporation of methanolic solutions of the reaction mixtures at

298 K. Crystallographic data were collected on a Bruker Apex2 CCD diffractometer

with a graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at a detector

distance of 5 cm using APEX2 [19] at 100 K with the Oxford Cryosystem Cobra low-

temperature attachment. The collected data were reduced using SAINT program [19]

and the empirical absorption corrections were performed using SADABS program

[19]. A summary of the crystallographic data and structure determination parameters

of 7 and 8 are given in Table IV.3. Of 23830 and 97620 collected reflections, 5582

and 21568 unique reflections were recorded for 7 and 8, respectively. The structures

were solved by direct methods using SHELXTL [20]. All non-hydrogen atoms were

refined anisotropically, whereas the amide H atoms in 7 were located in difference

maps and isotropically refined. The remaining H atoms were placed in calculated

77

Table IV.3

Crystal data and structure refinement parameters for 7 and 8

Crystal parameters 7 8

CCDC No. 731868 732296 Formula C24H18N12Cd C48H32N24Hg4 Formula weight 586.90 1747.34 Crystal system Monoclinic Triclinic Space group C2/c P-1 a/Å 8.5640(1) 12.0261(2) b/Å 13.3161(1) 12.4997(2) c/Å 22.3754(2) 16.3737(3) α° 90.00 89.571(1) β° 94.31 88.673(1) γ° 90.00 88.576(1) V/Å3 2544.45(4) 2459.86(7) λ/Å 0.71073 0.71073 ρcalcd/gm cm-3 1.532 2.359 Z 4 2 T/K 100.0(1) 100 µ (mm-1) 0.896 12.508 F(000) 1176 1616 Crystal size (mm3) 0.30 × 0.28 × 0.21 0.41 × 0.21 × 0.19 θ ranges (°) 1.83 to 35.00 1.24 to 35.00 h/ k / l -13,13/-20,21/-35,36 -19,18/-20, 20/-24, 26 Reflections collected 23830 97620 Independent reflections 5582 21568 Tmax and Tmin 0.8369 and 0.7754 0.2032 and 0.0798 Data/restraints/parameters 5582/0/172 21568/0/685 Goodness-of-fit on F2 1.216 1.039

Final R indices [I>2σ(I)] R = 0.0208 and wR = 0.0569

R = 0.0391 and wR = 0.0994

R indices (all data) R = 0.0235 and wR = 0.0654

R = 0.0508 and wR = 0.1084

Largest peak and hole (eÅ-3) 0.517 and -0.682 6.047 and -1.795 Weighting scheme: R = Σ||Fo|-|Fc||/Σ|Fo|, wR = [Σw(Fo

2-Fc2)2/Σw(Fo

2)2]1/2,

calc. w = 1/[σ2 (Fo2) + (0.0358P)2 + 0.6835P] (7); calc. w = 1/[σ2 (Fo

2) + (0.0683P)2 +

1.9020P] (8); where P = (Fo2+2Fc

2)/3

positions with the C-H distance of 0.97 Å after checking their positions in the

difference map. The Uiso values were constrained to be 1.2Ueq of the carrier atoms for

all H atoms. In the final difference Fourier maps, the residual maxima and minima

were 0.517 and -0.682 eÅ-3 for 7 and 6.047 and -1.795 eÅ-3 for 8. Materials for

78

publication were prepared using SHELXTL, PLATON [21] and ORTEP-32 [22]

programs.

E. X-ray crystal structures

[Cd(L3)2(dca)2] (7)

The coordination polyhedron around cadmium(II) in 7 is best described as a

distorted octahedron with a CdN6 chromophore. The coordination includes (Figure

IV.5) two chelated tailored (−N=C−NH−C=N−) diimine ligated by four pyridine N

Table IV.4

Selected interatomic distances (Å) and angles (0) for 7

Bond distances

Cd1-N4 2.3097(9) Cd1-N1A 2.3183(9)

Cd1-N4A 2.3097(9) Cd1-N3 2.3418(10)

Cd1-N1 2.3183(9) Cd1-N3A 2.3418(10)

N4-C11 1.1595(14) N6-C12 1.1542(17)

Bond angles

N4-Cd1-N4A 95.24(5) N3-Cd1-N3A 175.32(4)

N4A-Cd1-N1 86.01(4) N1-Cd1-N1A 95.25(5)

N4A-Cd1-N1A 168.04(4) N4-Cd1-N1 168.04(4)

N4-Cd1-N3 93.12(4) N1-Cd1-N3A 98.78(3)

N4A-Cd1-N3A 90.04(3) N4-Cd1-N3A 90.04(3)

N1-Cd1-N3 78.01(3) C11-N4-Cd1 165.61(10)

N4-Cd1-N3 93.12(4) C11-N5-C12 121.59(11)

N4-C11-N5 172.56(13) N5-C12-N6 173.45(15)

atoms (N1, N3, N1A, N3A; the symmetry code for N1A and N3A: -x, y, 1/2-z) of two

different L3 units and two nitrile N atoms (N4, N4A; the symmetry code for N4A: -x,

y, 1/2-z) of two terminally bound dicyanamido moieties. The two pyridine N atoms

(N1, N1A) of two different L3 units and two nitrile N atoms (N4, N4A) of two

pendant dicyanamido units define the equatorial plane around cadmium(II), whereas

axial positions are occupied by two pyridine N atoms (N3, N3A) of two different

units of L3. Cd-N distances of 7 lie within a range of 2.3183(9)-2.3418(10) Å which

79

are greater than Cd-N(dca) [2.3097(9) Å] distance indicating the stronger anionic

pseudohalide coordination over neutral N atom of L3. The cisoid and transoid angles

(Table IV.4) around cadmium(II) are in line with strong distortion from an idealized

octahedron. The cadmium atom is almost in a least square plane (N1/N1A/N4/N4A)

as is reflected from the small deviation value [0.000(1) Å]. The two basal N atoms N1

Figure IV.5. ORTEP view of cis-[Cd(L3)2(dca)2] (7) with atom

labeling scheme (50% ellipsoid probability)

and N4 deviate 0.241(1) Å and 0.242(1) Å, respectively above the plane

(N1/N1A/N4/N4A) and their symmetry related atoms (N1A and N4A) deviate same

amount below the plane. The angle between the two chelate planes

[N3/C6/N2/C5/N1/Cd1 and N3A/C6A/N2A/C5A/N1A/Cd1] is 62.53º. L3 ligands are

not planar as the dihedral angle between two pyridine rings (N3/C6--C10 and N1/

C1--C5; N3A/C6A--C10A and N1A/C1A--C5A) is 27.11(6)°. Two dca units are in a

cis orientation with an angle of 95.24(5)°. In the terminal pseudohalides, the

N4-C11/N4A-C11A distance [1.1595(14) Å] is longer than the N6-C12/N6A-C12A

length [1.1542(17) Å] which indicates the nitrile nitrogen (N4/N4A) atom of dca is

coordinated to the metal center. The C11-N4-Cd1/C11A-N4A-Cd1 angle is

165.61(10)° which reflects the bending coordination nature of dca. The skeletal bond

80

angles N4-C11-N5 [172.56(3)o], C11-N5-C12/C11A-N5A-C12A [121.59(11)o] and

N5-C12-N6/N5A-C12A-N6A [173.45(15)o] of dca reflecting non-linearity of the

larger pseudohalide rod.

Figure IV.6. (a) A section of 1D chain in 7 formed through N-H···N hydrogen bonds

along crystallographic c-axis; (b) A view of C-H···N hydrogen bonded 2D sheet

structure in 7 parallel to ab-plane

In the crystal packing the individual [Cd(L3)2(dca)2] (7) units self-assemble

through hydrogen bondings between the uncoordinated nitrile N (N6) atom of

dca and -NH- group of L3 into 1D chain (Figure IV.6a) along the crystallographic

c-axis. These 1D chains are further engaged in C-H···N (Table IV.5) hydrogen bonds

involving coordinated nitrile N (N4) atom of dca and H atom of ligand framework in

an interwoven fashion parallel to ab plane (Figure IV.6b) to form a 3D

supramolecular network structure (Figure IV.7). The Cd···Cd separation between the

nearest neighbours in each 1D chain is 7.916 Å.

81

Table IV.5

Hydrogen bond distances (Å) and angles (°) for 7

D-H···A D-H H···A D···A D-H···A

N2-H1N2···N6i

C2-H2A···N4ii

0.89(2)

0.93

2.02(2)

2.62

2.9063(16)

3.4847(16)

177(2)

154

Symmetry codes: i = -1/2+x, 1/2-y, -1/2+z and ii = -1/2-x, -1/2+y, 1/2-z

Figure IV.7. 3D network structure in 7 formed through interwoven

N-H···N and C-H···N hydrogen bonds

[Hg4(L3-H)4(dca)4]n (8)

The 2D polymer comprises repeating tetranuclear unit, [Hg4(L3-H)4(dca)4]

(Figure IV.8), with four crystallographically independent mercury atoms (Hg1, Hg2,

Hg3 and Hg4). Two different L3-H units connect two mercury(II) centers (Hg1 and

Hg2/Hg3 and Hg4) through use of pyridine and amide N atoms (N3 and N2 for Hg1,

N6 and N5 for Hg2/N15 and N14 for Hg3, N12 and N11 for Hg4) of each L3-H in

bidentate chelating fashion forming four-membered chelate loops and the rest

pyridine N atom (N4 for Hg1 and N1 for Hg2/N10 for Hg3 and N13 for Hg4) of each

L3-H coordinating the second mercury center [2(b)]. Two subsequent Hg···Hg

separations are 4.074 Å (Hg1···Hg2) and 4.062 Å (Hg3···Hg4). Each pentacoordinated

82

Table IV.6

Selected interatomic distances (Å) and angles (0) for 8

Bond distances

Hg1-N4 2.082(4) Hg1-N3 2.087(3)

Hg1-N2 2.855(3) Hg1-N21 2.729(4)

Hg1-N24 2.753(4) Hg2-N6 2.073(4)

Hg2-N1 2.073(3) Hg2-N16 2.621(4)

Hg2-N7 2.650(4) Hg2-N5 2.782(4)

Hg3-N10 2.088(3) Hg3-N15 2.091(3)

Hg3-N18 2.647(4) Hg3-N9 2.673(4)

Hg3-N14 2.810(4) Hg4-N13 2.073(4)

Hg4-N12 2.077(3) Hg4-N22 2.675(4)

Hg4-N19 2.678(4) Hg4-N11 2.752(4)

Bond angles

N4-Hg1-N3 179.19(14) N6-Hg2-N16 96.32(14)

N1-Hg2-N7 86.21(14) N6-Hg2-N7 90.25(14)

N16-Hg2-N7 86.65(14) N1-Hg2-N16 88.01(14)

N6-Hg2-N1 174.24(14) N10-Hg3-N18 90.94(13)

N10-Hg3-N15 177.79(14) N10-Hg3-N9 86.88(14)

N15-Hg3-N18 90.08(13) N18-Hg3-N9 83.74(14)

N15-Hg3-N9 91.28(14) N13-Hg4-N22 85.87(14)

N13-Hg4-N12 177.73(14) N13-Hg4-N19 95.28(14)

N12-Hg4-N22 93.56(14) N22-Hg4-N19 88.48(13)

N12-Hg4-N19 86.90(14)

mercury(II) center of the tetranuclear coordination unit adopts a distorted trigonal

bipyramidal geometry as exemplified by their tau parameters (τ = 0.67 for Hg1, 0.51

for Hg2, 0.59 for Hg3 and 0.53 for Hg4) [23] with an HgN5 chromophore ligated

by two pyridine N (N3, N4 for Hg1, N1, N6 for Hg2, N10, N15 for Hg3 and N12,

N13 for Hg4) atoms of two different L3-H units, two nitrile N (N21, N24 for Hg1, N7,

N16 for Hg2, N9, N18 for Hg3 and N19, N22 for Hg4) atoms of two different µ1,5

83

Figure IV.8. ORTEP plot of the tetranuclear repeating unit of [Hg4(L3-H)4(dca)4]n (8)

with atom labeling scheme (20% probability)

(a) (b)

Figure IV.9. Perspective views of (a) 1D chain and (b) 2D sheet structures in 8

formed by µ1,5 dca linkages

84

bridged dca and one amide N (N2 for Hg1, N5 for Hg2, N14 for Hg3 and N11 for

Hg4) of one L3-H unit. The equatorial plane consists of two nitrile N (N21, N24 for

Hg1, N7, N16 for Hg2, N9, N18 for Hg3 and N19, N22 for Hg4) of two different µ1,5

bridged dca units and one amide N (N2 for Hg1, N5 for Hg2, N14 for Hg3 and N11

for Hg4) of L3-H unit, while two axial positions are occupied by two pyridine N

atoms (N3, N4 for Hg1, N1, N6 for Hg2, N10, N15 for Hg3 and N12, N13 for Hg4)

of two different L3-H units. The equatorial Hg–N(nitrile) and Hg–N(amide) distances

are in the range of 2.621(4)–2.753(4) Å and 2.752(4)–2.855(3) Å (Table IV.6),

respectively, whereas, the axial Hg–N(pyridine) distances are somewhat shorter and

are in the close range of 2.073(4)–2.051(3) Å. Hg1, Hg2, Hg3 and Hg4 centers

deviate 0.976, 0.799, 0.861 and 0.790 Å towards axial N4, N1, N10 and N13 atoms,

respectively. The two pyridine rings of each L3-H unit are to some extent folded as

reflected in the dihedral angles between the planar pyridine rings which are 40.76°,

41.86°, 44.88° and 41.52° for N1/C1–C5 and N3/C6–C10; N4/C11–C15 and

N6/C16–C20; N10/C23–C27 and N12/C28–C32; N13/C33–C37 and N15/C38–C42,

respectively. The two µ1,5 bridged dca units linked to each mercury(II) center are in

mutually cis orientation as is evident from the bond angle values of N21–Hg1–N24,

N7–Hg2–N16, N9–Hg3–N18 and N19–Hg4–N22 (vide Table IV.6). The two µ1,5 dca

bridges (N7–C21–N8–C22–N9) and (N16–C43–N17–C44–N18) facilitate the

propagation of the polymer into a zigzag 1D chain along b-axis by connecting Hg2

and Hg3 centers (Figure IV.9a). Again, Hg1 and Hg4 centers of the adjacent 1D

chains are further connected by two µ1,5 dca bridges (N19–C45–N20–C46–N21) and

(N22–C47–N23–C48–N24) along c-axis that results in a 2D sheet parallel to bc plane

(Figure IV.9b).

F. Thermogravimetric study

To examine thermal stabilities of the compounds 7 and 8, thermogravimetric

(TG) analyses were made between 40 and 750 ºC in the static atmosphere of nitrogen.

Compound 7 is stable up to 211 ºC and TG curve (Figure IV.10) indicates that

weight loss (observed, 75.04%; expected, 75.00%) between 211 and 605 ºC

corresponds to the departure of two chelating L3 ligands and two terminal

pseudohalides, dicyanamide. Compound 8 is stable upto 214 ºC and TG curve

85

(Figure IV.11) shows that decomposition of ligands and pseudohalides occurs in two

successive steps. The weight loss (observed, 9.74%; expected, 10.44%) in the first

step at 214-236 ºC is indicative of the release of one L3-H molecule and in the second

step at 284-411 ºC the weight loss (observed, 44.39%; expected, 56.07%) corresponds

to the departure of three L3-H and four dicyanamide units. Moreover, the

thermogravimetric analyses reveal that thermal stability of compound 7 and 8 is

comparable.

Figure IV.10. TG curve of 7 Figure IV.11. TG curve of 8

G. Luminescence spectra

The emission spectra of L3 and of complexes 7 and 8 are depicted in Figure

IV.12 and the data are listed in Table IV.7. Complexes with L3 feature two intense

absorptions at 266 nm and 314 nm which are assigned to intraligand π-π* transitions.

In DMF solutions the absorption spectra of the complexes are dominated by

intraligand π-π* transitions at 274 nm, 312 nm for 7 and 275 nm, 313 nm for 8. Upon

photoexcitation at 266 nm (for L3), 274 nm (for 7) and 275 nm (for 8) in DMF

solutions, free ligand exhibits fluorescent emission centered on 345 nm, whereas the

corresponding cadmium(II) and mercury(II) dicyanamide complexes show

photoluminescence with the main emissions at almost similar position (349 nm for

7 and 339 nm for 8) to that of free ligand. The appearance of luminescence [24] in the

complexes may be attributed to the intraligand 1(π-π*) fluorescence.

86

Table IV.7

Photophysical datac

Sample Absorption (nm) Emission (nm)

L3 266 345

7 274 349

8 275 339

cIn DMF solutions at 298 K

Figure IV.12. Fluorescence behaviours of free L3 and compounds

7 and 8 in DMF solutions at 298 K

H. Conclusion

We are able to prepare two new luminous materials of two Group 12 metal

ions in combination with a tailored diimine, L3 and dicyanamide. To the best of our

knowledge, 8 is the first example of a mercury(II) coordination polymer where each

L3 uses one pyridine and one amide N atoms to chelate one mercury center while the

rest pyridine N atom coordinates another metal center resulting a continuum in

87

combination with dca. The difference in binding of L3 in combination with dca to the

two Group 12 metal ions gives rise to different molecular architectures. The

preparation of such compounds illustrates a potentially versatile approach to the

construction of uncharged metal-organic frameworks, which is an emerging area of

research for the rational design of functional materials. In the next chapter we will

examine the variation of molecular and crystalline architectures of two cadmium(II)

chloride complexes in conjunction with two tetradentate Schiff bases (L4 and L5) of

the varied length of alkylenic arms.

IV.3. Experimental section

A. Preparation of the complexes

(a) Chemicals, solvents and starting materials

High purity 2,2'-dipyridylamine (Fluka, Germany), sodium dicyanamide

(Lancaster, UK), cadmium(II) chloride/nitrate/acetate (E. Merck, India), mercury(II)

acetate (E. Merck, India), sodium perchlorate (Lancaster, UK) and potassium

hexafluorophosphate (Fluka, Germany) were purchased from respective concerns and

used as received. All other chemicals and solvents were AR grade and used as

received. The synthetic reactions and work-up were done in open air.

(b) Complexes

[Cd(L3)2(dca)2] (7)

A methanolic solution (10 cm3) of L3 (0.114 g, 0.66 mmol) was added

dropwise to a solution of Cd(OAc)2.2H2O (0.088 g, 0.33 mmol) in the same solvent

(10 cm3). Dicyanamide (0.058 g, 0.66 mmol) in methanol (10 cm3) was added slowly

to this solution. The final colourless solution was filtered and kept undisturbed in air

for open evaporation. Colourless rectangular crystals that separated were washed with

dehydrated alcohol and dried in vacuo over silica gel indicator. Yield: 0.155 g (80%).

88

[Hg4(L3-H)4(dca)4]n (8)

To a methanolic solution (10 cm3) of Hg(OAc)2 (0.106 g, 0.33 mmol), L3

(0.057 g, 0.33 mmol) and dca (0.029 g, 0.33 mmol) were successively added

dissolving in the same solvent (10 cm3 each). The light yellow solution was filtered

and kept undisturbed in an open air for evaporation. Within 24 hours, the light yellow

rectangular crystals of 8 that separated were processed as above in 7. Yield: 0.108 g

(75%).

B. Characterization of the complexes

The complexes were characterized by microanalyses (C, H and N). Results

obtained with the help of a Perkin-Elmer 2400 CHNS/O elemental analyzer are set in

Table IV.8.

Table IV.8

Characterization data

Compound

(Mol formula)

Found

(Calcd)%

C H N

7

(C24H18N12Cd)

49.3

(49.1)

3.1

(3.1)

28.8

(28.6)

8

(C48H32N24Hg4)

32.7

(32.9)

1.8

(1.8)

19.2

(19.3)

C. Physical measurements

Elemental analyses (C, H and N) were measured using a Perkin-Elmer 2400

CHNS/O elemental analyzer. IR spectra (KBr discs, 4000-400 cm-1) were recorded

using a Perkin-Elmer FTIR model RX1 spectrometer. Thermal studies were made

with a Perkin-Elmer Diamond TG/DT analyzer heated from 40-750 ˚C under

nitrogen. UV-Vis (in DMF) spectra were recorded with a Jasco model UV-Vis-NIR

spectrophotometer. Fluorescence experiments were done using Hitachi fluorescence

spectroflurimeter F-4500.

89

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