Indian Journal of Chemistry Vol. 38A, December 1999, pp. 1277-1282

Copper(II) complexes with bis thiazole based ligands : Spectral, cyclic voltammetric and

EPR studies

Sa njib Kumar Das & Pavan Mathur*

Department of Chemistry. University of Delhi , Delhi 110007, India

Received 8 March 1999; revised 19 August 1999

A series of complexes with stoichiometry corresponding to [CuLX 2J has been synthesised, where L is a potentially tridentate li gand carrying pendant benzothiozolyl groups and X = CI04- .

cr. NO,-. C H,COO- and HCOO-. EPR spectra of the complexes examined as a frozen DMF solution reveal a di storted tetragonal geometry. The visible band energy for these copper(lI) complexes is found to fall in the range observed for N20 2 type donor environment. while the pl ot o f the reduction potential versus d-d band energy indi cates th at increasing covalency leads to a decrease in d/ ./ orbital energy and a concomitant anodic shift.

Coordination of hi stidine-imidazole and methionine thioether is well known to give ri se to a distorted geometry around Cu(Il) in a number of copper containing'enzymes, leading to a large change in their e lectronic structure 1.". In thi s note we describe some copper(U) complexes of a potentially tridentate ligand containing two benzothiozole moities , and one ether oxygen donor group. The five coordinate copper(lI) complexes with distorted geometry may have relevance to the type (II) copper site in enzymes such as tyrosin ase.

Experimental Solvents and reagents were of A R grade and used without further purification.

Preparation of ligand N,N'-bi s(benzothiozolyl )ether was prepared as

reported earlier6; the preparation method consisted of

the followin g steps: Diglycoli c acid 4 g (0:029 mol ) and 0-

aminothiophenol (7 .75 g, 0.058 mol) were mjxed and added to 25 ml of polyphosphoric acid, the temperature of the mixture was raised to 140°C and maintained at thi s value with efficient stirring for 4 hr. The reaction mixture was a.\lowed to cool to room temperature and poured into vigorously stirred water

(200 rnJ). A yellowish precipitate formed, was filtered and dried. This was recrystallized from ethanol. Pale yellow crystals, were isolated .

The ligand was characterized by elemental analysis (Table I) and lH N.M.R. spectroscopy. ,IH N.M.R. (CDCI3): 7.3 . (4 H, m), 6.9 (4 H, m) , 4.4 (4 H, s).

Synthesis of complexes

[Cu(DGBT)X2 j.nH20 , X = cr, N03- , OAC, HCOO-, CL04-

The ligand (DGBT), (312 mg I mmol) and hydrated copper chloride/nitrate (CuX2.nH20), (I mmol) were separately dissolved in methanol (30 ml), and mixed . The reaction mixture was stirred for 1 hr on a warm water bath, which resulted in the precIpitation of the desired compounds. The precipitate was collected, washed with methanol and dried over P20 S in vacuo. In the case of the perchlorate, acetate and formate, precipiated copper hydroxide was suspended in methanol and dissolved by addition of dilute (1 :5) perchloric acid, acetic acid and formic acid respectively . This solution of the copper salt was then reacted with the ligand in a manner similar to that reported above and desired products were isolated. Except the perchlorate complexes which analysed for a 1:2 metal-ligand system, a ll other complexes were of I: 1 type (Cu : li gand) .

Caution: Although no accident occurred with the present copper (II) perchlorate complexes during the experimental work, it should be remembered that perchlorates are potentially explosive.

Elemental analyses were obtained from microanalytical laboratory of RSIC, University of Punjab, Chandigarh, India. Copper was estimated by atomic absspectroscopy using a Shimadzu AA-64-13 instrument. Electronic spectra were measured using a KONTRON UVIKON 930 uv-vis spectrophotometer. I H NMR spectra were taken on a Bruker 300 Mhz

FT-NMR spectrometer at RSIC, Chandigarh . X-band EPR spectra were obtained on VARIAN E-112 ESR spectrometer with a variable temperature liquid nitrogen cryostat at lIT Madras, India. IR spectra were recorded in the solid state as KBr pellets except for the perchlorate complex in which case IR was

1278 I DIAN J CHEM, SEC. A, DECEMBER 1999

0 .60r------------------------------------------------------. \ \ \

\ b ..--.- -Q

?-.. ~ / / .. ", '~", u

c o

.0 ~

o III

.0

<t:

/ '. /'

/j ." .- .- __ .- - - z;-::-::-:'-. "- , .. 'fI ;_ .~

e~"''' . ' .-=-~)l "". "-"':::::"'-. - - d

-.........::: ._ .~ __ )C o L-____________________ ~ _____________________ L ________ ~ 400 600 80 0

Wa velength

Fig. 1- Vi ~ ib l e spect ra of (a) Cu(DGBT)(HCOO-)2.3H20; (b) Cu(DGBT)(N01h; (e) Cu(DGBT)(OACh2H20; (d) Cu (DGBTMCIO~); (e) Cu(DGBT)(CI)2 and all in DMSO

Tab le I -Microanalytical and optical data of copper(II ) comp lexes in DMSO

Found (Caled ). 'Ii Ilcff Amax

Compound C H

DGBT 60.5 (6 1.5) 2.9 (3 .8)

I Cu(DGBT) , ( CIO~ ), J 43 .39 (43 .34) 2.3 (2.7)

ICu(DGBT)CI 21 43.7 (43. 1) 2.2 (2.6)

ICu(DGBT)( NOd, j 39.2 (3 8.5) 1.9 (24)

I Cu(DGBT)(OAC)212H20 44.9 (45 .3) 3.9(4.15)

I c ur DGBT)( HCOO)2 J3H ,O 41.2 (4 1.6) 3.2 (3 .8)

taken in nuj o l on a Perkin-Elmer spectrum 2000 IT­IR spec tromete r.

Cyc li c-vo lt ammetric measurements were carried out using a BAS , CV -SOW e lec trochemica l ana lysing system. High purity , HPLC grade DMSO & CH1CN so lven t was e mpl oyed for the CV studies with 0. 1 M aClO~ as supporting e lectrolyte. A three e lectrode

confi gurati on was used, compri si ng pl atinum di sk working e lec trode, platinum wire counter electrode and Agi AgN01 references e lec trode . Elec trode performance was monitored by observing the ferrocen ium/fe rrocence (Fc+/Fc) couple in the above so lvent sys tem. Experiment s were carri ed out at room te mperature (20"C ) under dry nitrogen.

Results and discussion

E/euron ic .I'peuro (lnd eleclrochemislry Fi gure I shows the e lectro ni c spec tra of copper(lI )

't:0mplexes. the ir ba nd positions and ex tinc tion coeffici e nts are shown in Table I . The bands obtained

in UV reg ion are due to n-n* trans iti ons . These bands show blue shifts in the ir respec ti ve complexes and support the bindin g of be nzothi ozo lyl ring through

N M (BM) (11m) (log £ dm) mol- I cm- I)

9.2 (8.9) 298 (4.14), 362(Sh)

6.6 (6.3) 6.9 (7. 1) 1.79 288 (4.26), 892(2.08)

5.9 (6.2) 13.9 ( 14.2) 1. 86 289 (4.20), 880 (2.04)

11.1 (11.2) 12.5 ( 12.7) 1.83 290 (4.28) , 769 ( 1.98)

4.3 (5.2) 11.7 (11.9) 1.90 29 1 (4.26), 720 ( 1.88)

4.7 (5.3) 12.0 ( 12.2) 1.9 1 292 (4.2 1), 709 (2. 16)

ImIlle nitrogen to the meta l center. A ll the copper complexes show a majo r absorption band in the vis ible region, which , in genera l, is split indicat ing lowered site symmetry for copper(II) in the respect ive complexes. Thi s is further confirmed by the ir large ex tinction coeffic ient s wh ich are in the range of (75.4- 144 .5 LM- I cm- I

) .

Figure 2a-e shows cyc lic voltammograms of all the complexes. These were recorded in 2:8 DMSO:MeCN so lvent sys te m. Al l the complexes ex hibit a quasireversibl e CU ll-CUi reducti on wave, w ith peak to peak wid th much larger than that observed for the one-elec tron couple Fc+/Fc ion in the same solvent system. Larger peak width for the one-

I C 11 C I I ' I ' . e ec tron u ~ u coup e III t lelr respec tI ve I . b . II W comp exes IS not an uncommon 0 servatl on. e

were prompted to study the sc;an rate de pe ndence of the redox peak for the compound shown in Fig . 2(a).

P lot of i pc vs yl /2 (scan rate) is nearly linear which is

known to be observed for quasi- reversibl elreversible behaviourl 2. Further, a plot of ipalipc vs yl /2 shows a

near stra ight line suggesting that adsorpti on on e lec trode surface if any is neglig ibl e l 1

. Al l the present

.....

NOTES

+ 20 .0 r--------------, +4 .5r-------____ -.

b

+12.0

<t +4.0 :l..

<t +0.9 ::::I..

" " c C ~ -4.0 ~ -0.9 I­

:J

U

<t ::l..

" -f= (1) I-I-

:J U

-\2 .0

+9.0

+3.0

-3.0

-9.0

-0.5

.... :J U

- \5.0 L----'-~__:___:_I_::____!_.::__-,L-..J +6.0 +4.0 +2.0 0 ·0 -2.0 -0.4 -0.6

Potent lal I V

+ 12 .50r------_____ ~

« + 2 .50

:l..

" -~ -2 .50 .... .... :J U

-7. 50

e

-\2 .50L---'--...L..----L_---'L-_..J +0.40 +0.20 0 .00 -0 .20 -0.40 -0.60

Potentia I IV

-2.7

-4. 5 L--_L--~_=_--'---'---.l---.J +0.7 +O.~ +0.3 +0.1 -01 -0.3 -0.5

Potent ial / V

+12.0.---------------,

+8.0

<t +40 :J... . ~ c (1) l-

I- 0 . 0 :J U

-4.0

d

- 8 .0 '--_-'--_-'-_-'--_--'-_--'"---l

+0.5 +0.3 + 0 .1 -0. \ -0.3 - 0.5

Potential I V

1279

Fig. 2- (' Y. SpcClra 01 (a) Cu(DGBT)(Cl h ; (b) Cu(DGBT)(OACh.2H20 ; (c) Cu(DGBT)(NO.lh; (d) Cu(DGBT)(HCOO-)+2+.3 H20 ; k) Cu(DGBTh(CI)4h and all in 2:8 DMSO:CH3 solven!. Scan rate- l 00 mvs-I .

1280 INDIAN J CHEM, SEC. A, DECEMBER 1999

Table 2-Cyclic voltametric data of copper(II ) complexes at 298 K

Co mpound Erc(V) Ep.(V)

lCu(DGRT)2 (C IO~h ll l J -0.050 0 .1 28

lCu(DGBT)Cl 21 12] -0.05 1 0.12

lCu(DGBT)(N0 1)2 J l3J -0. 119 0.178

[Cu(DGBT)(OACh 12H20 [4] -0.098 0.11 0

ICu(DGBT)(HCOOh J31-1 20 lSI -0.099 0.083

L--~~j A (r--------1

DPPH

I I

2209 2600 3000 340t) · 380(} ' 4200 GAUSS

~ -'"

DPPH

E I/2 (V ) ip/irc

0.039 1.04

0.034 1.06

0.029 1.02

0.006 1.05

-0.008 0.95

DPPH

I I I 2200 2600 3000 34 0 0

GAUSS

/\.

) - L"::'L _ .....L._....J

DPPH

t1Ep 0. 178

0.171

0.207

0.208

0.182

3800 4200

2200 2600 :3000 3400 3600 4200 22~db~-'----:2::-:6::0::0:-"--::;3::00~0-L--::3:-40J'-::-::0-L--c:3,.J8L,0...,.0-.L-4-,J!Oef GAUSS

AA )

DPPH _.

2200 2600 3000 3400 3800 4200 GAUSS

DPV (V)

0.05

0.04

0.05

0.001

-0.005

Fig. 3-200G Scan of frozen DMF solution of [Cu(DGBT)(N03h]' Condi tions: receiver gain=2>< 1-2G; microwave power 20 mw; mi crowave frequency 8.95 Gl-lz modulation amplitude 0.5x 10 T=77K at x ·band; (b) 200G scan of frozen DMF solution of ICu(DGBT)(HCOO- h j. 3H20. Cond it ions: receiver gai n=1. 6x I05G; microwave power 20 mw; microwave frequency 8.95 Gl-lz modulalion am[1litude O.)x IO T=77K at x-band; (c) 200G scan of frozen DMF solution of [Cu(DGBT)(OACh].2H20. Condit ions: receiver gain=lix 102G: mi crowave power 20 mw; microwave freq uency 8.95 GHz modulation amplitude 0.5x I 0 T=77K at x-band; (d) 200G scan o f frozen DMF so lution of [Cu(DGBT)(CI04)2] ' Conditions: receiver gain=2.5xI02G; microwave power 20 mw; mi crowave frequ ency li.Y5 G Hz modu lation amplitude 0.5x10 T=77K at x-band and (e) 200G scan of frozen DMF solution of [Cu(DGBT)(Cl)2j. Conditions: receiver gain=2.5x I02G; mi crowave power 20 mw; microwave frequency 8.95 GHz modulation amplitude 0.5x 10 T=77K at x-band

....

NOTES 128 1

Table 3-EPR data of copper(I1 ) complexes

Compound gil

[Cu(DGBTh(CI04h l 2.38

[Cu(DGBT)CI2J 2.39

[ Cu(DGBT)( N0 3hl 2.40

I Cu(DGBT)(OACh J2H2O 2.35

[Cu(DG BT)( HCOOh I3H2O 2.45

complexes show a peak current ratio (ipa/ipc) near to unity [Shown in Table (2)]. However, the M p values are greater than the nersnti an value (M p "" 59 mv) for a one-electron redox system. This ' indicates a considerable reorgani sati on of the coordination sphere during electron transfer as has been observed for a number of other copper (II) complexes '4 . The followin g order of increas ing EI/2 values is found : HCOO- < CH1COO- < NO,- < cr < CI04- (Table 2). From thi s order one could di scern that the binding of an add itiona l DGBT li gand in the case of CI04-

complex causes the largest destabili sation of the copper(II) oxidati on state, poss ibl y because of greater tetrahedral distorti on of the equatorial pl ane, this being due to steri c constraints and a fac ial rearrangement of the li gands. Within the series of fi ve-coordinate complexes, the chloride has the largest des tabili sing effect, while fo rmate has the largest stabilising influence on the copper(lI) ox idati on state. The va ri ation in the E I/2 value together with visible band energy (d-d) indicates the retenti on of exogenous ani on with the central metal Ion III solution. The above stabilization/ destabi lizati on of the Cu" centre may further be unde rstood by comparing the covalency, d-d band energy and EI /2 value. Smaller the value of a 2

, larger wi ll be the cova lency and lower will be the replusion

between the dx2 / -CU" equatori al bond . Consequently,

a decrease in dx2 / orbital energy will cause an anodic shi ft in CU"-CUi reducti on potenti al ". This is evident fro m the graph plotted between EI/2 and d-d visible band energy which yields a straight line.

EPR spectroscopy

Fi gure ] shows a representati ve X-band EPR spectrum of the above complexes examined at 77 K as a frozen DMF solu tion. Since gil> gl. > 2.003 has been found in all the complexes, spectra are fa irly typical of tetragonally distorted copper(II) environment. The spin Hamiltonian parameters (gil &

g.l A ll gil/Ali X 104 a 2

2.00 123 194 0.56

2.08 120 199 0.57

2.07 120 200 0.59

2.02 140 167 0.62

2. 14 137 178 0.61

All) in all the complexes are very similar to those found in N-coordinated benzimidazole copper (II) complexes and are given in Table 3. Therefore, the benzothiozole ligand binds to copper(II) through the imine nitrogen atom in the equatorial pl ane, rather than the sulphur. This is because, a significant change in gil and A ll would have been observed if S-atom were to replace nitrogen atom in the equatorial plane in the present series of complexes7

.

The above complexes have a large gil and a small . A ll. The large gil / All ratio has been interpreted in terms of tetrahedral di storti on of the basal planes.9

and is also supported by the low energy d-d bands exhibited by the above complexes in the range of 709-892 nm region in DMF. The low value of All in case of chloride and nitrate complexes in comparison to acetate and formate complexes together with the fac t that the d-d bands are shi fted to longer wavelength in both chloride and nitrato complexes is indicati ve of stronger ax ial interaction than that in the corresponding acetate and fo rmate complexes 10 . It is, therefore, concluded that at least one anionic li gand attaches in the ax ial position. This would then place the tridentate ligand in a meridional arrangement rather than fac ial. No nitrogen superhyperfine splitting has been observed in any of these complexes. This may be due to loss of tetragonality, because of steric constraints of the li gand . EPR spectra of perchlorate complex [Fig. 3(d)] shows the presence of four prominent and three weak gil lines. This could be explained on the bas is of the presence ot at leas t two copper(lI ) spec ies in solution having similar gil and gl. va lues. This situation may ari se, if 1:2 perchl orate complex dissoc iates in solution to fo rm a small percentage of I: I spec ies. It is for this reason that the sti ck di agram in Fig. 3(d) indicates whi ch gil components have been utilised for the calcul ati on of A ll va lues .

Ill fi-(lred spectroscopy

IR spectra \,yere taken in KBr pellets. In free ligand ,

1282 INDIAN J CHEM, SEC. A, DECEMBER 1999

a strong band is found at ca 1460 cm- f with .another weaker band at ca 1490 cm- f

• By analogy with assigned bands for imidazole, the 1460 cm- I band is

attributed to VS (C=N-C =C), while the other band is assigned to an overtone or combination band l5 . These bands are shifted by ca 10-20 cm- I in the complexes. This result implies a direct coordination of the imine nitrogen atoms with the metal site. Bands at 1470 and 1320-1340 cm- I are assigned to Vasym (N02) Vsym (N02)

and Vasym (NO) respectively suggesting a un identate mode of binding of nitrate groupl7. In the copper(ll) perchlorate complex appearance of strong bands at 1080 and 910 cm- I suggests the presence of ionic perchlorate l8. The above spectral studies allow us to . propose a meridional structure for the five-coordinate complexes, containing one DGBT ligand and two anions (I), while a distorted octahedral geometry for the six coordinate, perchlorate complex with two DGBT ligands in a facial arrangement (II) as shown below:

++

0--1 II

Acknowledgement We thank the RSIC , lIT, Madras for the EPR

facility. On of us (S KD) is thankful to the CSIR, New Delhi , for the award of a senior research fellowship.

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