iJ1Qlan .Iotlmal of Chemistry VoL 38A, Febmary 1999, pp. 156-160

Synthesis and characterization of cobalt(II) chelates ofN,N'-ethylene bis-salicylaldimine and related quadridentate ligands

S Sailaja, M Radhakrishna Reddy, K Mohana Raju t & K Hussa in Reddy*

Department of Chemistry, Sri Krishnadevaraya University, Anantapur 515 003 , India

Received 7 August 1997. revised 18 November 1998

Cobalt(lI ) complexes with a series of as many as sixteen SALEN type of quadridentate ligands deri ved by condensing substituted carbonyls and different diamines have been synthesized and characterized by IR, IN NMR. molar conducti vity. magnetic susceptibility measurements and electronic spectral data. Subnormal magnetic moment values in fer the pn;,ellce of dimeric structure for the present complexes. However, electronic spectral data suggest a square phmar structure in :;oluti on. possibly due to the monomeric structure. Infrared spectral data suggest the bonding of ligands through two orlllO phenolic oxygen and two azomethine nitrogen atoms. The electrochemical behaviour of these complexes has been studied h) cyclic voltanlmetry. Additional phenolic groups present in these complexes are useful for the faci le synthesis of polymer supported cobalt complexes.

Although Schiff base metal complexes have broad applications as catalysts'-3 in chemical and petro­chemical industries, the analogous chemistr/ of cobalt complexes which have been proposed as model compounds for vitamin B'2 has also been reported. Considerable efforts5

-7 have been made in recent

years for the synthesis and characterization of macrocyc lic cobalt complexes since they are much in common with, metallo corrins . Transition metal complexes with N,N-ethyelene bis-sa licylaldimine (SALEN) type of ligands have been extensively studied8

. However, cobalt complexes with different substituted (or functionalized) SALEN analogues are scarce. Recently, we have reported the synthes is and characterization of a seri es of symmetrical and unsymmetrical macrocyc lic cobalt complexes9

"o. In continuation of our investigations on cobalt complexes and in the light of above discussion, herein we report the synthesis, characterization, spectral studies and electrochemistry of cobalt complexes of sixteen quadridentate ligands derived using sa li­cylaldehyde, resorcylaldehyde, resacetophenone and 2,4-dihydroxybenzophenolTe and different diamines viz. 1,2-diaminoethane, 1,3 -diaminopropane, 1,2-diaminopropane and 1 ,2-diaminobenzene (Structures 1-XVI). The studies pertaining to cobalt complexes of

fDepartment of Polymer Science, S K University, An antapur 5 15 003

SALEN and SALOPH are included for the sake of comparison.

Materials and Methods All . the chemicals used were of AR grade.

Resacetophenone " and 2,4-dihydroxybenzaldehyde ' 2

(resorcialdehyde) were prepared according to literature methods. Sa licy laldehyde (Sarabhai). 2,4-dihydroxybenzophenone (SISCO), 1,2-diaminoethane (Qualigens), 1,2-diaminopropane (Me rck), 1,3-

Struc.tu rcs I-XVI

I. RI = H , R2 = CZH. ,x : H , SALEN ;

II . RI = H, R2 = 1,3 .. C3H6 ,x =H SAL 1,3 -PN;

Iii . RI =H,R2=I,2-C3H6,X=H,SALI,2-PN;

IV . RI = H,R2 = 1,2 ·,C6 H4 ; X = H ,SALOPH ;

V. RI = H , R2 = C2H4 X = OH, RESEN ;

VI. II, = H,R2

= 1, 3..C3 H6 , X= OH,RES 1, 3-PN .

VII. RI = H,R2 = 1,2 .. C3 H, X = OH,RES 1,2 ' PN.

VIII . RI = H,R 2 = 1,2 - C6 H4 ; X = OH RESOPH;

IX . R,=CH3 ,R 2 =C2 H4 , X =OH,RAEN ;

X. RI = CH3

.R2

=I,3·-C3H6 ,X = OH,RA 1,3-P~I.

XI. RI = CH),R2 :I,2.£3H6 ,x = OH,RAI,2 - P N;

XII . RI = CH3 ,R2: 1,2 -CsH4 , X = OH RAOPH ;

XIII. RI : C6HS,R2 = C2H4'X =OH, OBEN .

XIV. RI = CsHS'R2 = 1, 3 - C3H6 ,X=OH,DB 1,3 ··PN .

XV. RI = CsHS'R2 = 1,2 - C3H6' X oOH, 081,2 " PN;

XVI. RI = C6HS'R2 = 1. 2 - C6H4,X=OH,DBOPH

..

SAILAJA e/ af.: SYNTHESIS OF Co (ll ) COMPLEXES WITH QUADRIDENTATE LIGANDS 157

diaminopropane (Merck), 1,2-diaminobenzene (Merck) and cobalt acetate (Qual igens) were used III

the present study.

Synthesis of ligands The carbonyl compound and diamine are mixed in

2: I molar ratio in a 250-cmJ round bottom fl ask containing 50 ml of methanol. The reacti on mi xture was refluxed for 3 h. On coo ling to room temperature. a co loured product separated out. The crystalline compound was collected by fi Itration and washed with ice-cold methanol. The li gands were recrysta!l ized from methan o I.

Synthesis of complexes In a clean 250-cm' rou nd bottom flask, requI site

alT'JO unt of li gand (0.008 mol) dissolved in 50 1111 of methanol and cobalt acetate (2 g, 0.008 11101) dissolved in 10 ml of water were taken and the reaction mixture refluxed fo r 3 h. On coo ling to room temperature, a co loured co mplex separated out. The

product was co llected by filtrat ion and washed with hot water and finally with co ld meth anol.

The IR spectra were recorded in KBr pe ll ets in the range 4000-400 cm'l with Brucker IFS 66 V spectro­meter. The absorban ce measurements in DM F were carri ed out on Sh i madzu U V -160A spectro photo­meter usin g 10.mm quartz ce ll s. The IH NMR spectra were obtained using Varian XL-300 MH z hi gh reso lution NMR instrument in CDC IJ and DM SO-cfr, so lvents at room temperature. The conductance measurement s of - 10'.1 M solutions were made in DMF usin g a Systronic 303 direct readin g con­ductivity bridge. The magneti c momcnts of coba lt complexes were obtained at 298 K usin g a sophisticated EG & G Princeton Applied Research Vibrating Sample Magnetometer VSM ISS operating at field strength 0.3 to 0.8 T. The cyc lic vo ltammetry was performed with a BAS mode l CV-27 contro ll er and conventi onal three electrode configurati on with a glassy ca rbon working electrode, silver/sil ver chl or ide reference electode and pl atinum cOllllier e lectrode. Nitrogen was used as purge gas and all solutions were of 0.1 tv! co ncentrati on in DMF in TBAC IO.I supportin g electrolyte.

Results and Discussion A serics of tetrade nlJ te li gands were prepared

using different 011hohydr0xycarbonyl co mpounds viz. salcylaldehyde, ~-re so rcy l a ld c hyde, resacetophenone

and 2,4-dihydroxybenzophenone with several dia­mines viz. 1,2-diaminoethane, 1,2-diaminopropane, I ,3 -diaminopropane and 1,2-diaminobenzene. All the ligands were recrystalli sed from methanol. All the coba lt complexes of these are stab le at room temperature. The complexes are slightly soluble in water, methanol and ethanol and highly so luble in DMF, DMSO. Coba lt complexes of th e li gands derived using sa licylaldehyde are fairly so luble in chloroform . The molar conducti vity values (in DMF) suggest that a ll the complexes are non-electrolytes.

The IR spectra of I igands indicate the absence of peaks due to carbonyl and amino groups and the appearance of new peaks between 1660-1623 cm'l which may be ass igned to vC=N stretchin g vibration in dicating the condensation to result in the formati on of corresponding azomethine compounds. The characteri stic phenolic OH absorptions are also observed between 3450-3300 cm' l. The broad nature of these bands suggests the presence of a weak hyd rogen bond as a part of resonating ring system . The other characte ri sti c phenolic C- O stretching vi brations are observed between 1 174-1 142 cm'l in all the li gands.

125/JA

A

I 50}JA

0·2 - 0,3 -0-6 -1-3 -1-8

EIV

Fi g. I--Cyclic vollammograms for !\ . S!\L1~N and 13 . Co-S,\I.I ·:N in l)MF (DOO I .11 TB!\CI()4) ( j ( ' ckClwdc. !\g/!\g( '1 rci"crencc eleclrodc (scan rale 100 IllV S· I )

158 INDIAN J CHEM, SEC. A, FEBRUARY 1999

The IH NMR spectra of ligands derived from salicylaldehyde and J3-resorcylaldehyde indicate the presence of a strong singlet peak - 8.64-8.15 8 which can be assigned to methine protons of azomethine groups. In the case of ligands derived from resacetophenone, the peak corresponding to methyl group attached to azomethine is observed at 82.30.

The peaks corresponding to methine, methylene and methyl groups of the diamino moiety are observed at 83.85, 3.70 and 1.42 respectively. The downfield peak between 16.68 to 13.08 in all the ligands can be assigned to orthophenolic group whiclh is in chelation with azomethine nitrogen atom. The other characteristic peaks corresponding to the phenyl

Table l-CycI ic voltammetric data of cobalt complexes (10.3 M) to DN F

Complex Gain Redox couple Epc Epa E II2 E (V) (V )

Co-SALEN 0.1 11IIIV Oxi +0.05 0.20 0 . 125 150 1111 -1.28 -1.18 1.23 100

Co-SAL 1,3-PN 0.2 11/1 -1.53 -1.45 -1.5 75 111!11 -0.98

Co-SAL 1,2-PN 0.2 111111 -0.88 1111 -1.23 - 1.78 - 1.22 50 11/1 -1.38 11/1 - 2.0

"'IJI - 0.45

Co-SALOPH 0.2 11/1 red -1.18 - 1.05 - 1.11 125 -1 .73 - 1.63 -1.7 50

III/IV Oxi +0.3

Co-RESEN 0.2 "'II -1.2 1111 -1.68 ",/IV +0.18

Co-RES 1,3-PN 0.2 1111 -1.65

Co-RES 1,2-PN 0.2 1111 -1 .65

Co-RESOPH 0.2 IJII -1.18 -1.13 -1.15 50 1111 - 1.8 111m -0.33

Co-RAEN 0. 1 11/1 - 1.25 -1.4 - 1.45 150

",/I V +0. 13 Co-RA IJ-PN 0.2 11/1 -1.78

1I I1IV +0. 1

Co-RA 1,2-PN 0.2 1111 - 1.5 - 1.38 1.48 125 1111 +0. 15 IIIIIV +0. 15

II Co-RAO PH 0. 1 1111 - 1.8 11 1111 -0.58 111111 0.23

III Co-D BEN 0.2 I111 - 1.38 - 1.28 - 1.33 100 1111 - 1.65 1I111V +0.25

V Co-DB 1.3-PN 0.2 11/1 - -1. 65 1111 un

Co-DB I,2-PN 0.2 1111 - 138 -133 -135 50 - 1.825

II II IV +0.2

Co-DBOP II 0 .2 11 /1 - 1.23 111 / 11 0.6

...

SAn-AlA et al.: SYNTIIESIS OF Co (II) COMPLEXES WITH QUADRIDENT ATE LIGANDS 159

nucleus is also observed in all the ligands. The above spectral data is in confirmity with the structure proposed for the ligands.

The cyclic voltammetric data of the ligands suggest an irreversible reduction of azomethine (>C=N-) group present in all the ligands. Careful examination of cyclic voltammograms (eg., SALEN , SAL 1,2-PN ) reveals the presence of intermediates in the reduction of azomethine group. The cyclic voltammograms of SALEN and its coba\t(II) complex are given in Fig. I . A possible mechanism is proposed for the reduction of >C=N group in three different steps as shown in Scheme I , based on the CY data and in analogy with earlier observations lJ

.15

•

The cobalt(lI) complexes have magnet ic moment values in the range, 0.58-2 .82 BM . In general , hi gh spin 4-coordinate complexes are tetrahedral whil e corresponding low spin complexes have square pl anar geometry. Tetrahedral cobalt(lI) complexes have magnetic moment values in the range 4.2-4 .9 BM. Tetracoordinate low spin coba lt( lI ) compl exes show magnetic moments in the range 2. 7-2 .9 BM . The magnetic moment values of present complexes suggest to have a square planar confi gurati on. The subnormal magnetic moment va lues of compl exes

d· 1(, 17 may suggest a pentacoor lI1 ate structure ' con-taining two roughly planar units which are linked by out-of-plane Co-O bonds.\

The electronic spectra of the present coba lt( lI ) complexes were recorded in DMF. The spectra show intense bands in high energy region 33220-26525 cm·1 which can be ass igned to charge transfer L~M bands. In the e lectroni c spectra of Co-SA LEN and Co- RES 1,2-PN, low energy bands are identified at

Rl I

x~NH +

St' P 3!2 • . 2H

Rl I ~H-NH2

X OH

il ~H- N ; C

HO~X Slop 2! 2 • . 2H +

Rl H I

RzH - N- HC'r(3I

HO~.x

I. R ,=H, Rz=CzIl 4 • X=H SALEN: /I. R 1=II .Rz= U -C1 11 ". X=II SAL U-PN: XI/'. R I =Cb ll ~ , Rz= I.3 -C, lIb' X=OIl . Dn 1. 3- I'N: .\ '1'. R, =Cb ll ~ . R2~ U-C)Ho. X=() II. Dn U -I'N .

Scheme I

20325 and 19646 due to d-d transitions which is suggestive of squ:-lre planar structure. The present observation is in analogy with previous studies made for low spin square planar complexes with Co-N20 2

chromop~ore . The other cobalt complexes gave no information as the weak spin forbidden d-d transition is full y covered by the tail of broad and strong charge transfer band . The charge transfer is shi fted towards shorter wavelength (high energy region) in the spectra of cobalt complexes with ligand s derived us ing 1,2-phenyelenediamine.

Co-SALOPH (3 3222) > Co-SALEN (28985); Co­RESOPH (3 1 4~ 6) > Co-R ESEN (26881); Co-DBO PH (28901) > Co-DB EN (28818 cm·I).

In the IR spectra of free li gands, broad absorpti on bands in the region, 3450-33 00 cm·1 are observed due

to weak hydrogen bonded and free pheno lic v(O H) stretching vibrations. The latter bands are observed in the IR spectra of compl exes wi th liga nds contai ning additional or para-pheno lic groups at ca. 3430 cm· l

.

Strong and sharp band s in the spectra of complexes are observed in the ran ge 1640-1590 em' ! which are lowe r by 10-15 cm' ! when compared with li gand s and

ass igned to vC=N vibrati ons suggesting the parti c ipa­tion of azomethine nit rogen atom in coordination.

The bands characteri stic o f vC-O stretchin g vibrati on a re observed at hi gher frequency when compared w ith the ligands in the reg ion 1200- i 152 cm'! indicat ing the parti c ipati on of pheno lic oxygen in complex fo rmation. Additi ona l band s 469-435 cm' ! are a lso observed in the spectra o f a ll complexes

ass igned to vCo- N vibrati on which substantiate the parti c ipation o f azo methine nitrogen atom of Sc hi ff base in li gati OI1.

T he cyc l ic vo ltammograms of coba lt compl exes with SA LEN , SAL I J-PN and SA L 1,2-PN show

reversi ble Co(ll )/Co(l) couple with £ 1/2 == - 1.23, - 1.49 and - 1.21 Y respective ly (Tab le I). Co-SA LOPII shows two revrs ible Co( II )/Co( l) co upl e "vith E! 2 = - 1. 1 and - 1.7 Y respecti ve ly. The one-e lectron reduction is responsible fo r revers ible wave. Co­SA LEN complex is oxidi zed at E I/2 = +0.1 3 Y and it may be responsible for Co( III)/Co(lY ). S im ilar

e lectrochemica l behav iour is noticed fo r the coba lt complexes of RAEN , RA 1,2- PN. DBEN and DB 1.2-PN.

The cobalt comp:exes deri ved from RESEN. RES I J-PN and RES, 1,2- PN show irrevers ibl e red ucti oll waves at Ere = - 1.65 Y. But in a ll these co mplexes.

160 INDIAN J CHEM, SEC. A, FEBRUARY 1999

the cathodic peak is shifted to more negative potentials as the scan rate is increased. The Epc of Co­RESEN complex is -1.53, -1.60 and -1 .65 at the scan rates ISO, 300 and 450 respectively. Simiiar trend is observed with the complexes of RES I J-PN and RES 1,2-PN. This suggests that as the' scan rate increases, slow electron transfer kinetics makes the peak potential to shift in such a manner that they are no longer symmetric and these species may be highly irreversible. The complex derived from RESOPH has one reversible reduction wave at E1 /2 = - I. IS V corresponding to Co(II)/Co(l) couple.

Except cobalt complexes with ligands derived using salicylaldehyde, all complexes have free phenolic functiona l groups which can be linked to polymers contaInIng active groups such as chloromethyl and sulphonic acid groups to prepare polymer supported cobalt complexes . These polymer supports may be used as catalysts in certain organic reactions and this work is in progress .

Acknowledgement We are grateful to the DST, New Delhi (Grant No.

SP/S I/F-07/92) for financial suppurt.

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