Indian Journal of ChemistryVol. 22A. January 1983, pp. 35-38

Macrocyclic Metal Complexes: Part IV-Electrochemical Studieson Cobalt(III) Macrocyclic Complexes

J CHAKRABORTY, KUNJA B NAIK, (Mrs) K L SINGH & B SAHOO*

Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302

Received 13 May 1982; revised and accepted 5 October 1982

Electrochemical studies on fluoro-boro-bridged macrocyclic complexes of cobaltrl ll), Co[bofamcyclaene(l4) B'X] andCo[bofaphcyclaene(l4) B'X], and on their respective parent non-macrocyclic complexes have been done in aqueousdimethylformamide using tetraethylammonium perchlorate as the supporting electrolyte. Two well defined redox stepscorresponding to Co(JII)-Co(II) and Co(II)-Co(I) couples have been observed. For all the rnacrocycles. the redox potentials aremore anodic as compared with those observed for their respective parent complexes, the values being - 200-250 m V and-700mV respectively for the two steps. The anodic shift of E1 2 values for the first stage of reduction is due to decrease inelectron density on central cobalt atom due to the presence of electronegative > BF 2 groups at the bridge head of themacrocycles. The relative ease of reduction for Co(II)-Co(l) couple in the macrocycles implies greater stabilization of metal ionin the low oxidation state which is one of the pronounced characteristics of cyclic ligand systems.

Extensive polarographic studies on metal complexeswith z-acceptor ligands have been reported 1.2.Cobalt(II) complexes ofbis-2, 6-(2' -quinolinyljpyridineand dicyanobispyridyl complexes of cobalt(III) havebeen studied 3.4. The heteroligand complex,[Co(CN)z(Bipy)z] + is reduced at dropping mercuryelectrode (d.rn.e.) in OMSO and gives rise to threediffusion waves, each corresponding to the transfer ofone electron. Electrochemical studies of macrocycliccomplexes have also received considerable attention inrecent years":". Polarographic studies on macrocyclicmetal complexes of coba1t(lII) of the type,[CoL(NOzh] + where L represents a series of 14-membered macrocyclic ligands, viz., 1, 4, 8, 11-tetraazacyclo-tetradecane(cyclam), 2, 3-dimethyl-cyclam(OMC), 2, 3-dimethyl-l, 4,8, l l-tetraazacyclo-tetradeca-l , 3-diene(DIM) and 2,3,9, 10-tetramethyl-1, I, 8, l l-tetraazacyclotetradeca-I , 3, 8, 10-tetraene(TIM) have been done in methanol?

Fluoro-boro-bridged macrocyclic complexes ofcoba\t(III) of the type, Co(Cy)B'X, where Cyrepresents macrocyclic ligands, I, 8-diboro-l, 1, 8, 8-tetrafluoro-2, 7, 9, 14-tetraoxa-3, 6, 10, 13-tetraaza-4,5, 11, 12-tetramethylcyclotetradeca-3, 5) 10, 12-tetraene[abbr. bofamcyclaene(14)] and 1, 8-diboro-l,I, 8, 8-tetrafluoro-2, 7, 9, 14-tetraoxa-3, 6, 10, 13-tetraaza-4, 5, 11, 12-tetraphenylcyclotetradeca-3, 5,10, 12-tetraene [abbr. bofaphcyclaene(14)]; B'represents a pyridine base and X represents a halideion (B' and X occupying trans-axial positions) havebeen reported by USB. Electrochemical studies on thesecomplexes have been carried out with a view tocorrelating the half-wave potentials with changes instructural parameters and stabilities of the chelates.We have used dimethylformamide (OM Fj-water

mixture (80:20) as the solvent medium because of thesolubility of the complexes in the former solvent and itsaprotic nature. A part from the macrocyclic complexes,their parent compounds, Co(LHz)B'X, have been alsostudied polarographically to throw light on thechanges that occur on account of the macrocycliceffect.

Materials and MethodsOimethylformamide (BOH), perchloric acid (E

Merck, AR) and tetraethylammonium hydroxide (25%solution, Fluka) were used for polarographicmeasurements. Parent complexes, Co(OximeH)zB'X[OximeH = mono anion of I, 2-dimethyl-l, 2-dione dioxime (dimethylglyoxime) or I, 2-diphenyl-ethane-I, 2-dione dioxime (c-benzildioxime), B'-pyridine, fi-picoline or r-picoline and X = CI -, Br -, orI -] were prepared by the literature method". Theirfl uo r o- bor o- bridged macr ocyc lic deri va ti ves[Co(bofamcyclaene(14) B'X] and Co[bofa-phcyclaene(14)B'X] were prepared as described earlierby USB.

Polarographic curves were recorded with arecording polarograph LP 7 fitted with a EZ 7 linerecorder. A dropping mercury electrode (d.m.e.) wasused as an indicator electrode and an externalsaturated calomel electrode (S.C.E.) was used as thereference electrode.

The aqueous dimethylformamide solution of thecomplex (I x 10-3 M)(OMF: HzO=80:20) containing0.1 M tetraethylammonium perchlorate (TEAP) wasconnected with a saturated calomel electrode (S.C.E.).The capillary of d.m.e. had the characteristics, m= 6.6 mg see -1, t = 3.7 see for a mercury column ofheight 42 em. Half-wave potentials were measured at a

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INDIAN J. CHEM., VOL. 22A. JANUARY 1983

scanning rate of 100 m V/min, Tetraethylammoniumperchlorate(O.1 M), used as the supporting electrolyte,was prepared and dimethylformamide was purified asreported earlier!".

Results and DiscussionThe electrochemical data for the complexes/halo)

(pyridine)-bis(l, 2-dimethylethane-l, 2-dione dioxi-mato)cobalt(III) [Cotdmgl-ljj B'H] and (halo) (pyri-dine) bis( I, 2-diphenylethane-l, 2-dione dioximato)-cobalt(III) [Co(bdoH)zB'X], and their macrocyclicderivatives are presented in Tables I and 2. A re-presentative plot of log(ilid -I) vs E is shown in Fig. I.For the 14-membered macrocycles and their parentcomplexes (for the series containing X and B' as axialligands) two well defined reduction waves have beenobserved. These redox waves may correspond tospecies having the metal ion in the oxidation states + 2and + I respectively. In the macrocyclic complexes ofthe type Co[bofaph-cyclaene( I4)B'X] , a third wavewas observed in the potential range - 1.13- - 1.30 V.However, no such wave was observed for themacrocycles Co[bofamcyclaene(14)B'X] within thepotential range studied i.e. upto - 2.0 V. The thirdwave in the former series of macrocycles has not beenfully investigated; it may be either due to the reductionof the macrocyclic ligand or the red uction of the metalion from oxidation state + I to O. The first twopolarographic waves of these complexes were found tobe diffusion controlled at the dropping mercury

Table I-Electrochemical Data of Cobalt(IIOMacrocyc\es of the Type, Co[bofamcyc\aene(14)B'X] and

Their Parent Complexes

Complex Co'+ ¢Co+

Ef/2(Y) Slope-' E1I2(y) Slope -,(mY) (mY)

CoL,PyCI -0.788 68 -0.99 64CoL,PyBr -0.786 60 -0.988 69CoL,PyI -0.784 67 -0.984 86CoL,P-PicCI -0.823 67 -1.014 59CoL,p-PicBr -0.792 54 -0.968 75CoL,P-Piel -0.807 67 -0.992 88CoL,y-PicCI -0.820 74 -1.023 87CoLI y-PicBr -0.794 54 -0.992 75CoL,y-Piel -0.802 69 -0.989 88Co(dmgH),PyCI -1.028 93 -1.770 80Co(dmgH),PyBr -1.013 87 -1.761 88Co(dmgH),p-PicCI -1.055 86 -1.763 83Co(dmgH),p-PicBr -1.026 96 -1.798 100Co(dmgH),tJ-PicI -1.005 120 -1.778 88Co(dmgH),y-PicCI -1.028 75 -1.769 94Co(dmgH),y-PicBr -0.991 72 -1.756 67Co(dmgH)2y-PicI -0.979 82 -1.750 71

L, =bofamcyc1aene(14)

36

electrode as revealed by the linear plots of id versusJh:;r and id versus concentration, which pass throughthe origin.

The results of the electrochemical studies indicatesome salient features concerning the influence of thenature of the ligand on the nature of the electrodereaction. In terms of the first stage of reduction, theelectrochemical processes can be broadly divided intotwo groups. The first stage of reduction for themacrocyclic complexes, Co[bofamcyclaene(14) B'X]

'"o

o. First stop of r educt ion cetnn ...•Co(IIlb. Socond stop of reduction Co(IIl~Co(Il

0'8

0"6

0'2

.~ a

-0'2

-08 L-_-'-_---' __ ....l-_--'-__ -'--_ a- 075 - er77 - 0·79 - Otll -0'83 -0'85

~-~-~--~-~--~-- b-0,95 -0,97 -0'39 -1·01 -1'03 -1'05

E VS SCE/ v

Fig. I-Plot of log(i/id -I) versus E of I x 10-3 MCo [bofamcyclaene (14) PyCI]

Table 2-Electrochemical Data of Cobalt(III)Maqrocycles of the Type, Co[bofaphcyclaene(14)B'X] and

Their Parent Complexes

Complex Co3+ ¢Co2+ Co2+ ¢Co+

E,tiY) Slope " ' E,/,(Y) Siope-'(mY) (mY)

CoL,PyBr -0.514 80 -0.842 75CoL,PyI -0.537 64 -0.857 78CoL,P-PicCl -0.532 78 -0.787 79CoL,P-Piel -0.595 76 -0.903 88CoL,y-PicCI -0.561 70 -0.780 80CoL,y-PicBr -0.525 75 -0.790 70CoL,y-Piel -0.559 82 -0.940 68Co(bdoH),PyBr -0.755 71 -1.495 75Co(bdoH),PyI -0.776 77 -1.570 80Co(bdo H) 2 tJ- PiBr -0.750 65 -1.550 82Co(bdol1),y-PicBr -0.762 61 -1.601 69Co(bdoH), y-Piel -0.760 83 -1.524 83L, =bofaphcyclaene( 14)

CHAKRABORTY I!( (/1.: Co(lI!) MACROCYCLIC COMPLEXES

appears to be reversible11 at the d.m.e. (reciprocal ofthe slope lies within 58-68 mY) (Table I, Fig. I a) whilefor the macrocycles, Co[bofaphcyclaene(14) B'X] thefirst stage of reduction may be regarded as quasi-reversible as determined from the reciprocal of theslope of the plot of log (i/id-I) versus E (Table 2). Forother complexes the first and second reduction stagesare either irreversible or quasi-reversible (Fig. Ib). Thistype of redox processes occur for cobalt (III) cornp-plexes with bipyridine", phenanthroline and severalmacrocyclic ligands". Bis( I, 2-dimethylethane-l, 2-dione dioximato) cobalt(III) complexes have beenstudied polarographically by Schrauzer et al.12 Aseries of base adducts of different p K; values have beenstudied by Costa and coworkers 13. The latter workersobserved that the first reduction wave is irreversible orquasi-reversible for the pyridine complex and developswith half-wave potential, E1/2, between -0.33 and-0.89V (vs S.C.E.). For the present series of bis(l, 2-dimethylethane-I, 2-dione dioximato)coba1t(III) com-plexes, the first reduction wave, which develops withhalf-wave potential between -0.979 and -1.005 V inaqueous DMF (Table I), is more cathodic than theearlier reported values and is presumed to beinfluenced by the solvent molecules and/or ne-phelauxetic effect (vide infra).

It is striking to note that E1/2 values for Co(III)-Co(II) couple for the non-macrocyclic parentcomplexes are more cathodic than those for theirrespective macrocyclic derivatives and the waves areirreversible as evident from the magnitude of the slopes(Tables I and 2). The E1/2 values for the coba1t(II)-cobalt(I) couple for the macrocyC\ic complexes arefound to be more anodic than those for their respectiveparent complexes indicating greater stabilization ofCo(I) in the cyclic ligand system. Such behaviour hasbeen observed for a series of iron(lI), nickel(II) andcoba1t(II) complexes 7. It has been reported that for thebis(l, 2-diphenylethane-l, 2-dione dioximato)cobalt(III) complexes, the E 1/2 values for the first andsecond stages of reduction are more anodic relative tothose for the bis(l, 2-dimethylethane-l, 2-dionedioximato) cobalt(III) series of complexes reflectinghigher degree of ligand unsaturated in the formerseries. But polarographic studies on a series ofcobalt(IlI) complexes of the type [Co(MAC)(CH3CN)2]3 '. where MAC represents a series of 14-membered macrocycles varying in their degree andposition of ligand unsaturation and substitutions,show some striking variations from the presentfindings. For the MAC series of complexes, the half-wave potentials for the Co(II)/Co(I) couple vary in apredictable manner, related to ligand unsaturation,while the Co(IlO/Co(II) couple is insensitive to thesestructural variations.

Electrochemical reduction represents an electrontransfer to the lowest unoccupied molecular orbital.The value of E1/2 represents a thermodynamicquantity and provides a measure of the energy level ofthis orbital in the complex. It is striking to note thatvariation of E1/2 values for the first and second stagesof reduction for each series spans a very small range,~ 40 m V. The small range implies that the transferredelectron is delocalized embracing a commonunoccupied molecular orbital which is most likelydominated by planar ligand field including the metalion.

Half-wave potentials provide a measure of theelectron density on the metal ion 14. The anodic shift ofhalf-wave potential for Co(III)-Co(II) couple reflects adecrease in electron density on the cobalt ion due to thepresence of highly electronegative> BF 2 groups at thebridge head of the macrocycles which decrease theelectron density in the chelate ring and the drift of theelectrons takes place both in the zr-electron system andthe a-frame work.

Another significant aspect of the present electro-chemical studies is the difference in magnitudes of theEI/2 values for the first and second stages of reductionfor the macrocyclic complexes and their respectiveparent compounds. The second stage of reduction issubstantially easier in the macrocyc\ic complexes( ~ 200 mV) than in their respective non-macrocyclicparent complexes (~700 m V) as reflected from thepositions of the cathodic waves. The results showgreater metal-ligand interactions in the equatorialplane in the macrocycles, further implying a higherstabilization of the metal ion in the lower oxidationstate. A formal + I oxidation state of cobalt in theproduct and the ligand field strength in the equatorialplane would indicate a planar structural arrangement,a very favourable structure with a low spin d8 electronconfiguration.

Comparison of the present polarographic data withthe available E1/2 values of some macrocyclesilluminates a self consistent pattern. The E1/2 values ofmacrocyclic complexes of Co(Il0 with cyclam, DMC,DIM and TIM in methanol, for the first step, are foundto range from - 0.450 V to - 0.217 V and the E 1/2values for the second stage of reduction are - 0.841and -0.474 V for DIM and TIM complexesrespectively. The results show that an increase inligand unsaturation leads to an anodic shift of E1/2which has been attributed to greater nephelauxeticeffect of the unsaturated ligand. The Racah parameterB reflects the measure of nephelauxetic effect. The Bvalues lie in the range 350-450 ern -1 for the presentseries of complexes" while they are about 600 em -1 forthe cyclam, TIM, DIM and DMC complexes ofcobalt(lII). Relatively higher cathodic E1/2 values for

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INDIAN J. CHEM., VOL. 22A, JANUARY 1983

the present series of complexes are consistent withlower B values arising due to more electrophilic > BF 2bridges.

AcknowledgementThe authors are thankful to the UGC, New Delhi,

for the award of a Teacher Fellowship to JChakraborty, and to the CSIR, New Delhi for aFellowship to Kunja B. Naik.

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XXI CIRCE conference, Prague (Czechoslovak Academy ofScience), 1970, 5.

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3 Bonomo R P, Musumeci S, Rizzarelli E & Sammartano S, Jelectroanalyt Chern, 56 (1974) 435.

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4 Maki N, J electroanalyt Chern, 51 (1974) 353.5 Tait A M, Lovecchio F V & Busch D H, Inorg Chern, 16(1977)

2206.6 Drew M G B, Grimshaw J, Mellroy P D A & Nelson S M, J chem

Soc, Dalton, (1976) 1388.7 Higson B & McKenzie E, Inorg nucl chem Leu, 6 (1970) 209.8 Chakraborty J, Naik Kunja B & Sahoo B, Indian J Chern

(accepted).9 Schrauzer G N, Inorg Synth, II (1968) 62.

10 Mahapatra B K & Sahoo B, Indian J Chern, 21A (1982) 370.II Kodama M, Bull chem Soc Japan, 4S (1975) 3133.12 Schrauzer G N & Windgassen R J, J Am chem Soc, 88 (1966)

3738.13 Costa G, Mestroni G, Puxeddu A & Reisenhofer E, J chem Soc,

(A), (1970) 2870.14 Carter M J, Rillema D P & Basolo F, J Am chem Soc, 96 (1974)

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