Organocobalt Complexes of Diacetylmonoxime Buckled

Indian Journal of Chemistry Vol. 39A, December 2000, pp. 1312-1316

Organocobalt complexes of corresponding organo derivatives (IV-VII and IX­ diacetylmonoxime buckled with different XIII) are given in Fig. I. diamines as 'models' for vitamin B 12 Experimental derivatives All the solvents used were of AR grade, cobalt(II) chloride hexahydrate (Qualigens), I ,2-diamino K Hussai n Reddy', M Radhakrishna Reddy & K Mohana Raju t benzene and 3,4-diaminotoluene (FLUKA AG), Department of Chemistry, Sri Krishnadevaraya University, diacetylmonoxime (Spectrochem AR), iodomethane Anantapur 515 003, India (Merck), bromoethane (Merck), pyridine (Qualigens) Received 16 March 1999; revised 15 October 1999 and imidazole (Spectrochem, AR) were used in the present study. Macrocyclic chlorocobalt complexes have been synthesized using preformed li gands viz. bis(diacetyl monoxime)-1 ,2- phenylenediimine (I) and bis(diacetyl monoxime)-3 ,4-tolylene­ Synthesis of ligands I and I.l diimine (II). The chloro derivatives have been alkylated using In a I 00 cm3 short-necked round-bottomed flask, iodomethane and bromomethane in the presence of pyridine or I diacetyl-monoxime (3.76 g, 0.037 mol) was taken in and imidazole. The alkyl derivatives have been characterized by 3 elemental analysis, conductivity data, magnetic susceptibility methanol ( 15 cm ) and to this was added diamine measurements, electronic and infrared spectral data and they are (2 g, 0.0185 mol) in methanol and heated under reflux found to be I: I electrolytes. Various ligand fi eld parameters like for I h. The reaction mixture was cooled and Dq •Y, Dq'· and Dt have been calculated. Infrared spe<.:tra reveal the coloured compound obtained was recrystallized from 'trans effect' in these complexes. Electrochemical behaviour of these complexes has been studied by cyclic voltammetry.

There has been growing interest in organocobalt complexes which might serve as 'model compounds' for coenzyme B 12. Cobalt complexes of diacetylmonoxime buckled with diamines 1 appear to mimic the properties of coenzyme B 12 better than the complexes of tetradentate Schiff base li gands, perhaps, as in the former the equatorial ligand can act I ![ as ligand of the uninegative charge and posesses pseudo symmetry as corrin. Synthesis of organocobaloxime with modified structural features continues to be a fascinating area in the chemistry of model compounds of coenzyme B 12 . Recently, Gupta and Rol reviewed the synthetic aspects of organocobaloxime chemistry. In view of the above relevance and in continuation of our investigations on axial ligational properties of cobalt complexes3.4, herein we report the synthesis, characterization, spectral and electrochemical studies Jtl R = B = Cl Vlll R = B = Cl B C H N IX R ~ CH ; B = C H N on macrocyclic 'organocobalt complexes' of IV R = CH3 ; = 5 5 3 5 5 R = CH ; B:: c H N X CH ; B: c H N bis(diacetylmonoxime)-1 ,2-phenylenediimine (I) and v 3 3 4 2 R" 3 3 4 2 VI R = c H ; B: C H N XI R = C H ·;s= C H N bis-(diacetylmonoxime)-3,4-tolylenediimine (II). The 2 5 5 5 2 5 5 5 VII · R = c H ; B= c H N XII R. ~ c H ; B =c H N structures of chloro derivatives (Ill, VIII) and 2 5 3 4 2 2 5 3 4 2 Fig. !-Structures of ligands (I & II), chloro complexes (III & t Dcpartme~t of Polymer Science and Technology, S K University, VIII) and organocobalt complexes (IV-VII, IX-XII). III & VIII Anantapur, India are neutral complexes. C5H5N =Pyridine, C3H4N2 =Imidazole NOTES 1313 warm ethanol. The colours, melting points together The elemental analyses were performed by the with elemental analyses are given in Table I . HERAEUS (Mikro standard 8304071) elemental analyzer. The magnetic susceptibility measurements Synthesis of th e chloro complexes III and VIII were made using a vibrating sample magnetometer The reaction mixture containing ligand (0.004 (VSM) operating at a field strength of 2 KG to 8 KG . mol)and cobalt(II) chloride hexahydrate (0.004 mol) The molar conductance of complexes in (DMF) 3 3 in methanol (30 cm ) was refluxed with stirring for (Ca. -10- M) were determined at 27± 2°C using a 1 h. The resulted green solution was filtered and kept Systronic 303 direct reading conductivity bridge. The overnight at room temperature. The dark green 1H-NMR spectra were recorded on an AMX-400 compound which separated out was filtered and MHz high resolution NMR instrument in acetone-d6 washed repeatedly with acetone followed by diethyl solvent at room temperature. The electronic spectra ether to get the dark green compound. for all complexes in UV region ( 180-1100 nm) were recorded with Schimadzu UV -160A spectrophoto­ Synthesis of organocobalt complexes IV-VII and IX­ meter in methanol. The IR spectra were recorded in XII the ranges 4000-400 (in KBr) and 450-50 cm-1 (in In a Schlenk tube, a saturated methanolic solution polyethylene) using Brucker IFS 66V FT-IR of macrocyclic cobalt complex (III) or (VIII) (0.004 spectrophotometer. The cyclic voltammetry was mol) was taken and stirred under nitrogen performed with a BAS model CV -27 controller and a atmosphere. Alkylation reaction was carried out by conventional three electrode configuration with using CH31/C2H5Br as described in the standard glassy carbon working electrode, silver/silver 7 procedure . The colour, melting points, and molar chloride reference electrode and platinum counter conductance for all complexes are given in Table I. electrode. Nitrogen was used as a purge gas and all Table 1-Colour, elemental analyses and magnetic moment data of the cobalt complexes

(B.M.) Colour Found (Calcd) % rcJT11 c H N I C,4HI RN40 2 Yellow 61. 05 6.71 19.95 (178-180) (6 1.30) (6.6 1) (20.43)

II C,sH2oN40 2 Light pink 62.35 6.91 19.51 (169-171) (62.50) (6.94) ( 19.44) III [Co(C t4H t7N40 2)CI 2l Grey green 41.95 4.23 13.94 Diamagnetic (242D) (41.71) (4.25) ( 13.90)

IV [CH 3Co(C14H 17 N40 2)C5H5N]CI Dark green 51.92 5.55 15 .03 0.22 (>300) (51.89) (5.62) (15.13)

v [CH3Co(C,4H11N402)C3H4N2]Cl Snuff 47.71 5.48 18.75 0.72 (246 D) (47.75) (5.54) ( 18.61 )

VI [C2H3Co(C 14H 17 N40 2)C5H5N]CI Light brown 52.98 5.76 14.72 Di amagnetic (295 D) (52.89) (5.88) (14.69)

VII [C2H5Co(C,4Ht1N402)C3H4H2]Cl Brown 48.87 5.9 1 18.10 0.12 (206 D) (48.99) (5.80) (18.04)

VIII [Co(C15H ,~N402)Cl2 Brownish green 43 .12 4.62 13.35 0.48 (2 14-216) (43 .07) (4.79) (13 .40)

IX [CH3Co(C15 H ,~N 4 0 2 )C 5 H 5 N]Cl Dark brown 52.76 5.81 14.76 0.52 (290 D) (52.89) (5.88) (14.69)

X [CH 3Co(C15 H ,9N402)C3H4N2]Cl Black 49.06 5.91 17.96 0.77 (277 D) (48.99) (5.80) (18.04)

XI [C2HsCo(C 1sH ,9N402)C5HsN]Cl Dark brown 53.71 6.15 14.76 Diamagnetic (> 300) (53.83) (6.12) (14.60)

XII [C 2 H 5 Co ( C, 5 H,~N 4 02 )( C 3 H4N 2 ]CI Light brown 49.99 6.15 17.45 Diamagnetic (265 D) (50.66) (6.04) ( 17.52) 1314 INDIAN J CHEM., SEC. A, DECEMBER 2000 solutions were 0.1 M concentration in TBAP. residual amount of paramagnetic susceptibility (Table 1). This may be arised by the distortion of Results and discussion groups by their neighbours due to the existence of The infrared spectra of ligands I and II show no electron energy levels very close to that of ground characteristic absorption bands assignable to either state. C=O (or) NH2 groups indicating the formation of The electronic spectra of the dichloro complexes ligands. Strong bands appearing in the region 1584- are recorded in DMF. These complexes exhibit three 1 6 1576 cm- are attributed to v(C=N) in both ligands • d-d transitions, two in the visible and one in the near The strong as well as sharp bands at 1220 em-I are UV -region, characteristic of tetragonally distorted 7 due to the N-0 stretching vibration. The strong bands octahedral cobalt(III) complexes . The crystal field observed around -775 cm-1 are assigned to C=N-0 model of Wentworth and Piper16 has been employed deformation vibration. Bands characteristic of methyl to interpret the electronic absorption bands. In the groups of diacetylmonoxime are observed in the present study, the D 411 crystal field model has been 1454-1458 [Vasy(C-CH,)] and 1380-1386 cm-1 [Vasy chosen as its spectral characteristics are analogous to (C-CH3)]regions in the IR spectra of both ligands. those of complexes with D 411 microsymmetry. The phenyl ring exhibits several bands in the lower The two bands exhibiting in the visible region, energy region. Thus the bands at 1260, 1130 and 990 15552, 17090 cm- 1 and 21367, 22075 cm-1 are 1 cm- are attributed to the phenyl group. The broad attributable to the transitions 1A 1 .~: -~ 1E/ (vE) and 1 bands at around 2900 cm- are assigned to v(OH) 1 1 A 1K --t A 2 ~: (vA) in the dichloro complexes III and vibrations of the li gands. VIII respectively. The calculated DlY, Dt and Dqz Both ligands are characterized by high resolution based on these assignments are 2517, 2583; 570, 663 1 1 6 (400 MHz) H-NMR spectra in acetone-d6. These and 1357, 1590 cm- respectively. Jackels et al and ligands show high field signals at 2.91 and 3.03 as Tait et aC reportecj...that the field strength (Dq'Y) of doublets corresponding to the methyl protons. The the macrocyclic ligand increases steadily as the extent broad multiplets centered at 7.87 and 7.68 ppm are of unsaturation increases. In contrary to their observed in low field strength regions of spectra due observations, the Dq'Y obtained for the chloro to aromatic protons. The oxime protons give sharp complexes are not much higher than that of 1 singlet at I 0.66 and I 0.68 ppm in the spectra of I and cobalamin (2500 cm- ) and ethylenediamine 1 8 II ligands. A singlet in the spectra of ligand II is (2530 cm- ) complexes • observed at 2.58 corresponds to aromatic methyl In the absorption spectra of organocobalt group present on the phenyl group. complexes the most intense bands in the highest The synthesis of organocobalt complexes involves energy region have been assigned to rr-~rc · transition the oxidative-addition reaction of the in situ of equatorial ligand. The lowest energy band found in generated Co(l) species in solution by reduction of 20000-25000 cm-1 has been assigned to Co-C charge dichloro complex using sodium borohydride. In all transfer transition. This observation is in analogy with the complexes cobalt is found to be in +3 oxidation the spectra of cobaloximes and cobalamins. This band state. The ease with which cobalt(III) complexes are shifts to longer wavelength with increasing electron 8 formed by the ligand is to accommodate the smaller donating ability of alkyl groups and axial bases . cobalt(II) ion more readily than the larger cobalt(II) The Co-C CT bands(nm) are observed at 421 (IV), ion in its central cavity. 439(V), 423(VI), 445(VII), 402(IX), 450(X), All the complexes are stable at room temperature, 41 O(XI) and 413(XII)in the spectra of organocobalt non-hygoscopic, insoluble in water and readily complexes. The numbers of the complexes are given soluble in methanol, ethanol, DMF and DMSO. The in parentheses. It is observed that Co-C CT transitions molar conductivity data show that the dichloro of present alkyl cobalt complexes are comparable complexes are non-electrolytes and the alkyl with different popular in-plane ligands such as derivatives are l: I electrolytes. dimethylglyoxime and diphenyl glyoxime reported by 9 10 The magnetic moment values of dichloro and alkyl Schrauzer et at. and Gupta et a/. • cobalt complexes are found to be diamagnetic. In the IR spectra of the complexes, the broad band However some of the complexes are showing a small observed at 2900 cm-1 in the free ligands shifted to NOTES 1315

1 2400 cm- • This is attributed to the v(OH) of Table 2-Cyclic voltammetric data of chloro and alkyl cobalt hydrogen bond. Strong bands appeared at 1584 (I) complexes (-I o-3 M) in DMF containing TBAP at 1.0 V/S at 1 1 glassy carbon electrode, temp. 26° C and 1576 (II) cm- are shifted (tm 8-12 cm- ) to lower wave number suggesting the participation of Complex Redox £1" (V) Epa(Y ) £112 (V) !::,.£ azomethine nitrogen in coordination. Appearance of couple (mY) on Iy one band at 1220 em-I in the free Ii gands and III III/II -1.00 -0.86 -0.93 140 1 presence of two bands at -1220 and 1075 cm- in the III I -1.23 spectra of complexes account for two non-identical II/I -1.54 1.48 1.56 30 N-0-H linkages, i.e.C=N-0-H and >C=O-N ... H in the IV III/II -0.95 -0.94 -0.95 10 complexes. Bands characteristic of C-CH symmetric 3 II/I -1.28 and asymmetric deformations occur in the 1370-1384 II/I -1.46 and 1433-1451 cm-1 regions respectively. Bands in the 417-453 cm-1 region are assigned to v(Co-N) v 111/11 -0.62 -0.51 -0.57 II 0 II equatorial vibration. -1.34 III/II The organocobalt complexes exhibit absorption VI -0.95 II/I -1.30 bands characteristic of the axially bound bases. The broad v(NH)vibration appearing in free imidazole VII III/II -0.53 ligand is observed as sharp band at 3142 cm- 1 in all II/I -1.25 complexes. The NH in-plane deformation vibration II/I -1.46 occurring at 1540 em_, in the spectra of free VIII III/II -1.00 -0.90 -0.95 100 imidazole is not affected. A sharp band appearing in II/I -1.22 the region 1320-1325 cm- 1 in all imidazole complexes II/I -1.56 suggests the imidazole coordinate the cobalt ion IX II/I -1.30 through unsaturated ring nitrogen. The v(C=C) and III I -1.56 v(C=N) stretching vibrations and the CH deformation vibration of the pyridine are observed in 161 0-1620; X III/II -0.64 -0.56 0.60 80 II/I -1.19 1560-1567 cm-1 and 762-767 cm- 1 regions II/I -1 .38 respectively. The bands observed in 234-276 and 320- 348 cm-1 regions are tentatively assigned to v(Co-N) XI III/II 0.58 (axial base) and v(Co-C) vibrations, respectively". II/I -1.34 Among the axial bases the pyridine produces less XII III/II -0.65 0.59 -0.62 60 significant effect on the Co-N stretching frequency. II/I -1.40 This is due to the greater 'cr' donating ability of pyridine compared to imidazole. Pyridine may Co(ll) --7 Co(l) couple. Complex VIII is reduced at discourage electron donation by equatorial chelate to more negative potential in comparison with complex metal center thereby decreasing equatorial v(Co-N) III. This is due to the presence of an electron bond energy. 12 donating group (-CH3) on the phenylene diamine The cyclic voltammogram data of all the moiety of complex VIII. complexes are given in Table 2. The cyclic The cyclic voltammogram of organocobalt voltammograms of dichloro complexes (III and VIII) complexes exhibit two cathodic waves. All have two cathodic waves corresponding the complexes show reversible one electron reduction at 111 11 11 1 reductions, Co /Co and Co /Co . These two £ 112 in the range -0.57 to -0.95 V which corresponds 1 14 complexes (III and VIII) show a reversible reduction to the Co(III) --7 Co(II) coupleL • • The second wave 11 1 (Co(III) --7 Co(ll)) at £ 112 = -0.93 and -0.95 V corresponds Co /Co reduction couple which is respectively. Both complexes also have quasi­ observed irreversible in all complexes at Epc in the reversible wave at Epc = -1.22 V which is range -1.30 to 1.56 V respectively. The complexes irreversible. The complex III shows reversible one IV, VII, IX and X also shows quasi-irreversible electron reduction at £ 112 = -1 .56 V corresponding to peaks at the E pc in the range -I. I 9 to- I .25 V. 1316 INDIAN J CHEM., SEC. A, DECEMBER 2000

It is noticed that the !1E for the complex IV (I 00 4 Hussai n Reddy K, Radhakrishna Reddy M & Mohana Raju K, Indian J Chem, 38 ( 1999) 299. m V) is much lower than that of the complex V (Ito 5 Schrauzer G N, In : Advances in chemistry series, No I 00, mV). It suggests that the electron transfer process is 15 edited by R F Gould, (American Chemical Society, taking place more rapidly in the complex IV. It may Washington DC) ( 1971)p I. 3 be due to the weak binding of the base to the Co + ion 6 Jackels S C, Farmery K, Barefield E K, Rose N J & Busch 16 in the latter complex • Inspection of the data also DH, lnorg Chem, II ( 1972) 2893. reveals that l.:iE for the complex X derived using 7 Tait A M, Lovecchio F V & Busch D H. lnorg Chem, 16 imidazole is higher than that of the complex XII, ( 1977) 2206. indicating the slow rate of electron transfer for the 8 Davis W J & Smith J, J. chem Soc. A, ( 1971) 317. reduction of Co(III) to Co(II) in the former complex. 9 Schrauzer G N, Lee L P &. Sibert J W, J .Am chem Soc, 9 1 ( 1970) 2997. 10 Gupta B D, Qanugo K, Sing V & Shobini, Indian J Ch em, Acknowledgement 37 (1998) 707. The authors thank the Department of Science and II Tscano P J & Marzilli L G, Prog lnorg CllCin , 31 ( 1984) Technology, Government of India, New Delhi 105. (SP/S I /F-07 /92) for financial support. The authors 12 Christodoulou D. Kanatzidis M G & Conconvanis D. lnorg also thank RSIC, Madras and SIF, Bangalore for Chem, 29 (1990) 191. providing IR and 1H NMR spectra data respectively. 13 Lex a D & Saveant J M, JAm chem Soc, 98 ( 1976) 2652. 14 Elliott C M, Hershanhart E, Finke R G & Smith B L, JAm References chem Soc, I 03 ( 1981 ) 5338. I Choo P L, Mulichak AM, Jones R W, Bacon J W & Pett V 15 Kissinger P T & Heineman W R, Laboratory techniques in B, ln org chim Acta , 171 (1990), 183. electroanalytical chemistry (Dekker, New York) ( 1996) p 2 Gupta B D & Roy S, lnorg chim Acta, 146 ( 1988) 209. 683. 3 Radhakri shna Reddy M, Hussain Reddy K & Mohana Raju , 16 Detacconi N R, Lexa D & Saveant J M, JAm chem Soc, 101 K, Polyhedron, 17 ( 1998) 1355. (1979) 467.