Synthesis, characterization and spectrochemistry, electrochemistry and spectroelectrochemistry of bromo (Lewis base) cobalt(III) complexes of 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime

Abstract

A series of six coordinate cobalt(III) complexes of the type [LCo((DO)(DOH)bzo)Br] + (for abbreviations see V. Alexander and V. V. Ramanujam, Inorg. Chim. Acta 156 (1989) 125), where L=4-methyl-pyridine, 1-methylimidazole, benzimidazole, 5,6-dimethylbenzimidazole, 2-aminopyridine, triphenylphosphine, ethylamine, diethylamine and triethylamine, have been synthesized with the tetraaza quadridentate chelate 3,8-dimethyl-5,6-benzo4,7-diazadeca-3,7-diene-2,9-dione dioxime. The quadridentate ligand coordinates with cobalt in the equatorial position as a monoanion with the concomitant formation of an intramolecular hydrogen bond. The Lewis base and the bromide coordinate in the axial sites of cobalt. The synthesis of these complexes has been accomplished by making use of the lability of the axial sites of the dibromo complex, [Co((DO)(DOH)bzo)Br 2]. These complexes have been characterized by C, H, N and Co analysis, conductivity measurements, magnetic susceptibility and IR, 1H NMR and UVVis spectroscopy. Indirect evidence for the presence of a hydrogen bond in these complexes has been obtained by replacing the H-bonded proton by the BF 2 group. All these complexes exhibit three d-d transitions, two in the visible region and one in the near UV region, assignable to 1A 1g → 1E a g, 1A lg → 1A 2g and 1A lg → ( 1E b g + 1B 2g), characteristic of tetragonally distorted octahedral cobalt(III) complexes and have been assigned a D4a microsymmetry in analogy with the crystal field model of Wentworth and Piper. The ligand field parameters such as Dq xy , Dq z and D t have been computed and the field strength of the in-plane ligand is higher than that of the axial ligand. The electronic absorption spectra of low spin cobalt(III) in pseudo O h symmetry and in a D 4 h microsymmetry when Dq xy > Dq z , Dq z > Dq xy and Dq xy ⋍ Dq z are explained with the aid of a qualitative energy level diagram. The observed spectrochemical series of the Lewis bases for Co 3+ is Iz < py < 4-CH 3py < Et 2NH < MeIz < EtNH 2 < Et 3N < BIz < PPh 3. They exhibit two polarographic reduction waves corresponding to the Co(III) → Co(II) and Co(Il) → Co(I) reductions. The cyclic voltammograms in DMF at a GC disk electrode consist of two cathodic waves corresponding to the reductions, Co(III) → Co(II) and Co(Il) → Co(I) and two anodic waves corresponding to the oxidations, Co(I) → Co(II) and Co(II) → Co(III). Both Co(III)/Co(II) and Co(II)/Co(I) redox couples are irreversible as indicated by the △E p and i pa/ i pc values. △E p for the former couple is much higher than that of the latter indicating a slow rate of electron transfer for the reduction of Co(III) to Co(II) due to the strong binding of the Lewis base to the Co 3+ ion. The i pa/ i pc for the Co(II)/Co(I) couple is 0.5 indicating a 50% Co(I) yield. This suggests the involvement of a coupled chemical reaction such as an ECE mechanism. The mechanisms of the electrode processes are explained in the light of recent electrochemical investigations on similar Co(III) complexes. The generation of five coordinate [LCo II-((DO)(DOH)bzo)] + during the reduction of the Co(III) complex and the generation of the four coordinate Co(I) complex, [Co I((DO)(DOH)bzo)], during the reduction of the Co(II) complex are envisioned. The cyclic voltammetric E 1/2 value for the Co(III)/Co(II) redox couple depends on the spectrally determined field strength of the axial ligand in a predictable manner. The plot of Dq z versus E 1/2 is linear with a slope of 0.56 ± 0.02 cm −1 mV −1. The Co(II)/Co(I) E 1/2 is independent of the axial ligand and reflects the nature of the in-plane ligand. The dependence of the Co(m)/Co(II) E 1/2 value on the axial ligand reflects the transfer of an electron during the reduction of the Co(III) complex to the d z2 orbital which is sigma antibonding with respect to both axial and in-plane ligands while the independence of the Co(II)/Co(I) E 1/2 value on the axial ligand reflects the transfer of an electron during the reduction of the five coordinate Co(II) complex to the d x 2− y 2 orbital which is sigma anti-bonding with respect to the equatorial ligand only.