A new trinuclear mixed-valence Co(II)–Co(III) complex stabilized by a bis(salicylidene) based ligand

The present work deals with a class I mixed valence trinuclear complex [CoII(μ-OAc)2(μ-L)2Co2III(NCS)2] (1) formed by partial aerial oxidation (H2L = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene). The terminal CoIIIN3O3 centres exhibit a uniform facial arrangement of both N3 and O3 donor sets and the CoII centre is coordinated to six oxygen atoms. Variable temperature magnetic susceptibility measurement reveals that the intermediate Co(II) paramagnetic centre undergoes a very weak spin exchange (θ = −6.39 K, antiferromagnetic) interaction with the two terminal Co(III) diamagnetic centres over the range 300–2 K. A three line EPR spectra at 4 K was characterized with highly anisotropic g tensors and the one line broad spectra at 300 K indicates the strong influence of temperature on the metal spin states, consistent with a high spin Co(II) complex. Two anodic responses appeared at +0.85 (Co2+ → Co3+, reversible) and +1.7 V (Co3+ → Co4+, irreversible) corresponding to one and two electron transfer oxidation respectively. Another reversible signal attributed to Co3+ → Co2+ reduction occurred at −0.5 V, associated with a two electron stoichiometry. Compound 1 reacted with hydrazine to form a square planar orange-red [CoII(L)] species, 2. Inter conversion between 1 and 2 involves electron exchange coupled with proton transfer.

Reaction of Co(OAc)2 with H2L and NH4SCN in air afforded an octahedral mixed valent trimeric complex, 1 which was reacted with hydrazine resulting in a mononuclear Co(II) square planar complex, 2. Compound 2 regenerates 1 on treatment with suitable reagents. 1 displayed two oxidative and one reductive response in MeCN and the electron equivalents associated with the redox couples were ascertained from coulometry and DPV. A weak antiferromagnetic interaction (=−6.39 K) is operative in 1 at low temperature. An anisotropic EPR spectrum of 1, recorded at 4 K, suggests transitions between the ground state Kramers doublet. Synthetic strategies for 1 and other known related trimers have also been compared.Figure optionsDownload as PowerPoint slide