Structure and electrochemical properties of bis(pyridine)cobalt(III) complexes of N,N′-ethylenebis(salicylideneimine) homologues with an aryl substituent on the ethylene backbone. Crystal structures of [Co(salpen)(py)2]PF6·0.5 Et2O and [Co(salcppn)(py)2]PF6·2py (py = pyridine)

Abstract

Bis(pyridine)cobalt(III) complexes of salen homologues [H₂salen =N,N′-ethylenebis(salicylideneimine)] with the substituent groups R¹ and R² on one of the ethylene carbon atoms, [Co(L)(py)₂]PF₆(L = salen homologue with R¹= H or Me, R²= aryl), have been synthesized and structurally characterized. Single-crystal X-ray analyses for [Co(salpen)(py)₂]PF₆[salpen =N,N′-1-phenylethylenebis-(salicylideneiminate)] and [Co(salcppn)(py)₂] PF₆[salcppn =N,N′-2-(p-chlorophenyl)propane-1,2- diylbis(salicylideneiminate)] demonstrate six-co-ordination around the metal with pyridine molecules at the apical sites. The ethylene conformation is gauche for both complexes with the phenyl group in the equatorial orientation in the former and with the p-chlorophenyl group in the axial and the methyl group in the equatorial orientation in the latter. In [Co(salcppn)(py)₂]PF₆ the p-chlorophenyl group and one of the pyridine molecules are nearly parallel to each other so as to diminish the steric repulsion between them. The gauche conformation in solution is also deduced for the complexes in solution based on ¹H n.m.r. spectra. The Co^III–Co^II reduction potentials of the complexes are not affected by mono-substitution on the ethylene carbon but significantly shifted to the positive side by disubstitution (aryl and methyl groups) on the ethylene carbon. The conformations and electrochemical properties of the complexes are discussed in terms of the steric interaction between the substituents (R¹, R²) and the pyridine molecule co-ordinated at the apical site.