Bis-chelated-arylazoimidazole-1,10-phenanthroline–osmium(II) complexes. Structure, spectra and electrochemistry

[Os(phen)(RaaiR′)2](PF6)2 [phen = 1,10-phenanthroline, RaaiR′ = 1-alkyl-2-(arylazo)imidazole, p-R–C6H4–Ndouble bond; length as m-dashN–C3H2–NN–1-R′, where R = H (a), Me (b), Cl (c) and R′ = Me (2), Et (3), CH2Ph (4) have been synthesized from the reaction of cis–trans–cis-[OsBr2(RaaiR′)2] with phen in the presence of aqueous AgNO3 in ethanol. The structure of [Os(phen)(ClaaiEt)2](PF6)2 was confirmed by X-ray diffraction study. Electronic spectra exhibit a strong MLCT band at 490–512 nm along with weak transition at longer wavelength 865–880 nm. Cyclic voltammetry of the complexes shows two metal redox couples, Os(III)/Os(II) at 0.9–1.0 V and Os(IV)/Os(III) at 1.4–1.6 V versus SCE, and three successive ligand reductions. The EHMO calculation using crystallographic parameters of the complex has been compared with analogous Ru and Os complexes. A correlation between electronic properties and MO results is also reported.