Oxidation state analysis of a four-component redox series [Os(pap)2(Q)]n involving two different non-innocent ligands on a redox-active transition metal.

Complexes [Os(pap)(2)(Q)] (1-4) have been obtained and structurally characterized for pap = 2-phenylazopyridine and Q = 4,6-di-tert-butyl-N-aryl-o-iminobenzoquinone (aryl = phenyl (1), 3,5-dichlorophenyl (2), 3,5-dimethoxyphenyl (3), or 3,5-di-tert-butylphenyl (4)). The oxidized form (3)(ClO(4))(2) was also crystallographically characterized while the odd-electron intermediates Os(pap)(2)(Q) (1(+)-4(+)) and Os(pap)(2)(Q) (2(-)) were investigated by electron paramagnetic resonance (EPR) and UV-vis-NIR spectroelectrochemistry in conjunction with density functional theory (DFT) spin density and time-dependent DFT (TD-DFT) calculations. The results from the structural, spectroscopic, and electrochemical experiments and from the computational studies allow for the assignments Os(II)(pap(0))(2)(Q(0)), Os(II)(pap(0))(2)(Q(•-)), [Os(IV)(pap(•-))(2)(Q(2-))], and Os(II)(pap(•-))(pap(0))(Q(2-)), with comproportionation constants K(c) ≈ 10(3.5), 10(10), 10(18), and 10(5), respectively. The redox potentials and the comproportionation constants exhibit similarities and differences between Ru and Os analogues. While the Q-based redox reactions show identical potentials, the more metal-involving processes exhibit cathodic shifts for the osmium systems, leading to distinctly different comproportionation constants for some intermediates, especially to a stabilization of the neutral osmium compounds described in this article. The example Os(pap)(2)(Q) illustrates especially the power of combined structural and EPR analysis with support from DFT towards the valence state description of transition metal complexes incorporating redox non-innocent ligands.