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)(ClO4)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 [OsII(pap0)2(Q0)]2+, [OsII(pap0)2(Q•–)]+, [OsIV(pap•–)2(Q2–)], and [OsII(pap•–)(pap0)(Q2–)]−, with comproportionation constants Kc ≈ 103.5, 1010, 1018, and 105, 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)]n 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.