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

Abstract

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 [Os^II(pap⁰)₂(Q0)]²⁺, [Os^II(pap⁰)₂(Q^·-)]⁺, [Os^IV(pap^·-)₂(Q^2-)], and [Os^II(pap^·-)(pap⁰)(Q^2-)]^-, with comproportionation constants K_c ≈ 10^3.5, 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.