Effect of metal exchange (Os vs. Ru) and co-ligand variation (Cl−vs. acac−) on the oxidation state distribution in complexes of an o-phenylenediamido(2−)/o-quinonediimineredox system

Abstract

The compounds OsCl₂L₂ (1), RuCl₂L₂ (2) and Ru(acac)₂L (3), L° = N-phenyl-o-quinonediimine and acac⁻ = 2,4-pentanedionato, have been synthesised and electrochemically, spectroscopically and structurally characterised with the support of detailed DFT calculations. For the new osmium compound 1, the average values for CNH (1.331 Å), CNPh (1.345 Å) and CC(meta) bond lengths at 1.360 Å agree more with an o-semiquinonediimine than with an o-quinonediimine formulation for the coordinated ligand, whereas the latter assignment is more appropriate to describe the ruthenium analogue 2 (average bond lengths of 1.308, 1.324 and 1.348 Å, respectively). In turn, the corresponding average bond lengths in 3 (1.333, 1.352 and 1.350 Å, respectively) are suggestive of an o-semiquinonediimine ligand. DFT calculations confirm the structural data and describe the electronic structures related to the oxidation state assignments. The stabilisation of trivalent ruthenium by electron donating acac⁻ in 3 = Ru^III(acac)₂(L^•−) and the preference of osmium for higher oxidation states (here: Os^IVCl₂(L^•−)₂ in 1) as compared to ruthenium analogues (here: Ru^IICl₂(L°)₂ in 2) in otherwise similar setting is responsible for these results. Oxidation of the diamagnetic complexes proceeds to partly EPR and UV-VIS spectroelectrochemically detectable cations with metal-centered spin, suggesting the following formulations: [Os^IIICl₂(L°)₂]⁺ for 1⁺, [Ru^IIICl₂(L°)₂]⁺ for labile 2⁺, and [Ru^III(acac)₂(L°)]⁺ for 3⁺. In the case of 3, reversible metal-centered reduction results in ligand-based spin as compatible with a [Ru^II(acac)₂(L^•−)]⁻ situation for 3⁻.