Varying electronic structural forms of ruthenium complexes of non-innocent 9,10-phenanthrenequinonoid ligands

Abstract

Bis(acetylacetonato)ruthenium complexes [Ru(acac)₂(Q_1–3)], 1–3, incorporating redox non-innocent 9,10-phenanthrenequinonoid ligands (Q₁ = 9,10-phenanthrenequinone, 1; Q₂ = 9,10-phenanthrenequinonediimine, 2; Q₃ = 9,10-phenanthrenequinonemonoimine, 3) have been characterised electrochemically, spectroscopically and structurally. The four independent molecules in the unit cell of 2 are involved in intermolecular hydrogen bonding and π–π interactions, leading to a 2D network. The oxidation state-sensitive bond distances of the coordinated ligands Qⁿ at 1.296(5)/1.289(5) Å (C–O), 1.315(3)/1.322(4) Å (C–N), and 1.285(3)/1.328(3) Å (C–O/C–N) in 1, 2 and 3, respectively, and the well resolved ¹H NMR resonances within the standard chemical shift range suggest DFT supported variable contributions from valence formulations [Ru^III(acac)₂(Q˙⁻)] (spin-coupled) and [Ru^II(acac)₂(Q⁰)], respectively. Complexes 1–3 exhibit one oxidation and two reduction steps with comproportionation constants K_c ∼ 10⁷–10²² for the intermediates. The electrochemically generated persistent redox states 1ⁿ (n = 0, 1−, 2−) and 2ⁿ/3ⁿ (n = 1+, 0, 1−, 2−) have been analysed by UV-vis-NIR spectroelectrochemistry and by EPR for the paramagnetic intermediates in combination with DFT and TD-DFT calculations, revealing significant differences in the oxidation state distribution at the {Ru–Q} interface for 1ⁿ–3ⁿ. In particular, the diminished propensity of the NH-containing systems for reduction results in the preference for Ru^II(Q⁰) relative to Ru^III(Q˙⁻) (neutral compounds) and for Ru^II(Q˙⁻) over the Ru^III(Q²⁻) alternative in the case of the monoanionic complexes.