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 [RuIII(acac) 2(Q-)] (spin-coupled) and [RuII(acac) 2(Q0)], respectively. Complexes 1-3 exhibit one oxidation and two reduction steps with comproportionation constants Kc ∼ 107-1022 for the intermediates. The electrochemically generated persistent redox states 1n (n = 0, 1-, 2-) and 2 n/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 1 n-3n. In particular, the diminished propensity of the NH-containing systems for reduction results in the preference for Ru II(Q0) relative to RuIII(Q-) (neutral compounds) and for RuII(Q-) over the Ru III(Q2-) alternative in the case of the monoanionic complexes. © 2014 The Royal Society of Chemistry.