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

Bis(acetylacetonato) ruthenium complexes [Ru(acac) 2(Q(1-3))], 1-3, incorporating redox non-innocent 9,10-phenanthrenequinonoid ligands (Q(1) = 9,10-phenanthrenequinone, 1; Q(2) = 9,10-phenanthrenequinonediimine, 2; Q(3) = 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 p-p interactions, leading to a 2D network. The oxidation state-sensitive bond distances of the coordinated ligands Q(n) at 1.296(5)/1.289(5) A (C-O), 1.315(3)/1.322 (4) A (C-N), and 1.285(3)/1.328(3) A (C-O/C-N) in 1, 2 and 3, respectively, and the well resolved H-1 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)(Q(0))], respectively. Complexes 1-3 exhibit one oxidation and two reduction steps with comproportionation constants Kc similar to 10(7)-10(22) for the intermediates. The electrochemically generated persistent redox states 1n (n = 0, 1-, 2-) and 2(n)/3(n) (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)-3(n). In particular, the diminished propensity of the NH-containing systems for reduction results in the preference for RuII(Q(0)) relative to RuIII(Q(.-)) (neutral compounds) and for RuII(Q(.-)) over the RuIII(Q(2-)) alternative in the case of the monoanionic complexes.