Experimental and DFT Evidence for the Fractional Non-Innocence of a β-Diketonate Ligand

New compounds [Ru(pap)2(L)](ClO4), [Ru(pap)(L)2], and [Ru(acac)2(L)] (pap=2-phenylazopyridine, L-=9-oxidophenalenone, acac-=2,4-pentanedionate) have been prepared and studied regarding their electron-transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)2(L)](ClO4) and [Ru(acac)2(L)] were characterized by crystal-structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the RuIII/II couple, which was in agreement with the very different characteristics of the strongly p-accepting pap ligand and the s-donating acac- ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L- into L. could be deduced from the near-IR absorption of [RuIII(pap)(L.)(L-)]2+. Other intense long-wavelength transitions, including LMCT (L-?RuIII) and LL/CT (pap.-?L-) processes, were confirmed by TD-DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L-radical-involving forms, that is, [RuIII(pap0)2(L-)]2+?[RuII(pap0)2(L.)]2+ and [RuIII(pap0)(L-)2]+?[RuII(pap0)(L center dot)(L-)]+. Calculations of electrogenerated complex [RuII(pap.-)(pap0)(L-)] displayed considerable negative spin density (-0.188) at the bridging metal.