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 π-accepting pap ligand and the σ-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⋅)(L−)]+. Calculations of electrogenerated complex [RuII(pap.−)(pap0)(L−)] displayed considerable negative spin density (−0.188) at the bridging metal.