Evidence for Bidirectional Noninnocent Behavior of a Formazanate Ligand in Ruthenium Complexes

Redox series of the complexes [Ru­(L)­(L′)₂]ⁿ, L = 1,5-diphenyl-3-(4-tolyl)-formazanate and L′ = 2,4-pentanedionate (acac^–), 2,2′-bipyridine (bpy), or 2-phenylazopyridine (pap), were studied by cyclic and differential pulse voltammetry and by TD-DFT-supported spectroelectrochemistry (UV–vis–NIR, EPR). The precursors [Ru^III(L^–)­(acac^–)₂], [Ru^II(L^–)­(bpy)₂]­ClO₄, and [Ru^II(L^–)­(pap)₂]­ClO₄ were identified in their indicated oxidation states by X-ray crystal structure determination. The six-membered formazanato-ruthenium chelate rings have an envelope conformation with puckering of the metal. DFT calculations indicate a pronounced sensitivity of the N–N bond lengths toward the ligand oxidation state. Several electrochemically accessible charge states were analyzed, and the derived oxidation numbers Ru^II, Ru^III, or Ru^IV, L′ or (L′)^•–, and L^–, L^•2–, or the new formazanyl ligand L^• for the two-way noninnocent formazanate reflect the increasing acceptor effect of the ancillary ligands L′ in the series acac^– < bpy < pap.