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 [RuIII(L)­(acac)2], [RuII(L)­(bpy)2]­ClO4, and [RuII(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 RuII, RuIII, or RuIV, 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.