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.