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

Abstract

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.