Inorganic Chemistry, Vol.43, No.4, 1530-1538, 2004
Metal vs ligand reduction in complexes of dipyrido[3,2-a : 2',3'-c]phenazine and related ligands with [(C5Me5)CIM](+) (M = Rh or Ir): Evidence for potential rather than orbital control in the reductive cleavage of the metal-chloride bond
Complexes between the chlorometal(III) cations [(C5Me5)CIM](+), M = Rh or Ir, and the 1,10-phenanthroline-derived alpha-diimine (Nboolean ANDN) ligands dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4,7,10-tetraazaphenanthrene (tap), or 1,10-phenanthroline-5,6-dione (pdo) were investigated by cyclic voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry with respect to either ligand-based or metal-centered (and then chloride-dissociative) reduction. Two low-lying unoccupied molecular orbitals (MOs) are present in each of these three Nboolean ANDN ligands; however, their different energies and interface properties are responsible for different results. Metal-centered chloride-releasing reduction was observed for complexes of the DNA-intercalation ligands dppz and tap to yield compounds [(Nboolean ANDN)(C5Me5)M] in a two-electron step. The separation of alpha-diimine centered optical orbitals and phenazine-based redox orbitals is apparent from the EPR and UV-vis-NIR spectroelectrochemistry of [(dppz)(C5Me5)M](0/.-/2-). In contrast, the pdo complexes undergo a reversible one-electron reduction to yield o-semiquinone radical complexes [(pdo)(C5Me5)CIM](.) before releasing the chloride after the second electron uptake. The fact that the dppz complexes undergo a Cl--dissociative two-electron reduction despite the presence of a lowest lying pi* MO (b(1)(phz)) with very little overlap to the metal suggests that an unoccupied metal/chloride-based orbital is lower in energy. This assertion is confirmed both by the half-wave reduction potentials of the ligands (tap, -1.95 V; dppz, -1.60 V; pdo, -0.85 V) and by the typical reduction peak potentials of the complexes [(L)(C5Me5)CIM](PF6) (tap, -1.1 V; dppz, -1.3 V; pdo, -0.6 V; all values against Fc(+/0)).