Voltammetry of half-sandwich manganese group complexes of η6-PhC3B7H9 and η5-C60Bn2PhH2, two ligands that are cyclopentadienyl mimicks

The potentials of a series of one-electron oxidation and reduction reactions have been determined for manganese group half-sandwich complexes of the tricarbadecaboranyl ligand PhC3B7H9 and the penta-organo fullerene ligand C60Bn2PhH2 (Bn = benzyl). The anodic processes were studied in CH2Cl2 and the cathodic processes were studied in both CH2Cl2 and THF, the supporting electrolyte being [NBu4][B(C6F5)4]. The manganese complex Mn(CO)2(PMe3)(PhC3B7H9) (1) is a member of a three-electron transfer series which includes oxidation to 1+ (0.51 V versus ferrocene) and successive reductions to 1− (−1.66 V) and 12− (−1.77 V). Both the oxidation and reduction of the closely-related complex Mn(CO)2(PPh3)(PhC3B7H9) (2) are chemically irreversible under slow-scan cyclic voltammetry conditions. The rhenium complex Re(CO)2(PPh3)(PhC3B7H9) (3) oxidizes (E1/2 = 0.82 V versus ferrocene) to a radical cation which, unlike its cyclopentadienyl analogue, shows no evidence of dimerization. Oxidation of the fullerene-based complex Re(CO)3(C60Bn2PhH2) is more facile than that of its cyclopentadienyl analogue, in contrast to previous findings in this class of metal-fullerene derivatives. An electrochemical ligand factor, EL, of 0.63 is calculated for the PhC3B7H9 ligand in manganese group half-sandwich complexes.

The E1/2 potentials of both the oxidation and reduction of manganese and rhenium half-sandwich complexes have been measured in CH2Cl2 and THF containing [NBu4][B(C6F5)4]. Three tricarbadecaboranyl compounds and one penta-organo fullerene compound were studied. The one-electron oxidation of the Re compounds give radical cations that do not dimerize on the voltammetry time scale. A ligand electronic parameter of 0.63 has been determined for the PhC3B7H9 ligand in these complexes, 0.30 V positive of that of the cyclopentadienyl ligand.Figure optionsDownload as PowerPoint slide