Electrochemical behaviour of ditertiary phosphine and diphosphazane ligand-bridged derivatives of di-iron and diruthenium nonacarbonyl

Abstract

Cyclic voltammetric studies in benzonitrile, dichloromethane and acetone show that the oxidation of the di-iron derivatives [Fe 2(μ-CO)(CO) 4(μ-R 2PYPR 2) 2] (Y = CH 2, R = Me or Ph; Y = NEt, R = OMe, OEt, O iPr or OPh) generally proceeds via an EEC mechanism, the only exception being the oxidation of the Y = CH 2, R = Ph derivative in acetone, which proceeds via an EE mechanism. The chemical step in the EEC mechanism involves solvent attack at an iron atom with formation of a dicationic solvento species of the type [Fe 2(CO) 5(solvent)(μ-R 2PYPR 2) 2] 2+. The electrochemical oxidation of the diruthenium tetramethoxydiphosphazane ligand-bridged derivative [Ru 2(μ-CO)(CO) 4{μ-(MeO) 2PN(Et)-P(OMe) 2} 2] only proceeds via an EEC mechanism in the very weakly coordinating solvent dichloromethane; in benzonitrile and acetone oxidation is via an ECE mechanism for which the potential required to remove the second electron is lower than that for the removal of the first electron giving rise to an overall 2e-transfer reaction. Again the end-product of the oxidation process is a dicationic solvento species. Electrochemical oxidation in all three solvents of the diruthenium tetraisopropoxydiphosphazane ligand-bridged derivative [Ru 2(μ-CO)(CO) 4{μ-( iPrO) 2PN(Et)P(O iPr) 2} 2] is proposed to proceed via an ECEC mechanism for which the first chemical step involves a structural rearrangement and the second solvent attack at a ruthenium atom to form the dicationic solvento species. Significantly, the separation between the potentials required to remove the first and second electrons is small, i.e., < 0.5 V. Two pathways are utilized in the electrochemical oxidation of the mixed-ligand complex [Ru 2(μ-CO)(CO) 4{μ-(MeO) 2PN(Et)P(OMe) 2}{μ-( iPrO) 2PN(Et)P(O iPr) 2}], their nature being dependent on the choise of solvent. The ECE mechanism is adopted in all three solvents benzonitrile, acetone and dichloromethane; however, in the first solvent the second pathway is the EEC process whereas the second pathway adopted in acetone and dichloromethane is the ECEC process. Thus, the overall mechanism proposed for the electrochemical oxidation of the above derivatives of [Fe 2(CO) 9] and [Ru 2(CO) 9] allows for three pathways to a dicationic solvento species, the pathway adopted being dependent on the metal, the bridging ligand, in particular on its size, and on the coordinating ability of the solvent.