Electron transfer and chemical reactions associated with the oxidation of an extensive series of mononuclear complexes [M(CO)2(κ1-P-P)(κ2-P-P)X] and binuclear complexes [{M(CO)2(κ2-P-P)X}2(μ-P-P)] (M = Mn, Re; P-P = diphosphine or related ligand; X = Cl, Br)

The electrochemical oxidation of an extensive series of cis, mer-M(CO)(2)(kappa(1)-dpm)(kappa(2)-P-P)X (M = Mn, Re; dpm = Ph2PCH2PPh2; P-P = dpm, Ph2PCH2CH2PPh2 (dpe), o-(Ph2P)(2)C6H4-(dpbz); X = Cl, Br) complexes has been investigated on both the voltammetric and bulk electrolysis time scales. At short time domains or low temperatures, the manganese complexes undergo a reversible one-electron oxidation to cis,mer-[Mn(CO)(2)(kappa(1)-dpM)(kappa(2)-P-P)X](+). These compounds isomerize under slow scan rate voltammetric conditions at room temperature to give trans-[Mn(CO)(2)(kappa(1)-dpm)(kappa(2)-P-P)X](+), which on even longer bulk electrolysis time scales slowly lose X- to form a reactive trans-[Mn(CO)(2)(kappa(2)-dpm)(kappa(2)-P-P)](2+) intermediate. In turn, this complex is reduced either at the electrode surface (X = Cl) or in a homogeneous chemical reaction (X = Br) to form trans-[Mn(CO)(2)(kappa(2)-dpm)(kappa(2)-P-P)](+), which is the final product observed under conditions of bulk electrolysis. In contrast, the first oxidation step for the corresponding rhenium complexes involves oxidation of the pendant phosphorus atom and reaction with traces of water to give cis, mer-Re(CO)(2)(kappa(1)-dpmO)(kappa(2)-dpm)X and then cis-[Re(CO)(2)(kappa(2)-dpmO)(2)](+), with the rhenium center subsequently being oxidized at more positive potentials. Methylation of the pendant phosphorus of cis,mer-Re(CO)(2)(kappa(1)-dpm)(kappa(2)-dpm)X gives cis, mer- [Re(CO)(2)(kappa(1)-dpmMe)(kappa(2)-dpm)X](+). The ligand-based oxidation pathway is blocked by the methylation, so that only the usual metal-centered Re(I)/Re(II)-based oxidation processes are observed in the voltammetry of these compounds. The compounds cis, mer-Re(CO)(2)(kappa(1)-ape)(kappa(2)-ape)X and cis,mer-[Re(CO)(2)(kappa(1)-apeMe)(kappa(2)-ape)X](+) (ape = Ph2AsCH2CH2PPh2) exhibit electrochemistry similar to that of the dpm analogues. The major products of the interaction of dpe with the metal pentacarbonyl halides (2: 1) are the new binuclear species {M(CO)(2)(kappa(2)-dpe)X}(2)(mu-dpe), whose structures have been determined by IR and P-31 NMR spectroscopy and electrospray mass spectrometry (ESMS). The stereochemistry of the manganese complexes is cis,fac, and upon voltammetric oxidation, first one and then the second manganese atom are oxidized. Bulk oxidative electrolysis at low temperature leads to the generation of cis,fac-[{Mn(CO)(2)(kappa(2)-dpe)X}(2)(mu-dpe)](+) and cis,fac-[{Mn(CO)(2)(kappa(2)-dpe)X}(2)(mu-dpe)](2+). The stereochemistry of the oxidized metal centers isomerizes to trans(+) at room temperature, but the binuclear structure is retained. Bulk reductive electrolysis at low temperature after bulk oxidative electrolysis at room temperature leads to the characterization of trans-{Mn(CO)(2)(kappa(2)-dpe)X}(2)(mu-dpe). The binuclear rhenium complexes cis,mer-{Re(CO)(2)(kappa(2)-P-P)X}(2)(mu-P-P) (P-P = dpe, Ph2P(CH2)(3)PPh2 (dpp)) were isolated, and they each undergo two successive one-electron oxidations to generate the mono- and dications without any isomerization on the voltammetric time scale. ESMS provides independent proof of the binuclear nature of the oxidized species.