Synthesis and electrochemistry of 1,1′-bis(phosphino)cobaltocenium compounds

The electrochemistry of 1,1′-bis(diphenylphosphino)cobaltocenium hexafluorophosphate ([dppc][PF6]), 1,1′-bis(dicyclohexylphosphino)cobaltocenium hexafluorophosphate ([dcpc][PF6]), 1,1′-bis(di-iso-propylphosphino)cobaltocenium hexafluorophosphate ([dippc][PF6]), and 1-(di-tert-butylphosphino)cobaltocenium hexafluorophosphate ([1-dtbpc][PF6]) was examined in methylene chloride with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. A reversible reductive wave followed by an irreversible wave at more negative potentials was observed. Ten new phosphinothioyl ([dppcS2][PF6], [dcpcS2][PF6], [dippcS2][PF6], [1-dtbpcS][PF6], and 1,1′-bis(dicyclohexylphosphinothioyl)ferrocene) and phosphinoselenoyl derivatives ([dppcSe2][PF6], [dcpcSe2][PF6], [dippcSe2][PF6], [1-dtbpcSe][PF6], and 1,1′-bis(dicyclohexylphosphinoselenoyl)ferrocene) were prepared and characterized, and the structures of eight of these compounds were determined. The electrochemistry of these phosphinochalcogenyl cobaltocenium compounds, as well as the previously prepared [dppcO2][PF6], displayed two reversible reductive waves at potentials less negative than that of the free phosphines. A correlation was found to exist between the Hammett substituent constant σp and the reduction potentials of these compounds. In addition, the phosphinoselenoyl [dppcSe2][PF6], [dcpcSe2][PF6], and [dippcSe2][PF6] displayed an electrochemically irreversible oxidative wave, potentially indicating an intramolecular Se–Se bonded trication. The electrochemistry of three new and five previously reported transition metal complexes of the general formula [MnCl2(P∩P)][PF6] (M = Pd or Pt, n = 1, P∩P = dppc, dcpc or dippc; M = Au, n = 2, P∩P = dppc or dcpc)) was also examined displaying at least two reductive waves at potentials less negative than that of the free phosphines. Comparison of the electrochemical data with that previously obtained for analogous ferrocenes indicates that a correlation exists between the reduction potentials of the cobaltocenium phosphines and the potentials at which oxidation of the ferrocene phosphines occurs. In addition, the structure of [Au2Cl2(dppc)][PF6] was determined.