Synthesis, crystal structure and comparative electrochemistry of metallocenyldiphenylphosphines of ruthenocene, osmocene, ferrocene and cobaltocenium hexafluorophosphate

The metallocenyldiphenylphosphines [M(C5H5)(C5H4PPh2)] with M = Fe(II) (ferrocenyl = Fc), 1, Ru(II) (ruthenocenyl = Rc), 2, Os(II) (osmocenyl = Oc), 3, and Co(III)+PF6− (cobaltocenium hexafluorophosphate = [Cc][PF6]), 4, were synthesized and the crystal structure of RcPPh2, 2, (Z = 4, monoclinic, space group P21/c) was determined. The differences in reactivity of each metallocenyl derivative were such that 1 could be obtained from a Friedel Crafts reaction between ferrocene and PPh2Cl in the presence of AlCl3 as catalyst. Both the ruthenocene and osmocene derivatives 2 and 3 were obtained by reacting the monolithiated metallocene precursor with PPh2Cl. However, monolithiation of ruthenocene had to be achieved via a stoichiometric amount of tBuLi. For osmocene, monolithiation was achieved by a 20% excess of nBuLi. This was evidenced by the failure to isolate any bisphosphine, Oc(PPh2)2, during workup. Complex 4 could not be obtained via phosphination of free cobaltocenium hexafluorophosphate. Phosphine derivatisation of free cyclopentadiene prior to complexation with CoIII was required to form [CcPPh2][PF6], 4. The electrochemistry of all phosphines was studied by voltammetric techniques in CH2Cl2/0.1 mol dm−3 [N(nBu)4][B(C6F5)4]. A reversible one-electron transfer process for the ferrocenyl group of 1 was observed at 0.078 V vs. FcH/FcH+. The osmocenyl and ruthenocenyl derivatives exhibited irreversible metallocenyl oxidations at 0.355 and 0.476 V respectively. The cobaltocenium complex, 4, exhibited two reversible one-electron transfer reductions to liberate first a neutral CoII cobaltocene species at −1.062 V and then an anionic CoI cobaltocene species at −2.122 V. A single electrochemical irreversible, one-electron oxidation at the phosphorus centre which forms a quickly-decomposing phosphorus radical cation, Mc+Ph2Pradical dot+, was also observed at Epa > 0.754 V. The newly-formed Mc+Ph2Pradical dot+ species or its chemical decomposition products can be oxidized at Epa > 1.090 V vs. FcH/FcH+.