Reactivity of nickel (II) diphosphine complexes towards alkoxides: a new route to the synthesis of nickel(0) compounds through nickel(II) alkoxides

Reaction of [Ni(L-L)Cl2] [L-L = Ph2P(CH2)nPPh2, n = 2 (dppe) or 3 (dppp)] with NaOR (R = Me, Et or Pr(i)) under a dinitrogen atmosphere afforded [Ni(L-L)2], Ni(OR)2 and aldehyde (or acetone when R = Pr(i)) in 1 : 1 : 1 ratio, showing the peculiar reducing effect of alkoxide promoted by the chelating property of the phosphorus ligand. The reaction of NaOMe with [Ni(dcpe)Cl2] [dcpe = 1,2-bis(dicyclohexylphosphino)ethane] afforded mainly [Ni2(dcpe)3] when carried out in the absence of free diphosphine, and [Ni(dcpe)2] in the presence of free diphosphine. The reaction always yields the nickel(o) species [Ni(L-L)(CO)2] when it is carried out under a carbon monoxide atmosphere. The intermediate formation of unstable alkoxo(diphosphine)nickel(II) complexes has been demonstrated by recording the P-31 NMR spectra of the reacting solutions at low temperature; in the case of the reaction of [Ni(dcpe)Cl2] with NaOMe only, it was possible to isolate as a solid the [Ni(dcpe)(OMe)2] Complex, which has been fully characterized by analytical and spectroscopic (IR and H-1, P-31 NMR) methods. A possible route by which nickel(o) complexes could be formed is discussed.