Synthesis and structural characterisation of group 10 metal(ii) gallyl complexes: analogies with platinum diboration catalysts?

Abstract

Reactions of the anionic gallium(I) heterocycle, [:Ga{[N(Ar)C(H)]₂}]⁻ (Ar = C₆H₃Prⁱ₂-2,6), with a variety of mono- and bidentate phosphine, tmeda and 1,5-cyclooctadiene (COD) complexes of group 10 metal dichlorides are reported. In most cases, salt elimination occurs, affording either mono(gallyl) complexes, trans-[MCl{Ga{[N(Ar)C(H)]₂}}(PEt₃)₂] (M = Ni or Pd) and cis-[PtCl{Ga{[N(Ar)C(H)]₂}}(L)] (L = R₂PCH₂CH₂PR₂, R = Ph (dppe) or cyclohexyl (dcpe)), or bis(gallyl) complexes, trans-[M{Ga{[N(Ar)C(H)]₂}}₂(PEt₃)₂] (M = Ni, Pd or Pt), cis-[Pt{Ga{[N(Ar)C(H)]₂}}₂(PEt₃)₂], cis-[M{Ga{[N(Ar)C(H)]₂}}₂(L)] (M = Ni, Pd or Pt; L = dppe, Ph₂CH₂PPh₂ (dppm), tmeda or COD). The crystallographic and spectroscopic data for the complexes show that the trans-influence of the gallium(I) heterocycle lies in the series, B(OR)₂ > H⁻ > PR₃ ∼ [:Ga{[N(Ar)C(H)]₂}]⁻ > Cl⁻. Comparisons between the reactivity of one complex, [Pt{Ga{[N(Ar)C(H)]₂}}₂(dppe)], with that of closely related platinum bis(boryl) complexes indicate that the gallyl complex is not effective for the catalytic or stoichiometric gallylation of alkenes or alkynes. The phosphaalkyne, Bu^tCP, does, however, insert into one gallyl ligand of the complex, leading to the novel, crystallographically characterised P,N-gallyl complex, [Pt{Ga{[N(Ar)C(H)]₂}}{Ga{PC(Bu^t)C(H)[N(Ar)]C(H)N(Ar)}}(dppe)]. An investigation into the mechanism of this insertion reaction has been undertaken.