Phosphido-bis(borane) complexes of the alkaline earth metals

Abstract

The reaction between two equivalents of {(Me₃Si)₂CH}(Ph)PH(BH₃) (1) and Bu₂Mg, followed by two equivalents of BH₃·SMe₂, gives the corresponding phosphido-bis(borane) complex, which may be crystallised as two distinct chemical species: the complex [{(Me₃Si)₂CH}(Ph)P(BH₃)₂]₂Mg(THF)₄·THF (2a), and two different THF solvates (1 : 1 and 1 : 2) of the solvent-separated ion triples [{(Me₃Si)₂CH}(Ph)P(BH₃)₂]₂[Mg(THF)₆]·THF (2b) and [{(Me₃Si)₂CH}(Ph)P(BH₃)₂]₂[Mg(THF)₆]·2THF (2c). Similar reactions between two equivalents of 1 and either (4-tBuC₆H₄CH₂)₂Ca(THF)₄ or [(Me₃Si)₂CH]₂Sr(THF)₂, followed by two equivalents of BH₃·SMe₂, give the heavier alkali metal complexes [{(Me₃Si)₂CH}(Ph)P(BH₃)₂]₂M(THF)₄ [M = Ca (3), Sr (4)]. Surprisingly, compounds 2a, 3 and 4 adopt almost identical structures in the solid state, which differ only in the geometrical arrangement of the phosphido-bis(borane) ligands and the hapticity of the borane groups.