Phosphido-Borane and Phosphido-Bis(Borane) Complexes of the Alkali Metals, a Comparative Study

Treatment of the secondary phosphine {(Me3Si2CH}(Ph)PH with BH3 center dot SMe2 yields the phosphineborane {(Me3Si2)(2)CH}(Ph)PH(BH3) (12) as colorless crystals. The reactions between 12 and n-BuLi, PhCH2Na or PhCH2K yield the corresponding phosphido-borane complexes [[{(Me3Si)(2)CH}(Ph)P(BH3)]Li(THF)(2)]infinity (13), [{(Me3Si)(2)CH}(Ph)P(BH3)][Na(12-crown-4)(2)] (14), or [[{(Me3Si)(2)CH}-(Ph)P(BH3)]K(pmdeta)](2) (15), respectively, after crystallization in the presence of the appropriate co-ligand. While 13 crystallizes as a chain polymer, 14 crystallizes as a separated ion pair and 15 as a dimer. In both 13 and 15 the phosphido-borane ligands bind the alkali metal cations via both P-M and B-H center dot center dot center dot M contacts. Unexpectedly, NMR spectroscopy suggests that the separated ion pair 14 undergoes rapid inversion at phosphorus, while 13 and 15 do not. The phosphido-bis(borane) complexes [{(Me3Si)(2)CH}(Ph)P(BH3)(2)]Li(12-crown-4) (16b), [[{(Me3Si)(2)CH}(Ph)P(BH3)(2)]Na(THF)(2)](2) (17), and [[{(Me3Si)(2)CH}-(Ph)P(BH3)(2)]K(THF)(0.5)]infinity (18a) were prepared by treatment of the corresponding in situ-generated phosphido-borane complexes with BH3 center dot SMe2 and crystallization in the presence of the appropriate co-ligand. Compound 16b crystallizes as a monomer, while 17 crystallizes as a dimer and 18a crystallizes as a ribbon polymer. These crystallographic studies reveal entirely new binding modes for both phosphido-borane and phosphido-bis(borane) ligands and allow a direct comparison between these two ligand types.