Phosphido-borane and phosphido-bis(borane) complexes of the alkali metals, a comparative study

Treatment of the secondary phosphine {(Me(3)Si)(2)CH}(Ph)PH with BH(3)·SMe(2) yields the phosphine-borane {(Me(3)Si)(2)CH}(Ph)PH(BH(3)) (12) as colorless crystals. The reactions between 12 and n-BuLi, PhCH(2)Na or PhCH(2)K yield the corresponding phosphido-borane complexes [[{(Me(3)Si)(2)CH}(Ph)P(BH(3))]Li(THF)(2)](∞) (13), [{(Me(3)Si)(2)CH}(Ph)P(BH(3))][Na(12-crown-4)(2)] (14), or [[{(Me(3)Si)(2)CH}(Ph)P(BH(3))]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···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 [{(Me(3)Si)(2)CH}(Ph)P(BH(3))(2)]Li(12-crown-4) (16b), [[{(Me(3)Si)(2)CH}(Ph)P(BH(3))(2)]Na(THF)(2)](2) (17), and [[{(Me(3)Si)(2)CH}(Ph)P(BH(3))(2)]K(THF)(0.5)](∞) (18a) were prepared by treatment of the corresponding in situ-generated phosphido-borane complexes with BH(3)·SMe(2) 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.