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

Treatment of the secondary phosphine {(Me3Si)2CH}­(Ph)­PH with BH3·SMe2 yields the phosphine-borane {(Me3Si)2CH}­(Ph)­PH­(BH3) (12) as colorless crystals. The reactions between 12 and n-BuLi, PhCH2Na or PhCH2K yield the corresponding phosphido-borane complexes [[{(Me3Si)2CH}­(Ph)­P­(BH3)]­Li­(THF)2] (13), [{(Me3Si)2CH}­(Ph)­P­(BH3)]­[Na­(12-crown-4)2] (14), or [[{(Me3Si)2CH}­(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···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)2CH}­(Ph)­P­(BH3)2]­Li­(12-crown-4) (16b), [[{(Me3Si)2CH}­(Ph)­P­(BH3)2]­Na­(THF)2]2 (17), and [[{(Me3Si)2CH}­(Ph)­P­(BH3)2]­K­(THF)0.5] (18a) were prepared by treatment of the corresponding in situ-generated phosphido-borane complexes with BH3·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.