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

Treatment of the secondary phosphine {(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­PH with BH<sub>3</sub>·SMe<sub>2</sub> yields the phosphine<b>-</b>borane {(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­PH­(BH<sub>3</sub>) (<b>12</b>) as colorless crystals. The reactions between <b>12</b> and <i>n</i><b>-</b>BuLi, PhCH<sub>2</sub>Na or PhCH<sub>2</sub>K yield the corresponding phosphido<b>-</b>borane complexes [[{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)]­Li­(THF)<sub>2</sub>]<sub>∞</sub> (<b>13</b>), [{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)]­[Na­(12<b>-</b>crown<b>-</b>4)<sub>2</sub>] (<b>14</b>), or [[{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)]­K­(pmdeta)]<sub>2</sub> (<b>15</b>), respectively, after crystallization in the presence of the appropriate co<b>-</b>ligand. While <b>13</b> crystallizes as a chain polymer, <b>14</b> crystallizes as a separated ion pair and <b>15</b> as a dimer. In both <b>13</b> and <b>15</b> the phosphido<b>-</b>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 <b>14</b> undergoes rapid inversion at phosphorus, while <b>13</b> and <b>15</b> do not. The phosphido<b>-</b>bis­(borane) complexes [{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)<sub>2</sub>]­Li­(12<b>-</b>crown<b>-</b>4) (<b>16b</b>), [[{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)<sub>2</sub>]­Na­(THF)<sub>2</sub>]<sub>2</sub> (<b>17</b>), and [[{(Me<sub>3</sub>Si)<sub>2</sub>CH}­(Ph)­P­(BH<sub>3</sub>)<sub>2</sub>]­K­(THF)<sub>0.5</sub>]<sub>∞</sub> (<b>18a</b>) were prepared by treatment of the corresponding in situ<b>-</b>generated phosphido<b>-</b>borane complexes with BH<sub>3</sub>·SMe<sub>2</sub> and crystallization in the presence of the appropriate co<b>-</b>ligand. Compound <b>16b</b> crystallizes as a monomer, while <b>17</b> crystallizes as a dimer and <b>18a</b> crystallizes as a ribbon polymer. These crystallographic studies reveal entirely new binding modes for both phosphido<b>-</b>borane and phosphido<b>-</b>bis­(borane) ligands and allow a direct comparison between these two ligand types.