Heavier Alkali Metal Complexes of a Silicon- and Phosphine-Borane-Stabilized Carbanion

Treatment of the phosphine-borane adduct (Me3Si)2{Me2P(BH3)}CH (5) with either MeNa or MeK in
diethyl ether yields the corresponding alkali metal salts [[(Me3Si)2{Me2P(BH3)}C]ML]∝ [ML =
Na(THF)2·1/2PhMe (6a); K (7)] after recrystallization. Metathesis reactions between in situ prepared
[(Me3Si)2{Me2P(BH3)}C]Li and either rubidium or cesium 2-ethylhexoxide give the compounds [[(Me3Si)2{Me2P(BH3)}C]ML]n [ML = Rb, n = ∞ (10); Cs(pmdeta), n = 2 (11)] after recrystallization.
Compounds 6a, 7, 10, and 11 are the first heavier alkali metal complexes of a phosphine-borane-stabilized
carbanion to be isolated in the solid state. Compounds 5, 6a, 7, 10, and 11 have been characterized by
elemental analyses, multielement (1H, 13C{1H}, 11B{1H}, and 31P{1H}) NMR spectroscopy, and X-ray
crystallography. In the solid state compounds 6a, 7, 10, and 11 adopt dimeric or polymeric structures in
which the alkali metal cations are associated with both the carbanion centers and the borane hydrogen
atoms.