Alkali metal complexes of a phosphine–borane-stabilised carbanion: influence of co-ligands on structure

Abstract

The adducts [[(Me₃Si)₂{Me₂P(BH₃)}C]K(L)_n]_m [L = THF, n = 0.5, m = ∞ (2a); L = tmeda (2b), pmdeta (2c), n = 1, m = 2] may be synthesised by treatment of solvent-free [[(Me₃Si)₂{Me₂P(BH₃)}C]K]_∞ (2) with the corresponding Lewis base (tmeda = N,N,N′,N′-tetramethylethylenediamine; pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine). X-Ray crystallography reveals that, whereas 2 crystallises with a complex 2-dimensional sheet structure, 2a crystallises as a ribbon-type one-dimensional polymer and both 2b and 2c crystallise as dimers. The corresponding complex with 12-crown-4, [K(12-crown-4)₂][(Me₃Si)₂{Me₂P(BH₃)}C] (2d) crystallises as a separated ion pair. The complexes [[(Me₃Si)₂{Me₂P(BH₃)}C]M(pmdeta)]_n [M = Na, n = 1 (6); M = Rb, n = 2 (7)] may be synthesised by treatment of [(Me₃Si)₂{Me₂P(BH₃)}C]M with pmdeta. Whereas 6 crystallises as a discrete monomer, compound 7 crystallises as a dimer. Compounds 2, 2a–2d, 6, 7 and the corresponding caesium derivative [[(Me₃Si)₂{Me₂P(BH₃)}C]Cs(pmdeta)]₂ (5) provide an opportunity to consider the influence of the ionic radius of the metal and the nature of the co-ligands on the structures of alkali metal complexes of a phosphine–borane-stabilised carbanion.