Desolvation and aggregation of sterically demanding alkali metal diarylphosphides

The reaction between (Dipp)₂PH and one equivalent of n-BuLi, PhCH₂Na or PhCH₂K in THF gives the complexes [(Dipp)₂P]Li(THF)₃ (2a), {[(Dipp)₂P]Na(THF)₂}₂ (3a) and [(Dipp)₂P]K(THF)₄ (4a), respectively [Dipp = 2,6-iPr₂C₆H₃]. Exposure of these compounds to vacuum yields the alternative solvates [(Dipp)₂P]Li(THF)₂ (2b), [(Dipp)₂P]Na(THF)_1.5 (3b), and [(Dipp)₂P]K (4b), respectively; the alternative adduct [(Dipp)₂P]Na(PMDETA) (3c) was prepared by treatment of 3a with PMDETA. Treatment of (Dipp)(Mes)PH or (Mes)₂PH with one equivalent of n-BuLi in THF gives the complexes [(Dipp)(Mes)P]Li(THF)₃ (7a) and [(Mes)₂P]₂Li₂(THF)₂(OEt₂) (8a) after crystallisation from diethyl ether [Mes = 2,4,6-Me₃C₆H₂]; crystallisation of 8a from hexane gives the alternative adduct [(Mes)₂P]Li(THF)₃ (8b). Exposure of 7a, 8a and 8b to vacuum leads to loss of coordinated solvent, yielding the solvates [(Dipp)(Mes)P]Li(THF)₂ (7b) and [(Mes)₂P]Li(THF) (8c). The solid-state structures of complexes 2a, 3a, 3c, 4a, 7a, 8a, and 8b have been determined by X-ray crystallography. Variable-temperature ³¹P{¹H} and ⁷Li NMR spectroscopy indicates that 2b, 3b and 7b are subject to a monomer-dimer equilibrium in solution, where the monomeric forms are favoured at low temperature. In contrast, variable-temperature ³¹P{¹H} and ⁷Li NMR spectroscopy suggests that 8c is subject to a dynamic equilibrium between a dimer and a cyclic trimer in solution, where the trimer is favoured at low temperatures.