Synthesis, structures and stabilities of thioanisole-functionalised phosphido-borane complexes of the alkali metals

Abstract

Treatment of the secondary phosphine {(Me₃Si)₂CH}PH(C₆H₄-2-SMe) with BH₃·SMe₂ gives the corresponding phosphine-borane {(Me₃Si)₂CH}PH(BH₃)(C₆H₄-2-SMe) (9) as a colourless solid. Deprotonation of 9 with n-BuLi, PhCH₂Na or PhCH₂K proceeds cleanly to give the corresponding alkali metal complexes [[{(Me₃Si)₂CH}P(BH₃)(C₆H₄-2-SMe)]ML]_n [ML = Li(THF), n = 2 (10); ML = Na(tmeda), n = ∞ (11); ML = K(pmdeta), n = 2 (12)] as yellow/orange crystalline solids. X-ray crystallography reveals that the phosphido-borane ligands bind the metal centres through their sulfur and phosphorus atoms and through the hydrogen atoms of the BH₃ group in each case, leading to dimeric or polymeric structures. Compounds 10–12 are stable towards both heat and ambient light; however, on heating in toluene solution in the presence of 10, traces of free phosphine-borane 9 are slowly converted to the free phosphine {(Me₃Si)₂CH}PH(C₆H₄-2-SMe) (5) with concomitant formation of the corresponding phosphido-bis(borane) complex [{(Me₃Si)₂CH}P(BH₃)₂(C₆H₄-2-SMe)]Li (14).