Tris(borane) Adducts of Diphosphanylmethanides: The [H 3 BCH(PPh 2 BH 3 ) 2 ] – Anion and Its Alkali Metal Complexes

Abstract

The reactivity of lithium complexes that contain the borane-modified diphoshanylmethanide ligand [CH(PPh2BH3)2]– towards different Lewis base adducts of BH3 was studied to gain further insight into the mechanism of the isomerization of this derivative, which formally proceeds through a shift of one BH3 group from the phosphorus atom to the carbon atom. Whereas the use of BH3·THF in THF only resulted in the thf adduct of the starting material, [Li{CH(PPh2BH3)2}(thf)2] (1), the application of BH3·SMe2 in toluene resulted in the formation of the novel compound [(Li{H3BCH(PPh2BH3)2})∞] (2). The subsequent addition of ethereal ligands led to the isolation of [Li{H3BCH(PPh2BH3)2}(Me4thf)] (3) and [Li{H3BCH(PPh2BH3)2}(thf)3] (4). Treatment of these complexes with stronger Lewis bases such as N,N,N′,N′-tetramethylethane-1,2-diamine (tmeda) results in the removal of one phosphorus-bound BH3 molecule and the formation of the [Ph2PCH(BH3)PPh2BH3]– anion. These results indicate that the isomerization of [CH(PPh2BH3)2]– requires an additional BH3 source and a rather strong Lewis base. Complexes 1–4 and the related derivatives [Li{CH(PPh2BH3)2}(Me4THF)] (5) and [K{H3BCH(PPh2BH3)2}(dme)2] (6; dme = 1,2-dimethoxyethane) were characterized by multinuclear NMR spectroscopy and by single-crystal X-ray diffraction analysis.