Amidophosphine–Borane Complexes of Alkali Metals and the Heavier Alkaline-Earth Metals: Syntheses and Structural Studies

The N-benzhydrylamido-1,1-diphenylphosphine–borane
ligand [Ph2P­(BH3)­NH­(CHPh2)] (1-H) has been prepared via the reaction of an equimolar ratio
of the phosphine amine [Ph2PNH­(CHPh2)] and the
borane adduct [BH3·SMe2] at an ambient
temperature. The reaction of 1-H with ((trimethylsilyl)­methyl)­lithium
(neosilyllithium), [LiCH2SiMe3], afforded a
three-membered lithium amidophosphine–borane complex with the
composition ([η2-Ph2CHNP­(BH3)­Ph2)­Li­(THF)2] (2). Analogous
reactions with sodium and potassium bis­(trimethylsilyl)­amides at ambient
temperature yielded the respective alkali-metal amidophosphine–borane
complexes [{(Ph2CHNP­(BH3)­Ph2)­Na­(THF)2}2] (3) and [{(Ph2CHNP­(BH3)­Ph2)­K­(THF)2}2] (4), with the loss of hexamethyldisilazane, [(Me3Si)2NH]. The corresponding heavier alkaline-earth-metal complexes
[M­(THF)2{Ph2P­(BH3)­N­(CHPh2)}2] (M = Ca (5), Sr (6), Ba
(7)) can be obtained via the reaction of [M­{N­(SiMe3)2}2(THF)n] (M = Ca, Sr, Ba) and 1-H. The metal complexes 5–7 can also be prepared via a salt metathesis
route, where the alkali-metal salts 3 and 4 were reacted with the respective metal diiodides in THF at ambient
temperature. The molecular structures of 1-H and 2–7 have been established by X-ray diffraction
analyses, and from the solid-state structures of 3–7, it was confirmed that, in all of the compounds, the metal
ions are chelated through the nitrogen atom and the hydrogen atoms
of the borane group of ligand 1-H.