Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

Abstract

Reactions of B(C₆F₅)₃ with the β-diketiminato (BDI) alkaline-earth phosphidoborane complexes, 1a [(BDI)Ca(H₃B·PPh₂)] and 1b [(BDI)Mg(H₃B·PPh₂)]₂ (BDI = [HC{C(CH₃)N(2,6-iPr-C₆H₃)}₂]⁻) result in the formation of phosphinodiboronate complexes 4a [(BDI)Ca(η⁶-toluene){H₃B·PPh₂·B(C₆F₅)₃}] and 4b [(BDI)Mg{H₃B·PPh₂·B(C₆F₅)₃}]. Calcium complex 4a is stable in aromatic solvents at room temperature and does not display well-defined onward reactivity at elevated temperatures. Magnesium complex 4b undergoes a room temperature transformation to provide the known hydridoborate derivative 3b [(BDI)Mg{HB(C₆F₅)₃}] and an N,P,N’-ligated species, 5 [{HC(C(CH₃)N(2,6-iPr-C₆H₃))₂(H₂BPPh₂)}Mg{H₃B·PPh₂·B(C₆F₅)₃}] that results from interception of the putative phosphinoborane, H₂B = PPh₂, by the BDI ligand backbone following B(C₆F₅)₃-mediated hydride abstraction. NMR spectroscopic investigations were supported by DFT calculations, which suggested a mechanism involving B(C₆F₅)₃ migration and hydride abstraction within the coordination sphere of magnesium. Interception of H₂B = PPh₂ by B(C₆F₅)₃ is proposed to stabilise this species, whilst activating it towards ligand-centred nucleophilic attack. The significant stabilisation energy calculated for the Ca-π(toluene) interaction in 4a accounts for the contrasting outcomes between the two Ae-elements. The crystal structures of compounds 4a and 5 are presented and discussed.