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

Reactions of B(C6F5)(3) with the beta-diketiminato (BDI) alkaline-earth phosphidoborane complexes, 1a [(BDI) Ca(H3B center dot PPh2)] and 1b [(BDI)Mg(H3B center dot PPh2)](2) (BDI = [HC{C(CH3)N(2,6-iPr-C6H3)}(2)](-)) result in the formation of phosphinodiboronate complexes 4a [(BDI)Ca(eta(6)-toluene){H3B center dot PPh2 center dot B(C6F5)(3)}] and 4b [(BDI)Mg {H3B center dot PPh2 center dot 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 center dot PPh2 center dot 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-pi(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.