Synthesis and Reactivity of Calcium Diborane(6) and Diboranate Derivatives

Molecular β-diketiminato (BDI) calcium hydride and n-hexyl derivatives are shown to display contrasting reactivities toward bis(pinacolato)diboron (B ₂pin ₂). Whereas the dimeric constitution of the calcium hydride is retained during formation of an unusual diborane(6) dianion, the initially dimeric organocalcium reagent is converted to a calcium diboranate monomer, [(BDI)Ca{pinB-Bpin(n-hex)}]. Attempts to effect heterolysis of the B−B bond of this latter species in a manner reminiscent of a previously reported magnesium system were unsuccessful and resulted in either dismutation to the homoleptic β-diketiminate, [(BDI) ₂Ca], or, in the case of organic nitriles, the generation of N-donor adduct derivatives, [(BDI)Ca(NCR){pinB-Bpin(n-hex)}] (R=t-Bu, i-Pr, o-tol). This latter reactivity again contrasts with the previously reported behaviour of an analogous magnesium diboranate, observations that are rationalised by density functional theory (DFT) calculations to be effectively a consequence of the larger radius and lower resultant steric congestion, of the third-row group 2 cation.