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

Molecular -diketiminato (BDI) calcium hydride and n-hexyl derivatives are shown to display contrasting reactivities toward bis(pinacolato)diboron (B2pin2). 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 -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 to 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.