Hydride donating abilities of the tetracoordinated boron hydrides

The hydride donating ability (HDA), determined as Gibbs free energy (ΔG°H−) for the reaction of H− dissociation, was assessed via the DFT/M06/6-311++G (d,p) calculations for 90 tetracoordinated borohydrides Li [L3B-H] taking into account the solvent effects via the optimization in MeCN and CH2Cl2 under SMD model. Obtained this way, the HDAMeCN values vary from 118.2 to 13.4 kcal/mol and correlate well with the Lewis acidity parameters (AN, HA and FA) of parent trigonal boranes (L3B). These data show numerically how the variation of the substituents at the boron atom allows the fine-tuning the B–H bond reactivity (reduction power) in the reactions involving hydride transfer as well as the selectivity of the reduction processes. The analysis of the data obtained shows that by varying the number of substituents and their nature, it is possible not only to change the properties of neutral trisubstituted boranes from highly electrophilic (represented by halogenide- and pseudohalogenide-boranes) to highly nucleophilic (exemplified by alkoxy-an amidoboranes), but also to repolarize the boron-bound hydrogen and make the proton transfer process more favourable than the hydride transfer.