Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

Thermodynamic hydricity (HDA(MeCN)) determined as Gibbs free energy (Delta G degrees[H](-)) of the H(-)detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedralcloso-boranes dianions [BnHn](2-), and their lithium salts Li-2[BnHn] (n = 5-17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B2H6(HDA(MeCN)= 82.1 kcal/mol) and its dianion [B2H6](2-)(HDA(MeCN)= 40.9 kcal/mol for Li-2[B2H6]) can be selected as border points for the range of borane clusters' reactivity. Borane clusters with HDA(MeCN)below 41 kcal/mol are strong hydride donors capable of reducing CO2(HDA(MeCN)= 44 kcal/mol for HCO2-), whereas those with HDA(MeCN)over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B2H6, B4H10, B5H11, etc.). The HDA(MeCN)values ofcloso-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDA(MeCN).