Abstract
Thermodynamic hydricity (HDAMeCN) determined as Gibbs free energy (ΔG°[H]−) of the H− detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [BnHn]2−, and their lithium salts Li2[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 (HDAMeCN = 82.1 kcal/mol) and its dianion [B2H6]2− (HDAMeCN = 40.9 kcal/mol for Li2[B2H6]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDAMeCN below 41 kcal/mol are strong hydride donors capable of reducing CO2 (HDAMeCN = 44 kcal/mol for HCO2−), whereas those with HDAMeCN 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 HDAMeCN values of closo-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 HDAMeCN.
Highlights
Boron-based chemistry is vast, diverse, and fascinating due to the ability of boron to form electron-deficient structures of various shapes with delocalized electrons and multicenter bonding
Like polyhedral boron hydrides, require the presence of an excess of Lewis or Brønsted acids for generation of boron-centered quasi-borinium cations [36]. The reactivity of such compounds can be characterized in terms of their ability to hydride transfer as thermodynamic hydricity (HDA) through DFT calculations [32,34]
Our study and literature analysis show that borane clusters with Hydride donating ability in MeCN (HDAMeCN) below 41 kcal/mol are strong hydride donors capable of reducing CO2 (HDAMeCN = 44 kcal/mol for HCO2 − [31]); those with HDAMeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B2 H6 [58], B4 H10, B5 H11, etc. [59])
Summary
Boron-based chemistry is vast, diverse, and fascinating due to the ability of boron to form electron-deficient structures of various shapes (such as cages, clusters, etc.) with delocalized electrons and multicenter bonding. Thermodynamic hydricity, i.e., hydride donating ability (HDA), determined as Gibbs free energy (∆G◦ [H]− ) for the reaction of hydride ion, H− , detachment, is a very important characteristic of transition metal hydrides [31] and main group hydrides [32,33,34] that describes their reactivity and Molecules 2020, 25, 2920; doi:10.3390/molecules25122920 www.mdpi.com/journal/molecules. We have demonstrated the existence of an inversely proportional dependence of the hydride-transfer ability of Li[L3 B–H] on Lewis acidity of L3 B [34]. High values of HDA indicate high Lewis acidity of parent borane L3 B, whereas low HDA values indicate high hydride transfer ability of Li[L3 B–H]. The hydride donating ability (HDA) of boranes can be used as a measure of the Lewis acidity of parent neutral borane or boron cations
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