Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

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.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion

“…Polyhedral closo -boranes are characterized by rather high values of hydride donor ability (e.g., HDA MeCN = 71.9 kcal/mol for [B 6 H 6 ] 2− and 79.6 kcal/mol for [B 12 H 12 ] 2− ), which indicates the weak ability of the nonactivated B–H bond in closo -boranes to hydride transfer [ 29 ]. It is not surprising, therefore, that the most common method of functionalization of polyhedral boron hydrides follows the mechanism of electrophile-induced nucleophilic substitution (EINS) ( Scheme 1 ).…”

“…The [B 6 H 6 ] 2− is the smallest stable closo -borane dianion [ 29 , 55 , 56 ], where the bulk of electron density is located on the boron atoms of the octahedral skeleton. The high nucleophilicity of boron atoms [ 57 ] in [B 6 H 6 ] 2− leads to its easy protonation.…”

“…The changes in the reactivity of the B–H bond entailed by halogens’ introduction were evaluated via calculations of their hydride donor ability (HDA) and proton donor ability (PDA). Hydride donor ability (HDA)—i.e., thermodynamic hydridicity, defined as the Gibbs free energy (∆G°[H − ]) value for the reaction of hydride ion (H − ) detachment—is an important reactivity characteristic in transition metals [ 24 , 25 ] and the main group hydrides [ 20 , 26 , 27 , 28 , 29 ]. Similarly, the proton donor ability (PDA), i.e., thermodynamic acidity, was calculated as the Gibbs free energy (∆G°[H + ]) for the reaction of proton (H + ) detachment.…”

Fused 1,2-Diboraoxazoles Based on closo-Decaborate Anion–Novel Members of Diboroheterocycle Class

“…Boron clusters are versatile building blocks used for design compounds with given properties. Varying the structure of the boron cage and the nature of exo -polyhedral substituents, one can create molecules with plenty of useful properties such as high thermodynamic stability [ 7 , 8 ], magnetic properties [ 9 ], and low toxicity [ 10 , 11 , 12 ]. These properties cause a lot of boron cluster potential application in medicine [ 13 , 14 , 15 ], catalysis [ 16 , 17 ], the creation of luminescent materials [ 18 , 19 ], and in supramolecular chemistry [ 20 , 21 ].…”

Theoretical Study of closo-Borate Anions [BnHn]2− (n = 5–12): Bonding, Atomic Charges, and Reactivity Analysis

“…The closo-borate anions of general formula [B n H n ] 2− (n = 5-12) have been extensively studied theoretically and experimentally [1][2][3][4]. These cluster species have highly symmetrical structure and unusual chemical bonding, which cannot be described by classical Lewis structure [5][6][7].…”