Quantitative Assessment of B−B−B, B−H_b−B, and B−H_t Bonds: From BH₃ to B₁₂H₁₂²⁻

Abstract

We report the thermodynamic stabilities and the intrinsic strengths of three‐center‐two‐electron B−B−B and B−Hb−B bonds (Hb : bridging hydrogen), and two‐center‐two‐electron B−Ht bonds (Ht : terminal hydrogen) which can be served as a new, effective tool to determine the decisive role of the intermediates of hydrogenation/dehydrogenation reactions of borohydride. The calculated heats of formation were obtained with the G4 composite method and the intrinsic strengths of B−B−B, B−Hb−B, and B−Ht bonds were derived from local stretching force constants obtained at the B3LYP‐D2/cc‐pVTZ level of theory for 21 boron‐hydrogen compounds, including 19 intermediates. The Quantum Theory of Atoms in Molecules (QTAIM) was used to deepen the inside into the nature of B−B−B, B−Hb−B, and B−Ht bonds. We found that all of the experimentally identified intermediates hindering the reversibility of the decomposition reactions are thermodynamically stable and possess strong B−B−B, B−Hb−B, and B−Ht bonds. This proves that thermodynamic data and intrinsic B−B−B, B−Hb−B, and B−Ht bond strengths form a new, effective tool to characterize new (potential) intermediates and to predict their role for the reversibility of the hydrogenation/dehydrogenation reactions.

Estimation of Thermodynamic Properties of Metal Hydroborates

“…We have recently studied spectroscopic and thermodynamic properties of boron hydrogen species in the gas phase using anharmonic DFT calculations which show excellent agreement between theoretical and experimental vibrational spectra . Using the G4 composite method, we have also obtained the formation enthalpy of a series of neutral and charged boron hydrogen species …”

“…The G4 enthalpy of formation of these ions in the gas phase are −65.0, −107.6 and −428.6 kJ/mol respectively, and their entropy at 300 K as obtained from anharmonic DFT calculations is equal to 189.7, 342.9 and 352.9 J/mol.K respectively …”

“…The enthalpy of formation of NaBH 4 (s) and Na + (g) are given in the literature . We have recently obtained the value of Δ f H° for BH 4 − (g) from theoretical calculations (‐64.9 kJ/mol). The lattice energy for NaBH 4 (s) is thus calculated to be equal to –736.3 kJ/mol.…”

Metal–Halogen Bonding Seen through the Eyes of Vibrational Spectroscopy

“…The local mode analysis has been successfully applied to characterize covalent bonds [63,65,108,[116][117][118][119] and weak chemical interactions such as halogen [15,16,80,120], chalcogen [17,102,121], pnicogen [122][123][124], and tetrel interactions [64] as well as H-bonding [18,[125][126][127][128][129] and BH• • • π interactions [130,131]. A new metal-ligand electronic parameter (MLEP) was derived as quantitative measure of the intrinsic strength of metal-ligand bonding [129,[132][133][134][135].…”

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory

“…So far, the local mode analysis has been successfully applied to characterize covalent bonds [58,[61][62][63][64][65][66] and weak chemical interactions such as intra-and inter-molecular hydrogen bonding in various forms and systems [67][68][69][70][71][72][73], chalcogen [74][75][76], pnicogen [77][78][79] and tetrel interactions [80], and in particular halogen bonding [81][82][83][84]. Recently, we extended the local vibrational mode theory from molecular to periodic one-dimensional (1D) through three-dimensional (3D) systems [85].…”