G. Sean McGrady

The nature and driving forces behind the formation of dihydrogen bonds are explored for several textbook boranes, and their influence on the crystalline architecture is revealed.

This perspective describes the recent discovery and investigation of homopolar dihydrogen bonding, and focuses on the identification and characterisation of hydride–hydride interactions in compounds of the main group elements.

The design of molecular catalysts that mimic the behavior of enzymes is a topical field of activity in emerging technologies, and can lead to an improved understanding of biological systems. Herein, we report how the bulky arms of the cations in [(nC₄H₉)₄N]⁺[HCO₃]^‐give rise to a host scaffold that emulates the substrate binding sites in carbonic anhydrase enzymes, affording a unique glimpse of an arrested intermediate in the base‐mediated binding and activation of CO₂.

The design of molecular catalysts that mimic the behavior of enzymes is a topical field of activity in emerging technologies, and can lead to an improved understanding of biological systems. Herein, we report how the bulky arms of the cations in [(nC₄H₉)₄N]⁺[HCO₃]^‐give rise to a host scaffold that emulates the substrate binding sites in carbonic anhydrase enzymes, affording a unique glimpse of an arrested intermediate in the base‐mediated binding and activation of CO₂.

The synergic interplay of H⋯H and M–H interactions in the evolution of H₂from binary metal hydrides is revealed.

The structures adopted by a range of complexes AlH₃·nL, (n = 1 or 2), have been explored in detail to identify the factors that determine the value of n, and whether a monomeric or dimeric arrangement is preferred for the 1 : 1 complexes. Single-crystal X-ray diffraction, vibrational and NMR spectroscopies, and thermal analysis data have been collected, DFT calculations have been performed for AlH₃·nL species, and pK_a values have been collated for a series of amine and phosphine ligands L. The pK_a of the ligand L exerts an important influence on the type of complex formed: as the basicity of L increases, a monomeric structure is favoured over a dimeric arrangement. Dimeric amine complexes form if pK_a < 9.76, while monomeric complexes are preferred when pK_a > 9.99. The steric requirements of L also influence the structural preference: bulky ligands with large cone angles (>163°) tend to favour formation of monomers, while smaller cone angles (3·NMe₂Et, AlH₃·NMe₃ and AlH₃·nEt₂O.

The direct synthesis of NaAlH₄ has been studied, for the first time, by in situ²⁷Al and ²³Na wide-line NMR spectroscopy using high pressure NMR apparatus. Na₃AlH₆ formation is observed within two minutes of hydrogen addition, while NaAlH₄ is detected after a total of four minutes. This indicates the formation of the hexahydride does not proceed to completion before the formation of the tetrahydride ensues.

The AlH₃ adducts of TMEDA (Me₂NCH₂CH₂NMe₂), DIOX (O(CH₂CH₂)₂O), TEA (Et₃N), BDMA (PhNMe₂), and TMPDA (Me₂NCH₂CH₂CH₂NMe₂) have each been characterised by single-crystal X-ray diffraction at low temperature, by ¹H, ¹⁴N and ²⁷Al NMR and FT-Raman and FT-IR spectroscopy, and by DFT calculations and elemental analysis. Hence, AlH₃·TMEDA and AlH₃·DIOX are both shown to adopt a polymeric structure, with the bidentate ligand bridging two Al centres, each of which adopts a trigonal bipyramidal (TBP) arrangement with equatorial hydride moieties. The 1 : 2 adduct AlH₃·2BDMA is monomeric but the geometry at the Al centre resembles closely that of the polymeric TMEDA and DIOX complexes. AlH₃·TEA alone adopts a monomeric structure in which the Al centre is tetrahedrally coordinated by three hydride and one amine ligand. The Al–L bond distance of 2.0240(17) Å for AlH₃·TEA is the shortest of all the complexes in this study, and AlH₃·TEA also possesses the shortest Al–H bonds. AlH₃·DIOX has the shortest Al–L bond distance of the polymeric species (2.107(14) Å) on account of the higher electronegativity of the oxygen donor. The structure of AlH₃·TMEDA was determined at low temperature (monoclinic space group P2₁/c), and salient features are compared to the previous room temperature study, for which a highly disordered orthorhombic space group (P2₁2₁2₁) was reported. The polymeric structures appear to be stabilised by a number of intermolecular interactions and unconventional hydrogen bonds; these are most pronounced for AlH₃·DIOX, whose chains are connected by highly directional C–H⋯H–Al bonding with an H⋯H distance of 2.32(6) Å.

²⁷Al and ⁷Li wide-line NMR spectroscopy incorporating a high pressure NMR apparatus has allowed the first in situ study of the solvent mediated, direct synthesis of an alanate, thus overcoming the dearth of analytical techniques available to study phenomena occurring in a pressurised slurry. In contrast to the decomposition reaction, the elucidated hydrogenation pathway does not proceed through the hexahydride intermediate.