Lewis base complexes of AlH3: prediction of preferred structure and stoichiometry

Abstract

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 (<125°) encourage the formation of dimeric or 1 : 2 adducts. The steric bulk of the ligand appears to be more important for phosphine complexes, with smaller phosphines being unable to stabilise the complex at ambient temperatures even through dimerisation. Raman spectroscopy and DFT calculations have been particularly helpful in elucidating the stoichiometric preferences of complexes that have been contentious; these include AlH3·NMe2Et, AlH3·NMe3 and AlH3·nEt2O.

Efficient Synthesis of an Aluminum Amidoborane Ammoniate

“…Al MAS NMR spectrum (Figure 2b) verifies the formation of Al(NH2BH3)3·3NH3 and provides key information about its molecular structure. After reaction, only traces of the characteristic resonance for AlH3·OEt2 at 109.9 ppm are observed [23]. Two major resonances at 65.5 ppm and 33.6 ppm dominate the spectrum, clearly indicating that Al(NH2BH3)3·3NH3 contains equal amounts of aluminum in two very different coordination environments.…”

Aluminum hydride for solid-state hydrogen storage: Structure, synthesis, thermodynamics, kinetics, and regeneration

“…AlH 3 can react with a variety of organic compounds [119][120][121][122][123][124][125][126][127] . A variety of AlH 3 adducts of TMEDA (Me 2 NCH 2 CH 2 NMe 2 ), DIOX (O(CH 2 CH 2 ) 2 O), TEA (Et 3 N), BDMA (PhNMe 2 ), and TMPDA (Me 2 NCH 2 CH 2 CH 2 NMe 2 ) have been characterized by XRD, NMR, FT-Raman, FT-IR, DFT calculation, elemental analysis, and thermal analysis [126 , 127] .…”

Direct Hydrogenation of Aluminum via Stabilization with Triethylenediamine: A Mechanochemical Approach to Synthesize the Triethylenediamine ⋅ AlH3 Adduct

“…Among these ammonia borane as a particularly hydrogen rich material has been widely studied although its hydrogen release is not reversible exposing the spent material to hydrogen at technically feasible pressures and therefore chemical recycling would be needed . Related to ammonia borane, new compounds based on AlH 3 ⋅ nitrogen bases adducts have recently attracted significant interest and the material investigated in this study also belongs to this class of material.…”