Crystallographic snapshot of an arrested intermediate in the biomimetic 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 [(n C4 H9 )4 N](+) [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 .

Homopolar dihydrogen bonding in alkali metal amidoboranes: crystal engineering of low-dimensional molecular materials.

Hydrogen bonding is a predominant interaction in supramolecular chemistry. The absence of a conventional hydrogen bond donor in LiNMe(2)BH(3) and KNMe(2)BH(3) results in the formation of elaborate M···H-B polymeric arrays supported by heteropolar and homopolar H···H bonding, in a unique synergistic combination of unconventional intermolecular interactions.

In situ high pressure NMR study of the direct synthesis of NaAlH4.

The direct synthesis of NaAlH4 has been studied, for the first time, by in situ (27)Al and (23)Na 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.

Homopolar dihydrogen bonding in alkali-metal amidoboranes and its implications for hydrogen storage.

The solid-state structures of LiNH(2)BH(3) and NaNH(2)BH(3) have been shown recently to exhibit intricate M(δ+)···(δ-)H-B and N-H(δ+)···(δ-)H-B interactions. However, closer inspection of these structures reveals additional homopolar H···H interactions, viz., B-H(δ-)···(δ-)H-B and N-H(δ+)···(δ+)H-N, which contribute to the relative stability of the extended structures of these crystalline materials. In addition, an NMR study of the isotopomer LiND(2)BH(3) shows that a significant quantity of H(2) is desorbed thermally along with HD, which can only arise from hydride-hydride interactions, either directly from B-H(δ-)···(δ-)H-B moieties or indirectly through the participation of Li-H intermediates.

Ti-doped LiAlH4 for hydrogen storage: synthesis, catalyst loading and cycling performance.

The direct synthesis of LiAlH(4) from commercially available LiH and Al powders in the presence of TiCl(3) and Me(2)O has been achieved for the first time. The effects of TiCl(3) loadings (Ti/Al = 0, 0.01, 0.05, 0.2, 0.5, 1.0 and 2.0%) and various other additives (TiCl(3)/Al(2)O(3), metallic Ti, Nb(2)O(5), and NbCl(5)) on the formation and stability of LiAlH(4) have been systematically investigated. The yield of LiAlH(4) initially increases, and then decreases, with increasing TiCl(3) loadings. LiH + Al → LiAlH(4) yields above 95% were obtained when the molar ratios of Ti/Al were 0.05 and 0.2%. In the presence of a very tiny amount of TiCl(3) (Ti/Al = 0.01%), LiAlH(4) is still generated, but the yield is lower. In the complete absence of TiCl(3), LiAlH(4) does not form. Addition of metallic Ti, Nb(2)O(5), and NbCl(5) to commercial LiH and Al does not result in the formation of LiAlH(4). Preliminary tests show that TiCl(3)-doped LiAlH(4) can be cycled, making it a suitable candidate for hydrogen storage.

Structure and bonding of KSiH3 and its 18-crown-6 derivatives: unusual ambidentate behavior of the SiH3(-) anion.

Density functional theory (DFT) calculations of [K(18-crown-6)SiH(3)] (1) and KSiH(3) (2) have shown that both the classical tet and non-classical inv coordination modes of the [SiH(3)](-) anion to the K(+) ion are energetically accessible. Single-crystal X-ray structures of the tet and inv derivatives [K(18-crown-6)SiH(3)·THF] (1a) and [K(18-crown-6)SiH(3)·HSiPh(3)] (1b) confirm this conclusion, showing that small changes in the coordination sphere of the metal are sufficient to alter the orientation of the anion. A topological analysis of the calculated electron densities for 1 and 2 reveals that the K···Si interaction in the tet conformer of 2 possesses a significant amount of covalent character. In contrast, the inv form of 2 displays primarily electrostatic character for the K···Si and K···H interactions. Incorporation of the 18-crown-6 ligand in 1 reduces the polarizing power of the K(+) cation, hardening the cation-anion interaction in both conformers. The experimental structures of 1a and 1b bear out these conclusions, with the strongly bound tetrahydrofuran (THF) ligand softening the K(+) ion in 1a and favoring the tet conformer, while the weakly interacting HSiPh(3) ligand in 1b has minimal effect on the K(+) center, resulting in an inv orientation.

Insights into metal borohydride and aluminohydride bonding: X-ray and neutron diffraction structures and a DFT and charge density study of [Na(15-crown-5)][EH(4)] (E = B, Al).

The neutron and X-ray structures of [Na(15-crown-5)][BH(4)] and [Na(15-crown-5)][AlH(4)], respectively, are reported, along with a topological analysis of their DFT-computed charge densities that explores the bonding between the anionic complex hydride [EH(4)](-) (E = B, Al) and the counterion [Na(15-crown-5)](+). In each case, the interaction is weak and mainly electrostatic in nature; however, notable differences are observed in the manner in which [BH(4)](-) and [AlH(4)](-) bind to the metal, which explains their different coordination modes. A range of unconventional E-H···H-C contacts is revealed to play an important role in the overall bonding and crystal packing of both complexes. These interactions can be classified as weak dihydrogen bonds based on the atoms in molecules approach.

Facile cycling of Ti-doped LiAlH4 for high performance hydrogen storage.

LiH and Ti-doped Al react quantitatively with H(2) in Me(2)O solution to form LiAlH(4) under mild conditions. The solvent is easily vented along with excess H(2) on completion, leaving dry Ti-doped LiAlH(4); this releases approximately 7 wt % H(2) commencing at 80 degrees C with excellent kinetics.

Nature of the bonding in metal-silane sigma-complexes.

The nature of metal silane sigma-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [Cp'Mn(CO)(2)(eta(2)-HSiHPh(2))] 1 has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate a trigonal bipyramid; salient bond distances and angles are Mn-H(1) 1.575(14), Si-H(1) 1.806(14), Si-H(2) 1.501(13) A, and H(1)-Si-H(2) 148.5(8) degrees. This complex is similar to [Cp'Mn(CO)(2)(eta(2)-HSiFPh(2))] 2, whose structure and bonding characteristics have recently been determined by charge density studies based on high-resolution X-ray and neutron diffraction data. The geometry at the Si atom in these sigma-bond complexes is compared with that in other systems containing hypercoordinate silicon. The Mn-H distances for 1 and 2 in solution have been estimated using NMR T(1) relaxation measurements, giving a value of 1.56(3) A in each case, in excellent agreement with the distances deduced from neutron diffraction. Density functional theory calculations have been employed to explore the bonding in the Mn-H-Si unit in 1 and 2 and in the related system [Cp'Mn(CO)(2)(eta(2)-HSiCl(3))] 3. These studies support the idea that the oxidative addition of a silane ligand to a transition metal center may be described as an asymmetric process in which the Mn-H bond is formed at an early stage, while both the establishment of the Mn-Si bond and also the activation of the eta(2)-coordinated Si-H moiety are controlled by the extent of Mn --> sigma*(X-Si-H) back-donation, which increases with increasing electron-withdrawing character of the X substituent trans to the metal-coordinated Si-H bond. This delocalized molecular orbital (MO) approach is complemented and supported by combined experimental and theoretical charge density studies: the source function S(r,Omega), which provides a measure of the relative importance of each atom's contribution to the density at a specific reference point r, clearly shows that all three atoms of the Mn(eta(2)-SiH) moiety contribute to a very similar extent to the density at the Mn-Si bond critical point, in pleasing agreement with the MO model. Hence, we advance a consistent and unifying concept which accounts for the degree of Si-H activation in these silane sigma-bond complexes.

Temporal and spatial imaging of hydrogen storage materials: watching solvent and hydrogen desorption from aluminium hydride by transmission electron microscopy.

An in situ thermal desorption study of solvated aluminum hydride (alane) by transmission electron microscopy and selected area diffraction has permitted characterisation of the structural and morphological changes during desorption of solvent and hydrogen in real-time; this powerful technique for studying hydrogen storage materials complements several others already employed.

Elucidation of the bonding in Mn(eta2-SiH) complexes by charge density analysis and T1 NMR measurements: asymmetric oxidative addition and anomeric effects at silicon.

The bonding in Mn(eta2-SiH) complexes is interpreted in terms of an asymmetric oxidative addition whose extent is controlled by the substitution pattern at the hypercoordinate silicon centre, and especially by the ligand trans to the eta2-coordinating SiH moiety.

Solution-based routes to ammine metal borohydrides: formation of ammonia-borane.

Ammine metal borohydrides (AMBs) have recently commanded attention as low-temperature hydrogen sources. As an alternative to widely used mechanochemical synthesis that affords mixtures with salt co-products, we have been investigating solution synthesis routes to obtain pure AMBs. Here we show that reactions of MCln + nNaBH4 with ammonia in thf afford ammonia-borane (AB) via borane abstraction from M-coordinated borohydride. The amount of AB formed correlates roughly with the metal ion electronegativity and AMB thermal stability, except for reducible metals such as Ti, which affords nearly 3 equiv. of AB per Ti.

Ruthenium trihydrides with N-heterocyclic carbene ligands: effects on quantum mechanical exchange coupling.

Ruthenium trihydrides containing N-heterocyclic carbene ligands display large quantum mechanical exchange couplings in their 1H NMR spectra: DFT calculations are used to explore this phenomenon and to compare them to their phosphine congeners.

Homopolar dihydrogen bonding in main group hydrides: discovery, consequences, and applications.

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. A highlight of this programme has been an appreciation of the important role played by this interaction in the structural and thermochemical properties of these materials, and in the mechanisms through which they release hydrogen. A fuller understanding of this new class of H∙∙∙H interactions has also allowed us to explore their role in the supramolecular chemistry of hydrogen-rich compounds.

Agostic interactions in d0 metal alkyl complexes.

The phenomenon of agostic interactions is reviewed and the nature of the interaction is revisited. A historical perspective is followed by an overview of experimental techniques used to diagnose agostic behavior, and previous interpretations of agostic bonding are presented. A series of simple metal alkyl complexes is considered and a new model for the phenomenon in d(0) systems is developed which sets them apart from agostic late-transition-metal complexes. Factors such as the valence electron count and coordination number of the metal center are revealed to be unimportant in facilitating the interaction in most d(0) systems. The charge density distribution in several transition-metal alkyl complexes is explored by experimental and theoretical techniques, including the powerful "Atoms in Molecules" approach. Local charge concentrations are shown to play an important role in the agostic interaction. Finally, we demonstrate for the first time a way to manipulate and control the magnitude and disposition of such local charge concentrations, and hence the strength of agostic interactions in d(0) metal alkyl complexes.

Molecular Structures of Methyldifluoroarsine, CH(3)AsF(2), and Dimethylfluoroarsine, (CH(3))(2)AsF, in the Gas Phase As Determined by Electron Diffraction and ab InitioCalculations.

The structures of gaseous CH(3)AsF(2) and (CH(3))(2)AsF have been determined by electron diffraction incorporating vibrational amplitudes derived from ab initio force fields scaled by experimental frequencies and, for the difluoride, restrained by microwave constants. The following parameters (r(alpha) degrees structure, distances in pm, angles in degrees) have been determined for CH(3)AsF(2): r(As-C) = 194.6(4), r(As-F) = 173.1(1), angleCAsF = 95.2(1), angleFAsF = 97.0(1). For (CH(3))(2)AsF structural refinement gives r(As-C) = 195.1(1), r(As-F) = 175.4(1), angleCAsF = 95.3(5), and angleCAsC = 96.9(8). For the series (CH(3))(3)As, (CH(3))(2)AsF, CH(3)AsF(2), and AsF(3), both As-C and As-F bond lengths are shortened with increasing numbers of F atoms, but the angles CAsF and FAsF are almost invariant.

Desorption of hydrogen from light metal hydrides: concerted electronic rearrangement and role of H···H interactions.

A theoretical study of the desorption of hydrogen from rhombic Group 1 metal hydride dimers reveals a concerted reorganisation of the electron density for the M-H and H-H moieties as the reaction coordinate is traversed and a closed-shell H···H interaction evolves into a covalent H2 bond. The central role played by homopolar dihydrogen bonding in this process is revealed and analysed.

Lewis base complexes of AlH3: structural determination of monomeric and polymeric adducts by X-ray crystallography and DFT calculations.

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 (1)H, (14)N and (27)Al 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 P2(1)/c), and salient features are compared to the previous room temperature study, for which a highly disordered orthorhombic space group (P2(1)2(1)2(1)) 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.