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1.
Chem Commun (Camb) ; 52(12): 2581-4, 2016 Feb 11.
Article in English | MEDLINE | ID: mdl-26750101

ABSTRACT

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.

3.
Dalton Trans ; 44(21): 9718-31, 2015 Jun 07.
Article in English | MEDLINE | ID: mdl-25915435

ABSTRACT

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.

4.
Angew Chem Int Ed Engl ; 54(1): 164-8, 2015 Jan 02.
Article in English | MEDLINE | ID: mdl-25376525

ABSTRACT

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 .


Subject(s)
Bicarbonates/chemistry , Biomimetic Materials/chemistry , Carbon Dioxide/metabolism , Carbonic Anhydrases/chemistry , Quaternary Ammonium Compounds/chemistry , Bicarbonates/metabolism , Binding Sites , Biomimetic Materials/metabolism , Biomimetics , Carbonic Anhydrases/metabolism , Crystallography, X-Ray , Models, Molecular , Quaternary Ammonium Compounds/metabolism
5.
Chem Commun (Camb) ; 50(29): 3820-3, 2014 Apr 14.
Article in English | MEDLINE | ID: mdl-24589845

ABSTRACT

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.

6.
Dalton Trans ; 42(19): 6953-64, 2013 May 21.
Article in English | MEDLINE | ID: mdl-23508223

ABSTRACT

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.

7.
Dalton Trans ; 42(19): 6965-78, 2013 May 21.
Article in English | MEDLINE | ID: mdl-23508235

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.

8.
Phys Chem Chem Phys ; 15(17): 6179-81, 2013 May 07.
Article in English | MEDLINE | ID: mdl-23519072

ABSTRACT

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.


Subject(s)
Aluminum Compounds/chemical synthesis , Sodium Compounds/chemical synthesis , Aluminum/chemistry , Aluminum Compounds/chemistry , Magnetic Resonance Spectroscopy , Pressure , Sodium/chemistry , Sodium Compounds/chemistry
9.
J Am Chem Soc ; 135(7): 2439-42, 2013 Feb 20.
Article in English | MEDLINE | ID: mdl-23363072

ABSTRACT

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.

11.
Chem Commun (Camb) ; 48(20): 2597-9, 2012 Mar 07.
Article in English | MEDLINE | ID: mdl-22288073

ABSTRACT

The solid-state structure of ammonia borane is held together by an intricate N-H···H-B proton-hydride bonding network. These intermolecular interactions have long been considered to mediate the release of hydrogen from this material. Here we reveal the silent but important role played by B-H···H-B interactions in the thermal decomposition of this leading hydrogen storage candidate.

12.
Inorg Chem ; 50(21): 11222-7, 2011 Nov 07.
Article in English | MEDLINE | ID: mdl-21981304

ABSTRACT

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.

13.
J Am Chem Soc ; 133(41): 16598-604, 2011 Oct 19.
Article in English | MEDLINE | ID: mdl-21859070

ABSTRACT

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.


Subject(s)
Energy-Generating Resources , Metals, Alkali/chemistry , Hydrogen Bonding
14.
J Am Chem Soc ; 133(39): 15593-7, 2011 Oct 05.
Article in English | MEDLINE | ID: mdl-21863886

ABSTRACT

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.

15.
Dalton Trans ; 40(33): 8301-3, 2011 Sep 07.
Article in English | MEDLINE | ID: mdl-21713292

ABSTRACT

The extended structures of [K(1-aza-18-crown-6)]BH(4) and its 18-crown-6 analogue exhibits significantly different primary and secondary stabilizing interactions. However, their respective ion pairs display similar cation-to-anion interactions, in spite of the differences in the nature of the crown ether ligand.

16.
Inorg Chem ; 49(24): 11395-402, 2010 Dec 20.
Article in English | MEDLINE | ID: mdl-21090626

ABSTRACT

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.

17.
Chem Commun (Camb) ; 46(27): 4923-5, 2010 Jul 21.
Article in English | MEDLINE | ID: mdl-20544122

ABSTRACT

Preliminary results are reported for the extraction and catalytic hydrocracking of Alberta bitumen and oil sands using supercritical fluid mixtures; high levels of extraction and upgrading were attained using reaction conditions significantly milder than those previously reported.

18.
Dalton Trans ; 39(13): 3170-3, 2010 Apr 07.
Article in English | MEDLINE | ID: mdl-20449443

ABSTRACT

A series of complexes of general formula [CpMn(CO)(2)(eta(2)-HPR(1)R(2).BCl(3))] has been studied by DFT calculations and topological analyses of the charge density thus derived. The 21 complexes included in this study exhibit closely similar Mn-H-P geometries, in spite of a wide range of substituents (R(1); R(2)) at the phosphorus atom. Topological analysis of the electron density suggests that these are genuine sigma-bond complexes, albeit at a later stage on the oxidative addition reaction coordinate than corresponding silanes, with strong Mn-P and Mn-H bonding and a weak P-H interaction.

19.
Dalton Trans ; (30): 5851-7, 2009 Aug 14.
Article in English | MEDLINE | ID: mdl-19623384

ABSTRACT

The single-crystal X-ray structures of the complexes [CpCr(CO)3H] 1, [CpMo(CO)3H] 2 and [CpW(CO)3H] 3 are reported. The results indicate that 1 adopts a structure close to a distorted three-legged piano stool geometry, whereas a conventional four-legged piano stool arrangement is observed for 2 and 3. Further insight into the equilibrium geometries and potential energy surfaces of all three complexes was obtained by DFT calculations. These show that in the gas phase complex 1 also prefers a geometry close to a four-legged piano stool in line with its heavier congeners, and implying strong packing forces at work for 1 in the solid state. Comparison with their isolelectronic group 7 tricarbonyl counterparts [CpM(CO)3] (M = Mn 4 and Re 5) illustrates that 1, 2 and 3 are sterically crowded complexes. However, a surprisingly soft bending potential is evident for the M-H moiety, whose order (1 approximately = 2 < 3) correlates with the M-H bond strength rather than with the degree of congestion at the metal centre, indicating electronic rather than steric control of the potential. The calculations also reveal cooperative motions of the hydride and carbonyl ligands in the M(CO)3H unit, which allow the M-H moiety to move freely, in spite of the closeness of the four basal ligands, helping to explain the surprising flexibility of the crowded coordination sphere observed for this family of high CN complexes.

20.
J Am Chem Soc ; 131(14): 5032-3, 2009 Apr 15.
Article in English | MEDLINE | ID: mdl-19296609

ABSTRACT

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.

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