4. Hydrogen Release from Ammonia-Borane 4087 4.1. General Requirements for Use of Ammonia-Borane as a Fuel for the Production of H 2 in Fuel Cells 4087 4.2. Thermal Decomposition of Ammonia-Borane 4087 4.2.1. Thermal Decomposition of Ammonia-Borane in the Solid State 4087 4.

Hydrogen storage for transportation applications requires high volumetric and gravimetric storage capacity. B-N compounds are well suited as storage materials due to their light weight and propensity for bearing multiple protic (N-H) and hydridic (B-H) hydrogens. This critical review briefly covers the various methods of hydrogen storage, and then concentrates on chemical hydrogen storage using B-N compounds. The simplest B-N compound, ammonia borane (H3NBH3), which has a potential 19.6 wt% hydrogen storage capacity, will be emphasised (127 references).

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The catalytic dehydrocoupling/dehydrogenation of N-methylamine-borane, MeNH(2)·BH(3) (7), to yield the soluble, high molecular weight poly(N-methylaminoborane) (8a), [MeNH-BH(2)](n) (M(W) > 20 000), has been achieved at 20 °C using Brookhart's Ir(III) pincer complex IrH(2)POCOP (5) (POCOP = [μ(3)-1,3-(OPtBu(2))(2)C(6)H(3)]) as a catalyst. The analogous reaction with ammonia-borane, NH(3)·BH(3) (4), gave an insoluble product, [NH(2)-BH(2)](n) (8d), but copolymerization with MeNH(2)·BH(3) gave soluble random copolymers, [MeNH-BH(2)](n)-r-[NH(2)-BH(2)](m) (8b and 8c). The structures of polyaminoborane 8a and copolymers 8b and 8c were further analyzed by ultrahigh resolution electrospray mass spectrometry (ESI-MS), and 8a, together with insoluble homopolymer 8d, was also characterized by (11)B and (1)H solid-state NMR, IR, and wide-angle X-ray scattering (WAXS). The data indicate that 8a-8c are essentially linear, high molecular weight materials and that the insoluble polyaminoborane 8d possesses a similar structure but is of lower molecular weight (ca. 20 repeat units), presumably due to premature precipitation during its formation. The yield and molecular weight of polymer 8a was found to be relatively robust toward the influence of different temperatures, solvents, and adduct concentrations, while higher catalyst loadings led to higher molecular weight materials. It was therefore unexpected that the polymerization of 7 using 5 was found to be a chain-growth rather than a step-growth process, where high molecular weights were already attained at about 40% conversion of 7. The results obtained are consistent with a two stage polymerization mechanism where, first, the Ir catalyst 5 dehydrogenates 7 to afford the monomer MeNH═BH(2) and, second, the same catalyst effects the subsequent polymerization of this species. A wide range of other catalysts based on Ru, Rh, and Pd were also found to be effective for the transformation of 7 to polyaminoborane 8a. For example, polyaminoborane 8a was even isolated from the initial stage of the dehydrocoupling/dehydrogenation of 7 with [Rh(μ-Cl)(1,5-cod)](2) (2) as the catalyst at 20 °C, a reaction reported to give the N,N,N-trimethyl borazine, [MeN-BH](3), under different conditions (dimethoxyethane, 45 °C). The ability to use a variety of catalysts to prepare polyaminoboranes suggests that the synthetic strategy should be applicable to a broad range of amine-borane precursors and is a promising development for this new class of inorganic polymers.

The B all and N all: Soluble, linear, high molecular weight polyaminoborane homopolymers and copolymers have been synthesized by iridium‐catalyzed dehydrocoupling of readily available amine–borane adducts RNH2⋅BH3 (R=H, Me, nBu).

The efficient catalytic dehydrocoupling of a range of amine-borane adducts, R'RNH x BH(3) (R' = R = Me 1a; R' = R = (i)Pr 1b; R' = Me, R = CH(2)Ph 1c) by a series of group 4 metallocene type precatalysts has been demonstrated. A reduction in catalytic activity was detected upon descending the group and also on substitution of the cyclopentadienyl (Cp) ligands with sterically bulky or electron-donating substituents. Precatalysts Cp(2)TiCl(2)/2(n)BuLi and Cp(2)Ti(PMe(3))(2), which are believed to act as precursors to [Cp(2)Ti], were found to promote the transformation of 1a to [Me(2)N-BH(2)](2) (3a) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me(2)NH-BH(2)-NMe(2)-BH(3) (2a) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer 3a. Synthesis of the (2)H-isotopologues of 1a allowed the extraction of phenomenological kinetic isotope effects for 1a --> 2a and 2a --> 3a from initial rate data, which permitted the proposal of a catalytic cycle with plausible intermediates. Support for the presence of an active Ti(II) catalyst was provided by the lack of reactivity of Ti(III) complexes TiCl(3) and Cp(2)TiCl or Ti(0) in the form of THF soluble colloids or bulk Ti powder toward 1a or 1b. Modeling of the rates of consumption of 1a and formation of 3a during catalysis by Cp(2)Ti(PMe(3))(2) supported this conclusion and allowed the proposal of a two cycle, four step reaction mechanism. The proposed first cycle generates 2a in a two step process. In the second cycle, interaction of 2a with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form 3a, also in a two step process.

A transportation device can be tunably controlled by the ultraviolet actuation of a liquid crystal elastomer.

Irradiation of silicon-bridged [1]ferrocenophane [Fe(eta-C(5)H(4))(2)SiMe(2)] (1) in the presence of substitutionally labile Lewis bases such as 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy) initiates ring-opening polymerization and oligomerization via cleavage of an iron-cyclopentadienyl bond. A distribution of cyclic polyferrocenylsilane c-PFS (PFS = [Fe(eta-C(5)H(4))(2)SiMe(2)](n)) and a series of cyclic oligomers (2(2)-2(7)) were isolated by column chromatography and fully characterized. Varying temperature and concentration were found to influence the molecular weight distribution and the ratio of polymer to oligomer products, enabling the formation of c-PFS with molecular weights >100 kDa. Cyclic polymer samples were found to possess lower hydrodynamic radii and viscosity and higher glass transition temperatures than those of their linear PFS counterparts (l-PFS) of comparable molecular weight. Compared with crystalline samples of l-PFS of similar molecular weights, c-PFS formed smaller spherulites, as observed by polarizing optical microscopy. While the wide-angle X-ray scattering (WAXS) patterns from lower molecular weight l-PFS were found to differ from those from higher molecular weight samples, those obtained for lower and higher molecular weight samples of c-PFS are identical and resemble diffraction patterns of high molecular weight l-PFS. The electrochemical behavior of each cyclic oligomer 2(2)-2(7) was studied by cyclic and differential pulse voltammetry and was found to depend on whether the oligomer contains an odd or even number of ferrocene units. In contrast to linear analogs, two reversible redox processes of varying intensities were observed for cyclic oligomers containing an even number of iron centers, while three reversible redox processes of varying intensities were observed for cyclic oligomers containing an odd number of iron centers. As the oligomer chain length increased, the electrochemical behavior of all cyclic oligomers approached that of both cyclic and linear high molecular weight polymers.