Poisoning of Heterogeneous, Late Transition Metal Dehydrocoupling Catalysts by Boranes and Other Group 13 Hydrides

Rh–M (M: Co, Cu, and Fe) nanoclusters as highly efficient and durable catalysts for the methanolysis of ammonia borane

Herein, we report the preparation, characterization, and employment of hydroxyapatite-supported rhodium-based Rh–M (RhCo, RhCu, and RhFe) nanoclusters as cost-effective, highly efficient and reusable catalysts for hydrogen evolution from ammonia borane methanolysis.

Relativistic heavy atom effect on the 31 P NMR parameters of phosphine chalcogenides. Part 1. Chemical shifts

Four-component density functional theory calculations of ³¹ P NMR chemical shifts have been performed for the representative series of 56 phosphine chalcogenides in order to investigate an influence of different functional groups on the heavy atom relativistic effect on the NMR chemical shifts of light phosphorous atoms (Heavy Atom on Light Atom effect). The validity of the 4-component density functional theory approach used for the wide-scale calculations of the phosphorous chemical shifts in a wide series of phosphine chalcogenides has been confirmed on a small series of 5 representative compounds with the aid of high-quality coupled cluster singles and doubles calculations taking into account solvent, vibrational, and the relativistic corrections in comparison with the experiment.

Enhanced catalytic activity of the nanostructured Co-W-B film catalysts for hydrogen evolution from the hydrolysis of ammonia borane

In this work, nanostructured Co-W-B films are successfully synthesized on the foam sponge by electroless plating method and employed as the catalysts with enhanced catalytic activity towards hydrogen evolution from the hydrolysis of ammonia borane (NH₃BH₃, AB) at room temperature. The particle size of the as-prepared Co-W-B film catalysts is varied by adjusting the depositional pH value to identify the most suitable particle size for hydrogen evolution of AB hydrolysis. The Co-W-B film catalyst with the particle size of about 67.3 nm shows the highest catalytic activity and can reach a hydrogen generation rate of 3327.7 mL min^-1 g_cat^-1 at 298 K. The activation energy of the hydrolysis reaction of AB is determined to be 32.2 kJ mol^-1. Remarkably, the as-obtained Co-W-B film is also a reusable catalyst preserving 78.4% of their initial catalytic activity even after 5 cycles in hydrolysis of AB at room temperature. Thus, the enhanced catalytic activity illustrates that the Co-W-B film is a promising catalyst for AB hydrolytic dehydrogenation in fuel cells and the related fields.

Iron Catalyzed Dehydrocoupling of Amine- and Phosphine-Boranes

Catalytic dehydrocoupling methodologies, whereby dihydrogen is released from a substrate (or intermolecularly from two substrates) is a mild and efficient method to construct main group element-main group element bonds, the products of which can be used in advanced materials, and also for the development of hydrogen storage materials. With growing interest in the potential of compounds such as ammonia-borane to act as hydrogen storage materials which contain a high weight% of H2, along with the current heightened interest in base metal catalyzed processes, this review covers recent developments in amine and phosphine dehydrocoupling catalyzed by iron complexes. The complexes employed, products formed and mechanistic proposals will be discussed.

Micelle-templated synthesis of Pt hollow nanospheres for catalytic hydrogen evolution

We report an efficient, mild and simple strategy for the fabrication of colloidal hollow platinum nanospheres with the ability to tune wall-thickness and void-space over several nanometers, for application in hydrogen evolution from ammonia–borane.

Probing the second dehydrogenation step in ammonia-borane dehydrocoupling: characterization and reactivity of the key intermediate, B-(cyclotriborazanyl)amine-borane

While thermolysis of ammonia-borane (AB) affords a mixture of aminoborane- and iminoborane oligomers, the most selective metal-based catalysts afford exclusively cyclic iminoborane trimer (borazine) and its B-N cross-linked oligomers (polyborazylene). This catalysed dehydrogenation sequence proceeds through a branched cyclic aminoborane oligomer assigned previously as trimeric B-(cyclodiborazanyl)amine-borane (BCDB). Herein we utilize multinuclear NMR spectroscopy and X-ray crystallography to show instead that this key intermediate is actually tetrameric B-(cyclotriborazanyl)amine-borane (BCTB) and a method is presented for its selective synthesis from AB. The reactivity of BCTB upon thermal treatment as well as catalytic dehydrogenation is studied and discussed with regard to facilitating the second dehydrogenation step in AB dehydrocoupling.

Amine-capped Co nanoparticles for highly efficient dehydrogenation of ammonia borane

Highly efficient heterogeneous catalysts are desired for the development of new energy storage materials. The rational choice and use of capping ligands are of significant importance for performance optimization of metal nanoparticle (NP) catalysts. By exploiting amine-rich polyethylenimine (PEI) and graphene oxide (GO) as a NP support, we demonstrate that as a capping ligand, PEI deposited on GO provides a novel pathway able to simultaneously control the morphology, spatial distribution, surface active sites of cobalt (Co) NPs, and remarkably enhances their catalytic properties for the hydrolytic dehydrogenation of ammonia borane (AB). Such a synergistic effect enables the synthesized PEI-GO/Co catalysts to reveal extremely high dehydrogenation activities under atmosphere condition. A total turnover frequency of 39.9 molH2 min(-1) molCo(-1) and an apparent activation energy of 28.2 kJ mol(-1) make the catalytic performance of these PEI-GO/Co catalysts comparable to those of noble metal-based catalysts, including bimetallic and multimetallic catalysts.

Tetraglyme-mediated synthesis of Pd nanoparticles for dehydrogenation of ammonia borane

Palladium nanoparticles (PdNPs) were conveniently prepared in tetraglyme (TG) solution using a variety of palladium precursors. At 140 °C, TG promoted Pd(3)(OAc)(6) to produce irregular shaped PdNPs with an average size of 4 nm. When these PdNPs were re-dispersed in TG and used for the dehydrogenation of ammonia borane (AB) at 85 °C, remarkably enhanced catalytic performance was achieved to release 2.3 equiv. of H(2) in 1 h.

Retracted Article: Honeycomb-like Co–B amorphous alloy catalysts assembled by a solution plasma process show enhanced catalytic hydrolysis activity for hydrogen generation

A novel solution plasma process was developed to assemble honeycomb-like Co–B amorphous alloy catalysts with 253.33 m² g⁻¹ specific surface area using triethanolamine (TEA) as a soft template.

Probing the intrinisic structure and dynamics of aminoborane coordination at late transition metal centers: mono(σ-BH) binding in [CpRu(PR3)2(H2BNCy2)]+

Aminoboranes, H(2)BNRR', represent the monomeric building blocks from which novel polymeric materials can be constructed via metal-mediated processes. The fundamental capabilities of these compounds to interact with metal centers have been probed through the coordination of H(2)BNCy(2) at 16-electron [CpRu(PR(3))(2)](+) fragments. In contrast to the side-on binding of isoelectronic alkene donors, an alternative mono(σ-BH) mode of aminoborane ligation is established for H(2)BNCy(2), with binding energies only ~8 kcal mol(-1) greater than those for analogous dinitrogen complexes. Variations in ground-state structure and exchange dynamics as a function of the phosphine ancillary ligand set are consistent with chemically significant back-bonding into an orbital of B-H σ* character.

Dehydrogenation of saturated CC and BN bonds at cationic N-heterocyclic carbene stabilized M(III) centers (M = Rh, Ir)

Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)(2)(H)(2)Cl [M = Rh, Ir; NHC = IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes = N,N'-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)(2)(H)(2)Cl (1) with Na[BAr(f)(4)] in fluorobenzene generates [Ir(IPr)(2)(H)(2)](+)[BAr(f)(4)](-) (4) in which the iridium center is stabilized by a pair of agostic interactions utilizing the methyl groups of the isopropyl substituents. After a prolonged reaction period C-H activation occurs, ultimately leading to the dehydrogenation of one of the carbene (i)Pr substituents and the formation of [Ir(IPr)(IPr'')(H)(2)](+)[BAr(f)(4)](-) (5), featuring the mixed NHC/alkene donor IPr'' ligand. By contrast, the related IMes complexes M(IMes)(2)(H)(2)Cl (M = Rh, Ir), which feature carbene substituents lacking beta-hydrogens, react with Na[BAr(f)(4)] in fluorobenzene to give rare examples of NaCl inclusion compounds, viz., [M(IMes)(2)(H)(2)Cl(Na)](+)[BAr(f)(4)](-) (M = Rh, 6; M = Ir, 7). Intercalation of the sodium cation between the mesityl aromatic rings of the two NHC donors has been demonstrated by crystallographic studies of 7. Synthetically, 6 and 7 represent convenient yet highly reactive sources of the putative 14-electron [M(NHC)(2)(H)(2)](+) cations, readily eliminating NaCl in the presence of potential donors. Thus 7 can be employed in the synthesis of the dinitrogen complexes [Ir(IMes)(2)(N(2))(2)](+)[BAr(f)(4)](-) (8a) and [Ir(IMes)(2)(N(2))THF](+)[BAr(f)(4)](-) (8b) (albeit with additional loss of H(2)) by stirring in toluene under a dinitrogen atmosphere and recrystallization from the appropriate solvent system. The interactions of 6 and 7 with primary, secondary, and tertiary amineboranes have also been investigated. Although reaction with the latter class of reagent simply leads to coordination of the amineborane at the metal center via two M-H-B bridges {and formation, for example, of the 18-electron species [M(IMes)(2)(H)(2)(mu-H)(2)B(H).NMe(3)](+)[BAr(f)(4)](-) (M = Rh, 9; M = Ir, 10)}, the corresponding reactions with systems containing N-H bonds proceed via dehydrogenation of the BN moiety to give complexes containing unsaturated aminoborane ligands. Thus, for example, 6 catalyzes the dehydrogenation of R(2)NH x BH(3) (R = (i)Pr, Cy) in fluorobenzene solution (100% conversion over 6 h at 2 mol % loading) to give R(2)NBH(2); the organometallic complex isolated at the end of the catalytic run in each case is shown to be [Rh(IMes)(2)(H)(2)(mu-H)(2)BNR(2)](+)[BAr(f)(4)](-) (R = (i)Pr, 11; R = Cy, 12). In contrast to isoelectronic alkene donors, the aminoborane ligand in these complexes (and in the corresponding iridium compounds 13 and 14) can be shown by crystallographic methods to bind in end-on fashion via a bis(sigma-borane) motif. Similar dehydrogenation chemistry is applicable to the primary amineborane (t)BuNH(2) x BH(3), although in this case the rate of rhodium-catalyzed dehydrogenation is markedly slower. This enables the amineborane complex [Rh(IMes)(2)(H)(2)(mu-H)(2)B(H) x NH(2)(t)Bu](+)[BAr(f)(4)](-) (15) to be isolated at short reaction times (ca. 6 h) and the corresponding (dehydrogenated) aminoborane system [Rh(IMes)(2)(H)(2)(mu-H)(2)BNH(t)Bu](+)[BAr(f)(4)](-) (16) to be isolated after an extended period (ca. 48 h). As far as further reactivity is concerned, aminoborane systems such as 14 show themselves to be amenable to further dehydrogenation chemistry in the presence of tert-butylethylene leading ultimately to the dehydrogenation of the boron-containing ligand and to the formation of a directly Ir-B bonded system described by limiting boryl (Ir-B) and borylene (Ir=B) forms.

Zeolite-confined ruthenium(0) nanoclusters catalyst: record catalytic activity, reusability, and lifetime in hydrogen generation from the hydrolysis of sodium borohydride

Sodium borohydride, NaBH4, has been considered the most attractive hydrogen-storage material for portable fuel cell applications, as it provides a safe and practical means of producing hydrogen. In a recent communication (Zahmakiran, M.; Ozkar, S. Langmuir 2008, 24, 7065), we have reported a record total turnover number (TTON) of 103 200 mol H2/mol Ru and turnover frequency (TOF) up to 33 000 mol H2/mol Ru x h obtained by using intrazeolite ruthenium(0) nanoclusters in the hydrolysis of sodium borohydride. Here we report full details of the kinetic studies on the intrazeolite ruthenium(0) nanoclusters catalyzed hydrolysis of sodium borohydride in both aqueous and basic solutions. Expectedly, the intrazeolite ruthenium(0) nanoclusters show unprecedented catalytic lifetime, TTON = 27 200 mol H2/mol Ru, and TOF up to 4000 mol H2/mol Ru x h in the hydrolysis of sodium borohydride in basic solution (5% wt NaOH) as well. More importantly, the intrazeolite ruthenium(0) nanoclusters are isolable, bottleable, redispersible, and yet catalytically active. They retain 76% or 61% of their initial catalytic activity at the fifth run with a complete release of hydrogen in aqueous and basic medium, respectively. The intrazeolite ruthenium(0) nanoclusters were isolated as black powder and characterized by using a combination of advanced analytical techniques including XRD, HRTEM, TEM-EDX, SEM, XPS, ICP-OES, and N2 adsorption.

Transition Metal-Catalyzed Dissociation of Phosphine−Gallane Adducts: Isolation of Mechanistic Model Complexes and Heterogeneous Catalyst Poisoning Studies

Attempts to induce the catalytic dehydrocoupling of the phosphine-gallane adduct Cy2PH.GaH3 (Cy=cyclohexyl) (1) by treatment with ca. 5 mol% of either the Rh(I) complex [{Rh(mu-Cl)(1,5-cod)}2] (cod=cyclooctadiene) or the Rh(0) species Rh/Al2O3 and [Oct4N]Cl-stabilized colloidal Rh led to catalytic P-Ga bond cleavage to generate the phosphine, H2, and Ga metal. Interestingly, subsequent treatment of the reaction mixtures with Me2NH.BH3 failed to lead to the formation of [Me2N-BH2]2 via Rh-catalyzed dehydrocoupling, which suggested that catalyst deactivation was taking place. Poisoning studies involving the treatment of the active Rh(0) catalyst with Cy2PH, PMe3, or GaH3.OEt2 showed that deactivation indeed occurred as the dehydrocoupling of Me2NH.BH3 either dramatically decreased in rate or did not take place at all. The X-ray photoelectron spectroscopy analysis of colloidal Rh(0) that had been treated with Cy2PH and PMe3 confirmed the presence of phosphorus on the catalyst surface in each case, consistent with catalyst poisoning via phosphine ligation. A mechanism for the Rh-catalyzed P-Ga bond cleavage reaction of 1 and Me3P.GaH3 (2) is proposed and involves the initial reaction of Ga-H bonds with the Rh colloid surface, which weakens and ultimately breaks the P-Ga bond. The reasonable nature of this mechanism is supported by a model reaction between the zerovalent group 9 complex Co2(CO)8 and 2 which afforded Me3P.Ga[Co(CO)4]3 (3). Consistent with the elongated and thus weakened P-Ga bond in 3, solutions of this species in Et2O subsequently form the known complex [(Me3P)Co(CO)3]2 (4) and Ga metal after 4 h at 25 degrees C.

Highly Efficient Colloidal Cobalt- and Rhodium-Catalyzed Hydrolysis of H3N·BH3 in Air

The compound H3N.BH3 (1) is currently attracting considerable attention as a potential hydrogen storage material. Group 9 catalysts which rapidly and conveniently hydrolyze aqueous 1 in air are described. When treated with 1 mol % [{Rh(mu-Cl)(1,5-cod)}2] (cod=cyclooctadiene) in air, aqueous 1 undergoes rapid hydrolysis to afford the ionic species [NH4][BO2] in approximately 40 s. Higher catalyst loadings (3 mol %) result in a reduction in reaction time to 10 s. Quantification of the hydrogen evolved revealed that, on average, 2.8 of a maximum possible 3.0 equivalents (93%) were generated during the course of the reaction. Rh(0) species (e.g., Rh black, Rh stabilized on alumina, aqueous Rh colloids) were also found to be active hydrolysis catalysts, and evidence for a heterogeneous mechanism is provided. Significantly, although Ir(0) colloids are less active, aqueous Co(0) colloids are also effective catalysts for this process. This result is particularly important as Co, a first-row metal, is considerably more economical than the precious metal catalysts typically employed.

Transition-Metal-Catalyzed Dehydrocoupling: A Convenient Route to Bonds between Main-Group Elements

The development of transition-metal-catalyzed dehydrocoupling reactions as a synthetic method for the formation of main-group element-element bonds provides an increasingly attractive and convenient alternative to traditional routes such as salt metathesis/elimination-type reactions. Since the first reported examples in the early 1980s, there has been a rapid expansion of this field, with extensions to a wide variety of metal-mediated homonuclear and heteronuclear bond-forming processes. Applications of this new chemistry in molecular and polymer synthesis, materials science, hydrogen storage and the transfer hydrogenation of organic substrates are attracting growing attention. An overview of this emerging area is presented in this Concepts article with a focus on recent results.

Efficient Catalysis of Ammonia Borane Dehydrogenation

In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.