Ammonia–Borane and Amine–Borane Dehydrogenation Mediated by Complex Metal Hydrides

Two pathways of proton transfer reaction to (triphos)Cu(η(1)-BH4) via a dihydrogen bond [triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane]

The interaction of the η(1)-tetrahydroborate copper(i) complex (triphos)Cu(η(1)-BH4) () with proton donors [CF3CH2OH (TFE), (CF3)2CHOH (HFIP), (CF3)3COH (PFTB), PhOH, p-NO2C6H4OH (PNP), p-NO2C6H4N[double bond, length as m-dash]NC6H4OH (PNAP), CF3OH] was a subject of a combined IR spectroscopic and theoretical investigation. Spectral (Δν) and thermodynamic (ΔH) parameters of dihydrogen bond (DHB) formation were determined experimentally. The terminal hydride ligand (characterized by the basicity factor Ej(BH) = 0.87 ± 0.01) is found to be a site of proton transfer which begins with nucleophilic substitution of BH4(-) by the alcohol oxygen atom on the copper center (BH pathway). The activation barrier computed for (CF3)2CHOH in CH2Cl2 - ΔG = 20.6 kcal mol(-1) - is in good agreement with the experimental value (ΔG = 20.0 kcal mol(-1)). An abnormal dependence of the reaction rate on the proton donor strength found experimentally in dichloromethane is explained computationally on the basis of the variation of the structural and energetic details of this process with the proton donor strength. In the second reaction mechanism found (CuH pathway), DHB complexes with the initial ROH coordination to the bridging hydride lead to B-Hbr bond cleavage with BH3 elimination. "Copper assistance" via the CuO interaction is not involved. This mechanism can be evoked to explain the occurrence of proton transfer in coordinating solvents.

General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-Based Organic Polymers

Bimetallic Pd-Au nanoparticles (NPs) usually show superior catalytic performances over their single-component counterparts, the general and facile synthesis of subnanometer-scaled Pd-Au NPs still remains a great challenge, especially for electronegative ultrafine bimetallic NPs. Here, we develop an anion-exchange strategy for the synthesis of ultrafine Pd-Au alloy NPs. Simple treatment of main-chain imidazolium-based organic polymer (IOP) with HAuCl₄ and Na₂PdCl₄, followed by reduction with NaBH₄ generated Pd-Au alloy NPs (Pd-Au/IOP). These NPs possess an unprecedented tiny size of 1.50 ± 0.20 nm and are uniformly dispersed over IOP. The electronic structure of the surface Pd and Au atoms is optimized via electron exchange during alloying, a net charge flowing resulting from counteranions is injected into Au and Pd to form a strong ensemble effect, which is responsible for a remarkably higher catalytic activity of Pd-Au/IOP in the hydrolytic dehydrogenation of ammonia borane than those of monometallic counterparts.

Cu/Ni nanoparticles supported on TiO2(B) nanotubes as hydrogen generation photocatalysts via hydrolysis of ammonia borane

Cu_0.64Ni_0.36-TiO₂(B) NTs with a lower bandgap energy show a high H₂ generation rate in the hydrolysis of AB.

Dehydrogenation of dimethylamine-borane mediated by Group 1 pincer complexes

Alkali metal carbazolido complexes are precatalysts for the dehydrogenation of Me₂NH·BH₃, where the cation plays a vital role in the reaction outcome.

Phosphine-free cobalt pincer complex catalyzed Z-selective semi-hydrogenation of unbiased alkynes

A molecularly defined NNN-type cobalt pincer complex catalyzed semi-hydrogenation of unbiased alkynes to Z-selective alkenes is reported. The reaction operates at a low temperature (50–80 °C), phosphine ligand-free, and base-free conditions with no additive required.

SE2 reaction in noncarbon system: Metal-halide catalysis for dehydrogenation of ammonia borane

An electrophilic substitution (SE) reaction of BN isosteres has been investigated for the dehydrogenation of ammonia borane (AB) by metal chlorides (MCl2) using various ab initio calculations. In contrast to the typical SE reaction occurring at the carbon atom, the nitrogen atom in AB serves as the reaction center for the SE reaction with the boron moiety as the leaving group when the MCl2 approaches the AB. The SE2 backside reaction is favored as a trigger step for the dehydrogenation of AB by the MCl2 The SE2 reaction is found for 3d-transition-metal chlorides (e.g., FeCl2, CoCl2, NiCl2, CuCl2, and ZnCl2), while PdCl2 leads to the dehydrogenation of AB by a direct B-H σ-bond activation, similar to most organometallic catalysts. Interestingly, the polymerization of AB promoted by MCl2 can be explained with the similar SE2 mechanism, and the dehydrogenation of the BN derivative 3-methyl-1,2-BN-cyclopentane (CBN) bearing a carbon backbone ring also follows the SE2 reaction. In particular, the experimental observation that the use of metal-chloride catalysis decreases the by-products obtained during the hydrogenation of AB can be explained by our mechanism involving the SE2 reaction. This work is helpful for the development of novel metal-halide catalysts for practical hydrogen storage materials, including the BN moiety.

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.

Group 4 Half-Sandwich Tris(trimethylsilylmethyl) Complexes: Thermal Decomposition and Reactivity with N,N-Dimethylamine-Borane

The thermal decomposition of group 4 trimethylsilylmethyl derivatives [M(η⁵-C₅Me₅)(CH₂SiMe₃)₃] (M = Ti (1), Zr (2), Hf (3)) in solution and their reactivity with N,N-dimethylamine-borane were investigated. Heating of hydrocarbon solutions of compounds 2 and 3 at 130-200 °C results in the elimination of SiMe₄ and the clean formation of the singular alkylidene-alkylidyne zirconium and hafnium compounds [{M(η⁵-C₅Me₅)}₃{(μ-CH)₃SiMe}(μ₃-CSiMe₃)] (M = Zr (4), Hf (5)). The reaction of 2 and 3 with NHMe₂BH₃ (≥1 equiv) at room temperature affords the dialkyl(dimethylamidoborane) complexes [M(η⁵-C₅Me₅)(CH₂SiMe₃)₂(NMe₂BH₃)] (M = Zr (6), Hf (7)). Compounds 6 and 7 are unstable in solution and decompose with formation of the alkyl(dimethylamino)borane [B(CH₂SiMe₃)H(NMe₂)] (8), SiMe₄, and other minor byproducts, including the tetranuclear zirconium(III) octahydride complex [{Zr(η⁵-C₅Me₅)}₄(μ-H)₈] (9) in the decomposition of 6. Addition of NHMe₂BH₃ to the titanium tris(trimethylsilylmethyl) derivative 1 gives the trinuclear mixed valence Ti(II)/Ti(III) tetrahydride complex [{Ti(η⁵-C₅Me₅)(μ-H)}₃(μ₃-H)(μ₃-NMe₂BH₂)] (10) at 45-65 °C. While the complete conversion of 1 under argon atmosphere requires excess NHMe₂BH₃ (up to 15 equiv), complex 10 is readily prepared with 3 equiv of NHMe₂BH₃ under a hydrogen atmosphere indicating that the formation of 10 involves hydrogenolysis of 1 in the presence of (NMe₂BH₂)₂. In absence of amine-borane, the reaction of 1 with H₂ leads to the tetranuclear titanium(III) octahydride [{Ti(η⁵-C₅Me₅)}₄(μ-H)₈] (11), which upon addition of NHMe₂BH₃ and subsequent heating at 65 °C affords complex 10. The X-ray crystal structures of 2, 4, 5, 10, and 11 were determined.

H/D scrambling in a chromium-catalyzed dehydrocoupling reaction of a borane-dimethylamine adduct

H/D scrambling took place in a chromium-catalyzed dehydrocoupling reaction of a deuterium-labeled borane-dimethylamine adduct. In the hydrogen elimination of BH₃·NDMe₂ (1a-dN), H₂, HD and D₂ were generated in 65 : 30 : 5 ratio, and 62% of deuterium atoms were incorporated into the major product, the dimethylaminoborane dimer. Proton and deuteron nuclei were thus concentrated into the evolved dihydrogen and aminoborane dimer, respectively. The mechanism of H/D scrambling is understood based on the reaction pathway of the dehydrocoupling of 1a, which was previously proposed based on DFT calculations. The H/D distribution in the products is explained by the energy difference according to the deuterated position in an intermediate of the dehydrocoupling reaction.

Isolable zirconium hydride species in the reaction of amido complexes with amine-boranes

Mono-, di- and trinuclear zirconium hydride species have been isolated in the treatment of amido complexes [Zr(η⁵-C₅Me₅)(NMe₂)_nCl_3-n] (n = 3, 1) with amine-borane adducts NHR₂BH₃ (R₂ = Me₂, HtBu). The reactions involve the formation of amidoborane ligands with ZrH-B interactions which readily undergo β-hydride elimination to give hydride functions.

Dehydrogenative B-H/C(sp3 )-H Benzylic Borylation within the Coordination Sphere of Platinum(II)

The first examples of stoichiometric dehydrogenative B-H/C(sp³ )-H benzylic borylation reactions, which are of relevance to catalytic methylarene (di)borylation, are reported. These unusual transformations involving a (κ² -P,N)Pt(η³ -benzyl) complex, and either pinacolborane or catecholborane, proceed cleanly at room temperature. Density functional calculations suggest that borylation occurs via successive σ-bond metathesis steps, whereby a Pt^II -H intermediate engages in C(sp³ )-H bond activation-induced dehydrogenation.

Probing a General Rule towards Thermodynamic Stabilities of Mono BN-doped Lower Polyenes

The BN-doped organic analogues are interesting as aliphatic amineboranes for hydrogen storage, precursors for aromatic borazines and adsorbent cage azaboranes. However, BN-doped aliphatic polyenes remained undeveloped. Herein, we perform theoretical calculations on two mono BN-doped aliphatic lower polyenes, 1,3-butadiene and 1,3,5-hexatriene. A general rule is proposed, i.e., isomers with terminal nitrogen and directly BN-connected, N-B(R), in particular, are of significant thermodynamic stability as compared with their inverse analogues (where boron is at the terminal position). The N-B(R) type isomers are found to be the most stable ones in both polyenes. Isomers with terminal B and N are of intermediate stability. Highly destabilized isomers are those with one terminal methylene group and one terminal heteroatom in the butadiene series, and two terminal methylene groups in the hexatriene series. Rules established here may lead researchers to synthesize isomers with particular thermodynamic stability.

Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility

One of the greatest challenges in using H₂ as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density (19.6 wt %) and ease of handling and transport. Hydrogen release from ammonia borane is mediated by either hydrolysis, thus giving borate products that are difficult to rereduce, or direct dehydrogenation. Catalytic AB dehydrogenation has thus been a popular topic in recent years, motivated both by applications in hydrogen storage and main group synthetic chemistry. This Account is a complete description of work from our laboratory in ruthenium-catalyzed ammonia borane dehydrogenation over the last 6 years, beginning with the Shvo catalyst and resulting ultimately in the development of optimized, leading catalysts for efficient hydrogen release. We have studied AB dehydrogenation with Shvo's catalyst extensively and generated a detailed understanding of the role that borazine, a dehydrogenation product, plays in the reaction: it is a poison for both Shvo's catalyst and PEM fuel cells. Through independent syntheses of Shvo derivatives, we found a protective mechanism wherein catalyst deactivation by borazine is prevented by coordination of a ligand that might otherwise be a catalytic poison. These studies showed how a bidentate N-N ligand can transform the Shvo into a more reactive species for AB dehydrogenation that minimizes accumulation of borazine. Simultaneously, we designed novel ruthenium catalysts that contain a Lewis acidic boron to replace the Shvo -OH proton, thus offering more flexibility to optimize hydrogen release and take on more general problems in hydride abstraction. Our scorpionate-ligated ruthenium species (12) is a best-of-class catalyst for homogeneous dehydrogenation of ammonia borane in terms of its extent of hydrogen release (4.6 wt %), air tolerance, and reusability. Moreover, a synthetically simplified ruthenium complex supported by the inexpensive bis(pyrazolyl)borate ligand is a comparably good catalyst for AB dehydrogenation, among other reactions. In this Account, we present a detailed, concise description of how our work with the Shvo system progressed to the development of our very reactive and flexible dual-site boron-ruthenium catalysts.

Elongated Dihydrogen versus Compressed Dihydride in Osmium Complexes

Small modifications on the co-ligands of complexes containing two coordinated hydrogen atoms can determine the elongated dihydrogen versus compressed dihydride nature of these species and therefore their chemical behavior. 2,6-diphenylpyridine favors the formation of the osmium(IV) cation [OsH₂ (C₆ H₄ pyPh)(PiPr₃ )₂ ]⁺ , whereas 2-phenoxy-6-phenylpyridine, which contains an oxygen atom between the heterocycle and one of the phenyl groups, stabilizes the osmium(II) elongated dihydrogen species [Os(C₆ H₄ pyOPh)(η² -H₂ )(PiPr₃ )₂ ]⁺ . In contrast to the latter, the former shows a marked tendency to undergo reductive elimination of the heterocycle.

Copper(i)-catalysed transfer hydrogenations with ammonia borane

Highly Z-selective alkyne transfer semihydrogenations and conjugate transfer hydrogenations of enoates can be effected by employing a readily available copper(i)/N-heterocyclic carbene (NHC) complex, [IPrCuOH], in combination with ammonia borane as a H₂ equivalent.

Boron–nitrogen main chain analogues of polystyrene: poly(B-aryl)aminoboranes via catalytic dehydrocoupling

High molecular weight B-arylated polyaminoboranes are obtained via catalytic dehydropolymerisation of B-aryl amine–boranes and represent the first inorganic polystyrene analogues with a B–N main chain.

The superior desorption properties of MgCl2-added ammonia borane compared to MgF2-added systems—the unexpected role of MgCl2 interacting with [NH3] units

MgCl₂ unexpectedly interacts with [NH₃] units in NH₃BH₃, analogous to the amine complex of Mg(NH₃)_xCl₂, resulting in superior desorption.

Disilanes as oxygen scavengers and surrogates of hydrosilanes suitable for selective reduction of nitroarenes, phosphine oxides and other valuable substrates

In this report, we demonstrate that the reaction of nitroarenes with hexamethyldisilane under various conditions affords a different range of compounds with excellent selectivity.

Bond dissociation energy controlled σ-bond metathesis in alkaline-earth-metal hydride catalyzed dehydrocoupling of amines and boranes: a theoretical study

Alkaline-earth-metal could catalyse the dehydrocoupling procedure of N–H and B–H bond due to the low Ae–H bond energy. The direct σ-bond metathesis procedure is proved to be unfavourable.