Metal-free dehydrogenation of amine-boranes by an N-heterocyclic carbene

Syntheses, Structures, and Reactivities of N-Heterocyclic Carbene-Coordinated Aminoborane Complexes

Recent research has attracted considerable attention toward N-heterocyclic carbene-coordinated boranes (NHC-borane) and their B-substituted derivatives because of their unique characteristics. In the present work, we focused on the syntheses, structures, and reactivities of such types of amine complexes, [NHC·BH₂NH₃]X ((NHC = IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and IMe (1,3-dimethylimidazol-2-ylidene); X = Cl, I, OTf). We have developed a synthetic method to access NHC·BH₂NH₂ through the reaction of NaH with [IPr·BH₂NH₃]I, which was synthesized by the reaction of IPr·BH₂I with NH₃. As a Lewis base, NHC·BH₂NH₂ could further react with HCl or HOTf to produce the corresponding salts of [IPr·BH₂NH₃]⁺. IPr·BH₂NH₂BH₂X (X = Cl, I) were synthesized by the reaction of HCl/I₂ with IPr·BH₂NH₂BH₃ and then converted to [IPr·BH₂NH₂BH₂·IPr]X (X = Cl, I) by reacting with IPr. The IMe-coordinated boranes reacted quite similarly. The preliminary results revealed that the introduction of the NHC molecule has a considerable impact on the solubility and reactivities of aminoboranes.

Visible-Light-Induced Dual C(sp3)-H Bond Functionalization of Tertiary Amine via Hydrogen Transfer to Carbene and Subsequent Cycloaddition

Herein, we describe the dual C(sp³)-H bond functionalization of a tertiary amine through hydride-transfer-induced dehydrogenation, followed by cycloaddition, using an easily preparable diazoester as a new type hydride-acceptor precursor under mild, redox-neutral conditions. With carbene as a hydrogen acceptor, this method was demonstrated by the preparation of a broad range of functionalized isoxazoldines in moderate to good yields.

Syntheses, formation mechanisms and structures of a series of linear diborazanes

Both anti and gauche conformations are observed in the linear diborazanes, which are stabilized by inter- and intramolecular DHBs, respectively.

NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

The catalytic dehydrocoupling of amine–boranes has recently received a great deal of attention due to its potential in hydrogen storage applications. The use of aluminum catalysts for this transformation would provide an additional cost-effective and sustainable approach towards the hydrogen economy. Herein, we report the use of both N-heterocyclic imine (NHI)- and carbene (NHC)-supported Al(III) hydrides and their role in the catalytic dehydrocoupling of Me2NHBH3. Differences in the σ-donating ability of the ligand class resulted in a more stable catalyst for NHI-Al(III) hydrides, whereas a deactivation pathway was found in the case of NHC-Al(III) hydrides.

Dehydrogenation of Amine-Boranes Using p-Block Compounds

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors

The divalent carbene carbon centre in cyclic (alkyl)(amino)carbenes (CAACs) is known to exhibit transition-metal-like insertion into E-H σ-bonds (E = H, N, Si, B, P, C, O) with formation of new, strong C-E and C-H bonds. Although subsequent transformations of the products represent an attractive strategy for metal-free synthesis, few examples have been reported. Herein we describe the dehydrogenation of phosphine-boranes, RR'PH·BH3, using a CAAC, which behaves as a stoichiometric hydrogen acceptor to release monomeric phosphinoboranes, [RR'PBH2], under mild conditions. The latter species are transient intermediates that either polymerise to the corresponding polyphosphinoboranes, [RR'PBH2]n (R = Ph; R' = H, Ph or Et), or are trapped in the form of CAAC-phosphinoborane adducts, CAAC·H2BPRR' (R = R' = tBu; R = R' = Mes). In contrast to previously established methods such as transition metal-catalysed dehydrocoupling, which only yield P-monosubstituted polymers, [RHPBH2]n, the CAAC-mediated route also provides access to P-disubstituted polymers, [RR'PBH2]n (R = Ph; R' = Ph or Et).

An exceptionally stable NHC complex of indane (InH3)

A study of the decomposition of the indium trihydride complex, [InH₃(IPr)], has identified a decomposition pathway mediated by proximal alkyl C–H bonds on the ligand. This prompted the synthesis of [InH₃(IPr*)], which exhibits vastly superior thermal and air stability relative to the reported InH₃ complexes.

Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources

An NHC-stabilized three-coordinate hydrosilylene dehydrogenates ammonia borane and forms more stable complexes with BH₃, BPh₃, BBr₃ and BPhBr₂ but less stable ones with BF₃, and BCl₃ for which ligand scrambling occurs.

Ionic Liquid Facilitated Dehydrogenation of tert-Butylamine Borane

The current work reports ionic liquid (IL) facilitated dehydrogenation of tert-butylamine borane (TBAB) at 90 and 105 °C. For the screening of potential IL solvent, solubility predictions of TBAB in ILs were performed by the conductor-like screening model segment activity coefficient (COSMO-SAC) model. The COSMO-SAC model predicted a logarithmic infinite dilution activity coefficient of -6.66 and -7.31 for TBAB in 1-butyl-3-methylimidazolium acetate [BMIM][OAc] and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate [TDTHP][Phosph], respectively. Hydrogen (1.95 equiv) was seen to release from TBAB/[BMIM][OAc] at 105 °C, whereas TBAB/[TDTHP][Phosph] produced 1.63 equiv of hydrogen after 360 min of dehydrogenation. The proton nuclear magnetic resonance (1H NMR) characterization of TBAB/IL systems revealed the structural integrity of ILs during dehydrogenation. Further characterization through the boron NMR (11B NMR) technique disclosed the time-resolved formation and stability of the starting compound, intermediate boron moieties, and product distribution. The 11B NMR characterization also revealed the fact that the TBAB/[TDTHP][Phosph] mixture dehydrogenates via bimolecular addition of TBAB by forming borohydride anion (-BH4 -). It was seen to oligomerize with the subsequent addition of TBAB in the oligomer chain. For the TBAB/[BMIM][OAc] system, the 11B NMR characterization could not identify the borohydride anion but confirmed a faster formation of the B=N moiety when compared to the TBAB/[TDTHP][Phosph] system. On the basis of the NMR characterization, IL-facilitated dehydrogenation mechanism of TBAB is proposed.

Generation of aminoborane monomers RR'N=BH2 from amine-boronium cations [RR'NH-BH2L](+): metal catalyst-free formation of polyaminoboranes at ambient temperature

Protonation of MeRNH·BH3 (R = Me or H) with HX (X = B(C6F5)4, OTf, or Cl), followed by immediate, spontaneous H2 elimination, yielded the amine-boronium cation salt [MeRNH·BH2(OEt2)][B(C6F5)4] and related polar covalent analogs, MeRNH·BH2X (X = OTf or Cl). These species can be deprotonated to conveniently generate reactive aminoborane monomers MeRN=BH2 which oligomerize or polymerize; in the case of MeNH2·BH3, the two step process gave poly(N-methylaminoborane), [MeNH-BH2]n.

Donor-acceptor chemistry in the main group

This Perspective article summarizes recent progress from our laboratory in the isolation of reactive main group species using a general donor-acceptor protocol. A highlight of this program is the use of carbon-based donors in combination with suitable Lewis acidic acceptors to yield stable complexes of parent Group 14 element hydrides (e.g. GeH2 and H2SiGeH2). It is anticipated that this strategy could be extended to include new synthetic targets from throughout the Periodic Table with possible applications in bottom-up materials synthesis and main group element catalysis envisioned.

Interplay of donor–acceptor interactions in stabilizing boron nitride compounds: insights from theory

The chemical bonds in donor–acceptor stabilized linear and cyclic (BN)_n (n = 1–3) adducts are examined using natural bond orbital (NBO), atoms-in-molecules (AIM), and energy decomposition (EDA-NOCV) analyses.

Reaction of a bulky amine borane with lanthanide trialkyls. Formation of alkyl lanthanide imide complexes

Lanthanide imidoborane complexes with terminal alkyl groups are synthesized from the reactions of simple lanthanide trialkyls with a bulky amine borane.

Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2N=BCl2 was characterized by single crystal X-ray diffraction.