Dehydrocoupling of Ammonia/Amine Boranes and Related Transformations Catalysed by Group 4 Metal Complexes

Ammonia borane and amine boranes are main group analogues of alkanes, which are characterised by their large gravimetric hydrogen content. This hydrogen can be released in dehydrocoupling and dehydropolymerisation reactions to obtain B-N oligomers and polymers that are of importance as precursors for functional B-N materials. Furthermore, amine boranes are potent reagents for application in transfer hydrogenation reactions, representing a versatile, easy-to-handle alternative to the use of gaseous hydrogen for the reduction of organic compounds. Compared to late transition metals, complexes of readily available and comparatively inexpensive electropositive group 4 metals have been used to a much lesser extent. This review summarises the developments in the field of dehydrocoupling of amine boranes and transfer hydrogenation with these reagents, catalysed by complexes of group 4 metals. We analyse the background for these developments using examples and reaction mechanisms and provide an outlook for future developments in this field of research.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Dehydropolymerization of Amine-Boranes using Bis(imino)pyridine Rhodium Pre-Catalysis: σ-Amine-Borane Complexes, Nanoparticles, and Low Residual-Metal BN-Polymers that can be Chemically Repurposed

The sigma amine-borane complexes [Rh(L1)(η2 :η2 -H3 B⋅NRH2 )][OTf] (L1=2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine, R=Me, Et, n Pr) are described, alongside [Rh(L1)(NMeH2 )][OTf]. Using R=Me as a pre-catalyst (1 mol %) the dehydropolymerization of H3 B ⋅ NMeH2 gives [H2 BNMeH]n selectively. Added NMeH2 , or the direct use of [Rh(L1)(NMeH2 )][OTf], is required for initiation of catalysis, which is suggested to operate through the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed at the end of catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. [Rh(L1)(NMeH2 )][OTf] is shown to operate at 0.025 mol % loadings on a 2 g scale of H3 B ⋅ NMeH2 to give polyaminoborane [H2 BNMeH]n [Mn =30,900 g/mol, Ð=1.8] that can be purified to a low residual [Rh] (6 μg/g). Addition of Na[N(SiMe3 )2 ] to [H2 BNMeH]n results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane [H2 BNMeH]3 : the chemical repurposing of a main-group polymer.

Easy Access to Tertiary Amines from Carbonyl Compounds with Substituted Amine-Boranes: A Substrate, Catalyst, and Additive-Free Approach Under Mild Conditions

Tertiary amines are ubiquitous and play an essential role in organocatalysis, pharmaceuticals, and fine chemicals. Amongst various synthetic procedures known for their synthesis, the reductive amination of carbonyl compounds has been found to be a proficient method. Over the past few decades, different synthetic strategies for reductive amination have been developed. Most of them suffer from the use of transition metals and/or harsh reaction conditions. Herein, we present an efficient, operationally simple protocol for the chemoselective transformation of carbonyl compounds to tertiary amines under benign conditions. The strategy encompasses a broad substrate scope under the metal-free condition at room temperature and does not require any solvent. A detailed mechanistic investigation was performed with the aid of control experiments and computational study to shed light on the reaction pathway.

Mechanistic insights into H3B·NMeH2 dehydrogenation by Co-based complexes: a DFT perspective

Mechanistic insights into Co-catalyzed H3B·NMeH2 dehydrogenation and polyaminoborane formation are carefully investigated using density functional theory.

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

The dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir(ⁱPr-PN^HP)} fragment [ⁱPr-PN^HP = κ³-(CH₂CH₂PⁱPr₂)₂NH] is reported. Neutral Ir(ⁱPr-PN^HP)H₃ or Ir(ⁱPr-PN^HP)H₂Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F₂C₆H₄ solution. In contrast, addition of [NMeH₃][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) to Ir(ⁱPr-PN^HP)H₃ immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with M_n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir(ⁱPr-PN^HP)(H)₂(H₂)][BAr^F₄]; σ-amine-borane [Ir(ⁱPr-PN^HP)(H)₂(H₃B·NMe₃)][BAr^F₄]; and [Ir(ⁱPr-PN^HP)(H)₂(NMeH₂)][BAr^F₄]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal–ligand cooperativity, and this is supported experimentally by N–Me substituted [Ir(ⁱPr-PN^MeP)(H)₂(H₃B·NMe₃)][BAr^F₄] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H₃B·NMeH₂ with the N–H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir(ⁱPr-N^HP)(H)₂(H₂)][BAr^F₄], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH₂ (via B–N bond cleavage), results in the formation of essentially inactive Ir(ⁱPr-PN^HP)H₃, with a coproduct of [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺. While reprotonation of Ir(ⁱPr-PN^HP)H₃ results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺/catalyst in insoluble polyaminoborane. The role of [NMeH₃]⁺/[H₂B(NMeH₂)]⁺ as chain control agents is also noted.

Calix[4]pyrrolato Aluminate Catalyzes the Dehydrocoupling of Phenylphosphine Borane to High Molar Weight Polymers

High molar weight polyphosphinoboranes represent materials with auspicious properties, but their preparation requires transition metal-based catalysts. Here, calix[4]pyrrolato aluminate is shown to induce the dehydropolymerization of phosphine boranes to high molar mass polyphosphinoboranes (up to Mn =43 000 Da). Combined GPC and 31 P DOSY NMR spectroscopic analyses, quantum chemical computations, and stoichiometric reactions disclose a P-H bond activation by the cooperative action of the square-planar aluminate and the electron-rich ligand framework. This first transition metal-free catalyst for P-B dehydrocoupling overcomes the problem of residual d-block metal impurities in the resulting polymers that might interfere with the reproducibility of the properties for this emerging class of inorganic materials.

Catalytic and mechanistic studies of a highly active and E-selective Co(II) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Herein we report the application of a Co(II) PNNH pincer catalyst system (PNNH = 2-(5-(t-butyl)-1H-pyrazol-3-yl)-6-(dialkylphosphinomethyl)pyridine) for the highly E-selective transfer semihydrogenation of internal diaryl alkynes using methanol and ammonia borane...

Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ³-(ⁱPr₂PCH₂CH₂)₂NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H₂BNMeH)_n, by dehydropolymerization of H₃B·NMeH₂. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H₃ (3), forms, which sits as an outer-sphere adduct 3·H₃BNMeH₂ as the resting state. At the end of catalysis, dormant Rh(L)H₂Cl (2) is formed. Reaction of 2 with H₃B·NMeH₂ returns 3, alongside the proposed formation of boronium [H₂B(NMeH₂)₂]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative "PNHP" ligand templates dehydrogenation, releasing H₂B═NMeH (ΔG^‡_calc = 19.6 kcal mol^-1). H₂B═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M_n ∼71 000 g mol^-1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H₂B(NMe₂H)₂]Cl as a chain transfer agent, M_n = 37 900-78 100 g mol^-1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H₃B·NMeH₂. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

Mechanistic insights into dehydrocoupling of amine boranes using dinuclear zirconocene complexes

Catalytic dehydrocoupling of H₃B·NMe₂H using Cp₂Zr(Cl)(μ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied. Spectroscopic monitoring and stoichiometric experiments show the formation and interconversion of several catalytically active Zr species.

Dehydropolymerisation of methylamine borane using highly active rhodium(iii) bis(thiophosphinite) pincer complexes: catalytic and mechanistic insights

The bis(thiophosphinite) pincer complexes [(^RPSCSP^R)Rh(py)(H)(Cl)] (^RPSCSP^R = C₆H₄–2,6-(SPR₂)₂ with R = ⁱPr, 2a and R = Ph, 2b) are highly active precatalysts for the dehydropolymerisation of methylamine borane.

Reactions of organoplatinum complexes with dimethylamine-borane

Reactions of organoplatinum complexes with dimethylamineborane are reported.