Catalytic Synthesis, Characterization, and Properties of Polyaminoborane Homopolymers and Random Copolymers

Manganese catalysed reduction of nitriles with amine boranes

The room temperature reduction of various nitriles using amine boranes (ABs) catalysed by a manganese(i) alkyl complex is described. Based on experimental findings, a plausible mechanistic scenario is presented. This includes the presence of two catalytic cycles, one for productive reduction of nitriles and one for hydrogen evolution.

Catalyst-Free Transfer Hydrogenation from Amine-Borane Small Oligomers

Amine-borane dimers and oligomers with varied steric and electronic profiles were prepared via capping agent-controlled AA/BB polycondensations. They were used for transfer hydrogenations to aldehydes, ketones, imines as well as electron-poor alkene/alkyne moieties. The amine-borane Lewis-paired oligomers and the congested bis(amine-borane)s provided the highest yields. This was likely helped by facilitated dissociation (oligomers) or H-bond assistance. In the case of the oligomers, the second equivalent of H2 present was also engaged in the reaction. Solid-state NMR characterization provides evidence that the boron-containing materials obtained after transfer dehydrogenation are highly similar to those obtained from thermal dehydrogenation. The oligomers bridge the gap between simple amine-borane molecular reductants and the poly-amine-boranes and provide a full picture of the reactivity changes at the different scales.

A Robust, Divalent, Phosphaza-bicyclo[2.2.2]octane Connector Provides Access to Cage-Dense Inorganic Polymers and Networks

While polymers containing chain or ring motifs in their backbone are ubiquitous, those containing well-defined molecular cages are very rare and essentially unknown for the inorganic elements. We report that a rigid and dinucleophilic cage (PNSiMe₃)₂(NMe)₆, which is chemically robust and accessible on a multi-gram scale from commercial precursors, serves as a linear and divalent connector that forms cage-dense inorganic materials. Reaction of the cage with various ditopic P(III) dihalide comonomers proceeded via Me₃SiCl elimination to give high molecular weight (30 000-70 000 g mol^-1), solution-processable polymers that form free-standing films. The end groups of the polymers could be tuned to engender orthogonal reactivity and form block copolymers. Networked cage-dense materials could be accessed by using PCl₃ as a tritopic P(III) linker. Detailed mechanistic studies implicate a stepwise polycondensation that proceeds via phosphino-phosphonium ion intermediates, prior to Me₃SiCl loss. Thus, metathesis between the dinucleophilic cage and polyhalides represents a general strategy to making cage-dense polymers, setting the stage for systematically understanding the consequences of the three-dimensional microstructure on macroscopic material properties.

Polyphosphinoborane Block Copolymer Synthesis Using Catalytic Reversible Chain-Transfer Dehydropolymerization

An amphiphilic block copolymer of polyphosphinoborane has been prepared by a mechanism-led strategy of the sequential catalytic dehydropolymerization of precursor monomers, H₃ B ⋅ PRH₂ (R=Ph, n-hexyl), using the simple pre-catalyst [Rh(Ph₂ PCH₂ CH₂ PPh₂ )₂ ]Cl. Speciation, mechanism and polymer chain growth studies support a step-growth process where reversible chain transfer occurs, i.e. H₃ B ⋅ PRH₂ /oligomer/polymer can all coordinate with, and be activated by, the catalyst. Block copolymer [H₂ BPPhH]₁₁₀ -b-[H₂ BP(n-hexyl)H]₁₁ can be synthesized and self-assembles in solution to form either rod-like micelles or vesicles depending on solvent polarity.

Boron Nitride-Doped Polyphenylenic Organogels

Herein, we describe the synthesis of the first boron nitride-doped polyphenylenic material obtained through a [4 + 2] cycloaddition reaction between a triethynyl borazine unit and a biscyclopentadienone derivative, which undergoes organogel formation in chlorinated solvents (the critical jellification concentration is 4% w/w in CHCl₃). The polymer has been characterized extensively by Fourier-transform infrared spectroscopy, solid-state ¹³C NMR, solid-state ¹¹B NMR, and by comparison with the isolated monomeric unit. Furthermore, the polymer gels formed in chlorinated solvents have been thoroughly characterized and studied, showing rheological properties comparable to those of polyacrylamide gels with a low crosslinker percentage. Given the thermal and chemical stability, the material was studied as a potential support for solid-state electrolytes. showing properties comparable to those of polyethylene glycol-based electrolytes, thus presenting great potential for the application of this new class of material in lithium-ion batteries.

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.

Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane

An intramolecular phosphine-borane frustrated Lewis pair (FLP) chelate, ⁱPr₂P(C₆H₄)BCy₂ or PB (Cy = cyclohexyl), was used to coordinate aminoborane (H₂BNH₂) and iminoborane (HBNH) units via donor-acceptor stabilization. Attempts to induce dehydrogenation from these B-N adducts with known metal catalysts (or pre-catalysts) have been unsuccessful thus far, and related observations were noted with an H₂BNH₂ complex supported by a modified FLP chelate bearing a geometrically constrained bicyclic 9-borabicyclo(3.3.1)nonane (BBN) unit. Treatment of the iminoborane adduct [PB{HBNH}] with a chlorinating agent led to ligand activation via B-C bond cleavage instead of the expected H/Cl exchange at boron to give [PB{ClBNH}]. Nucleophilic attack at the boron center in [PB{HBNH}] was observed upon addition of BnK (Bn = benzyl), yielding the amidoborate complex [PB{H(Bn)BNH}{K(THF)₂}].

Switching Selectivity in Copper-Catalyzed Transfer Hydrogenation of Nitriles to Primary Amine-Boranes and Secondary Amines under Mild Conditions

A simple and efficient copper-catalyzed selective transfer hydrogenation of nitriles to primary amine-boranes and secondary amines with an oxazaborolidine-BH₃ complex is reported. The selectivity control was achieved under mild conditions by switching the solvent and the copper catalysts. More than 30 primary amine-boranes and 40 secondary amines were synthesized via this strategy in high selectivity and yields of up to 95%. The strategy was applied to the synthesis of ¹⁵N labeled in 89% yield.

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.

Access to adducts of parent iminoborane isomers, HBNH and NBH2, using frustrated Lewis pair chelation

Adducts of the parent iminoborane isomers, HBNH and NBH₂, have been prepared, each stabilized by the frustrated Lewis pair (FLP) chelate ⁱPr₂P(C₆H₄)BCy₂ (PB). PB{HBNH} was accessed via dehydrohalogenation, while the corresponding isomer PB{NBH₂} was obtained from the borylation of the formal nitrene-FLP complex PB{NH}.

Facile Synthetic Method of Na[BH3(NH2BH2)2H] Based on the Reactions of Sodium Amidoborane (NaNH2BH3) with NiBr2 or CoCl2

The reactions of sodium amidoborane (NaNH₂BH₃) with NiBr₂ have been investigated, and the results showed that black precipitate 1 including the NiBNH_x composites could be obtained. From the aqueous solution of the precipitate 1, the hydrolysis product Ni-B (2) was isolated and characterized. Both the in situ formed precipitate 1 and the hydrolysis product 2 can catalyze the formation of Na[BH₃(NH₂BH₂)₂H]. CoCl₂ showed comparable performance with NiBr₂. Based on these results, a facile method for the synthesis of Na[BH₃(NH₂BH₂)₂H] has been developed. This work provides insights into studying experimental methods for the synthesis of long B/N chain complexes and developing boron and nitrogen chemistry.

Dehydropolymerisation of Methylamine Borane and an N-Substituted Primary Amine Borane Using a PNP Fe Catalyst

Dehydropolymerisation of methylamine borane (H3 B⋅NMeH2 ) using the well-known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH2 CH2 PiPr2 )2 ) (1) gives poly(aminoborane)s by a chain-growth mechanism. In toluene, rapid dehydrogenation of H3 B⋅NMeH2 following first-order behaviour as a limiting case of a more general underlying Michaelis-Menten kinetics is observed, forming aminoborane H2 B=NMeH, which selectively couples to give high-molecular-weight poly(aminoborane)s (H2 BNMeH)n and only traces of borazine (HBNMe)3 by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H3 B⋅NMe2 H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H3 B⋅N(CH2 SiMe3 )H2 was studied in homo- and co-dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

A simple cobalt-based catalyst system for the controlled dehydropolymerisation of H3B·NMeH2 on the gram-scale

A simple Co-based catalyst system promotes the efficient and controlled dehydropolymerisation of amine–boranes on scale.