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

Rising Opportunities in Catalytic Dehydrogenative Polymerization

This article gives a perspective on various types of catalytic dehydrogenative polymerization reactions (including organic and main group polymers) while introducing "hydrogen-borrowing polymerization" and "acceptorless dehydrogenative polymerization" to this class. Limitations and future opportunities of each method have been discussed.

Selective PNP Pincer-Ir-Promoted Acceptorless Transformation of Glycerol to Lactic Acid and Hydrogen

The catalytic transformation of glycerol (GLY) using [(iPr2PNHP)Ir(COD)]Cl [iPr2PNHP = κ3-(iPr2PCH2CH2)2NH] affords hydrogen and lactic acid (LA), trapped as its sodium salt (Na[LA]) with high yield (96%) and selectivity (99%) in the presence of an equivalent of in situ generated NaOEt at 140 °C within 4 h. A diminution in activity was observed when the PNMeP ligand was used instead of PNHP, or when Cl- was replaced by [BArF4]-. An Ir to Rh substitution also resulted in poor activity. Kinetic studies show a first-order dependence of the initial rate of turnovers on the concentrations of [(iPr2PNHP)Ir(COD)]Cl, NaOEt, and glycerol. An outer-sphere mechanism does not explain the activity of [(iPr2PNMeP)Ir(COD)]Cl, and DFT studies support an inner-sphere mechanism, with oxidative addition of glycerol to the 14-electron intermediate [(iPr2PNHP)Ir]Cl determined as the rate-determining step (RDS). A kH/kD of 2.7 obtained with glycerol-d8 shows a major contribution from O-H activation in the RDS. The kinetics of the reaction become favorable (ΔG140⧧ = 27.01 kcal/mol) when one of the terminal O-H's of glycerol is hydrogen bonded to the N-H of the pincer backbone, in contrast to cases where no hydrogen bonds are invoked (ΔG140⧧ = 31.96 kcal/mol) or are not possible [(iPr2PNMeP)Ir]Cl (ΔG140⧧ = 30.36 kcal/mol).

Platinum(II) Phenylpyridyl Schiff Base Complexes as Latent, Photoactivated, Alkene Hydrosilylation Catalysts

Photoactivated catalysts for the hydrosilylation of alkenes with silanes offer temporal control in manufacturing processes that require silicone curing. We report the development of a range of air-stable Pt(II) (salicylaldimine)(phenylpyridyl), [Pt(sal)(ppy)], complexes as photoinitiated hydrosilylation catalysts. Some of these catalysts show appreciable latency in thermal catalysis and can also be rapidly (10 s) activated by a LED UV-light source (365 nm), to give systems that selectively couple trimethylvinylsilane and hexamethylsiloxymethylsilane to give the linear hydrosilylation product. Although an undetectable (by NMR spectroscopy) amount of precatalyst is converted to the active form under UV-irradiation in the timescale required to initiate hydrosilylation, clean and reliable kinetics can be measured for these systems that allow for a detailed mechanism to be developed for Pt(sal)(ppy)-based photoactivated hydrosilylation. The suggested mechanism is shown to have close parallels with, but also subtle differences from, those previously proposed for thermally-activated Karstedt-type Pt(0) systems.

The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

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.

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.

Transition-Metal-Free Dehydropolymerization of Phosphine-Boranes at Ambient Temperature

Stoichiometric reaction of phosphine-borane adducts RR'PH⋅BH₃ (R=Ph, R'=H, Ph, Et, and R=R'=^t Bu) with the strong acid HNTf₂ (Tf=SO₂ CF₃ ) leads to H₂ elimination and the formation of the triflimido derivatives, RR'PH⋅BH₂ (NTf₂ ). Subsequent deprotonation by using bases, such as diisopropylethylamine or the carbene IPr (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), led to the formation of P-mono- or -disubstituted polyphosphinoboranes [RR'P-BH₂ ]_n . Evidence for the intermediacy of transient phosphinoborane monomers, RR'PBH₂ , was provided by trapping reactions.

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.

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

Dimeric aminoboranes, [H₂BNR₂]₂ (R = Me or CH₂CH₂) containing B₂N₂ cores, can be activated by I₂, HNTf₂ (NTf₂ = [N(SO₂CF₃)₂]), or [Ph₃C][B(C₆F₅)₄] to form isolable H₂B(μ-NR₂)₂BHX (for X = I or NTf₂). For X = [B(C₆F₅)₄]⁻ further reactivity, presumably between [H₂B(μ-NMe₂)₂BH][B(C₆F₅)₄] and aminoborane, forms a B₃N₃-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H₂B(μ-NMe₂)₂BH(I) and [(μ-NMe₂)BH(NTf₂)]₂ indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf₂ binding. While the iodide congener reacted very slowly with alkynes, the NTf₂ analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)₂BNR₂ species and R₂NBH(NTf₂) as the major products. Further studies indicated that the B₂N₂ core is maintained during the first hydroboration, and that it is during subsequent steps that B₂N₂ dissociation occurs. In the mono-boron systems, for example, ⁱPr₂NBH(NTf₂), NTf₂ is N-bound; thus, they have less steric crowding around boron relative to the B₂N₂ systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B₂N₂-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B₂N₂ electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H₂B(μ-N₂C₃H₃)]₂ analogues, making them less useful for the directed diborylation of a single substrate.

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.

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.