Theoretical Investigation on the Chemistry of Entrapment of the Elusive Aminoborane (H 2 NBH 2 ) Molecule

A neutral N-heterocyclic phosphenium complex of manganese was synthesised by a metathesis approach and characterised by IR, NMR, and XRD studies. The short P-Mn distance suggests a substantial metal-ligand double bond character. Reaction with a hydride produced an anionic phosphine complex, which was also characterised by IR and NMR spectroscopy and, after anion exchange, a single-crystal XRD study. Protonation of the anion occurs at the metal to yield a neutral phosphine metal carbonyl hydride, which releases dihydrogen upon irradiation with UV light. These reactions confirm the electrophilic nature of the phosphenium ligand and suggest that the title complex might undergo reactions displaying metal-ligand cooperativity. Surprisingly, reaction with ammonia borane (AB) did not proceed under transfer hydrogenation of the Mn=P double bond but through the catalytic dehydrogenation of AB. The phosphenium complex behaves here as a class II catalyst, which dehydrogenates AB to NH BH that was trapped with cyclohexene. Computational model studies led to the identification of two possible catalytic cycles, which differ in the regioselectivity of the initial AB activation step. In one case, the activation proceeds as cooperative transfer hydrogenation of the Mn=P bond, whereas in the other case a H /H pair is transferred to the phosphorus atom and a nitrogen atom of the phosphenium unit, resulting in a ligand-centred reaction in which the metal fragment acts merely as stabilising substituent. Unexpectedly, this pathway, which constitutes a new reaction mode for phosphenium complexes, seems to be better in accord with experimental findings on the course of the catalysis.

Section: Resultsmentioning

confidence: 99%

N‐Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation

Gediga

Feil

Schlindwein

et al. 2017

“…Schneider has reported threefold improvements in ToN for H 3 B⋅NH 3 dehydropolymerization using Fe and Ru pincer systems upon addition of a catalytic amount of NMe 2 Et . The added amine prevents catalyst deactivation by BH 3 binding, produced from the rearrangement of 2 H 2 B=NH 2 to BH 3 ⋅THF and HB(NH 2 ) 2 …”

Section: Mechanismmentioning

confidence: 99%

“…Baker proposed an elegant probe for these two extremes using entrapment of aminoborane by hydroboration of exogenous cyclohexene . There are caveats associated with this approach though, as the trapping is dependent on the relative rates of hydroboration versus polymerization, where nucleophilic assistance of the solvent can be key . Interestingly Paul has calculated B−N head‐to‐tail bond forming polymerization using the IrH 2 (POCOP) catalyst to be lower in energy than cyclohexene hydroboration; suggesting that even if free aminoborane is formed in this specific system, polymerization could well be favored over hydroboration in the experimental system.…”

Section: Mechanismmentioning

confidence: 99%

See 1 more Smart Citation

Amine–Borane Dehydropolymerization: Challenges and Opportunities

Colebatch

Weller

2018

The dehydropolymerization of amine–boranes, exemplified as H 2 RB⋅NR′H 2 , to produce polyaminoboranes (HRBNR′H) n that are inorganic analogues of polyolefins with alternating main‐chain B−N units, is an area with significant potential, stemming from both fundamental (mechanism, catalyst development, main‐group hetero‐cross‐coupling) and technological (new polymeric materials) opportunities. This Concept article outlines recent advances in the field, covering catalyst development and performance, current mechanistic models, and alternative non‐catalytic routes for polymer production. The substrate scope, polymer properties and applications of these exciting materials are also outlined. Challenges and opportunities in the field are suggested, as a way of providing focus for future investigations.

“…Ideally, when the final product of AOB oligomerization is PBZ , three equivalents of H 2 can be released. AOB is a crucial primitive immediate perceived as the monomer of fast oligomerization, which had been isolated experimentally in argon matrices at −156 °C . The oligomerization of AOB has been extensively studied by various groups .…”

Section: Resultsmentioning

confidence: 99%

Designing Metal‐Free Frustrated Lewis Pairs Catalyst for the Efficient Dehydrogenation of Ammonia Borane

Song

2018

Ammonia borane (AB) has been in the spotlight for the chemical storage of hydrogen over the past decade. However, the development of methods for efficient and controlled hydrogen release from AB under mild conditions is still underway. Herein, using density functional theory (DFT) computations, we designed a metal-free frustrated Lewis pair (FLP) catalyst o-(BPh )C H (NiPr ) (M1) that can efficiently dehydrogenate AB to release more than two equivalents of H under mild conditions. Catalyst M1 can dehydrogenate not only AB to H N=BH (AOB) and H , but also oligomers of AOB with rather low free-energy barriers. The high dehydrogenation activity of M1 is the key of new oligomerization routes to the efficient dehydrogenation of AB to borazine (BZ) or H B-(NH=BH) -NH (PIB) and finally to polyborazylene (PBZ) so that more than two equivalents of H can be released. A first-principle kinetic Monte Carlo (KMC) study reveals that the activity of our catalytic system can be tuned by varying the initial concentration of M1 and AB. This work can guide the design of catalyst for the highly efficient utilization of AB as a hydrogen storage material.