Unfolding the crucial role of a nucleophile in Ziegler–Natta type Ir catalyzed polyaminoborane formation

Abstract

formation through dehydropolymerization of ammonia-borane by Brookhart's iridium pincer catalyst has been under intense scrutiny but a sound molecular level understanding has remained elusive. Herein, using DFT the mechanism outlined by us for IrH2POCOP catalyzed formation underscores the importance of generation of nucleophiles, in particular that of the metal bound NH2BH2 moiety armed with a nitrogen lone pair for chain initiation and chain propagation steps.

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…A recent computational study explored the mechanism of The Catalytic Dehydrocoupling of Amine-Boranes and Phosphine-Boranesthe polymerisation of H 2 B¼NH 2 by 30 (although concomitant dehydrogenation of the amine-borane was not probed) and also implicated a chain growth mechanism, as suggested experimentally by Manners. The proposed mechanism for propagation involves end chain growth; the lone pair on the NH 2 end of the chain interacts with the Lewis acidic BH 2 group of the entering H 2 B¼NH 2 molecule [29]. This suggested mechanism contrasts with a coordination insertion mechanism, in which a transient aminoborane inserts into a growing polymer chain at the metal centre, similar to Ziegler-Natta olefin polymerisation.…”

“…Although cyclohexene trapping is still regarded as a useful method for detecting free aminoboranes, more recent studies have suggested that the absence of hydroboration does not necessarily reflect an absence of free aminoborane. It has been suggested that, if borazine formation (from aminoborane trimerisation/dehydrogenation) or hydroboration of cyclohexene are not kinetically competitive with metal-based BN oligomerisation/ polymerisation processes, Cy 2 B¼NH 2 will not be observed even if H 2 B¼NH 2 is present [21,28,29]. [30].…”

Amine–Borane Dehydropolymerization: Challenges and Opportunities

“…Paul has suggested, using DFT calculations, a mechanism for dehydropolymerization catalyzed by IrH 2 (POCOP), which resembles a nucleophilic head‐to‐tail polymerization (Scheme C) . Separate metal centers (i.e., “bicatalyst”) are responsible for the dehydrogenation and polymerization activity.…”

“…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.…”

“…[69,70] Paul has suggested, using DFT calculations,amechanism for dehydropolymerization catalyzed by IrH 2 (POCOP), which resembles an ucleophilic head-to-tail polymerization (Scheme5C). [71] Separate metal centers (i.e.," bicatalyst") are responsible for the dehydrogenation [61] and polymerization activity.T his latter process involves metal-initiated chain-growth in which the metal binds one end of the polymer chain, and aminoborane monomer units are added by nucleophilic attack of the amine terminus lone pair.T his type of mechanism is proposed for systems which display modified chain-growth behavior, [27,36] in which there is no clear correlation between molecular weight and catalyst loading,o na ccount of the dual role of the metal centers in dehydrogenation and polymerization processes, whichm akes it difficult to resolve the catalystc ontribu-tion to polymer growth kinetics due to potential complications stemming from the presence of multiple, competinge quilibria. This mechanism is similart ot hat proposed by Schneider for RuH 2 (PMe 3 ){NH((CH 2 ) 2 PiPr 2 ) 2 }, in which different Ru sites catalyze dehydrogenation and BÀNc oupling, relianto nc ooperative assistance from the PNP ligand.…”