The Mechanism of Borane–Amine Dehydrocoupling with Bifunctional Ruthenium Catalysts

Abstract

Borane-amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia-borane (H3N-BH3, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B-N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo's catalyst) results in formation of borazine and 2 equiv of H2, while 1 equiv of H2 and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by (11)B MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru(H)2PMe3{HN(CH2CH2PtBu2)2}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H-NH2BH3 bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru(H)PMe3{N(CH2CH2PtBu2)2}] does not undergo the same mechanism as was previously proposed in a theoretical study.

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…Interestingly, a significant solvent effect on the relative rate of dimerisation has also been noted, with acetonitrile accelerating the process [13,20]. The less bulky congeners H 2 B¼NH 2 [21] and H 2 B¼NMeH [22,23], however, have not been directly observed as intermediates in dehydrocoupling, although they have been isolated coordinated to a transition metal fragment, being formed from dehydrogenation of the corresponding amine-borane [24]. In 2008, Baker, Dixon and co-workers proposed that H 2 B¼NH 2 liberated from the metal results in the eventual production of borazine, whereas H 2 B¼NH 2 (or derivatives thereof) remaining bound to the metal results in oligomeric or polymeric products [25].…”

“…Additionally, the authors noted that the induction period was approximately twice as long using H 3 B · NMe 2 D compared with H 3 B · NMe 2 H, whereas no change was observed using D 3 B · NMe 2 H. This implied that N-H activation was rate limiting in the formation of the active species, which is proposed to be an amido-boryl complex 48. These, and other observations, led to a proposed catalytic cycle applicable for both the dehydropolymerisation of H 3 [21]. The methylation of the pincer nitrogen atom in 57 prevents the bifunctional reactivity that is thought to be key in rationalising the high activities of these complexes.…”

“…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].…”

“…Homogeneous catalysts can operate via inner-sphere or outersphere mechanisms. Outer-sphere mechanisms allow dehydrogenation of amineboranes without the direct coordination to the metal centre by using metal-ligand cooperativity [21,30,[56][57][58][59]. By contrast, inner-sphere mechanisms involve initial coordination of the amine-borane to the metal forming a sigma complex, followed by dehydrogenation of the amine-borane (Scheme 11).…”

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

“…They are formed by the dehydropolymerization of amine‐boranes (H 3 B⋅NRH 2 ; R=H or Me, for example; Scheme 1 A),1 and metal‐catalyzed routes to polyamino‐boranes offer the potential for fine control over molecular weight and polymer stereochemistry. There is recent evidence that these processes occur at a metal center in which the catalyst needs to perform two roles: 1) formal dehydrogenation of amine‐borane to form a latent source of amino‐borane (H 2 B=NRH), and 2) subsequent B−N bond formation 2, 3, 4, 5, 6. For some systems a coordination/insertion mechanism is proposed, although the precise structure of the propagating species is currently unresolved (Scheme 1 B) 3, 5, 6.…”