Mechanistic Studies of the Dehydrocoupling and Dehydropolymerization of Amine–Boranes Using a [Rh(Xantphos)]⁺ Catalyst

Abstract

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2B═NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…Examples have also been isolated in which multiple amine-borane moieties are bound to a metal centre, similar to intermediates often invoked for dimerisation and polymerisation mechanisms (vide infra) [12,62,[68][69][70] [72]. In 2013, Weller and MacGregor reported the first well-characterised example of the B-B homocoupling of an amine-borane to yield the diborane (4) Me 3 N · BH 2 BH 2 · NMe 3 ligand sigma bound to rhodium [62].…”

“…2.2 and 2.3). Consequently, H 3 B · NMe 2 H is often used as a model for the dehydrocoupling of H 3 B · NH 3 and H 3 B · NMeH 2 [12,13], the products of which are often insoluble or poorly defined polymeric or oligomeric materials [14]. The cyclic dimer [H 2 BNR 2 ] 2 (R¼e.g.…”

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

“…Thus, it is likely that similar active species are present in THF or 1,2‐F 2 C 6 H 4 . The lack of induction period is in direct contrast to xantphos‐based rhodium catalysts, which show induction periods for H 3 B⋅NMeH 2 dehydrocoupling in C 6 H 5 F,5, 15 suggesting that a different kinetics regime or mechanism is in operation.…”

“…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.…”

Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications

“…Among the most studied of the early main group metal hydrides are those of magnesium and zinc, primarily using either bulky monoanionic ligands such as -diketiminate (nacnac) and its derivatives, Running in parallel to this research has been that of the amine boranes (RNH2· BH3; R2NH· BH3), which contain protic and hydridic hydrogen atoms in close proximity to one another, making them primed for hydrogen release under the correct conditions, 8 as well as precursors for the synthesis of B-N oligomers 9 and polymers. 10 Again, the use of main group metals in this area for environmental and economic reasons is pervasive with prominent contributions arising from the groups of Harder,11 Hill 12 and Wright. who have reported both neutral and cationic NHC-stabilized zinc hydride species (specifically with a focus on alkyl zinc/zinc halide reagents), some of which are effective catalysts for hydrosilylation reactions (Figure 2).…”