Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine-boranes using a [Rh(Xantphos)]⁺ catalyst.

Heather C Johnson,Erin M Leitao,Guy C Lloyd-Jones,Andrew S Weller,George R Whittell,Ian Manners

doi:10.1021/ja503335g

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.

Highlights

Catalytic routes for the formation of main–group/main–group bonds are important for the targeted construction of new molecules and materials
Encouraged by the [Rh(Xantphos)]+ fragment’s ability to B–B homocouple amine– boranes we report its use in a detailed stoichiometric, catalytic and mechanistic/kinetic investigation into the dehydropolymerisation of H3B·NMeH2 to form polyaminoborane
A detailed mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)]+ fragment suggests that similar mechanisms operate for both, that only differ in that B–N bond formation is favoured for H3B·NMeH2 but not H3B·NMe2H

Summary

Introduction

Catalytic routes for the formation of main–group/main–group bonds are important for the targeted construction of new molecules and materials. Enabling catalytic methodologies for such bond forming events lag behind those developed for the construction of C–C and C–X bonds.[1] The development of reliable, robust and controllable processes is an important challenge.[2,3,4,5] Catalytic dehydropolymerization[6] of amine–boranes to give polyaminoboranes presents one such opportunity, as this produces new BN polymeric materials that are isoelectronic with technologically pervasive polyolefins.

Results

Discussion

Conclusion