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

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) (Mn = 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 (Mn = 2 800 g mol–1, PDI = 1.8) or THF solvent (Mn = 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.