Controlled Amine-Borane Dehydropolymerization Enabled by Mechanistic Insight Using the Ir(tBu-POCOP)H2 Catalyst

A detailed kinetic, mechanism, and operando speciation study on the controlled, catalytic, cascade-like dehydropolymerization of H3B·NMeH2 to form N-methyl polyaminoborane [H2BNMeH]n (Me-PAB) using Ir(tBu-POCOP)H2 as a precatalyst is reported. Catalyst speciation, as monitored using online FlowNMR, shows the formation of a borohydride complex Ir(tBu-POCOP)(H)(BH4) during an induction period, that rapidly speciates to Ir(tBu-POCOP)H4 during productive turnover. Kinetic studies in the roles of NMeH2, trace water, and [H3B·NMeH2] on the induction period, productive catalysis and speciation reveal a complex set of processes that provide a framework for numerical and computational (DFT) modeling. Collectively these combine to support a mechanism for the dehydrogenation of H3B·NMeH2 to form the actual monomer, H2B═NMeH, that involves concerted B–H/N–H activation. By understanding the factors that control the maximum rate of dehydrogenation [ν(max)] and catalyst speciation, and deploying temperature variation and amine additives, a wide-range of Me-PAB molecular weights (Mn = 35,000–191,200 g·mol–1) can be achieved; in addition to low catalyst loadings (21 ppm), multigram scales (16 g) and water/air tolerance. The development of such systems which operate to selectively produce Me-PAB, using as-supplied substrates and simple catalysts, that also work on a scale useful for materials testing, promotes the wider exploitation of Me-PAB as a general preceramic precursor to hex-boron nitride.