Dehydropolymerization of HB·NMeH To Form Polyaminoboranes Using [Rh(Xantphos-alkyl)] Catalysts.

A systematic study of the catalyst structure and overall charge for the dehydropolymerization of HB·NMeH to form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR groups are selected from Et, Pr, and Bu. The most efficient systems are based upon {Rh(Xantphos-Pr)}, i.e., [Rh(κ-P,O,P-Xantphos-Pr)(H)(η-HB·NMe)][BAr], 6, and Rh(κ-P,O,P-Xantphos-Pr)H, 11. While H evolution kinetics show both are fast catalysts (ToF ≈ 1500 h) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (M = 28 000 g mol, Đ = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (M = 9000 g mol, Đ = 2.9). For 6 isotopic labeling studies suggest a rate-determining NH activation, while speciation studies, coupled with DFT calculations, show the formation of a dimetalloborylene [{Rh(κ-P,O,P-Xantphos-Pr)}B] as the, likely dormant, end product of catalysis. A dual mechanism is proposed for dehydropolymerization in which neutral hydrides (formed by hydride transfer in cationic 6 to form a boronium coproduct) are the active catalysts for dehydrogenation to form aminoborane. Contemporaneous chain-growth polymer propagation is suggested to occur on a separate metal center via head-to-tail end chain B-N bond formation of the aminoborane monomer, templated by an aminoborohydride motif on the metal.