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A systematic study of the catalyst structure and overall charge for the dehydropolymerization of H 3 B·NMeH 2 o form N-methyl polyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R)} fragments in which PR 2 groups are selected from Et, i Pr, and  Bu. The most efficient systems are based upon {Rh(Xantphos- i Pr)}, i.e., [Rh(κ 3 -P,O,P-Xantphos- i Pr)(H) 2 (η 1 -H 3 B·NMe 3 )][BAr F  4 ], 6, and Rh(κ 3 -P,O,P-Xantphos- i Pr)H, 11. While H 2 evolution kinetics show both are fast catalysts (ToF ≈ 1500 h -1 ) and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neutral 11 (M n = 28 000 g mol -1 , Đ = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (M n = 9000 g mol -1 , 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 3 -P,O,P-Xantphos- i Pr)} 2 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.

Dehydropolymerization of H3B·NMeH2 To Form Polyaminoboranes Using [Rh(Xantphos-alkyl)] Catalysts

Dehydropolymerization of H₃B·NMeH₂ To Form Polyaminoboranes Using [Rh(Xantphos-alkyl)] Catalysts