The dehydropolymerization of H3B•NMeH2 to form polyaminoboranes using [Rh(Xantphos–alkyl)] catalysts

Journal article

A systematic study of catalyst structure and charge for the dehydropolymerization of H3B•NMeH2 to form N-methylpolyaminoborane is reported using catalysts based upon neutral and cationic {Rh(Xantphos-R}} fragments, in which PR2 groups are selected from Et, iPr and tBu. The most efficient systems are based upon {Rh(Xantphos-iPr}}, i.e. [Rh(3–P,O,P–Xantphos–iPr)(H)2(2–H3B•NMe3)][BArF4], 6, and Rh(3–P,O,P–Xantphos–iPr)H, 11. While H2 evolution kinetics show both are fast catalysts (ToF ~ 1500 hr–1), and polymer growth kinetics for dehydropolymerization suggest a classical chain growth process for both, neu-tral 11 (Mn = 28,000 g mol–1, Ð = 1.9) promotes significantly higher degrees of polymerization than cationic 6 (Mn = 9,000 g mol–1, Ð = 2.9). For 6 isotopic labelling studies suggest a rate determining NH activation, while speciation studies, coupled with DFT cal-culations, show the formation of a dimetalloborylene [{Rh(3–P,O,P–Xantphos–iPr)}2B]+ as the, likely dormant, end product of ca-talysis. A dual mechanism is proposed for dehydropolymerization, in which neutral hydrides (formed by hydride transfer in cation-ic 6 to form a boronium co–product) are the active catalysts for dehydrogenation to form amino–borane. Contemporaneous chain–growth polymer propagation occurs on a separate metal center via head-to-tail end chain B–N bond formation of the aminoborane monomer, templated by an aminoborohydride–containing catalyst.

Authors: Adams, GM; Colebatch, AL; Skornia, JT; McKay, AI; Johnson, HC

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Chemical Society
• Journal: Journal of the American Chemical Society
• Volume: 140
• Issue: 4
• Pages: 1481–1495
• Publication date: 2017-12-29
• Acceptance date: 2017-12-29
• DOI: 10.1021/jacs.7b11975
• EISSN: 1520-5126
• ISSN: 0002-7863