Dehydropolymerization of H3B•NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2

[Rh(kappa(2)-PP-DPEphos){eta(2)eta(2)-H2B(NMe3)(CH2)(2)Bu-t}][BAr4F] acts as an effective precatalyst for the dehydropolymerization of H3B center dot NMeH2 to form N-methylpolyaminoborane (H2BNMeH)(n). Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1-1 mol %, an inverse relationship) and use of the chainmodifying agent H-2: with M-n ranging between 5 500 and 34 900 g/mol and (sic) between 1.5 and 1.8. H-2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh](TOTAL). Speciation studies at 10 mol % indicate the initial formation of the amino-borane bridged dimer, [Rh-2(kappa(2)-PP-DPEphos)(2)(mu-H)(mu-H2BN=HMe)][BAr4F], followed by the crystallographically characterized amidodiboryl complex [Rh-2(cis-kappa(2)-PP-DPEphos)(2)(sigma,mu-(H2B)(2)NHMe)][BAr4F]. Adding similar to 2 equiv of NMeH2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh](TOTAL) is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M-n = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(kappa(2)-DPEphos)(NMeH2)(2)][BAr4F] and [Rh(kappa(2)-DPEphos)(H)(2)(NMeH2)(2)][BAr4F], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(kappa(2)-PP-DPEphos){eta(2)-H2B(NMe3)(CH2)(2)Bu-t}][BAr4F], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.