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, andBu. 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.

The μ‐amino–borane complexes [Rh2(LR)2(μ‐H)(μ‐H2B=NHR′)][BArF 4] (LR=R2P(CH2)3PR2; R=Ph, iPr; R′=H, Me) form by addition of H3B⋅NMeR′H2 to [Rh(LR)(η6‐C6H5F)][BArF 4]. DFT calculations demonstrate that the amino–borane interacts with the Rh centers through strong Rh‐H and Rh‐B interactions. Mechanistic investigations show that these dimers can form by a boronium‐mediated route, and are pre‐catalysts for amine‐borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.

[Rh(κ 2 -PP-DPEphos){η 2 η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ] acts as an effective precatalyst for the dehydropolymerization of H 3 B·NMeH 2 to form N -methylpolyaminoborane (H 2 BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H 2 : with M n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H 2 evolution studies (1,2-F 2 C 6 H 4 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 (κ 2 -PP-DPEphos) 2 (μ-H)(μ-H 2 BN=HMe)][BAr F 4 ], followed by the crystallographically characterized amidodiboryl complex [Rh 2 ( cis -κ 2 -PP-DPEphos) 2 (σ,μ-(H 2 B) 2 NHMe)][BAr F 4 ]. Adding ∼2 equiv of NMeH 2 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 NMeH 2 acts to form the  precatalysts [Rh(κ 2 -DPEphos)(NMeH 2 ) 2 ][BAr F 4 ] and [Rh(κ 2 -DPEphos)(H) 2 (NMeH 2 ) 2 ][BAr F 4 ], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ 2 -PP-DPEphos){η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ], which has been doped with amine. This promoting effect of added amine in situ is shown to be genera...

The archetypal single electron transfer reductant, samarium(II) diiodide (SmI 2 , Kagan’s reagent), remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis. While the chemistry of SmI 2 is very often unique, and thus the reagent is indispensable, it is almost invariably used in superstoichiometric amounts, thus raising issues of cost and waste. Of the few reports of the use of catalytic SmI 2 , all require the use of superstoichiometric amounts of a metal coreductant to regenerate Sm(II). Here, we describe a SmI 2 -catalyzed intermolecular radical coupling of aryl cyclopropyl ketones and alkynes. The process shows broad substrate scope and delivers a library of decorated cyclopentenes with loadings of SmI 2 as low as 15 mol %. The radical relay strategy negates the need for a superstoichiometric coreductant and additives to regenerate SmI 2 . Crucially, our study uncovers an intriguing link between ketone conformation and efficient cross-coupling and thus provides an insight into the mechanism of radical relays involving SmI 2 . The study lays further groundwork for the future use of the classical reagent SmI 2 in contemporary radical catalysis.

Cationic σ-dihydrogen, σ-amine-borane and neutral hydride complexes, based upon {Rh(PONOP)}, are all shown play a role in the dehydrocoupling of H3B·NMe2H. Movement between the three is promoted by free amine, NMe2H.