Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst

Abstract

The air tolerant precatalyst, [Rh­(L)­(NBD)]Cl ([1]­Cl) [L = κ3-( i Pr2PCH2CH2)2NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H2BNMeH) n , by dehydropolymerization of H3B·NMeH2. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh­(L)­H3 (3), forms, which sits as an outer-sphere adduct 3·H 3 BNMeH 2 as the resting state. At the end of catalysis, dormant Rh­(L)­H2Cl (2) is formed. Reaction of 2 with H3B·NMeH2 returns 3, alongside the proposed formation of boronium [H2B­(NMeH2)2]­Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative “PNHP” ligand templates dehydrogenation, releasing H2BNMeH (ΔG ‡ calc = 19.6 kcal mol–1). H2BNMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M n ∼71 000 g mol–1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H2B­(NMe2H)2]Cl as a chain transfer agent, M n = 37 900–78 100 g mol–1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H3B·NMeH2. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]­Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

“…Notably, the second observed signal at δ 11B = 27.7 while close to that reported for (Me 2 N) 2 BH (δ 11B 26–29, 1 J BH = 135–139 Hz), 31 exhibited a 1 J BH = 180 Hz. This is more consistent with the formation of 11 ( Scheme 2 ), which is closely comparable in spectroscopic data to 9 .…”

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

“…In this contribution, we demonstrate that the mechansim of amine-borane dehydropolymerization continues to offer unexpected and nuanced differences depending on the choice of metal/ligand fragment. , While the perceived wisdom would be that neutral precatalysts such as 1-H 3 , that offer MLC mechanistic pathways, would be effective catalysts for amine-borane dehydrogenation, we show here that they are not fully consistent with Fagnou’s very brief observation nearly 15 years ago on related systems . That the cationic catalyst manifold offers a lower energy pathway and produces polymer selectively (albeit at 1 mol % catalyst loading) via a non-MLC route was also initially surprising. ,,, Moreover, the role of NMeH 2 , formed from slow B–N bond cleavage, is different from other recently reported systems. Here, it has an inhibitory role, by promoting the formation 1-H 3 , in contrast to other systems where its role is accelatory by bringing a cationic precatalyst onto the catalytic cycle through promoting hydride transfer to form an active, neutral, catalyst.…”

“…Interestingly, these are significantly longer than those measured for the 1-H 3 / [NMeH 3 ][BAr F 4 ] catalysis (M n = 18,700 g/mol), which supports the role of ammonium/boronium as chain control agents in dehydropolymerization. 27,52 The GPC analysis also showed an additional, sharper, signal that has previously been identified as being due to entrained [BAr F 4 ] − in the polymer sample, likely as the [NMeH 3 ] + or [H 2 B(NMeH 2 ) 2 ] + salt. 25 The [BAr F 4 ] − anion coelutes with the polymers, complicating the analysis of the molecular weight distribution, meaning that degrees of polymerization should be considered as approximate only.…”

“…4 ] showed no activity, in contrast to the analogous Rh system, where [Rh(κ 3 -{(CH 2 CH 2 P i Pr 2 ) 2 NH}(NBD)]Cl was observed to be an excellent precatalyst, albeit after an induction period to form the active catalyst Rh( i Pr-PN H P)H 3 . 52 Both [1-H 3 B•NMe 3 ]-[BAr F 4 ] and in situ prepared [1-H 4 ][BAr F 4 ] are competent, but rather slow, precatalysts (Table 1), promoting quantitative conversion to poly(N-methylaminoborane) (H 2 BNMeH) n after 6 h. The polymer was formed selectively, however, with only trace N-trimethylborazine or other BN products formed, as shown by a broad signal observed in the 11 B NMR spectrum at δ −6.1, alongside a sharp signal at δ −6.6 for [BAr F 4 ] − in the crude, unprecipitated, polymer/catalyst mixture, cf. Figure 2.…”

“…While the precatalyst Co( i Pr-PN H P)Cl 2 20 promotes rapid and selective dehydropolymerization, mechanistic studies were frustrated because of the paramagnetic nature of the species present, although the NMe variant, Co( i Pr-PN Me P)Cl 2 , was inactive suggesting a cooperative ligand mechanism. In contrast, by using diamagnetic [Rh( i Pr-PN H P)(NBD)]Cl as a cationic precatalyst, the generation of an active neutral dehydrogenation catalyst, Rh( i Pr-PN H P)H 3 , was revealed, 52 and dehydrogenation operates via a cooperative ligand mechanism. It was proposed that Rh( i Pr-PN H P)H 3 forms from an NMeH 2promoted hydride transfer (C, Scheme 2) 48,49 from H 3 B• NMeH 2 to in situ generated Rh( i Pr-PN H P)H 2 Cl.…”