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

Claire N Brodie,Steven Huband,Guy C Lloyd-Jones,Stuart A Macgregor,Timothy M Boyd,David E Ryan,Lia Sotorríos,James S Town,Eimear Magee,Andrew S Weller,David M Haddleton

doi:10.1021/jacs.1c10888

Abstract

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ3-(iPr2PCH2CH2)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·H3BNMeH2 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 (Mn ∼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, Mn = 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.

Highlights

Polymers with alternating B−N main-chain backbones, the polyaminoboranes (H2BNRH)n, are inorganic analogues of polyolefins (H2CCHR)n, through the simple isosteric relationship between BN and CC (Scheme 1A).[1−4] Polyolefins are technologically and societally ubiquitous, and their catalyticScheme 1. (A) Polyolefins versus Polyaminoboranes; (B) Synthesis of (H2BNMeH)n Using Ir(POCOP)H2 synthesis from alkene monomers, and the resulting macromolecular structures, can be controlled using mechanism-led catalyst design.[5−9] Despite ill-defined (H2BNH2)n being known for decades from thermolysis of H3B·NH3,10 polyaminoboranes are significantly less well-developed
We report here a reliable, scalable, conveniently synthesized, and air-tolerant catalyst system, [1]Cl, for the dehydropolymerization of H3B·NMeH2 to selectively form N-methyl polyaminoborane, (H2BNMeH)n
Upon the basis of our detailed mechanistic studies, we propose that this selective, controlled, cascade polymerization results from an ensemble of processes that are all geared around the concentration of H3B·NMeH2

Summary

INTRODUCTION

Polymers with alternating B−N main-chain backbones, the polyaminoboranes (H2BNRH)n, are inorganic analogues of polyolefins (H2CCHR)n, through the simple isosteric relationship between BN and CC (Scheme 1A).[1−4] Polyolefins are technologically and societally ubiquitous, and their catalytic. A likely coproduct of this hydride transfer, boronium [H2B(NMeH2)2][BArF4], was shown to act as a chain-length control agent, being proposed to protonate the amine-terminus of a growing polymer chain that arises from the low barrier head-to-tail B−N chain-forming events (e.g., Scheme 2) Despite these detailed insights, the synthetically accessible precursor to [Rh(DPEphos)H2]2, Rh(DPEphos)(η3-H2CPh),[44] is not straightforward to prepare as it is air and thermally sensitive, while the precise mechanism of amine-borane dehydrogenation remains unresolved. We demonstrate the straightforward synthesis on 10 g scale of (H2BNMeH)n at low catalyst loadings (0.01 mol %) and the exploration of a variety of materials processing methods for the resulting (H2BNMeH)n, such as melt extrusion and thin-film deposition

RESULTS AND DISCUSSION

CONCLUSIONS

■ REFERENCES