Edinburgh Research Explorer Amine–Borane Dehydropolymerization Using Rh-Based Precatalysts: Resting State, Chain Control, and Efficient Polymer Synthesis

A detailed study of H 3 B·NMeH 2 dehydropolymerization using the cationic pre-catalyst [Rh(DPEphos)(H 2 BNMe 3 CH 2 CH 2t Bu)]- [BAr F4 ], identifies the resting state as dimeric [Rh(DPEphos)H 2 ] 2 and boronium [H 2 B(NMeH 2 ) 2 ] + as the chain-control agent. [Rh(DPEphos)H 2 ] 2 is conveniently generated in situ from Rh(DPEphos)(benzyl), and catalyzes polyaminoborane formation, M n = 15000 g mol –1 ]. Closely-related Rh(Xantphos)(benzyl) operates on gram scale, at 0.1 mol%, to afford higher molecular weight polymer [ M n = 85000 g mol –1 ] with low residual [Rh], 81 ppm. This insight offers a mechanistic template for dehydropolymerization. The catalyzed dehydropolymerization of amine-boranes, arche-typically H 3 B·NMeH 2 , is an atom-efficient methodology for the synthesis of polyaminoboranes (H 2 BNRH) n (Scheme 1A), forming H 2 as the only

2][3][4] This new class of maingroup polymer 5 is based upon BN main-chain units, and is isosteric with technologically-mature polyolefins.These mainchain B-N units suggest, in addition to unexplored material and chemical properties, potential applications as piezoelectric materials, 6,7 or as precursors to boron-based ceramics and h-BN. 1,8,92][13][14][15][16] While non-catalytic routes have been reported, 10,17 in terms of overall efficiency, scalability, substrate scope, and control of the polymer characteristics, catalytic routes offer the broadest opportunity for the tailored synthesis of polyaminoboranes.

Scheme 1. (A) Amine-Borane Dehydropolymerization, (B) Exemplar Pre-Catalyst Systems.
A wide range of pre-catalyst systems have been described for amine-borane dehydropolymerization (Scheme 1B).After the original report of high 3 molecular weight polymer formed using 1,11 systems based on group-4 metallocenes B, 18,19 cooperative ligands C, 14,16,20,21 and cationic [RhL2] + pre-catalysts (L2 = e.g., Ph2P(CH2)3PPh2, DPEphos, Xantphos) D, [22][23][24] have been described.For the Rh-based catalysts we have reported speciation, kinetics and degree of polymerization studies.These are broadly generalized by: an induction period, a non-living chaingrowth propagation, an inverse relationship between catalyst loading and degree of polymerization, and H2 acting as a chain controlling agent to reduce polymer chain length, 15,[22][23][24] Scheme 2. We have also reported on the key role of NMeH2, formed by B-N bond cleavage in H3B•NMeH2. 21,25Exemplified using the [Rh(DPEphos)(H2BNMe3CH2CH2 t Bu)][BAr F 4] pre-catalyst, 23 1 [Ar F = 3,5-(CF3)2C6H3], the amine NMeH2 removes the induction period, increases the degree of polymerization, and simplifies the kinetics, allowing a half order dependency on [Rh]TOTAL to be determined.However, the structure of the active catalyst is undetermined, with insight limited to the detection of a single species at d( 31 P) 41.3 [J(RhP) = 150 Hz].Also lacking is a robust explanation for the relationship between [Rh]TOTAL, and Scheme 2: Exemplar Complex 1 and Prior Observations.H2, on the degree of polymerization.Despite these advances, the precise details of initiation, propagation and termination remain to be determined for these diverse catalyst systems, 3 while identification of resting states is rare 14,16 and challenging. 18Herein we report on an investigation of the [Rh(DPEphos)] + pre-catalyst system, 1, in which a study of the kinetics, speciation and synthesis has allowed identification of the active catalyst, as well as the polymer-growth/termination processes to be interrogated.These insights are then harnessed in the design of a new, efficient, Rh-based catalyst that produces polyaminoborane on scale.A simple protocol is also described to significantly reduce the levels of residual catalyst in the isolated polymer.
We have previously reported that when 1 is employed as precatalyst, the monocationic hydrido-aminoborane dimer [Rh2(DPEphos)2(μ-H)(μ-H2B=NHMe)][BAr F 4] 2 is formed during the early stages of reaction. 23We propose this arises via an amine-promoted B-H hydride transfer 26  ].Solutions of complex 3 in 1,2-F2C6H4, or in THF, irreversibly lose H2 on degassing, to form an insoluble yellow/brown powder, analyzing as [Rh(DPEphos)H]∞, likely to be a coordination polymer with Rh-H-Rh linkages.While the Rh-polymer does not dissolve on addition of H2, the soluble complex 2 is regenerated when [H2B(NMeH2)(OEt2)][BAr F 4] is added. 10Thus, when using a cationic pre-catalyst (i.e. 1 or 5), persistent NMeH2 will favor soluble neutral hydride via equilibration with complex 2 (k3, Scheme 3).When using neutral pre-catalyst 4, a high initial concentration of amine-borane, e.g.[H3B•NMeH2]0 = 0.446 M in THF, inhibits the formation of a precipitate.Presumably, the amine-borane intercepts Rh(DPEphos)H before it oligomerizes.Thus, dimeric, neutral, hydride 3 is observed as the common resting state, irrespective of the pre-catalyst, or solvent.The half order dependence in [Rh]TOTAL points to a rapid endergonic equilibrium between dimer and monomer, prior to the turn-over limiting step.This has been noted in other Rh2Hx systems, 32,36,37 and the data are thus consistent with the resting state being dimeric 3.An important difference between neutral versus cationic pre-catalysts, is that the latter generate a boronium coproduct, which has important implications for the dehydropolymerization, as discussed next.Neutral pre-catalyst 4 was deployed in the dehydropolymerization of H3B•NMeH2 at a variety of catalyst loadings, Table 1.In 1,2-F2C6H4 as solvent, kinetics measurements were hampered by formation of the insoluble precipitate.In THF, the eudiometric measurements on H2 production were less reliable due to solvent volatility.Nevertheless, polymerization goes to completion in both solvents, selectively forming [H2BNMeH]n, Figure 1A. 38A plot of conversion versus Mn (Figure 1B, relative to polystyrene standards) 3,11,16 is characteristic of a non-living chain-growth polymerization: at low conversions the polymer is formed with high Mn and H3B•NMeH2 dominates.Variations in catalyst loading did not affect the degree of polymerization of the resulting polyaminoborane, in either 1,2-F2C6H4 (Figure 1C, Mn = 15000 g mol -1 ) or THF solutions (Mn = 17000 g mol - 1 ), under 'open conditions' with a slow Ar-flow.This is different to cationic pre-catalysts, such as 1, where Mn scales inversely with [Rh]TOTAL: e.g.6400 (1 mol%), 34900 g mol -1 (0.2 mol%). 23However, 'closed conditions' that allow for build-up of H2 result in very low molecular weight oligomers being signal.e 5 M, 1.1 g scale.
formed (1 mol % 4, less than 1000 gmol -1 by GPC, 11 B NMR spectroscopy 31 ); The cationic pre-catalyst 1 behaves analogously. 22he neutral and cationic pre-catalyst systems differ by the presence of a boronium co-product with the latter, the relative concentration of which will scale with [Rh]TOTAL. 39Given the underlying insensitivity to the degree of polymerization to [Rh]TOTAL when using neutral 4, we thus considered whether with cationic pre-catalysts boronium [H2B(NMeH2)2][BAr F 4] can act as a chain-control agent to modify Mn.To test this, [H2B(NMeH2)2][BAr F 4] was doped (0.25 to 1 mol%) into 1 mol% 4 / H3B•NMeH2, to selectively form polyaminoborane ( 11 B NMR).Although GPC analysis of the resulting polymer using refractive index detection is affected by the co-eluting [BAr F 4] -masking the lower molecular weight region (Figure 1D), 15 there is a qualitative trend of decreasing Mp with increasing [H2B(NMeH2)2][BAr F 4], Table 1.This outcome is consistent with boronium acting as a chain-control agent.Chain Length Distribution (ln-CLD) analysis of high molecular weight fractions in GPC has been shown to be useful where there is overlap between distributions of polymer and transfer agents, such as that noted here, allowing for chain control processes to be probed. 40A Mayo-type plot of [boronium]/[H3B•NMeH2] versus the ln-CLD slope indicates an inversely linear relationship (Figure 1E and F), further supporting the conclusion that the boronium functions as a rapid chain control agent in the dehydropolymerization.Collectively, the analysis above facilitates the construction of a mechanistic landscape for dehydropolymerization, Scheme 5, that is consistent not only with the results herein, but also with our previous observations on cationic Rh-based systems. 15,22,23,28,41Thus, dehydrogenation of amine-borane to give the reactive monomer, H2B=NMeH, occurs at a neutral [Rh-H] species, in an H2-mediated equilibrium with dimer 3.
Dehydrogenation to form H2B=NMeH, via BH/NH activation (Scheme 5A) could be facilitated by a hemilabile DPEphos ligand (e.g.k 2 and k 3 coordination 42 ) as previously suggested.Initiated by a formal hydride-transfer from the rhodium hydride, 44 that is now playing a dual role in both dehydrogenation and initiation, 11,14 H2B=NMeH then undergoes rapid head-totail end-chain nucleophilic B-N bond formation, as proposed previously (Scheme 5B). 12,13,15,16 hain-control by protonation of the terminus nucleophilic amine of the polymeryl group by boronium returns a cationic pre-catalyst, aminoborane and NMeH2, that are rapidly recycled (Scheme 5C). 25 A related intramolecular proton transfer has been proposed by Paul and coworkers for Ir(POCOP)H2 systems. 13We speculate that, in the absence of boronium, chain transfer to pre-monomer H3B•NMeH2 controls chain-length, Scheme 5C.Whatever the precise mechanism for these chain-control processes, they result in relatively narrow dispersities of the final isolated polymer, and constant degrees of polymerization during propagation (Figure 1B). 45H2 loss from 3, and related systems, 32,35 occurs readily on degassing.The position of the initial monomer/dimer equilibrium is thus expected to be sensitive to [H2], impacting on the rate of dehydrogenation as well initiator concentration.This, we suggest, is the origin of the low degrees of polymerization observed under 'closed conditions'.In support of this, for a system where hydride-bridged dimer formation is disfavoured due to sterics, Rh(Xantphos-i Pr)H, H2 does not act to modify the degree of 15 The precise gearing of all of these interconnected relationships is therefore pre-catalyst, coproduct (e.g.boronium) and solvent specific.The use of new pre-catalysts based upon neutral 4 demonstrates wider applicability (Figure 2, Table 1).For example, the Xantphos benzyl complex, 6, is an effective pre-catalyst for dehydropolymerization (1 mol%, 88000 g mol -1 , Ð 1.6), and the degree of polymerization can be controlled by [H2B(NMeH2)2][BAr F 4], e.g. 1 mol% Mn = 21000 g mol -1 .Complex 6 can be used at low loadings and high [H3B•NMeH2] (0.1 mol%, 5 M in THF, using commercially sourced amine-borane), to produce high 3 molecular weight polyaminoborane on gram scale (85000 g mol -1 , 1.1 g).Use of activated charcoal in the polymer work-up reduces the [Rh]-content from 195 ppm (no workup) to 81 ppm.This is considerably lower than reported for other Rh and Co dehydropolymerization systems. 15,21,46The simple benzyl-dppp-catalyst 7 also promotes formation of high molecular weight polyaminoborane (98000 g mol -1 ).In summary, the identification of the catalyst resting state, the events that lead to its formation, and thus the role that co-products such as boronium and H2 likely play in chain-control, have provided important insights into the complex and nuanced set of interconnected processes that are required for selective amine-borane dehydropolymerization using Rh(bisphosphine)based catalysts.While the detailed elucidation of the elementary steps awaits further study, Scheme 5 provides a testable framework for the analysis and design of catalyst systems for controlled amine-borane dehydropolymerization.

Supporting Information
Full experimental, structural, kinetics data and details of the simulated model.The Supporting Information is available free of charge on the ACS Publications website.