Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine-boranes using a [Rh(Xantphos)]⁺ catalyst.

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2B═NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.


Introduction
Catalytic routes for the formation of main-group/main-group bonds are important for the targeted construction of new molecules and materials. However, enabling catalytic methodologies for such bond forming events lag behind those developed for the construction of C-C and C-X bonds. 1 The development of reliable, robust and controllable processes is thus an important challenge. [2][3][4][5] Catalytic dehydropolymerization 6 of amine-boranes to give polyaminoboranes presents one such opportunity, as this produces new BN polymeric materials that are isoelectronic with technologically pervasive polyolefins. Such new materials have potential applications as high performance polymers and as precursors to BN-based ceramics and single layer hexagonal BN thin films (white graphene). 7 Although ill-defined branched, oligomeric materials that have been termed "polyaminoborane" have historically been prepared by non-catalytic methods, [8][9][10][11] it is only recently that high molecular weight, essentially linear polyaminoboranes have been produced by catalytic methods from amine-boranes such as H 3 15 In 2008 the former group 16 also described that the dehydrooligomerization of H 3 B·NMeH 2 at low relative concentrations of amine borane, or mixtures of the latter with H 3 B·NH 3 , gave low molecular weight but soluble oligomers (M n less than ca. 2,500 g mol -1 ). Independently in 2008, Manners and co-workers 17 reported the production of high molecular weight [H 2 BNMeH] n (M n = 55,200 g mol -1 , PDI = 2.9) and related materials at low catalyst loadings (0.3 mol%) using both high and low concentrations of substrates. 15 18 as have Mn(h 5 -C 5 H 5 )(CO) 3 , Cr(h 6 -C 6 H 6 )(CO) 3 and Cr(CO) 6  also catalyze polyaminoborane formation, the latter at very low (less than 0.1 mol%) loadings. Ionic liquids have also been shown to support the formation of polyaminoboranes from H 3 B·NH 3 when used in conjunction with metal-based catalysts. 24 It is also noteworthy that anionic oligomerization approaches to both linear and branched short chain aminoboranes have recently been described. 25,26 Mechanistic studies focussing on the dehydropolymerization of H 3 B·NMeH 2 or H 3 B·NH 3 substrates are few in number. Nevertheless important observations and overarching rationales have been suggested from these studies. This relative dearth can be compared to studies with H 3 B·NMe 2 H, which are considerably more numerous, and often demonstrate subtle differences in likely mechanistic pathways depending on identity of the metal-ligand fragment. 2,18,[27][28][29][30][31][32][33] The polymer growth kinetics (molecular weight versus conversion) using the Ir( t BuPOCOP t Bu)H 2 / H 3 B·NMeH 2 system suggest the operation of a modified chain-growth mechanism that involves both a slow metalbased dehydrogenation of amine-borane and faster insertion/polymerization of the resulting amino-borane. 15 Using the same system, sigma-bound amine-borane intermediates for catalytic redistribution of oligomeric diborazanes have recently been proposed on the basis of kinetic modelling. 34 Using catalyst systems based upon Fe(PhNCH 2 CH 2 NPh)(Cy 2 PCH 2 CH 2 PCy 2 ) / H 3 B·NH 3 an initiation mechanism that invokes an Fe-amido-borane has been suggested, which then undergoes dehydrogenative insertion of additional H 3 B·NH 3 to form polyaminoborane. 22 For Ru(PNP)(H)(PMe 3 ) / H 3 B·NH 3 a mechanism is proposed, based upon experimental and DFT studies, in which amino-borane is formed in a low, but steady state, concentration that undergoes catalysed polymerization by an enchainment reaction that relies upon metal-ligand cooperatively. 23 Kinetic studies using the Ir( t BuPOCOP t Bu)H 2 16 and Ru(PNP)(H)(PMe 3 ) 23 systems demonstrate a first order dependence on both amineborane and catalyst concentrations, although for the latter catalyst when H 3 B·ND 3 was used there was a zero-order dependence on this substrate suggesting a change in the turnover limiting step. A number of apparently homogeneous 35 37 Catalyst systems in which amino-borane is suggested to not be released from the metal do not form the hydroborated product during dehydropolymerization, while for those that form borazine from trimerization of free amino-borane, or when aminoborane is produced thermally in the absence of a metal-ligand fragment, 34 the hydroborated product is observed in significant quantities. However, recent experimental and computational studies using Ir( t BuPOCOP t Bu)H 2 or Ru(PNP)(H)(PMe 3 ) suggest that if hydroboration or borazine formation are not kinetically competitive with metalpromoted B-N coupling then Cy 2 B=NH 2 will not be observed, even if free amino-borane is formed transiently. 23,34 Adding to this complexity, hydrogen redistribution reactions can also occur, in which amino-boranes take part in hydrogen-transfer with amineboranes, 34,39 while a nucleophilic solvent (e.g. THF) can also potentially catalyse polyaminoborane formation from amino-boranes. 40 Mechanistic insight that comes from the direct observation of intermediates in dehydropolymerization is also very rare, although off-cycle products have been reported. 13,29,41 The product of the first insertion event of H 3  These studies lead to an overall mechanistic framework for dehydropolymerization using transition metal fragments that supports, and puts detail upon, the dehydrogenation/coordination/insertion mechanism proposed by others. 15,22,23,28,37 This insight leads the to gross control of the degree of dehydropolymerization, allowing for both low and higher molecular weight polyaminoborane to be obtained.  69 We have recently 39 shown that when the amino-borane H 2 B=NH t Bu is released from a metal center it undergoes trimerisation to form [HBN t Bu] 3 by an (unresolved) mechanism in which hydrogen redistribution processes are occurring, 34 and it is possible that such processes are also operating here. As found for 5, complex 7 undergoes a second, slower, dehydrogenation.

Stoichiometric
This process is a little faster than for 5, taking 2 hours to fully consume 7 to afford III and an equilibrium mixture of 6/1. trimethylborazine III, and there is an induction period observed before catalysis. These observations suggest additional mechanistic considerations need to be adopted under the conditions of high ratios of amine-borane to metal-ligand fragment, and these are discussed next.  Figure 1a). 15 No significant signals were observed around d 0 which might indicate chain branching, 23 although such a feature if small could be lost in the peak width of the main signal. To the detection limit of 11

Effect of Solvent on Polymerization
Changing the solvent to THF produced polyaminoborane (catalyst = 1, 0.2 mol%) with higher molecular weight (M n = 52 200 versus 22 700 g mol -1 ) than for C 6 H 5 F solvent, but now taking a significantly longer time to reach near completion (19 hr versus 2 hr, Table   1). This suggests THF slows the rate of dehydropolymerization, possibly by the reversible formation of an adduct (cf 4), and this may also have a role to play in attenuating any chain termination events if competitive with H 2 binding 71 (see below).
Alternatively, more of the catalyst could sit as the simple adduct species 4 leading to fewer active metal sites, and thus longer polymer chains growing from the metal. THF may also solvate the growing polymer better leading to longer chain growing from the metal. Only a very small quantity of trimethylborazine, III, was observed (1-2%). THF solvent might also result in a change in mechanism to one which involves hydride donation to the metal to form a THF-stabilized borenium, i.e. [(NMeH 2 )(THF)BH 2 ] + . 32 Polymer growth kinetics and control over molecular weight using hydrogen.
A plot of number-averaged degree of polymerization, DP n [DP n = M n / M w (H 2 B=NMeH)] versus conversion for the dehydrocoupling of H 3 B·NMeH 2 using 1 (0.2 mol%, open system) shows a relationship that is suggestive of a predominately chain growth mechanism for the growing polymer (Scheme 11). Such a process has been proposed previously for the [Ir( t BuPOCOP t Bu)H 2 ] catalyst system for which a modified chain growth mechanism is invoked, in which slow dehydrogenation to form amino-borane is followed by faster metal-mediated polymerization of this unsaturated fragment. 15 This suggestion is on the basis of the polymer conversion kinetics that show that high molecular weight polymers are present at low (less than 40%) conversion; coupled with the observation that higher catalyst loadings lead to higher molecular weight polymer. A similar mechanism has been proposed for the dehydropolymerization of ammoniaborane using bifunctional Ru-catalysts. 23 Our polymer conversion kinetics suggest a similar mechanism is operating, in that there is a high degree of polymerization at low conversion (M n = 30 800 g mol -1 , PDI = 1.4 at 20% conversion; M n = 25 300 g mol -1 , PDI = 1.6 at 100% conversion). 72 However, in contrast to the [Ir( t BuPOCOP t Bu)H 2 ] systems, when the catalyst loading is increased (i.e. x 5 the loading, 1 mol%) the polymer that results is now of significantly lower molecular weight, but similar polydispersity, (M n = 7 500 g mol -1 , PDI = 1.5). This strongly suggests a metal-centered process, as initially This shows that the catalyst remains active directly after catalysis has finished, but it is not a living system and there must be some chain transfer/termination process occurring.
In a closed system (Youngs flask, ~ 30 cm 3 volume, stirred) dehydropolymerization also proceeds essentially to completion (Scheme 11, Table 6), but over a much longer timescale than in an open system (24 hrs versus 2 hrs) The resulting isolated solid is waxy in appearance, suggesting a lower M n polymer compared with the free flowing solid produced in an open system. A 11 B{ 1 H} NMR spectrum of this material shows a broad, poorly resolved peak centred around d -5 that also shows evidence for shorter chain oligomeric species, cf. H 3 B·NMeHBH 2 ·NMeH 2 , 39 by an overlaid sharper signal that becomes a broad triplet in the 11 B NMR spectrum (Figure 1b). There is also a smaller intensity signal ca. d -18 in the region associated with BH 3 groups, 29  As discussed in the Introduction, the hydroboration of exogenous cyclohexene has previously been shown act as a marker for the presence of free amino-borane H 2 B=NMeH in dehydropolymerization reactions. 22,34,37 In the presence of cyclohexene using 50 mol% of 1 with H 3 B·NMeH 2 , the hydroborated product Cy 2 B=NMeH is observed as the major boron-containing product, alongside III as the minor product (Scheme 12). This suggests that at low substrate concentration free amino-borane is generated, that has sufficient lifetime for reaction with cyclohexene. By contrast, at high substrate concentrations (0.2 mol% 1) no hydroborated product is observed. Instead polymer is produced, interestingly with a significantly higher molecular weight than formed in the absence of cyclohexene (M n = 38 600 g mol -1 , PDI = 1.8). A small amount of cyclohexane is also formed (~5% conversion). This suggests that under this concentration regime free amino-borane is not produced in concentrations that allow for hydroboration of cyclohexene. As 2 has been reported to reduce cyclohexene to cyclohexane while becoming a Rh(I) species, 50      which has sufficient stability to be crystallographically characterized (Figure 2), alongside 3, in a 7:1 ratio. very similar kinetic behavior in their consumption during catalysis, although the final products differ. This suggests that there is a common mechanistic framework that links the two, although certain details will be different, for example in the final products of the B-N bond forming event. Any mechanistic scenario suggested is required to satisfy a number of criteria that flow from our observations on these two systems: • There is a slow induction period, that is proposed to involve N-H activation; • Catalysis appears to occur in the Rh(III) oxidation state, rather than a Rh(I)/Rh(III) cycle; • Polymer kinetics support a predominately chain growth process, there is a singlesite model for polymer propagation, and the catalyst is not living; • Chain transfer/termination is modified by H 2 and THF, the former resulting in shorter polymer chains, the latter in longer chains; • Saturation kinetics operate during the productive phase of catalysis, i.e. a pseudo zero order in substrate during the early phase of productive catalysis; • In a sealed system (i.e. under H 2 ) turnover is slower and follows a first order decay (as measured for H 3 B·NMe 2 H). This inhibition by H 2 is reversible, as opening the closed system (i.e. release of H 2 ) results in an increase in relative rate.
• At low substrate concentration borazine forms and exogenous cyclohexene is hydroborated, indicating free amino borane; • At high substrate concentration no borazine forms and cyclohexene is not hydroborated; • Catalytic turnover proceeds via a resting state that is suggested to be an amidoborane; • Immediately at the end of catalysis activity is retained in both closed and open systems.
We propose the mechanism shown in Scheme 18 as one that best fits the available data.
Addition of amine borane to 1 results in rapid dehydrogenation and hydrogen transfer to the metal, presumably via a transient sigma complex A, to give a Rh(III) dihydride (e.g.

5)
. This can also be accessed by direct addition of amine-borane to the preformed The key feature of this suggested mechanism is the generation of an active catalyst, proposed to be an amido-borane, that then reversibly binds additional amineborane so that saturation kinetics operate during catalysis. B-N bond formation (with H 3 B·NMeH 2 ) or elimination of amino-borane (with H 3 B·NMe 2 H) follows, in which N-H activation is proposed to be turn-over limiting. Importantly, for the dehydropolymerization of H 3 B·NMeH 2 we also demonstrate that polymer formation follows a chain growth processes from the metal, and that control of polymer molecular weight can be also achieved by using H 2 or THF solvent. Hydrogen is suggested to act as a chain transfer agent, leading to low molecular weight polymer, THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. Although the molecular weights of polymeric material obtained are still rather modest compared to the previously reported Ir( t BuPOCOP t Bu)(H) 2 system, the insight available from using the valence isoelectronic [Rh(Xantphos)(H) 2 ] + fragment leads to a mechanistic framework that explains the experimental observations and polymer growth kinetics. The suggested mechanism for dehydropolymerization is one in which the putative amido-borane species dehydrogenates an additional H 3 B·NMeH 2 to form the "real monomer" H 2 B=NMeH that then undergoes insertion into the Rh-amido bond to propagate the growing polymer chain on the metal. This is directly analogous to the chain growth mechanism for single-site olefin polymerization. 1 A future challenge is thus to use this insight to develop catalysts capable of living polymerization and/or control of polymer tacticity as so elegantly demonstrated with polyolefin chemistry; and it will be interesting to see if the mechanistic themes discussed here are applicable in a more general sense to other catalyst systems.
Supporting Information. Experimental and characterization details, including NMR data, X-ray crystallographic data, polymer characterization data and kinetic plots. This material is available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data have been deposited with the Cambridge Crystallographic Data Center (CCDC) and can be obtained via www. ccdc.cam.ac.uk/data_request/cif.

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.