The Catalytic Dehydrocoupling of Amine– and Phopshine– Boranes This is the final accepted copy (without the corrections to the proof) for: H. C. Johnson, T. N. Hooper and A. S. Weller The Catalytic Dehydrocoupling of Amine– and Phosphine– Boranes Top. Organomet. Chem. 2015, 49, 153 Heather C. Johnson, Thomas N. Hooper and Andrew S. Weller Department of Chemistry, University of Oxford, Oxford, OX1 3TA, UK. Email: [email protected] Abstract Mechanistic studies into the catalysed dehydrocoupling of amine– and phosphine–boranes have seen a rapid development over the last 5 years. The primary driver for this intense research effort has been the development of catalysts that might offer significant benefits with regard to the kinetics of hydrogen release, for potential use when linked with a fuel cell. Secondary to this, although becoming increasingly important, is the use of dehydrocoupling approaches to afford well–defined polymeric materials with B–N or B–P backbones that offer potential as high–performance polymers, pre–ceramic materials and as precursors to white graphene. There have been many systems studied, using catalysts incorporating metals from across the periodic table. This review attempts to bring together the insight revealed from these studies, which shows a rich and complex mechanistic landscape for the dehydrocoupling of phosphine– and amine–boranes. Keywords: Amine–Borane; Phosphine–Borane; Mechanism; Dehydrocoupling; Catalysis 1 Contents 1 Introduction 2. Transition metal catalysed dehydrocoupling of amine-boranes 2.1 General considerations 2.2 Aminoboranes: observation and trapping 2.3 Linear diborazanes 2.4 Early examples of metal-catalysed dehydrocoupling 2.5 Heterogeneous catalysts for the dehydrocoupling of amine-boranes 2.6 Transition metal catalysed dehydrocoupling of H B·NH promoted by ionic 3 3 liquids 2.7 Homogeneous dehydrocoupling of amine-boranes 2.7.1 Sigma complexes of amine-boranes 2.8 Mechanistic Studies on Homogeneous Dehydrocoupling Systems 2.8.1 Early Transition Metals 2.8.2 Mid–transition metals 2.8.3 Late Transition Metals 2.8.4 Dehydrocoupling of Amine-Boranes Involving Ligand Cooperativity 2.8.5 The role of solvent in dehydrocoupling using late transition metals. 2.9 Generic mechanisms for dehydrocoupling of H B·NMe H using transition 3 2 metals 2.10 Main Group Element Catalysed Dehydrocoupling of Amine-Boranes 2.10.1 Main Group Amidoboranes, Stoichiometric Studies 2.10.2 Group 2 Metal Catalysed Dehydrocoupling of Amine-Boranes 2.10.3 Group 3 Metal Catalysed Dehydrocoupling of Amine-Boranes 2.10.4 P-Block Catalysed Dehydrocoupling of Amine-Boranes 2.10.5 Frustrated Lewis Pair Dehydrogenation of Amine-Boranes 3. Dehydrocoupling of Phosphine-Boranes 3.1 Transition Metal Catalysed Dehydrocoupling of Phosphine-Boranes 3.2 Determination of the Active Catalytic Species: Hetero or Homogeneous 3.3 Sigma-Complexes and B-agostic Interactions of Phosphine-Boranes 3.3 Sigma-Complexes and B-agostic Interactions of Phosphine-Boranes 3.4 Stabilised Phosphinoboranes 2 3.5 Group 8 Catalysed Dehydrocoupling of Phosphine-Boranes 3.6 Mechanistic Investigations into the Rhodium Catalysed Dehydrocoupling of Secondary Phosphine-Boranes 3.7 Mechanistic Investigation into the Rhodium Catalysed Dehydrocoupling of Primary Phosphine-Boranes 3.8 Lewis Acid Catalysed Dehydrocoupling of Phosphine-Boranes 4. Future Prospects. Acknowledgments 3 1 Introduction The transition metal catalysed dehydrocoupling of amine-boranes and, to a lesser extent phosphine-boranes, has received much attention in recent years [1, 2]. For amine-boranes, the parent compound, H B·NH , is an air-stable solid containing a 3 3 high weight percentage of hydrogen (19.6%), and thus has been explored extensively as a potential candidate for chemical hydrogen storage vectors [3]. Although H B·NH can release dihydrogen on heating to temperatures above 120 °C, leading to 3 3 mixtures of products including polyborazylene and polyaminoboranes, metal catalysts have led to more efficient and controlled dehydrogenation [4]. Amine–boranes have also been studied with respect to the formation of BN–based materials. In particular polyaminoboranes, which are isoelectronic with societally and technologically ubiquitous polyolefins, have potential applications as piezoelectric materials or as precursors to BN-based ceramics[5] or white graphene [6]. Likewise the analogous dehydrocoupling of phosphine-boranes produces oligomeric and polymeric materials that show promise as electron beam resists and precursors to semi-conducting boron- phosphide. [7]. In this review we outline recent developments to elucidate, and thus harness, the mechanism of catalytic dehydrocoupling of amine– and phosphine–boranes. Although there is yet to be developed a common, detailed, overarching mechansim that encompasses all catalysed systems, we hope that this contribution serves to mark the current state–of–the art in the field and provide a background to aid future developments in the area. It is the control of these processes, to either afford well– defined final products, or the maximum rate and yield of hydrogen evolution, that makes catalytic routes attractive for dehydrocoupling. This is not the first time that dehydrocoupling of amine– and phosphine–boranes has been reviewed, and there 4 have been recent overviews dealing with their general chemistry and properties,[1, 2] role in hydrogen storage applications,[4, 8, 9] as well as dehydrocoupling processes [5, 7, 10, 11]. We do not attempt to review the extensive literature on the catalysed hydrolysis of amine–boranes to produce H as the principal product of interest [3] 2 2. Transition metal catalysed dehydrocoupling of amine-boranes 2.1 General considerations A generalised scheme for the products observed from amine–borane dehydrocoupling is shown in Scheme 1. The parent H B·NH can also lose over 2 equivalents of 3 3 hydrogen to form polyborazylene as well as often insoluble oligomeric and polymeric materials that arise from loss of less than two equivalents of H [8]. Primary amine– 2 boranes, H B·NRH , can undergo loss of one equivalent of hydrogen during 3 2 dehydrocoupling to afford polyaminoboranes [H BNRH] (R = H, Me, nBu); while 2 n borazines, [HBNR] (R = H, Me, nBu), can result from the loss of 2 equivalents of 3 dihydrogen and should be considered to be the thermodynamic product of the dehydrocoupling of primary amine-boranes. Secondary amine-boranes H B·NR H 3 2 can lose one equivalent of dihydrogen and dehydrocouple through soluble and well- defined intermediates; for H B·NMe H, the most commonly observed are the 3 2 aminoborane H B=NMe and the linear diborazane H B·NMe BH ·NMe H (see 2 2 3 2 2 2 Section 2.2). Consequently, H B·NMe H is often used as a model for 3 2 dehydrocoupling of H B·NH and H B·NMeH ,[12, 13] the products of which are 3 3 3 2 often insoluble or poorly defined polymeric or oligomeric materials [14]. The cyclic dimer [H BNR ] (R = e.g. Me, Et) is generally formed as the major dehydrocoupling 2 2 2 5 product of H B·NR H. With bulky N-substituents, e.g. iPr or Cy, dimerisation is 3 2 prevented and instead the aminoborane H B=NR results [15, 16]. 2 2 Scheme 1 Simplified dehydrocoupling pathway for H B·NMe H, H B·NMeH and 3 2 3 2 H B·NH . The generation of intermediate aminoboranes is not shown, but is implicit 3 3 for many processes. 2.2 Aminoboranes: observation and trapping The dehydrocoupling of amine-boranes is often proposed to proceed via formation of aminoboranes H B=NRR’ that arise from initial dehydrogenation; and aminoboranes 2 such as e.g. H B=NtBuH and H B=NMe have been directly observed as 2 2 2 intermediates in catalytic dehydrocoupling of their respective amine-boranes [17, 18]. The kinetics for the ‘off-metal’ dimerisation of H B=NMe to form [H BNMe ] have 2 2 2 2 2 been explored, and found to be a second order process with a large negative entropy of activation [19]. Interestingly, a significant solvent effect on the relative rate of dimerisation has also been noted, with acetonitrile accelerating the process [13, 20]. The less bulky congeners H B=NH [21] and H B=NMeH,[22, 23] however, have not 2 2 2 been directly observed as intermediates in dehydrocoupling, although they have been isolated coordinated to a transition metal fragment, being formed from dehydrogenation of the corresponding amine–borane [24]. In 2008, Baker, Dixon and 6 coworkers proposed that H B=NH liberated from the metal results in the eventual 2 2 production of borazine, whereas H B=NH (or derivatives thereof) remaining bound 2 2 to the metal results in oligomeric or polymeric products [25]. To detect free aminoborane, cyclohexene was added to reaction mixtures, as cyclohexene can be hydroborated by H B=NRH (R = H, Me), forming Cy B=NRH (Scheme 2) thereby 2 2 acting as a useful marker for free aminoboranes. Scheme 2 Trapping of aminoboranes by cyclohexene. R = Me, H. Accordingly, when cyclohexene was added to a reaction mixture of H B·NH and 3 3 [Rh(1,5-cod)(µ-Cl)] , Cy B=NH was the major product observed instead of the 2 2 2 expected borazine (see Section 2.4[26]). However, upon addition of cyclohexene to a solution of H B·NH and catalyst Ir(tBuPOCOPtBu)(H) ,[27] the same oligomeric 3 3 2 products were observed as in the absence of cyclohexene, i.e. no hydroboration product was observed (Section 2.8.3). Although cyclohexene trapping is still regarded as a useful method for detecting free aminoboranes, more recent studies have suggested that the absence of hydroboration does not necessarily reflect an absence of free aminoborane. It has been suggested that, if borazine formation (from aminoborane trimerisation/dehydrogenation) or hydroboration of cyclohexene are not kinetically competitive with metal-based BN oligomerisation/polymerisation processes, Cy B=NH will not be observed even if H B=NH is present [21, 28, 29]. 2 2 2 2 2.3 Linear diborazanes 7 Another intermediate often observed in the dehydrocoupling of H B·NMe H is the 3 2 linear diborazane H B·NMe BH ·NMe H [15]. Schneider has calculated that the 3 2 2 2 pathway for B—N bond cleavage of H B·NMe BH ·NMe H to generate H B·NMe H 3 2 2 2 3 2 and H B=NMe is close to thermoneutral (ΔG = –2.3 kcal mol-1) [30]. Therefore if 2 2 this process is reversible the position of the equilibrium (and hence whether H B·NMe BH ·NMe H is observed in catalysis) is likely to be dependent upon their 3 2 2 2 relative concentrations and rates of formation with a particular catalyst. A general pathway for the dehydrocoupling of H B·NMe H with Schneider’s ruthenium 3 2 catalysts (see Section 2.8.4) was developed (Scheme 3), suggesting that the formation of H B·NMe BH ·NMe H is a metal–based process. The role of this diborazane in the 3 2 2 2 dehydrocoupling of H B·NMe H has been further discussed by others [15, 19, 28, 31, 3 2 32]. Scheme 3 Schneider’s early model for dehydrocoupling H B·NMe H to form 3 2 [H BNMe ] . 2 2 2 Weller and Manners have since reported that the diborazane H B·NMeHBH ·NMeH , 3 2 2 the product of one dehydrooligomerisation of H B·NMeH , can be formed by 3 2 catalytic methods[33] and its role as a possible intermediate in dehydropolymerisation has been further explored (see Section 2.8.3) [28]. Shore and co-workers have also reported the synthesis by stoichiometric methods of the H B·NH analogue, 3 3 8 H B·NH BH ·NH ,[34] while Sneddon and co-workers have reported the synthesis of 3 2 2 3 triborazanes, such as H B·(NH BH ) ·NH ,[35] which are implicated in 3 2 2 2 3 dehydropolymerisation processes [36] 2.4 Early examples of metal-catalysed dehydrocoupling The first example of transition metal catalysed dehydrocoupling was reported in 1989 by Roberts and co-workers. The amine-borane H B·NtBuMeH was dehydrogenated at 3 120 °C by 10% Pd on charcoal to form the aminoborane H B=NtBuMe, which 2 dimerised to form [H BNtBuMe] [37]. In 2001 Manners and co-workers reported that 2 2 RhI or RhIII precursors catalytically dehydrocoupled secondary amine-boranes H B·NR H (R = Me , cyclo-C H ) to yield the corresponding cyclic dimer 3 2 2 2 4 8 [H BNR ] (Scheme 4). The RhI precursor was also an effective catalyst for the 2 2 2 dehydrocoupling of H B·NH and H B·NMeH to form their respective borazines, 3 3 3 2 although in both cases insoluble material, indicative of oligomeric chains, was also observed in the reaction mixtures [26] Scheme 4 Dehydrocoupling of amine-boranes by [Rh(1,5-cod)(µ-Cl)] and 2 RhCl ·3H O. 3 2 9 2.5 Heterogeneous catalysts for the dehydrocoupling of amine-boranes Various systems act as heterogeneous catalysts for amine-borane dehydrocoupling by the formation in situ of catalytically active nanoparticles, although the nature of the actual catalytic component can be the subject of debate. Nonetheless, heterogeneous catalysis is attractive due to facile separation of the products and catalyst. The dehydrocoupling of H B·NMe H by [Rh(1,5-cod)(µ-Cl)] showed a reaction profile 3 2 2 with an induction period, during which a black precipitate was observed to form. Tests, originally developed by Finke,[38] were performed to probe for heterogeneous catalysis. For example, both filtration and catalyst poisoning with mercury halted catalysis (Figure 1), suggesting a heterogeneous system in which the dehydrocoupling is catalysed by rhodium nanoparticles [39]. Figure 1 Left: Addition of mercury to the reaction mixture. Right: The effect of filtration and poisoning with PPh . Both figures reprinted (adapted) with permission 3 from C. A. Jaska and I. Manners, J. Am. Chem. Soc., 2004, 126, 9776-9785. Copyright 2004 American Chemical Society. Later EXAFS studies by Autrey and co-workers suggested that, instead, soluble Rh 6 clusters are responsible for the dehydrocoupling activity in this system [40]. 10