NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis
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inorganics Communication NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis Catherine Weetman 1 , Nozomi Ito 2, Masafumi Unno 2 , Franziska Hanusch 1 and Shigeyoshi Inoue 1,* 1 WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, 85748 Garching bei München, Germany 2 Department of Chemistry and Chemical Biology, Gunma University, Kiryu 376-8515, Japan * Correspondence: [email protected]; Tel.: +49-89-289-13596 Received: 26 June 2019; Accepted: 12 July 2019; Published: 24 July 2019 Abstract: The catalytic dehydrocoupling of amine–boranes has recently received a great deal of attention due to its potential in hydrogen storage applications. The use of aluminum catalysts for this transformation would provide an additional cost-effective and sustainable approach towards the hydrogen economy. Herein, we report the use of both N-heterocyclic imine (NHI)- and carbene (NHC)-supported Al(III) hydrides and their role in the catalytic dehydrocoupling of Me2NHBH3. Differences in the σ-donating ability of the ligand class resulted in a more stable catalyst for NHI-Al(III) hydrides, whereas a deactivation pathway was found in the case of NHC-Al(III) hydrides. Keywords: aluminum; amine–borane; dehydrocoupling; homogeneous catalysis; N-heterocyclic carbenes; N-heterocyclic imines 1. Introduction Main group chemistry has seen a resurgence of interest in recent years, driven by the need for more economically viable and eco-friendly processes. Whilst catalytic transformations using transition metals are well established, the long-term sustainability of these naturally low-abundance metals is limited. In contrast, use of p-block elements such as aluminum allows for the use of earth-abundant and environmentally benign elements, with aluminum being the third most abundant element in the Earth’s crust. Owing to their high stability and Lewis acidity, Al(III) catalysts in the form of trialkyl or trihalide derivatives have traditionally been used in Zieglar–Natta [1] and Friedel–Crafts [2] reactions, respectively. In comparison, the chemistry of low-oxidation and/or coordinate Al complexes (and p-block complexes) is still in its infancy, and advances in past decades have shown that main group complexes can act as transition metals [3–5]. Few examples of low-oxidation and/or coordinate Al + complexes in catalysis have been reported, such as the use of aluminum ions, R2Al , in polymerization systems [6,7] and the recent example of dialumene—a compound with a neutral aluminum–aluminum double bond—for the catalytic reduction of CO2 [8]. At the forefront of main group catalysis, with the exception of frustrated Lewis pair (FLP) chemistry [9–11], is the use of metal hydrides, whereupon non-redox-based catalytic cycles have been proposed using a sequence of σ-bond metathesis and insertion reactions. Examples of both s- and p-block metal hydrides for the hydroelementation of unsaturated organic substrates, including CO2, have been reported in recent years [12,13]. In terms of aluminum hydride catalysis, the use of a β-diketiminate-supported Al(III) hydride (LAlH2) was made more reactive by increasing its Lewis acidity upon reaction with MeOTf (Tf = SO2CF3) to yield LAl(H)(OTf). The retention of the Al–H moiety and increased positive charge at Al drew comparisons to transition metal catalysts, as it showed excellent activity towards the hydroboration of carbonyl-containing substrates [14]. Other recent Inorganics 2019, 7, 92; doi:10.3390/inorganics7080092 www.mdpi.com/journal/inorganics Inorganics 2019, 7, 92 2 of 11 Inorganics 2019, 7, x FOR PEER REVIEW 2 of 12 examplesrecent examples of Al hydrides of Al hydrides in catalysis in catalysis have shown have that shown commercially that commercially available available Al–H-containing Al–H‐containing reagents such as LiAlH and DIBAL-H are active towards the hydrogenation of imines using 1 bar of H [15] reagents such 4as LiAlH4 and DIBAL‐H are active towards the hydrogenation of imines using 1 bar2 of and hydroboration of alkynes [16], respectively. H2 [15] and hydroboration of alkynes [16], respectively. InterestInterest inin dehydrocouplingdehydrocoupling reactions reactions has has recentlyrecently risen, risen, asas itit providesprovides aa cleanclean and atom-eatom‐efficientfficient routeroute toto newnew element–elementelement–element bonds,bonds, withwith thethe concomitantconcomitant formationformation ofof dihydrogen,dihydrogen, whichwhich hashas numerousnumerous applicationsapplications inin organicorganic synthesissynthesis andand materialsmaterials chemistrychemistry [17[17–19].–19]. TheThe formationformation ofof dihydrogendihydrogen in in these these typestypes ofof reactionsreactions hashas gainedgained thethe mostmost attentionattention duedue toto the potential for hydrogen storage and therefore sustainable energy sources. In this regard, ammonia–borane (NH BH ) has been storage and therefore sustainable energy sources. In this regard, ammonia–borane (NH33BH33) has been earmarkedearmarked asas anan idealideal candidatecandidate forfor aa futurefuture “hydrogen“hydrogen economy”economy” duedue toto itsits highhigh weightweight percentagepercentage ofof hydrogenhydrogen [ 20[20,21].,21]. As As such, such, catalysts catalysts from from across across the the periodic periodic table table have have shown shown their their potential potential in the in catalytic release of H from amine–boranes. Whilst transition metals dominate the field [22], examples the catalytic release2 of H2 from amine–boranes. Whilst transition metals dominate the field [22], fromexampless-block from [23 –s26‐block], f -block [23–26], [27, 28f‐],block and metal-free[27,28], and [29 metal–31] based‐free [29–31] systems based have alsosystems contributed have also to furtheringcontributed our to furthering understanding our understanding of these complex of these mechanisms. complex Centralmechanisms. to most Central of the to metal-based most of the systemsmetal‐based is the systems formation is ofthe metal formation hydrides of [metal22] and hydrides their subsequent [22] and reactivitytheir subsequent in enabling reactivity turnover. in Inenabling terms of turnover. Al hydrides In terms in dehydrocoupling of Al hydrides catalysis, in dehydrocoupling Wright and co-workers catalysis, haveWright reported and co the‐workers use of Al(III)have reported amide pre-catalysts the use of Al(III) for amine–borane amide pre‐catalysts [32–36] and for amine–silaneamine–borane [37 [32–36]] dehydrocoupling, and amine–silane in which [37] thedehydrocoupling, active Al hydride in which forms inthe situ. active Al hydride forms in situ. RecentRecent workwork withinwithin ourour groupgroup hashas shownshown thethe useuse ofof N-heterocyclicN‐heterocyclic iminesimines (NHIs)(NHIs) forfor thethe stabilizationstabilization of of group-13 group‐13 metal metal hydrides hydrides [38]. [38]. This classThis ofclass ligand of isligand comparable is comparable to that of N-heterocyclicto that of N‐ carbenesheterocyclic (NHCs), carbenes which (NHCs), are widely which established are widely as ligands established for many as catalytic ligands transformations for many catalytic [39], as theytransformations can both be considered [39], as they to can be strong both be 2σ -electronconsidered donors to be (Figure strong1 2).σ‐ Theelectron strong donors donating (Figure capabilities 1). The ofstrong NHI donating ligands has capabilities allowed of for NHI the stabilizationligands has allowed and subsequent for the stabilization isolation of and a variety subsequent of aluminum isolation hydridesof a variety which of aluminum have been usedhydrides for a which number have of key been transformations, used for a number such of as key the activation transformations, of elemental such sulfuras the [ 40activation] and the dehydrogenativeof elemental sulfur coupling [40] andand subsequent the dehydrogenative formation of acoupling rare aluminum and subsequent chalcogen double-bondedformation of a rare species aluminum [41]. Notably, chalcogen the NHI-supporteddouble‐bonded aluminumspecies [41]. hydrides Notably, proved the NHI to be‐supported efficient catalystsaluminum in thehydrides hydroboration proved to of be terminal efficient alkynes catalysts and in carbonyl the hydroboration compounds [of42 terminal]. Thus, our alkynes attention and turnedcarbonyl towards compounds the use [42]. of Thus, NHI-supported our attention aluminum turned towards hydrides the for use dehydrocoupling of NHI‐supported catalysis aluminum and comparisonshydrides for withdehydrocoupling NHC aluminum catalysis hydrides. and comparisons with NHC aluminum hydrides. FigureFigure 1.1. ArchetypalArchetypal ligand ligand systems systems comprising comprising the the five-membered five‐membered N-heterocycle N‐heterocycle structural structural motif motif and resonanceand resonance forms. forms. NHC: NHC: N-heterocyclic N‐heterocyclic carbene; carbene; NHI: NHI: N-heterocyclic N‐heterocyclic imine. imine. 2.2. Results and Discussion ToTo compare compare the the catalytic catalytic potential potential of NHI- of andNHI NHC-stabilized‐ and NHC‐stabilized Al(III) hydrides, Al(III) ahydrides, direct comparison a direct betweencomparison the between two ligand the classes two ligand bearing classes the bearing same substituents the same substituents was targeted. was For targeted. ease of For synthesis, ease of 2,6-diisopropylphenylsynthesis, 2,6‐diisopropylphenyl (Dipp) and 2,4,6-trimethylphenyl(Dipp) and