NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

Home , Aluminium hydride, Borane, Boranes, Frustrated Lewis pair

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 2,4,6‐trimethylphenyl (Mes) substituents (Mes) were substituents used for were both NHI used and for NHCboth NHI ligands. and FurtherNHC ligands. comparisons Further within comparisons the NHC within ligand the class NHC were ligand drawn class between were drawn aryl and between alkyl groups.aryl and Synthesis alkyl groups. of the Synthesis six diﬀerent of the Al(III) six hydridesdifferent (Al(III)1–6) used hydrides in this ( study1–6) used (Figure in this2) were study prepared (Figure according2) were toprepared literature according procedures to and literature then subsequently procedures trialed and in dehydrocouplingthen subsequently catalysis. trialed in dehydrocoupling catalysis.

Inorganics 2019, 7, x FOR PEER REVIEW 3 of 12

Inorganics 2019, 7, 92 3 of 11 Inorganics 2019, 7, x FOR PEER REVIEW 3 of 12

Figure 2. Series of Al(III) hydrides utilized in the dehydrocoupling of amine–boranes.

Notable differences between the two ligand classes are apparent upon comparison of the FigureFigure 2. 2. SeriesSeries of of Al(III) Al(III) hydrides hydrides utilized utilized in in the the dehydrocoupling dehydrocoupling of amine–boranes. structures. Due to the presence of the imine moiety in NHIs, the steric protection is removed from the metalNotable center, didifferencesﬀerences and as between thebetween NHI the ligands the two two ligand can ligand act classes as aclasses multifunctional are apparent are apparent upon ligand, comparison upon it forms comparison of both the structures. σ‐ ofand the π‐ structures.Duedonor to thebonds presenceDue to toAl, the ofresulting thepresence imine in moietyofa dimericthe imine in NHIs, Al–H moiety2 the complex steric in NHIs, protection which the prevailssteric is removed protection in solution from is the removedand metal solid center, fromstate. theandInitial metal as attempts the center, NHI ligandsof and using as canthe catalysts actNHI as ligands a1 multifunctionaland 3 canin a act series as a ligand,of multifunctional silane–amine it forms both (PhSiH ligand,σ- and3 andit formsπ HNiPr-donor both2) bonds and σ‐ and silane– to π‐ Al, resultingborane (PhSiH in a dimeric3 and HBpin) Al–H dehydrocouplingcomplex which prevails reactions in solutiondid not result and solid in turnover, state. Initial even attempts at elevated of donor bonds to Al, resulting2 in a dimeric Al–H2 complex which prevails in solution and solid state. usingtemperatures. catalysts 1However,and 3 in a the series use of of silane–amine dimethylamine–borane (PhSiH and (Me HNiPr2NHBH) and3) resulted silane–borane in turnover (PhSiH underand Initial attempts of using catalysts 1 and 3 in a series of silane–amine3 (PhSiH2 3 and HNiPr2) and silane–3 HBpin)mild conditions. dehydrocoupling reactions did not result in turnover, even at elevated temperatures. However, borane (PhSiH3 and HBpin) dehydrocoupling reactions did not result in turnover, even at elevated the use of dimethylamine–borane (Me NHBH ) resulted in turnover under mild conditions. temperatures. However, the use of dimethylamine–borane2 3 (Me2NHBH3) resulted in turnover under 2.1. Catalysis mild conditions. 2.1. Catalysis The initial reaction of 5 mol % of 1 with Me2NHBH3 showed a minor formation of H2 after 24 h 2.1.at room CatalysisThe initialtemperature. reaction Increasing of 5 mol % the of 1temperaturewith Me2NHBH to 503 showed°C for the a minor same formationtime period of also H2 after resulted 24 h atin roomminor temperature. consumption Increasing of Me2NHBH the temperature3. Upon increasing to 50 ◦C the for temperature the same time to period 80 °C, also visible resulted gas evolution in minor The initial reaction of 5 mol % of 1 with Me2NHBH3 showed a minor formation of H2 after 24 h consumptionwas noted to occur, of Me2 andNHBH monitoring3. Upon increasingby both 1H theand temperature 11B NMR spectroscopy to 80 ◦C, visible showed gas the evolution formation was of at room temperature. Increasing the temperature1 11 to 50 °C for the same time period also resulted in notedthe cyclic to occur, dimer and [Me monitoring2NBH2]2 (A by) bothand minorH and formationB NMR of spectroscopy the borane showed species the(HB(NMe formation2)2 (B of)). the In minor consumption of Me2NHBH3. Upon increasing the temperature to 80 °C, visible gas evolution cyclic dimer [Me NBH ] (A) and minor formation of the borane species (HB(NMe ) (B)). In contrast, wascontrast, noted upon to occur, the2 useand2 of2monitoring NHC catalyst by both 3, turnover 1H and 11 wasB NMR achieved spectroscopy under milder showed 2conditions2 the formation and theof uponformation the use of A of was NHC noted catalyst to occur3, turnover at 50 °C. wasFurther achieved screening under using milder catalysts conditions 1–6 (Table and 1) the also formation revealed the cyclic dimer [Me2NBH2]2 (A) and minor formation of the borane species (HB(NMe2)2 (B)). In of A was noted to occur at 50 C. Further screening using catalysts 1–6 (Table1) also revealed lower contrast,lower but upon generally the use prolonged of NHC◦ catalyst reaction 3, timesturnover for wasNHC achieved‐supported under Al(III) milder hydrides, conditions whilst and NHI the‐ but generally prolonged reaction times for NHC-supported Al(III) hydrides, whilst NHI-supported formationsupported of catalysts A was noted required to occur shorter at 50 times °C. Further but higher screening temperatures. using catalysts Catalysts 1–6 1 (Table–6 performed 1) also revealed at about catalysts required shorter times but higher temperatures. Catalysts 1–6 performed at about the same lowerthe same but rategenerally in comparison prolonged to previousreaction reportstimes for of Al(III)NHC‐ supportedhydride catalysis Al(III) ofhydrides, Me2NHBH whilst3 [33,34]. NHI ‐ rate in comparison to previous reports of Al(III) hydride catalysis of Me NHBH [33,34]. supported catalysts required shorter times but higher temperatures. Catalysts2 1–63 performed at about Table 1. Dehydrocoupling of Me2NHBH3 with 5 mol % Al(III) hydride catalysts (1–6). the same rate in comparison to previous reports of Al(III) hydride catalysis of Me2NHBH3 [33,34]. Table 1. Dehydrocoupling of Me2NHBH3 with 5 mol % Al(III) hydride catalysts (1–6).

Table 1. Dehydrocoupling of Me2NHBH3 with 5 mol % Al(III) hydride catalysts (1–6).

Catalyst Temp/ C Time/h Conversion a/% b 1 Catalyst Temp/°C◦ Time/h Conversion a/% TOFTOF b/h−/1h − 1 1 8080 34 90 90 0.53 0.53 2 80 34 84 0.49 2 80 34 84 a 0.49b −1 Catalyst3 Temp/°C50 Time/h 65 Conversion 80 /% TOF /h 0.25 413 508050 34 2665 9080 92 0.530.25 0.74 524 508050 34 6526 8492 93 0.490.74 0.29 635 505050 65 4065 8093 92 0.250.29 0.46 6 50 40 92 0.46 4 c 50 26 92 0.74a Reaction conditions 5 mol % 1–6 (0.012 mmol), Me2NHBH3 (0.24 mmol), 0.5 mL C6D6. conversion of Me2NHBH3 c a determinedReaction conditions by 11B NMR 5 relative mol % integrals; 1–506 (0.012 b TOF mmol),= (conversion65 Me2NHBH/catalyst3 93(0.24 loading) mmol),/time. 0.5c Catalyst0.29 mL C6D loading6. conversion determined of 11 b perMe molecule2NHBH3 relative determined to6 amine–borane by B NMR50 concentration. relative40 integrals; TOF92 = (conversion/catalyst0.46 loading)/time. c Catalyst loading determined per molecule relative to amine–borane concentration. Reaction conditions 5 mol % c 1–6 (0.012 mmol), Me2NHBH3 (0.24 mmol), 0.5 mL C6D6. a conversion of

Me2NHBH3 determined by 11B NMR relative integrals; b TOF = (conversion/catalyst loading)/time. c Catalyst loading determined per molecule relative to amine–borane concentration.

Inorganics 7 Inorganics 2019, 20197, x FOR, , 92 PEER REVIEW 4 of 124 of 11

Differences between the two ligand classes were found upon inspection of the product Differences between the two ligand classes were found upon inspection of the product distribution distribution (Figure 3 and Table S1), indicating likely subtle changes in the stability of reaction (Figure3 and Table S1), indicating likely subtle changes in the stability of reaction intermediates or intermediates or different mechanisms. Both ligand classes resulted in the formation of the cyclic different mechanisms. Both ligand classes resulted in the formation of the cyclic dimer (A) as the dimer (A) as the major species, proceeding with the initial formation of BH4− and major species, proceeding with the initial formation of BH4− and [Me2NBH2N(Me)2BH3]− anions (D). [Me2NBH2N(Me)2BH3]− anions (D). However, only NHI‐supported Al(III) hydrides (1,2) resulted in However, only NHI-supported Al(III) hydrides (1,2) resulted in the formation of the borane (B) species the formation of the borane (B) species and a trace amount of dimethylamino–borane, Me2N=BH2 (C), and a trace amount of dimethylamino–borane, Me N=BH (C), which can be considered an alkene which can be considered an alkene analog. In the case2 of NHC2 catalysis, both the free and metal‐ analog. In the case of NHC catalysis, both the free and metal-bound [Me2NBH2N(Me)2BH3]− anions bound [Me2NBH2N(Me)2BH3]− anions (D and D’, respectively) were observed. Differences within the (D and D’, respectively) were observed. Differences within the ligand classes were marginal in the case ligand classes were marginal in the case of NHI ligands, with the steric demands being slightly of NHI ligands, with the steric demands being slightly removed from the Al center and thus minor removed from the Al center and thus minor differences in the TOFs were observed. However, in the differences in the TOFs were observed. However, in the case of NHC complexes (3–6), where the steric case of NHC complexes (3–6), where the steric demands of the ligand substituents were in closer demands of the ligand substituents were in closer proximity to the Al center, differences in TOFs were proximity to the Al center, differences in TOFs were apparent. The use of less‐sterically demanding apparent. The use of less-sterically demanding Mes (4) or methyl (6) substituents resulted in much Mes (4) or methyl (6) substituents resulted in much higher rates of reaction in comparison to those higher rates of reaction in comparison to those bearing isopropyl substituents (3 and 5). bearing isopropyl substituents (3 and 5).

34 h A A B D 65 h C D+D’ D + D’

18 h 40 h

4 h 18 h

1 h 1 h

0 h − BH4 0 h BH4−

I – NHI catalysis (1) II – NHC catalysis (3)

FigureFigure 3. In 3.situIn reaction situ reaction monitoring monitoring by 11B by NMR11B spectroscopy NMR spectroscopy for the forcatalytic the catalytic dehydrocoupling dehydrocoupling of

Me2NHBHof Me23 NHBHwith 13 with(I, left)1 ( I,and left) 3 and (II, 3right).(II, right). A: [MeA:2 [MeNBH2NBH2]2; B2:] 2HB(NMe; B: HB(NMe2)2; C2): 2;MeC:2N=BH Me2N2=; BHD: 2; D: − − [Me2NBH[Me2NBH2N(Me)2N(Me)2BH3]2;BH D’3: ]Al−; D’bound: Al bound [Me2NBH [Me22N(Me)NBH22N(Me)BH3] . 2BH3]−.

2.2. Mechanistic2.2. Mechanistic Studies Studies A seriesA series of stoichiometric of stoichiometric reactions reactions were were undertaken undertaken in in order order to to provide furtherfurther insight insight into into these thesereactions reactions and and the the role role of of the the supporting supporting ligands. ligands. 2.2.1. NHI Mechanism, Catalysts 1 and 2 2.2.1. NHI Mechanism, Catalysts 1 and 2 The initial reaction of 1 eq. of Me2NHBH3 with compound 1 resulted in no reaction at room The initial reaction of 1 eq. of Me2NHBH3 with compound 1 resulted in no reaction at room temperature, in line with catalytic reduction. Upon heating to 80 C, a small emergence of A was temperature, in line with catalytic reduction. Upon heating to 80 °C, a◦ small emergence of A was noted in the 11B NMR spectrum after 1 h, whilst the 1H NMR spectrum appeared unchanged (Figures noted in the 11B NMR spectrum after 1 h, whilst the 1H NMR spectrum appeared unchanged (Figure S13 and S14). Further heating overnight led to the complete consumption of Me2NHBH3, as noted S13 and S14). Further heating overnight led to the complete consumption1 of Me2NHBH3, as noted by by the loss of Me2NHBH3 CH3 signal at δ 1.78 ppm in the H NMR spectrum. Two NHI-containing the loss of Me2NHBH3 CH3 signal at δ 1.78 ppm in the 1H NMR spectrum. Two NHI‐containing species were identiﬁed via 1H NMR spectroscopy, with the major species being catalyst 1 (85% by species were identified via 1H NMR spectroscopy, with the major species being catalyst 1 (85% by relative integrals). The 11B NMR spectrum showed the formation of compounds A and B, and a third relative integrals). The 11B NMR spectrum showed the formation of compounds A and B, and a third species which had a minor shift of δ 0.5 ppm in comparison to Me2NHBH3 (Figure S15). We propose species which had a minor shift of δ 0.5 ppm in comparison to Me2NHBH3 (Figure S15). We propose that this new minor NHI-containing species is the ﬁrst step in the catalytic cycle, whereupon σ-bond that this new minor NHI‐containing species is the first step in the catalytic cycle, whereupon σ‐bond metathesis of Al–H with the protic N–H bond of Me2NHBH3 occurs to yield 1 eq. of H2 and compound metathesis of Al–H with the protic N–H bond of Me2NHBH3 occurs to yield 1 eq. of H2 and compound 7 (Figure4). Unfortunately, attempts to isolate compound 7 through varying stoichiometries failed and 7 (Figure 4). Unfortunately, attempts to isolate compound 7 through varying stoichiometries failed resulted in the isolation of 1 along with dehydrocoupling products A and B. and resulted in the isolation of 1 along with dehydrocoupling products A and B.

Inorganics 2019, 7, 92 5 of 11 InorganicsInorganics 2019 2019, ,7 7, ,x x FOR FOR PEER PEER REVIEW REVIEW 55 ofof 1212

Figure 4. Stoichiometric reduction of Me2NHBH3 with catalyst 1. Figure 4. Stoichiometric reduction of Me 2NHBHNHBH33 withwith catalyst catalyst 11..

AdditionalAdditionalAdditional supportsupport for forfor the thethe proposed proposedproposed ﬁrst firstfirst step stepstep (i.e., (i.e.,(i.e., formation formationformation of ofof7 ) 77 is)) isis found foundfound from fromfrom consideration considerationconsideration of oftheof thethe formation formationformation of the ofof the dehydrocouplingthe dehydrocouplingdehydrocoupling product productproductB. The BB. formation. TheThe formationformation of B is ofof proposed BB isis proposedproposed to occur toto via occuroccurβ-hydride viavia β‐ β‐ 2 2 2 3 − 3 hydrideeliminationhydride eliminationelimination from the metal-coordinatedfromfrom thethe metalmetal‐‐coordinatedcoordinated [Me2NBH2 N(Me)[Me[Me2NBHNBH2BH23N(Me)N(Me)]− anion2BHBH along3]]− anionanion with along BHalong3 elimination; withwith BHBH3 0 0 elimination;thiselimination; has been this observedthis hashas been inbeen both observedobserved groups 2 inin and bothboth 3 d groupsgroupsmetal-based 22 andand catalysis 33 dd0 metalmetal [23,43‐‐basedbased,44]. Alternatively, catalysiscatalysis [23,43,44].[23,43,44]. Wright Alternatively,andAlternatively, co-workers WrightWright proposed andand that coco‐‐Bworkersworkersis also the proposedproposed result of thatthat the B deprotonationB isis alsoalso thethe resultresult of Me 2ofofNHBH thethe deprotonationdeprotonation3 by the hydridic ofof 2 3 MeB–HMe2NHBHNHBH rather3 thanbyby thethe Al–H hydridichydridic [33]. B–HB–H However, ratherrather in thanthan the Al–HAl–H latter [33].[33]. case However,However,B was observed inin thethe latterlatter to form casecase at BB the waswas start observedobserved of the tocatalysis,to formform atat with thethe startstart the concentration ofof thethe catalysis,catalysis, remaining withwith thethe static concentrationconcentration throughout remainingremaining the catalytic staticstatic reaction throughoutthroughout [32]. thethe On catalyticcatalytic further 11 11 reactioninspectionreaction [32].[32]. of the OnOn furtherfurtherB NMR inspectioninspection data obtained ofof thethe from 11BB NMR theNMR in situdatadata monitoring obtainedobtained fromfrom of the thethe catalytic inin situsitu reaction monitoringmonitoring (Figure ofof thethe3I, catalyticFigurescatalytic S2 reactionreaction and S4), (Figure(Figure both NHI 3I,3I, FiguresFigures catalysts S2S2 showed andand S4),S4), the bothboth initial NHINHI formation catalystscatalysts showed ofshowed the [Me thethe2NBH initialinitial2N(Me) formationformation2BH3 of]of− 2 2 2 3 − theanionthe [Me [Me with2NBHNBH the2N(Me)N(Me) formation2BHBH3 of]]− anionanionB occurring with with the the in formation theformation latter stages of of B B occurring occurring of the catalysis. in in the the latter latter Therefore, stages stages it of of is the the highly catalysis. catalysis. likely Therefore,thatTherefore, this mechanism itit isis highlyhighly proceeds likelylikely thatthat via thisthis an mechanism intermediatemechanism proceedsproceeds akin to compound viavia anan intermediateintermediate7, as outlined akinakin to into compound thecompound proposed 77,, asmechanismas outlinedoutlined inin (Figure thethe proposedproposed5). mechanismmechanism (Figure(Figure 5).5).

Figure 5. Proposed catalytic mechanism for the dehydrocoupling of Me22NHBH33 with NHI-supported FigureFigure 5. 5. Proposed Proposed catalytic catalytic mechanism mechanism for for the the dehydrocoupling dehydrocoupling of of Me Me2NHBHNHBH3 with with NHI NHI‐‐supportedsupported Al(III) hydrides (1, 2). Al(III)Al(III) hydrides hydrides ( (11, ,2 2).). 2.2.2. NHC Mechanism 2.2.2.2.2.2. NHCNHC MechanismMechanism A similar investigation was carried out for the NHC-supported Al(III) hydrides, again using the AA similar similar investigation investigation was was carried carried out out for for the the NHC NHC‐‐supportedsupported Al(III) Al(III) hydrides, hydrides, again again using using the the diisopropylphenyl substituent (catalyst 3, IDippAlH3). Upon reaction of 3 with 1 eq. of Me2NHBH3, 3 2 3 diisopropylphenyldiisopropylphenyl substituentsubstituent (catalyst(catalyst 33,, IDippAlHIDippAlH3).). UponUpon reactionreaction ofof 33 withwith 11 eq.eq. ofof MeMe2NHBHNHBH3,, two new NHC-containing species were observed to form along with a notable lack of H2 formation in two new NHC‐containing species were observed to form along with a notable lack of H2 formation twothe 1newH NMR NHC spectrum.‐containing One species of these were species observed was to identiﬁed form along as the with dihydroaminal, a notable lack IDippHof H2 formation, which 1 2 in the 1 H NMR spectrum. One of these species was identified as the dihydroaminal, IDippH2, which inhas the been H NMR previously spectrum. reported One from of these the species NHC-induced was identified dehydrocoupling as the dihydroaminal, of MeNH BH IDippH[30].2 The, which11B 2 3 11 has been previously reported from the NHC‐induced dehydrocoupling of MeNH2BH3 [30]. The 11 B hasNMR been spectrum previously again reported showed from a minor the shiftNHC of‐induced approximately dehydrocouplingδ 0.5 ppm, of indicating MeNH2BH the3 formation[30]. The ofB NMRNMR spectrumspectrum againagain showedshowed aa minorminor shiftshift ofof approximatelyapproximately δ δ 0.50.5 ppm,ppm, indicatingindicating thethe formationformation ofof the NHC-stabilized [Al]-N(Me)2BH3 complex 8, similar to that proposed for the NHI mechanism (7). 2 3 thethe NHCNHC‐‐stabilizedstabilized [Al][Al]‐‐N(Me)N(Me)2BHBH3 complexcomplex 88,, similarsimilar toto thatthat proposedproposed forfor thethe NHINHI mechanismmechanism ((77).). Additional heating of this sample led to the increased formation of IDippH2 and a new NHC-containing 2 AdditionalAdditional heatingheating ofof thisthis samplesample ledled toto thethe increasedincreased formationformation ofof IDippHIDippH2 andand aa newnew NHCNHC‐‐ 1 11 containingcontaining compoundcompound inin thethe 1HH NMRNMR spectrum.spectrum. TheThe 11BB NMRNMR spectrumspectrum diddid notnot showshow anyany

Inorganics 2019, 7, 92 6 of 11 compoundInorganics in the 20191H, 7 NMR, x FOR PEER spectrum. REVIEW The 11B NMR spectrum did not show any dehydrocoupling6 of 12 products, but the formation of two broad signals at δ 13 and 17 ppm was evident. Subsequent dehydrocoupling products, but the formation of two broad− signals −at δ −13 and −17 ppm was evident. scale-up ofSubsequent this stoichiometric scale‐up of this reaction stoichiometric and XRD-analysis reaction and XRD revealed‐analysis therevealed unexpected the unexpected formation of compound formation9 (Figure of6 ).compound 9 (Figure 6).

N(4) B(2) C(1) B(1) Al(1)

N(3)

Figure 6. MolecularFigure 6. Molecular structure structure of compound of compound9 in 9 in the the solid solid state state with with thermal thermal ellipsoids ellipsoids set at the set 50% at the 50% probabilityprobability level. Hydrogen level. Hydrogen atoms atoms (except (except those those on on B(1), B(1), B(2), B(2), and and Al(1)) Al(1)) are omitted are omitted for clarity, for and clarity, and diisopropylphenyldiisopropylphenyl substituents substituents on the on NHC the NHC ligand ligand are are depicteddepicted in in wireframe wireframe for simplicity. for simplicity. Selected Selected bond lengths (Å) and angles (°): C(1)–B(1) 1.628(3), B(1)–N(3) 1.555(6), N(3)–Al(1) 2.026(5), N(4)–Al(1) bond lengths (Å) and angles ( ): C(1)–B(1) 1.628(3), B(1)–N(3) 1.555(6), N(3)–Al(1) 2.026(5), N(4)–Al(1) 1.948(6), B(2)–N(4) 1.599(6),◦ C(1)–B(1)–N(3) 115.6(2), B(1)–N(3)–Al(1) 108.0(3), N(3)–Al(1)–N(4) 1.948(6), B(2)–N(4)112.89(2), 1.599(6), Al(1)–N(4)–B(2) C(1)–B(1)–N(3) 95.7(3). 115.6(2), B(1)–N(3)–Al(1) 108.0(3), N(3)–Al(1)–N(4) 112.89(2), Al(1)–N(4)–B(2) 95.7(3). Compound 9 was identified as NHC–BH2–N(Me)2–AlH2–N(Me)2–BH3 complex, wherein one NHC–AlH3 reacted with two molecules of Me2NHBH3. The AlH2 unit is pseudo‐tetrahedral, with the Compound 9 was identified as NHC–BH2–N(Me)2–AlH2–N(Me)2–BH3 complex, wherein one N(3)–Al(1) bond longer than N(4)–Al(1), suggesting some dative bonding character from the lone NHC–AlH3 reacted with two molecules of Me2NHBH3. The AlH2 unit is pseudo-tetrahedral, with pair on N(3) to the empty orbital of Al(1). The NHC–BH2 bond lengths (1.628(3) Å) were in line with the N(3)–Al(1)previously bond reported longer thanNHC–BH N(4)–Al(1),2‐containing suggesting compounds, someand also dative fit with bonding the 11B NMR character data matching from the lone pair on N(3)the to broad the empty signal at orbital δ −17 ppm of [30,45,46]. Al(1). The Interestingly, NHC–BH the2 bond27Al NMR lengths spectrum (1.628(3) showed Å) a triplet were (δ in85 line with ppm, JAl–H = 354 Hz), which has a comparable shift to that observed by Wright and11 co‐workers for previously reported NHC–BH2-containing compounds, and also fit with the B NMR data matching the broad signaltheir Al–H at δ‐containing17 ppm dehydrocoupling [30,45,46]. Interestingly, catalyst (δ 82.8 the ppm,27Al d, J NMRAl–H = 288 spectrum Hz). showed a triplet (δ 85 We propose− that the formation of compound 9 (Figure 7) occurs in a similar sequence to that ppm, JAl–H reported= 354 Hz), by Rivard which and has co‐workers a comparable for the formation shift to of thatNHC–BH observed2–N(H)(Me)–BH by Wright3 [30]. andFirstly, co-workers upon for their Al–H-containingreaction of 3 with dehydrocoupling Me2NHBH3, the reaction catalyst occurs (δ 82.8at the ppm, NHC–Al d, bondJAl–H rather= 288 than Hz). Al–H (Figure 7a). We proposeHere the that NHC the acts formation as a base, ofresulting compound in the 9formation(Figure of7) IDippH occurs2 (confirmed in a similar by sequence1H NMR), to that dimethylamino–borane (C), and AlH3 species. In the second step (Figure 7b), C inserts into the NHC– reported by Rivard and co-workers for the formation of NHC–BH2–N(H)(Me)–BH3 [30]. Firstly, AlH3 bond, resulting in NHC–BH2 formation. Finally, the second molecule of Me2NHBH3 undergoes upon reactionσ‐bond of metathesis3 with Me of its2NHBH N–H with3, thethe terminal reaction Al–H occurs bond, resulting at the NHC–Al in the formation bond of rathercompound than Al–H 1 (Figure7a).9 Here (Figure the 7c). NHC acts as a base, resulting in the formation of IDippH 2 (confirmed by H NMR), dimethylamino–borane (C), and AlH3 species. In the second step (Figure7b), C inserts into the NHC–AlH3 bond, resulting in NHC–BH2 formation. Finally, the second molecule of Me2NHBH3 undergoes σ-bond metathesis of its N–H with the terminal Al–H bond, resulting in the formation of compound 9 (Figure7c). The viability of compound 9 as a catalyst in the dehydrocoupling reaction was trialed, however only further decomposition to IDippH2 was noted to occur, suggesting that this is a deactivation pathway in the active catalyst cycle. This would also account for the increased reaction times with

3. To confirm the viability of the active cycle, that is, via compound 8 (NHC–Al(H)2NMe2BH3), the use of a more sterically demanding secondary amine–borane was tested. Reaction of 3 with iPr 1 eq. of diisopropylamine–borane (iPr2NHBH3) resulted in the formation of 8 , with a reduced amount of IDippH2, thus confirming that this is the likely first step of the active catalytic cycle. It is further proposed that due to the increased steric congestion at the Al center with NHC ligands, the δ-hydride elimination is kinetically preferred over β-hydride elimination from the Al-coordinated Inorganics 2019, 7, 92 7 of 11

[Me2NBH2N(Me)2BH3]− anion, therefore accounting for the absence of borane product (B) in the case of 3–6Inorganics. 2019, 7, x FOR PEER REVIEW 7 of 12

FigureFigure 7. Proposed 7. Proposed sequence sequence of of reactions reactions whichwhich result in in the the formation formation of ofcompound compound 9. (a9) .(Initiala) Initial deprotonation of Me2NHBH3 by NHC rather than Al–H. (b) Insertion of C into NHC–Al bond. (c) deprotonation of Me2NHBH3 by NHC rather than Al–H. (b) Insertion of C into NHC–Al bond. (c) Deprotonation of Me2NHBH3 by Al–H bond for formation of compound 9. Deprotonation of Me2NHBH3 by Al–H bond for formation of compound 9.

A proposedThe viability mechanism of compound is outlined 9 as a catalyst in Figure in 8the, taking dehydrocoupling both the active reaction and was deactivation trialed, however pathways only further decomposition to IDippH2 was noted to occur, suggesting that this is a deactivation into account. Dimethylamino–borane (C) could either (i) react with compound 8 to form the pathway in the active catalyst cycle. This would also account for the increased reaction times with 3. metal-bound [Me2NBH2N(Me)2BH3]− anion (D’) (which was observed to form during the course of To confirm the viability of the active cycle, that is, via compound 8 (NHC–Al(H)2NMe2BH3), the use the catalysis)of a more or sterically (ii) insert demanding into the NHC–AlH secondary3 amine–boraneof the catalyst, was leading tested. to Reaction deactivation of 3 with and formation1 eq. of of 9. Therefore,diisopropylamine–borane over the course (iPr of2 theNHBH catalysis,3) resulted the in remaining the formation “active” of 8iPr, species with a reduced likely diminishes amount of and resultsIDippH in prolonged2, thus confirming reaction that times. this The is the diﬀ likelyerences first in step the of reaction the active times catalytic observed cycle. with It is catalysts further 3–6 wereproposed therefore that likely due dueto the to increased the strength steric congestion of the NHC–Al at the Al bond center with with strong NHC ligands, donors the such δ‐hydride as IMe 4 (6) resultingelimination in the higheris kinetically TOF due preferred to the decreased over β‐ likelihoodhydride elimination of deactivation. from Thisthe wasAl‐coordinated also reﬂected in [Me2NBH2N(Me)2BH3]− anion, therefore accounting for the absence of borane product (B) in the case Inorganicsthe use of2019 the, 7, stronger x FOR PEERσ-donating REVIEW NHI ligands, as no deactivation pathways were observed. 8 of 12 of 3–6. A proposed mechanism is outlined in Figure 8, taking both the active and deactivation pathways into account. Dimethylamino–borane (C) could either (i) react with compound 8 to form the metal‐ bound [Me2NBH2N(Me)2BH3]− anion (D’) (which was observed to form during the course of the catalysis) or (ii) insert into the NHC–AlH3 of the catalyst, leading to deactivation and formation of 9. Therefore, over the course of the catalysis, the remaining “active” species likely diminishes and results in prolonged reaction times. The differences in the reaction times observed with catalysts 3–6 were therefore likely due to the strength of the NHC–Al bond with strong donors such as IMe4 (6) resulting in the higher TOF due to the decreased likelihood of deactivation. This was also reflected in the use of the stronger σ‐donating NHI ligands, as no deactivation pathways were observed.

Figure 8. ProposedProposed mechanism for the dehydrocoupling of Me2NHBH33 withwith NHC NHC-supported‐supported Al(III) Al(III) hydrides ( 3–6).

3. Conclusions In conclusion, differences in the use of NHI and NHC ligands for the stabilization of Al(III) hydrides and influence on catalytic activity were found. Whilst NHI‐supported complexes required increased thermal activation, these systems were more stable in comparison to their NHC counterparts. Through a series of stoichiometric reactions, the isolation of compound 9 revealed a catalyst deactivation pathway that is prevalent in the NHC systems.

4. Materials and Methods

4.1. General Methods and Instruments All manipulations were carried out under argon atmosphere using standard Schlenk or glovebox techniques. Glassware was heat‐dried under vacuum prior to use. Unless otherwise stated, all chemicals were purchased from Sigma‐Aldrich (Steinheim, Germany) and used as received. Me2NHBH3 was sublimed three times prior to use. Benzene, toluene, and n‐hexane were refluxed over standard drying agents (benzene/hexane over sodium and benzophenone), distilled, and deoxygenated prior to use. Deuterated benzene (C6D6) was dried by storage over activated 4 Å molecular sieves. All NMR samples were prepared under argon in J. Young PTFE tubes. Catalysts 1–6 were synthesized according to procedures described in the literature [47]. NMR spectra were recorded on a Bruker AV‐ 400 spectrometer (Rheinstetten, Germany) at ambient temperature (300 K). 1H, 11B, 13C, and 27Al NMR spectroscopic chemical shifts δ are reported in ppm relative to tetramethylsilane. δ(1H) and δ(13C) were referenced internally to the relevant residual solvent resonances. Details on XRD data are given in the Supplementary Materials.

4.2. General Catalytic Procedure for Catalysis of Me2NHBH3

5 mol % aluminum hydride catalyst (0.012 mmol) and Me2NHBH3 (0.24 mmol) in 0.5 mL of C6D6 were added to a J. Young PTFE NMR tube. The 1H and 11B NMR spectra were recorded and then placed in an oil bath at either 50 or 80 °C. Reaction progress was monitored by 1H/11B NMR spectroscopy until the consumption of Me2NHBH3.

4.3. General Procedure of Stoichiometric Reactions

Aluminum hydride catalyst (1: 40 mg, 0.046 mmol; 3: 40 mg, 0.095 mmol) and 1 eq. of Me2NHBH3 (1: 3 mg, 0.046 mmol; 3: 5.6 mg, 0.095 mmol) in 0.5 mL of C6D6 were added to a J. Young PTFE NMR tube. The 1H and 11B NMR spectra were recorded and then they were placed in an oil bath at either

Inorganics 2019, 7, 92 8 of 11

3. Conclusions In conclusion, diﬀerences in the use of NHI and NHC ligands for the stabilization of Al(III) hydrides and inﬂuence on catalytic activity were found. Whilst NHI-supported complexes required increased thermal activation, these systems were more stable in comparison to their NHC counterparts. Through a series of stoichiometric reactions, the isolation of compound 9 revealed a catalyst deactivation pathway that is prevalent in the NHC systems.

4. Materials and Methods

4.1. General Methods and Instruments All manipulations were carried out under argon atmosphere using standard Schlenk or glovebox techniques. Glassware was heat-dried under vacuum prior to use. Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich (Steinheim, Germany) and used as received. Me2NHBH3 was sublimed three times prior to use. Benzene, toluene, and n-hexane were reﬂuxed over standard drying agents (benzene/hexane over sodium and benzophenone), distilled, and deoxygenated prior to use. Deuterated benzene (C6D6) was dried by storage over activated 4 Å molecular sieves. All NMR samples were prepared under argon in J. Young PTFE tubes. Catalysts 1–6 were synthesized according to procedures described in the literature [47]. NMR spectra were recorded on a Bruker AV-400 spectrometer (Rheinstetten, Germany) at ambient temperature (300 K). 1H, 11B, 13C, and 27Al NMR spectroscopic chemical shifts δ are reported in ppm relative to tetramethylsilane. δ(1H) and δ(13C) were referenced internally to the relevant residual solvent resonances. Details on XRD data are given in the Supplementary Materials.

4.2. General Catalytic Procedure for Catalysis of Me2NHBH3

5 mol % aluminum hydride catalyst (0.012 mmol) and Me2NHBH3 (0.24 mmol) in 0.5 mL of 1 11 C6D6 were added to a J. Young PTFE NMR tube. The H and B NMR spectra were recorded and 1 11 then placed in an oil bath at either 50 or 80 ◦C. Reaction progress was monitored by H/ B NMR spectroscopy until the consumption of Me2NHBH3.

4.3. General Procedure of Stoichiometric Reactions

4.4. Synthesis of Diisopropylamineborane, iPr2NHBH3 This synthesis was adapted from general literature procedures [48], with the exception that this was solely carried out in hexanes rather than THF. Diisopropyl amine (1.00 g, 1.3 mL, 9.88 mmol) was placed in a Schlenk ﬂask followed by 10 mL of hexanes. The reaction mixture was cooled in an ice bath to approximately 0 C. BH dms (dms = dimethylsulﬁde) (0.75g, 0.94 mL, 9.88 mmol) was added ◦ 3· dropwise via syringe over 5 min. The reaction was allowed to stir for 10 min at 0 ◦C, before warming to room temperature and stirring overnight. Volatiles were removed under reduced pressure to leave a colorless oil (0.83 g, 73%). 1 H NMR (400 MHz, 298 K, C6D6): δH 3.19 (1H, s, NH), 2.85 (2H, m, NCH(CH3)2), 2.16–1.48 (3H, 11 br, q. BH3), 1.06 (6H, d, JHH = 6.6 Hz, CH(CH3)2), 1.00 (6H, d, JHH = 6.7 Hz, CH(CH3)2). B NMR (128 MHz, 298 K, C D ): δB 20.49 (q, J = 96.5 Hz, BH ). 6 6 − BH 3 Inorganics 2019, 7, 92 9 of 11

4.5. Synthesis of Compound 8iPr

IDippAlH3 (3: 100 mg, 0.24 mmol) was dissolved in 3 mL of toluene and transferred to a Schlenk ﬂask, followed by the addition of 1 eq. of iPr2NHBH3 (35 µL, 0.24 mmol). This was allowed to stir at room temperature for 3 h. Solvent and volatiles were removed under reduced pressure to leave a pale-yellow gummy solid. This was subsequently washed with 3 10 mL of n-hexane to yield a × colorless solid, 80 mg, 67% yield. 1 H NMR (400 MHz, 298 K, C6D6): δH 7.25 (2H, m, Ar p-H), 7.12 (4H, m, Ar m-H), 6.48 (2H, s, NCH), 3.63 (2H, br.s. AlH2), 2.66 (6H, overlapping multiplets, CH(CH3)2 and NCH(CH3)2), 2.60–1.52 (3H, q, BH3), 1.43 (12H, d, JHH = 6.8 Hz, CH(CH3)2), 1.04 (12H, d, JHH = 6.9 Hz, CH(CH3)2), 0.96 (6H, 13 1 d, JHH = 6.6 Hz, NCH(CH3)2), 0.82 (6H, d, JHH = 6.6 Hz, NCH(CH3)2). C{ H} NMR (100 MHz, 298 K, C6D6): δC 145.7 (Ar C), 134.8 (Ar C), 130.8 (Ar C), 124.3 (Ar C) 124.0 (NCH), 51.8 (NCH(CH3)2), 29.1 11 (CH(CH3)2), 25.1 (CH(CH3)2), 23.4 (CH(CH3)2), 20.7 (NCH(CH3)2), 18.8 (NCH(CH3)2). B NMR (128 MHz, 298 K, C D ): δB 20.13 (q, J = 95.9 Hz, BH ). 27Al NMR (78 MHz, 298 K, C D ): δAl 109.2 6 6 − BH 3 6 6 (br.s). Due to the presence of IDippH2, satisfactory elemental analysis results were not obtained.

4.6. Synthesis of Compound 9

IDippAlH3 (3: 200 mg, 0.48 mmol) and 2 eq. of Me2NHBH3 (56 mg, 0.96 mmol) were weighed in a Schlenk tube. To this, 5 mL of toluene was added, and the Schlenk tube was placed in an oil bath at 50 ◦C for 24 h. Solvent and volatiles were removed under reduced pressure to leave a pale-yellow gummy solid. This was subsequently washed with 3 10 mL of n-hexane to yield a colorless solid, 102 × mg, 40% yield. Crystals suitable for single-crystal X-ray diﬀraction were grown from a concentrated toluene solution at 30 ◦C. 1 − H NMR (400 MHz, 298 K, C6D6): δH 7.19 (2H, m, Ar p-H), 7.11 (4H, m, Ar m-H), 6.53 (2H, s, NCH), 3.00 (4H, sept, JHH = 6.8 Hz, CH(CH3)2), 2.13 (12H, s, NCH3), 1.46 (12H, d, JHH = 6.7 Hz, 13 1 CH(CH3)2), 0.99 (12H, d, JHH = 6.9 Hz, CH(CH3)2). C{ H} NMR (100 MHz, 298 K, C6D6): δC 146.2 (Ar-C), 135.4 (Ar-C), 130.7 (Ar-C), 124.4 (Ar m-C), 123.2 (NCH), 54.6 (NCH3), 29.1 (CH(CH3)2), 26.3 11 (CH(CH3)2), 22.5 (CH(CH3)2). B NMR (128 MHz, 298 K, C6D6): δB 13.5 (br.s. BH3), 17.0 (br.s. 27 − − BH2). Al NMR (78 MHz, 298 K, C6D6): δAl 85 (t, JAl–H = 354 Hz, AlH2). Due to presence of IDippH2, satisfactory elemental analysis results were not obtained.

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/8/92/s1. Table S1 and Figures S1–S12: Catalysis data. Figures S13–S15: NHI mechanism NMR spectra. Figures S16–S20: NHC mechanism NMR spectra. Figures S21–S24: NMR spectra for compound 9. Table S2: Crystallographic details of 9 (CCDC 1936588). Copies of this information may be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif . Supplementary material includes the CIF and checkCIF files for 9. Author Contributions: C.W. and N.I. performed the experiments. F.H. measured and solved the SC-XRD data. C.W. and S.I. supervised the complete project and wrote the manuscript. C.W., N.I., M.U., F.H. and S.I. discussed the results and commented on the manuscript. Funding: This project received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 754462 and TUM University Foundation (Fellowships CW), as well as the European Research Council (SILION 637394) and WACKER Chemie AG. Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Boor, J., Jr. Zieglar-Natta Catalysts and Polymerisations; Academic Press Inc.: New York, NY, USA, 1979. 2. Olah, G.A. Friedel-Crafts and Related Reactions; Wiley: New York, NY, USA, 1963. 3. Power, P.P. Main-group elements as transition metals. Nature 2010, 463, 171–177. [CrossRef][PubMed] 4. Weetman, C.; Inoue, S. The Road Travelled: After Main-Group Elements as Transition Metals. ChemCatChem 2018, 10, 4213–4228. [CrossRef] 5. Melen, R.L. Frontiers in molecular p-block chemistry: From structure to reactivity. Science 2019, 363, 479. [CrossRef][PubMed] Inorganics 2019, 7, 92 10 of 11

+ 6. Kim, K.-C.; Reed, C.A.; Long, G.S.; Sen, A. Et2Al Alumenium Ion-like Chemistry. Synthesis and Reactivity toward Alkenes and Alkene Oxides. J. Am. Chem. Soc. 2002, 124, 7662–7663. [CrossRef][PubMed] 7. Khandelwal, M.; Wehmschulte, R.J. Deoxygenative Reduction of Carbon Dioxide to Methane, Toluene, + and Diphenylmethane with [Et2Al] as Catalyst. Angew. Chem. Int. Ed. 2012, 51, 7323–7326. [CrossRef] [PubMed]

8. Weetman, C.; Bag, P.; Silvási, T.; Jandl, C.; Inoue, S. CO2 Fixation and Catalytic Reduction by a Neutral Aluminium Double bond. Angew. Chem. 2019.[CrossRef][PubMed] 9. Stephan, D.W.; Erker, G. Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More. Angew. Chem. Int. Ed. 2009, 49, 46–76. [CrossRef] 10. Stephan, D.W.; Erker, G. Frustrated Lewis Pair Chemistry: Development and Perspectives. Angew. Chem. Int. Ed. 2015, 54, 6400–6441. [CrossRef] 11. Stephan, D.W. The broadening reach of frustrated Lewis pair chemistry. Science 2016, 354, aﬀ7229. [CrossRef] 12. Hill, M.S.; Liptrot, D.J.; Weetman, C. Alkaline earths as main group reagents in molecular catalysis. Chem. Soc. Rev. 2016, 45, 972–988. [CrossRef] 13. Hadlington, T.J.; Driess, M.; Jones, C. Low-valent group 14 element hydride chemistry: Towards catalysis. Chem. Soc. Rev. 2018, 47, 4176–4197. [CrossRef][PubMed] 14. Yang, Z.; Zhong, M.; Ma, X.; De, S.; Anusha, C.; Parameswaran, P.; Roesky, H.W. An Aluminum Hydride That Functions like a Transition-Metal Catalyst. Angew. Chem. Int. Ed. 2015, 54, 10225–10229. [CrossRef] [PubMed]

15. Elsen, H.; Färber, C.; Ballmann, G.; Harder, S. LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis. Angew. Chem. Int. Ed. 2018, 57, 7156–7160. [CrossRef][PubMed] 16. Bismuto, A.; Thomas Stephen, P.; Cowley Michael, J. Aluminum Hydride Catalyzed Hydroboration of Alkynes. Angew. Chem. Int. Ed. 2016, 55, 15356–15359. [CrossRef][PubMed] 17. Leitao, E.M.; Jurca, T.; Manners, I. Catalysis in service of main group chemistry oﬀers a versatile approach to p-block molecules and materials. Nat. Chem. 2013, 5, 817. [CrossRef][PubMed] 18. Melen, R.L. Dehydrocoupling routes to element-element bonds catalysed by main group compounds. Chem. Soc. Rev. 2016, 45, 775–788. [CrossRef][PubMed] 19. Slootweg, C.; Jupp, A.; Boom, D. Dehydrogenation of Amine–Boranes using p-block Compounds. Chem. Eur. J. 2019.[CrossRef] 20. Keaton, R.J.; Blacquiere, J.M.; Baker, R.T. Base Metal Catalyzed Dehydrogenation of Ammonia Borane for − Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844–1845. [CrossRef][PubMed] 21. Hamilton, C.W.; Baker, R.T.; Staubitz, A.; Manners, I. B–N compounds for chemical hydrogen storage. Chem. Soc. Rev. 2009, 38, 279–293. [CrossRef] 22. Rossin, A.; Peruzzini, M. Ammonia–Borane and Amine–Borane Dehydrogenation Mediated by Complex Metal Hydrides. Chem. Rev. 2016, 116, 8848–8872. [CrossRef] 23. Liptrot, D.J.; Hill, M.S.; Mahon, M.F.; MacDougall, D.J. Group 2 Promoted Hydrogen Release from NMe H BH : Intermediates and Catalysis. Chem. Eur. J. 2010, 16, 8508–8515. [CrossRef][PubMed] 2 · 3 24. McLellan, R.; Kennedy, A.R.; Orr, S.A.; Robertson, S.D.; Mulvey, R.E. Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes. Angew. Chem. Int. Ed. 2017, 56, 1036–1041. [CrossRef][PubMed] 25. Bellham, P.; Hill, M.S.; Kociok-Köhn, G. Alkali metal-mediated dehydrocoupling of Me NH BH . Dalton 2 · 3 Trans. 2015, 44, 12078–12081. [CrossRef][PubMed] 26. Bellham, P.; Anker, M.D.; Hill, M.S.; Kociok-Köhn, G.; Mahon, M.F. The signiﬁcance of secondary interactions during alkaline earth-promoted dehydrogenation of dialkylamine–boranes. Dalton Trans. 2016, 45, 13969–13978. [CrossRef][PubMed] 27. Erickson, K.A.; Kiplinger, J.L. Catalytic Dehydrogenation of Dimethylamine Borane by Highly Active Thorium and Uranium Metallocene Complexes. ACS Catal. 2017, 7, 4276–4280. [CrossRef] 28. Cui, P.; Spaniol, T.P.; Maron, L.; Okuda, J. Dehydrogenation of Amine-Borane Me NH BH Catalyzed by a 2 · 3 Lanthanum-Hydride Complex. Chem. Eur. J. 2013, 19, 13437–13444. [CrossRef][PubMed] 29. Stubbs, N.E.; Robertson, A.P.M.; Leitao, E.M.; Manners, I. Amine–borane dehydrogenation chemistry: Metal-free hydrogen transfer, new catalysts and mechanisms, and the synthesis of polyaminoboranes. J. Organomet. Chem. 2013, 730, 84–89. [CrossRef] Inorganics 2019, 7, 92 11 of 11

30. Sabourin, K.J.; Malcolm, A.C.; McDonald, R.; Ferguson, M.J.; Rivard, E. Metal-free dehydrogenation of amine–boranes by an N-heterocyclic carbene. Dalton Trans. 2013, 42, 4625–4632. [CrossRef] 31. Lui Melanie, W.; Paisley Nathan, R.; McDonald, R.; Ferguson Michael, J.; Rivard, E. Metal-Free Dehydrogenation of Amine-Boranes by Tunable N-Heterocyclic Iminoboranes. Chem. Eur. J. 2016, 22, 2134–2145. [CrossRef] 32. Hansmann, M.M.; Melen, R.L.; Wright, D.S. Group 13 BN dehydrocoupling reagents, similar to transition metal catalysts but with unique reactivity. Chem. Sci. 2011, 2, 1554–1559. [CrossRef]

33. Cowley, H.J.; Holt, M.S.; Melen, R.L.; Rawson, J.M.; Wright, D.S. Catalytic dehydrocoupling of Me2NHBH3 with Al(NMe2)3. Chem. Commun. 2011, 47, 2682–2684. [CrossRef][PubMed] 34. Less, R.J.; Simmonds, H.R.; Dane, S.B.J.; Wright, D.S. Stoichiometric and catalytic reactions of LiAlH4 with Me2NHBH3. Dalton Trans. 2013, 42, 6337–6343. [CrossRef][PubMed] 35. Less, R.J.; Simmonds, H.R.; Wright, D.S. Reactivity and catalytic activity of tert-butoxy-aluminium hydride reagents. Dalton Trans. 2014, 43, 5785–5792. [CrossRef][PubMed] 36. Less, R.J.; García-Rodríguez, R.; Simmonds, H.R.; Allen, L.K.; Bond, A.D.; Wright, D.S. Use of crown ethers to isolate intermediates in ammonia–borane dehydrocoupling reactions. Chem. Commun. 2016, 52, 3650–3652. [CrossRef][PubMed] 37. Allen, L.K.; García-Rodríguez, R.; Wright, D.S. Stoichiometric and catalytic Si–N bond formation using the

p-block base Al(NMe2)3. Dalton Trans. 2015, 44, 12112–12118. [CrossRef][PubMed] 38. Ochiai, T.; Franz, D.; Inoue, S. Applications of N-heterocyclic imines in main group chemistry. Chem. Soc. Rev. 2016, 45, 6327–6344. [CrossRef][PubMed] 39. Nesterov, V.; Reiter, D.; Bag, P.; Frisch, P.; Holzner, R.; Porzelt, A.; Inoue, S. NHCs in Main Group Chemistry. Chem. Rev. 2018.[CrossRef] 40. Franz, D.; Inoue, S. Activation of Elemental Sulfur by Aluminum Dihydride: Isolation of Mono-and Bis(hydrogensulﬁde) Complexes of Aluminum. Chem. Eur. J. 2014, 20, 10645–10649. [CrossRef] 41. Franz, D.; Szilvasi, T.; Irran, E.; Inoue, S. A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes. Nat. Commun. 2015, 6, 10037–10042. [CrossRef] 42. Franz, D.; Sirtl, L.; Pothig, A.; Inoue, S. Aluminum Hydrides Stabilized by N-Heterocyclic Imines as Catalysts for Hydroborations with Pinacolborane. Z. Anorg. Allg. Chem. 2016, 642, 1245–1250. [CrossRef] 43. Spielmann, J.; Bolte, M.; Harder, S. Synthesis and structure of a magnesium–amidoborane complex and its role in catalytic formation of a new bis-aminoborane ligand. Chem. Commun. 2009, 6934–6936. [CrossRef] [PubMed] 44. Hill, M.S.; Kociok-Köhn, G.; Robinson, T.P. Group 3-centred dehydrocoupling of Me NH BH . Chem. Comm. 2 · 3 2010, 46, 7587–7589. [CrossRef][PubMed] 45. Malcolm, A.C.; Sabourin, K.J.; McDonald, R.; Ferguson, M.J.; Rivard, E. Donor–Acceptor Complexation and Dehydrogenation Chemistry of Aminoboranes. Inorg. Chem. 2012, 51, 12905–12916. [CrossRef][PubMed] 46. Dübek, G.; Franz, D.; Eisenhut, C.; Altmann, P.J.; Inoue, S. Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources. Dalton Trans. 2019, 48, 5756–5765. [CrossRef][PubMed] 47. Francis, M.D.; Hibbs, D.E.; Hursthouse, M.B.; Jones, C.; Smithies, N.A. Carbene complexes of Group 13 i i trihydrides: Synthesis and characterisation of MH3{CN(Pr )C2Me2N(Pr )}, M = Al, Ga or In. J. Chem. Soc. Dalton Trans. 1998, 3249–3254. [CrossRef] 48. Coles, N.T.; Mahon, M.F.; Webster, R.L. Phosphine– and Amine–Borane Dehydrocoupling Using a Three-Coordinate Iron(II) β-Diketiminate Precatalyst. Organometallics 2017, 36, 2262–2268. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).