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Bis-NHC Aluminium and Gallium Dihydride Cations [(NHC)2EH2] (E=Al,Ga) Andreas Hock,[A] Luis Werner,[A] Melanie Riethmann,[A] and Udo Radius*[A]

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Bis-NHC Aluminium and Gallium Dihydride Cations [(NHC)2EH2] (E=Al,Ga) Andreas Hock,[A] Luis Werner,[A] Melanie Riethmann,[A] and Udo Radius*[A] EurJIC Full Paper European Journal of Inorganic Chemistry doi.org/10.1002/ejic.202000720 Main Group Element Hydrides + Bis-NHC Aluminium and Gallium Dihydride Cations [(NHC)2EH2] (E=Al,Ga) Andreas Hock,[a] Luis Werner,[a] Melanie Riethmann,[a] and Udo Radius*[a] Abstract: The NHC alane and gallane adducts (NHC)·AlH2I 13 NHC complexes with an additional equivalent of the corre- Me Me (NHC = Me2Im 7, iPr2Im 8, iPr2Im 9) and (NHC)·GaH2I sponding NHC afforded cationic aluminium and gallium hy- Me Me + Me Me (NHC = Me2Im 10, iPr2Im 11, Dipp2Im 12; R2Im = 1,3-di- drides [(NHC)2·AlH2] I− (NHC = Me2Im 13, iPr2Im 14, iPr2Im + Me Me organyl-imidazolin-2-ylidene; Dipp = 2,6-diisopropylphenyl; 15) and [(NHC)2·GaH2] I− (NHC = Me2Im 16, iPr2Im 17) and Me iPr = isopropyl; Me2Im = 1,3,4,5-tetra-methyl-imidazolin-2-yl- the normal and abnormal NHC coordinated compound idene) were prepared either by the simple yet efficient reaction [(Dipp2Im)·GaH2(aDipp2Im)]+I− 18. Compounds 7–18 were iso- of the NHC adduct (NHC)·AlH3 with elemental iodine or by the lated and characterized by means of elemental analysis, IR and treatment of (NHC)·GaH3 with an excess of methyl iodide at multinuclear NMR spectroscopy and by X-ray diffraction of the room temperature. The reaction of one equivalent of the group compounds 7, 9, 10, 15, 16 and 18. Introduction Aluminum and gallium hydrides are important reagents in or- ganic and inorganic synthesis, e.g. for reductions and hydride transfer,[1,2] as well as important materials, in particular for hydride storage.[3] The majority of aluminum and gallium hydride species are either neutral or anionic, whereas cationic Lewis-base stabilized aluminium and especially gallium hydrides are rather scarce in the literature. Atwood and co-worker syn- thesized and fully characterized the six- and five-coordinated aluminium hydride cations [{MeNCH2CH2N(Me)CH2CH2CH2}2· + + AlH2] VIII and [{MeN(CH2CH2NMe2)}·AlH2] VII as their alumin- – [4] ium hydride [AlH4] salts (see Scheme 1, VII & VIII). The first + four-coordinated aluminium hydride cation [(NMe3)2·AlH2] III was reported by Roesky et al., synthesized from a rather unfore- seeable reaction. A mixture of (NMe3)·AlH2Cl, (NMe3)·AlH3 and tBuC≡CLi leads to formation of [(NMe3)2·AlH2]2[(AlH)8- (CCH tBu) ], in which the [AlH ]+ cation is stabilized by two 2 6 2 Scheme 1. Lewis-base stabilized aluminium and gallium cations. Lewis-bases NMe3 (see Figure 1, III) and the carbaalanate cluster 2– counterion [(AlH)8(CCH2tBu)6] , formed in course of the reac- tion, serves as a bulky, non-coordinating anion.[5] The first au- salts [(NtBuH ) GaH ]+ and [(NsBuH ) GaH ]+ (see Figure 1, IV) thenticated cationic gallium hydrides were presented by Par- 2 2 2 2 2 2 as their chloride salts.[6] sons and co-workers.[6] Treatment of lithium gallium hydride Aluminium or gallium hydride cations stabilized by N-hetero- with [NtBuH ]Cl or [NsBuH ]Cl leads to the formation of the 3 3 cyclic carbenes (NHCs) are less common. Recently we reported the synthesis of a six-coordinated aluminium cation [a] A. Hock, L. Werner, M. Riethmann, Prof. Dr. U. Radius + [(iPr2Im)4·AlH2] IX as a side product of the reaction of lithium Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg aluminium hydride with an excess of iPr2Im (iPr2Im = 1,3-diiso- Am Hubland, 97074 Würzburg, Germany [7] E-Mail: [email protected] propyl-imidazolin-2-ylidene, see Scheme 1, IX). Stephan et al. http://www.ak-radius.de presented recently a dinuclear dicationic aluminium hydride 2+ Supporting information and ORCID(s) from the author(s) for this article are [(Dipp2Im)·AlH(μ-H)]2 VI in [(Dipp2Im)·AlH(μ-H)]2[B(C6F5)4]2 available on the WWW under https://doi.org/10.1002/ejic.202000720. {Dipp2Im = 1,3-bis(2,6-diisopropyl-phenyl)imidazolin-2-ylidene} © 2020 The Authors. European Journal of Inorganic Chemistry published by and a three-coordinated mononuclear dication [(Bn Im) ·AlH]2+ Wiley-VCH GmbH · This is an open access article under the terms of the 2 2 Creative Commons Attribution License, which permits use, distribution and (Bn2Im = 1,3-dibenzylimidazolin-2-ylidene) in [(Bn2Im)2·AlH]2- [8] reproduction in any medium, provided the original work is properly cited. [B(C6F5)4] II (see Scheme 1, VI & II). Jones and Stasch et al. Eur. J. Inorg. Chem. 2020, 4015–4023 4015 © 2020 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH EurJIC Full Paper European Journal of Inorganic Chemistry doi.org/10.1002/ejic.202000720 tion of such cationic bis-NHC aluminium and gallium hydrides from the reaction of (NHC)·AlH2I and (NHC)·GaH2I with addi- Me Me tional NHC for the carbenes Me2Im , iPr2Im, iPr2Im , and Dipp2Im. Results and Discussion Cationic bis-NHC stabilized aluminium and gallium hydrides + [(NHC)2EH2] (E = Al, Ga) should be electronically and sterically saturated. Thus, our strategy was to synthesize adducts of the type (NHC)·EH2I (E = Al, Ga) with a good leaving group iodide as precursor. Syntheses of NHC stabilized aluminium and gallium hydride chlorides are well known in the literature. NHC stabi- lized mono- and dichloroalanes and gallanes (NHC)·EH2Cl and (NHC)·EHCl2 (E = Al, Ga) are either accessible by the reaction of the NHCs with in situ generated “EH2Cl” and “EHCl2”orby dismutation between (NHC)·EH3 and (NHC)·ECl3 (E = Al, Ga).[9,12d,15] For the synthesis of NHC-stabilized iodoalanes only a few examples are available in the literature,[9,16] e.g. a report by Jones and Stasch et al. on the synthesis of (NHC)·AlH2I ad- ducts (NHC = Mes2Im, Dipp2Im) by dismutation between two equivalents of (NHC)·AlH3 and one equivalent (NHC)·AlI3.For Me Me Figure 1. Molecular structure of (Me2Im )·AlH2I 7 (left), (iPr2Im )·AlH2I 9 the synthesis of diiodoalanes (NHC)·AlHI2 (NHC = Mes2Im, Me (right) and (Me2Im )·GaH2I 10 (down) in the solid state (ellipsoids set at the Dipp2Im) the corresponding NHC stabilized aluminium hydrides 50 % probability level). Selected bond lengths [Å] and angles [°]: 7: C1–Al1 were treated with an excess of methyl iodide. However, for the 2.025(2), Al1–H1 1.61(4), Al1–H2 1.66(4), Al1–I1 2.586(8); H1–Al1–H2 109.4(18), H1–Al1–I1 108.7(12), H2–Al1–I1 106.5(14), C1–Al1–H1 114.3(14), C1–Al1–H2 heavier gallium analogues only one example of a NHC stabi- 106.5(14), C1–Al1–I1 105.88(7); 9: C1–Al1 2.039(3), Al1–H1 1.58(5), Al1–H2 lized iodogallane was presented in the literature so far, i.e. 1.60(4), Al1–I1 2.5960(10); H1–Al1–H2 104(2), H1–Al1–I1 110.5(17), H2–Al1–I1 (Mes2Im)·GaH2I, which was prepared from the reaction of meta- 111.1(14), C1–Al1–H1 106.7(18), C1–Al1–H2 120.2(14), C1–Al1–I1 103.77(9); I [16] stable “Ga I” with (Mes2Im)·GaH3. 10: Ga–C1 2.036(4), Ga–H1 1.52(6), Ga–H2 1.44(7), Ga–I 2.6415(6). C1–Ga– We were interested to develop a simple synthetic route to H1 109(2), C1–Ga–H2 108(2), C1–Ga–I 100.07(13), H1–Ga–H2 122(3), H1–Ga–I Me Me 110(2), H2–Ga–I 105(2). obtain the NHC adducts (NHC = Me2Im , iPr2Im, iPr2Im )of iodoalane and iodogallane (NHC)·MH2I (M = Al, Ga) of sterically less demanding NHCs in a way that may be transferred to large reported the monocationic aluminium hydride quantity synthesis. Therefore, we reacted the compounds + Me Me [(Dipp2Im)·AlH2(aDipp2Im)] V (“a” denotes “abnormal coordi- (NHC)·AlH3 (NHC = Me2Im 1, iPr2Im 2, iPr2Im 3) with nation”) which is stabilized by one normal and one abnormal 0.5 equivalents of elemental iodine I2 in toluene at room tem- coordinated NHC (see Scheme 1, V).[9] Furthermore, Krossing perature. Straight after the addition of iodine, dissolved in mini- and Jones et al. disclosed recently the synthesis of a “naked” mum amounts of toluene, to a toluene solution of 1–3, respec- I + Ga cation [(Dipp2Im)2Ga] I stabilized by a sterically demanding tively, the solution discolored. After 15 min stirring at room NHC and a non-coordinating anion as counterion in temperature, all volatiles were removed in vacuo and the com- [10] Me Me [(Dipp2Im)2Ga][Al(OC(CF3)3)4] (see Scheme 1, I). Thus, NHC pounds (NHC)·AlH2I(NHC=Me2Im 7, iPr2Im 8, iPr2Im 9) stabilized cationic aluminium hydrides are rather scarce and were isolated as colorless powders in excellent yields (7:88%, NHC stabilized cationic gallium hydrides are currently unknown. 8:97%,9: 86 %) and purity (see Scheme 2). [7,11] [12] We and others investigate currently group 13 metal For the reaction of (NHC)·GaH3 with elemental iodine, how- compounds (M = Al, Ga, In) which are stabilized with strong σ- ever, consistently the formation of small amounts of (NHC)·GaI3 donating NHC-ligands. The coordination of NHCs, for example, was observed, independent on the stoichiometry used and to alanes and gallanes leads to the synthesis of stable NHC even after short reaction times. Therefore, the gallane com- [7,11,12] Me Me metal hydride adducts. Thermally stable examples such pounds (NHC)·GaH2I (NHC = Me2Im 10, iPr2Im 11, Dipp2Im as (Dipp2Im)·MH3 (M = Al, Ga, In) have been prepared particu- 12) were synthesized by the reaction of the NHC gallane adduct larly for sterically demanding NHCs. Moreover, the syntheses (NHC)·GaH3 with an excess of methyl iodide at room tempera- of some low-valent, NHC-stabilized dinuclear hydrodialane and ture.
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