Reactivity of NHC Alane Adducts towards N‐Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al−H Bond Activation

Abstract

The synthesis of mono‐NHC alane adducts of the type (NHC)⋅AlH3 (NHC=Me2Im (1), Me2ImMe (2), iPr2Im (3 and [D3]‐3), iPr2ImMe (4), Dipp2Im (10); Im=imidazolin‐2‐ylidene, Dipp=2,6‐diisopropylphenyl) and (NHC)⋅AliBu2H (NHC=iPr2Im (11), Dipp2Im (12)) as well as their reactivity towards different types of carbenes is presented. Although the mono‐NHC adducts remained stable at elevated temperatures, ring expansion occurred when (iPr2Im)⋅AlH3 (3) was treated with a second equivalent of the carbene iPr2Im to give (iPr2Im)⋅AlH(RER‐iPr2ImH2) (6). In 6, {(iPr2Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp2Im with the formation of (iPr2Im)⋅AlH2(ROR‐Dipp2ImH2)H2Al⋅(iPr2Im) (9). In 9, two {(iPr2Im)⋅AlH2} moieties stabilize the ring‐opened Dipp2Im. If two hydridic sites are blocked, the adducts are stable with respect to further ring expansion or ring opening, as exemplified by the adducts (iPr2Im)⋅AliBu2H (11) and (Dipp2Im)⋅AliBu2H (12). The adducts (NHC)⋅AlH3 and (iPr2Im)⋅AliBu2H reacted with cAACMe by insertion of the carbene carbon atom into the Al−H bond to give (NHC)⋅AlH2/iBu2(cAACMeH) (13–18) instead of ligand substitution, ring‐expansion, or ring‐opened products.

Imidazolyl Alanes – Synthesis, Structures, and Reactivity Studies

“…The Al–C NHC bond [ d C1–Al1 = 207.3(2) pm] is elongated as compared to the Al–C aNHC bond [ d C5–Al2 = 199.1(2) pm], which is most likely induced by steric repulsion of the Mes and t Bu substituents. Nevertheless, these bond lengths are in line with reported aluminum NHC and aNHC compounds . In contrast to the Al– C c arbene bonds, the aluminum nitrogen distances of d Al1–N3 = 196.0(2) pm d Al2–N2 = 194.4(2) pm differ only slightly.…”

“…Hence, a proton‐coupled rearrangement has occurred, which has, however, already been reported by others. [14o], [23e] 2) We were able to isolate and characterize compound 3 (Scheme ), which can be viewed as adduct of monomeric imidazolyl alane with t Bu 2 AlBr. Compound 3 is stable for several months at ambient temperature under Schlenk conditions, as isolated compound, as well as in solution (C 6 D 6 , hexane).…”

B−B Cleavage and Ring‐Expansion of a 1,4,2,3‐Diazadiborinine with N‐Heterocyclic Carbenes

“…Heating of 2c under the same conditions does not induce any CÀNb ond activation/RER,w hich is analogoust oo ther works on p-block element hydridea dducts of cAAC Me . [25] To probe the Lewis acidity of the two unique boron atoms in these new B,N-heterocycles, one additional equivalent of NHC Me was added to 3a and 3b,r esulting in the formation of adducts 4a and 4b in moderatey ields (Scheme 3). Monitoring this reaction by 11 BNMR, the B1ÀH1 resonances of 3a/3b disappear,w hereasn ew peaks emerge at À8.8 and À16.8 ppm for 4a and 4b,r espectively.T his assignment is further support- Atomicd isplacemente llipsoids are depicted at 50 %p robability and omitted at the ligand periphery.Hydrogena toms are omittedf or clarity,except for those bound to boron.S elected bond lengths []a nd angles [8]: for 2b B2À B1 1.699(2), B1ÀN1 1.408(2),B 2 ÀN2 1.570(2), B2ÀC1 1.633(2), B1ÀH1 1.12(2), N1-B1-B2 119.0(1), N2-B2-B1 110.4(1), N2-B2-C1 114.1(1), B1-B2-C1 114.6(1), B1-B2-C1-N3 33.5(2);for 2c:B1 ÀH1 1.18(2), N1ÀB1 1.419(4), B1ÀB2 1.678 (5) Atomic displacemente llipsoidsa re depicted at 50 %p robability and omitted at the ligand periphery.Most hydrogen atoms are omitted for clarity.S elected bond lengths []and angles [8]: for 3a:N2 ÀB2 1.443(2), B2ÀN4 1.411(2), N1ÀB1 1.403(2), B1ÀC1 1.574(2), C1ÀB2 1.588(2), C1ÀN3 1.474(2), B1ÀH1 1.11 (2), N2-B2-C1 116.6(1), B2-C1-B1105.…”

Bis‐NHC Aluminium and Gallium Dihydride Cations [(NHC)2EH2]+ (E = Al, Ga)

“…Recently we reported the synthesis of a six-coordinated aluminium cation [(iPr 2 Im) 4 •AlH 2 ] + IX as a side product of the reaction of lithium aluminium hydride with an excess of iPr 2 Im (iPr 2 Im = 1,3-diisopropyl-imidazolin-2-ylidene, see Scheme 1, IX). [7] Stephan et al presented [8] Jones and Stasch et al 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 106.5 (14), C1-Al1-I1 105.88(7); 9: C1-Al1 2.039(3), Al1-H1 1.58 (5), Al1-H2 1.60(4), Al1-I1 2.5960 (10); H1-Al1-H2 104(2), H1-Al1-I1 110.5 (17), H2-Al1-I1 111.1 (14), C1-Al1-H1 106.7 (18), C1-Al1-H2 120.2 (14), C1-Al1-I1 103.77(9); 10: Ga-C1 2.036(4), Ga-H1 1.52 (6), Ga-H2 1.44 (7), Ga-I 2.6415 (6). C1-Ga-H1 109(2), C1-Ga-H2 108(2), C1-Ga-I 100.07 (13), H1-Ga-H2 122(3), H1-Ga-I 110(2), H2-Ga-I 105(2).…”

“…The coordination of NHCs, for example, to alanes and gallanes leads to the synthesis of stable NHC metal hydride adducts. [7,11,12] Thermally stable examples such as (Dipp 2 Im)•MH 3 (M = Al, Ga, In) have been prepared particularly for sterically demanding NHCs. Moreover, the syntheses of some low-valent, NHC-stabilized dinuclear hydrodialane and hydrodigallene compounds were presented, [13] for example the dialane [(Dipp 2 Im)•AlH 2 ] 2 which was reported by Jones et al [12,14] However, the formation of cationic aluminium and gallium dihydrides is rare and stabilization of [EH 2 ] + with two strong σ-donating NHC-ligands should lead to accessible hydroalane and hydrogallane cations.…”