Alane-Centered Ring Expansion of N-Heterocyclic Carbenes

Abstract

The β-diketiminato aluminum dihydrides, [HC­{(Me)­CNAr}2AlH2] [4: Ar = 2,6-di-isopropylphenyl (Dipp), 5: 2,4,6-trimethyl­phenyl (Mes)] react directly with N-aryl-substituted N-heterocyclic carbenes (NHCs) by C–N bond activation to afford aluminum amido-alkyl derivatives of the form [HC­{(Me)­CNAr}2AlCH2(N­(Ar′)­CH)2]. The more sterically congested alane (4), bearing N-Dipp substitution, does not react with either 1,3-bis­(2,6-di-isopropylphenyl)­imidazol-2-ylidene (IPr) or 1,3-bis­(2,4,6-trimethyl­phenyl)­imidazol-2-ylidene (IMes), even under forcing conditions. In contrast, in situ generation of 1,3-bis­(phenyl)­imidazol-2-ylidene through deprotonation of the corresponding imidazolium tetrafluoroborate by KN­(SiMe3)2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield [HC­{(Me)­CNDipp}2AlCH2(N­(Ph)­CH)2]. Although compound 5 also does not react with IPr, relaxation of the steric demands of the supporting β-diketiminate ligand to N-mesityl substitution enables analogous ring opening of IMes, with the formation of [HC­{(Me)­CNMes}2AlCH2(N­(Mes)­CH)2] (7), when the reaction is heated to 80 °C. DFT calculations performed on model systems suggest that in comparison to the parent alane (AlH3) the enhanced propensity of these systems to induce NHC ring cleavage is a consequence of the relative stability of the initially formed five- and four-coordinate adducts as well as the augmented hydridic character of the Al–H bonds within the β-diketiminate-supported molecules.

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

“…The NHC−aluminum bond distance of 2.0709(18) Å corresponds to the one observed in ( i Pr 2 Im) ⋅ AlH 3 ( 3 ; 2.0405(17) Å). The Al−C RER‐NHC bond length of 1.999(2) Å and the Al−N RER‐NHC of 1.8514(17) Å do not deviate from the known literature data for Al−C (1.99 Å) and Al−N (1.88 Å) single bonds.…”

“…NHC aluminum chemistry has been developed to some extent and it is known that Lewis acid–base adducts of aluminum hydrides are rather unstable. Besides simple decomposition of the compounds to unidentified molecules, NHC to a NHC ( a NHC=abnormal N‐heterocyclic carbene) rearrangement in the coordination sphere of aluminum and ring‐expansion reaction of an NHC in β‐diketiminatoaluminum dihydrides have been observed . However, the utilization of carbenes in aluminum chemistry also facilitates the isolation of interesting substances with new molecular structures, such as (Dipp 2 Im) ⋅ H 2 Al−AlH 2 ⋅ (Dipp 2 Im), obtained by magnesium(I)‐mediated dehydrogenative coupling of (Dipp 2 Im) ⋅ AlH 3 , or a cAAC‐stabilized neutral aluminum radical and the NHC‐stabilized aluminum cation [{(Dipp 2 Im) ⋅ AlH( μ 2 ‐H)} 2 ] 2+ 2[B(C 6 F 5 ) 4 ] − .…”

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

“…[7] Calculations have shown that the five coordinated bis-NHC adducts of group 13 hydrides are higher in energy and less stable than the four coordinated tetrahedral mono NHC adducts and are often intermediates of further reactions. [18] Instead of NHC ring expansion we observe in the case of (NHC) 2 •EH 2 I stabilization by an elimination of the good leaving group iodide to give the four-coordinated tetrahedral aluminium or gallium cation, respectively (see Scheme 4, ii). The strong σ-donating alkyl-substituted NHCs stabilize the cationic electron deficient metal centre in its 8VE count.…”

“…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).…”

Insertion of Group 12–16 Hydrides into NHCs: A Theoretical Investigation

“…Indeed, we have very recently reported the first observation of the insertion of an aluminumh ydride at room temperature, [36] which was assisted by the use of aN acNac ligand that servest oi ncreaset he hydridic character in comparison with NHC ligands.…”