Trichlorido(1,3-dimethyl-2,3-dihydro-1H-imidazol-2-ylidene-κC 2)aluminium(III)

The title compound, [Al(C5H8N2)Cl3], was prepared by a thermolytic decomposition under high-vacuum conditions and presents a formal adduct of an Arduengo carbene, 1,3-dimethyl-1H-imidazol-2-ylidene, and aluminium trichloride. The Al atom adopts a pseudo-tetrahedral CCl3 coordination environment. All N and C atoms, the Al atom, one of the Cl atoms, and all aromatic H atoms of the molecule lie on a mirror plane. As a result of the mirror symmetry of the molecule, the H atoms of all methyl groups are disordered between symmetry-equivalent positions.

The title compound, [Al(C 5 H 8 N 2 )Cl 3 ], was prepared by a thermolytic decomposition under high-vacuum conditions and presents a formal adduct of an Arduengo carbene, 1,3dimethyl-1H-imidazol-2-ylidene, and aluminium trichloride. The Al atom adopts a pseudo-tetrahedral CCl 3 coordination environment. All N and C atoms, the Al atom, one of the Cl atoms, and all aromatic H atoms of the molecule lie on a mirror plane. As a result of the mirror symmetry of the molecule, the H atoms of all methyl groups are disordered between symmetry-equivalent positions.
Of interest, in all four structurally characterized X 3 Al-AC adducts, the Al atoms are in a tetrahedral coordination environment as shown by an analysis of the structures compiled in the Cambridge Structural Database (Allen, 2002) [CSD; Version 5.34, release May 2013; 4 entries, 4 fragments].
Preparation of all Al complexes mentioned above includes, as a step, generation of a free AC by deprotonation of a corresponding imidazolium salt with a strong base under mild conditions. This seriously limits the method due to the known thermal instability of sterically non-hindered ACs even in solution. Recently, we developed a facile route to BF 3 and PF 5 adducts with sterically non-hindered ACs by a thermolytic decomposition of related imidazolium salts with Compound (I) presents a formal adduct of an Arduengo carbene, 1,3-dimethyl-1H-imidazol-2-ylidene, and aluminium trichloride. The Al-atom adopts a pseudo-tetrahedral coordination environment, defined by the three Cl atoms and the carbene C atom (Table 1). All N-and C-atoms, the Al-atom, one of the Cl-atoms, and all aromatic H-atoms of the molecule lie on a mirror plane at (x, 1/4, z). The H-atoms of methyl groups are disordered between symmetry equivalent positions ( Fig. 1).

Experimental
Acetonitrile and toluene solvents were kept over and distilled from CaH 2 and/or Na/K alloy under Ar atmosphere, respectively. AlCl 3 was sublimed in high vacuum prior to use. 1 H NMR spectra were recorded on a Varian 400 INOVA instrument in CD 3 CN at 400 MHz and 298 K, with the signal of the residual solvent protons [δ(H) = 1.94 p.p.m.] used as an internal reference.
The imidazolium salt (II) was prepared by a reaction of AgF (1.27 g, 10.0 mmol) and 1,3-dimethyl-1H-imidazolium iodide (2.24 g, 10.0 mmol) in distilled water (25 ml). The precipitate of AgI was filtered off, the filtrate was concentrated till dryness, and dried on the high-vacuum line (1.3-2.0×10 -1 Pa) at 323 K. Recrystallization from an ethanol/acetone mixture (1: 1) followed by double recrystallization from dry acetonitrile gave 0.93 g (6.8 mmol, 68%) of (II) as colorless Compound (I): Crystalline (II) (0.93 g, 6.8 mmol) and AlCl 3 (0.91 g, 6.8 mmol) were placed into a small apparatus for distillation of high-melting compounds, the system was connected to the high-vacuum line (1.3-2.0×10 -1 Pa) through a liq. N 2 cooled trap, evacuated, and heated. At approximately 423 K, the signs of melting were observed and then a strongly exothermic reaction accompanied by a gas evolution started. The temperature of the reaction mixture was then increased up to 573 K and the crude product was collected in the receiver as a reddish oil (reaction vessel temperature 573-673 K). The crude compound of (I) (1.03 g, 4.5 mmol, 66%) was allowed to crystallize in a refrigerator (270 K, 7 days). 1 H NMR δ p.p.m.: 3.98 (s, 6H, CH 3 ), 7.24 (s, 2H, CH=CH). Single crystal of (I) suitable for the X-ray diffraction analysis were grown by recrystallization of crude (I) from dry toluene, mounted in a Lindemann glass capillary (Ø 0.5 mm, glove box, N 2 atmosphere) and sealed off.

Refinement
The H atoms were treated as riding atoms with distances C-H = 0.96 (CH 3 ), 0.93 Å (C Ar H) and U iso (H) = 1.5 U eq (C), 1.2 U eq (C), respectively. The H-atoms of the methyl groups are disordered due to the mirror symmetry of the aromatic ring; hance they were refined with half-occupancy.

Trichlorido(1,3-dimethyl-2,3-dihydro-1H-imidazol-2-ylidene-κC 2 )aluminium(III)
Crystal data [Al(C 5  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.