Synthesis, characterization and crystal structure of a 2-(diethylaminomethyl)indole ligated dimethylaluminium complex

The title aluminium complex was prepared by methane elimination from the reaction of 2-(diethylaminomethyl)indole and trimethylaluminium. Each of the two crystallographically independent molecules has a four-coordinate aluminium center that has pseudo-tetrahedral geometry.


Chemical context
Organoaluminium chemistry has a long history of active research that has led to numerous applications in industry (Mason, 2005). Organoaluminium compounds have garnered much attention in recent years for their use in the formation of polyactides, (Liu et al., 2010;Chisholm et al., 2003Chisholm et al., , 2005Zhang et al., 2014;Chen et al., 2012;Schwarz et al., 2010) and hydroamination (Koller & Bergman, 2010a,b;Khandelwal & Wehmschulte, 2012). While many varieties of ancillary ligands on aluminium have been employed in such reactions, a majority of these systems have nitrogen-donor arms as a component. Our group is interested in particular in the use of 2-(dialkylaminomethyl)indoles (Nagarathnam, 1992) as ligands for organoaluminium complexes. Herein we report the synthesis, characterization and crystal structure of the first 2-(dialkylaminomethyl)indolyl-aluminium complex, [Al(CH 3 ) 2 (C 13 H 17 N 2 )].

Structural commentary
The asymmetric unit of the title complex contains two independent molecules (Fig. 1). They are structurally different with regard to the chelate rings that are formed around the aluminium atoms by the indolyl moiety. The most obvious difference between the two crystallographically independent molecules is the displacement of the Al atom from the plane of the chelate ring. Al1 deviates by 0.6831 (5) Å from the plane The crystal packing is illustrated in Fig. 2. In the crystal, molecules associate via three different types of C-HÁ Á Á interactions, as shown in Figs. 3 and 4. There is one interaction between the methyl proton H5A and the centroid of the (C12A-C17A) aromatic ring of 2.57 Å (Table 1) and another between the methylene proton H4D and the aromatic C14 of 2.88 Å . The third interaction is between H2B and the centroid of C12A i -C17A i [ Table 1; symmetry code: (i) 1 À x, À 1 2 + y, 1 À z]. This interaction links the two independent molecules in the asymmetric unit into chains that extend along the b-axis direction.

Figure 1
A view of the asymmetric unit of the title compound, showing the atom labeling. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity.

Figure 2
Crystal packing diagram of the title compound viewed along the a axis. C-HÁ Á Á interactions between molecules in the asymmetric unit.

Figure 4
All C-HÁ Á Á interactions between molecules of the title compound.

Synthesis and crystallization
To a 100 mL side-arm flask was added 2-(diethylaminomethyl)indole (0.402 g, 2.0 mmol) and 25 mL of toluene. A toluene solution of trimethylaluminium (1.0 mL, 2.0 M, 2.0 mmol) was added via syringe. The reaction solution turned bright yellow, which darkened as the solution was stirred for 12 h. The solvent was then removed in vacuo resulting in a yellow solid, which was dissolved in a mixture of 10 mL of hot toluene, followed by cooling to 243 K for 48 h. The resulting yellow crystalline material was isolated by filtration. Yield: 0. X-ray quality crystals were grown from a concentrated solution in hot toluene followed by slow cooling to room temperature followed by storage at 243 K for 72 h.