[N,N′-Bis(2,4,6-trimethylphenyl)ethane-1,2-diimine-κ2 N,N′]tetracarbonylchromium(0)

The octahedral coordination of the Cr0 atom in the title compound, [Cr(C20H24N2)(CO)4], displays some distortion. This is manifested by an exocyclic torsion angle C(mesitylene)—N—Cr—C(carbonyl) that deviates by more than 20° from planarity. Another structural feature is the significant distortion from linearity of the Cr—C—O angles of the two carbonyl groups that interact with both ortho-methyl groups of the two mesitylene rings. The remaining two carbonyl groups overlap with the centres of the mesitylene rings themselves and are linear within <3°.


Related literature
For the synthesis of similar complexes, see: Baxter & Connor (1995). The MLCT (metal-to-ligand charge-transfer) band was observed at 570 nm for an analogous complex; see: Ruminski & Wallace (1987).
The synthesis of similar complexes, using disubstituted 2,2′-bipyridine compounds as coordinating ligands to study solvatochromism of these and other group 6 metal derivatives, was reported previously (Baxter & Connor, 1995). In the UV spectrum of (I), a stronger MLCT band is observed at 595 nm. The intense colour of the complex is ascribed to this metal-to-ligand transition. For an analogous complex, viz. [Cr(CO) 4 (dpp)], dpp = 2,3-bis(2-pyridyl)pyrazine, the MLCT band was observed at 570 nm with CHCl 3 as solvent (Ruminski & Wallace, 1987). The transition around 300 nm was assigned as a dpp π* → π intraligand transition.
The solid state structure of the title compound revealed that the molecule packs with the mesitylene rings close to parallel to the (ac)-plane, with the angle between the mean planes formed by these rings at 19.81 (12)°. The 5-membered (Cr-N-C-C-N) ring is planar and approximately parallel to the (bc)-plane. This mean plane is almost perpendicular with the mean planes formed by the mesitylene rings, with the values for these angles being 85.42 (11)° and 76.87 (11)°.
The distorted octahedral geometry around the Cr 0 atom ( Fig. 1) is manifested by the exocyclic torsion angle C21-N2-Cr1-C1 with a value of -21.81 (19)°. Another structural feature is the significant distortion from linearity, by 8.0° and 9.8°, of the Cr-C-O bond angles of the two carbonyl groups that interact with the ortho methyl groups of the two mesitylene rings. These two Cr-CO bond lengths are similar at 1.890 (3)  The crystal packing is without any other significant features, but the value of 18.5 Å 3 per non-H atom is indicative of the efficient packing of the molecules in the unit cell.

Experimental
Cr(CO) 6 (3 mmol, 0.66 g) and N,N′-(ethane-1,2-diylidene)bis(2,4,6-trimethylaniline) (3 mmol, 0.60 g) were added to a microwave container and 30 ml of dichloromethane added. The container was sealed and the vessel inserted into the microwave oven. The reaction was left at 700 Watt for 1.5 h. The resulting solution was dark blue in colour. Solvent was supplementary materials removed in vacuo. The product was isolated on a silica gel column using 1:

Refinement
All hydrogen atom positions were obtained from difference Fourier maps but were included in the refinement as riding on the atom to which they are bonded. Isotropic displacement parameters for the hydrogen atoms were set at 1.2 times the equivalent isotropic displacement parameter of the atom to which each hydrogen atom is bonded (1.5 times for the methyl H atoms).

Figure 1
View of the asymmetric unit of (I). Displacement ellipsoids are shown at the 50% probability level.

[N,N′-Bis(2,4,6-trimethylphenyl)ethane-1,2-diimine-κ 2 N,N′]tetracarbonylchromium(0)
Crystal data [Cr(C 20 (14) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.