1,4-Bis(4-bromo-2,6-diisopropylphenyl)-1,4-diazabuta-1,3-diene

The molecule of the title compound, C26H34Br2N2, lies on a crystallographic inversion center and hence the two imine groups are s-trans. The dihedral angle between the central 1,4-diazabuta-1,3-diene unit and the attached substituted phenyl ring is 88.4 (7)°. The structure features a C—H⋯N close contact. The crystal selected for this study proved to be a non-merohedral twin with a minor component of 21.8 (3)%.

We gratefully acknowledge Bruker AXS sponsorship of this publication.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BX2248). DAB ligands have also been used extensively in d-block coordination chemistry, particularly within the field of olefin polymerization catalysis (Ittel et al. 2000). Jung et al. (2007) recently used the title compound as a precursor to an N-heterocyclic carbene in the synthesis of a catalytically active cationic (η 3 -allyl)(NHC)palladium complex.
The molecule of (I) resides on a crystallographic inversion center and hence the two imine groups are s-trans. The dihedral angle between the central 1,4-diazabuta-1,3-diene moiety and the attached substituted phenyl ring is 88.4 (7)°.
The molecular symmetry approaches C 2h , however, the positions of the isopropyl groups break the mirror plane symmetry: both H atoms on the tertiary C atoms of the two symmetry-independent i Pr groups point toward atom N1, but reside on the opposite sides of the phenyl ring. This is illustrated with two disparate but "would be equivalent" torsion angles, one for each i Pr group: C2-C3-C8-C9 (-96.5 (8)°) and C2-C7-C11-C13 (163.6 (7)°). This geometry differs from that of the unbrominated congener of (I), 1,4-bis(2,6-diisopropyl-phenyl)-1,4-diazabuta-1,3-diene, (II). For related structures, see: Müller et al. (2003), Schaub et al. (2006. Compound (II), structurally characterized at 173 K by Berger et al. (2001) and at 193 K by Laine et al.(1999), crystallizes with the molecule of (II) on an inversion center. The H atoms of the tertiary C atom of the isopropyl groups point toward the N atom and, in contrast to (I), are located on the same side of the phenyl ring.
The overall symmetry of (II) is much closer to C 2h as the i Pr groups are oriented very similarly: in the 193 K structure of (II) two "would be equivalent" Me-C(H)-C-C(N) torsion angles measured 144.6 and 145.4°. The C-Br distance of 1.897 (6) Å is in excellent agreement with the value of 1.899 (11) Å obtained by averaging 2303 C-Br bond lengths from 1736 relevant compounds reported to the Cambridge Structural Database (Allen, 2002).
Experimental 4-Bromo-2,6-di-iso-propyl aniline was prepared according to the literature procedure (Liu et al. 2005). To a stirred solution of 4-bromo-2,6-di-iso-propyl aniline (3.0 g,11.71 mmol) in methanol (40 cm 3 ) containing 4 drops of formic acid was added glyoxal (0.85 g, 5.80 mmol, 40% aqueous soln.) slowly dropwise. The reaction mixture was stirred for 24 h at room temperature, filtered, and the precipitate washed with cold MeOH (2 x 10 mL). This yellow solid was dried in vacuo and recrystallized from EtOH to give a crop of pale yellow needles suitable for X-ray diffraction analysis. Yield 3.53 g, 56%.

Refinement
All H-atoms were placed in idealized locations with C-H distances 0.93 -0.98 Å and refined as riding with appropriate thermal displacement coefficients U iso (H) = 1.2 or 1.5 times U eq (bearing atom). The crystal of (I) selected for this study proved to be a non-merohedral twin. The two twin components are related by a 179.9° rotation about the [001] direction in reciprocal space with the minor component contribution of 21.8 (3)%. Fig. 1. Molecular structure of (I). The thermal ellipsoids are shown at 30% probability level. Atoms labeled with the suffixes A and unlabeled are generated by the symmetry operation (-x+1, -y+1, -z+1).

Figures
1,4-Bis(4-bromo-2,6-diisopropylphenyl)-1,4-diazabuta-1,3-diene  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 Rfactors(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.