Di-tert-butyl 3,3′-(2,2′-bi-1H-imidazole-1,1′-diyl)dipropanoate

In the title compound, C20H20N4O4, the complete molecule is generated by a crystallographic centre of symmetry. The conformation is stabilized by two intramolecular C—H⋯N links.

In the title compound, C 20 H 20 N 4 O 4 , the complete molecule is generated by a crystallographic centre of symmetry. The conformation is stabilized by two intramolecular C-HÁ Á ÁN links.

Comment
Biimidazole is a potentially polydentate ligand, but its chemistry is less developed in comparison to the imidazole chemistry.
The reason may be a limited solubility of biimidazole in common organic solvents. Although several new disubstituted 2,2'-biimidazoles have been recently synthesized (Barnett et al., 1999;Barnett et al., 2002), a few metal complexes based on biimidazole derivatives are reported (Zhang, Zhang, Ren et al., 2009). In the course of our ongoing study, we have successfully synthesized a series of biimidazole derivatives with terminal carboxylic, hydroxyl, phosphino, imino groups which can be used as ligands in the coordination chemistry Zhang, Zhang, Xu et al., 2009) and cross-coupling reactions . These ligands exhibit rich coordination patterns and catalytic properties. Here we report the synthesis and the crystal structure of the title compound which is an intermediate of those above mentioned ligands. As shown in Fig. 1, the biimidazole ring atoms (C6, C7, C9, N1, N2 and their inversion-related partners) exhibit essentially coplanar mutual orientation [the dihedral angle is 0.00 (1)°], and the value of the torsion angle C9-N1-C10-C5 is -77.53 (30)°. In the crystal structure, there are weak C-H···N interactions (Tab. 1, Fig. 2).

Experimental
The title compound was prepared according to a published procedure (Barnett et al., 1999). 0.2 g (5 mmol) of NaOH was added to a suspension of 3 g (22.4 mmol) of 2,2'-biimidazole in 100 ml of DMF (dimethylformamide) at 80°C. The resulting mixture was stirrred for 30 min. In the course of this time the mixture gradually turned into a clear pale yellow solution.
7.12 g (55.6 mmol) butyl acrylate in 10 ml of DMF was added dropwise in several minutes to the solution and the reaction was stirred at 80°C for 8 h until the heating was stopped. The DMF was removed via vacuum distillation in a hot oil bath at 100°C. The resulting black brown oil was dissolved in water (30 ml) and extracted with CH 2 Cl 2 . The organic layer was washed with water, and then evaporated under reduced pressure to yield a white product (7.4 g, 85%). The product was dissolved in 95% ethanol (30 ml) and cooled slowly in a refrigerator to afford colourless block crystals of average size 1.5 mm×1.2 mm×0.5 mm that were suitable for the X-ray analysis.

Refinement
All the hydrogens were discernible in the difference electron density map. Nevertheless, the hydrogens were situated into the idealized positions. The C-H distances were constrained to 0.93, 0.96 and 0.97 Å for aryl, methylene and methyl hydrogens, respectively. U iso (H) = 1.2 U eq (C aryl ); U iso (H) = 1.2 U eq (C methylene ); U iso (H) = 1.5 U eq (C methyl ).
supplementary materials sup-2 Figures   Fig. 1. View of the title molecule with the displacement ellipsoids at the 45% probability level. The labelling of the non-H atoms is also given. The symmetry code (i):-x+1, -y+1, -z+1. Di-tert-butyl 3,3'-(2,2'-bi-1H-imidazole-1,1'-diyl)dipropanoate  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.