Di-μ-chlorido-bis[chlorido(1,4,6-trimethyl-6-nitro-1,4-diazepine)copper(II)]

The title neutral copper complex, [Cu2Cl4(C8H17N3O2)2], shows a binuclear center with a Cu—(μ-Cl)2—Cu core, in which each copper ion is coordinated by the N,N,O donor atoms of the tridentate ligand 1,4,6-trimethyl-6-nitro-1,4-diazepine (meaaz-NO2) and three chloride exogenous ligands. Each metal ion is facially coordinated by meaaz-NO2 through N,N,O donor atoms, whereas two bridging and one terminal chloride ions occupy the other face of the highly Jahn–Teller-distorted octahedron. Two N atoms from tertiary amine groups of the meaaz-NO2 ligand and two exogenous Cl atoms with short Cu—N and Cu—Cl distances define the equatorial plane. The coordination around each CuII ion is completed by another Cl atom and an O atom from the NO2 group, in the axial positions. The binuclear complex exhibits a centrosymmetric structure with point symmetry .


Comment
Tridentate ligands that are able to force facial geometry, such as 1,4,7-tacn (1,4,7-triazacyclononane), daza (1,4-diazepan-6-amine) (Romba, et al., 2006), tach (cis,cis-1,3,5-triaminocyclohexane) (Hegg & Burstyn, 1998), play an important role in the stabilization of a great number of structural motifs in coordination compounds and in biological systems (Peralta et al., 2005). Copper complexes with this kind of ligand have been reported over the past few years with a view to the study of the hydrolysis of phosphate esters, proteins and DNA (Deal & Burstyn, 1996;Fry et al., 2005). Indeed such copper(II) complexes exhibit high catalytic reactivity in the hydrolysis of DNA model diesters as bis(4-nitrophenyl)phosphate with rate constants of ≈ 10 -4 s -1 (Belousoff et al., 2006). In this context we report herein the synthesis and X-ray analysis of a new dinuclear copper complex with the tridentate ligand meaaz-NO 2 .
The seven-membered chelate ring of the meaaz-NO 2 ligand restricts the N-Cu-N angle to 77.35 (8)°, which is about 6°s maller than the respective angles formed by nine-membered ring in the Cu-tacn complexes.
As described in Rodriguez, et al. (1999), there are three kinds of configurations for copper complex containing the Cu-(µ-Cl) 2 -Cu core: Type I, in which two square pyramids share one base-to-apex edge with the two bases nearly perpendicular to one another; Type II, square pyramids sharing one base-to-apex edge but with parallel basal planes and Type III, square pyramids sharing a basal edge with coplanar basal planes. The configuration of the Cu centers reported here are Type II given that the axial positions of one copper(II) center is directed toward the top and the same axis of the adjacent center is in the anti position. Although the Cu centers in the complex are hexacoordinate, it can be considered as type II, because the sixth coordination bond is very long (Cu-O2 = 2.845 (2)Å).

supplementary materials sup-2
The packing is mainly governed by weak C-H···O and C-H···Cl interactions with average D···A distances of 2.99 Å and 3.74 Å, respectively. In addition, the packing analysis reveals that the molecules are accomodated in layers parallel to the (001) plane and are stacked along crystallographic a axis (Fig. 2).
Copper complex was synthesized by adding 187 mg of the ligand meaaz-NO 2 (1.0 mmol) to a CH 3 CN solution containing CuCl 2 .2H 2 O (171 mg, 1.0 mmol). The solution was then concentrated under magnetic stirring and was allowed to stand at room temperature for a few days, yielding a small number of dark green crystals which were suitable for the single-crystal X-ray analysis.

Refinement
H atoms were placed at their idealized positions with distances of 0.97 and 0.96 Å and U eq fixed at 1.2 and 1.5 times U iso of the preceding atom for CH 2 and CH 3 , respectively.

Special details
Experimental. Absorption correction: SADABS (Bruker, 2006) was used to scale the data and to perform the multi-scan semi-empirical absorption correction.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (4)