supplementary materials

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-NO₂.

This neutral copper complex exhibits a centrosymmetric structure (Fig. 1) with a highly distorted octahedral environment around the copper center. Each metal ion is facially coordinated by meaaz-NO₂ through N₂O donors atoms, whereas two bridged and one terminal coordinated chlorines occupy the other face of the distorted octahedron. Two amine nitrogen atoms (N3 and N6) of the ligand and two exogenous chlorines (Cl1, Cl2) lie in the equatorial plane. The coordination sphere of Cu1 is completed by another chlorine (Cl2') and an oxygen atom (O2) from the NO₂ group, in the axial positions. In the equatorial plane, the Cu—N and Cu—Cl bond lengths are Cu1—N6 2.064 (2) Å, Cu1—N3 2.122 (2) Å, Cu1—Cl2 2.2686 (7) Å and Cu1—Cl1 2.2694 (7) Å), respectively. The longer bond lengths Cu1—Cl2' (2.7611 (8) Å) and Cu1—O2 (2.845 (2) Å) are associated with the two apical positions, as expected for a (4 + 2) distorted geometry, as is common for Cu^II. The Cu—N and the Cu—Cl bond lengths in the equatorial plane are comparable to those found for other copper complexes [Cu(tacn)Cl₂] (Cu—N2 2.063 (4) Å, Cu—N3 2.038 (4) Å, CU—Cl1 2.268 (1) Å and Cu—Cl2 2.312 (1) Å) (Schwindinger, et al., 1980), [Cu₂(µ-Cl)₂(Me-bpa)₂(ClO₄)₂] (Me-bpa = N-methyl-bis(2-pyridylmethyl)amine) (Cu1—N36 1.983 (2) Å, Cu1—N10 2.036 (2) Å, Cu1—N26 1.989 (2) Å and Cu1—Cl1 2.2587 (6) Å) (Astner et al., 2008) and [Cu(me₃tacn)Cl₂] (Cu1—N1 2.100 (2) Å, Cu1—N2 2.111 (2) Å, Cu1—Cl2 2.2558 (9) Å and Cu1—Cl1 2.3050 (8) Å) (Steed et al., 2007). The seven-membered chelate ring of the meaaz-NO₂ ligand restricts the N—Cu—N angle to 77.35 (8)°, which is about 6° smaller 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).

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).