Crystal structure of (perchlorato-κO)(1,4,7,10-tetraazacyclododecane-κ4 N)copper(II) perchlorate

The crystal structure of (perchlorato-κO)(1,4,7,10-tetraazacyclododecane-κ4 N)copper(II) perchlorate is reported. The crystal was grown from a solution of methanol at ambient temperature which resulted in no co-crystallization of solvent.


Chemical context
Azamacrocycle ligands, including 1,4,7,10-tetraazacyclododecane (cyclen), are of significant importance in research due to their ability to form stable metal complexes, allowing for their use in a wide range of applications. Some of these complexes have been studied for their use as chemical sensors, contrast agents in MRI and PET, antimicrobial agents and as biomimetic catalysts (De Leó n-Rodríguez et al., 2010;Yoo et al., 2005). Copper-cyclen complexes have been studied extensively for their ability to perform catalytic DNA cleavage and peptide hydrolysis (Zhang et al., 2016;Li et al. 2014;Hormann et al., 2015). Although the synthesis of a similar Cu II complex has been reported previously, no crystal structure of the complex, [Cu(1,4,7,](ClO 4 ) 2 , has previously been published (Kruppa et al., 2006).

Structural commentary
In the title complex (Fig. 1), the copper(II) ion coordinated by the four nitrogen atoms of the cyclen ligand and one oxygen ISSN 2056-9890 atom of a perchlorate ligand. The five-coordinate cupric ion shows a nearly ideal square-pyramidal geometry ( 5 = 0.049; Addison et al., 1984). The Cu-N bond lengths range from 2.004 (1) to 2.015 (1) Å , which are typical values. The Cu II ion exhibits a tetragonal distortion that leads to a longer apical bond with Cu1-O1 = 2.266 (1) Å , which is 0.12 Å longer than the average Cu-O distance (Clay et al., 1979;Rohde & Merzweiler, 2010). The average N-Cu-O bond angle is 103.8 (8) . Three hydrogen bonds are present within the asymmetric unit, with two extending from O2 and O3 of the bound perchlorate anion to N1-H1 and N2-H2, respectively. The third hydrogen bond extends from N2-H2 to O8 of the unbound anion; the numerical details are given in Table 1.

Supramolecular features
The crystal structure exhibits three unique symmetry elements: an inversion center, a twofold screw axis and a glide plane. The complex cations of two asymmetric units hydrogenbond across an inversion center, which is clearly visible when viewed along the a axis ( Fig. 2), creating a dimer. These hydrogen bonds (N3-H3Á Á ÁO1, N3-H3Á Á ÁO4, N4-H4Á Á ÁO5) have an average NÁ Á ÁO distance of 3.16 Å (Fig. 3). The complexes assemble in rows parallel to the b axis ( Fig. 4) due in part to weak electrostatic interactions between the bound perchlorate anion and a neighboring cyclen ligand. A hydrogen bond between the cyclen ligand and a neighboring perchlorate anion (N1-H1Á Á ÁO3) allows the building units to assemble parallel to the a axis (Fig. 5).

Database survey
A database survey resulted in several similar Cu-cyclen complexes with five-coordinate copper(II). Four structures chosen for further analysis contained a copper(II) ion coordinated by either five nitrogen atoms or four nitrogen atoms and one oxygen atom (Rohde & Merzweiler, 2010;Sarma et al., 2010;Pé ré z-Toro et al., 2015;Guo et al., 2008). Where applicable, the complexes have similar Cu-O bond lengths to that of the title complex, with only slight deviations. The title complex and surveyed complexes have similar Cu-N distances with a standard deviation of 0.018 Å .

Figure 2
View of the unit cell along the a axis. An inversion center (yellow dots) exists between two asymmetric units, creating the dimeric unit defined at the center of the unit cell. Hydrogen bonds are drawn in blue.

Figure 3
A view of hydrogen bonding within a dimer pair. Hydrogen bonds are drawn in blue. Carbon and hydrogen atom labels have been omitted for clarity. copper(II) perchlorate hexahydrate (527 mg, 1.4 mmol) were separately dissolved in 2.8 mL anhydrous methanol each and combined. The resulting purple solution formed a precipitate. The reaction mixture was heated to reflux for 30 min then filtered. The filtrate was evaporated to dryness to yield a purple amorphous solid. X-ray quality crystals were grown by dissolving the solid in a minimum amount of methanol followed by slow evaporation at ambient temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Packing of the complex cations, as viewed along the c axis of the unit cell. The a axis is drawn in red and the b axis is drawn in green.

Figure 5
Hydrogen bonding between complex cations and anions, as viewed along the c axis. Hydrogen bonds are drawn in blue. The a axis is drawn in red and the b axis is drawn in green. Crystal structure of (perchlorato-κO)(1,4,7,10-tetraazacyclododecaneκ 4 N)copper(II) perchlorate Jessica L. Gray, Deidra L. Gerlach and Elizabeth T. Papish

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.