Chlorido(2,2′-{[2-(1-methyl-1H-imidazol-2-yl-κN 3)imidazolidine-1,3-diyl-κN]bis(methylene)}bis(1-methyl-1H-imidazole-κN 3))copper(II) perchlorate

The molecular and crystal structure of a novel copper(II) complex bearing a new tetradentate 1-methyl-imidazole-containing imidazolidine ligand is described.

In the crystal structure of the title complex, [CuCl(C 17 H 24 N 8 )]ClO 4 , the copper(II) metal exhibits an N 4 Cl pentacoordinate environment in a distorted square-pyramidal geometry. Coordination to the metal centre occurs through the three 1-methylimidazole N atoms from the pendant groups, one amine N atom from the imidazolidine moiety and one chlorido anion. Intermolecular interactions take place at two of the 1-methyl-imidazole rings in the form of parallel-displacedstacking interactions forming chains parallel to the a axis. Three O atoms of the perchlorate anion are rotationally disordered between two orientations with occupancies of 0.5.

Chemical context
Copper ions play a key role in many natural processes, as they are found in the active site of enzymes involved in electron and O 2 transfers, oxidation and reduction, being a target for the obtaining of biomimetic or bioinspired compounds (Stephanos & Addison, 2014). As a result of the redox characteristics of the copper ion, the versatility of ligands to which it coordinates, and the geometries it is capable of forming, copper complexes have attracted attention as catalysts for different transformations, mainly involving the activation and reduction of oxygen (Elwell et al., 2017). For the hydrogen evolution reaction (HER), the obtaining of homogeneous copper catalysts is limited by the dissociation of copper(II) because of the more negative potentials required for the reduction of protons (Zhang et al., 2014;Du et al., 2016). However, different copper complexes have been obtained and evaluated as catalysts for HER, showing promising results (Zhang et al., 2016;Haddad et al., 2017;Khusnutdinova et al., 2018). The use of bioinspired tripodal tetradentate ligands in the construction of metal complexes catalysts can provide a unique feature, the presence of cis-labile sites for substrate coordination that may be a requisite for its catalytic activity, facilitating electron/atom transfer processes.

Structural commentary
The title complex crystallizes in the monoclinic system, space group P21/n. The asymmetric unit comprises one complex cation and one disordered perchlorate anion (Fig. 1). The copper(II) ion has an N 4 Cl pentacoordinated environment formed by one ligand molecule and one chlorido ion. Coordination of the ligand to the metal centre occurs through the three 1-methyl-imidazole nitrogen atoms (N Me-im ) and one of the tertiary amine nitrogen atoms from the imidazolidine moiety (N am ). A distorted square-pyramidal geometry is observed ( = 0.39), with the basal plane composed of the chlorido ion, the amine nitrogen and the two equivalent 1-methyl-imidazole nitrogen atoms N11/N21. The third 1-methyl-imidazole nitrogen N31 occupies the apical position. Distortion of the geometry is evidenced by the bond angles in the coordination sphere, ranging from 77.43 (11) to 113.64 (12) and 147.65 (12) to 171.21 (8) for the cis and trans angles, respectively. This highly distorted square-pyramidal geometry may arise from the formation of a seven-membered chelate ring (Cu1/N1/C5/N4/C7/C32/N31) that is less tensioned than the four or five-membered rings, allowing a more flexible arrangement. As a consequence of the geometry distortion, the copper(II) ion lies 0.2565 (13) Å above the Cl1/ N21/N1/N11 basal plane towards the apical position. Squarepyramidal copper complexes exhibiting a smaller geometry distortion tends to show a slighter displacement of the metal centre from the basal plane. In the similar [Cu(bpqa)Cl] + ( = 0.16) and [Cu(tmqa)Cl] + ( = 0.06) complexes [bpqa = 1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)-N-(quinolin-2-ylmethyl)methanamine; tmqa = tris(quinolin-2-ylmethyl)amine; Wei et al., 1994], the copper(II) ion is 0.189 (2) and 0.045 (3)  The Cu-N Me-im bond lengths in the title compound range from 1.976 (3) to 2.173 (3) Å , the longest one being formed by the 1-methyl-imidazole nitrogen N11. For the Cu-N am bond, a distance of 2.137 (3) Å was found. Similar values were reported for the 1-methyl-imidazole-containing complexes [Cu(Hhis-im 2 )Cl] + (Higa et al., 2007)

Features of related complexes
In pentacoordinated copper(II) complexes containing tripodal N 4 donor ligands similar to the title compound, the Cu-Cl bond length seems to be directly related to the type and degree of geometry distortion around the metal centre. In complexes exhibiting a square-pyramidal geometry, as in the title compound, the Cu-Cl bond length has a range of 2.27-2.29 Å . For complexes in a trigonal-bipyramidal geometry, the Cu-Cl distance is around 2.23 Å (Karlin et al., 1982;Oberhausen et al., 1990;Wang et al., 1995). This difference may be related to the ligand spatial orientation, resulting from the geometric arrangements around the metal centre. The trigonal-bipyramidal geometry imposes a vertical positioning of the coordinated ligand rings parallel to the axial direction, which minimizes the repulsion between the electronic clouds of the chloride ion and the tripodal ligand. This arrangement allows a greater approach of the chloride ion to the metal centre and consequently a shorter bond distance. In the case of complexes in a square-pyramidal geometry, the coordinated rings are oriented parallel to the basal plane, increasing the chloride/ligand repulsion effect, which makes the Cu-Cl bond more elongated. Curiously, copper complexes in both geometries with tripodal ligands showing steric hindrance exhibit intermediate Cu-Cl bond distances among those found for complexes with non-hindered ligands on squarepyramidal and trigonal-bipyramidal geometries, indicating a balance of repulsive and stabilizing chloride/ligand interactions that is geometry independent (Wei et al., 1994;Jitsukawa et al., 2001).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The perchlorate anion is rotationally disordered over two orientations sharing the O1 oxygen atom with site occupancy factors of 0.5. The two disordered positions were refined by applying SADI restraints on the Cl-O bond lengths and OÁ Á ÁO separations. The U ij parameters of the Cl2 atom were restrained to an approximate isotropic behaviour.

Chlorido(2,2′-{[2-(1-methyl-1H-imidazol-2-yl-κN 3 )imidazolidine-1,3-diyl-κN]bis(methylene)}bis(1-methyl-1Himidazole-κN 3 ))copper(II) perchlorate
Crystal data [CuCl(C 17  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. Refinement. All H atoms were placed geometrically and refined using a riding atom approximation, with C-H = 0.93-0.98 Å, and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl groups.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )