{1,1′-Bis[(pyridin-2-yl)methyl]-2,2′-bipiperidyl}(perchlorato)copper(II) perchlorate

The title complex, [CuII(ClO4)(mesoPYBP)](ClO4) {PYBP = 1,1′-bis[(pyridin-2-yl)methyl]-2,2′-bipiperidyl, C22H30N4}, was prepared and found to crystallize with two crystallographically independent complex salt moieties. The metal atoms of the cations adopt a pseudo-square-pyramidal coordination geometry, where the tetradentate aminopyridine ligands (PYBP) are wrapped around the Cu atoms in the equatorial plane.

The title complex, [Cu II (ClO 4 )(mesoPYBP)](ClO 4 ) {PYBP = 1,1 0 -bis[(pyridin-2yl)methyl]-2,2 0 -bipiperidyl, C 22 H 30 N 4 }, was prepared and found to crystallize with two crystallographically independent complex salt moieties. The metal atoms of the cations adopt a pseudo-square-pyramidal coordination geometry, where the tetradentate aminopyridine ligands (PYBP) are wrapped around the Cu atoms in the equatorial plane. The Cu-O bonds involving an O atom of the coordinating perchlorate anion are approximately perpendicular to the plane. The two remaining non-coordinating perchlorate anions are involved in several C-HÁ Á ÁO hydrogen bonds with the PYBP ligand and balance the total charge of the complex salt. The two crystallographically independent moieties are related to each other via a pseudo-translation along the a-axis direction. Exact translational symmetry is broken by (i) a difference in the conformation of one of the piperidine rings, featuring a chair conformation in one of the cations, and a sterically disfavored boat conformation in the other; and (ii) by modulation of the non-coordinating perchlorate anions.

Chemical context
The design and synthesis of a family of linear tetradentate aminopyridine ligands, featuring a diamine derivative backbone (e.g. 1,2-cyclohexyldiamine or 2,2 0 -dipyrrolidyl) and two picolyl arms attached to the amine nitrogen atoms, have frequently been discussed (Murphy & Stack, 2006;Yazerski et al., 2014). Common examples of linear tetradentate aminopyridine ligands are shown in Fig. 1 Common examples of linear tetradentate aminopyridine ligands.

. The Fe and Mn complexes
bearing this type of ligand show good catalytic activity for olefin epoxidation (Lyakin et al., 2012;Mikhalyova et al., 2012), as well as aromatic (Makhlynets & Rybak-Akimova, 2010) and aliphatic (Ottenbacher et al., 2015) C-H activation. Related copper(II) complexes with aminopyridine ligands have also been synthesized and characterized (Singh et al., 2017;Kani et al., 2000;Liebov et al., 2011). Potential applications of these complexes include fluorescent sensing of NO. The copper(II) ion in complexes with an appended fluorophore is readily reduced by nitric oxide with concomitant fluorescence enhancement (Kumar et al. 2013a,b).
The tetradentate mesoPYBP ligand surrounds the metal ion in the basal plane (Fig. 2). One of the two remaining octa-hedral sites is occupied by the oxygen atom of a coordinating perchlorate anion, while the other site remains vacant. Another perchlorate anion in the outer sphere balances the net charge and connects nearby complex cations via C-HÁ Á ÁO hydrogen bonds. The two chemically equivalent moieties are related to each other via a pseudo-translation by half a unit cell along the a-axis direction (Fig. 3). Similar to recently discussed crystal structures of Cu-N 2 /Py 2 complexes (Singh, et al. 2017), the exact translational symmetry is broken by slightly different conformations of the two complex cations.
As shown in Fig. 3, one of the cations (the red Cu1 moiety) has both piperidine rings in a chair conformation, while the other complex cation (the green Cu2 moiety) has one piperidine ring in a sterically disfavored boat conformation (shown in light green). The reason the second cation adopts this unfavorable conformation can be tentatively traced back to the packing interactions of the cations and perchlorate anions. An ORTEP diagram of the molecular structure of [Cu(mesoPYBP)-(ClO 4 )](ClO 4 ), showing the atom-labeling scheme, with ellipsoids drawn at the 50% probability level. H atoms have been omitted for clarity.

Figure 3
Crystal packing of the title complex viewed along a axis: the Cu1 (red) and Cu2 (green) moieties are related by pseudo-translation along the a axis. The molecular parts contributing to the pseudosymmetry are highlighted. H atoms and all atom labels have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Supramolecular features
Details of hydrogen-bonding parameters are listed in Table 1. There are in total twelve C-HÁ Á ÁO hydrogen bonds, between aromatic and aliphatic C-H units and perchlorate O atoms (Fig. 4). Among these hydrogen bonds, only three involve the inner-sphere perchlorato ligand (C6-H6AÁ Á ÁO3 ii ; C17-H17BÁ Á ÁO4 ii ; C28-H28BÁ Á ÁO12 iv ); all of these hydrogen bonds are intermolecular, linking with the hydrogen atoms on the pyridine -carbons of the adjacent Cu-mesoPYBP cations.
The perchlorate close to the Cu1 moiety forms six hydrogen bonds with four adjacent complex cations (both Cu1 and Cu2), while that close to the Cu2 moiety only forms three hydrogen bonds with two adjacent complex cations (Cu2 only). This difference in hydrogen-bonding environments of the two outer-sphere perchlorates breaks the symmetry between them and between the cation moieties. All CÁ Á ÁO distances of the C-HÁ Á ÁO interactions (3.08-3.29 Å ) are roughly equal to or shorter than the sum of van der Waals radii of the corresponding atoms (3.25 Å ), indicating normal strength interactions.

Synthesis and crystallization
The synthesis of the mesoPYBP ligand involves two steps. A detailed synthetic procedure for (2R,2 0 S)-2,2 0 -bipiperidine-1,1 0 -diium dibromide (mesoBPÁ2HBr) via reductive hydrogenation of 2,2 0 -dipyridyl was reported by Herrmann et al. (2006) and Yang et al. (2013). 1.81 g mesoBPÁ2HBr was dissolved in 8 mL H 2 O, and 8 mL of 5 M NaOH solution was added, followed by addition of 10 mL of CH 2 Cl 2 . With vigorous stirring, 4 mL of an aqueous solution containing 1.86 g picolyl chloride hydrochloride was added dropwise, and the reaction mixture was stirred for about four days. The two layers were separated, and the aqueous layer was extracted with CH 2 Cl 2 . The organic layers were combined and the solvent was evaporated under vacuum.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed at calculated geometries and allowed to ride on their parent C atoms. The C-H distances were set to 0.99 Å for CH 2 , 1.00 Å for CH and 0.95 Å for aromatic CH bonds. Isotropic displacement parameters were set to 1.2 times of the equivalent isotropic displacement parameter of the parent atom. Data collection: APEX2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2006) and OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: publCIF (Westrip, 2010).

{1,1′-Bis[(pyridin-2-yl)methyl]-2,2′-bipiperidyl}(perchlorato)copper(II) perchlorate
Crystal data 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.