Crystal structure, spectroscopic characterization and Hirshfeld surface analysis of aquadichlorido{N-[(pyridin-2-yl)methylidene]aniline}copper(II) monohydrate

The title complex contains four distorted square-pyramidal molecules in the asymmetric unit, each of which interacts with another molecule located in an adjacent unit cell by means of two hydrogen-bonded water molecules of crystallization, thus forming symmetric dimers that govern the supramolecular features of the infinite lattice.

The reaction of N-phenyl-1-(pyridin-2-yl)methanimine with copper chloride dihydrate produced the title neutral complex, [CuCl 2 (C 12 H 10 N 2 )(H 2 O)]ÁH 2 O. The Cu II ion is five-coordinated in a distorted square-pyramidal geometry, in which the two N atoms of the bidentate Schiff base, as well as one chloro and a water molecule, form the irregular base of the pyramidal structure. Meanwhile, the apical chloride ligand interacts through a strong hydrogen bond with a water molecule of crystallization. In the crystal, molecules are arranged in pairs, forming a stacking of symmetrical cyclic dimers that interact in turn through strong hydrogen bonds between the chloride ligands and both the coordinated and the crystallization water molecules. The molecular and electronic structures of the complex were also studied in detail using EPR (continuous and pulsed), FT-IR and Raman spectroscopy, as well as magnetization measurements. Likewise, Hirshfeld surface analysis was used to investigate the intermolecular interactions in the crystal packing.

Chemical context
Cu II ions coordinated by diimine N-donor ligands (-N C-C N-) are of great interest since they combine structural flexibility with other desired characteristics, such as ease of preparation, photophysical (Barwiolek et al., 2016) and photobiological (Banerjee et al., 2016) properties, and catalytic activity (Dias et al., 2010), as well as the capability to mimic active protein sites (Gupta & Sutar, 2008) and stabilize both metal oxidation states common in biological systems. These complexes also exhibit a broad spectrum of pharmacological properties including anti-inflammatory, antibacterial, antioxidant and antimetastatic (Chaviara et al., 2005) activities. In particular, they are promising metallotherapeutic drugs for the treatment of cancer, given their ability to induce apoptosis or generate reactive oxygen species (ROS) in oxidative stress, resulting in DNA damage and strand breaks in cancerous cells (Trudu et al., 2015).
In particular, bidentate pyridinylimine (C 5 H 4 N-CH 2 -NH-C 6 H 5 ) and pyridinylmethylamine (C 5 H 4 N-CH N- ISSN 2056-9890 C 6 H 5 ) Schiff base derivatives have attracted increasing attention because of their close structural relationship with the protein A aggregate p-I-stilbene [I-C 6 H 4 -CH N-C 6 H 4 -R, R = N(CH 3 ) 2 ] and thus their potential use for the development of metal chelators for the attenuation of metalinvolved neurodegeneration in Alzheimer's disease (DeToma et al., 2012). These ligands can therefore act as chemical reagents that can target metal-associated amyloid-(A) species and modulate metal-induced A aggregation and neurotoxicity in vitro and in living cells (Braymer et al., 2012).
Based on their relevant structural features and promising biological activity, we have begun to explore novel metal complexes coordinated with diimine ligands (Schiff bases). We report here the synthesis and structural characterization of the complex [Cu(H 2 O)Cl 2 (C 12 H 10 N 2 )]ÁH 2 O where C 12 H 10 N 2 = N-(pyridin-2-ylmethylene)aniline. This compound is formed by the reaction of copper chloride dihydrate with the C 12 H 10 N 2 Schiff base to afford bright-green crystals suitable for X-ray diffraction studies.

Supramolecular features
As expected, both the water molecule of crystallization and the aqua ligand play a significant role in the crystal packing of the complex. This is governed by the presence of symmetric cyclic dimers formed between complex molecules in adjacent

Figure 2
Symmetric cyclic dimers in the crystal structure of the title complex formed by dual ClÁ Á ÁH-O-HÁ Á ÁCl interactions (dotted blue lines) between the chlorine ligands and the water molecules.

Figure 1
ORTEP representation of the title complex with the atom numbering. Displacement ellipsoids are drawn at 50% probability level. The hydrogen bond to the water molecule of crystallization is shown as a dashed blue line.
unit cells along the a-axis direction (see Fig. 2

Hirshfeld surface analysis
In order to investigate and visualize the role of weak intermolecular interactions, a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out and the asso- Hirshfeld surface of the title complex mapped over electrostatic potential in the range 0.5 to 1.5 atomic units.

Figure 3
The crystal packing in a view along the a + b vector showing the stacking of symmetric cyclic dimers.

CW-EPR/Pulsed-EPR and PPMS characterization
In order to obtain in-depth information on the spin properties of this unpaired spin complex (d 9 , 2S + 1 = 2), electron paramagnetic resonance (EPR) continuous-wave (CW) experiments were performed on a X-Band Bruker ELEXSYS E500 spectrometer operating at 9.8 GHz. The powdered sample was inserted in a quartz tube and the spectra were recorded at room temperature and 100 K under non-saturated conditions: microwave power of 0.63 mW and modulation amplitude of 2 G. Pulsed EPR was studied at 5 K with a Bruker ELEXSYS E580 spectrometer equipped with a helium flow cryostat. Two-pulse echo field sweep acquisitions were performed using a standard Hahn echo sequence 90 À À the first quadrant (+,À) where A > 2n I (n I being the nuclear frequency) corresponds to the strong hyperfine coupling A between the I nucleus and the unpaired electron and the second quadrant (+,+) where A < 2n I corresponds to weaker interactions. Magnetization measurements were performed with a physical property measurement system (PPMS) Quantum Design Dynacool of 9 T and the vibrating sample magnetometer (VSM) option. To verify that both samples, i.e. powder and crystal, correspond to the same compound, a comparison between the X-ray powder diffraction pattern and the simulated single X-ray diffraction pattern is presented in Fig. 6. A strong isotropic signal characteristic of Cu II was detected by EPR spectroscopy with g iso = 2.13 and a line width of 130 G at room temperature. No sign of anisotropic behaviour was detected in the low-temperature continuous wave EPR spectra recorded at 100 K (Fig. 7) and 8 K. Further efforts to reveal possible minor anisotropic behavior through Q-band (34 GHz) low-temperature (8 K) measurements still showed a single strong band characteristic of the Cu II ion in an isotropic environment. The isotropic signal was so dominant in the powder spectrum that the anisotropic features were invisible. A similar observation was made by Xavier & Murugesan (1998). 98 mg of Cu II were quantified in the complex sample by continuous wave EPR using copper sulfate with a known mass as standard. To obtain more information about the surroundings of the copper(II) centre, a pulsed EPR experiment was performed using HYSCORE. In quadrant (+,+), a unique signal characteristic of hydrogen was detected with a Larmor frequency of 14.6 MHz corresponding to a weak interaction between the copper unpaired electron and the hydrogen nucleus (Fig. 8). Interactions between the copper and the nitrogen atoms were not observed. It is probable that the interaction is too strong to be detected by the HYSCORE sequence.
Magnetic susceptibility measurements were performed to verify the nature of coupling between the cupric ions. The   CW-EPR spectrum for the title complex measured at 100 K. temperature dependence of the molar magnetic susceptibility M and the corresponding inverse susceptibility 1/ M measured at a magnetic field of 0.1 T in the temperature range of 2-400 K is shown in Fig. 9. Fig. 10 shows the dependence of magnetization on the magnetic field at 2 K, 100 K and 300 K. At higher temperature, the magnetization manifest Curie-Weiss-like behaviour. The magnetization curves of the sample have features typical of a paramagnetic contribution between magnetic centers

Database survey
A survey of the Cambridge Structural Database (CSD, Version 5.40, Oct 2019; Groom et al., 2016) reveals that crystal structures have been reported for coordinated Cu II and Zn II complexes containing N-(pyridin-2-ylmethyl)aniline and its deprotonated form N-(pyridin-2-ylmethylene)aniline, respectively (Braymer et al., 2012). For the former complex, [Cu(C 12 H 12 N 2 )Cl 2 ], a nearly square-planar geometry between the bidentate Schiff base and two chloro ligands was reported; while for the latter, [Zn(C 12 H 10 N 2 )Cl 2 ], a distorted tetrahedral geometry was observed. A detailed revision of the CIF file reported for the [Cu(C 12 H 12 N 2 )Cl 2 ] complex, however, reveals that the crystal packing of this compound can be best described as comprising polymeric chains of complex units consisting of slightly distorted square-pyramidal [()Cu-Cu(C 12 H 12 N 2 )Cl 2 ] where the apical position is occupied by a bridged Cu II ion.

Synthesis and crystallization
The N-(pyridin-2-ylmethylene)aniline ligand, C 12 H 10 N 2 , was prepared by condensation reaction between 2-pyridinealdehyde (Sigma-Aldrich, 99%) and aniline (Sigma-Aldrich, 99%) in dry methanol (Merck, HPLC grade) at reflux temperature for 4 h under atmospheric pressure and constant stirring. The stoichiometry used in this reaction was 1:1 mmol. The released water vapour was prevented from returning to the reaction vessel by placing a condensation trap containing methanol in the lower base of the reflux column. No byproduct was formed during the reaction. The purity and molecular weight of the ligand was confirmed by GC/MS spectrometry using an Agilent 6850 series II gas chromatograph (CG) coupled to an Agilent 5975B VL MSD mass spectrometer (MS) equipped with a split/splitess injection port (533 K, split ratio 15:1), with an Agilent 6850 series automatic injector and an Agilent 19091S-433E HP-5MS column.
The title Cu II complex was prepared by reacting the C 12 H 10 N 2 ligand with copper chloride dihydrate, CuCl 2 Á2H 2 O (Merck, 99.9%), in dry methanol for 8 h at room temperature, under atmospheric pressure and constant stirring. The Cu II complex precipitated in methanol as a green solid, which was then separated from the solvent by rotoevaporation. Crystallization was carried out from a saturated solution of the Cu II complex in methanol at 313 K, which was allowed to cool to Two-dimensional HYSCORE spectrum of the title complex recorded at 5 K.

Figure 9
Temperature dependence of molar magnetic susceptibility M and inverse susceptibility 1/ M .

Figure 10
Dependence of magnetization on the magnetic field at 2, 100 and 300 K. room temperature and then hexane was added until reaching a 1:1 methanol/hexane ratio, followed by storage at 277 K. Crystals of the title Cu II complex were separated by subtled decantation and evaporation of the solvents at room temperature.
An infrared (IR) spectrum in attenuated total reflectance (ATR) was acquired from a ground crystal using a Shimadzu Prestigie-21 spectrophotometer with Fourier Transform (FTIR), equipped with a Michelson-type interferometer, a KBr/Ge beam-splitter, a ceramic lamp and DLATGS detector. The FTIR spectrum was measured in the 4000-500 cm À1 range with a resolution of 3.0 cm À1 and 30 scans. Likewise, Raman spectra of the title complex were obtained using a LabRAM HR confocal Raman microscope (Horiba Scientific) operating in a backlit orientation and equipped with a cryogenic detector and laser lines of 473, 532 and 633 nm of 18, 30 and 17 mW maximum power, respectively. The micro-Raman spectra of the complex were taken through an Olympus 50Â long-working-distance microscope objective (NA = 0.5, WD = 10.6 mm), in the range from 3500 to 100 cm À1 , with a resolution of 4 cm À1 and a laser power of around 3.0 mW.

Aquadichlorido{N-[(pyridin-2-yl)methylidene]aniline}copper(II) monohydrate
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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )