Polynuclear 2D Copper(II) Complex of Pyrazine Containing Perchlorate As a Counter-Ion: Crystal Structure and DFT Calculations

A new polynuclear cationic complex of copper(II) (Cu(II)) with pyrazine as a ligand and perchlorate acting as a counter-ion was obtained. X-ray diffraction results accounted for a 2D polymer array of cations, in which the metal-ions were located on an ideal squarepyramidal coordination environment defined by four nitrogen atoms of pyrazines and one chloride. The polymer compound, which extended along the ab plane of the structure, showed Cu(II) displaced 0.9 Å from the plane defined by the N-atoms and chloride. Perchlorate ions were situated in cavities interacting with pyrazine by anion-π weak interactions. These low-energy bonds emerged as a consequence of the π-acidity of pyrazine upon coordination to Cu(II). Density-functional theory (DFT) calculations at the M06-2X level of theory using 6-31G(d) and 6-31+G(d) were conducted aimed at achieving an electronic description of selected properties of title complex. M06-2X/631G(d), in particular, proved to be an acceptable performer in dealing with geometric aspects keeping the computational cost very low.

As a part of our interest to increase basic knowledge on structural properties of Cu(II) complexes containing diimine ligands, the synthesis and structure of a polymer array of Cu(II), pyrazine, and chloride containing perchlorate as a counter-ion are presented. The network extends along the ab plane of the structure, in which the metal-ion is observed slighty displaced from the plane defined by the N-donor atoms. Anions are hosted by the structure in cavities via noncovalent anion-π interaction involving coordinated pyrazine. To shed light into the electronic properties, geometry optimization and population analysis were performed with the density-functional theory (DFT) hybrid meta exchange-correlation functional M06-2X [17] combined with 6-31G(d) [18,19,20,21,22,23] and 6-31+G(d) [24].

Materials and instrumentation
Reagents were obtained from major suppliers and used without further purifications. Elemental analyses for carbon, hydrogen, and nitrogen were performed on a Thermo Scientific Flash 2000 analyzer. FTIR spectra were recorded for the complex dispersed in KBr pellets on a Shimadzu IRPrestige-21 spectrometer.

Synthesis
A solution of 47 μL (0.60 mmol) of pyrazine in MeOH (8 mL) was carefully layered on top of an aqueous solution (8 mL) of Cu(II) chloride (51 mg, 0.30 mmol) and sodium perchlorate (74 mg, 0.60 mmol). Light-blue crystals were obtained after seven days (yield 47%). Anal. Calcd for CuC 8   detector with Cu-K α radiation (λ = 1.54178Å). APEX 3 software was used for data collection and reduction, and multiscan absorption correction was applied [25]. The structure was solved by iterative methods using SHELXT. Structure refinement was done using SHELXL-2018 [26,27] within SHELXLE [28] by the full-matrix least squares on F2 method. Nonhydrogen atoms were refined using anisotropic displacement parameters, whereas hydrogen atoms were geometrically positioned and refined with the riding model. For carbon-bonded hydrogen-atoms, thermal parameters were set to 1.2 times the U eq (equivalent isotropic displacement factor) of the carbon atom they are bonded to. The molecular graphics were prepared using MERCURY [29]. Selected crystallographic data, experimental details, and refinement results are summed up in Table 1.

Hirshfeld surface analysis
For the further understanding of the intermolecular interactions driving the hosting of perchlorate anion in the 2D polynuclear complex, the Hirshfeld surface around the anion was constructed. CRYSTAL EXPLORER17 was used to determine the surface [30,31], which was based on the CIF file. Normalized contact distances were mapped in the surface, where normalized contact distance (d norm ) is defined as where d e and d i represent the distances from a point on the surface to the nearest nucleus outside and inside the surface, respectively, and r vdW corresponds to the van der Waals (vdW) radii of the atoms involved [32]. The bidirectional graph of d e and d i distances within the surface (and their frequency)-2D fingerprint plot-provided quantitative information about the type of interactions [33].

Theoretical calculations
All calculations were conducted at the DFT. Geometry optimizations starting from molecular structure determined by X-ray diffraction were carried out. In all cases, M06-2X [17] in combination with 6-31G(d) [18,19,20,21,22,23] was used, and an ultrafine grid featured by 90 radial shells and 590 angular points per shell was considered. Diffuse functions were also taken into account (by using 6-31+G(d) [24]) aimed at inferring the influence of counter-ions on electronic properties. Thus, the system [CuCl(pyz) 4 ] + · · · ClO 4 was studied by using M06-2X in combination with 6-31+G(d) in the gas phase. To explore how pyrazine (as a bridgingligand) affects electronic properties, the model complex {CuCl(μ-pyz) 4 [CuCl(NH 3 ) 3 ] 4 } 5+ was considered, in which pyrazine of terminal units was substituted by NH 3 . Any attempts to optimize the geometry of this model complex with M06-2X/6-31G(d),6-31+G(d) were unsuccessful. Thus, we decide to consider a diamagnetic substitution approach in the model complex by replacing all Cu(II)-terminal cationic units by the neutral ones 3 ] 4 } + , whose geometry was optimized with M062X/6-31G(d) (further details are in Section 3.3). The nature of the stationary point was verified through vibrational analysis (no imaginary frequencies at the minimum). Natural population analysis (NPA) calculations were performed with the NBO code [34,35,36,37] included in the program package GAUSSIAN09, Rev. D01 [38], which was used for all theoretical studies reported in this contribution.   (3) 2.547 (2) Bond angle (3) 95.07(8) Figure 1: 2D array of the title complex (perspective view along the c-axis). Thermal ellipsoids are drawn at the 50% probability level.

Results and discussion
[CuCl(pyz) 2 ] n (ClO 4 ) n . The mononuclear subunit shows the metal-ion seated on an ideal square-pyramidal coordination environment [39,40], in which Cu(II) is bonded to a chloride and to four nitrogen atoms of the pyrazine-ligand with distances of 2.547(2) and 2.052(2)Å, respectively. Selected inequivalent bond lengths and angles are presented in Table 2; bond lengths and angles generated by symmetry and otherwise equivalent to the ones presented are excluded for clarity. It deserves to be mentioned that distance Cu−N was in line with the one observed in complexes of Cu(II) with pyrazine-containing ligands [41,42]. In this regard, complexes of Cu(II) with pyridylalkylamide ligands N -(pyridin-2-ylmethyl)pyrazine-2-carboxamide and N -(2-(pyridin-2-yl)ethyl)pyrazine-2-carboxamide, taken as an example into account, exhibit Cu−N bond distances in the range from 1.997Å to 2.067Å [41]. The polymer array extends along the ab plane of the structure (Figure 1), in which the metal-ion is observed displaced 0.9Å from the plane defined by the N-donor atoms, and chloride appears apically coordinated in an alternate fashion ( Figure 2   which points this ligand to present an important π-acidity in the title complex. Thus, this π-acidity is expected to prevail in the interior of all cavities (i.e., voids which were obtained upon the formation of the polynuclear 2D crystal structure) in which perchlorate ions are located. Consequently, anionπ weak interactions emerge and perchlorate ions appear connected to pyrazine (aided by the tilted orientation of this ligand) (Figure 3). Distance O 3 Cl−O · · · pyz (centroid) of 3.146Å is observed, and this metric parameter appears in line with the ones observed in related systems [44,45,46].

Hirshfeld surface analysis
In order to better understand the position of the perchlorate anion in the lattice, and how it functioned as a template for the construction of the 2D polynuclear complex, a Hirshfeld surface was constructed around it. The d norm mapped through the surface confirms that the predominant intermolecular interaction with the 2D polynuclear complex is anion-π interactions (Figure 4). These are sufficiently strong to generate red hotspots throughout the surface, where the interatomic distance is significantly shorter than the sum of the van der Waals radii of the atoms indicating the existence of a bond-type interaction. The largest hotspots are surrounding the O atoms on the side oriented towards the adjacent pyrazine ligand. Moreover, when the 2D fingerprint ( Figure 4) is calculated, the most frequent interactions (painted green and red in the fingerprint plot)  correspond to atom-atom distances between 2.0Å and 3.2Å, corresponding to Cl−O · · · H Cl−O · · · C contacts in the Cl−O · · · pyz interaction.

Geometry optimization and NPA results
Results from X-ray diffraction measurements account for a 2D-array of Cu(II), chloride, and pyrazine having perchlorate as a counter-ion, which is observed hosted in cavities interacting with the aromatic units by anion-π interactions. Thus, we decided to optimize one model complex [CuCl(pyz) 4 Table 3).
Population analysis for [CuCl(pyz) 4 ] + obtained with M06-2X/6-31+G(d) shows the metal center carrying a charge of +1.021, which is markedly smaller than the formal charge of +2 as a result of a donation process, in which chloride ends with a charge of −0.665. While pyrazine shows a natural charge of −0.421 on nitrogen atoms, its coordination induces a polarization in the πsystem as indicated by the charge of −0.519 on coordinated nitrogen atoms, and of −0.372 on the uncoordinated donor atoms.
Changes are observed by the inclusion of counter-ion in system [CuCl(pyz) 4 ] + · · · ClO 4 -. While copper appears as Cu +1.036 , NPA results for nitrogen seems to be dependent on position and role of this donor atom. Population results for coordinated nitrogen of pyrazines interacting with ClO 4 are of −0.536, with the charge of those donor atoms in opposite position being of −0.376. The presence of noncovalent anion-π interactions also affects the natural charge of all atoms on noninteracting pyrazine. Coordinated nitrogen appears with a charge of −0.499, with the ones remaining uncoordinated ending with a charge of −0.388.  Optimized geometry of the polynuclear model complex is displayed in Figure 6, and cartesian coordinates of the mononuclear one are included in the Supplementary Material (Table S1).
In going from the mononuclear complex to the model system, no important changes in geometric parameters are detected. Distances Cu−Cl and Cu−N of 2.349Å and 2.079Å, respectively, are observed in [CuCl(pyz) 4 ] + , with these parameters remaining practically unchanged upon coordination of pyrazine to Zn(II) (d(Cu−Cl) = 2.341Å; d(Cu−N) = 2.077-2.081Å). As in the case of 6-31+G(d), the computational nonexpensive 6-31G(d) basis set accounts for an underestimation of Cu−Cl distance with respect to the experimental results (deviation of −0.198Å), while an overestimation but reasonable match is obtained in Cu−N distance (deviation of +0.027Å).
Population analysis of selected atoms in {CuCl(μ-pyz) 4 [ZnCl 2 (H 2 O) 3 ] 4 } + was also conducted. In this regard, NPA results of pyrazine seems to show dependence on metal ion. While nitrogen atoms bonded to Cu(II) display a charge of −0.541, the ones coordinated to Zn(II) exhibit a charge of −0.464. The coordination process promotes Cu(II) to carry a charge of +1.162 and chloride ends as Cl −0.689 , with both results being practically identical to the ones observed in mononuclear system.

Concluding remarks
A new bidimensional array of Cu(II), pyrazine, chloride, and perchlorate as a counter-ion was obtained in the solid state. X-ray diffraction results exhibited the metal-center seated on an ideal square-pyramidal coordination environment given by four N-donor atoms and chloride, which appears apically coordinated. Pyrazine was observed tilted to each other along the structure, and this relative orientation promoted this ligand to participate in noncovalent anion-π weak interactions with perchlorate. This anion was located in cavities obtained upon formation of the 2D polymer array, in whose interior is expected to prevail the effects of the π-acidity displayed by coordinated pyrazine.
Theoretical studies at the DFT theory were conducted using M06-2X in combination with 6-31G(d), and diffuse functions were added (through 6-31+G(d)) to explore the influence of counter-ions in geometry and population analysis. The effect on geometric parameters of pyrazine as bridging-ligand was explored-taken M06-2X/6-31G(d) into account-by comparing [CuCl(pyz) 4 ] + with a system, in which pyrazine connects Cu(II) to diamagnetic and neutral units {ZnCl 2 (H 2 O) 3 }.
The presence of ClO 4 induced a twist of the aromatic ring of pyrazine in order to promote the anion-π interactions experimentally observed. Even when the inclusion of counter-ion and diffuse function seemed to be necessary to get a good description of equilibrium geometry, M06-2X/6-31G(d), applied to the simple [CuCl(pyz) 4 ] + complex, proved to be an acceptable performer in dealing with geometric aspects keeping the computational cost very low. This conclusion also emerged for those systems in which pyrazine acts as bridging-ligand.
Finally, we hope that our work contributes with new elements to the design of coordination polymers of interest, and to the study of the packing forces responsible of the solid state structure. Coordinates (x, y, z) of optimized systems in the gas phase employing M06-2X in combination with different basis sets at T =298 K.

Conflict of interest
The authors declare that they have no conflict of interest.