SYNTHESIS, CRYSTAL STRUCTURE, HIRSHFELD SURFACE ANALYSIS AND QUANTUM CHEMICAL CALCULATIONS OF [Cu2(C6H9N3S)2(ClO4)2] Π- COMPLEX WITH 2-ALLYLAMINO-5- METHYL-1,3,4-THIADIAZOLE

This work is focused on the synthesis and structure characterization of the novel Cu(I) π-complex [Cu2(Thiaz1)2(ClO4)2] (1) with 2-allylamino-5-methyl-1,3,4-thiadiazole (Thiaz1) ligand. The crystals of the compound were obtained by means of the alternatingcurrent electrochemical technique and studied using single crystal X-ray diffraction. The crystal structure of the complex 1 is constructed from the centrosymmetric dimers, in which two copper(I) ions are coordinated by two Thiaz1 molecules through thiadiazole N atoms and allylic C=C bond. Energy framework computational analysis for structure 1 has been performed.

infrared spectrum for Thiaz1 was recorded on a Bruker Vertex 70 FTIR spectrometer in KBr pellets. Diffraction data for 1 were collected on a Rigaku AFC7 diffractometer equipped with a Mercury CCD area detector, graphite monochromatized MoK α radiation. Energy framework calculations were performed on the DFT/B3LYP/6-31G(d, p) level using the CrystalExplorer 17.5 software [7,8].
Thiaz1 was prepared according to the reported previously procedure [9] starting from the 4-allylthiosemicarbazide. Thiaz1: NMR1Н (400 MHz, CDCl 3  Crystals of the coordination compound [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] (1) were obtained via alternating current electrochemical synthesis [10]. Equimolar quantities (1 mmol) of Cu(ClO 4 ) 2 ·6H 2 O and Thiaz1 were dissolved in 5 mL of 95% ethanol. The solution was placed into a glass tube where it was sealed with a minimal volume of residual air using a rubber stopper equipped with two copper wire electrodes. Alternating voltage of 0.5 V with frequency of 50 Hz was applied to the electrodes for three weeks. Single crystals appropriate for X-ray diffraction measurements were collected from the electrodes. The yield was about 30%.
The collected diffraction data were processed with the Rigaku CrystalClear software suite program package [11]. The structure was solved using SIR-92 program and refined by least squares method on F 2 by ShelXL program with the graphical user interface of WinGX [12][13][14]. Atomic displacements for non-hydrogen atoms were refined using an anisotropic model. The hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. Crystal parameters, data collection and the refinement parameters are summarized in Table 1. Table 1 Selected crystal data and structure-refinement parameters of 1

RESULTS AND DISCUSSION
Compound [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] (1) crystallizes in the centrosymmetric space group P`1, with one organic molecule and one Cu(I) ion in the asymmetric unit. Thiaz1 molecule acts as bridging chelate π, σ-ligand being attached to Cu(I) by means of the C=C bond of allyl group and two N atoms of 1,3,4-thiadiazole ring (Fig. 1). The copper(I) ion adopts trigonal-pyramidal (τ 4 =0.79, τ 4 -four-coordinate geometry index) surrounding, including in the basal plane of the polyhedron the N2 atom and the allylic C=C bond of one Thiaz1 molecule, the N1 atom of neighboring heterocyclic ring. The apical position of the pyramid is occupied by O1 atom of the anion. Thus, metal ions connect two Thiaz1 ligands into centrosymmetric [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] fragment with one six-member {Cu 2 N 4 } cycle and two six-member {CuN 2 C 3 } rings. Similar centrosymmetric dimers were found previously in the structure of CuNO 3 and Cu(CF 3 SO 3 ) π-complexes with Thiaz1 [9] as well as CuNO 3 and CuClO 4 π-complexes based on 2-allylthio-5-phenyl-1,3,4-oxadiazole ligand [15]. The strength of Cu(I)-(C=C) interaction in 1 is confirmed by the fact that allylic C=C bond is elongated to 1.355(5) Å (compared with a nominal value of 1.33 Å and a value of 1.340(7) Å in Thiaz1 structure itself [5]) due to back-donation from an occupied 3d metal orbital to a low-lying empty π* orbital of the olefin. Short Cu-m of 1.957(3) Å (m -a middle point of C5=C6 bond) distance and moderately large C-Cu-C of 38.2(2)° angle also confirm this conclusion. The Cu(I) ion deviates from the base of the trigonal pyramid only by 0.08(3) Å, while the angle between the C=C bond and the polyhedron basal plane is 12.2(1)°.

Continued table 1
Through N -H···O hydrogen bonds between the H atom of the ligand amino group and the O toms of the ClO 4 anion neighbouring [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] fragments are linked into hydrogen-bonded stair-like chains (Fig. 1).  For the deeper analysis of the intermolecular interactions among the fragments of 1, Hirshfeld surface analysis through the mapping of the normalized contact distance (d norm ) as well as calculation of the interaction energies were performed using the CrystalExplorer software. The most prominent interactions N -H···O among the amino-group H atoms and anion O atoms of neighboring moieties can be seen in the Hirshfeld surface plot as the strongly red areas (Fig. 2(a)). The other poorly red and white areas on the surface plot correspond mainly to the C -H···O interactions. Fingerprint plots were produced to show the intermolecular surface bond distances with the regions highlighted for H···O and H···S interactions. The contribution to the surface area for H···H contacts is 24.5%. The energy framework calculations discussed in this paper were performed on the DFT/B3LYP/6-31G(d, p) level. All the calculations were provided for clusters of [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] fragments within a radius of 3.8 Å, which were generated around a single fragment. This method provides a qualitative picture of the 3D-topology of the main interactions in the crystal packings. The values of interaction energy calculated between the dimeric fragments in 1 are tabulated in Table 3 and visualized in Fig. 3. The cylinders in the energy framework represent the relative strengths of molecular packing in the different directions -interaction energies are proportional to the thickness of cylinders joining the centroids of fragments. According to the calculations (Fig.4 & Table 3), the main intermolecular interactions, which corresponds to the N -H····O hydrogen bonding with two dimers I, cover the total energy of -151.6 kJ/mol, with significant influence of electrostatic interaction. The interaction energy with the next neighboring fragments II, which are located above and below the main fragment, is -111.3 kJ/mol. The last value indicates significant influence of the C -H····O hydrogen bonding between mentioned moieties. In contrast to I, II and IV, interactions of the main fragment with III are mostly determined by dispersion interactions of ligand Thiaz1 methyl group. The total energy of all interaction between the dimers in 1 appears to be -379.4 kJ/mol.

CONCLUSIONS
We presented the synthesis and structure characterization of the novel π-complex [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] (1) with 2-allylamino-5-methyl-1,3,4-thiadiazole (Thiaz1) ligand. Organic molecule reveals quite common coordination behaviour of allyl 1,3,4-thiadiazoles, being attached to the metal ion by means of allylic C=C bond and two heterocyclic N atoms. Energy frameworks computational analysis confirms the major role of N -H···O interactions (with predominance of electrostatic factor) in the crystal structure organization: through N -H···O hydrogen bonds neighboring [Cu 2 (Thiaz1) 2 (ClO 4 ) 2 ] fragments are linked into hydrogen-bonded stair-like chains. The energy of C -H···O interactions were also quite high which confirms their considerable impact in stabilizing the structure of 1.

SUPPLEMENTARY MATERIAL
CCDC number 2052890 (1) contains the supplementary crystallographic data for this paper. Copies of the data can be obtained free of charge on applications to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: int. code +(1223)336-033; e-mail for inquiry: fileserv@ccdc.cam.ac.uk).