Synthesis, Experimental and Theoretical Characterization of ( μ 4 -oxo)hexakis( μ 2 -chloro)- tetrakis[1-(allyl)-1 H -imidazole]tetracopper(II)

: A new [Cu 4 OCl 6 L 4 ] cluster [L = 1 -(allyl)-1 H -imidazole] has been isolated and its crystal structure and spectroscopic properties determined. The compound crystallizes as a yellow solid in the monoclinic system, space group P 2 1 , with cell constants a = 10.6937(7) Å, b = 13.1004(7) Å, c = 13.2452(9) Å, β = 93.537(6)°, and two formula units per cell. The title tetranuclear complex has a central µ 4 - oxide ion surrounded tetrahedrally by four Cu II atoms. Each Cu atom is connected to three others via bridging Cl atoms. The fifth coordination position, located on the central Cu─O axis on the outside of the cluster, is occupied by an N atom of the monodentate imidazole ligand. The resulting coordi nation geometry of the metal ion is a slightly distorted trigonal bipyramid with the O and N atoms in the axial positions. Th e molecular structure and spectroscopic data were obtained using density functional theory (DFT/HSEH1PBE) method with the cc - pVDZ basis set for C, H, N, O and Cl atoms, and the LANL2DZ basis set for the Cu atoms, and compared with the experimental data. Consequently, the exp erimental data well coherences with the theoretical ones.


INTRODUCTION
OLYNUCLEAR copper complexes have received increasing attention in recent years, owing to their applications in metallobiochemistry, [1−3] material science, [4−6] theoretical chemistry, [7,8] and magnetism. [9,10]−15] The clusters have the same structural framework, with four copper atoms at the corners of a tetrahedron around the central μ4-bridging oxygen and six μ-bridging halide atoms over each edge of the tetrahedron.
These clusters have been studied largely because of their unique magnetic properties. [9,16]Most of these compounds show a magnetic susceptibility which has a maximum in the plot of the magnetic moment versus temperature. [17]In some, a strong anti-ferromagnetic coupling has been associated with the presence of more than one oxo-Cu bridge. [14,18]Oxo-Cu(II) species are important in many copper-catalyzed reactions that take place in biological systems which involve a reduction of oxygen molecules by Cu(I). [19,20]Model studies in this field have encouraged cluster syntheses.Potential applications in pharmacology and industrial catalytic processes have also been resulted. [21,22]lthough tetranuclear copper(II) complexes with imidazole are known in the literature, [9,16,23−32] to the best of our knowledge, no theoretical studies on the structure and spectroscopic properties of these complexes have been reported so far.Therefore, we have synthesized a new tetranuclear μ4-oxo-bridged copper(II) complex with the monodentate 1-(allyl)-1H-imidazole ligand, and characterized it by FT-IR and UV-vis.spectroscopy.The molecular structure of the complex has been determined by single crystal X-ray diffraction.In addition, the theoretical calculations have been performed by using DFT method.

General Remarks
All chemicals were obtained from commercial suppliers and used without further purification.Infrared spectra were recorded on an ATR Spectrum-II, PerkinElmer spectrometer.The UV-spectra were measured on a Shimadzu UV-1900i UV-VIS spectrometer in 10 −5 M CHCl3.Quartz cuvettes of 1 cm path length were used for the measurements.Melting points were measured with a Stuart SMP30 melting point apparatus.Elemental analyses were performed by ODTU Microlab (Ankara, Turkey).

X-Ray Crystallography
Data collection was performed on a STOE IPDS II diffractometer at room temperature (296 K) using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) by applying the ω-scan method.The structure was solved by direct methods using SHELXS-2013 [33] and refined with fullmatrix least-squares calculations on F 2 using SHELXL-2014 [33] implemented in WinGX [34] program suit.Hydrogen atoms were placed in idealized positions with isotropic thermal parameters fixed at 1.2 times the value of the attached atom.In the compound, the carbon atoms of the allylimidazole moieties (as a whole or only allyl part) were disordered over two positions, and the refined site-occupancy factors of the disordered parts are 0.72(5)/0.28(5)for C5-C6, 0.78(2)/0.22(2)for C7-C12, 0.74(6)/0.26(6) for C17-C18 and 0.61(4)/0.39(4)for C19-C24.The disordered atoms were refined using the SIMU, DELU, SADI and FLAT restraints of SHELXL-2014, which resulted in 608 restraints.Data collection: X-AREA, [35] cell refinement: X-AREA, data reduction: X-RED32. [35]Crystal data, data collection and structure refinement details are summarized in Table 1.The general-purpose crystallographic tool PLATON [36] was used for the structure analysis and presentation of the results.Molecular graphic was generated by using ORTEP-3. [34]CDC 1063735 contains the supplementary crystallographic data for the compound reported in this article.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.

Computational Procedure
The Cartesian coordinates of the X-ray structure were used as the starting geometry for the calculations.Quantumchemical computations were carried out with the GaussView 5 [37] molecular visualization program and the Gaussian 09 program package. [38]The optimized geometry and other electronic properties were obtained using the HSEH1PBE density functional method [39−42] with the cc-pVDZ basis set [43] for C, H, N, O and Cl atoms, and the LANL2DZ basis set [44−46] for the metal atoms.The calculated vibrational wavenumbers without imaginary frequencies were scaled by 0.962.The electronic absorption spectra were obtained using the time-dependent density functional theory (TD-DFT) [47] at the same level, in which solvent effect was inserted by using the default method. [48]

Experimental and theoretical structures
A perspective view showing the complex and the labeling scheme appears in Figure 2(a), while the DFT-optimized structure is drawn in Figure 2(b).Also, some of the experimental and theoretical findings are given in Table 2.The basic moiety of the molecular structure contains a core comprising a cluster and four 1-allyl-1H-imidazole ligands.
The cluster core consists of four Cu, one µ4-O and six µ2-Cl atoms, in which the cupric ions bonded to a single central oxo ion tetrahedrally.The Cu4(µ4-O) tetrahedron is almost regular with the Cu─O─Cu angles changing from 108.3(6) to 110.9(6)°.In the optimized structure, the Cu─O─Cu angles are found between 95.52 and 115.61°.In addition, the intermetallic distances span the ranges 3.095(3)-3.127(4)Å in the X-ray structure and 2.921-3.232Å in the theoretical structure, while the Cu in the axial positions.These bonds have been calculated at 2.357-2.574Å, 1.900-1.974Å and 1.968-2.005Å, respectively.The distortion of a five coordination can be best described by the structural parameter τ5. [49]The τ5 value can be conveniently utilized to estimate the degree of distortion from square-pyramidal to trigonal-bipyramidal structures.In the case of an ideal square-pyramidal geometry, the τ5 value is equal to zero, while it becomes unity for a perfect trigonal-bipyramidal geometry  5)°.The corresponding values of these parameters in the theoretical structure are found to be 3.946-6.066Å, 2.859-3.214Å, 82.73-96.67°and 171.17-174.35°,respectively.−32]   As can be seen from the Table 2, coherence between the optimized and X-ray structures are not good in general, especially in some parameters.Bond distances agree within ca.0.18 Å, while the largest deviation in the bond angles reaches up to 12.78°.When the X-ray structure is compared with its optimized counterpart (Figure 3), as expected, some conformational discrepancies are also observed between them, and the root mean square deviation (RMSD) obtained by superimposing two structures (except for H atoms) is 0.471 Å. Anyway, the differences are related to the fact that the theoretical calculations are based on the isolated molecule in the gas phase, while the experimental results are based on the molecule in the solid state.
In the crystal structure, there are no classical hydrogen bonds.The crystal packing is stabilized by Van der Waals interactions.

Spectroscopy
The FT-IR spectral data of the allyl-imidazole ligand and its corresponding tetranuclear complex exposed different bands in the 400-4000 cm −1 region (Figure 4).The characteristic bands of allyl-imidazole ligand are ν(C═C) at 1645 and 1421 cm −1 and ν(C═N) at 1505 cm −1 , which are calculated at 1674, 1404 and 1509 cm −1 , respectively.The band indexed by the allyl-imidazole ligand spectrum relating to the C═N bond is shifted on the IR spectrum of the studied complex [ν(C═N) = 1523 cm −1 ], which confirms the participation of the N atom in the coordination sphere of the complex studied.This band are appeared at 1521 cm −1 in the theoretical spectrum of the complex.The aromatic C─H stretching, in-plane bending and out-of-plane bending vibrations usually appear in the region 3150-2900, 1500-1100, and 1000-750 cm −1 , respectively.These vibrations are monitored at 3130, 1438, and 831 cm −1 in the FT-IR spectrum and calculated at 3168, 1252 and 703 cm −1 , respectively.In addition, the complex exhibits strong IR absorptions at 564 cm −1 , which can be associated with ν(Cu─O) frequencies originating from vibrations in the Cu4O core. [10]The band observed at 403 cm −1 in the theoretical spectrum is attributed to ν(Cu─O) vibration.
The electronic spectra of [1-(allyl)-1H-imidazole] and [Cu4(μ4-O)(μ2-Cl)6L4] complex were recorded in CHCl3 solution at room temperature.The ligand shows shoulders at 280 nm with absorption due to ligand-centered n -π* / π -π* (LL) transitions.The UV-vis spectrum of the complex shows three absorption peaks at 272, 754 and 874 nm (Figure 5).The absorption band at 272 nm is attributed to a ligand-to-metal charge transfer (LMCT) transition.This demonstrates metal coordination with the ligand.−53] The TD-DFT calculations predict an absorption peak at 287 nm for the ligand and two absorption peaks at 264 and 803 nm for the complex.

CONCLUSIONS
In this study, a new μ4-oxo-bridged cluster of formula [Cu4(μ4-O)(μ2-Cl)6L4] (in which L = 1-(allyl)-1H-imidazole) has been synthesized and characterized by elemental analysis, FT-IR and UV-vis spectroscopies.The structure of the complex has been determined by single-crystal X-ray diffraction (XRD) method.The result of the elemental analysis was consistent with the expected structure.The complex has a tetrameric structure of four copper(II) ions connected to a central oxo group in an approximately tetrahedral arrangement.Copper coordination is completed by three μ2-chloride ions and an imidazole nitrogen atom, defining a trigonal-bipyramidal environment around each copper ion.The Cu─O frequency of the complex, caused by vibration in the Cu4O core, exhibited strong IR absorption at 564 cm −1 .The UV-Vis spectrum of  the complex showed three absorption bands at 272, 754 and 874 nm, respectively, which are attributed to a ligandto-metal charge transfer (LMCT) transition and the d-d transition between copper(II) ions.Computational studies of the complex were also done at the HSEH1PBE/cc-pVDZ/LANL2DZ level.The theoretical structure is in agreement with the experimental one by a root mean square deviation of 0.471 Å.The theoretical spectroscopic values support the experimental findings as well.

Figure 1 .
Figure 1.Synthesis pathway of the title complex.

Figure 2 .
Figure 2. Molecular structure diagram of the title cluster, showing atom-numbering scheme and 20% displacement ellipsoids.Hydrogen atoms are omitted for clarity and only the major parts of the disordered fragments are shown (a).Calculated structure of the title complex (b).

Figure 3 .
Figure 3. Atom-by-atom superimposition of the calculated structure (grey) over the X-ray structure (black) for the title complex (c).