Synthesis, Structure and Electrochemistry of Tetranuclear Oxygen-Centered Copper(II) Clusters with Acetylacetone and Benz-pyrazole Hydrolyzed Derivatives as Ligand

Two copper(II) clusters Cu4OCl6(pyrazole)4 (1) and Cu4OBr6(Br-pyrazole)4 (2) have been synthesized by reacting acetylacetone and benzohydrazide (1:1 ratio) with CuX2 (X = Cl for 1 and X = Br for 2) in methanol solution. The structures of both clusters have been established by X-ray crystallography. The clusters contain four Cu, one O, six μ2-X atoms, and four pyrazole ligands. The pyrazole was prepared in situ by the reaction of acetylacetone with benzohydrazide in methanol under reflux. In 2, the methine hydrogen of the pyrazole ligand has been replaced by bromine atom. The four copper atoms encapsulate the central O atom in a tetrahedral arrangement. All copper atoms are five-coordinate and have similar coordination environments with slightly distorted trigonal bipyramidal geometry. The cyclic voltammogram of the clusters 1 and 2 show a one-electron quasi-reversible reduction wave in the region 0.485 to 0.731 V, and a one-electron quasi-reversible oxidation wave in the region 0.767 to 0.898 V. In 1, one irreversible oxidative response is observed on the positive of side of the voltammogram at 1.512 V and this can be assigned to Cu(II) to Cu(III) oxidation.


Introduction
The synthesis of multi-metal clusters by spontaneous self-assembly of organic-inorganic ligands and transitionmetal ions have attracted special attention due to their potential application in magnetic, electrochemical, catalytic studies and coordination chemistry. [1][2][3][4][5][6][7][8][9] A feature of modern coordination chemistry is its development of metal bio-sites modeling for biology systems. 10 Self-assembly is one of the few practical strategies for making such compounds. The spontaneous self-assembly of well-defined and complex molecular entities from constituent subunits occurs in solution. 11 In biology systems, self-assembly occurs by weak inter-or intra-molecular interactions of non-covalent bonds, while self-assembly in coordination chemistry occurs through the formation of covalent coordinate bonds. [11][12][13][14][15] It is well known that the constituent ligands and metals play important roles in the structures and properties of the multi-metal clusters. The synthesis and characterization of high nuclearity metal clusters with oxo bridges has been the goal of much research. [15][16][17][18][19][20] Oxo-copper(II) halide clusters have been extensively studied. [20][21][22][23] Bertrand reported the first structure in 1966. 24 The usual arrangement in tetranuclear Cu 4 OX 6 L 4 complexes consists of a central oxygen atom bound to four tetrahedrally arranged copper(II) atoms. At the same time, each copper(II) atom adopts a trigonal bipyramidal coordination with one terminal axial position occupied by ligand L. Many novel complexes or coordination polymers based on ligands generated in situ have been synthesized through one-pot reaction under hydrothermal or solution reaction conditions. [25][26][27] Here we report the synthesis, characterization and electrochemistry of tetranuclear copper(II) clusters 1 and 2, obtained by reaction of CuX 2 (X = Cl and Br) and pyrazole ligands generated in situ under hydrothermal conditions.

1. Reagents and Physical Measurements
All chemicals were used as supplied by Merck and Fluka without further purification. Infrared spectra were taken with an Equinox 55 Bruker FT-IR spectrometer using KBr pellets in the 400-4000 cm -1 range. Absorption spectra were determined in the solvent methanol using a GBC UV-Visible Cintra 101 spectrophotometer with 1 cm quartz, in the range of 200-800 nm at 25 °C. Elemental analyses (C, H, N) were performed using a CHNS-O 2400II PERKIN-ELMER elemental analyzer. Cyclic voltammetry was carried out using an Autolab potentiostate/galvanostate (PGSTAT-302N) instrument. The measurements were performed at 300 K in acetonitrile solutions containing 0.1 M tetrabutylammonium perchlorate (TBAP) and 0.1 mM copper(II) complexes deoxygenated by bubbling with nitrogen. The working, counter, and reference electrodes used were glassy carbon electrode, platinum wire and Ag/AgCl (3.0 KCl), respectively.

X-ray Crystallography
Diffraction images were measured at 200 K on a Nonius Kappa CCD diffractometer using Mo Kα, graphite monochromator (λ = 0.71073 Å). Data were extracted using the DENZO/SCALEPACK package. 28 Structures were solved by direct methods with the use of SIR92 and refined on F 2 by full matrix last-squares techniques using the CRYSTALS program package. 29,30 Atomic coordinates, bond lengths and angles, and displacement parameters were deposited at the Cambridge Crystallographic Data Centre. Crystallographic details are summarized in Table 1.

3. Syntheses of Tetranuclear Copper(II) Clusters
The cluster 1 was prepared as previously reported. 26 The cluster 2 was synthesized similar to cluster 1, acetylacetone (1.05 mL, 10 mmol) was added to a methanol solution (25 mL) of benzohydrazide (1.36 g, 10 mmol), and the mixture was heated to reflux for 5 h. A solution of Cu-Br 2 (2.23 g, 10 mmol) in methanol was added to the above-mentioned bright yellow solution. The green solution was stirred at room temperature for 2 h. Cluster 2: A green-brown precipitate was obtained upon the slow evaporation of the solvents at room temperature over two days. Dark-green block crystals were obtained by slow evaporation of the mother liquor and were washed with methanol and dried in air. Yield 25%. Anal. Calcd. for C 20

1. Synthesis
The synthetic route of the clusters is shown in Scheme 1. Clusters 1 and 2 were synthesized by a two-stepsone-pot reaction with the initial formation of the pyrazole (without its isolation) and then the addition of a methanolic solution of CuX 2 (X = Cl and Br). The pyrazole was prepared in situ from the reactions between acetylacetone and benzohydrazide in methanol under reflux. Initially, (3,5-dimethyl-1H-pyrazol-1-yl)(phenyl)methanone (benz-pyrazole) was obtained by the reaction of equimolar amount of acetylacetone and benzohydrazide. 31 The resulting solution was refluxed for 5 h and then was used for the synthesis of the complex without further purification.
Cluster 1, Cu 4 Cl 6 O(pyrazole) 4 , was obtained by the reaction between CuCl 2 and benz-pyrazole in association with hydrolysis at room temperature (Scheme 1). A search of the literature revealed that the cluster had been reported previously by Ja}imovi} et al. in 2007. 32 They synthesized the green crystals of the copper(II) cluster from the one-

2. Crystal Structures
The structure of cluster 1 is similar to the structure that has been previously reported. 32 In the Ja}imovi} report, the space group was triclinic P1 -. This crystal structure contained a region of electron density external to the atoms of the complex molecule which suggested the presence of ethanol solvate molecules. Attempts to find a satisfactory structural model for these molecules however failed, and SQUEEZE was used to correct the data set for the electron density in this region of the cell. Our determination is in the monoclinic space group P2/n. It also contains disordered solvate species, though not ethanol. This same cell was reported to the Cambridge Structure Database as a private communication by Stibrany and Potenza in 2007. 33 Crystallographic data for our determination of the structure of cluster 1 were given previously. 26 Selected interatomic distances and angles for 1 are given in Table 2 to allow direct comparisons values for cluster 2. Both structures were run at the same temperature (200 K).

Cyclic Voltammetry
The electrochemical behavior of the Cu(II) complexes has been studied by cyclic voltammetry (CV) in CH 3 CN solution. Tetrabutylammonium perchlorate (TBAP, 0.1 M) was used as the supporting electrolyte. The electrochemical data are summarized in Table 3 and the cyclic voltammograms of the copper(II) complexes are depicted in Fig. 4. As shown in figure 4a, cluster 1 shows a pair of redox peaks with a cathodic peak potential (E pc ) at 0.485 V and an anodic peak potential (E pa ) at 0.767 V. The half-wave potential (E 1/2 = 1/2(E pa + E pc )) and the peak-to-peak potential separation (ΔE p ) were obtained as 0.626 V and 0.282 V, respectively, indicating that the electrochemical behavior of 1 is quasi-reversible corresponding to the redox process of all four copper (Cu(II)/Cu(I)). [39][40][41] The crystal structures of complex 1 shows the presence of four similar Cu(II) centers in the tetranuclear cluster, so we can assume that in solution the four copper ions exhibit similar coordination environments as in the solid state. One irreversible oxidative response is observed on the positive of side of voltammogram at 1.512 V and this can be assigned to Cu(II) to Cu(III) oxidation.
plexes have similar structures, namely a tetranuclear cluster containing four Cu, one μ 4 -O, six μ 2 -X atoms (X = Cl, 1 and Br, 2), and four pyrazole ligands. The electrochemical behavior of clusters 1 and 2 are quasi-reversible corresponding to the redox process of all four copper (Cu(II)/Cu(I)).

Supplementary Material
The deposition number of the studied cluster 2 is CCDC 1404320. These data can be obtained free-of-charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data-request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033.  The cyclic voltammogram for copper(II) complex 2 was recorded at anodic potential in the range -0.2 to + 1.6 V. The cyclic voltammogram for 2 is shown in Fig. 4b. On changing the halogen from Cl to Br more redox peaks are observed. This usually happens when Cl is changed to Br or I in similar core structures. [42][43][44] The cyclic voltammetry of 2 shows two anodic peaks potential at 0.716 and 0.898 V and two cathodic peaks at 0.731 and 0.530 V. We could not get any wave for the oxidation of copper(II) to copper(III) in the potential range up to 1.6 V.

Conclusion
The present work describes the synthesis by a twosteps-one-pot reaction, characterization, and electrochemical behavior of copper(II) clusters 1 and 2. Single crystal X-ray diffraction studies revealed that the two com-