SYNTHESIS AND PHOTOCATALYTIC PROPERTIES OF A NEW PADDLE-WHEEL Cu(II) COMPLEX: AN INTEGRATED EXPERIMENTAL AND THEORETICAL INVESTIGATION

A new paddle-wheel Cu(II) complex having formula [Cu(L)2(DMF)] (1) (HL = 2-fluorobenzoic acid) has been synthesized and characterized using FTIR and single crystal X-ray diffraction. The single crystal Xray diffraction study indicates that the carboxylic acid ligand adopt bridging bidentate modes to coordinate Cu(II) centers thereby forming a binuclear copper(II)carboxylate type structure. The complex has been utilized as photocatalyst to photodegrade model aromatic dyes viz. methyl violet (MV) and rhodamine B (Rh B) in UV light. The photocatalytic results indicated that1 offered moderate photocatalytic activity and offers good stability after performing photocatalysis. The plausible mechanism through which 1 exerted photcatalytic property had been proposed by using density of states (DOS) and partial DOS calculations.


INTRODUCTION
The photocatalytic materials which are based on coordination complexes and metal-organic frameworks (MOFs) are gradually becoming a promising area of investigation as this class of materials offers good photocatalytic efficiency [1][2]. In such materials, varied interactions operating between inorganic/clusters and organic linkers led to variety of charge transfer transitions which is responsible for inducing better photocatalytic activities [3][4]. In view of these peculiar features associated with coordination complexes and MOFs, the photocatalytic performances of such materials have been explored extensively. For example, a 3D anionic framework was constructed using O-donor ligand, which exhibited good photocatalytic effects for degrading methyl orange (MO) [5]. Hou et al. have explored the polynuclear MOF based materials and tested them as photocatalyst for degradation of organic dyes [6]. These results indicated that the coordination complexes have high photocatalytic activities towards dye degradation due to different kinds of nuclei which form the cluster units [6]. Hence, in view of these pertinent findings the photocatalytic activities of MOF shaving varied type of frameworks should be explored further [7][8].
As an important class of ligand, carboxylates can adopt diverse coordination modes, viz. monodentate, monodentate bridging, bidentate chelating and bidentate bridging and hence can lead to the formation of different two-and three-dimensional structures in the resulting complexes and MOFs. With these aspects in mind, we had made an attempt to explore the coordination properties of 2-fluorobenzoic acid [9] with copper(II) cation. In the present investigation, we report the synthesis and structure of a new Cu(II) based carboxylate complex having formula[Cu(L) 2 (DMF)] (1) (HL = 2-fluorobenzoic acid). Further, the complex 1 had been used as photocatalytic materials for the degradation of aromatic dyes. The results of these investigations are presented herewith.

General considerations
All reagents were commercially available and used without further purification. The powder Xray diffraction (PXRD) data for the complex was collected on Bruker Advance X-ray diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å) at 50 kV, 20 mA with a scanning rate of 6°/min and a step size of 0.02°. FTIR spectrum for the complex as KBr pellet was recorded using Nicolet Impact 750 FTIR in the range of 4000-400 cm -1 . The photocatalytic activity studies were carried out in a Shimadzu UV-Vis 2501PC spectrophotometer.

X-ray crystallography
The single crystal X-ray diffraction data was collected using Bruker SMART APEX diffractometer having graphite monochromated Mo radiation (λ = 0.71073 Å) by using scan technique. The intensities were corrected for absorption effects by using SADABS. The crystal structure of the complex was solved by direct method (SHLEXS-2014) and was refined using full-matrix least-squares procedures based on F 2 (SHELXL-2014) [10]. All the hydrogen atoms were generated geometrically and refined isotropically using a riding model. All nonhydrogen atoms were refined by applying anisotropic displacement parameters. CCDC number: 1898139.

Photocatalytic method
The photocatalytic reactions were performed as follows: The binuclear Cu(II) complex 1 (80 mg) was dispersed in 50 mL aqueous solution containing methyl violet (MV)or rhodamine B (Rh B) (10 mg/L). The mixture was stirred in dark for 30 min to ensure the establishment of adsorption-desorption equilibrium. The photocatalytic degradation of MV/Rh B was conducted on an XPA-7 type photochemical reactor equipped with a 100W mercury lamp (mean wavelength 365 nm) with light intensities at quartz tube positions of 12.7 mW/cm 2 . During experiment, aliquots of 5.0 mL was taken out at specified time intervals and the clear solution was separated through centrifugation and then subsequently analyzed by using a UV-visible spectrophotometer. Additionally, control experiments were also conducted under similar reaction conditions in the absence of complex 1.

Computational details
The probable mechanism dealing with the photocatalytic properties of 1 has been addressed using theoretical calculations. For this the gas phase geometry of the binuclear Cu(II) complex was fully optimized using the B3LYP functional [11][12] using 6-31G** basis set. All the calculations were performed using Gaussian 09 program [13]. After achieving global minimum the density of states (DOS) plot and partial DOS plots for the complex 1 was obtained using GaussSum 3.1 [14].

Synthesis and spectroscopy
If the N,N'-dioxide-3,3'-benzocinnoline was absentin the reaction medium the desired complex was not obtained. Thus, the N,N'-dioxide-3,3'-benzocinnoline behave as template during the reaction between HL and Cu(II) salt. The IR spectra of 1 exhibits the main characteristic absorption bands in the range of 1462-1633 cm -1 which is mainly attributed to the asymmetric and symmetric stretching vibrations of the carboxylate groups. Also, the Δν [ν as (COO)ν s (COO)] value of 150 cm -1 indicates the coordination of carboxylate groups to the central Cu(II) center.

Crystal structure description
The complex crystallizes  [15][16]. The Cu···Cu distance in dinuclear core is 2.630(5) Å, and the Cu-Cu distance between adjacent metal-complexes within each hydrogen-bonded chain is 9.6 Å. The shortest separation through space inter-chain distance between Cu(II) ions is 6.2 Å. The Cu···Cu separation in Cu 2 X 4 L 2 type systems is usually longer when the axial ligand is nitrogen instead of an oxygen atom.  (Figure 1b).

Photocatalysis
The complex 1 was used as photocatalyst to photo-decompose model organic dyes pollutants viz. methyl violet (MV) and rhodamine B (Rh B). This had previously been established that in due course of degradation, MV and Rh B can be broken down into non-polluting compounds under light irradiation [18]. To monitor the photocatalytic degradation of MV and Rh B, the characteristic UV-Vis absorption band maxima of MV and Rh B observed at ~550 nm were selected. As presented in Figure 2  The powder X-ray diffractogram (PXRD) for 1 was recorded after performing photocatalytic experiments. The obtained diffractogram indicated resemblance of diffraction pattern with that of simulated and as-synthesized 1. This indicates that 1 maintains its crystalline nature after photocatalysis which further proves stability and reusability ( Figure 3a). As confirmed by PXRD and SEM, the structure integrity and surface morphology of 1 are well preserved which indicates its potential as a robust photocatalyst for photodegrading the organic dyes in wastewater (Figure 3b-3c).
To establish the plausible photocatalytic mechanism through which the photo-degradation of the aromatic dyes have been executed by 1, band structure calculations of the binuclear Cu(II) complex which is based on density functional theory (DFT) method has been performed. The band structure which is precisely represented in terms of density of states (DOS) and partial  Figure 4. The DOS and partial DOS plots for the complex indicates that the valence band present just below the Fermi level is having contributions from aromatic carbonsand the carboxylate oxygens of the L ligand with small contribution from the Cu(II) center. Also, the conduction band in the case of 1, lying just above the Fermi level in the range of -1.2 to -0.5 eV have been derived from aromatic carbons with small contributions from oxygen and metal centers. Hence, in 1, the electronic transitions are mainly operating from aromatic center to another aromatic region (ligand-to-ligand) some mixing from Cu(II) and carboxylate oxygen centers.   During photoexcitation of 1, charge transfer transitions from HOMO→LUMO takes place. After charge transfer, the electron deficient HOMO requires an electron to revert back to its stable state. Hence, an electron is captured from water molecule, which gets oxygenated into •OH active species. These •OH radicals decomposes the dye effectively to complete the photocatalytic process.The mechanistic scheme suggested to operate during the photocatalytic decomposition of dyes with the aid of binuclear complex 1 can be summarised as follows [ O 2 ·¯/ HO 2 · + S ℎ → oxidation products (5) The complex 1 gets photo-excited to produce electron-hole pair (equation 1). This photo-excited molecule undergo reaction with H 2 O (i.e. oxidative hole trapping) to produce hydroxyl (OH · ) radical (equation 2). The hydroxyl radical reacts subsequently with aromatic dyes in variety of ways to produce oxidized products [23]. The dioxygen molecule (O 2 ) behave as very effective oxidant for the reduced 1(e¯) and its main action is to regenerate the reduced photocatalyst 1.

CONCLUSION
In summary, s a new copper(II) complex with paddle-wheel unit has been presented. The complex behaved like a photocatalyst to photo-degrade organic dyes of MV and Rh B. However, the photocatalytic results were modest, but using precise carboxylate ligands having pre-tuned electronic functions can develop suitable Cu(II) based complexes, which can offer better catalytic properties.