Synthesis , crystal structures and spectroscopic investigation of new Cu / Schiff-base complexes

3068 | P a g e N o v e m b e r 1 3 , 2 0 1 4 Synthesis, crystal structures and spectroscopic investigation of new Cu/Schiff-base complexes Ahmed Toumi, Mohamed Rzaigui, Hichem Ben Jannet* Laboratory of Heterocyclic Chemistry, Natural Products and Reactivity; Team: Medicinal Chemistry and Natural Products, Faculty of Science of Monastir, University of Monastir, Monastir, Tunisia mido_toumi@yahoo.fr Laboratory of Material Chemistry, Faculty of Sciences of Bizerte, 7021 Zarzouna-Bizerte, Tunisia mohamed.rzaigui@fsb.rnu.tn Laboratory of Heterocyclic Chemistry, Natural Products and Reactivity; Team: Medicinal Chemistry and Natural Products, Faculty of Science of Monastir, University of Monastir, Monastir, Tunisia hich.benjannet@yahoo.fr ABSTRACT Three novel Copper complexes, [Cu(L1)2][CuCl2] (1), [Cu(L2)Cl] (2) and [Cu2(L3)3Cl2] (3), have been prepared by reaction of CuCl with the Schiff-base ligands L1: N,N’-bis(thiophen-2-ylmethylene)-ethane-1,2-diamine, L2: N,N’-bis(1Hpyrrol-2-ylmethylene)ethane-1,2-diamine and L3: N,N’-bis(2-nitrobenzylidene)-ethane-1,2-diamine in acetonitrile. The solid-state structures of these complexes were determined by X-ray diffraction from single crystal data and characterized by 1H and 13C NMR, IR and UV/Vis spectroscopies. This study shows that (1) is an ionic complex with a Cu(I)-centered cation and an isolated linear dichlorocuprate(I) anion, (3) is a dinuclear neutral complex of Cu(I) while (2) is a mononuclear neutral complex of Cu(II). In the three complexes, Cu is tetracordinated in different geometrical environments. The atomic arrangements and spectroscopic properties of the three complexes are reported. Complexes 13 exhibit, in the solid state at room temperature, photoluminescence between 320 and 550 nm.


Introduction
Luminescent metal complexes have attracted an increasing attention in many areas of chemistry, biology, medical and material science [1][2][3][4][5][6][7][8][9]. Their uses as emitter material in luminescence devices and luminescent chemo sensor are particularly prominent [10][11][12]. One of simple strategy for designing luminescent coordination compounds is to organize d10 electronic configuration metal ions with chromophore ligand. The origins of emission arise from the transitions of π*→π within ligand. This emission could be efficiently enhanced in coordination complex due to increasing of the rigidity of the ligand and reducing of energy loss by radiation less thermal vibrations [13,14]. The design and synthesis of desired structure luminescent complex is a challenge because the assembly of metal ion with ligand is sensitive to the delicate synthetic conditions [15]. Schiff-bases become one of the research hotspots owing to their strong coordination capability, biological activities, photochromic characteristics, etc. [16]. Copper complexes with various bis(Schiff-base) halide donor ligands are of growing interest owing to their wide variation in structural format and rich photo-physical and chemical properties. The steric, electronic, and conformational effect imparted by the coordinated ligands play an important role in modifying the properties of the prepared metal complexes. Although structural reports on [Cu (N4)]+ complex cations are numerous [17][18][19], there are a limited number of studies on the copper(I) complexes with isolated linear dihalogenocuprate(I) anions [20][21][22]. In this work, we select three flexible aromatic bis(Schiff-base) ligands (L1-L3) as Ndonor ligands to explore the structures, electronic and spectroscopic properties of their complexes with copper.

Starting materials
All reagents were used as received from Sigma-Aldrish without further purification. Solvents used for the reactions were purified by literature methods [23].

Synthesis
All syntheses and preparations for spectroscopic were carried out under purified Ar atmosphere using Schlenk techniques.

Synthesis of Ligands
we synthezised and characterized the three ligands L1-L3 as described in our previous work [24].

Reaction of diimine ligands with copper chloride
The three complexes 1-3 were synthesized in accordance with a previously published procedure [25]. To a 100 mL Schlenk flask containing CH3CN (5 mL) and a stir bar were added (99 mg, 1.0 mmol, 1.0 equiv) of copper chloride and (1.0 mmol, 1.0 equiv) of the corresponding ligand (L1 or L2) and (198 mg, 2.0 mmol, 2.0 equiv) of copper chloride and (453 mg, 3.0 mmol, 3.0 equiv) of L3, each dissolved in 5 mL CH2Cl2 at room temperature. All starting materials dissolved to give a yellow solution. The flask was sealed, and the atmosphere was purified of dioxygen by evacuation argon. The reaction was allowed to stir for 20 minutes at room temperature. The solution was then cannula-filtered into a Schlenk flask containing diethyl ether (50 mL), and the yellow solid that precipitated was allowed to settle for 30 minutes. The supernatant was cannula filtered off, and the product was dried under dynamic vacuum, giving complex (1) (95%), complex (2) (96%) and complex (3)

Physical measurements
NMR spectra were obtained on a BRUKER AM, 300 MHz spectrometer in DMSO-d6. 1 H and 13 C chemical shifts δ are reported in part per million (ppm) relative to the non deuterated residual DMSO. Coupling constants are given in Hertz. IR spectra were determined as KBr pellets on a Perkin Elmer FT-IR Spectrometer BX I. The UV/Vis spectra were recorded on a 6705 UV/Vis spectrophotometer JENWAY with quartz cells (1 cm path length). Excitation and emission spectra were measured with a Perkin-Elmer LS55 Fluorimeter using solid samples at room temperature.

Crystallographic data collection and structure determination
Suitable single crystals of [Cu(L1)2][CuCl2] (1), [Cu(L2)Cl] (2) and [Cu2(L3)3Cl2] (3) were used for x-ray diffraction analysis. Diffraction data were collected at room temperature on an Enraf-Nonius Mach3 diffractometer using graphitemonochromated AgK ( = 0.56087 Å) radiation. The structures were solved by direct methods and refined by the fullmatrix least-squares based on F2 using SHELXTL-97 program [26]. For the three compounds, the non-hydrogen atoms N o v e m b e r 1 3 , 2 0 1 4 were refined anisotropically and the hydrogen atoms of organic ligands were generated geometrically. Crystal data and structural refinement parameters for 1-3 are summarized in (Table 1).

Synthesis
The copper(I) complexes were prepared by reacting the copper(I) CuCl with bis(Schiff-bases), in a mixed acetonitriledichloromethane (1:1) solution. The synthesis of these complexes was carried out in an inert atmosphere. The stability of these complexes in solution depends on the solvent used.
In complex (1) (Fig. 1), central metal is surrounded by 4 nitrogen atoms from two bis-imine L1. In complex (2) (Fig. 1), the stereochemistry (syn and anti) of the ligand allowed the copper to be chelated only by 3 nitrogen atoms belonging to the same bis-imine L2, the nitrogen atom of the second pyrrol in L2 was far from the zone of central attack of metal due to the Z configuration. Otherwise, in complex (3) (Fig. 1), the presence of the NO2 group in ortho position in the aromatic system could present a steric obstruction preventing the complexation of the metal with four nitrogen from two ligands L3, then one copper atom was chelated with two nitrogen atoms belonging to the first ligand L3 and with a third one from a second ligand L3. Finally, two copper atoms react with three ligands L3 to form binuclear Cu(I) complex 3.

Structure elucidation
The structures of the obtained products were determined by X-ray diffraction and characterized by 1H and 13C NMR, IR, UV/Vis and photoluminescence.

[Cu(L2)Cl] (2)
Single-crystal structural analysis of (2) shows that this material is a neutral mononuclear complex with two formula units in the asymmetric unit. An ORTEP [28] view of the basic structural unit with the corresponding numbering system is depicted in (Fig. 4). Each copper ion in this basic structural unit is bound to one chloride ion and three nitrogen atoms (comprised of one deprotonated pyrrole nitrogen donor, and two imine nitrogen donors) of the deprotonated tridentate ligand (L2), giving an ClN3 coordination sphere. N o v e m b e r 1 3 , 2 0 1 4 The charge balance and X-ray analysis demonstrate that, in this case where L2 is partially deprotonated, the copper is in the Cu(II) oxidation state. In fact, despite the absence of oxygen in the reaction medium, a redox reaction occurred between Cu(I) and H+ which results from the deprotonating of a pyrrolic nitrogen, to form the complex (2) and H2, which emerges. This is happened with L2 only due to the possible deprotonating of its secondary amine function. L1 and L3 do not present this possibility. The pyrrolic ring linked to copper(II) has an aromatic character similar to the cyclopentadienyl one in ferrocene. The calculation of bond valence sum, using an empirical formula of bond valence, S = exp[(Ro-R)/B] (S = bond valence, R bond length) [29,30], indicates that the oxidation state of both Cu sites in the asymmetric unit of complex (2), was 2+ (2.013 and 2.030 valence units for Cu1 and Cu2 atoms respectively). This oxidation state is identical with the charge balance consideration. In the complex (2), the geometry of the coordination polyhedron around each copper(II) cation is square planar (with Σangles = 360.04° and 359, 61° at Cu1 and Cu2, respectively). Bond lengths and angles between these four-coordinate atoms and the central copper(II) atoms are comparable. For the square-plane

Spectroscopic properties (IR and NMR)
The IR absorption bands for the imines in the Schiff-base ligands (CH=N stretching vibration) were observed at 1629 -1637 cm-1. Once, the coordination is done, the frequency of the imine group decreases. This observation is attributable to the decrease in the sp2 character of the imine group by the donation of the π-electron.
The 1H NMR spectra data and peak assignments are presented in the experimental section for each complex. The sharp singlet at about 3.98-4.11 ppm is assigned to the CH2-CH2 protons. 1H NMR chemical shifts and IR absorption bands of the copper complexes correlate with the basicity of the original Schiff-bases depending on the electronic characters of the Schiff-base.
The degree of donation of π-electrons in imine groups was evaluated from the difference between the IR absorption band and the 1H NMR chemical shifts, of the imine group in the free ligands and the corresponding complexes.
The δ ppm values from the 1H NMR spectroscopic data increased as the increase in the electron-donating ability (i.e. the Hammett's ζ π values) of the aromatic [31]. The shifts towards low magnetic fields agree with the cases of the coordination of the nitrogen ligand to metals [31,32], as in the case of the protonation of amines [33] and imines [29], by which the electron densities of the nitrogen atoms decrease [33]. This shift towards low magnetic fields is attributable to the strong donation of the π-electron on the nitrogen atoms included in the conjugated π systems consisting of the aromatic rings and the conjugated diimines to copper atom.

Optical properties
The optical properties of the Schiff-base ligands (L1-L3) and their complexes (1-3) were explored using UV/Vis and photoluminescence. Table 2 summarizes the absorption data for (1-3) dissolved in DMSO. The complexes (1-3) have N o v e m b e r 1 3 , 2 0 1 4 similar patterns with different characteristic bands. By comparing the absorption spectrum of each complex with that of the corresponding free ligand, the high intensity bands can be ascribed to π-π* and n-π* transitions [34]. The luminescent properties of the free ligands and their complexes were investigated in the solid state at room temperature. This study constitutes an independent evidence of complexation between the ligands and the copper ions. Fig. 8 represents the comparison between the excitation and emission spectra of the free ligand L1 and the corresponding complex 1. The emission spectra of the free ligand (Fig. 8d) and that of the corresponding complex 1 (Fig. 8c) were obtained by excitation at similar wavelengths from their excitation spectra given in Figure 8a a nd Figure 8b, respectively. Ligand L1 and complexes 1, emit between 300 and 550 nm upon the UV excitations ( Table 3). As shown in Fig. 8, under comparable excitation conditions, the complex 1, gave an emission spectrum similar to its corresponding ligand L1, which suggests that these emissions can be ascribed to the intra-ligand π*→π and π*→ n transitions of Schiff-bases (ILCT) [35][36][37]. The lower energy bands were attributed to the transitions π*-π and the highest energy one to the π*→ n charge transfer of Shif-bases [37]. On the other hand, under comparable excitation conditions, the complexes 2 and 3 gave also emission spectra similar to those of their corresponding ligands L2 and L3 (Table 3). The enhancement of the fluorescence of the complexes compared to their respective ligands appears to be related to the L-L π…π stacking interactions, which result in a decrease in the HOMO-LUMO energy gap of the complex [38]. In addition, the complexes 1 and 3 exhibit an emission shoulder at 442 and 441 nm, respectively that do not appear on the spectra of their corresponding ligands (L1 and L3). This emission can be ascribed to be a Cu(I) to Schiff-base ligand, i.e. metal-to-ligand, charge transfer (MLCT) [35].

Conclusion
The construction of supramolecular architectures is currently of great interest due to their intriguing network topologies and interesting electric, magnetic, catalytic and optical properties. Three novel copper supramolecules with different structural motifs were prepared by reaction of CuCl with the Schiff-base ligands L1-L3 in acetonitrile. The structural motifs of these species depend mainly of the coordination geometry of the central atom and the flexibility and stereochemistry (syn and anti) of ligand molecules. In the complexes (1) and (3) the central atom is a copper(I) which has a d 10 diamagnetic configuration, while in (2) the central atom is a copper(II), which has a d 9 paramagnetic configuration. These two copper oxidation states commonly exhibit tetrahedral and square planar coordination geometries depending upon the nature of the ligand(s) L. Both geometries occur with a coordination number of four. Square planar is favoured when the Crystal Field Splitting Parameter (Δ) is large as this produces maximum electronic stabilization, whereas tetrahedral geometry is favoured for smaller values of Δ. The larger the value of Δ, the larger is the tendency for the d electrons to pair up in the lower lying orbitals, overcoming the repulsive pairing energy associated with two electrons occupying the same orbital. This has the effect that certain dn electron configurations favour certain geometries. The photoluminescence behavior of complexes 1-3 and their ligands L1-L3 respectively, could be of fundamental interest and also significant for different applications of photoactive materials.