Synthesis , Characterization and DFT-Based Investigation of a Novel Trinuclear Singly-Chloro-Bridged Copper ( II )-1-Vinylimidazole Complex

A novel trinuclear copper(II) complex [Cu3(μ-Cl)2Cl4(1-Vim)6] with monodentate 1-vinylimidazole (1-Vim) and chloro ligands has been prepared and experimentally characterized by elemental analysis, thermogravimetry (TGA, DTG, DTA), X-ray single crystal diffractometry, TOF-MS and FT-IR spectroscopies. The electronic and structural properties of the complex were further investigated by DFT/TD-DFT methods. Density functional hybrid method (B3LYP) was applied throughout the calculations. The calculated UV-Vis results based on TD-DFT approach were simulated and compared with experimental spectrum. Based on the data obtained, DFT calculations have been found in reasonable accordance with experimental data.


Introduction
2][3] These prominent features essentially stem from the existence and the adjacent positions of two or more metallic centers in the same molecular unit.6][17] Generally, the use of polydentate ligands that can present different coordination behaviours according to nature of the metal ions is straightforward for aforementioned versatilities.In case of our study reported herein, a novel mono-chloro-bridged trinuclear complex consisting of two discrete metal(II) centers connected through simple bridging chloro ligands and coordinated with monodentate 1-vinylimidazole ligands and monodentate chloro ligands were prepared and fully characterized.The interesting topology of the the complex was probed with X-ray single-crystal data.Qualitative picture of spectroscopic and structural properties of the complex beyond the experimental data were further investigated in the framework of density functional theory (DFT) and its time-dependent extension (TD-DFT).It is well known from the literature that DFT methods, especially the latter introduced hybrid ones that predict almost all molecular properties from simple organic molecules to more complex systems such as transition metal complexes are superior to other wave-function based electron correlation methods in preference.[20]

Physical Measurements
All the reactants and solvents were obtained from commercial suppliers and used without further purification.FT-IR spectrum was recorded on a BRUKER 2000 spectrometer as KBr slice.UV-Vis spectrum was measured with a UNICAM UV2 spectrometer in 200-800 nm range in methanolic solution.ESI mass spectrum was recorded with Agilent LC/MS-TOF spectrometer using methanolic solution.Elemental analysis was performed with a Costech ECS 4010 CHNS Elemental analyzer.Simultaneous thermal analyses were conducted by using a SIIO-Exstar 6000 Thermal analyzer within 35-1000 °C temperature range, by a heating rate of 10 °C/min.and in static nitrogen atmosphere.
Intensity data were collected using a STOE IPDS 2 diffractometer with graphite monochromated Mo K α radiation (λ = 0.71073 Å) at 293(2) K.The structure was solved by direct methods 21 and refined with full-matrix least-squares procedure on F 2 using SHELXL97. 22All non-hydrogen atoms were refined anisotropically.Hydrogens bonded to carbon atoms were positioned geometrically and refined with a riding model with U iso 1.2 times that of attached carbon atoms.The positions of water hydrogens were found by difference fourier map and refined isotropically.

3. Computational Procedure
All computations reported herein were carried out using Gaussian 03W suit of program 23 under C i constrained symmetry within unrestricted formalism.Spin-doub-let gas-phase B3LYP optimizations of the complex starting from experimental X-ray geometry was performed employing triple-zeta 6-311G(d) basis set for Cu atoms, double-zeta 6-31G(d) basis set for H, C and N atoms and 6-31G+(d) basis set for more diffuse chlorine atoms.Vibrational frequency analysis was calculated at the optimized structure at the same level of theory to ensure that the final geometry is a local minimum having no imaginary frequency.
TD-DFT excited state calculation was performed at the geometry optimized structure using the flexible LANL2DZ basis set for all atoms considering the large size of the system.
Investigation of natural charge distributions, population analysis of valence core orbitals and non-covalent energetic stabilizations by second order perturbation theory analysis of Fock matrix were carried out using natural bond orbital (NBO) analysis at the UB3LYP optimized structure by NBO version 3.1 implemented in Gaussian 03W package.
The percentage molecular orbital contributions (MOCs) from atoms and groups to the related molecular orbitals (MOs), were extracted from single point energy (SPE) calculation of the optimized structure using VMOdes software. 24

1. Crystal Structure
A perspective plot with the atom numbering scheme of the complex is shown in Figure 1 and the crystallographic data are given in Table 1.The crystal consists of neutral trimeric units and the symmetric unit of the network consists of two geometrically different copper(II) centers one of which (Cu2 and Cu2 i i: 1 -x, -y, 1 -z) is distorted square-pyramidal and the other (Cu1) is distorted squareplanar geometry as shown in Figure 2. In the trimer unit, the central copper(II) ion located on inversion center possessing a quasi-octahedral coordination geometry in which the two singly-bridging chloro ligands (Cl1, Cl1 i ) together with two N atoms (N1, N1 i ) of monodentate 1vim molecules are located on square plane.The two monodentate chloro ligands (Cl2, Cl2 i ) weakly coordinated and reside in the apical positions.Symmetry related copper(II) terminal has more common penta-coordination with the two monodentate chloro ligands (Cl2, Cl3), two N atoms (N3, N5) of 1-vim molecules of basal plane and one axially coordinated mono-bridging chloro ligand (Cl1).The coordination number 5 for copper(II) ions usually presents either a square pyramidal (SP) or trigonalbipyramidal (TBP) geometry (or an intermediate geometry).The Addison distortion index, τ used for the evaluation of the distortion of coordination geometry from TBP to SP was calculated as 0.

2. TOF-MS Spectroscopy
Successive MS measurements of the compound sample demonstrated almost same fragmentation products.The most abundant signal at m/z = 991.5 [Na+complex] in TOF-MS spectrum of the complex in Figure 4 is related to sodiated adduct of the trinuclear structure since the spectrum was taken in positive ion mode.Designation of other significant peaks is not possible due to probably different stable adducts and fragments in solution medium.Nevertheless, the expected sodium adduct of the complex is the most abundant one among them (Figure 4).

Thermogravimetry
The thermogram of the complex comprising simultaneous TG and DTA curves is depicted in Figure 5. Endothermic DTA signal observed immediate before decomposition onset denotes to melting phenomenon at c.a 137 °C.After the completion of the melting, the ligands endothermically release between 146-262 °C and 262-932 °C.The mismatch between the observed (84.20%) and calculated (75.36% for CuO final product, and 80.33% for Cu residue) total mass losses is only attributable to additional carbon remains with either two final products.

4. Geometry Optimization
The ground state geometry of the complex was optimized on spin unrestricted doublet state applying tight SCF procedure.The main geometrical parameters related to optimized structure are listed in Table 2 together with experimental ones.Calculated MOCs from atoms and group were given in Table 3 and frontier molecular orbitals were depicted in Figure 6.The calculated SOMO (singly occupied molecular orbital (from unrestricted  doublet calculation))-LUMO gap is 3.132 eV.Of the distributions of MOs, β-spin LUMO and LUMO+1 are mainly localized on copper(II) ions (%51.8 for β-LUMO and %53.2 for β-LUMO+1) while α-spin LUMO is composed mainly of central copper(II) ion (%54.9).The very low contributions from metal ions to occupied MOs indirectly drawn attention to a predominant coulombic interaction between metal centers and coordinated atoms.Superimposition of experimental and gas-phase optimized structures was presented in Figure 7.In general, there is a pleasant consistence between optimized and X-ray geometries according to the results and as expected, the general tendency of gas-phase optimizations in favour of somewhat extending the bond distances was introduced.

5. NBO Analysis and TD-DFT Calculation
Valence core electron populations of the molecule are presented in Table 4.The calculated charges of central metal ions are 0.66 and 0.70 respectively and lower than the oxidation state +2 and indicates the total charge donations from ligands to Cu1 and Cu2 ions equals.The nearest Cl3 atom to metal ion hold the least negative charges (-0.74) as expected.The bridging Cl1 and so-called monodentate Cl2 have almost even charges due to their similiar positions between two metals.Consequently, the natural charges of the ligands and metals are comparatively expedient to their corresponding distances among each other.Selected non-covalent interactions and corresponding second order energies are listed in Table 5.The presence of natural bond orbitals between metal centers and coordinated atoms according to NBO results indicate also the covalency of corresponding bonds in addition to coulomp type interactions except for the longest and the weakest two Cu2-Cl1 and Cu2-Cl2 bonds incorporating none of natural bond orbitals.Therefore, no remarkable contribution to the second order energy lowering comes from these weak covalent bonds as understood from Table 5. ned as π→π/d (β-HOMO→β-LUMO) transition by full population analysis of ground state MOs.Since none of transitions with non-zero oscillator strength in 600-800 nm region was found in TD-DFT aside from the aforementioned transitions (excitations under 298 nm not participate within 100 calculated ones and were not be able to involve because of scratch file size limitation of the program on 32 bit operating system).

Conclusions
Succesfully prepared a novel singly-chloro-bridged trinuclear monomeric [Cu 3 (μ-Cl) 2 Cl 4 (1-Vim) 6 ] complex has been structurally characterized by X-ray crystallograpy.The revealed X-ray structure clearly shows that the molecule consists of two different copper(II) coordination sites, a distorted square-pyramidal terminal copper(II) ions and either distorted (Jahn-Teller elongated) octahedral or square-planar central copper(II) ion.Since copper(II) ion within significant amount of its complex compounds is mostly prone to give these coordination geometries.Interestingly, single chlorine atom in the trimer connects two copper(II) centers in the complex otherwise two chlorine atoms participate in bridging coordination in most of the other reported chlorine-bridged polynuclear species. 34,35The comprehensive computational studies were executed by the most efficient Hiybrid-density functional method (DFT-B3LYP) in a tolerable computation time despite the large size of the molecule.The data obtained from DFT, TD-DFT and NBO analysis succesfully represented the experimental trends.

Figure 2 .
Figure 2. Two different coordination polyhedra around two different copper(II) centers

Table 3 .
Calculated MOCs from atoms and groups HOMO-1, etc. L, LUMO; L+1, LUMO+1, etc. the numbering of 1-Vim molecules was made by concerning preceding number of atoms involved α spin LUMO is considered as SOMO and so not given in the table again S, SOMO; H, HOMO; H-1,