Synthesis , X-ray Structural Characterization , and DFT Calculations of Binuclear Mixed-ligand Copper ( II ) Complexes Containing Diamine , Acetate and Methacrylate Ligands

The dinuclear Cu(II) complexes [Cu(en)(MAA)(μ-CH3COO)]2 (1) and [Cu(pn)(MAA)(μ-CH3COO)]2 (2) where MAA, en and pn are methacrylate, ethylendiamine and 1,3-propylendiamine, respectively, have been synthesized and characterized by elemental analysis, FT-IR and UV-Vis spectroscopy. The structures of the complexes have been determined by single-crystal X-ray diffraction analyses. In the dinuclear complexes 1 and 2 the two copper centers are five-coordinated and exhibit distorted square pyramidal geometries. The theoretical geometries of the studied compounds have been calculated by means of density functional theory (DFT) at the B3LYP/6-311+G(d,p)/LanL2DZ level considering effective core potential (ECP). Keyword: Copper complex; Dinuclear; Methacrylate; Diamine; DFT


Introduction
1][12][13][14] The carboxylates and their derivatives exhibit various possible bonding modes when coordinating to metal ions such as monodentate and bidentate either by forming bridges or chelation.Nevertheless, carboxylate ligands commonly act as bidentate ligand in the transition metal complexes.[19] Copper is an essential element to biological functions.The exchangeable portion of copper in blood plasma occurs mainly as a result of mixed-ligand formation involving copper-nitrogen interactions.Cu(II) mixed-ligand antineoplastic agents, containing diamine ligands exhibit cytotoxicity, genotoxicity, and antitumor effects. 20,21n the present work, we report the synthesis, spectroscopic characterization, structural aspects and density functional theory (DFT) calculations for two new mixedligand Cu(II) complexes containing diamine, acetate and methacrylate ligands.The complexes are synthesized by reaction of diaminum-methacrylic acid salt (diamine are ethylendiamine, en, and 1,3-propylendiamine, pn) with Cu(II) acetate (Scheme 1).
Here, the carboxylate ligands (acetate from the initial metal acetate input and methacrylate from diaminummethacrylic acid salt) are of particular interest, since the carboxylate can coordinate to metals in different modes.The carboxylate ligands are coordinated to the copper(II) Vafazadeh et al.: Synthesis, X-ray Structural Characterization, ... ion in both monodentate and bidentate modes, and binuclear copper complexes can be formed.

1. Materials and Methods
All chemicals were of analytical reagent grade and were used 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 of methanol using GBC UV-Visible Cintra 101 spectrophotometer with 1 cm quartz in the range of 200-800 nm.Elemental analyses (C, H, N) were performed by using a CHNS-O 2400 II PERKIN-ELMER elemental analyzer.

X-ray Crystallography
Diffraction images 1 and 2 were measured at 150 K on Agilent Xcalibur and SuperNova diffractometers using Mo Kα (λ = 0.71073 Å) and Cu Kα (λ = 1.54180Å) radiation, respectively.Data were extracted using the CrysAlis PRO package. 22The structures were solved by direct methods with the use of SIR92. 23The structures were refined on F 2 by full-matrix last-squares techniques using the CRYSTALS program package. 24The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98,N-H = 0.87, O-H = 0.82 Å) and with U iso (H) in the range 1.2-1.5 times U eq of the parent atom, after which the positions were refined with riding constraints for those bonded to C and without constraints for those bonded to N or O. Atomic coordinates, bond lengths and angles and displacement parameters have been deposited at the Cambridge Crystallographic Data Centre.Crystallographic data and refinement details for the complexes are given in Table 1.Details of the refinement procedures for the structures are given in the Supplementary Information.

3. Theoretical Calculations
All computations were performed by means of standard DFT method using the Gaussian09 (G09) pro-gram package. 25,26The geometries of the studied complexes have been optimized at the B3LYP level of theory. 279][30] Special care was taken to select the (global) minimum energy conformation via systematic conformational searching at this level.The nature of each stationary point was established by frequency calculations at the same level of B3LYP/6-31G(2df,p)/Lan-L2DZ.The geometry optimizations have been completed in the absence of solvent molecules and other impurities, and the optimized structures were compared with the crystalline structures.Charges on atoms have been calculated using Natural Bond Orbital (NBO) theory at the higher level of B3LYP/6-311+G(2df,p)/LanL2TZf. 31,32he diaminum-methacrylic acid salts L 1 and L 2 , were prepared by reaction between two equivalents of methacrylic acid (20 mmol, 1.70 mL) and one equivalent of related diamine, 1,2-ethylendiamine (10 mmol, 0.67 mL) and 1,3-propandiamine (10 mmol, 0.84 mL) in methanol medium (40 mL), respectively.The resulting bright yellow solution was heated to reflux for two hours.After two days, solid yellow powder obtained was filtered, washed with acetone and acetonitrile, and dried in air. L

4. 1. Synthesis of Copper(II) Complexes
Cu(CH 3 COO) 2 .H 2 O (2.00 mmol, 0.399 g) was slowly added to a methanol solution (40 mL) of the related ligand (L 1 , 2.00 mmol, 0.464 g and L 2 , 2.00 mmol, 0.492 g) and the resulting solution was stirred for two hours at room temperature.The color of solution turned to blue and after two days solid blue powder was obtained.
Yield: 0.99 g (93%).The blue solid product was recrystallized from acetonitrile/toluene (3:1 v/v).Blue crystals appeared at the bottom of the vessel upon slow evaporation of the solvents, which were filtered and dried in air.Anal

1. Syntheses and Characterization of the Complexes
The diaminum-methacrylic acid salt ligands was obtained by reaction of related diamine (ethylendiamine, en, and 1,3-propylendiamine, pn) and methacrylic acid in methanol under reflux.Copper(II) complexes 1 and 2 were obtained from the reaction mixture of the related ligand with the corresponding Cu(CH 3 COO) 2 .H 2 O salt in equimolar ratio in methanol as a solvent at room temperature.The reaction of copper(II) acetate with L 1 and L 2 ligands leads to the formation of dinuclear complexes 1 and 2.
The most significant IR bands for ligands and complexes are given in the experimental section.The IR spectra of the free ligands, L 1 and L 2 , shows ν(N-H) bands at 3500 and 3393, ν(C=C) bands at 1530 and 1543, respectively.The two strong bands at 1650 and 1455 cm -1 (for L 1 ) and 1646 and 1455 cm -1 (for L 2 ) corresponding to stretching frequencies of the carboxylate group: asymmetric ν asym (COO -) and symmetric ν sym (COO -), respectively.
The absorption spectra of the free ligands L 1 and L 2 in methanol solution show band n-π* transition at 226 and 216 nm, respectively.The electronic spectra of the copper complexes 1 and 2 in methanol solution show a broad band at 633 and 643 nm and a sharper signal at 255 and 246 nm, which arise from a spin-allowed d-d transition of the copper(II) ion (d 9 electronic configuration) and a charge transfer transition, respectively. 35,36

2. Description of X-ray Crystal Structures 1 and 2
The molecular structures of 1 and 2 are shown in Fig. 1.Both complexes 1 and 2 have dimeric structure.Compounds crystallize in triclinic space group P1 and there is one molecule in the unit cell (Z = 1).The single crystal X-ray diffraction data for compounds 1 and 2 are li-  1. Selected bond lengths and angles as well as interatomic distances are summarized in Table 2.
The oxygen atoms of carboxylate groups (acetate and methacrylate ions) and the NH 2 amine groups of the diamine ligand (ethylendiamine for 1 and 1,3-propylendiamine for 2) play a significant role in intramolecular and intermolecular hydrogen-bonding interactions (Figs. 2 and 3).In 1, the hydrogen atoms of the coordinated ethylendiamine molecule are involved in an intermolecular hydrogen-bonding interaction with the oxygen atoms of a neighboring coordinated methacrylate ion and the oxygen atoms of uncoordinated methacrylate and acetate ions.Also, there is an intramolecular hydrogen bonding between the hydrogen atom H151 of the NH of the ethylendiamine molecule with the oxygen atom O8 of uncoordinated methacrylate ion.In 2, there are intermolecular hydrogen bonding interaction between the hydrogen Table 3. Hydrogen bonding (Å) and angles (°) in complexes 1 and 2
In general, the predicted bond lengths are slightly longer in comparison with the values based upon the X-ray crystal structure data.The geometrical differences might atoms of the coordinated 1,3-propylendiamine molecule with the uncoordinated oxygen atom O2 of the bridging acetate ligand and coordinated oxygen atom O3 of the methacrylate ion.Also, there are intramolecular hydrogen bonding between the hydrogen atom of the NH of the 1,3propylendiamine molecule with the uncoordinated oxygen atoms O2 and O4 of acetate and methacrylate ions, respectively.Full details of the hydrogen bonding are given in Table 3.

DFT Optimized Geometries
The geometry optimization of copper complexes were carried out in their singlet and triplet spin states.The optimized geometric parameters at their most stable, triplet states, is shown in Fig. 4.
As shown in Table 4, the calculated bond lengths for the studied complexes agree well with the X-ray experimental data.For example, the calculated Cu1-N12, Cu1-N15, Cu-O6 and Cu-O2 bond lengths for the dinuclear complex 1 are 2.00, 2.05, 1.96 and 2.00 Å, and they correlate nicely with the experimental values of 2.00, 1.99, 1.95 and 1.97 Å, respectively.be a result of crystal packing forces which have an influence on the molecules as expected for the experimental ones (solid state), but the calculated geometries are in the gas phase. 40,42The crystal packing forces, which have an influence on the molecules, as expected for the experimental parameters (solid state), are a reason for the difference of calculated bond lengths in the gas phase and solid phase.
The calculated charges on the metal centers in complexes 1 and 2 are +0.875 and +0.873 respectively, and these values are greatly lower than the formal charge of +2.These differences are a result of charge donation from the donor atoms of ligands.

Conclusion
The reaction of copper(II) acetate with L 1 and L 2 ligands led to the formation of dinuclear copper(II) complexes 1 and 2. The crystal structures were determined for two studied complexes.An acetate oxygen bridge, a relatively rare bridging mode of the carboxylate group, has been found in dinuclear complexes 1 and 2. Coordination geometry for each copper ion was square pyramid.The optimized structure of complexes have been studied using the B3LYP/6-31G(d)/LanL2DZ level of theory.The calculated molecular geometries are in a very good agreement with the experimental data.It has been revealed that the triplet state for copper complexes 1 and 2 are more stable than their singlet state.

Supplementary Material
The deposition numbers of the studied complexes 1 and 2 are CCDC 1481551 and 1481552, respectively.These data can be obtained free-of-charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing datarequest@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033.

Acknowledgement
MN is thankful to Prof. Michelle L. Coote and Research School of Chemistry, The Australian National University for the offer of campus visiting position and for the Gaussian calculations.MC is grateful to the graduate school of Yazd University for the post-graduate scholarships.MN and RV are also grateful to Yazd University and the Australian National University for their valuable support.

Fig. 5 .
Fig. 5. Atom-by-atom superimposition of the calculated structures (black) over the X-ray structure (red); hydrogen atoms have been removed for clarity.

Table 1 .
Crystallographic data and structural refinement for complexes

Table 4 .
Selected geometric parameters from X-ray and DFT-B3LYP calculations