Synthesis, X-ray Structural Characterization, and DFT Calculations of Mononuclear Nickel(II) Complexes Containing Diamine and Methacrylate Ligands

The mononuclear Ni(II) complexes [Ni(en)2(H2O)2](MAA)2 (1) and [Ni(pn)2(MAA)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 spectroskopy. Structures of the complexes have been determined by single-crystal X-ray diffraction analyses. In the nickel(II) complexes 1 and 2 nickel(II) ion is six-coordinate and has a distorted octahedral geometry. Ni(II) is bonded to four nitrogen atoms of the two diamines and additionally to two oxygen atoms of aqua ligand in 1, and two oxygen atoms of methacrylate ligands in 2. 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 and considering effective core potential (ECP). The comparison of the results indicates that the employed DFT method yields good agreement with experimental data. Keyword: Nickel(II) complex; mononuclear; methacrylate; diamine; DFT


Introduction
2][3][4][5] There have been numerous of investigation on polydentate amines.6][7][8][9] On the other hand, the complexes with organic, inorganic carboxylates and their derivatives are widely used in coordination chemistry.The carboxylates in the complexes exhibit various possible bonding modes, mono-and bidentate by forming chelation or bridges in coordinating to the transition metal.][12][13][14][15] In our previous work, we reported the synthesis, spectroscopic characterization, structural aspects and density functional theory (DFT) calculations for two Cu(II) complexes containing diamine, acetate, and methacrylate ligands. 15In order to investigate the effect of the metal on the structural complexes with these ligands, we Scheme 1. Synthesis of the complexes 1 and 2 carried out the synthesis of two Ni(II) complexes by reaction of diaminum-methacrylic acid salt (diamines are ethylendiamine and 1,3-propylendiamine) with Ni(II) acetate (Scheme 1).The structures of the complexes have been determined by single-crystal X-ray diffraction analyses and calculated by density functional theory.

1. Starting Materials and Physical Measurements
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 dimethylformamide (DMF) 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 2400II PERKIN-ELMER elemental analyzer.

X-Ray Crystallography
Diffraction images of 1 and 2 were measured at 150 K on Agilent Xcalibur and SuperNova diffractometers using Cu Kα (λ = 1.54180Å) and Mo Kα (λ = 0.71073 Å) radiation, respectively.Data were extracted using the CrysAlis PRO package. 16The structures were solved by direct methods with the use of SIR92. 17The structures were refined on F 2 by full matrix last-squares techniques using the CRYSTALS program package. 18Atomic 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) program package. 19,20The geometries of the studied complexes have been optimized at the B3LYP level of the theory. 21The basis set of 6-31G(2df,p) was used for the C, H, N, and O atoms as recommended by Curtiss and his co-workers, 22 while the basis set of LanL2DZ was employed for Ni atom considering the size of complexes and hardware limitations [23][24][25] 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)/LanL2DZ.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 B3LY-P/6-311+G(2df,p)/LanL2TZf. 26,27

4. Syntheses of Compounds L 1 and L 2
The compounds L 1 and L 2 were prepared as previously reported elsewhere by us 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 (40 mL), respectively. 15The resulting bright yellow solution was heated to reflux for two hours.After two days, the solid yellow powder obtained was filtered, washed with acetone and acetonitrile, and dried in air.

5. Synthesis of Nickel(II) Complexes
Ni(CH 3 COO) 2 .4H 2 O (2.00 mmol, 0.496 g) was slowly added to an ethanol solution (40 mL) of the corresponding compound (L 1 , 2.00 mmol, 0.464 g; L 2 , 2.00 mmol, 0.492 g) and the resulting solution was stirred for two hours at room temperature.Two days upon evaporation of the solvents, a blue-green oil formed.The oil form obtained was re-suspended in ether and stirred at room temperature until a precipitate formed.The solid product

1. Syntheses and Characterization of the Complexes
The diaminum-methacrylic acid salt ligands were obtained by reaction of related diamine (ethylendiamine, en, and 1, 3-propylendiamine, pn) and methacrylic acid in methanol under reflux.The reaction of nickel(II) acetate with L 1 and L 2 leads to the formation of mononuclear complexes 1 and 2, while in the reaction of copper(II) acetate with these ligands dinuclear copper complexes were formed.
The most significant IR bands for ligands and complexes are given in the experimental section.In the IR spectra of the compounds L 1 and L 2 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.
In IR spectra of 1, [Ni(en) 2 (H 2 O) 2 ](MAA) 2 , the appearance of two bands at 1628 and 1450 cm -1 due to asymmetric ν asym (COO − ) and symmetric ν sym (COO − ), respectively, reveal the uncoordinated methacrylate ions.In contrast, complex 2, [Ni(pn) 2 (MAA) 2 ], shows two strong bands at 1632 and 1381 cm −1 corresponding to stretching frequencies of the carboxylate group: asymmetric ν asym (COO − ) and symmetric ν sym (COO − ), respectively.The difference between asymmetric and symmetric frequencies (Δ[ν asym (COO − ) − ν sym (COO − )] > 200 cm −1 ) indicates a monodentate coordination mode for the methacrylate ion (see the description of X-ray crystal structures section). 10,12,28,29he absorption spectra of the compounds L 1 and L 2 in methanol solution show band n-π* transitions at 226 and 216 nm, respectively.Electronic spectra of 1 and 2 show broad bands at 1009 and 631 nm (for 1) 1041 and 630 nm (for 2), respectively.These spectral features are consistent with six-coordinate octahedral geometry for Ni(II).These bands arise from spin-allowed d-d transitions of the nickel(II) ion in a distorted octahedral environment where two maxima observed in the visible region result from 3 A 2g → 3 T 1g and 3 A 2g → 3 T 2g transitions, respectively. 30The sharp a signal at 370 (for 1), and 377 nm (for 2) can be assigned to be charge transfer transition.Two bands at 284 and 225 nm (for 1) and 284 and 226 nm (for 2) assigned to intraligand π-π* transitions.
In 1, there is a disorder pattern in the packing of the -C(CH 3 ) =CH 2 group over two positions, with relative occupancies of 52%:48% (Fig. 1b).However, in 2 two methacrylate ions are coordinated to the Ni(II) ion.

D-H•••A D-H H•••A D•••A D-H•••A Symmetry code
N1-H811 Crystal structures of complexes 1 and 2 both show hydrogen bonding interactions.In 1 one hydrogen atom of the coordinated water molecule is involved in a intramolecular hydrogen bonding interaction with the oxygen atom O3 of a methacrylate anion, and the other water H atom is hydrogen bonded to O2A of the methacrylate anion (symmetry code: x, -y + 1, z + ½), with donor(D)-acceptor(A) distances of 2.747(4) and 3.214(4) Å and D-H•••A angles of 173(5) and 139(4)°, respectively.Also, there is a hydrogen bonding interaction between the hydrogen atoms of the NH 2 of the ethylendiamine ligands with the oxygen atoms of a methacrylate anion.In 2, there are hydrogen bonding interaction, between the hydrogen atoms bonded of the 1,3-propylendiamine with oxygen atoms of the methacrylate ligand.Full details of the hydrogen bonding are given in Table 3.

DFT Optimized Geometries
The geometry optimization of nickel(II) complexes was carried out in their singlet and triplet spin states.The optimized geometric parameters at their most stable spin states, which are triplet for complexes 1 and 2 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.
The differences between optimized geometrical parameters and experiment are less than 0.05 Å (bond distances) and 2° (bond angles) in most cases (Fig. 5).In general, the predicted bond lengths are slightly longer in comparison with the values based on the X-ray crystal structure data.The geometrical differences might 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. 34,35The crystal packing forces, which have an influence on the molecules, as expected for the experimental parameters (solid state), is a reason for the difference of calculated bond lengths in the gas phase and solid phase.
The vibrational frequency calculations were performed based on the optimized structures of complexes.The calculated and experimental IR spectra of complexes are in good agreement with experimental data (Fig. 6 and Fig. S1).
The energies of the HOMO and LUMO molecular orbitals have been also calculated.The experimental values of HOMO-LUMO gap (∆) for the complexes of 1 and 2 based on UV-Vis spectra are 5.51 and 5.49 eV, respectively, which corresponds to n → π* transitions.The theoretical The calculated charges on the metal centers in complexes 1 and 2 are +1.306 and +1.267 respectively, and these values are greatly lower than the formal charge of +2.These differences are as a result of charge donation from the donor atoms of ligands.

Conclusion
The reaction of nickel(II) acetate with L 1 and L 2 ligands led to the formation of mononuclear complexes 1 and 2. The crystal structures were determined for two studied complexes.In mononuclear nickel(II) complexes 1 and 2, metal centers are hexacoordinated with a distorted octahedral geometry.The optimized structure of the complexes has been studied using the B3LYP/6-31G(d)/Lan-L2DZ 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 nickel(II) 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 1481553 and 1481554, respectively.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.

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.BS 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. 1 .
Fig. 1.The ORTEP view of complex 1 (a) with one methacrylate anion (b), showing 30% probability thermal ellipsoids The ethylendiamine and 1,3-propylendiamine ligands form with Ni(II) atom five-membered and six-membered chelate rings, respectively.The Ni−N bond lengths in the complex 2 are at distances 2.104(2) and 2.105(2) Å, which are longer than Ni−N bond lengths (2.088(3) and 2.099(3) Å) in the complex 1, possibly due to the increased chelate rings formed with the Ni(II) atom.The main difference between the two complexes is that in 1 where are two water molecules coordinated to the Ni(II) ion and two methacrylate ions are not coordinated to the Ni(II) ion and acts only as counter anions, whereas in 2 the two methacrylate ions are coordinated to the center ion.The Ni−O bond length of complex 1 (2.159(3)Å) is longer than the corresponding bond of complex 2 (2.1225(19) Å).This variation is consistent with the anionic nature of the methacrylate ligands.The chelating N−Ni−N angle is 83.51(13)° for 1 and 86.69(9)° for 2, whereas the non-chelating N−Ni−N angles are 96.49(13)° and 93.31(9)° for 1 and 2, respectively.Selected bond lengths and angles, as well as interatomic distances, are summarized in Table

Fig. 3 .
Fig. 3. Various hydrogen bonding interactions in complexes 1 and 2, other hydrogen atoms are omitted for clarity.

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.