Synthesis and Computational Investigations of Ruthenium(II) Complexes Containing Hydrazine Schiff Base Ligands

Three new heteroleptic ruthenium(II) complexes containing hydrazine schiff base as ligands were synthesized and characterized by using elemental analysis, FT-IR, H, C NMR, and mass spectroscopic techniques. FT-IR study showed that the substituted phenylhydrazine ligands behave as a monoanionic bidentate O and N donors (L) coordinate to ruthenium via the deprotonated phenolic oxygen and the azomethine nitrogen. They possess excellent thermal stabilities, evident from the thermal decomposition temperatures. Absorption, emission and electrochemical measurements were carried out and the structures of the synthesized complex were optimized using density functional theory (DFT). The molecular geometry, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energies, Mulliken atomic charges and molecular electrostatic potential (MEP) of the molecules are determined using B3LYP method and standard 6-311++G (d, p) basis set.


INTRODUCTION
The design and synthesis of organic-inorganic hybrid complexes based on strong coordinate bonds and multiple weak non-covalent forces has become a rapidly expanding field of research in coordination chemistry and crystal engineering. These complexes not only possess fascinating structural features but also show interesting properties as new functional materials, which can be used in the areas of luminescence, sensors, separation, adsorption, catalysis and biological chemistry [1,2]. Polypyridyl ruthenium(II) complexes have been extensively studied attributable to their thermal stability, absorbing photochemical, photophysical and electrochemical properties, which can be adjusted by the insertion of various Lewis bases [3][4][5][6]. These features have driven the development of many applications of ruthenium(II) complexes catalysis [7][8][9] artificial photosynthetic system [10,11], and sensors [12]. Among these applications, dye sensitized solar cells have got significant consideration as an alternative to conventional silicon photovoltaic devices [5,13]. Dye-sensitized solar cells (DSSCs) are an attractive solar energy conversion technology and their advantages include their low cost of manufacture, ease of fabrication and modifiable aesthetic features, such as colour and transparency [14]. A large number of molecular sensitizers, such as ruthenium-complex sensitizers and zinc porphyrin sensitizers, has been *Correspondence Address to this author at the Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli-12, Tamil Nadu, India; Tel: 09994164804; Fax: 0462 2334363; E-mail: swarnalatha@msuniv.ac.in synthesized for the improvement of conversion efficiency of DSCs because the solar cell performances of the DSCs depend strongly on the photosensitizing ability of these sensitizers [15][16][17]. The electron transfer reactivity of the MLCT (metal to ligand charge transfer) excited states in polypyridyl-based complexes have long been used in many processes including electron transfer reagents and as dyes for solar photocells [18]. One of the most important requirements for the highly efficient sensitizers is a wide absorption, which covers whole visible and near-IR region as high as nearly 1000 nm, with a large molar absorptivity. In addition, suitable energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitals (LUMO) are also important requirements for the highly efficient sensitizers because these enable effective electron-transfer reactions in the DSCs such as an electron injection from the photoexcited sensitizers into the conduction band of TiO 2 and a regeneration of the resulting oxidized sensitizers by the redox mediator in the electrolyte solution [17]. And some of them have been characterized and tested in DSSCs. The C=N unit is a polar double bond, which could not only increase the conjugation degree of the molecules but also effectively promote the transmission of the excited electron to prevent the electronic quenching and improve the photoelectric conversion efficiency. Azomethines are isoelectronic to their vinyl analogue and it has been demonstrated that azomethines possess similar optoelectronic properties. These materials have received very little attention as materials for organic electronics such as organic field effect transistors, electrochromic devices and organic photovoltaic devices. Monomer Schiff bases have been synthesized and regarded as dye sensitizers in solar cells, and showed a good performance [19][20][21][22]. Schiff base and their metal complexes that have optoelectric properties, high catalytic activity and could be used in photovoltaics with improvements have been listed. Schiff base compounds are very promising because of their biodegradability, non-toxicity, good electrical conductivity in conjugated compounds, and cheaper and easier production [23]. Copper and chrome are both vital and toxic for many biological systems. Thus, in the separation of Cu 2+ and Cr 3+ from aqueous solution using schiff base ligands. Hydrazines are also useful for the synthesis of metal complexes as they easily form stable complexes with most transition metal ions. Hydrazines and their metal complexes have gained a special attraction for widespread application in technology and analytical chemistry [24,25]. Realizing the importance of the hydrazines and its derivatives as ligands, we have designed and proposed new ruthenium(II) complexes containing the hydrazine dervitative ligands. In the present paper, the selective synthesis and characterization of a three dinitro phenyl hydrazines containing ruthenium are reported, these synthesized compounds have theoretically investigated by using the more popular DFT methods, B3LYP, in 6-31++G(d,p) basis set. In order to make sense between the experimental and theoretical results, the electronic properties such as HOMO-LUMO energies, Mulliken atomic charges and molecular electrostatic potential (MEP) were calculated and the results were discussed. Moreover, the optical properties, thermal properties and electrochemical properties of metal complexes are also investigated in this paper.

Materials and Instrumentation
Commercially available RuCl 3 .3H 2 O was used as supplied from Himedia Pvt. Ltd. 2,2'-bipyridine, 2,4dinitrophenylhydrazine, 2, 4-dihydroxybezaldehyde, 4-(diethylamino)-2-hydroxybenzaldehyde, 2-hydroxy naphthaldehyde, lithium chloride were purchased from Sigma-Aldrich and were used as received. All other reagents used were chemically pure and analytical grade. Reagent grade organic solvents were purified and dried by recommended procedures [26] and degassed before use. Electrochemical measurements were carried out using CHI600E Electrochemical Analyzer. A three-electrode assembly comprising glassy carbon as working electrode, Pt wire as counter electrode and Ag/AgCl as reference electrode was used. The supporting electrolyte, tetrabutyl ammonium perchlorate (n-BuNClO), was dried in vacuum prior to use. cis-Ru(bpy) 2 Cl 2. 2H 2 O was prepared following a published procedures [27]. Elemental analysis was done using a Perkin-Elmer elemental analyzer. A JASCO FT/IR-410 Spectrometer was used to record the IR spectra in the range of 4000-400 cm -1 using KBr pellets. The electronic spectra were recorded on a Perkin Elmer Lambda-25 UV-VIS spectrometer. 1 H NMR spectra was recorded on a Bruker AV III 400 MHZ instrument using TMS as an internal reference. Electron spray ionization mass spectra of the compound were recorded on a JEOL GCMATEII mass spectrometer using a positive mode in DMF solvent. The emission spectra were recorded by spectrofluorometer SL174 with Xenon lamp and the emission was fed into a monochromator where the emission intensity was recorded as a function of the wavelength. Thermal properties of the ruthenium complexes were studied by thermogravimetric analysis (TGA) which were carried out between 25 ºC and 900 ºC in nitrogen atmosphere at a heating rate of 10 ºC min -1 using NETZSCH STA 409 C/CD TGA instrument.

Synthesis of [Ru(bpy) 2 L1] (ML1)
Schiff base ligand L1 (0.0657g, 1 mmol) was added to solution of Ru(bpy) 2 Cl 2 (0.100g, 1 mmol), in 50 mL of DMF. The resulting mixture was refluxed under a nitrogen atmosphere for 7 hours. After refluxing, 50mL of acetone was added to the refluxing mixture. The purple solution turned reddish brown and was kept in the refrigerator for overnight. The brown precipitate formed was filtered and washed thoroughly with diethyl ether. This solid was crystallized from DMF/hexane mixture. (Yield 65%). Anal.calcd. for C 33

Computational Methods
All calculations were performed using Gaussian 09 software package [28], at the B3LYP/6-311++G(d,p) level of theory. The DFT methods are low computational cost and more advantageous owing to their accuracy. These properties make DFT more sensible and realistic for the computations of different molecules. Transitions to the lowest excited singlet electronic states of ruthenium complexes were computed by using the gradient corrected DFT with the three-parameter hybrid functional Becke3 (B3) for the exchange part and the Lee-Yang-Parr (LYP) correlation function, HOMO-LUMO energy level calculations and Geometry optimization have been carried out in the present investigation, using 6-311++G(d,p) Basis set with Gaussian 09W program package [29]. The chemical reactivity descriptors were calculated using DFT. These are very important physical parameters to understand the chemical and physical activities of the ruthenium complexes. The calculated HOMO-LUMO orbital energies can be used to estimate the ionization energy (IE) = -! HOMO , electron affinity (EA) = -! LUMO , electronegativity (") = (IE + EA)/2, LUMO electronic chemical potential (#) = -", chemical hardness ($)=(IE-EA)/2, chemical softness (%) = 1/$, electrophilicity index(&) = #2/2$ [29,30].

RESULTS AND DISCUSSION
Analytical and spectroscopic data for the ligands and its complexes indicate a 1:1 metal-ligand stoichiometry for all the complexes. The synthetic route of the complexes and the proposed structure of the complexes are shown in Scheme 1. The complexes are soluble in most common organic solvents like DMF and DMSO.

FT-IR Spectral Features
The IR spectra of free Schiff bases were compared with those of the ruthenium complexes in order to ascertain the binding mode of the Schiff base ligands to the ruthenium metal ion in the complexes (Table S1, respectively [34][35][36]. The IR spectra of all the complexes therefore confirms the coordination mode of the phenyl hydrazine schiff base ligand to ruthenium(II) ion via the azomethine nitrogen and the phenolic oxygen. The most important conclusion drawn from the infrared spectral evidence is that the Schiff base ligand acts as a bidentate ligand towards the central ruthenium ions via one azomethine nitrogen atoms and one negatively charged oxygen atoms.

C NMR Spectra Analysis
Coordination of the ligands with the metal atom was further confirmed by using 1 H NMR spectral analyses (Table S2, S3. Figures a1-a12, Supporting material). The spectra of the free ligands have showed a signal at 11.6-11.8 ppm characteristics of phenolic -OH proton which have disappeared in their complexes, suggesting the coordination of deprotonated phenolic oxygen [37]. A singlet observed at 8.7-9.2 ppm in the spectra of the free ligands has been assigned to azomethine proton which has undergone a downfield shift around 9.8-9.1 ppm in the complexes indicating the coordination of azomethine nitrogen to ruthenium metal [37][38][39][40]. The complexes have shown their aromatic protons of the phenolic group and bipyridine ligands have exhibited their signal in the region 6.1-8.9 ppm The above observations made it clear that the ligands have coordinated to the ruthenium(II) ion.

Mass Spectra Analysis
The mass spectra of the ruthenium(II) complexes is in good agreement with the proposed molecular structure and the mass spectra of the complex is shown in Figures a13-a15

Electronic Spectra
Electronic spectra of all the nuclear Ru(II) complexes showed intense absorptions in the ultraviolet and visible region. The electronic spectra of all the complexes have been recorded in DMF and displayed four intense absorptions in the region 800-200 nm. The electronic spectral data of complexes 1-3 are listed in (Table 1) and selected spectra is shown in Figure 2. The absorption in the visible region at 530-546 nm is due to d-d transition and the bands observed in the region 302-378 nm are due to n-π* transition of non-bonding electrons present on the nitrogen of the azomethine group in the ruthenium(II) complexes. The band observed around 254-260 nm is assigned to ππ* transitions of the ligand. The pattern of the electronic spectra of all the complexes indicate the presence of an octahedral environment around ruthenium(III) similar to that of other ruthenium(II) octahedral complexes [41][42][43].

Emission Spectra
Photoluminescent spectra of the three ruthenium complexes in DMF solution in room temperature is shown in Figure 3. The excitation wavelengths were set to the absorption maxima from the UV-vis absorption spectra, as 525 nm. It can be seen that the broad emission band of complexes ML1, ML2 and ML3 is 604, 609, 613 nm respectively.

Thermal Analysis
The thermal stability data of the complexes are listed in Table 2. The ML1, ML2 and ML3 complexes undergo the similar type of decompositions (Figures  a16-a18).

Electrochemical Properties
The electrochemical properties of the ruthenium complexes were studied in DMSO solution by cyclic voltammetry using a glassy carbon electrode, tetrabutylammonium perchlorate as supporting electrolyte and the potentials are expressed with reference to Ag/AgCl. And the cyclic voltammograms are given in The reduction and oxidation potentials of ruthenium complex (ML1) were measured to be E red = -0.8645V and E ox = 0.9695 V, respectively, and the energy bandgap was 1.834 eV, the energy value of the HOMO was calculated to be -5.3695eV and for LUMO was -3.5355 eV. The oxidation and reduction potentials revealed in cyclic voltammograms showed the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, which correspond to ionization potentials and electron affinities, respectively. Similarly the redox potential values for ML2 and ML3 are determined and tabulated in Table 3 [45].

COMPUTATIONAL RESULTS
As mentioned earlier, all the calculations were applied in B3LYP method in 6-311++G(d,p) basis set. The optimized structure of ruthenium complexes along with labeling of atoms is shown in Figure 5 as ball and stick model. The geometry optimization yields a non planar structure. Also, the most optimized structural parameters of complexes ML1, ML2 and ML3 calculated by B3LYP/6-311++G (d,p) are presented in Table S4.

Electronic Properties
The electronic absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron. Both HOMO and LUMO are the main orbitals that take part in chemical stability. The HOMO-LUMO energies are computed at B3LYP/6-311G(d) level of theory [46]. These orbitals play an important role in the electric properties and determine the way the molecule interacts with other species. Both the HOMO and the LUMO are the main orbital taking part in chemical reaction. While the energy of the HOMO is directly related to the ionization potential, LUMO energy is directly related to the electron affinity. Also, the frontier orbital gap, the energy gap between [tetra butyl ammonium perchlorate as supporting electrolyte, Scan rate = 0.1V/s].   HOMO and LUMO, represents stability of structures and helps to characterize some significant issues including the kinetic stability as well as chemical reactivity of the molecule [47,48]. Chemical hardness is associated with the stability and reactivity of a chemical system. The values of HOMO-LUMO energy gap reflect the chemical activity of the molecule [49]. In order to evaluate the energetic behavior of the title compounds, we carried out calculations for ligand L1 complex ML1. The HOMO and LUMO energy calculated by B3LYP method in 6-311++G(d,p) basis set is presented in Table 4, Figure 6A, 6B shows the frontier orbitals shape. The plots of frontier molecular orbitals for L1 are shown the HOMO is localized on whole the azomethine (N25) nitrogen (-CH=N-NH-), hydrazine nitrogen (N27) and almost the phenolic oxygen (O29), and benzene ring. The weakly contribution on dinitro phenyl ring, The LUMO is localized mainly on the nitro group and dinitro phenyl ring, The weakly contribution on azomethine nitrogen and hydrazine nitrogen.
The frontier energy gap implies an electron density transfer to the benzene ring from the azomethine nitrogen (N25) and hydrazine nitrogen (N27) and phenolic oxygen and also explains the eventual charge transfer occurs within the compound. According to calculation, the energy band gap of HL1 reveals about 3.2293 eV by B3LYP method at the mentioned basis set. The detail result in B3LYP is HOMO energy = -5.6761eV, LUMO energy = -2.4467 eV. The HOMO density of octahedral form of the complex ML1 is distributed over the metallic element (73Ru), phenolic ring, phenolic oxygen (71O), azomethine nitrogen (60N), nitrogen atom in hydrazine (66N), nitro group and two bipyridine ring. The HOMO is mainly localized on phenolic ring, phenolic oxygen (71O), azomethine nitrogen (60N) and nitrogen atom in hydrazine (66N) with strong contribution, on the metal (73Ru) and carbon atoms of two bipyridine ring with a weak contribution, The LUMO is localized on the metal (73Ru), two bipyridine ring, and nitro group (65N, 68O, 69O) with great contribution, on carbon atoms phenolic ring, phenolic oxygen (71O, azomethine nitrogen (60N) and nitrogen atom in hydrazine (66N) with weak contribution. According to calculation, the energy band gap of Complex ML1 reveals about 1.7145 eV by B3LYP method at the mentioned basis set. The detail result in B3LYP is HOMO energy = -3.5074 eV.
LUMO energy = -1.7929 eV HOMO-LUMO energy gap = 1.7145 eV This small energy gap confirms the compounds with high chemical reactivity as well as high polarizability.

Mulliken Atomic Charge
We carried Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system because of atomic charges effect electronic structure, molecular polarizability, and dipole moment more a lot of properties of molecular systems [50]. The charge changes with methods presumably occur due to variation of the hybrid functional. Illustration of atomic charges plotted is shown in Figures a19-a21. In the complex ML1 all the hydrogen atoms have a net positive charge. The central metal (Ru73) atom has positive charge (1.2832e). The obtained atomic charge shows that the H49 atom has bigger positive atomic charge (0.4494e) than the other hydrogen atoms. This is due to the presence of electronegative oxygen atom (O72), the positive charges of hydrogen atom (H49) attracts the oxygen atom (O72). The carbon (C19) atom has bigger negative atomic charge -0.3055e) than the other carbon atoms. This is due to the presence of electronegative oxygen atom (O71, O72). The oxygen (O70) atom of nitro group has bigger negative atomic charge (-0.7185e) than the other oxygen atoms. The charge of the nitrogen atom (N60) in imine group is -0.4633e and the charge of oxygen atom (O71) in phenolic group is -0.5010e. The results illustrate that the charge of the azomethine groups exhibits a negative charge, which are donor atoms. Considering the applied method used in the atomic charge calculation, the oxygen atoms in phenolic group and nitrogen atoms in azomethine groups exhibit a negative charge, which are donor atoms.

Molecular Electrostatic Potential (MEP)
The 3-D plot of molecular electrostatic potential (MEP) maps provides the isosurface values with the location of negative and positive electrostatic potentials. The MESP surface diagram is used to understand the reactive behavior of a molecule, the differences between nuclophilicity and electrophilicity may affect the proton donating or accepting ability of a compound [51]. While the negative electrostatic potential corresponds to an attraction of the proton by the concentrated electron density in the molecule (and is colored in the shades of red on the EPS surface), the positive electrostatic potential corresponds to the repulsion of the proton by atomic nuclei in regions where low electron density exists and the nuclear charge is incompletely shielded (and is colored in shades of blue). MEP was calculated at the B3LYP/6-311G(d,p) optimized geometry of the ligands and complexes [52]. The MEP plots of L1 and ML1 are portrayed in Figure 7A, 7B. The red color is correlated with electron rich area whereas the blue color represents the electropositive sites [52][53][54]. MEP of L1 plots clearly indicate that on the maximum negative (red) region is localized on the azomethine nitrogen (N25) probably due to the attachment with hydrazine nitrogen atom (N27), minimum value of -9.390e-3. maximum positive (blue) region is localized on dinitro benzenene ring due to the hydrogen atoms, with a maximum value of 9.390e-3 and the green represents regions of zero potential.
In EPS of ML1 indicates that on deprotonation of phenolic oxygen (O71), the additional negative charge mostly distributed over bipyridine nitrogen atoms. The negative (red) region is localized on the phenolic oxygen, with a minimum value of -8.416e-3. However, maximum positive (blue) region is localized on the bipyridine nitrogen probably due to the hydrogen atoms, with a maximum value of 8.416e-3, and the green represents regions of zero potential. The optimized structures of complexes and ligands L1, L2, L3, ML2 and ML3 also have similar types of MEP and may have the same interactions, excepting that the molecular electrostatic potential value is varied.

CONCLUSION
We have synthesized and reported the characterization, photophysical, electrochemical and thermal studies of three new ruthenium complexes containing hydrazine schiff base ligands. Spectroscopic techniques including FT-IR, 1 H and 13 C NMR, and mass analysis were used to identify the synthesized products. The spectral results suggest that the presence of an octahedral geometry around the ruthenium metal. The three materials based on ML1, ML2, and ML3 have good stabilities and their thermal decomposition temperatures are 196°C, 192°C, and 198°C respectively. To study the structural and electronic properties of the synthesized molecules, HOMO-LUMO energy gap, the Mulliken atomic charges as well as molecular electrostatic potential of title compounds were determined. All theoretical calculations were carried out by the more popular DFT methods, B3LYP, at 6-31+G(d,p) level of theory. The HOMO-LUMO energy gap as an important value for stability index revealed high chemical reactivity of synthesized compounds in chemical reactions.

ACKNOWLEDGEMENT
One of the authors S. Kamalesu thank Dr. K. Muralidharan, Associate Professor and UGC-Networking Centre, School of Chemistry, University of Hyderabad, for awarding the visiting research fellowship to use the facilities at School of Chemistry, University of Hyderabad, Hyderabad.

SUPPLEMENTARY MATERIALS
The supplementary materials can be downloaded from the journal website along with the article.