Mono and binuclear ruthenium(II) complexes containing 5-chlorothiophene-2-carboxylic acid ligands: Spectroscopic analysis and computational studies

Accepted Manuscript Mono and binuclear ruthenium(II) complexes containing 5-chlorothiophene-2carboxylic acid ligands: Spectroscopic analysis and computational studies Kalaiyar Swarnalatha, Subramaniam Kamalesu, Ramasamy Subramanian PII:

S0022-2860(16)30728-1

DOI:

10.1016/j.molstruc.2016.07.048

Reference:

MOLSTR 22751

To appear in:

Journal of Molecular Structure

Received Date: 4 May 2016 Revised Date:

12 July 2016

Accepted Date: 13 July 2016

Please cite this article as: K. Swarnalatha, S. Kamalesu, R. Subramanian, Mono and binuclear ruthenium(II) complexes containing 5-chlorothiophene-2-carboxylic acid ligands: Spectroscopic analysis and computational studies, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.07.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Mono and Binuclear Ruthenium(II) Complexes containing 5-chlorothiophene-2-carboxylic acid ligands: spectroscopic analysis and computational studies Kalaiyar Swarnalathaa*, Subramaniam Kamalesua, Ramasamy Subramanianb Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli-12, Tamilnadu, India.

b

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a

Centre for Scientific and Applied Research, School of Basic Engineering and Sciences, PSN College of

Engineering and Technology, Tirunelveli, Tamil Nadu 627012, India

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Abstract

New Ruthenium complexes I, II and III were synthesized using 5-chlorothiophene-2-carboxylic acid (5TPC), as

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ligand and the complexes were characterized by elemental analysis, FT-IR, 1H, 13C NMR, and mass spectroscopic techniques. Photophysical and electrochemical studies 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 and Mulliken atomic charges of the molecules are determined at the B3LYP method and standard 6-311++G (d,p) basis set starting from

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optimized geometry. They possess excellent stabilities and their thermal decomposition temperatures are 185 °C, 180 °C and 200 °C respectively, indicating that the metal complexes are suitable for the fabrication processes of optoelectronic devices.

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Keywords: Ruthenium complexes, Optimized structures, DFT calculation.

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1. Introduction Polypyridyl complexes of ruthenium(II) and their photochemistry have been widely studied, particularly with regard to potential application to storage of light energy, light activated switches, solar energy conversion,

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catalysis, and photoinduced chemical reactions [1, 2]. The easy modification of its photophysical and electrochemical properties, achieved by varying the coordination environment, makes these compounds more attractive as photosensitizers, dyes in emissive layers in OLEDs, photocatalysts, and also as biophysical sensors and immunoassays [3-8]. The electron transfer reactivity of the MLCT (metal to ligand charge transfer) excited states in

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polypyridyl-based complexes have long been used in many processes including electron transfer reagents and as dyes for solar photocells [9]. Dye-sensitized solar cells (DSSCs) have recently received great attention because of

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their ease of fabrication and cost-effectiveness compared to silicon (Si) based photovoltaic devices [10, 11]. Numerous reports on ruthenium-based dye sensitized solar cells have indicated that the conversion efficiency of absorbed photons is sufficiently high, and offers little room for improvement [12, 13]. Thus, research efforts should be focused on improving photon absorption and charge injections into the conduction band of the semiconductor. It can be achieved by manipulating the dye’s molecular structure thereby tuning its electronic and optical properties, which either increases the degree of absorption of incident photons within the functional wavelength, or extend the

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functional range within near infrared range [14]. The central metal atom of an organometallic and coordination complexes can readily coordinate with conjugated ligands and undergo metal–ligand orbital overlap facilitating effective electronic transport and electronic transitions between the metal ion and the ligand, leading to large

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changes in the dipole moment between the excited states [15]. Diruthenium(II,III) chemistry has been actively studied and numerous tetracarboxylatediruthenium(II,III) compounds have been synthesized [16]. Numerous dyes

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designed for DSSCs, the ruthenium-based sensitizers incorporating thiophene derivatives have been proven to be excellent candidates to realize highly efficient and robust devices [17-19]. Another important dye employing thiophene derivatives is CYC-B1, which exhibits a remarkably high light-harvesting capacity of up to 2.12 x 104 L mol-1cm-1 [20, 21]. After the development of the CYC-B1 dye, several ruthenium dyes were synthesized by incorporating thiophene derivatives into the ancillary ligand and DSSC cells based on these dyes exhibited excellent photovoltaic performances [21]. 5-chloro thiophene-2-carboxylic acid is an excellent ligand, which can coordinate to metal ions in bidentate chelating and bidentate bridging form [22] and have biological activity. Piotr Drozdzewski [23] reported that several thiophene derivatives are known to show biological activities in point of pharmaceutical

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and clinical aspects. The major role of 2-thiophene carboxylates is connected with drug production. From the clinical point of view, thiophenecarboxylates have also been interesting for treatment of osteoporosis as the inhibitors of bone resorption in the tissue culture [23]. It has been found that the biological activities of 2-thiophene

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carboxylic acid are considerably increased when they are bonded into metal complex molecules [22]. In the present paper, the selective synthesis and characterization of a 5-chlorothiophene-2-carboxylic acid ligand containing ruthenium mononuclear and binuclear complexes are reported and also carried out the DFT studies. The synthesized compounds have theoretically investigated by using the more popular DFT method,

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B3LYP, in 6-311++G(d,p) basis set. In order to make sense between the experimental and theoretical results, the electronic properties such as HOMO-LUMO energies, and Mulliken atomic charges were calculated and the results

2. Experimental details 2.1. Materials and Instrumentation

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were correlated.

Commercially available RuCl3.3H2O was used as supplied from Himedia Pvt. Ltd. 2,2'-bipyridine, 5-chloro thiophene 2-carboxylic acid (5-tpc), lithium chloride were purchased from Sigma–Aldrich and were used as received. All the reagents used were chemically pure and analytical grade. Reagent grade organic solvents were

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purified and dried by recommended procedures [25] and degassed before use. The supporting electrolyte, tetra butyl ammonium perchlorate (n-BuNClO), was dried in vacuum prior to use. Cis-Ru(bpy)2Cl2.2H2O was prepared following a published procedure [26]. Elemental analysis was done using a Perkin-Elmer elemental analyzer. IR

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spectra were recorded as KBr pellets in the 400–4000 cm-1 region using a JASCO FT-IR 410 spectrometer. The electronic spectra were recorded on a Perkin Elmer Lambda-25 UV–Vis spectrometer. 1H NMR spectra was

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recorded on a Bruker AV III 400 MHZ instrument using TMS as an internal reference. Electron ionization mass spectrum of the compound was recorded on a JEOL GCMATEII mass spectrometer. The emission spectrum was 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 monometallic and bimetalic ruthenium complexes were studied by thermogravimetric analysis (TGA) using NETZSCH STA 409 C/CD TGA instrument between 25 ºC and 1000 ºC in nitrogen atmosphere at a heating rate of 10 ºC min-1.

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2.2. Synthesis of monometallic complex Ru(5-tpc)(H2O)2Cl2 [I] [Ru(5-tpc)(H2O)2Cl2] complex was prepared by the addition of Ru(III) chloride hydrate (0.100g, 1.0mmol) to a methanolic solution of 5-chloro thiophene-2-carboxylic acid, (0.062g, 1mmol) (Scheme 1). The resulting mixture

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was refluxed under a nitrogen atmosphere for 8 hours. After refluxing the brown solution turned green. The resulted solution was kept in the refrigerator for overnight. Dark black-green precipitate was formed, filtered off and then washed with diethyl ether (Yield 70%). Analytical calculation for Ru complex: C(16.25%); H(1.64%); S(8.68%).

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Found: C(15.18%); H(1.63%); S(8.90%).

Scheme 1 Synthetic route of ruthenium complexes I, II and III

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2.3 Synthesis of [Ru(bpy)2(5-tpc)] [II] Ru(bpy)2Cl2 (0.200g, 1.0mmol) was refluxed in 40ml CH2Cl2/ethanol (80:20) with 1:2 equivalent of 5-chloro thiophene-2-carboxylic acid ligand (0.067g, 1.0mml) about 6 hours, the initial purple solution turned red , was dried,

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and the product was precipitated from acetone / diethyl ether (Yield 72%). Analytical calculation for Ru complex: C(52.22%); H(3.16%); N(9.74%); S(5.58%). Found: C(51.90%); H(3.60%); N(9.22%); S(5.64%). 2.4 Synthesis of bimetallic complex [Ru2(5-tpc)4(H2O)2] [III]

[Ru2(5-tpc)4(H2O)2] was prepared by the addition of Ru(III) chloride hydrate (0.200g, 1.0 mmol) to a methanolic

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solution of 5-chloro thiophene 2-carboxylic acid,( 0.249g, 2.0mmol) (Scheme 1). The resulting mixture was refluxed under a nitrogen atmosphere for 8 hours. After refluxing the brown solution was turned green and kept in the

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refrigerator for overnight. The formed dark black-green precipitate was filtered off and then washed with diethyl ether (Yield 68%). Analytical calculation for Ru complex: C(27.16%); H(1.37%); S(14.50%). Found: C(26.53%); H(1.36%); S(14.43%). 2.5. Computational Methods

All calculations were performed using Gaussian 09 software package [27], at the B3LYP/6-311++G(d,p) level of theory. The DFT methods are more advantageous owing to their accuracy and low computational cost. These

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properties make DFT more practical and feasible for the computations of different molecules [28-31]. 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)

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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 [32]. The chemical

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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 [33], electron affinity [33], electronegativity [34], electronic chemical potential [34], molecular hardness [35], molecular softness [35], and electrophilicity index [36] using the following equations: ionization energy (IE) = -ߝHOMO, electron affinity (EA) = -ߝLUMO, electronegativity (߯) = (IE + EA)/2, LUMO electronic chemical potential (ߤ) = -߯, chemical hardness (ߟ)=(IE- EA)/2, chemical softness (ߪ) = 1/ߟ, electrophilicity index(߱) = ߤ2/2ߟ [32].

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3. Results and discussion 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, methanol, ethanol, DMF and DMSO.

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3.1. FT-IR spectral features The coordination mode of the 5-chlorothiophene-2-carboxylic acid ligand to the ruthenium(II) centre in the complexes was investigated by using FT-IR spectroscopy (Table 1). The samples were studied as powder dispersed in KBr pellets. This comparison allowed to identify the sites involved in the formation of bonds in the complex. The

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IR spectra of the free ligand display a band at 1697cm-1 corresponding to the carboxylate group. IR spectra of the new complexes I,II,III shows the symmetric and asymmetric stretching vibration of carboxylates in the range of

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1330-1490 cm-1 and 1440-1650 cm-1, which is the indicative of the coordination of the carboxylate oxygen atom to the metal ion [10, 37], the asymmetric and symmetric stretching bands of carboxylate groups are shifted to lower frequency for all the complexes, which reveals the formation of a linkage between the metal ion and carboxylato oxygen atom (Fig. 1). Moreover, the difference (<150 cm-1) between the asymmetric and symmetric stretching modes indicates the bidentate binding of the carboxylate group in the complexes [38,39]. Fig. 2 shows the possible modes of coordination of the carboxylic acid. The new broad band appeared at region 3500-3300 cm-1[40, 41] can

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be attributed to the stretching vibration of the coordinated water molecules. The ruthenium-oxygen bonding is confirmed by the newly formed band that appear at 450-475 cm-1 is assigned to the M-O vibration and the mode of coordination is found to be given in Fig 2(b) [39, 42]. The observed bands in the region 460–475 cm -1 in the mono

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nuclear complexes were tentatively assigned to the ν M-Cl vibrations [43]. The stretching frequencies of the

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complexes are listed in Table 1.

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Fig. 1 FT-IR Spectrum of complexes I (a), II (b), and III (c)

Fig. 2 Possible mode of coordination of carboxylic acid in Ruthenium complexes

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Table 1 FT-IR Spectral data of complexes I, II and III vasym (COO-)

vsym (COO-)

v(H2O)

v(M-O)

v(M-Cl)

5TPC (L)

1697

1675

-

-

-

RuL(H2O)2Cl2

1569

1482

3397

Ru(bpy)2L

1568

1481

-

Ru2L4(H2O)2

1563

1432

3362

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Compound

472

457

-

474

-

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456

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3.2. 1H and 13C NMR spectra analysis

NMR spectra of the ligand and the complexes were recorded in DMSO-d6 solution and the 1H NMR spectra of the free ligand showed a singlet in the region δ 11.2 for the carboxylic acid COOH proton, whereas doublet in the region δ 6.80–8.00 for the thiophene CH proton in the spectra of the ligand. In the 13C NMR spectra of the ligand, the signal at δ 167 corresponds to the COOH carbon and the signal at signal in the region δ 125–138 is due to the

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aromatic carbon atom.

Multiplets observed in the region δ 8.5 - 7.0 ppm in all the complexes I and II have been assigned to the aromatic protons of thiophene ligands. The broad singlet that appeared for the -COOH proton of the free ligand in

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the region δ 11.2 ppm is absent in all the complexes, supporting coordination of the ligand to the Ru(II) ion via the carboxalate oxygen. The signal observed in the region δ 4.8 for the H2O proton [44]. The 13C NMR of all the Ru(II)

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complexes showed resonances in the expected regions (Fig. a1-a4, Supporting material). The complexes (δ 162-161 ppm) revealed a upfield shift of the carboxalate carbon (COO) relative to the free ligands O-C=O (δ 167 ppm) indicating coordination of the carboxalate oxygen to the metal centre results from the reduced bond order (O-C-O) upon coordination [45].

3.3. Mass spectral analysis

The mass spectrum of the ruthenium(II) complexes is in good agreement with the proposed molecular structure and the mass spectrum of the complex is shown in Fig. 3a-3c. The molecular ion peak, [M+H] appears at m/z = 370.01, 574.99 and 885.4 confirms the stoichiometry of the complexes I, II and III respectively.

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Fig. 3 Mass spectrum of the complexes I (3a), II (3b), and III (3c)

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3.3. Electronic spectra Absorption and emission spectral features The electronic absorption spectra of the free ligand and the corresponding ruthenium complexes were measured in

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methanol solution (C=1×10-4 M) and are shown in Fig. 4a. The spectrum displayed an absorption maximum at 252 nm for the free ligand, which is attributable to π–π* absorption of the aromatic ring. The λ max value of the ruthenium complexes I,II, III are 255, 272,269, 275, 285, 295 nm respectively which is attributed to π–π*/LLCT transition , the band at 332 nm (I), 336 nm (II), 386 nm (III) which have been assigned to metal to ligand charge

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transfer transitions with admixture of d-d character [3, 46]. The band at 525,670 nm is assigned to pure MLCT transition [20]. The photoluminescence spectra of the three ruthenium complexes in DMF solution in room

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temperature is shown in Fig. 4b, It can be seen that the broad emission band of complexes I, II and III is 400, 420,

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620 nm respectively on excitation at their corresponding absorption maximum.

Fig. 4a Absorption spectrum of the complexes I, II and III (DMF)

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3.4 Thermal analysis

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Fig. 4b Emission spectrum of the complexes I, II and III. (Excitation at 332, 525 and 336 nm)

The thermal stability of the complexes was analyzed and the data are listed in Table 2. The [Ru(L)(H2O)2(Cl)2] I and [Ru2(L)4(H2O)2] III complexes undergo the same type of decompositions mainly in two stages (Fig. a5-a7,

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Supporting material). The complex [Ru(bpy)2L] decomposition in single stage. The first stage taking place in the 185–270 °C range corresponds to the dehydration of two coordinated water molecules. The final decomposition step

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is represented by the total removal of the organic ligand moiety in the 380–525 °C range with the formation of the corresponding metal oxide as the final product. The single decomposition step is represented by the total removal of the organic ligand moiety in the 200–525 °C range with the formation of the corresponding metal oxide as the final product. The TG curve of the complex I shows a weight loss 30.2% (calculated – 29.0%) in the temperature range 185–270 °C. This is due to the loss of coordinated water molecules. The second decomposition step of the complex is in the temperature range 380-520°C bringing a weight loss of 34.3% (calculated 35.1%) which corresponds to the loss of organic ligand. Above this temperature, a horizontal thermal curve has been observed due to the formation of the metal oxide. The TG curve of the complex II shows a weight loss 5.2% (calculated – 4.1%) in the temperature

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range 180-260 °C. This is due to the loss of coordinated water molecules. The second decomposition step of the complex is in the temperature range 380-520°C bringing a weight loss of 67.7% (calculated 65.5%) which correlates with the loss of coordinated organic ligand. The TG curve of the complex III shows a weight loss 76.5% (calculated

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– 75.4%) in the temperature range 200-525 °C. This is due to the loss of coordinated organic ligand molecule. The corresponding mass loss is due to the decomposition of the organic ligand molecule and it is in agreement with the calculated mass loss. The final residue is qualitatively proved to be anhydrous metal oxides [39, 41]. The proposed

% weight loss Obs.(calcd.)

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Table 2 Thermogravimetric data of ruthenium complexes Complexes Temperature Range T(˚C)

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structure of the metal complexes received further support from the thermal studies.

Process

185-270, 380-520, >520

30.2(29.0) 34.3(35.1) 35.5(36.0)

-2H2O(coord.), loss of organic moiety, Ru2O3

Ru(bpy)2L

200-525 >525

76.5(75.4) 23.5(24.5)

loss of organic moiety, Ru2O3

Ru(L)4(H2O)2

180-260, 380-520, >520

5.2(4.1) 67.7(65.5) 29.4(28.2)

-2H2O(coord.), loss of organic moiety, Ru2O3

4.2. Computational results

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Ru(L)(H2O)2(Cl)2

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As mentioned earlier, all the calculations were carried out 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 Fig. 6 as ball and stick model.

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The geometry optimization yields a non planar structure [24]. Also, the most optimized structural parameters of complexes I, II and III calculated by B3LYP/6- 311++G(d,p) are presented in Table 3.

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Fig. 6 Optimized structure of Ligand and complex I, II, III within numbering of atoms obtained at B3LYP/6-311++G(d,p) level of theory

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Table 3 Calculated structural parameters of the complexes I, II, III Bond length(A°)

Value (A°)

Bond angle(A°)

Value (A°)

18O-19Ru

2.1075

16O-19Ru-17O

62.8139

15O-19Ru

2.1073

7Cl-19Ru-8Cl

17O-19Ru

2.1293

8Cl-19Ru-18O

16O-19Ru

2.1241

7Cl-19Ru-15O

8Cl-19Ru

2.3795

7Cl-19Ru

2.3809

97.2388

88.5853 88.5812

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Complex II

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Complex I

1.5041

50O – 51Ru – 49O

70.9388

51Ru – 49O

1.89008

47N – 51Ru – 45N

90.0156

51Ru – 45N

1.8516

45N – 51Ru – 48N

77.2996

51Ru – 46N

1.9254

51Ru – 47N

1.9254

51Ru – 48N

1.9261

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51Ru – 50O

Complex III

2.3339

41O-48Ru-46O

87.30139

48Ru - 46O

2.13402

43O-48Ru-38O

87.31559

48Ru - 46O

2.07581

44O-47Ru-37O

87.46354

47Ru -37O

2.1326

45O-47Ru-42O

87.21999

47Ru -42O

2.0762

41O-48Ru-39O

73.67206

48Ru -41O

2.1342

37O-47Ru-40O

73.73674

48Ru -39O

2.1892

45O-47Ru-40O

73.59516

48Ru -38O

2.0763

43O-48Ru-39O

73.58064

47Ru -44O

2.0758

47Ru -45O

2.1324

47Ru - 40O

2.1899

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48Ru - 47Ru

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4.2.1. Electronic properties The electronic absorption corresponds to the transition of electrons 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

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unoccupied molecular orbital (LUMO). The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are computed at B3LYP/6-311++G(d,p) level of theory [47]. These orbitals play an important role in the electric properties and determine the way the molecule interacts with other species. Both the HOMO and LUMO are the main orbitals taking part in chemical reaction. The energy of the

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HOMO is directly related to the ionization potential and the LUMO energy is directly related to the electron affinity. Also, the frontier orbital gap, the energy gap between HOMO and LUMO, represents stability of structures and

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helps to characterize some significant issues including the kinetic stability as well as chemical reactivity of the molecule [48, 49]. Chemical hardness is associated with the stability and reactivity of a chemical system. The larger the HOMO-LUMO energy gap, the harder and more stable and less reactive the molecule. The smaller the HOMOLUMO energy gap, the softer and lesser stable and more reactive the molecule. The values of LUMO HOMO energy gap reflect the chemical activity of the molecule [32, 50].

In order to evaluate the energetic behavior of the title compounds, we carried out calculations for compound 1. The

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HOMO and LUMO energy calculated by B3LYP method in 6-311++G(d,p) basis set is presented in Table 4, Fig. 7, Fig. 7A, shows the frontier orbitals shape. The HOMO density of octahedral form of the complex is distributed over the metallic element (Ru19), carboxalate oxygen (O16, O17) atoms, chlorine (Cl7, Cl8), oxygen atoms in H2O and

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the thiophene ring. The HOMO is localized also on the metal (Ru19) but with a weak contribution, on the carboxalate oxygen (O16, O17), chlorine (Cl7, Cl8), and thiophene group with a great contribution. The LUMO is

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localized mainly on the metal (Ru19), carboxalate oxygen (O16, O17) and chlorine (Cl7, Cl8). Using B3LYP method, the energy bandgap of the complex I was found to be 2.119 eV and the HOMO and LUMO energies are 5.5606 eV and

-3.4415 eV respectively. This small energy gap confirms the compounds with high chemical

reactivity as well as high polarizability. The various energetic properties of complexes I, II and III are listed in Table 4.

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Fig. 7 Atomic orbital compositions of the frontier molecular orbital of complex I

Fig. 7A Frontier molecular orbitals for ligand and the three complexes: (a) HOMO of ligand (b) LUMO of ligand, (c) HOMO of complex I , (d) LUMO of complex I, (e) HOMO of complex II, (f) LUMO of complex II, (g) HOMO of complex III, (h) LUMO of complex III.

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Table 4 Energetic properties, dipole moments, energies of HOMO, LUMO orbitals and energy gap.

I

-5.5606

-2.1456

-3.4415

Energy gap

4.1559

2.1191

µ

(η)

(σ)

(ω)

Etotal

Dipole moment

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(a.u.)

(Debye)

6.3015

2.145 6

4.2235 5

4.2235 5

2.0779 5

0.4812

4.2923

-1200.89

0.514

5.5606

3.441 5

4.5010 5

4.5010 5

0.9438

9.5604

-6685.88

2.6116

2.9455 5

2.9455 5

2.3001 5

0.4348

1.8860

-6590.19

8.2054

3.4947

-3.4947

0.6051

1.6526

10.0917

-13776.8

0.061

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-6.3015

LUMO (eV)

(χ)

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Ligand

HOMO (eV)

EA

1.0595 5

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Compound

IP

II

-5.2457

-0.6454

4.6003

5.2457

0.645 4

III

-4.0998

-2.8896

1.2102

4.0998

2.889 6

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Table 5 Mulliken atomic charge of complex I Atom

Charge

Atom

Charge

C

-0.56219

11

H

0.44031

2

C

0.55115

12

H

0.42732

3

C

-0.10588

13

H

0.4248

4

C

-0.62443

14

H

0.4038

5

C

-0.14479

15

O

-0.59233

6

Cl

0.26087

16

O

-0.4507

7

Cl

-0.34841

17

O

-0.44253

8

Cl

-0.40019

18

O

-0.57611

9

H

0.22265

19

Ru

0.50297

10

H

0.26695

20

S

0.74674

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4.2.2 Mulliken atomic charge Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system because atomic charges affect dipole moment, molecular polarizability, electronic structure and lot

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of properties of molecular systems [51]. The calculated Mulliken charge values of ruthenium complexes I, listed in Table 5. The charge changes with methods presumably occur due to variation of the hybrid functionals. Illustration of atomic charges plot is shown in Fig. 8. The central metal (Ru19) atom has positive charge (0.5029e) and all the hydrogen atoms have a net positive charge. The obtained atomic charge shows that the H11 atom has bigger positive

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atomic charge (0.4403e) than the other hydrogen atoms. This is due to the presence of electronegative oxygen atom (O15), the hydrogen atom (H11) attracts the positive charge from the oxygen atom (O15). The carbon (C4) atom has

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bigger negative atomic charge (-0.6244e) than the other carbon atoms. This is due to the presence of electronegative chlorine atom (5Cl). The oxygen (O15) atom has bigger negative atomic charge (0.5923 e) than the other oxygen atoms. The results illustrate that the charge of the oxygen atoms in carboxylate groups exhibits a negative charge, which are donor atoms. Considering the applied method used in the atomic charge calculation, the oxygen atoms

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(donor atoms) in water and carboxylate groups exhibit a negative charge.

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5. Electrochemical properties

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Fig. 8 Mulliken atomic charge distribution of ligand (8a), complex I (8b), complex II (8c), complex III (8d),

Fig. 5 shows the cyclic voltammograms of the synthesized ruthenium complexes in DMSO solution by cyclic voltammetry using a glassy carbon electrode, tetrabutylammonium perchlorate as supporting electrolyte and the

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potentials are expressed with reference to Ag/AgCl. The oxidation and reduction potentials revealed in cyclic voltammograms showed the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital

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(LUMO) levels. From the oxidation potentials (Eox) and the reduction potentials (Ered) of the complexes, HOMO, and LUMO energy levels as well as the energy gap of the complexes were calculated according to the following equations, the corresponding electrochemical data are listed in Table 6. HOMO = - (Eox + 4.40) (eV) LUMO = - (Ered + 4.40) (eV) The reduction and oxidation potentials of ruthenium complex (I) were measured to be Ered = 0.5961V and Eox

=

1.1043 V, respectively, and the energy bandgap was 1.7004 eV, the energy value of the HOMO was calculated to be -5.5043eV and the energy value of the LUMO was calculated to be -3.8039 eV [52]. The energy levels calculated

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from DFT studies for complex I was in good agreement with the experimental data and slight variation are observed

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in complex II & III and are due to the solvent influence (DMSO).

Fig. 5 CV curves of complex I, II, and II measured in DMSO solution. [tetra butyl ammonium perchlorate as supporting electrolyte, Scan rate = 0.1V/s]

Complexes

Eoxonset (V)

Eredonset (V)

HOMO (eV)

LUMO (eV)

Eg (eV)

1.1043

-0.5961

-5.5043

-3.8039

1.7004

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I

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Table 6 Cyclic voltammetric results of complexes I, II, and III

II

1.0438

-0.5095

-5.4438

-3.8905

1.5533

III

1.1569

-0.6453

-5.5569

-3.7547

1.8022

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6. Conclusion The syntheses, characterization, photophysical, electrochemical and theoretical studies of three new ruthenium complexes containg 5-chlorothiphene carboxylic acid as ligand were reported. Spectroscopic techniques including

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FT-IR, 1H and 13C NMR, and mass analysis were used to identify the products. The synthesized complexes I, II, and III have good stabilities and their thermal decomposition temperatures are 185° C, 180° C, and 200° C respectively. HOMO-LUMO energy gap as well as the Mulliken atomic charges of the title compounds was determined. All the theoretical calculations were carried out by the DFT method, B3LYP, at 6-311++G(d,p) level of theory. The

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HOMO-LUMO energy gap is an important value for stability index revealed high chemical reactivity of synthesized compounds in chemical reactions and the theoretical and experimental values 2.1191 eV and 1.7004 eV for complex

Acknowledgement

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I is in good correlation.

We thank Department of Science and Technology, New Delhi, India for financial support in the form of research grant (DST Project No.SR/FT/CS-53/2010). Reference

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Figure captions Scheme 1 Synthetic route of ruthenium complexes I, II and III Fig. 1 FT-IR Spectrum of complexes I (a), II (b), and III (c)

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Fig. 2 Possible mode of coordination of carboxylic acid in Ruthenium complexes Fig. 3 Mass spectrum of the complexes I (3a), II (3b), and III (3c) Fig. 4a Absorption spectrum of the complexes I, II and III (DMF)

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Fig. 4b Emission spectrum of the complexes I, II and III. (Excitation at 332, 525 and 336) Fig. 5 CV curves of complex I, II, and II measured in DMSO solution. [tetra butyl

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ammonium perchlorate as supporting electrolyte, Scan rate = 0.1V/s]

Fig. 6 Optimized structure of Ligand and complex I, II, III within numbering of atoms obtained at B3LYP/6-311++G(d,p) level of theory

Fig. 7 Atomic orbital compositions of the frontier molecular orbital of complex I

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Fig. 7A Frontier molecular orbitals for ligand and the three complexes: (a) HOMO of ligand (b) LUMO of ligand, (c) HOMO of complex I , (d) LUMO of complex I, (e) HOMO of complex II, (f) LUMO of complex II, (g) HOMO of complex III,

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(h) LUMO of complex III.

Fig. 8 Mulliken atomic charge distribution of ligand (8a), complex I (8b), complex II (8c),

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complex III (8d),

Supplementary Material Fig. a1–a2

1

Fig. a3–a4

13

Fig. a5-a7

H NMR spectra of the ruthenium(II) complexes I and II C NMR spectra of the ruthenium(II) complexes I and II

TGA curve of the complex I. II and III

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Mono and Binuclear Ruthenium(II) Complexes containing 5-chlorothiophene-2-carboxylic acid ligands: spectroscopic analysis and computational studies

Highlights

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Manuscript ID: MOLSTRUC-D-16-00936


New mono- and binuclear Ru(II) complexes containing 5-TPC ligands were synthesized

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DFT calculations were performed using B3LYP/6-311++G(d,p) level of theory

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Orbital gap, HOMO, LUMO energies and Mulliken atomic charges were studied