Synthesis , Spectral and Electrochemical Properties and DFT Studies of Two Complexes of Ruthenium with Bis-2 , 3-( 2-Pyridyl ) Pyrazine and 4-Picoline

Reaction of bis-2,3-(2-pyridyl)pyrazine (L) with [Ru(dmso)4Cl2] in presence of 4-picoline (L1) affords mixedligand complexes of the type [Ru2L(L1)4Cl4] in which ligand, L binds to the metal center in the bridging, bidentate N,N fashion forming two five-membered chelate rings. Along with [Ru2L(L1)4Cl4], another complex of the type [Ru(L1)2(dmso)2Cl2] is formed. All the complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. DFT studies reveal that the HOMO of the complexes is mostly based on the metal center while the LUMO has maximum contribution from the bonded 4-picoline ligand. Cyclic voltammetry on these complexes shows a Ru(II)-Ru(III) oxidation on the positive side of SCE followed by a reduction of the coordinated 4-picoline ligand on the negative side of SCE.


MATERIALS AND METHODS
Commercial ruthenium trichloride was purchased from Arora Matthey, Kolkata, India.The ligands 4-picoline and bis-2,3-(2-pyridyl)pyrazine were obtained from sigma-aldrich.[Ru(dmso) 4 Cl 2 ] was prepared following a reported procedure 42 .All other chemicals and solvents were reagent grade commercial materials and were used as received.Purification of acetonitrile and dichloromethane, and preparation of tetrabutylammonium perchlorate (TBAP) for electrochemical work were performed as reported in the literature 43,44 .Microanalyses (C, H, N) were performed using a Heraeus Carlo Erba 1108 elemental analyzer.Mass spectra were recorded with a Micromass LCT electrospray (Qtof Micro YA263) mass spectrometer by electrospray ionization method. 1 H NMR spectra were recorded in CDCl 3 solution on a Bruker Avance DPX 300 NMR spectrometer using TMS as the internal standard.IR spectra were obtained on a Shimadzu FTIR-8300 spectrometer with samples prepared as KBr pellets.Electronic spectra were recorded on a JASCO V-570 spectrophotometer.Optimization of ground state structures and energy calculations for all the complexes were carried out by density functional theory (DFT) method using the GAUSSIAN 03 package 45 , where B3LYP was chosen as the basis function and 631g(d,p) basis set was taken for H, C and N, and SDD basis set for Ru.Electrochemical measurements were made using a CH Instruments model 600A electrochemical analyzer.A platinum disc working electrode, a platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were used in the cyclic voltammetry experiments.All electrochemical experiments were performed under a dinitrogen atmosphere.All electrochemical data were collected at 298 K and are uncorrected for junction potentials.

Synthesis of Complexes S y n t h e s i s o f [ R u 2 L ( L 1 ) 4 C l 4 ] a n d [Ru(L 1 ) 2 (dmso) 2 Cl 2 ]
The ligand bis-2,3-(2-pyridyl)pyrazine (28.45 mg, 0.12 mmol) was dissolved in ethanol (40 mL) and 4-picoline (56.53 mg, 0.61 mmol) was added to it followed by the addition of [Ru(dmso) 4 Cl 2 ] (100 mg, 0.30 mmol).The resulting mixture was refluxed for 4 h to produce a yellow solution.Evaporation of this solution gave a yellow solid, which was purified by thin layer chromatography on a silica plate using 1:5 acetonitrile-benzene as the eluant.A major yellow band separated followed by an orangish yellow band which was extracted with acetonitrile.

RESULTS AND DISCUSSION
Reaction of [Ru(dmso) 4    The [Ru 2 L(L 1 ) 4 Cl 4 ] and [Ru(L 1 ) 2 (dmso) 2 Cl 2 ] complexes are readily soluble in dichloromethane, chloroform, acetone, etc., and sparingly soluble in methanol, ethanol, acetonitrile, etc., producing yellow and orangish yellow solutions respectively.Electronic spectra of the complexes have been recorded in dichloromethane solution.Spectra of the complexes are shown in figure 1 and spectral data are presented in table 1.The complexes show several intense absorptions in the visible and ultraviolet region.The absorptions in the ultraviolet region are due to transition within the ligand orbitals.
To have an insight into the nature of absorptions in the visible region, DFT calculations have been performed after the structures of the [Ru 2 L(L 1 ) 4 Cl 4 ] and [Ru(L 1 ) 2 (dmso) 2 Cl 2 ] complexes were optimized.The DFT optimized structure of [Ru 2 L(L 1 ) 4 Cl 4 ] is shown in figure 2 and that of [Ru(L 1 ) 2 (dmso) 2 Cl 2 ] is displayed in figure 3. The computed bond parameters are presented in table 2.

Fig. 1 :Fig. 2 :Fig. 4 :Fig. 5 :Fig. 6 : 1 Current
Scheme 1: Reaction of bis-2,3-(2-pyridyl)pyrazine with [Ru(dmso) 4 Cl 2 ] in refluxing ethanol doublets and a triplet also appeared in an overlapped manner in the region 8.41-8.43ppm and 7.75-7.80ppm.The spectrum of [Ru(L 1 ) 2 (dmso) 2 Cl 2 ] shows a distinct peak for the methyl protons of the bonded 4-picoline group at 2.46 ppm.Two clear doublets corresponding to the aromatic protons of the ligand were seen at 8.18 and 7.10 ppm.The methyl protons of the coordinated dimethylsulfoxides also appear as two distinct singlets around 3.10 ppm and 3.13 ppm.Thus the 1 H NMR spectra of the complexes correlates well with their compositions.
c Shoulder.df DE p = (E pa -E pc ).g E pc value.[