Synthesis and Spectral Studies of Ru(II) Carbonyl Schiff Base Complexes

The synthesis and characterization of hexa co-ordinated ruthenium(II) complexes of the type [Ru(CO)(B)(L)] (where B = PPh 3 , AsPh 3 , py or pip and L = dibasic tetradentate Schiff base ligand) were synthesized from the reaction of [RuHCl(CO)(B)(EPh 3 ) 2 ] (where E = P, B = PPh 3 , py or pip, E = As, B = AsPh 3 ) with different tetradentate ligands. The ligands were derived by the condensation of 5-chloro-4-methyl-2-hydroxy benzophenone with ethylenediamine, propylenediamine and o -phenylene-diamine in 1:1 molar ratio. All the compounds have been characterized by elemental analysis and spectral (FT-IR, UV-VIS and 1 H-NMR) methods. An octahedral environment around Ru(II) ion has been tentatively proposed for all the complexes


INTRODUCTION
The interest in the synthesis and characterization of transition metal complexes containing Schiff bases lie in their biological and catalytic activity in many reactions 1,2 . Transition metal complexes, with ligands containing both hard and soft donor groups, have been used extensively in coordination and organometallic chemistry. The majority of such ligands are functionalized phosphines where the phosphorus is a soft donor and either oxygen or nitrogen is a hard donor 3,4 .
Over the past few years N 2 S 2 and N 2 O 2 Schiff base ligands have been extensively investigated with regard to their numerous applications in organic synthesis as well as medicine 5 . Schiff bases have played a special role as chelating ligands in main group and coordination chemistry, mainly due to their stability under a variety of oxidative and reductive conditions ease of preparation, and to the fact that imine ligands are borderline between hard and soft Lewis bases [6][7][8] .
Schiff base complexes have been particularly valuable for designing complexes of widely different structural type. The possibility of having lone pair of electrons in either  or sp 2 hybridized orbital or trigonally hybridized nitrogen in the azomethine group is of fundamental chemical and biological importance.
A large number of transition metal phosphine complexes have been used in catalysis due to the characteristic steric and electronic properties of the tertiaryphosphine ligands 9 In the past decade there has been a growing interest in reactions that include these type of ligands and complexes, which have proved to be active catalysts in processes such as the hydrogenation of aldehydes 10 and aromatic ketones 11 or the carbonylation of metal methyl bonds 12 , as therapeutic agents in anticancer drugs 13 or as structural fragments or polystyreneimmobilized catalysts 14 .
Further, the interest in the chemistry of ruthenium [15][16][17][18][19] is primarily due to its fascinating redox, electron transfer, and energy transfer properties and its ability to have a wide range of oxidation states (-2 to +8). Due to these characteristics large variety of ruthenium complexes have found wide applications including artificial photosynthesis, photo molecular devices electron transfer properties of proteins and DNA, catalytic oxidation of water and organic substrates. Furthermore, transition metal complexes of tetradentate Schiff base ligands find applications in catalysis [20][21][22] and as biomimetic enzyme models [23][24][25][26][27] .
In view of growing interest in the Schiff base metal complexes, the synthesis and characterization of a series of ruthenium(II) carbonyl complexes containing Schiff bases and triphenylphosphine/arsine have been carried out in this paper.

EXPERIMENTAL METHODS
Commercially available RuCl 3 ·3H 2 O was used as supplied from Loba Chemie. All the reagents used were chemically pure and annular grade. The solvents were freshly distilled before use. The analysis of carbon, hydrogen and nitrogen were performed at Sophisticated Test and Instrumentation Centre (STIC) Cochin University, Kochi. Infra-red spectra of complexes were recorded in KBr pellets with a Perkin -Elmer 597 spectrophotometer in the range 4000-400 cm -1 . Electronic spectra of the complexes were recorded in CHCl 3 solution with a Cary 300 Bio UV-Vis Varian spectrophotometer in the range 800-200 nm. The 1 H-NMR spectra were recorded in CDCl 3 with Bruker 400MHz instrument using TMS as internal reference. Melting Point was recorded in the Boeties micro heating table and is uncorrected.

Preparation of Schiff base ligands
Scheme I: Preparation of 4-chloro-3-methyl phenyl benzoate 7.13 g of 4-chloro-3-methyl phenol was dissolved in 75 ml of 10 % NaOH solution contained in a reagent bottle or conical flask of about 200 ml capacity. Add 8.64 ml of redistilled benzoyl chloride was added, cork the vessel securely and the mixture was shaken vigorously for 15-20 min. At the end of this period the reaction was usually completed and the solid product was obtained. Filtered if the solid ester with solution breaks up if any lumps on the filter, washed thoroughly with water and drained well. The crude ester was recrystallized from rectified spirit, a quantity of hot solvent was used trice the minimum volume required for complete solution.
The product was filtered as a colourless crystals.

Scheme II: Preparation of 5-chloro-4-methyl-2-hydroxybenzophenone
Anhydrous AlCl 3 (7.5 g) was heated in an oil bath at 140 °C for 5 min and the compound 4-chloro-3-methyl phenyl benzoate (2 g) was added with stirring. The temperature was allowed to rise to 170-180 °C and maintained for 20 min. After cooling, the compound was added to 200 ml of 2N HCl.
The insoluble material was filtered, washed and dried. The crude product was recrystallised from hot ethanol to give pale green needle shape crystals.
The reaction mixture colour changed gradually and the reactions were monitored by TLC. The solution was concentrated to about 3 ml and pet. ether (60-80 C) was added to obtain solid which was separated by filtration. The crude product of the resulting complexes was purified by column chromatography using chloroform/acetone mixture as eluent. On evaporation of the resulting solution, pure compounds were obtained which was dried under vacuum. Using similar procedure the following complexes were prepared.

Reflux
International Letters of Chemistry, Physics and Astronomy Vol. 8

(2) [Ru(CO)(py)(L 1 )]
This is prepared from [RuHCl(CO)(py)(PPh 3 ) 2 ] (100 mg) and L1 (67.45 mg) in benzene (20 ml). Yield: 33 mg, Colour: Green, Melting point: 143 °C.   All the new tetradentate Schiff base ruthenium(II) carbonyl complexes are green and brown in colour, stable in air, non-hygroscopic in nature and highly soluble in common solvents such as dichloromethane, acetonitrile and chloroform producing intense green and brown solutions. The analytical data are given in Table 1 and are good agreement with general molecular formula proposed.

FT-IR SPECTRA
In order to study the binding mode of the Schiff bases to ruthenium in the new complexes, the IR spectra of the Schiff bases were compared to the spectra of the ruthenium(II) complexes.
The free Schiff bases show a very strong absorption around 1615-1600 cm -1 which is characteristic of the azomethine (C=N) group 1 . If the Schiff bases coordinate through the nitrogen atom, it is expected that there must be a reduction in the azomethine (C=N) frequency due to the lowering of electron density upon coordination. In the IR spectra of all the complexes azomethine group (C=N) is observed at a lower region around (1625-1580 cm -1 ), indicating the coordination of nitrogen atom of the Schiff base to the ruthenium 2 and also this coordination is further supported when the energies of the azomethine (C=N) stretching vibration of the ligands are compared with those of their respective ruthenium(II) complexes, a shift of 4-28 cm -1 to lower energies is observed in all complexes, a consequence of metal binding to the imine nitrogen atoms, sustaining the coordination of the metal to the Schiff base ligands 3,4 .
A strong band observed around 1340-1386 cm -1 in the free Schiff bases has been assigned to phenolic C-O stretching. On complexation, this band has been shifted to higher frequency 1412-1440 cm -1 showing the other coordination site is the phenolic oxygen.This was further supported by disappearance of band around 3379-3438 cm -1 due to (OH) of free ligands in the spectra of all the complexes, indicating the deprotonation of Schiff bases prior to coordination through its oxygen atom.
For all the complexes, the IR spectra showed a strong band in the region 1937-1959 cm -1 due to terminally coordinated carbonyl group which occurs at higher frequency than the starting precursors.

Ligands
 C=N (cm -1 ) International Letters of Chemistry, Physics and Astronomy Vol. 8 Table 3. IR spectral data for Ru(II) Schiff base complexes.
For all the complexes, the IR spectra showed a medium intensity band in the region around 1090 cm -1 which is characteristic of coordinated nitrogen bases. Characteristic bands for the coordination of triphenylphosphine were also present around 1432-1436 cm -1 in the spectra of all the complexes.

ELECTRONIC SPECTRA
The electronic spectra of all the complexes were recorded in dichloromethane and spectral data are presented in Table 2. All the new Schiff base ruthenium(II) complexes are These assignments are in accordance with the assignment for similar ruthenium(II) octahedral complexes based on molar extinction coefficients.

H-NMR SPECTRA
The ligand to metal binding in the new ruthenium(II) complexes is further supported by the 1 H-NMR spectra.  The multiplet observed in the region around  6.8-7.8 ppm in all the listed complexes have been assigned to the aromatic protons of the phenyl group of the triphenylphosphine or Arsine, the Schiff base ligands and the heterocyclic nitrogen bases. The signal in the region 2.6-3.3 ppm is due to methylene protons nearer to nitrogen. As the methyl protons are attached to the phenyl ring the signal for these protons in the complexes appear in the upfield region  1.6-1.8 ppm and the selected spectra are shown. The absence of a resonance in the region  12-14.0 ppm in the complexes indicate deprotonation of the phenolic (-OH) group of the Schiff bases on complexation and coordination to ruthenium through the phenolic oxygen.