versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
Metal complexes of a new ligand derived from 2,3-quinoxalinedithiol and 2,6-bis(bromomethyl)pyridine
J. R. ANACONA*, THAIS MARTELL and IVONNE SANCHEZ
Departamento de Química, Universidad de Oriente, Apartado Postal 208, Cumaná, Venezuela.
The synthesis of a new ligand containing quinoxaline and pyridine subunits is described. The reaction of 2,3-quinoxalinedithiol with 2,6-bis(bromomethyl)pyridine leads to the isolation of bis(2-thio-3-mercaptoquinoxalino)-2,6-dimethylpyridine (L), which is a macrocyclic ligand precursor. The reaction of transition metal ions with L gives [M(L)X]X complexes (where M = Mn(II), Co(II), Cu(II), Ag(I), Zn(II), or Cd(II), and X = Cl or Br). The compounds were characterized by physical and spectroscopic measurements which indicated that the ligand is probably acting as a pentadentate NS4 chelating agent
Transition metal complexes containing macrocycles have been of considerable interest in terms of structural chemistry and biological function. This field has undergone spectacular growth during the past years and extensive work has been carried out in many areas [1-10]. This growth has largely been due to the synthesis of multidentate ligands and the complexes such ligands form with metal ions.
Since the recognition of the importance of complexes containing macrocyclic ligands, considerable effort has been directed towards the development of reliable syntheses of these interesting and important compounds, and for many years the results were largely unsuccessful because of the low yields, the many side products of the reactions, and the large volumes of solvents that were required to give sufficient dilution to minimize polymerization and favour cyclization.
The function of the metal ion in in situ reactions has been the source of much discussion since the developments of macrocyclic syntheses of this type, and when the directive influence of the metal ion controls the steric course of a sequence of stepwise reactions, the kinetic template effect is considered to be operative. As an example of this, and continuing with previous studies [11-15], we report here the isolation, characterization and crystal structure of a macrocyclic complex precursor containing the ligand bis(2-thio-3-mercaptoquinoxalino)-2,6-dimethylpyridine (L).
MATERIALS AND METHODS
All necessary precautions were observed to exclude oxygen and moisture during the synthesis and handling of the compounds. Analytical reagent grade chemicals were used as received for all the experiments. Fourier transform infrared (FTIR) spectra of the ligand and its metal complexes as KBr pellets were recorded in the spectral range 4000 - 400 cm-1 range with a Perkin-Elmer Series 2000 apparatus. FTIR spectra as polyethylene pellets were recorded between 450 - 120 cm-1 using a Bruker IFS 66V spectrophotometer. EPR spectra were recorded on a Bruker ECS 106 spectrometer operating in the X-band (9.76 GHz). a-a´-Diphenyl-b-picrylhydrazide free radical was used as the g marker. U.v.vis. spectra were recorded on a Perkin-Elmer spectrometer. The C, H, N, and S content was analyzed on a LECO CHNS 932 model microanalytical instrument. Some complexes were analyzed for their metal content with a Perkin-Elmer atomic absorption analyser, after decomposition with a mixture of HNO3 and HCl followed by H2SO4. Magnetic susceptibilities were measured on a Johnson Matthey Magnetic Susceptibility balance at room temperature using HgCo(NCS)4 as calibrant. 1H NMR spectra were recorded in DMSO-d6 solution on a Bruker AM-500 NMR spectrometer and TMS was used as an internal reference. Thermogravimetric analyses were performed with a Cahn RG electromicrobalance in an air atmosphere at a heating rate of 4 C min-1 up to 800 C.
Synthesis of ligand
Metallic sodium (1 g) and 2,3-quinoxalinedithiol (2 mmol) prepared from 2,3-dihydroxiquinoxaline by the procedure described previously , were added to refluxing degassed 2,6-bis(bromomethyl)pyridine (1 mmol), prepared by a method described elsewhere , in a MeOH medium. The contents were refluxed for two hours. The solid ligand was separated and dried under reduced pressure. It was purified by recrystallization from the same solvent (yield 72%). The structure of ligand L is shown in Figure 1.
Fig 1. Structure of ligand L Synthesis of complexes
An example of the general preparative method for the complexes is provided by the synthesis of the manganese(II) complex. 2,3Quinoxalinedithiol (2 mmol), manganese dichloride tetrahydrate (1 mmol) and metallic sodium (1 g) were dissolved in hot MeOH (10 cm3) and solid 2,6-bis(bromomethyl)pyridine (1 mmol) was added with stirring. The solution was refluxed for 8 h and its volume was reduced in a rotary evaporator until a light yellow precipitate appeared. After cooling, the solid was filtered off, washed with water, MeOH and ether, and dried under reduced pressure at room temperature (yield 30-45%).
RESULTS AND DISCUSSION
The manganese(II), cobalt(II) and copper(II) complexes are maroon. The cadmium(II) complex is light yellow, while the zinc(II) and silver(I) complexes are dark yellow. They are air stable solids, soluble in DMSO and DMF, slightly soluble in MeCN and insoluble in MeOH and H2O. The elemental analyses agree well with a 1:1 metal-to-ligand stoichiometry for all the complexes (Table 1). The conductivity values of the complexes measured in DMSO and DMF at room temperature fall in the range expected for 1:1 electrolytes , suggesting that one halide ion is not coordinated to the metal ion.
Thermogravimetric analysis of the silver(I) complex shows a mass loss equivalent to two water molecules in the 150170 C range, suggesting that these molecules are coordinated to the metal ion. All other complexes have lattice water only. Attempts to form complexes of well defined stoichiometry, under the above mentioned conditions, with chromium(III), iron(III), nickel(II), copper(I), mercury(II) and tin(II) ions were unsuccessful.
The IR spectroscopy can provide valuable information as to whether or not reaction has occurred. Disappearance of the absorption bands due to n(C-Br) in the 550-600 cm-1 range from 2,6-bis(bromomethyl) pyridine, in addition to the appearance of new bands in the 440-475 and 350-395 cm-1 ranges attributed to n(M-N) and n(M-S) vibrations, respectively , seen in the spectra of the metal complexes suggest that the product has been formed. The complexes show bands in the 1410-1460, 1070-1100 and 720-740 cm-1 regions that can be assigned to phenyl ring vibrations . A strong band in the 990-1100 cm-1 region due to C-S stretching vibrations is suggestive of the chelating character of the ligands in the case of metal complexes . Another C-S stretching mode was observed in the 1240-1320 cm-1 region in agreement with the literature . However, the n(M-X) band could not be distinguished unambiguously. In addition, the complexes show significant bands at 1570 and 1470 cm-1 which may be assigned to two high energy pyridine vibrations [23,24]. In the spectra of all complexes the low energy pyridine bands assigned to the in plane and out of plane deformation modes of the pyridine ring are seen at 640 and 420 cm-1, respectively, suggesting coordination of pyridine . Medium intensity bands appearing in the 2830-2950 cm-1 region correspond to aliphatic n(CH), while aromatic n(C-H) appear in the 3000-3100 cm-1 region. The IR spectrum of the silver(I) complex shows the characteristic strong absorption at 1360 cm-1 and a sharp medium band at 830 cm-1 typical of noncoordinated nitrate ions.
Even though the reactions were carried out in a nitrogen atmosphere, air oxidation may presumably occur and the ligand L having a delocalized pTT-orbital system would be able to coordinate to the central metallic ion under the form of semithioquinone, allowing complex paramagnetic compounds to be obtained. In order to test the formation of radical species, corrected magnetic moments have been calculated from the molar magnetic susceptibilities using Pascal's constants . The values for the d10 metal complexes are limited within the interval 0.10-0.20 B.M. (Table 1), suggesting that no oxidation occurs. These values clearly indicate that these complexes have no unpaired electrons. They are diamagnetic and show no EPR signals at room temperature.
The magnitudes of the magnetic moments for paramagnetic complexes fall within the ranges associated with high spin ions in octahedral fields, and they are unlikely to be of value in discriminating between the metal ions in Oh and D5h field symmetries. The manganese(II) complex has a magnetic moment of 5.66 B.M. as predicted for a d5 system with five unpaired electrons, while the EPR spectrum shows no hyperfine splitting due to 55Mn (100% natural abundance, nuclear spin I = 5/2). Our g = 2.15 agrees well with other published values for d5 Mn(II) ions (27). The cobalt(II) complex has a magnetic moment of 4.07 B.M., typical of d7 systems with three unpaired electrons indicating a quartet state. The EPR spectrum recorded at the X-band frequency at room temperature shows a single line with no hyperfine splitting due to 59Co (100% natural abundance, nuclear spin I = 7/2). Our g = 2.01 agrees quite well with other high spin cobalt(II) complexes. The copper(II) complex has a magnetic moment of 1.63 B.M., indicating that it has one unpaired electron, while the room temperature EPR spectrum of the powder sample shows a single broad signal with poor resolution of the hyperfine structure on both sides of the main signal with spectral characteristics g_ = 2.26 and g^ = 2.06, resembling those of copper(II) in other six and seven coordinate geometries. The trend g_ > g^ > ge (free ion value, 2.0023) and the axial symmetry value, G = 4.3, which falls in the 2.89-5.18 range, indicate that the unpaired electron is present in the d x2-y2 orbital .
The electronic spectra of the ligand show a 1B band of the phenyl ring at 229 nm. This high intensity band shifts slightly toward lower wavelengths in in the 220-236 nm range on complex formation. The spectra of the ligand as well as the complexes show the broad bands in the regions 275-320 and 320-380 nm range due to the TTp pTT* (>C=N-) transition of the chromophore and the secondary band of the benzene ring, respectively. The manganese(II) complex shows weak absorptions in the visible region, probably due to spin-orbit forbidden transitions. It is well known that the electronic spectra of cobalt(II) in its octahedral complexes, which are usually high spin, lead to a weak band (e < 10) near 500 nm, as was seen . The copper(II) ion has a d9 electronic configuration and gives rise to only one spectral term in the electronic spectra, 2D. The d-d electronic transition in an octahedral field is 2Eg ®2T2g, while in a pentagonal bipyramidal field it is 2A'1 ® 2E'2 (very low energy) and 2A'1 ® 2E''1. Therefore, a single band in the electronic spectra is expected for d9 configuration with a possible unfolding of this band because of Jahn-Teller distortion. In fact, the electronic spectrum of the copper(II) complex shows only one band centered at 590 nm, which has no value in discriminating between Oh and D5h symmetries. In the spectrum of the cadmium(II) complex a smooth band was seen at 560 nm which could be assigned as a charge transfer band. It has been reported  that a metal is capable of forming d p bands with ligands containing nitrogen as the donor atoms. The cadmium ion has its 5d orbitals completely vacant and hence L ® M bonding can take place by the acceptance of a lone pair of electrons from the donor nitrogen atom of the ligand.
1H NMR spectra
Proton magnetic resonance spectral data of the ligand and the complexes have been recorded. Experimental results for the paramagnetic complexes show that the peaks of protons belonging to different groups were very broad and could not be distinguished. The 1H NMR spectra of diamagnetic complexes slightly changed compared to those of the corresponding ligand, and the signals appeared downfield, as expected, due to increased conjugation on coordination. The signal at 3.8 ppm in the case of ligand L are due to the S-H proton, which is easily identified by deuterium exchange. This signal, however, is still present in the spectra of the complexes, indicating that no deprotonation occurs on complexation with the metal ions. The resonance assigned to aromatic ring protons (6.8 - 8.2 ppm) remains almost unaffected in the complexes. The protons of the methylene groups appear as a sharp singlet at 2.5-2.6 ppm [31,32]. The integrated relative intensities of the aromatic ring protons to methylene protons were in good agreement with the required 11:4 ratio.
Structure of complexes
The ligand has several potential donor atoms, but due to steric constraints the ligand can provide a maximum of five donor atoms at any one time for coordinating to a metal. On the assumption that four sulfur atoms and one nitrogen atom of the ligand are coordinated to the metal, as seems likely from an inspection of molecular models, it follows that the complexes would be five coordinate with respect to the ligand. As the ligand is potentially pentadentate, it is quite feasible that the metal ions are six coordinate with one halide ion at the vertices of an octahedron, as shown in Figure 2. When the energy of the complexes was minimized, using MM2 calculations, reasonable bond distances and bond angles around the metal were obtained, suggesting that an octahedral or distorted octahedral configuration around the metal is possible.
The possibility of a square pyramidal configuration around the metal has been discarded from the conductivity measurements, which show that the complexes are 1:1 electrolytes. Despite the crystalline nature of the products none proved suitable for X-ray structure determination.
The authors express their sincere thanks to Comisión de Investigación of the Universidad de Oriente for financial support, as well as to Lic. Erasto Bastardo for elemental analyses.
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