Synthesis and Characterization of a New Cobalt(II) Complex with 2-(2-Pyridyl)Imino-N-(2-Thiazolin-2-yl)Thiazolidine (PyTT)

The compound aquanitrate-кObis[2-(2-pyridy)-imin-кN-N-(2-thiazin-кN-2-y)thiazidine]cbat() nitrate has been isolated and characterized by single crystal X-ray diffraction, IR spectroscopy, UV-Vis-NIR diffuse reflectance and magnetic susceptibility measurements. The environment around the cobalt atom may be described as a distorted octahedral geometry with the ligand-metal-ligand bite angles varying between 84.07(8)° and 98.66(8)°.The metallic atom is coordinated to two thiazoline nitrogens [av. Co-N =2.067 Å], two imino nitrogens [av. Co-N =2.122 Å], one oxygen atom of the nitrate group monodentate [Co-O(1)= 2.249(2) Å] and the oxygen atom of the water molecule [Co-O(IW)= 2.105(2) Å]. Electronic UV-Vis-NIR spectral data and the calculated magnetic moment are indicative of octahedral Co(ll) complexes. In the same way as other PyTT complexes, the organic moiety preserves the imino-thiazolidine form detected in the structure of PyTT.


INTRODUCTION
Research interest in thiazoline and thiazolidine derivatives has increased during recent years, due to their biological and pharmaceutical activity. 2-thiazolines have been synthesized for many years and they are especially interesting because this heterocycle can be easily converted into thiazole or penicillin derivatives. A lot of new synthetic compounds have emerged due to investigation for improving their therapeutic effect.
It has been proposed that penicillin inhibits the formation of bacterial cellular wall by acting as a structural analog of the terminal D-alanyI-D-alanine residue of the peptidoglycan strand. It is suggested, therefore, that the transpeptidase would first react with the substrate to form an acyl enzyme intermediate, with the elimination of D-alanine, and that this active intermediate would then react with another strand to form the cross-link and regenerate the enzyme. The penicillin fits the substrate-binding site with the 13-1actam ring in the same position as the bond involved in the transpeptidation. It therefore acylates the enzyme, forming a penicilloyl enzyme, and thereby inactivates it/1/. In this mechanism, metallic ions might act as catalysts in the field of the coordination chemistry, because in the formation of substrate-enzyme complex metallic ions might provide all or some binding sites through formation of an intermediate enzyme-metalsubstrate type bridge complex.
Likewise, many of these compounds containing thiazoline and thiazolidine rings present antitumoral/2/, anti-HIV /3/ and antiinflammatory activity/4/, as well as being known for their application in the treatment of diabetes/5/. However, the biological activity could be modified or even improved thanks to coordination to metallic ions or atoms.
The synthesis of new siderophore analogues constitutes another example of pharmacological application. Microbial siderophores are relatively low molecular weight compounds synthesized in order to solubilize and transport iron (Iii) into the procariotes cells in the necessary concentrations/6/. New siderophore analogues are prototypes for the synthesis of chelating agents employed for clinical use/7/. Thus, siderophores have been characterized containing thiazoline rings as desferrithiocin or pyochelin. These compounds have been used in the treatment of acute iron poisoning and in the chronic iron overload resulting from transfusion theraphy of 13-thalasemia/8/. Likewise, some siderophores form strong chelates with other metal ions like copper(ll), nickel(ll), zinc(ll) /9/, calcium(ll), magnesium(ll) and cobalt(ll) /6/. The research on new siderophores will conduct to new clinical drugs for certain metal detoxification/10/.

Instrumental procedures
Chemical analyses of carbon, hydrogen, nitrogen and sulphur were performed by means of microanalytical methods using a Leco CHNS-932 microanalyser. A UV-Vis-NIR reflectance spectrum in the 200-1000 nm range was obtained from a pellet of the sample, using a Shimadzu UV-3101 PC spectrophotometer and BaSO4 as a reference. Magnetic measurements were made at room temperature on a magnetometer with pendulum MANICS DSM8, equipped with helium continous-flow cryostat and an electromagnetometer DRUSCH EAF 16 UE. IR spectra were recorded on a Perkin-Elmer FT-IR 1720 spectrophotometer, from KBr pellets in the 4000-370 cm 1 range and on a Perkin Elmer FT-IR 1700X spectrophotometer, from polyethylene pellets in the 500-150 cm range.

RESULTS AND DISCUSSION
The analytical results agree with the molecular formula C22H26CoNI007S4 for the red Co(ll) complex.
Description of the structure The X-ray study revealed that the crystals of [Co(NO3)(PyTT)2(H20)]NO3 are made up of triclinic units cells, containing [6(NO3)(PyTT)2(H20)] + cations and nitrate anions. Figure shows the molecular structure of the cation complex. The most relevant bond lengths and angles are given in Table 2.
C (17) ) s(4) The environment around the cobalt(ll) atom may be described as a distorted octahedral geometry with the ligand-metal-ligand bite angles varying between 84.07 (8) [N(7)-Co-O(I)] and 98.66 (8)  As in other PyTT complexes/11,12/, the organic moiety preserves the imino-thiazolidine form detected in the structure of PyTT. However, a drastic change is observed in the conformation upon complexation. Thus, the thiazoline rings are rotated around the C(1)-N(2) and C(12)-N(6) bonds, respectively, which permit the donation through N(I) and N (5)  The presence of hydrogen bonds is observed. Thus, the water molecule of coordination acts as donor of hydrogen, while oxygen atoms 0(4) of nitrate ion and 0(2) of nitrate ligand monodentate act as acceptors.

Physical measurements
The observed molar magnetic susceptibility for [Co(NO3)(PyTT)2(H20)]NO3 was corrected for diamagnetism and temperature-independent paramagnetism to provide the fully corrected magnetic moment at room temperature (la 5.0 BM). This value is consistent with the octahedral environment around cobalt(ll) The diffuse reflectance spectrum shows one intense band at 31250 cm -, which is indicative of one charge-transfer transition. Moreover, the spectrum exhibits two broad bands with considerable fine structure that have intensities consistent with spin-allowed d-d transitions. These bands may be assigned as: In the 1700-1800 cm region of the IR spectrum, the pattern of the v+v4 combination bands was studied. The value of the v t+v4 splittings (A 33 cm) indicates, according to Lever's criteria/31/, the presence of one monodentate nitrate group. Likewise, a third peak was observed in this region, which allows to recoghize a ionic nitrate group. Moreover, the IR spectrum shows other bands at 1288, 1028, 806, 721 and 704 cmthat can be assigned to the monodentate nitrate group whereas the bands at 1380 and 826 cm can be assigned to the nitrate ion group [31][32][33][34][35][36].
Only four bands assignable to Co-ligand stretching vibrations are observed in the low frequency range, although the C symmetry of the complex cation predicts the appearance of six such bands. The broad band at 346 cmis assignable to the v(Co-OH2) mode of the coordinated water/22,37,38/, whereas the band at 324 cm is assigned to v(Co-Nimine), in good accord with literature/39-42/.

CONCLUSIONS
From the structural study of this complex some conclusions can be drawn. First, it can be expected that this complex presents little or no antimicrobial activity due to the fact that the complex is octahedral and presents a complete coordination sphere/46/, which hinders the entry of new ligand to form the complex enzyme-metal-substrate complex. Secondly, since cobalt intoxications induce cardiomyopathy, allergic dermatitis and asthma/47/, it can be possible to use PyTT as siderophore for the treatment of poisoning with this metal due to the formation of a complex with chelate rings.

SUPPLEMENTARY MATERIAL
CCDC 209791 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 IEZ, UK; fax: +44 1223 336033; or deposit@ccdc,cam.ac.uk).