Synthesis and Characterization of Zinc b-Diketonate Complex Extended to the Macromolecular Polymers

A mononuclear complex of [Zn(tta)2(H2O)2]-(3)-(tta = deprotonated of 1-thenoyl-4,4,4trifluoro acetone-(1)-has been prepared by the reaction of 1-thenoyl-4,4,4-trifluoro acetone (H-tta: 1) with Zn(OAc)2.4H2O (OAc = O2CMe) in a 2:1 molar ratio complex 3 can be extended to form a coordination polymers of general formula [Zn(tta)2(X)]n (X = 4,4'-bipy (4), pz (5) by the reaction of zinc atom in 3 with s-donor ligand such as 4,4-bipyridine (4,4'-bipy) and pyrazin (pz). The reaction completion was controlled via FTIR and elemental analysis.


INTRODUCTION
Macromolecular coordination polymers including b-diketonate fragments have attracted widespread attention because of their potential applications as a high quality advanced materials 1 .The design and synthesis of such macromolecular coordination polymer chains can be constructed by binding of metal atom with s-donor rod-like bridging ligands such as 4,4'-bipyridine and pyrazine [2][3][4] .It has been widely observed that such polymeric materials can be influenced by the choice of the metal ion and the bridging ligand species 5 .

General remarks
All chemicals were purchased from commercial providers (Fluka Company) and were used as received.

Physical measurements
Infrared spectra were recorded using a Perkin-Elmer FTIR 1000 spectrometer.Melting points were determined using analytically pure samples with a Gallenkamp MFB 595 010M melting point apparatus.Microanalyses were performed using a Thermo FLASHEA 1112 Series instrument.
Thermogravimetric studies were carried out with the Perkin Elmer System Pyris TGA 6 with a constant heating rate of 8 K min -1 under N 2 (20.0 dm 3 h -1 ).

Synthesis of [Zn(tta) 2 (H 2 O) 2 ] (3)
Complex 3 is accessible by the reaction of Zn(OAc) 2 .4H 2 O (98.8 mg, 0.45 mmol), dissolved in 50 ml hot ethanol and tta (199.9 mg, 0.9 mmol).The reaction was stirred in ethanol for 5 hours.Enough distal water was added to precipitate the product and then washed several times with petroleum ether.The product dried under vacuum several days.A white (Zn) solid was obtained in the yield of 88 %.Mp: 177 -180 °C.IR (KBr), cm -1 : 3426 (

Synthesis and characterization
The reaction of 1-thenoyl-4,4,4-trifluoroacetone (H-tta: 1) with Zn(OAc) 2 .4H 2 O (OAc = O 2 CMe) in a 2:1 molar ratio gave [Zn(tta) 2 (H 2 O) 2 ] (3: tta = deprotonated of 1) complex in ethanol, which was isolated white solid after precipitation upon treatment with aqua (Scheme 1).The produced complex is soluble with most common organic solvents including tetrahydrofuran, acetonitrile, and ethanol.However, in water and non-polar solvents 3 is not soluble.This complex is stable in both solution and solid state under the normal conditions.This stability may due to the presence of intermolecular forces between the mononuclear complex spheres of 3, as shown in the reported structures 14 .The gentle heating of the title complex solid, in an oven up to 180 ºC, change the solubility to be non soluble in various organic solvents.The poor solubility indicates for turning into the di-or polynuclear ones by oligomerization through the bridging oxygen atoms of diketonate unit as known and observed of such systems 22 .
Complex 3 can be extended to infinite coordination polymers of [Zn(tta) 2 (X)] n (X = 4,4'-bipy (4), pz ( 5)) by the reaction of zinc atom in 3 with 4,4bipyridine (4,4'-bipy) and pyrazin (pz) in warm ethanol in a 1:1 molar ratio (Scheme 1).These polymers can also be prepared directly by stoichiometric reacting of Zn(OAc) 2 .4H 2 O (OAc = O 2 CMe) with 1 and s-donor ligand in 1:2:1 molar ratio, respectively for 6 hours of reaction stirring (Scheme 1).The aqua ligands in 3 are eliminated by a strong s-donor bridging ligand forming 4-5, wherein the complex and polymeric structures were proposed according the reported of similar materials 13, 14, 16 -21 .The solutions and solids of polymers are stable in air.After appropriate workup, polymers 4 -5 could be isolated as white solid.They are none dissolving in most common organic solvents including tetrahydrofuran, acetonitrile, and ethanol.However, in water and non-polar solvents also are not soluble.
The elemental analyses of 3 -5 agree with their formula as shown in Experimental section.The chemical nature is characterized by Ft-IR Scheme 1: Synthesis 3-5 The reaction progress of 1 with 2 could additionally be controlled by IR spectroscopy, since the characteristic absorptions of the free non-coordinated b-diketone H-tta disappeared during the course of the reaction and new bands characteristic for metal b-diketonato species were observed (Experimental).IR spectrum of 3 shows prominent absorptions at 3426 cm -1 (ascertains the presence of coordinated aqua ligands 23 ) and 1601 -1410 cm -1 (typical for metal b-diketonate complexes 24 -25 .IR spectroscopy can also be used to monitor the elimination of the aqua ligands in 3 by thermal treatment because the very characteristic vibrations of the aqua ligands continuously disappear with progress of the reaction 14,24 .The peaks at 1601, 1578, 1540, and 1508 are assigned to the keto-enol tautomerism chelating ring of  C=O and  C=C stretching vibrations 18 .The observed downfield shift, going from free ligand (1:  C=O = 1652 and  C=C = 1580 cm -1 ) to the corresponding vibrations in 3 indicates for the complexation, which found in consistent with the reported one 19 .The presence of these bands and their shifts ( C=O ) should be regarded as a characteristic stretching vibrations of keto-enol tautomerism chelating ring of tta ligands with Zn(II) centre as a whole, as in case of benzene 20 .The appearance of a vibration at 3104 cm -1 indicates the formation of hydrogen bonds in 3 26 , this can be considered as evidence for the formation of supramolecular complexes in solid state.The stretching vibration of  C-F , for the coordinated tta, is found at 723 cm -1 (3).This vibration is shifted to a somewhat lower frequency (for comparison H-tta (1): 732, 746 cm -1 ).The stretching vibration of the C-H out-of-plane of thienyl ring at 803 cm -1 in 1 is shifted to the lower frequency upon complexation in 3, which is observed at 790 cm -1 .
The reaction progress of 3 with s-donor bridging ligand can be controlled by IR spectroscopy, since the characteristic absorptions of the coordinated aqua ligands in 3 14 disappeared during the course of the reaction.This indicates the successful substituting the terminal aqua ligands in 3 by 4,4'-bipy and pz ligands forming 4 -5.As result of changing the aqua ligands in 3 with strong p-donor bridging ligand such as 4',4-bipyridine ligand, the stretching vibration of C-CF 3 shifted to the lower frequency (712 cm -1 ) compared to 3 (723 cm -1 ).Furthermore, the stretching vibrations of C-H of thenoyl ring shifted to the lower frequencies (3105 cm -1 ) compared to 3 (3102 cm -1 ).This is ascribed to an increase in the -donating and in the back-donating form Zn(II)  tta.IR spectrum shows the prominent absorptions in the range of 1600 -1413 cm -1 (typical for metal b-diketonate complexes 12,14,[27][28][29] ).
Due to the poor solubility of 4 -5 polymers the UV-Vis spectroscopy cannot be measured.

CONCLUSION
The mononuclear complex 3 has been successfully prepared and characterized by FTIR.The produced complex is extended to infinite metal-organic coordination polymer of [Zn(tta) 2 (X)] n (X = 4,4'-bipy (4), pz (5)).Complex 3 is formed in enol-tta system.The formation of hydrogen bonds can be considered as evidence for the formation of supramolecular complex of 3 in solid state.