Scientific paper Cd(II) and Zn(II) Coordination Polymers Assembled from Benzoyltrifluoroacetone and 1,2-Bis(4-Pyridyl)Ethane Ancillary ligands Farzin Marandi,1'* Shabahang Teimouri1 and Hoong-Kun Fun2'3 1 Department of Chemistry, Payame Noor University, 19395-3697 Tehran, I. R. of Iran 2 X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia 3 Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, Riyadh 11451, Saudi Arabia * Corresponding author: E-mail: f.marandi@gmail.com Received: 04-01-2013 Abstract Two novel cadmium(II) and zinc(II) metal-organic coordination polymers with a yS-diketone and N-donor ancillary li-gands, [Cd(bpe)(btfa)2]n (1) and [Zn(bpe)(btfa)2]n (2), (Hbtfa = benzoyltrifluoroacetone and bpe = 1,2-bis(4-pyridyl)et-hane), have been prepared and characterized by elemental analysis, IR and 1H NMR spectroscopy, and studied by thermal gravimetric analysis as well as single crystal X-ray diffraction. The crystal and molecular structures of 1 and 2 have been solved by X-ray diffraction and they turned out to be one-dimensional polymers with zigzag (1) and linear (2) dispositions of the metal atoms. These one-dimensional polymers are further connected to form a 3D supramolecular network by C-H—O and C-H—F interactions. Thermal stabilities of these polymeric complexes have also been investigated. Keywords: Coordination polymers; yS-Diketone; 1,2-Bis(4-pyridyl)ethane 1. Introduction The designed construction of metal-organic complexes from various molecular building blocks connected by coordination bond, supramolecular contacts (hydrogen bond, n-n stacking, etc.), or their combination, is an interesting research area not only because of their tremendous potential properties as functional solid materials in the areas of catalysis,1 gas adsorption,2 luminescence,3 non-linear optics,4 magnetism5 and ion exchange,6 but also for their intriguing structural diversities and new topologies. The combination of organic ligand 'spacers' and metal ion 'nodes' has been regarded as the most common synthetic approach to produce such coordination polymers. As dw metal ions, Cd2+ and Zn2+are particularly suited for the construction of coordination polymers and networks, since their spherical d10 configuration is associated with a flexible coordination environment so that tetrahe-dral, five-coordinated or octahedral geometries are possible and severe distortions of the ideal polyhedrons can easily occur. A successful approach to build these metal coordinated networks is to select suitable multi-dentate li-gands as spacers and, among these, flexible bipyridyl-ba-sed ligand with pyridine rings linked by aliphatic chain, such as 1,2-bis(4-pyridyl)ethane (bpe), can freely rotate to meet the requirement of coordination geometries of metal ions in the assembly process. This ligand is good candidate to produce unique structural motifs with beautiful aesthetics and useful functional properties.7 In contrast, unsaturated metal complexes of P-diketonates are of interest as building blocks of supramolecular structures. Several crystalline products have been synthesized using [M(P-dike-tonato)2] complexes (M = Pb, Cd, Ag) and different bridging and chelating ligands.8 However, we must not forget that a variety of weak interactions and subtle factors can play a decisive role in orienting the outcome of the crystallization processes, so that a 'true' engineering of polymeric networks, both from a structural and a functional point of view, still remains a quite difficult challenge. In the present work, 1,2-bis(4-pyridyl)ethane li-gands was employed to react with Cd(II) and Zn(II) to produce one-dimensional coordination polymers in the presence of benzoyltrifluoroacetonate anion (Scheme 1). The structures reported here differ from others in that other bipyridyl analogous ligands have used (4,4'-bipyridyl) that do not act as spacers resulting in H-bond superstructures being built instead.8cd and 9 Hbtfa or Htfpb: 4,4,4-trifluoro-1-phenyl-1,3-butanedione bpe: 1,2-bis(4-pyridyl)ethane Scheme 1: The ligands Hbtfa and bpe 2. Experimental 2. 1. Material and Measurements All chemicals were reagent grade and used without further purification. FT-IR spectra were collected on a Mattson 1000 spectrophotometer using KBr pellets in the range of 450-4000 cm1. Elemental analyses (CHN) were performed using a Carlo ERBA model EA 1108 analyzer whereas 1H NMR spectra were obtained using a Bruker spectrometer at 250 MHz in [D6]DMSO. Thermogravi-metrical analyses (TGA) were performed in N2 atmosphere with a flow rate of 20 ml/min on a Seiko Instruments thermal analyzer from 20 to 800 °C, with a heating rate of 10 °C/min in the ceramic crucibles. 2. 2. Crystallography Diffraction data for 1 and 2 were collected at room temperature and 100 K (with an Oxford Cyrosystem Cobra low-temperature attachment), respectively. The data were collected using a Bruker SMART APEXII CCD dif-fractometer with graphite monochromatic MoK radiation (X = 0.71073 A) at a detector distance of 5 cm and an AsPEXII software.10 The collected data were reduced using SAINT program,10 and the empirical absorption corrections were performed using SADABS program.10 Structures were solved using direct methods and were refined using the least-squares method from SHELXTL software package.11 All non-hydrogen atoms were refined anisotropically unless otherwise noted. Hydrogen atoms were located and included at their calculated positions. Materials for publication were prepared using SHELXTL11 and ORTEP III.12 All fluorine atoms in trif-luoromethyl group in 1 are rotationally disordered about the three C-F bonds and the ratio of occupancies for three components are fixed to 0.40 : 0.30 : 0.30; both minor components of fluorine atoms were refined isotropically. Full crystallographic data, in CIF format, may be obtained from the Cambridge Crystallographic Data Centre (CCDC-915370 for 1 and CCDC-915371 for 2) via . The crystal data and structure refinement of compounds 1 and 2 are summarized in Table 1. Selected bond lengths and angles of 1 and 2 are listed in Table 2. 2. 3. Synthesis of [Cd(bpe)(btfa)2]n (1) 1,2-bis(4-pyridyl)ethane (0.184 g, 1.0 mmol) was placed in one of the arms of a branched tube13 whereas cadmium(II) acetate (0.115 g, 0.50 mmol) and benzoyl-trifluoroacetone (0.216 g, 1.0 mmol) were placed in the other arm. Methanol and water in a ratio of 2:1 were carefully added to fill both arms. The tube was then sealed and the ligand-containing arm was immersed in a bath at 60 °C whereas the other was maintained at ambient temperature. After 2 days, crystals that were deposited in the cooler arm were filtered, washed with acetone and ether, and dried in air. Yield: 0.25 g (70%). Analysis: Found: C 52.49%, H 3.10%, N 3.55%, Calculated for C32H24CdF6N2O4: C 52.82%, H 3.30%, N 3.85%. IR (cm-1) selected bands: 3059(w), 2938(w), 2857(w), 1618(s), 1585(s), 1517(s), 1473(s), 1348(m), 1316(m), 1248(m), 1187(m), 1015 (m), 767(m), 649(w), 512(w). 1H NMR (DMSO, 5): 8.42(d, 4H, pyridyl of bpe), 7.82(d, 4H, pyridyl of bpe), 7.2-7.6(m, 10H, phenyl of btfa"), 6.19 (s, 2H, =CH- of btfa") and 2.91 (d, -CH2- of bpe). 2. 4. Synthesis of [Zn(bpe)(btfa)2]n (2) Complex 2 was synthesized in the same way as complex 1 using zinc(II) acetate instead of cadmium(II) acetate. Yield: 0.19 g (56%). Analysis: Found: C 56.49%, H 3.80%, N 4.45%, Calculated for C32H24F6N2O4Zn: C 56.47%, H 3.53%, N 4.12%. IR (cm-1) selected bands: 3055(w), 2927(w), 2864(w), 1625(s), 1582(s), 1522(s), 1491(s), 1352(m), 1256(m), 1194(m), 1014(m), 785(m), 666(w). 1H NMR (DMSO, 5): 8.43(d, 4H, pyridyl of bpe), 7.84(d, 4H, pyridyl of bpe), 7.25-7.60(m, 10H, phenyl of btfa"), 6.22 (s, 2H, =CH- of btfa") and 2.92 (d, 4H, -CH2-of bpe). 3. Results and Discussion 3. 1. Spectroscopic and Thermal Analysis IR spectra confirm the presence of organic ligands used in the syntheses (through the typical vibrations of pyridine aromatic rings, and diketonate groups). The presence of the bpe ligand is provided by the signals vas(CH2) Table 1. Crystal data and structure refinement for 1 and 2 2 1 Identification code [Cd(bpe)(btfa)2]n [Zn(bpe)(btfa)2]n Empirical formula C32H24CdF6N2O4 C32H24F6N2O4Zn Formula weight 726.93 679.90 Crystal system Orthorhombic Monoclinic Space group P ccn P 21/c Unit cell dimensions a = 17.9986(10) À a =14.9131(2) À b = 11.2127(6) À b = 17.6978(3) À c = 15.6293(9) À c = 11.61820(10) À ß = 99.069(1)° Volume 3154.2(3) À3 3028.05(7) À3 Temperature (K) 293(2) 100(2) Z 4 4 Density (calculated) 1.531 g cm-3 1.491 g cm-3 Absorption coefficient 0.766 mm-1 0.888 mm-1 F(000) 1456 1384 8 range for data collection 3.18-35.89 1.80-31.07 Index ranges -29 < h < 29 -21 < h < 21 -11 < k < 18 -18 < k < 25 -25 < l < 23 -16 < l < 16 Reflections collected 45332 37566 Independent reflections 7387 [R(int) = 0.0322] 9680 [R(int) = 0.0419] Completeness to theta 99.5 % 99.7 % Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data / restraints / parameters 7387/0/231 9680/0/409 Goodness-of-fit on F2 1.033 1.042 R\, wR2 [/0 > 2o{/0)] 0.0386, 0.1127 0.0420, 0.0898 Rj, wR2 (all data) 0.0719, 0.1439 0.0757, 0.1035 Largest dF peak, hole 0.80, -0.54 e. À-3 0.44, -0.45 e. À-3 at 2938 (1) and 2927 (2) cm-1; vs(CH2) at 2857 (1) and 2864 cm-1 (2). The bpe ligand is characterized by vas(CC) at 1585 (1) and 1582 cm-1 (2); vs(CC) at 1515 (1), 1491 cm-1 (2) vibrations, and the combined effect of the v(CC) and ¿(CCH) stretchings at 1015 (1) and 1014 cm-1 (2). The relatively weak absorption bands at around 3059 and 3055 cm-1 are due to the C-H modes involving the aromatic ring hydrogen atoms. The IR spectra of compounds showed strong bands at 1618, 1625 cm-1 and at 1517, 1522 cm-1, assigned to the v(C=O) and v(C=C) stretching of btfa" anions. These bands are at significantly lower energies than those found for free Hbtfa (1655 cm-1) and are indicative of /-diketonate chelation to Cd(II) and Zn(II). The absorption bands in the frequency range 1260-1130 cm-1 correspond with the C-F modes of the /3-diketonates.14 The 1H NMR spectra of the DMSO solu- a) b) Figure 1. The TGA curves of a) 1 and b) 2. tions of 1 and 2 display three different protons of the bpe ligand at 8.4, 7.8 (aromatic protons) and 2.9 (aliphatic protons) ppm and the singlets at 6.19 (1) and 6.22 (2) ppm of =CH- protons of btfa" anions. In 1 and 2, distinct peaks at 7.25-7.60 ppm were assigned to the five protons of the phenyl ring of the btfa" anions. In order to investigate the thermal stability of both complexes, TGA was performed in a N2 atmosphere. The TGA curves show that complexes 1 and 2 exhibit similar decomposition pathways. The TGA curves illustrate no weight loss up to 265 °C for 1 and up to 275 °C for 2, demonstrating that 1 and 2 are retained up to these temperatures. The thermal decomposition of the compounds occurs in two steps: the first step (sharp endothermic decomposition) in the temperature range 265-300 °C (almost 70% weight) for 1 and 280-300 °C (almost 70% weight) for 2, the second step in the temperature range 300-650 °C almost 12% and 16% weight for 1 and 2, respectively, corresponds to the decomposition of the compounds (Fig. 1). The mass loss calculations as well as the microanalyses of the solid residues suggest that the residue left as a final decomposition product of the complexes is CdO and ZnO, and the total mass loss of 18.12% for 1 (calc. 18.93%) and 13.25% for 2 (calc. 11.98%), respectively, agrees well with the proposed structures. These results indicate that 1 and 2 have medium thermal stability and in accord with the conclusion reported that a fluorinated substituent of a ligand leads to an improved thermal and oxi-dative stability.15 3. 3 Description of the Crystal Structures of 1 and 2 Single crystal structure determinations of 1 and 2 indicate the formation of one-dimensional coordination polymers (Figs. 2(a) and 3). Utilizing the bridging N,N'-donor ligand bpe, two 1D complexes of 1 and 2 are obtained. Polymer 1 crystallizes in space group P ccn and the asymmetric unit of compound 1 is constrained with two crystallographic symmetry elements. The asymmetric fragment of 1D polymer has crystallographic symmetry that is imposed with two-fold axes going through the cadmium atom and as well imposed inversion centre between C16 and its symmetry related carbon atom. As shown in Figure 2, Cd1 is coordinated by four oxygen atoms of two btfa anions and two cis bpe nitrogen atoms to give a Cd-O4N2 distorted octahedral environment (& = 69.42, 6 = 54.59, p = 172.98;