Published: October 07, 2011 r2011 American Chemical Society 5434 dx.doi.org/10.1021/cg201028e | Cryst. Growth Des. 2011, 11, 5434–5440 ARTICLE pubs.acs.org/crystal Three New Isostructural Coordination Polymers with Cd(II) Clusters as the SBU: Synthesis, Structural Characterization, and Luminescence Properties Prem Lama and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India b S Supporting Information ’ INTRODUCTION Coordination polymers have attracted considerable attention in recent years because of their potential applications as func- tional materials as well as the intriguing nature of molecular connectivities and topologies. 1 The design possibilities of organic ligands and the coordination tendencies of metal ions besides crystallization conditions have led to a large number of coordina- tion polymeric structures quite often endowed with novel structural features as well as unique properties. 2 Various nitrogen 3 and carboxylate 4 donating ligands have been used for this purpose. Heterocycles incorporating poly-nitrogen donors such as tetrazoles 5 and triazoles 6 have been demonstrated to be very good ligands for the construction of three-dimensional (3D) coordination polymers exhibiting a great structural diversity. So, our main aim was to prepare a ligand that contains both tetrazole as well as carboxylate groups as donors. The in situ hydrothermal synthesis to initiate [2 + 3] cycloaddition of azides with nitriles in presence of metal ions to form tetrazoles are known in the literature. 7 In situ ligand synthesis, first proposed by Champness and Schroder in 1997, 8 is regarded as a new and efficient approach for the synthesis of both organic compounds and coordination polymers. Recently, this technique has been devel- oped rapidly in coordination polymers, as it usually generates unexpected structures, which cannot be obtained by the direct reaction of ligands with metal salts. 9 Using this approach, we have synthesized a new tetrazole-based semiflexible ligand that forms coordination polymers with Cd(II) metal ion (Scheme1). Herein, we report the synthesis and crystal structural studies of three coordination polymers of Cd(II) having the formulas {Cd 1.33 L(Cl) 0.67 (H 2 O) 0.67 } n (1), {Cd 1.33 L(Br) 0.67 (H 2 O) 0.67 } n (2), and {Cd 1.33 L(I) 0.67 (H 2 O) 0.67 } n (3) using an in situ generated tetrazole ligand. ’ EXPERIMENTAL SECTION Materials and Methods. 4-Fluorobenzonitrile, ethyl isonipeco- tate, sodium azide, and Cd(II) salts were acquired from Aldrich and used as received. All solvents and K 2 CO 3 were procured from S. D. Fine Chemicals, India. All solvents were purified prior to use. Infrared spectra were obtained (KBr disk, 400À4000 cm À1 ) on a Perkin-Elmer model 1320 spectrometer. 1 H NMR and 13 C NMR spectra were recorded on a JEOL-ECX 500 FT (500 and 125 MHz, respectively) instrument in CDCl 3 and DMSO-d 6 with Me 4 Si as the internal standard. ESI-mass spectra were recorded on a WATERS Q-TOF premier mass spectrometer. Microanalyses for the compounds were obtained using a CE-440 elemental analyzer (Exeter Analytical Inc.) while thermogravimetric analyses (TGA) were obtained using a Mettler Toledo star system (heating rate of 5 °C/min). Solid-state photoexcitation and emission spectra were recorded on double UVÀ vis-NIR spectrophotometer (Varian model Cary 5000) and Jobin Yvon Horiba Fluorolog-3 spectrofluorimeter at room temperature. Synthesis. The ligand was synthesized in several steps as described below. Synthesis of 1-(4-Cyano-phenyl)-piperidine-4-carboxylic Acid Ethyl Ester ( L 1 ). Ethyl isonipecotate (2 mL, 12.9 mmol) and dry K 2 CO 3 (1.9 g, 13.8 mmol) were taken in a 50 mL round-bottom flask under an inert atmosphere, and then, 10 mL of dry DMF was added to it. The mixture Received: August 8, 2011 Revised: September 30, 2011 ABSTRACT: Three new isostructural coordination polymers, {Cd 1.33 L(Cl) 0.67 (H 2 O) 0.67 } n (1), {Cd 1.33 L(Br) 0.67 (H 2 O) 0.67 } n (2), and {Cd 1.33 L(I) 0.67 (H 2 O) 0.67 } n (3), have been synthesized using a semiflexible in situ generated tetrazole ligand. All the complexes crystallize in cubic space group P4 1 32. In complex 1, three Cd 2+ ions along with two N atoms each from three tetrazole units form a trinuclear unit where each metal ion is additionally bridged by carboxylate O and μ 3 -Cl units. This Cd(II) cluster along with the ligand form a trigonal arrangement that extends to form a two-dimensional sheet that further undergoes extensive interpenetration to generate an overall three-dimensional coordination polymer. All com- plexes exhibit thermal stability up to ∼330 °C losing coordinated water molecules at ∼140 °C. Upon excitation at 340 nm, the complexes exhibit solid-state luminescence with an emission peak at ∼415 nm that shows slight red shift with broadening of the band with changing the halide anion from chloride to bromide to iodide.