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The reaction of CoCl2 ·6H2O with m-BDTH2 (1,3-benzeneditetrazol-5-yl) leads to [Co(C8H6N8)2(H2O)2(CH3CN)2]Cl2 (1). Both tetrazolic groups remain protonated existing in a 1H tautomeric form. A trifurcated chlorine atom and stacking interactions assemble compound 1 into a three-dimensional network.
Tetrazoles are a class of organic heterocyclic compounds which consist of a five-membered ring of four nitrogen atoms and one carbon atom (Scheme 1). In the design of drug molecules, tetrazoles and tetrazole derivatives have generally been avoided because their explosive and flammable nature makes them a safety concern for process-scale synthesis. Nevertheless, tetrazoles such as Losartan  and Candesartan  are angiotensin II receptor antagonist drugs used mainly to treat high blood pressure (hypertension). Moreover, a well-known tetrazole is dimethyl thiazolyl diphenyl tetrazolium salt (MTT), which is used in the MTT assay of the respiratory activity of live cells in cell culture . 5-substituted-1H-tetrazoles (RCN4H) usually can be utilised as metabolism-resistant isosteric replacements for carboxylic acids (RCO2H) in SAR-driven medicinal chemistry analogue syntheses, while it has been found that they have comparable pKa values to the corresponding carboxylic acids (RCO2H) . Therefore, the study of the structures of tetrazoles is relevant to several aspects of medicinal chemistry, as, indeed, is their coordination chemistry given the increasing importance of metal-based drugs incorporating known therapeutic organic agents .
5-substituted-1H-tetrazoles, often termed tetrazolic acids, can be found in neutral, anionic, or cationic form, and they can act as ligands in metal complexes, form salts, and can be both acceptors and donors for hydrogen bonding. Such tetrazolic acids exist in an approximately 1:1 ratio of the 1H- and 2H-tautomeric forms (Scheme 1). It has been reported that the two positional isomers 1 and 2 may be differentiated on the NMR timescale [6, 7], while theoretical calculations show that the 2H-tautomers are the more stable isomers, although they were found to have a larger degree of electron delocalization than 1H-tautomers . Over the last decade, the synthetic procedures of such compounds have been improved [9, 10]; consequently more attention has been focused on the coordinating behaviour of such compounds. Thus, the 5-substituted-1H-tetrazoles have been used as versatile building blocks for molecular coordination networks, also called coordination polymers or metal organic frameworks [11, 12] as well as hydrogen bonded frameworks [13–16].
Recently, we embarked on a study of the coordination behaviour of various 5-substituted-1H-tetrazoles [13–16], and this article describes part of this systematic study where we report the synthesis and crystal structure of [Co(m-BDTH2)2(H2O)2(CH3CN)2]Cl2 (1) where m-BDTH2 = 1,3-benzeneditetrazol-5-yl.
All chemicals and solvents used for the synthesis were obtained from commercial sources and were used as received. The reaction was carried out under aerobic conditions. Elemental analysis (C, H, N) was performed at the Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, using an Elementar Vario EL analyzer. Fourier transform IR spectra were measured on a Perkin-Elmer Spectrum One spectrometer with samples prepared as KBr discs.
m-BDTH2 was prepared as described previously . A solution of m-BDTH2 (0.021g, 0.1mmol) in CH3CN/EtOH (ratio 1:1, total volume 20ml) was added dropwise to a stirred solution of CoCl2 6H2O (24mg, 0.1mmol). The resulting dark blue solution was refluxed for 2 hours, filtered and left to evaporate slowly. Orange crystals of [Co(C8H6N8)2(H2O)2(CH3CN)2]Cl2 (1) were formed after slow evaporation of the solution (Yield: 27mg, 80%). Calc. for C20H22Cl2CoN18O2, (1): C, 35.55; H, 3.28; N, 37.34. Found C, 35.69; H, 3.31; N, 37.32%. IR (KBr, cm−1) = 3338(s), 3053(w), 2896(m), 2846(m), 2822(m), 2737(w), 2317(m), 2290(m), 1711(w), 1621(w), 1553(s) 1481(s), 1459(s), 1374(m), 1256(m), 1169(m) 1144(w) 1127(w) 1084(s), 1029(s) 999(m) 906(w), 872(m) 802(s), 762(s), 733(s), 705(s), 684(s).
Single-crystal X-ray crystallographic data of 1 were collected at 100K on a Bruker SMART Apex CCD diffractometer using graphite-monochromated Mo-Kα radiation. Crystallographic data and details of the measurement and refinement are summarized in Table 1. Semiempirical absorption corrections were made using SADABS . The structures were solved using direct methods followed by full-matrix least-squares refinement against F 2 (all data) using SHELXTL . Anisotropic refinement was used for all non-H atoms; all H atoms were refined (both coordinates and isotropic temperature factors) without restraints, except for the methyl H-atoms on the acetonitrile solvent molecules, which were refined as a rigid tetrahedral group, but with the torsional angle allowed to refine (AFIX 137 in SHELXTL). The crystallographic data and refinement parameters are listed in Table 1.
Compound 1 was characterized crystallographically and found to belong to the triclinic P-1 space group. Selected distances (Å) and angles (°) are presented in Table 2. The Co(II) atom is six-coordinate with an octahedral geometry. The coordination sphere is occupied by two nitrogen N(3) atoms provided by two m-BDTH2 ligands, two acetonitrile, and two water molecules. Both tetrazolic groups remain protonated existing in a 1H tautomeric form. To our knowledge this is the first example showing the neutral form. Only one of the two aryl moieties is coordinated to the Co(II) atom through the nitrogen atom N3 (Figure 1). The dihedral angles between the planes through the coordinated tetrazole, the central aryl moiety, and the noncoordinated tetrazole are 5.87° and 9.8°, respectively.
In compound 1 there are two different kinds of interactions that lead to the formation of a 3D supramolecular architecture, namely, hydrogen bonding and stacking interactions. The chlorine atom is part of a trifurcated hydrogen bonding arrangement (Table 3) between three different cations. In this way a layer parallel to the a axis is formed (Figure 2). The first H-bond involves hydrogen atom H(5) of the noncoordinated tetrazolic group, the second involves hydrogen atom H(12) of the coordinated water molecule and the third involves hydrogen atom H(1) of the coordinated tetrazole group. In addition, a fourth hydrogen bond is located between hydrogen atom H(11) of water molecule and nitrogen atom N(7) of the noncoordinated tetrazolic group, forming an infinite 1D-chain perpendicular to the a axis. These 1D chains interact with each other through weak π − π stacking interactions involving all the aryl moieties (Table 3). A stronger interaction occurs between the central ring and the noncoordinated tetrazolic group, while there is also an interaction with an adjacent coordinated tetrazolic group belonging to a third 1D chain (Figure 3).
The IR spectrum of 1 displays broad bands centred at 3338cm−1, in agreement with the presence of H-bonded water molecules. The bands at 2317(m), 2290(m), can be attributed to the ν(CN) of the acetonitrile while the bands at 1553, 1481,1459, 802, 762, 733, 705, and 684cm−1 are characteristic of the nondeprotonated ligand .
In this article, we have examined the interaction of m-BDTH2 = 1, 3-benzeneditetrazol-5-yl with CoCl2·6H2O. The absence of any base in the reaction mixture prevents deprotonation of the ligand, and in addition to the fact that the ligand coordinates via the nitrogen atom at position 2 on the ring, the hydrogen atom on the nitrogen at position 1 could be located and refined giving clear evidence that the tetrazole is in the 1H tautomeric form (Scheme 1, left). Both the tetrazolic groups of the ligand remain protonated and in the 1H tautomeric form and, to our knowledge, this is the first example of a coordination compound with this neutral form. The trifurcated hydrogen bonding arrangement at the chloride counterion and stacking interactions assemble compound 1 into a three-dimensional supramolecular network. The present finding provides new structural data, which could enhance the understanding of the structural aspects of 5-substituted-1H-tetrazoles. This work represents a part of our systematic efforts to determine new synthetic pathways in the binary system metal/5-substituted-1H-tetrazoles, and further studies on the influence of other parameters on the hydrogen-bonded structural motifs are in progress.
CCDC 768261 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: ku.ca.mac.cdcc@tisoped.
The authors are grateful to the DFG Center for Functional Nanostructures for financial support. This work is dedicated to Professor Nick Hadjiliadis for his retirement.