The three-dimensional coordination network in poly[di-μ₃-cyanido-μ₅-[5-(pyridin-4-yl)tetra­zol­ato]-tricopper(I)]

The unique coordination abilities of 5-substituted 1H-tetrazoles such as 5-(2-, 3- or 4-pyridyl)-1H-tetrazole (abbreviated as 2-Hptz, 3-Hptz or 4-Hptz, respectively) to transition metal ions have been used extensively in the construction of functional coordination complexes [metal–organic frameworks (MOF), fluorescent and ferroelectric materials etc. (Xiong et al., 2002; Jiang et al., 2004; Ye et al., 2005; Li et al., 2009; Ouellette & Zubieta 2009; Song et al., 2010)]. By controlling the reaction conditions, such as the temperature, the pH of the medium and the solvent used, the tetrazole group can be coordinated to metal centres in µ₁-, µ₂-, µ₃- or µ₄-modes (Wang et al., 2005; Yang et al., 2009). However, µ₄-bridging coordination complexes are apparently less [common?] than the other three, which may be due to the spatial hindrance of the tetrazole group. A search for 5-substituted tetrazolate metal–organic complexes in the Cambridge Structural Database (CSD, version 5.33 plus Feb 1 2012 update; Allen, 2002) yielded 1183 hits, of which there were only 24 complexes where the tetrazolate anions adopt a µ₄-bridging mode (ca 2.0% of all the complexes). With the aim of preparing novel µ₄-bridge-based tetrazolate metal–organic complexes where the tetrazole group can be generated from an in situ reaction (Yang et al., 2009), we have used 4-cyanopyridine, NaN₃ and CuCN as the raw materials under hydrothermal conditions to give the title novel metal coordination polymer, (I), and report its crystal structure here.

Complex (I) crystallizes in the monoclinic C2/c space group, with the asymmetric unit consisting of one and a half Cu^I cations, half a 4-ptz anion and a whole CN^- anion, with atoms Cu2, N3, C3 and C4 residing on a twofold rotation axis (Fig. 1). The pyridyl group in the 4-ptz anion is moderately twisted away from the tetrazole group, with a dihedral angle of 45.0 (1)° between them. For the central Cu atoms, the Cu1 coordination environment is a distorted tetrahedron, with Cu—N/C bond lengths in the range 1.973 (3)–2.090 (2) Å. As far as atom Cu2 is concerned, although a very long inter-atomic contact of 2.870 (3) Å exists between Cu2 and N2(x - 1/2, -y + 5/2, z - 1/2), its coordination environment can be best described as planar–trigonal, with the metal atom residing completely in the plane and with bond distances of 2.018 (3) and 2.068 (3) Å for Cu2—N2(x - 1/2, y + 1/2, z - 1) and Cu2—C5(-x + 3/2, y + 1/2, -z + 3/2), respectively (Table 1). It is worth mentioning that all five N atoms in the 4-ptz anion are coordinated to these Cu^I centres, with the four tetrazole N atoms coordinated only to Cu1 and the pyridyl N atom bonded only to Cu2. Atom N4 of the CN^- anion is coordinated to Cu1 at one end and, at the other end, to Cu1 and Cu2 simultaneously through atom C5, with a Cu1···Cu2 distance of only 2.531 (1) Å, which is comparable with some analogues (Qin et al., 2011; Zhou et al., 2009).

In the crystal packing of (I), a three-dimensional coordination network is formed (Fig. 2a). The network can be analysed in terms of three aspects. Firstly, via the coordination of only a 4-ptz anion to Cu1 and Cu2, these two types of ionic components are linked into a one-dimensional chain running parallel to the [101] direction (Fig. 2b). Secondly, via the µ₂ coordination of cyano atom C5 to adjacent [101] chains, a two-dimensional layer structure runs parallel to the (101) plane, in which the Cu^I cations are arrayed alternately, separated by 4-ptz anions (Fig. 2b). Thirdly, adjacent (101) layers are joined together through the coordination of up and down cyano atoms N4 of the (101) plane to the Cu1 centres, forming a three-dimensional network.

To better understand this network from a topological view, the Cu1, Cu2, CN^- and 4-ptz ions can be regarded as 4-, 3-, 3- and 5-connected nodes. Thus, the whole network can be simplified as a (3,3,4,5)-connected topological network with the Schläfli symbol (4.6.8⁴)₂.(4^2.6.8⁷).(6.8²)₃ (Fig. 3). This is a new kind of topological network, according to analysis by TOPOS (Blatov, 2006). In comparison, Wang et al. (2009) have recently conducted an experiment using almost identical reactants to this work, with only CuCN replaced by CuBr. However, they obtained a completely different three-dimensional metal–organic polymer with a generally observed (3,4)-connected topological network. From this, we conclude that subtle alteration of the reaction conditions in a hydrothermal reaction may lead to entirely different products.

The spectroscopic characteristics of (I) were also investigated. The absorption band around 3437 cm^- in its IR absorption spectrum can be assigned to the stretching vibration of the aromatic C—H bonds, and the band at 2062 cm^-1 to the classical stretching vibration of the C≡N bond of the CN^-1 anion. In its solid luminescent spectrum upon excitation at 346 nm, (I) shows two emission peaks at 363 and 435 nm. The former can be attributed to ligand-to-metal charge transfer (LMCT) and the latter should be largely attributed to intra-ligand π–π^* fluorescent emission (Huang et al., 2006).

For related literature, see: Allen (2002); Blatov (2006); Huang et al. (2006); Jiang et al. (2004); Li et al. (2009); Ouellette & Zubieta (2009); Qin et al. (2011); Song et al. (2010); Wang et al. (2005, 2009); Xiong et al. (2002); Yang et al. (2009); Ye et al. (2005); Zhou et al. (2009).