supplementary materials

Thallium reagents, despite their inherent toxicity and cost, have played a conspicuous role in the development of modern inorganic and organometallic chemistry. Thallium(I) chemistry is very interesting due to a variety of reasons. (a) Thallium salts and complexes are often anhydrous. (b) The lone pair on thallium may or may not be stereochemically active. (c) High coordination number presents because of large size of Tl^I ion. (d) Thallium(I) complexes have potential ability to form metal–metal bonds and thallium(I) also forms complexes with aromatic hydrocarbons (Akhbari & Morsali, 2008).

The deprotonated forms of benzene-1,2,4,5-tetracarboxylic acid (btcH₄) can act not only as hydrogen bond acceptors but also as hydrogen bond donors, depending on the deprotonated carboxylate groups, to give different supramolecular adducts (Fabelo et al., 2005). There is an instance of benzene-1,2,4,5-tetracarboxylate coordinated to thallium in a mixed ligand system (Day & Luehrs, 1988). However, there are some coordination polymers reported that contain an anionic coordination polymer together with a cationic part, such as metal–organic framework-based hydrogen-bonded porous solids, [(pipzH₂)M(btc)(H₂O)₄.4H₂O]_n (M = Co^II, Ni^II, Zn^II; pipz = piperazine) (Murugavel et al., 2000). As the recent examples of this category, Cu^II and Zn^II anionic coordination polymers with ethylenediaminium and propane-1,2-diaminium (pn) as counter ions, {(enH₂)[Cu(btc)].2.5H₂O}_n (Rafizadeh et al., 2007a) and {(pnH₂)[Zn(btc)].4H₂O]}_n (Rafizadeh et al., 2007b), have been synthesized.

In the title compound (Fig. 1), the coordination behavior of carboxylate groups of btc are different. Compared with another Tl^I complex, [Tl(pydcH)]_n (pydcH₂ = pyridine-2,6-dicarboxylic acid), with the bond lengths of Tl—O being 2.853 (6) and 3.019 (6) Å (Rafizadeh et al., 2005), the Tl—O bond lengths of the title compound are in a more extended range [2.702 (5) to 3.350 (5) Å] (Table 1). In the crystal structure, O—H···O, N—H···O and C—H···O hydrogen bonds are present. Moreover, an interesting Tl···π interaction is found that is classified as cation···π interaction at a Tl–centroid distance of 3.537 (1) Å, as shown in Fig. 2. These interactions make all components assemble together in a packing arrangement.

As illustrated in Fig. 1, coordination number of the Tl^I atom is seven, with all coordinated atoms forced into one side of Tl^I and other side is left empty. This can be caused by the stereochemically active lone pair on Tl^I center. Based on crystal data available in the Cambridge Structural Database, stereochemistry of Pb^II complexes has been reviewed (Shimoni-Livny et al., 1998). Evidently, in the case of Pb^II complexes when the lone pair appears to have no steric effects, the bonds with ligand donor atoms are arranged throughout the surface of encompassing sphere (holodirected coordination) and there are no marked differences in the Pb—L bond lengths. But the Pb^II complexes, in which the lone pair is stereochemically active, have hemidirected coordination and the Pb—L bonds are directed only to a part of the coordination sphere, leaving a gap in the distribution of bonds to the ligands. There are shorter Pb—L bonds away from the proposed site of the lone pair and longer Pb—L bonds adjacent to this site of the lone pair (Li et al., 2008). Here also, the Tl^I atom shows the same behavior. In effect, the Tl1—O3ⁱⁱ and Tl1—O1Wⁱⁱⁱ (symmetry codes: (ii) -1+x, y, z; (iii) -x, -y, 1-z), that are apparently longer than other bonds (see Fig. 1 and Table 1), lie on the side of the putative lone pair and the shorter bonds lie away from the site of the lone pair.