supplementary materials

In the past decade, metal-organic coordination polymers have attracted considerable interest due to their potential applications and intriguing architectures (Batten & Robson, 1998). The saturated aliphatic dicarboxylate ligands, which exhibit conformational and coordination versatility due to single-bonded carbon chains, are also an attractive choice and considered as important flexible spacer ligand (Rao, et al., 2004). As one of the lower members in the α,ω-dicarboxylate family, the succinate anions play a special role. Under ambient conditions, the linkage of transition metal cations by succinate anions may lead to linear polymeric chains, two-dimensional open networks and three-dimensional framework coordination polymers (Zheng, et al., 2004). Some metal-organic coordination polymers are open-frameworked and there are guest species occluded by open-framework host in the structures. Water molecules are the commonly encountered guest species, and they usually play an important role in the stabilization of the host, on the other hand, the configuration of water clusters depends on the surrounding environment of the host (Wei, et al., 2006). A variety of water clusters observed in a number of hosts have been structurally characterized to help us gain insight into the nature of water-water interactions. Herein, we report the presence of a new type (H₂O)₁₂ water cluster in the structure of succinato bridged dinuclear complex [Cu₂(bpy)₄(C₄H₄O₄)](C₄H₄O₄).12H₂O.

The title compound consists of succinato bridged dinuclear [Cu₂(bpy)₄(C₄H₄O₄)]²⁺ complex cations, succinate anions and crystal water molecules. As illustrated in Fig. 1, Cu²⁺ in the complex cations are each square pyramidally coordinated by one O atom of the succinato ligand and four N atoms of two bpy ligands with the N3 atom at the apical position (d(Cu—O) = 1.981 (3) Å; equatorial d(Cu—N) = 2.000 (4)–2.033 (3) Å; axial d(Cu—N) = 2.172 (3) Å). The Cu atom is shift by 0.179 (2) from the equatorial plane through N1, N2, N4 and O4 atoms towards the apical N3 atom. The succinato group bis-bidentately bridges two Cu ions to form the dinuclear complex cation. Such bridging succinato ligand and the noncoordinating succinate anion are also exhibit trans conformation with the backbone C atoms in a plane. The dehedral angles between the two pyridine rings are twisted by 8.34° and 10.08° in the two different bpy ligands. The complex cations are forced to be aligned in such ways that the symmetry related bpy ligands are orientated parallelly and face the opposite directions with the interplanar distanceds varying from 3.533 Å to 3.541 Å. Obviously, sucn π-π stacking interactions are responsible for the supramolecular assembly of the complex molecules into two-dimensional layers parallel to (010). The resulting complex cationic layers are found to be stabilized by weak C(bpy)-H···O(carboxylate) hydrogen bonds with d(H4A···O2^II) = 2.47 Å (II: -x, -y + 2, -z + 2) (Fig. 2).

The most interesting feature of the solid-state structure of the title complex is the hydrogen-bonding interactions of the water molecules and the succinate anion, in which twelve water molecules form a (H₂O)₁₂ cluster associated by O—H···O hydrogen bonds (as shown in Fig 3). The geometric parameters of the clusters are summarized in Tables 1. The O···O distances range from 2.738 (6)–2.876 (7) Å and the angles of the O—H···O are vary from 145° to 175°. Interestingly, such (H₂O)₁₂ water clusters are farther hydrogen bond interacting with succinate anions to complete two-dimensional layers parallel to (010) with (d(O···O) = 2.682 (5)–2.758 (7) Å; <O—H···O = 149–176°).