Crystal structure of [tris(2-benzimidazolylmethyl)amine-κ⁴N,N′,N′′,N′′′]-[(pyridine-2,6-dicarboxylato-κ²O,N)]cadmium(II)–methanol (1:3) C₃₄H₃₆CdN₈O₇

Source of material

Tris(2-benzimidazolylmethyl)amine (ntb) was prepared according to the literature method [1]. To a stirred methanol solution (20 mL) containing cadmium perchlorate hexahydrate (0.066 g, 0.2 mmol) and ntb (0.08 g, 0.2 mmol), a methanol solution (10 mL) of 2,6-pyridinedicarboxylic acid (H₂dipic, 0.033 g, 0.2 mmol) and triethylamine (0.022 g, 0.2 mmol) was added dropwise. After stirring for 3 h, the filtrate of the resulting solution was allowed to stand at room temperature for several weeks to give colorless crystals of the title compound, in a yield of 55%.

Experimental details

The crystal structure was refined using the SHELX-18/3 package [3, 4] . Hydrogen atoms were positioned geometrically and allowed to ride on their parent atoms. The U_iso values of the hydrogen atoms of methyl groups were set to 1.5U_eq(C) and the U_iso values of all other hydrogen atoms were set to 1.2U_eq(C).

Discussion

Imidazole is a typical heterocyclic ligand as a component of biologically important molecules [5]. Moreover, the derivates of imidazole have long been used in coordination chemistry [6]. For example, tris(2-benzimidazylmethyl)amine (ntb) was used in the coordination of transition metals to mimic the active sites of superoxide dismutase by Nie’s group [7]. The tetradentate tripodal ligand ntb contains three aromatic N-donors on its arms, which can each rotate freely around an N(apical)-C bond. Thus, multicomponent complexes or coordination polymeric networks can be formed from this ligand with metal ions of low coordination number [8]. Compared to other metal complexes, cadmium complexes of ntb are rarely reported [9, 10] . On the other hand cadmium(II) complexes of ntb using 2,6-pyridinedicarboxylate as coligand have never been reported. In this article, we report the structures of a new 2,6-pyridinedicarboxylate-containing cadmium(II) complex of ntb, which exhibits hydrogen-bonding forming a chain structure.

The title compound consists of one [Cd(ntb)(dipic)] complex, and three methanol molecules. The Cd(II) ion is octahedrally coordinated by four N atoms of ntb, one carboxylate oxygen and pyridine nitrogen of the 2,6-pyridinedicarboxylate ligand. The equatorial plane is defined by three benzimidazoles N(1), N(2), N(6) of ntb and one carboxylate oxygen atom O(1). Equatorial bond angles are ranging from 69.7° to 139.8°. The bond distance of Cd(1)—N(1) = 2.602(2) Å, lies trans-to O(1) atom, is significantly longer than Cd(1)—N(2) = 2.272(2) Å, Cd(1)—N(6) = 2.268(2) Å, and Cd(1)—O(1) = 2.3571(19) Å. The axial positions are occupied by N(4) and N(8) with N(4)—Cd(1)—N(8) angle of 139.91°. The bond length of Cd(1)—N(4) is 2.352(3) Å and the distance of Cd(1)—N(8) is 2.336(2) Å. The bond distance Cd(1)—N(1) and the angle N(4)—Cd(1)—N(8) reflect the distortions of the coordination octahedron. Other significant distortions are observed in the angles, N(1)—Cd(1)—O(1) = 139.82°, N(2)—Cd(1)—N(6) = 136.32°. In addition, intermolecular H-bond of type N—H⋯O [N(3)H(3)⋯O(3) = 2.72 Å N(3)H(3)⋯O(3) 168°] between the NH group of ntb and the uncoordinated carboxylate oxygen atom of pyridinedicarboxylate anion bridges adjacent molecules to form a dimeric unit. These dimers are further connected via two hydrogen bonds: one between the NH group of ntb and methanol oxygen N—H⋯O [N(7)—H(7 A)⋯O(5) = 2.775 Å N(7)—H(7 A)⋯O(5) 152.94°] and the other between OH of methanol and the uncoordinated carboxylate oxygen of the picolinato ligand O—H⋯O [O(5)—H(5 A)⋯O(2) = 2.752 Å O(5)—H(5 A)⋯O(2) 161.23°] to form two antiparallel chains. Geometric parameters of the ligands and coordination modes are in perfect accord with the literature [11].