Crystal structure of dichlorido(2,2′:6′,2′′-terpyridine-κ3 N,N′,N′′)zinc: a redetermination

The crystal structure of the title compound, [ZnCl2(C15H11N3)], was redetermined based on modern CCD data. In comparison with the previous determination from photographic film data [Corbridge & Cox (1956 ▶). J. Chem. Soc. 159, 594–603; Einstein & Penfold (1966 ▶). Acta Cryst. 20, 924–926], all non-H atoms were refined with anisotropic displacement parameters, leading to a much higher precision in terms of bond lengths and angles [e.g. Zn—Cl = 2.2684 (8) and 2.2883 (11) compared to 2.25 (1) and 2.27 (1) Å]. In the title molecule, the ZnII atom is five-coordinated in a distorted square-pyramidal mode by two Cl atoms and by the three N atoms from the 2,2′:6′,2′′-terpyridine ligand. The latter is not planar and shows dihedral angles between the least-squares planes of the central pyridine ring and the terminal rings of 3.18 (8) and 6.36 (9)°. The molecules in the crystal structure pack with π–π interactions [centroid–centroid distance = 3.655 (2) Å] between pyridine rings of neighbouring terpyridine moieties. These, together with intermolecular C—H⋯Cl interactions, stablize the three-dimensional structure.


Related literature
The title compound is dimorphic, with one polymorph (form I) crystallizing in space group No. 15, and the second polymorph (type II) crystallizing in space group No. 14 (Corbridge & Cox, 1956). The crystal structure of the title compound was originally determined by Corbridge & Cox (1956) from photographic data (final R value = 0.24) and was later re-refined by Einstein & Penfold (1966) based on the original intensity data but using more advanced least-squares procedures (R = 0.14). In both reports, the setting in P2 1 /a of space group No. 14 was used. For background to terpyridine-based materials, see: Fermi et al. (2014); Song et al. (2014). For the biocompatibility of zinc compounds, see: Gao et al. (2009 Table 1 Hydrogen-bond geometry (Å , ).  (Sheldrick, 2008) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

S1. Experimental
A solution of 2,2′:6′,2′′-terpyridine (0.23 g, 1 mmol) in acetonitrile (20 ml) was mixed with a solution of zinc chloride (0.14 g, 1 mmol) in methanol (5 ml) and refluxed for 4 h. The reaction mixture was the cooled to room temperature and filtered into a large test tube. Colorless crystals were obtained at room temperature after two weeks. Yield: 75%.

S2. Refinement
All hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C -H = 0.93 Å and U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. The arrangement of the molecules in the crystal structure of (I), showing π-π interactions (dashed red lines) and C-H···Cl hydrogen bonds (dashed green and turquoise lines).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.