Bis(benzenethiolato)(2,2′-biquinoline)zinc(II)

The title compound, [Zn(C6H5S)2(C18H12N2)], was prepared as a model for future complexes that will be incorporated into light-harvesting arrays. The ZnII atom lies on a twofold rotation axis and the ligands are arranged tetrahedrally around this atom. The benzenethiolate ligand and the biquinoline ligand are nearly perpendicular to one another, making a dihedral angle of 84.09 (5)°. The biquinoline ligand is nearly planar, with a maximum deviation of 0.055 (3) Å from the mean plane of the ring system. In the crystal, the molecules pack in a manner such that the biquinoline ligands are parallel to one another, with a π–π interaction [interplanar distance = 3.38 (1) Å] with the neighboring biquinoline ligand.

The title compound, [Zn(C 6 H 5 S) 2 (C 18 H 12 N 2 )], was prepared as a model for future complexes that will be incorporated into light-harvesting arrays. The Zn II atom lies on a twofold rotation axis and the ligands are arranged tetrahedrally around this atom. The benzenethiolate ligand and the biquinoline ligand are nearly perpendicular to one another, making a dihedral angle of 84.09 (5) . The biquinoline ligand is nearly planar, with a maximum deviation of 0.055 (3) Å from the mean plane of the ring system. In the crystal, the molecules pack in a manner such that the biquinoline ligands are parallel to one another, with ainteraction [interplanar distance = 3.38 (1) Å ] with the neighboring biquinoline ligand.

S1. Comment
Deeply colored, luminescent complexes of zinc(II) arise when this d 10 ion is coordinated with mixed ligands of benzenethiol anions and dinitrogenpolypyridyl ligands (Koester, 1975;Crosby et al., 1985;Highland et al., 1986). In many cases, the lowest-lying excited state of this class of molecules has been assigned to a charge transfer from the thiol to a π * molecular orbital on the nitrogen heterocycle (Koester, 1975;Highland et al., 1986). The straightforward synthesis of the complexes, the cost, and the variety of ligand substitutions that are possible, enable numerous ways to tune the energy of this ligand-to-ligand charge transfer state. The closed-shell zinc(II) complex of this study absorbs strongly in the visible region of the spectrum and serves as a model for future complexes that will be incorporated into light-harvesting arrays.
The molecular structure of the title compound is illustrated in Fig. 1. The ligands are arranged tetrahedrally around the zinc atom, which lies on a 2-fold rotation axis. The benzenethiolate ligand and the biquinoline ligand are nearly perpendicular to one another, making a dihedral angle of 84.09 (5)°. The benzenethiolate ligands make a 72.30 (5)° angle with one another. The biquinoline ligand is nearly planar, with a maximum deviation of 0.055 (3) Å from the mean plane of the ring system.
In the crystal of the title compound the molecules pack in a manner such that the biquinoline ligands of all molecules are parallel. An exhaustive study has been made (Janiak, 2000) of structures in the Cambridge Structural Database (Allen, 2002) which show π-π interactions between quinoline ring systems. This study showed that parallel ring systems which interact are offset by an amount related to the distance between ring centroids. In the present study, the planes of the quinoline rings related by π-π interactions are ca. 3.38 Å apart. The centeroids of the pyridine ring and the benzene ring are ca. 3.68 Å apart, and the centroid-centroid line makes an angle of 23.3° with the normal to the plane of the quinoline rings. These values are in agreement with those found in the Janiak study. The π-π interactions may account for the nearplanarity of the biquinoline ligand.

S2. Experimental
The complex was synthesized via a general procedure (Crosby et al., 1985). Zn(OAc) 2 .2H 2 O (0.0566 g) was dissolved in 5 ml absolute ethanol and heated. benzenethiol (0.0582 g) was dissolved in 4 ml absolute ethanol and heated. The benzenethiol solution was then added dropwise to the zinc(II) solution with vigorous stirring at reflux. 2,2′-biquinoline (0.0640 g) dissolved in 5 ml absolute ethanol was then slowly added to the refluxing solution. The solution turned orange and was allowed to sit overnight. An orange crystalline solid (0.0848 g) was collected in 47% yield via vacuum filtration and washed with cold ethanol. The compound was characterized by 1 H NMR, UV-VIS absorption spectroscopy, and room temperature and 77 K emission spectroscopy.

S3. Refinement
The H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.93 Å, with U iso (H) = 1.2U eq (parent C-atom).  Crystal packing diagram of the title compound, showing the π-π interactions between the biquinoline ligands.