Crystal structure and DFT study of a zinc xanthate complex

In the title compound, the ZnII ion is coordinated by two N atoms of the N,N,N′,N′-tetramethylethylenediamine ligand and two S atoms from two 2-methoxyethyl xanthate ligands. Two C—H⋯O and two C—H⋯S intramolecular interactions occur. In the crystal, molecules are linked by C—H⋯O and C—H⋯S hydrogen bonds, forming a three-dimensional supramolecular architecture.


Chemical context
Xanthates (dithiocarbonates) are related to the dithiolate family. Xanthate is a bidentate monoanionic sulfur-sulfur donor ligand. It stabilizes complexes of most of the transition elements and can bind metal centers in monodentate, isobidenate, anisobidenate or ionic modes. Xanthates have the ability to inhibit the replication of both RNA and DNA viruses in vitro (Friebolin et al., 2005). They have been used as accelerators in the vulcanization of rubber (Gupta et al., 2012), in cellulose synthesis (Tiravanti et al., 1998), as collectors in the froth flotation of metal sulfide ores (Lee et al., 2009) and as reagents for heavy-metal sedimentation in waste-water treatment (Chakraborty et al., 2006). In our previous work, we prepared and structurally characterized nickel(II) and zinc(II) n-propylxanthate complexes containing N,N,N 0 ,N 0 -tetramethylethylenediamine as a neutral ligand. Both complexes showed a distorted octahedral environment around the metal center (Qadir & Dege, 2019). In this paper, we report the synthesis and crystal structure of a zinc(II) 2-methoxyethylxanthate complex containing N,N,N 0 ,N 0 -tetramethylethylenediamine, [Zn(S 2 COC 2 H 4 OCH 3 ) 2 (tmeda)], which was investigated by a DFT study.

Figure 1
The molecular structure of the title complex, with the atom labelling.

Frontier molecular orbital analysis
The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are named as frontier molecular orbitals (FMOs). The FMOs play an important role in the optical and electric properties. The frontier orbital gap characterizes the chemical reactivity and the kinetic stability of the molecule. A molecule with a small frontier orbital gap is generally associated with a high chemical reactivity, low kinetic stability and is also termed a soft molecule. The density functional theory (DFT) quantumchemical calculations for the title compound were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). Fig. 3 illustrates the HOMO and LUMO energy levels of the title compound. The small HOMO-LUMO energy gap (3.19 eV) in this compound indicates the chemical reactivity is strong and the kinetic stability is weak, which in turn increases the non-linear optical activity. As a result, with the small HOMO-LUMO energy gap, this compound could potentially be used in optoelectronic applications.

Molecular electrostatic potential (MEP)
The MEP map of the title molecule was calculated theoretically at the B3LYP/6-311G(d,p) level of theory and is illustrated in Fig. 4. The blue-coloured region is electrophilic and electron poor, whereas the red colour indicates the nucleophilic region with rich electrons in the environment and provide information about interactions that can occur between molecules (Tankov & Yankova, 2019). In the title compound, the reactive sites are localized near the C-O group: this is the region having the most negative potential spots (red colour), all over the oxygen atom because of the The electron distribution of the HOMO and LUMO energy levels of the title compound.

Figure 4
The total electron density three-dimensional surface mapped for the title compound with the electrostatic potential calculated at the B3LYP/6-311 G(d,p) level.
C-HÁ Á ÁO interactions in the crystal structure. The negative potential value of À0.092 a.u. indicates the region that shows the strongest repulsion (electrophilic attack). In addition, the most positive region is located at the hydrogen atoms and shows the strongest attraction (nucleophilic attack) sites, which involve the N,N,N 0 ,N 0 -tetramethylethylenediamine moiety.

Synthesis and crystallization
Tetramethylethylenediamine (10 mmol, 1.16 g) was added to a hot solution of Zn(CH 3 CO 2 )Á2H 2 O (10 mmol, 2.20 g) in 2-methoxyethanol. A hot solution of potassium 2-methoxyethylxanthate (20 mmol, 3.81 g) in 2-methoxyethanol was added and the mixture was stirred for 30 min. Water was added to the mixture and a white precipitate was formed. The product was recrystallized from acetone.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-bound H atoms were positioned geometrically and refined using a riding model, with C-H = 0.98 and 0.99 Å and with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) otherwise. All atoms of the amine ligand are disordered and were modelled as two orientations with relative occupancies of 0.538 (6) and 0.462 (6). The diffuse electron density of half an acetone solvent molecule was removed with the solvent-mask procedure implemented in OLEX2 (Dolomanov et al., 2009). There are two voids of 122.4 Å 3 in the unit cell and the electron count was 18.2 per void.

Crystal data
[Zn(C 4 H 7 O 2 S 2 ) 2 (C 6 H 16 N 2 )]·0. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (