Crystal structure and DFT study of a zinc xanthate complex

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′,N′-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′,N′-tetra­methyl­ethylenedi­amine, [Zn(S₂COC₂H₄OCH₃)₂(tmeda)], which was investigated by a DFT study.

Structural commentary

The mol­ecular structure of the title compound is illustrated in Fig. 1 ▸. The Zn ^{ion is coordinated by two N atoms of the} N,N,N′,N′-tetra­methyl­ethylenedi­amine mol­ecule and two S atoms from two 2-meth­oxy­ethylxanthate mol­ecules. The Zn1—N1, Zn1—N2, Zn1—S1 and Zn1—S3 bond lengths are 2.141 (5), 2.123 (5), 2.3107 (9) and 2.3050 (9) Å, respectively (Table 1 ▸). These bond distances are similar to those reported in the work of Cusack et al. (2004 ▸). The C7—O8 and C13—O14 bond lengths are similar [1.344 (3) and 1.346 (3) Å, respectively], while the C9—O8 and C15—O14 bonds are also not significantly different [1.454 (3) and 1.459 (3) Å, respectively]. In the same way, the C10—O11 [1.417 (3)] and C16—O17 [1.418 (4)] bond lengths are similar to each other. All of the C—O bonds show single-bond character. In the {S₂C} section of the xanthate ligands, the carbon-to-sulfur S1 distance is 1.731 (3) Å, which is typical of a single bond whereas the carbon-to-sulfur S2 distance of 1.647 (3) Å is typical of a carbon-to-sulfur double bond. In the mol­ecule, weak C1—H1C⋯O8, C2A—H2AB⋯O11, C5A—H5AA⋯S1 and C6—H6C⋯S4 intra­molecular inter­actions are observed (Table 2 ▸).

Figure 1

The mol­ecular structure of the title complex, with the atom labelling. Only the major component of the disordered amine ligand is shown. Displacement ellipsoids are drawn at the 50% probability level.

Supra­molecular features

The crystal packing of the title compound (Fig. 2 ▸) features inter­molecular hydrogen bonds (C6—H6B⋯O11^{, C3}A—H3AB⋯S2^{, C6}A—H6AA⋯S1^{, C4}A—H4AA⋯O17^{, C4}A—H4AB⋯S3^{, C9—H9}A⋯O17^{, C9—H9}B⋯S2 ^{and C18—H18}B⋯S2^{; symmetry codes as in Table 2} ▸), which connect the mol­ecules into a three-dimensional supra­molecular architecture.

Figure 2

A view of the crystal packing of the title complex. Dashed lines denote the inter­molecular hydrogen bonds (Table 2 ▸). Symmetry codes: (i) x − , −y + , z − ; (ii) x + , −y + , z + .

Database survey

Previously reported complexes related to the title complex are [Cd(S₂COCH₂CH₂OMe)₂(bipy)] [CSD (Groom et al., 2016 ▸) refcode BENDII; Chen et al., 2002 ▸], [Ni(C₄H₇O₂S₂)₂(C₆H₁₆N₂)] (NADTAQ; Qadir, 2016 ▸), [Ni(moexa)₂phen] (unsolvated form) and [Ni(moexa)₂phen] (benzene solvate), moexa = O-methoxy­ethyl­xan­thato-S,S′ (with refcodes SICTUT and SICVAB, respectively; Edwards et al., 1990a ▸), [Ni(moexa)₂bpy]; forms I and II (with refcodes VETVIZ and VETVIZ01, respectively; Edwards et al., 1990b ▸) and [Cd(S₂COCH₂CH₂OCH₃)₂(4,7-Me₂phen)] (WACPOG; Chen et al., 2003 ▸). The Cd—S and Cd—N bond lengths range from 2.489 to 2.796 Å and 2.334 to 2.406 Å, respectively. Similarly, the Ni—S and Ni—N bond lengths range from 2.432 to 2.458 Å and 2.070 to 2.172 Å, respectively. In these complexes, compared with the Zn ^{complex, the metal-to-ligand distances with} M—S/N bond lengths follow the order Zn ^{< Ni < Cd in the corresponding complexes.}

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) quantum-chemical 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.

Figure 3

The electron distribution of the HOMO and LUMO energy levels of the title compound.

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 &amp; 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 C—H⋯O inter­actions in the crystal structure. The negative potential value of −0.092 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′,N′-tetra­methyl­ethylenedi­amine moiety.

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.

Synthesis and crystallization

Tetra­methyl­ethylenedi­amine (10 mmol, 1.16 g) was added to a hot solution of Zn(CH₃CO₂)·2H₂O (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 ^{in the unit cell and the electron count was 18.2 per void.}