Synthesis and crystal structure of a solvated CoIII complex with 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazone ligands

The synthesis, crystal structure and spectroscopic characterization of the novel and, according to our knowledge the first to be obtained in crystalline form, CoIII complex with a multidentate NSO-containing mixed-ligand − 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazone – is reported.


Chemical context
In recent years, Schiff bases have played a vital role in the progress of modern coordination chemistry, in the improvement of the areas of magnetism, luminescence, chirality, catalysis, cytotoxicity and ferroelectricity (Andruh et al., 2015;Mishra et al., 2016;Aazam & El-Said, 2014). Thiosemicarbazones represent an important class of Schiff base sulfur-donor ligands, particularly for many transition-metal ions. These metal complexes have received considerable attention, primarily because of their bioinorganic relevance (Gupta et al., 2003;Singh et al., 2000): they are promising drug candidates, biomarkers and biocatalysts (Hayne et al., 2014;Lim et al., 2010). It has been noted that some metal(II) complexes with thiosemicarbazone-derived ligands have the ability to induce apoptosis in cancerous cell lines (Ferrari et al., 2004;Santini et al., 2014).
Despite the attention towards Schiff bases, thiosemicarbazones and their metal complexes, very few studies have been devoted to the synthesis and crystal-structure determinations of Co complexes. In this work, we present the synthesis, crystal structure and spectroscopic characterization of the novel and, according to our knowledge, the first to be obtained ISSN 2056-9890 in crystalline, form Co III complex with the multidentate NSOcontaining mixed-ligand 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazone.

Structural commentary
The title complex crystallizes in the triclinic space group P1. The asymmetric unit (Fig. 1) consists of two independent mononuclear complex cations, a dithionate anion as counteranion and seven solvent molecules of crystallization (four dimethylmethanamide and three methanol). Each Co III ion is coordinated by two monodeprotonated (by the phenol group) ONS tridentate thiosemicarbazone ligands through the phenoxo oxygen, imine nitrogen and thione sulfur atoms. Thus, the coordination geometry around each Co III ion can be described as moderately distorted octahedral with an S 2 N 2 O 2 coordination sphere with N,O,N and S atoms in the equatorial plane and O and S atoms in the apical positions.
In the title compound, the two Co-N, Co-O and Co-S distances are each almost identical (the mean values being 1.89, 1.92 and 2.22 Å , respectively) to those in an analogous chromium complex with a similar ligand (CCDC refcode YIMPER;Chumakov et al., 2013). At the same time, the Co-O and Co-N distances in the title complex are shorter than in analogous Co II complexes with related semicarbazone ligands (Co-N = 2.041 Å and Co-O = 2.056 Å in VAYZUT, VAYZON and VAZBAC; Wu et al., 2017). The Co-S distances in the title complex are in the range 2.2202 (19)-2.2269 (17) Å , which is generally comparable to the range 2.23-2.24 Å observed for a Co III complex (VENDIB; Burstein et al., 1988) and shorter than was found for the Co II complex of glyoxylic acid with thiosemicarbazone (2.419-2.424 Å ; ODOWUC; Huseynova et al., 2018).
Despite the ligands coordinating to the Co III cations through the thione sulfur atoms, the C-S bond length of the thiosemicarbazone moiety (average length of 1.71 Å ) approaches the standard C S double-bond value and differs only slightly from the distance observed in the corresponding neutral ligand [1.688 Å in BIZYAL (Zhao et al., 2008) and 1.697 Å in BIZYAL01 (Vrdoljak et al., 2010)].

Supramolecular features
The solid-state organization of the complex can be described as an insertion of the anions and solvent molecules within the crystallographically independent complexes (Fig. 2). In the crystal, the components are linked by numerous N-HÁ Á ÁO and O-HÁ Á ÁO contacts (Table 1), giving a three-dimensional hydrogen-bonded network. Overall, the amino groups of the coordinated ligands are involved in eleven N-HÁ Á ÁO contacts: N8-H8AÁ Á ÁO8, N2-H2Á Á ÁO3 and N2-H2Á Á ÁO4 are contacts between ligands through the nitrogen of the secondary amino group and methoxy group oxygen; N11-H11Á Á ÁO14, N5-H5AÁ Á ÁO11 and N12-H12BÁ Á ÁO9 are contacts between the nitrogen of the secondary and The molecular structure of the title compound with the atom-labeling scheme. Displacement ellipsoids are drawn at the 30% probability level. Solvent molecules (dimethylformamide and methanol) are omitted for clarity.

Figure 3
A fragment of the packing of the title compound demonstrating the N-HÁ Á ÁO contacts that link three complex cations and an S 2 O 6 2À anion as an hydrogen-bond acceptor. Hydrogen bonds are shown as dashed lines. Methanol solvate molecules bonded to S 2 O 6 2À by O-HÁ Á ÁO hydrogen bonds, dimethyformamide solvent molecules and C-bound hydrogen atoms are omitted for clarity.

Synthesis and crystallization
The title compound was prepared according to a previously published procedure (Rusanov et al., 2003) by slow interdiffusion of a solution of 0.086 g (0.26 mmol) of CoS 2 O 6 Á6H 2 O in 1ml of methanol and 0.117g (0.52 mmol) of the ligand in 1ml of dimethylformamide and 1ml of chloroform. Dark-brown crystals of the title compound, suitable for X-ray analysis, were formed within a few days (yield: 60%).
The IR spectrum of the title compound (as KBr pellets) is consistent with the above structural data. In the range 4000-400 cm À1 it shows all characteristic peaks: (CH) due to aromatic C-H stretching at 3000-3100 cm À1 , the aromatic ring vibrations in the 1600-1400 cm À1 region, weak absorption band at 738 cm À1 due to (C-S) vibrations and the characteristic peak at 1608 cm À1 assigned to azomethine (C N) group. The weak band at 3308 cm À1 can be assigned to the N-H group vibrations. All these data are in good agreement with literature data (Seena & Kurup, 2007;Kalaivany et al., 2014 The Co II dithionate used in this work was prepared by mixing aqueous solutions containing stoichiometric amounts of cobalt sulfate and BaS 2 O 6 Á2H 2 O. The white precipitate of BaSO 4 was removed by filtration and the solution containing the metal dithionate was evaporated to a small volume on a rotary evaporator and then cooled for crystallization. BaS 2 O 6 Á2H 2 O was prepared using the method described by Pfanstiel (1946).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. One of the methanol molecules is disordered over two positions with relative occupancies of 0.597 (17) and 0.403 (17) for the major and minor components. The hydrogen atoms bonded to carbon were included at geometrically calculated positions and as riding with U iso (H) = 1.2U eq (C) for aromatic CH and U iso (H) = 1.5U eq (C) for methyl groups. The H atoms of the NH and OH groups were also placed at calculated position using the corresponding AFIX instruction with U iso (H) = 1.2U eq (N) for NH/NH 2 and U iso (H) = 1.5U eq (O) for OH hydrogen atoms.

Funding information
Funding for this research was provided by a grant from the Ministry of Education and Science of Ukraine for prospective development of the scientific direction 'Mathematical sciences and natural sciences' at Taras Shevchenko National University of Kyiv.  Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016/4 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
[Co (C 9   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 )