Syntheses, crystal structures and Hirshfeld surface analyses of bis(2-mercaptobenzimidazole)bromo- and iodocopper(I) complexes

The title complexes feature distorted trigonal–planar CuS2 X (X = Br, I) coordination geometries for the metal ions. The presence of the acetone solvent molecule in the iodide complex plays an important role in the packing.


Chemical context
2-Mercaptobenzimidazole (C 7 H 6 N 2 S; bimztH 2 ) has many uses including as an antioxidant to prevent rubber deterioration (Moldovan & Alexandrescu, 2002), an absorbant of mercury from industrial waste water in the form of 2-mercaptobenzimidazole-clay (Manohar et al., 2002), as a modifier of electrode surfaces to increase the efficiency of electrochemical analysis (Berchmans et al., 2000), as an intermediate in the production of the anti-inflammatory drug lanzoprazole (Wongwattana, 2004) and as a Cu corrosion inhibitor (Finšgar, 2013).
The preparation of bimztH 2 involves the reaction between o-phenylenediamine and potassium ethyl xanthate in an ethanol-water mixture followed by reaction with acetic acid and water at 333-343 K (Vanallan & Deacon, 1971). The structure of bimztH 2 exhibits tautomerism between its thione and thiol forms (Rout et al., 1984) as shown in the scheme below.
We now describe the syntheses and crystal structures of bimztH 2 complexes with copper(I) halides, CuX (X = Br, I). It may be noted that the S atom of the ligand is a soft base and therefore favoured to form a coordinate bond with a soft acid such as copper(I). Hirshfeld surface analyses were performed to gain further insight into the intermolecular interactions in these structures.

Structural commentary
The mononuclear structures of [Cu(bimztH 2 ) 2 Br] (1) and [Cu(bimztH 2 ) 2 I]ÁCH 3 COCH 3 (2) are depicted in Fig. 1. Both complexes crystallize in the monoclinic system, space group P2 1 /c. The copper ions adopt distorted trigonal-planar coordination geometries with one Cu-X bond (X = Br, I) and two Cu-S bonds, the lengths of which lie between 2.2189 (15) and 2.5479 (7) Å , being close to those found in complexes with a trigonal-planar geometry such as [Cu 2 (mimtH) 5 ] 2+ (Atkinson et al., 1985) and [Cu(SC 6 H 5 ) 3 ] 2À (Coucouvanis et al., 1980). When comparing (1) and (2), the bond angles are distorted from the ideal values of 120 with greater distortion in (2) resulting from the presence of the acetone solvent molecule and an N4-H4AÁ Á ÁO1 hydrogen bond. The acetone molecules in (2) result in weaker C S bonds as supported by IR and 13 C NMR data (vide infra). Both complexes feature a pair of intramolecular N-HÁ Á ÁX hydrogen bonds as listed in Tables 1 and 2 for (1) and (2), respectively.

Hirshfeld surface analysis
The Hirshfeld surface (HS) analyses (HS mapped over d norm are shown in Fig. 6) and d e and d i fingerprint plots (Figs. 7 and 8) were generated using Crystal Explorer 17.5 (Turner et al., 2017). The red spots indicate the donors and acceptors of the hydrogen bonds, appearing close to H1A and S1 of the N1-H1AÁ Á ÁS1 bond for (1) and close to H2AÁ Á ÁS1 of the N2A--H2AÁ Á ÁS1 bond for (2). In addition, a red spot is found between H4A and O1 of the acetone solvent molecule for (2). The fingerprint plots for (1) show that the principal intermolecular contacts are HÁ Á ÁH at 34.6% (Fig. 7b) The intermolecularinteractions in the crystal packing of (1) plotted down the c axis.

Figure 3
The intra-and inter molecular hydrogen-bonding interactions of (2).

Figure 5
The intermolecularinteractions in the crystal packing of (2) plotted down the b axis.

Database survey
2-Mercaptobenzimidazole has been found to form a complex with Pt, the bond formation being via the sulfur atom only with a square-planar geometry [Cambridge Structural Database (Groom et al., 2016) refcode GURMOV; Jolley et al., 2001]. In the case of the Co II complex, two sulfur atoms are bonded with the metal atom in a tetrahedral coordination geometry (refcode ZOKYAZ; Ravikumar et al., 1995). Cu I complexes with 2-mercaptobenzimidazole derivatives have been investigated as a model of copper proteins (refcodes QORGUZ, QORHAG and QORHEK; Balamurugan et al., 2001). A series of polynuclear clusters containing Ni II and Co II (refcodes FOPVEN, FOPVIR and FOPXOZ; Han et al., 2015) of this ligand have been synthesized and the magnetic susceptibility of an Ni II complex (FOPVEN) has been reported. The photophysical properties of the rigid structure of a hexanuclear Cu I complex of 2-mercaptobenzimidazole constructed by S bridges has been studied (refcode COPNUT; Singh et al., 2017).

Synthesis, crystallization and chracterization
[Cu(bimztH 2 ) 2 Br] (1) A mass of 0.19 g (1.2 mmol) of bimztH 2 was placed in 30 ml of acetone at 318 K and stirred until completely dissolved to form a colourless solution. CuBr (0.09 g; 0.6 mmol) was added followed by further stirring for about 15 min to obtain a yellow solution, which was refluxed for 120 min at 353 K to become turbid with a light-yellow colour and then filtered. The colourless filtrate was left at room temperature for 3 days to form transparent needles and then filtered by vacuum suction to obtain 0.16 g of (1) (58% yield, m.p. 518-523 K). Elemental analysis (%): found (calculated); C = 38.32 (37.86), H = 2.78 (2.73), N = 12.17 (12.62), S = 14.71 (14.45).
FT-IR spectra Suzuki (1962) proposed that features in thioamide IR spectra could be assigned to band I at 1395-1570 cm À1 arising from the N-H deformation and C-N stretching; band II at 1270-1420 cm À1 from C-N stretching, N-H deformation and C-H bending, band III at 940-1140 cm À1 from C-N and C S stretching and band IV at 680-860 cm À1 due to C S stretching (compare Jolley et al., 2001). Additionally, Raper et al. (1988) studied absorption bands of thioamide in the complex prepared from bimzH 2 and copper(II) perchlorate and found them at 1470 cm À1 (band I), 1360 cm À1 (band II), 1180 cm À1 (band III) and 740 cm À1 (band IV) compared with those of the free ligand at 1468 cm À1 , 1357 cm À1 , 1181 cm À1 and 744 + 713 cm À1 , respectively. The broad absorption band at 3155 cm À1 is due to N-H stretching, which moves to a higher wavenumber and splits into two upon complexation.
For all our complexes, the FT-IR spectrum indicates the shift of bands I and II to a higher wavenumber, similar to the behaviour of N-H stretching due to the coordination through the sulfur atom and resulting charge transfer from N to S,  The fingerprint plots for (1).

Figure 8
The fingerprint plots for (2). which makes the N-H and C-N bonds stronger (Aslanidis et al., 2002). Band III of thioamide for all complexes shifts to a lower wavenumber but this is hard to quantify because this area also covers C-N stretching. Band IV for C S stretching changes significantly from 744 and 713 cm À1 in the free ligand to 734 cm À1 in the complex, reflecting copper-sulfur coordination. A change also occurs for the C-S bending mode at 602 cm À1 of C-S bending to lower wavenumber, corresponding with previous work (Raper et al., 1988 Table 3 IR peak assignments (cm À1 ) for the bimztH2 ligand and (1) and (2) (1) and (2).
(1) and 1384 cm À1 were found (figure not shown). After heating at 383 K for 10 minutes, these bands disappeared. Therefore these are due to C O stretching and C-H bending, respectively, indicating the presence of acetone in the compound. IR data are summarized in Table 3.
1 H NMR and 13 C NMR spectra 1 H NMR data for the ligand and (1) and (2) are listed in Table 4. The chemical shift at 13.28 ppm (br, s) belongs to two groups of N-H protons. The ratio of integration reveals that the two protons have the same environment. The ratio of N-H and aromatic protons is 1:2 without the signal of the S-H proton, indicating that both ligand and complex contain thione in DMSO-d 6 (Isab et al., 2003). Furthermore, the ligand exhibits chemical shifts around 7.49 ppm due to four methane protons on an aromatic benzene ring at positions H 4 , H 7 , H 5 , and H 6 , which change upon complex formation. The 13 C NMR spectra of the ligand and complexes (Table 5) reveal seven carbon signals, including that of the thiocarbonyl group at 168.34 ppm, four carbon atoms in the aromatic ring at 109.75 and 122.59 ppm for C 4,7 and C 5,6 , respectively, and two quarternary carbon atoms at 132.48 ppm. In the complex, C 2 and C 8,9 have upfield chemical shifts due to more electron shielding. The coordination via sulfur causes C S to be weaker as well as the electron density to change from nitrogen to C 2 , whereas C 4,7 and C 5,6 have downfield chemical shifts due to the electron transfer to C 8,9 , corresponding with the work of Isab et al. (2003). For (2), the carbonyl signal at 206.64 ppm and methane carbon at 30.86 ppm indicate the presence of acetone in the compound.

Bromidobis(2,3-dihydro-1H-1,3-benzodiazole-2-thione)copper(I) (1)
Crystal data 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 )