Crystal and molecular structures of a binuclear mixed ligand complex of silver(I) with thiocyanate and 1H-1,2,4-triazole-5(4H)-thione

The centrosymmetric binuclear complex features a tetrahedral AgI centre within a S4 donor set. The three-dimensional molecular packing is sustained by amine-N—H⋯S(thione), N—H⋯N(triazolyl) and N—H⋯N(thiocyanate) hydrogen bonds.


Chemical context
The title binuclear Ag I complex, (I), containing 1H-1,2,4triazole-5(4H-thione) and thiocyanate ligands has been synthesized and its crystal and molecular structures determined as part of our on-going studies in this area (Kodcharat et al., 2013). Interest in the 1,2,4-triazole-based heterocyclic thione derives from the various medical applications and extensive biological activity exhibited by Schiff base molecules derived from 1,2,4-triazoles. For example, these molecules are known for their anti-fungal, anti-bacterial, anti-tumour, anticonvulsant, anti-inflammatory and analgesic properties (Al-Soud et al., 2003;Walczak et al., 2004;Almasirad et al., 2004;Amir & Shikha, 2004;Turan-Zitouni et al., 2005). In addition, the synthesis and biological activities of coordination complexes of these molecules continue to attract significant attention as coordination often enhances the biological activity of the organic molecules (Dharmaraj et al., 2001;Singh et  1H-1,2,4-Triazole-5(4H-thione), the heterocyclic ligand in (I) and hereafter referred to as HtrzSH, has attracted relatively little attention in the literature although recently the anti-cancer potential of derivatives of this were described (Bü yü kekşi et al., 2018). The crystallographic study of (I) described herein is complemented by an analysis of the calculated HOMO and LUMO and an analysis of the calculated Hirshfeld surfaces and energy frameworks.

Structural commentary
The binuclear complex, [Ag(HtrzSH) 2 (SCN)] 2 (I), Fig. 1, crystallizes in the monoclinic space group P2 1 /n and is disposed about a crystallographic centre of inversion. The HtrzSH molecules only employ their exocyclic thione-sulfur atoms in coordination, there being no AgÁ Á ÁN contacts of note. Each Ag I atom is coordinated by a terminally bound HtrzSH molecule and by two thione-sulfur atoms derived from two 2 -bridging HtrzSH molecules. The coordination of each Ag I atom is completed by a terminal, S-bound thiocyanate anion. The geometry around the silver centre defined by the S 4 donor set is distorted tetrahedral with the S-Ag-S bond angles spanning about 25 , i.e. from a narrow 91.60 (2) for S1-Ag-S1 i , being subtended by the bridging S1 atoms, to a wide 127.43 (2) for S2-Ag-S3; symmetry operation (i): 1 À x, 1 À y, 1 À z. The Ag 2 S 2 core has the shape of a distorted rhombus as the Ag-S1 bond length of 2.5596 (7) Å is significantly shorter than the Ag-S1 i bond of 2.8188 (7) Å . The Ag-S bond lengths fall in two distinct classes, with the Ag-S1 b and Ag-S t (b = bridging, t = thiocyanate) bond lengths being similar and shorter than Ag-S1 i b (Table 1). Despite the different modes of coordination of the thione-S atoms, the C1-S1 and C3-S2 bond lengths are indistinguishable at 1.698 (3) Å . Each of the C1-S1 and C3-S2 bond lengths in (I) are marginally longer than 1.6836 (19) Å found in the structure of the free molecule (Bü yü kekşi et al., 2018). This small difference is reflected in the observation that no significant differences are evident in bond lengths within the five-membered rings in (I) and those in the uncomplexed molecule (Bü yü kekşi et al., 2018).
The five-membered rings lie prime to either side of the Ag 2 S 2 core, with the dihedral angles between the core and the N1-and N4-rings being 88.99 (11) and 85.16 (11) , respectively. The independent rings are close to being co-planar, exhibiting a dihedral angle of 8.38 (16) . Finally, the N1-amine is orientated to be in close proximity to the S3-thiocyanato atom, enabling the formation of an intramolecular amine-N-HÁ Á ÁS(thiocyanato) hydrogen bond (Table 2). While the N4amine is similarly oriented, the HÁ Á ÁS separation of 3.31 Å is not indicative of a significant interaction.  Table 1 Selected geometric parameters (Å , ).

Figure 1
The molecular structure of (I) showing displacement ellipsoids at the 70% probability level. The unlabelled atoms are generated by the symmetry operation 1 À x, 1 À y, 1 À z. The dashed lines represent intramolecular amine-N-HÁ Á ÁS(thiocyanato) hydrogen bonds.

Analysis of the Hirshfeld surfaces
The Hirshfeld surface analysis (McKinnon et al., 2004;Tan et al., 2019) of (I) was performed using Crystal Explorer 17 (Turner et al., 2017) to give further insight into the important intermolecular contacts normalized by van der Waals radii through a red-white-blue surface colour scheme where these colours denote the close contacts shorter than, equal to and longer than the sun of the respective van der Waals radii. As seen in Fig. 4(a) of the Hirshfeld surface plotted over d norm for (I), the red regions of the surface represent close contacts corresponding to the N-HÁ Á ÁS and N-HÁ Á ÁN hydrogen-bonding interactions mentioned above. An additional feature, i.e. SÁ Á ÁS contacts, are noted. The closest of these, i.e. S1Á Á ÁS1 iii = 3.2463 (9) Å [symmetry operation: (iii) 2 À x, 1 À y, 1 À z], link the binuclear molecules into chains along the a-axis direction. On the Hirshfeld surface mapped over electrostatic potential (DFT 3-21G) shown in Fig. 4(b), the faint-red and light-blue regions correspond to negative and positive electrostatic potential, respectively, The full and delineated (HÁ Á ÁH, NÁ Á ÁH/HÁ Á ÁN, SÁ Á ÁS, SÁ Á ÁH/HÁ Á ÁS and CÁ Á ÁH/HÁ Á ÁC) two-dimensional fingerprint plots are shown in Fig. 5(a)-(f), respectively. The NÁ Á ÁH/ HÁ Á ÁN contacts, at 35.8%, are the major contributor to the Hirshfeld surface. The SÁ Á ÁH/HÁ Á ÁS contacts (19.4%) also make a significant contribution. Other significant contributions come from the CÁ Á ÁH/HÁ Á ÁC (12.7%) and SÁ Á ÁS (8.3%) contacts with HÁ Á ÁH contacts, occurring at distances beyond the sum of the van der Waals radii, contributing only 7.6%. The next most significant contribution is made by NÁ Á ÁC/ Acta Cryst. (2020). E76, 42-47 research communications Figure 2 A view of the unit-cell contents of (I) in projection down the a axis, with N-HÁ Á ÁS and N-HÁ Á ÁN hydrogen bonds shown as orange and blue dashed lines, respectively.

Figure 3
The face-to-facestacking of (I).

Figure 4
A view of the Hirshfeld surface for (I) mapped over (a) d norm and (b) the electrostatic potential; the red and blue regions represent negative and positive electrostatic potentials, respectively. CÁ Á ÁN contacts (6.7%) arising in the main from thestacking interactions between triazolyl rings.
The energy frameworks were simulated (Turner et al., 2017) in order to analyse the specific intermolecular interactions identified above for each molecule-to-molecule contact. This was achieved by summing up four different energy components (Turner et al., 2017) for each pair of molecules, i.e. electrostatic (E ele ), polarization (E pol ), dispersion (E dis ) and exchange-repulsion (E rep ); these were obtained using the wave function calculated at the HF/3-21G level of theory. The results are summarized in Table 3. The greatest energy of attraction between molecules amounts to 138.4 kJ mol À1 , having a major electrostatic contribution (À142.4 kJ mol À1 ), and is associated with the following interatomic contacts: C2-H2Á Á ÁN7, C4-H4Á Á ÁN7 andstacking of between triazole rings. The next most significant contribution, with a total energy of À125.0 kJ mol À1 , arises from conventional hydrogen bonds, i.e. N1-H1NÁ Á ÁN5, N4-H4NÁ Á ÁN2 and N6-H6NÁ Á ÁN7 as well as C2-H2Á Á ÁS3 interactions. The next attractive interaction, with E tot = À48.9 and E dis = À120.3 kJ mol À1 , respectively, reflects the N3-H3NÁ Á ÁS2 hydrogen bonding and S1Á Á ÁS1 secondary bonding contact.
The magnitudes of intermolecular energies, i.e. the E ele , E dis and E tot components, are represented graphically in Fig. 6(a)-(c), respectively, by energy framework diagrams whereby the cylinders join the centroids of molecular pairs using a red, green and blue colour scheme; the radius of the cylinder is proportional to the magnitude of interaction energy.

Molecular orbital calculations
The HOMO and LUMO energies for the atom positions in the crystal structure of (I) were calculated using a pseudopotential plane-wave DFT method (Parr & Yang, 1994) implemented in the NWChem package (Valiev et al., 2010). The plane wave basis set and PBE exchange-correlation functional were chosen for the calculations on the experimental structure, i.e. without geometry optimization. The HOMO and LUMO of (I) are illustrated in Fig. S1 in the supporting information and their energies were calculated to be 3.011 and 6.173 eV, respectively. The HOMO is delocalized across the thiocyanato groups and the bridging region between the two dimers. The LUMO includes the delocalization around the triazole rings.  Table 3 Summary of interaction energies (kJ mol À1 ) calculated for (I).

Figure 6
The colour interaction mapping and energy frameworks for (I) showing the (a) electrostatic potential force, (b) dispersion force and (c) total energy diagrams. All cylindrical radii were adjusted to the same scale factor of 100 with a cut-off value of À50.0 kJ mol À1 within a 3 Â 3 Â 3 unit cell and their sizes are proportional to the relative strength of the corresponding energies.    Kang et al., 2013), which is a two-dimensional coordination polymer. As indicated above for Fe(NO) 2 (HtrzS)(HtrzSH)Á0.5H 2 O (EYABOV01; Aldoshin et al., 2008), mono-anionic forms of HtrzSH are known. Here, HtrzS functions as a monodentate thiolate-S ligand. A monodentate thiolate-S mode of coordination is also seen in (3-ClC 6 H 4 CH 2 ) 3 Sn(HtrzS) (SUXSAG; Keng et al., 2010). The three remaining structures feature a tridentate coordination mode leading to coordination polymers. In [Cu(HtrzS)] n (TEHYIQ; Zhang et al., 2012), this is achieved by bidentate, 2 -bridging by the thiolate-S atom and the participation of one of the triazolyl-N atoms in coordination. In [Pb(HtrzS)(NO 3 )OH 2 ] n (MOKKAA; Imran et al., 2015), the thiolate-S and two triazolyl-N atoms are involved in coordination. A similar coordination mode is found for one of the independent anions in [Cd 2 (HtrzS) 2 (SO 4 )] n (LOFJUM; Zhang et al., 2008). The second anion is tetradentate as the thiolate-S atom is bidentate, 2 -bridging. From the foregoing, it is evident that HtrzSH/HtrzS ligands adopt a wide range of coordination modes in the relatively few structures in which they have been characterized, suggesting further work in this area is warranted.