The effect of halogens in the coordination of 2-pyridinethioamide to gold centers

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Polyhedron 226 (2022) 116114

Available online 1 September 2022

The effect of halogens in the coordination of 2-pyridinethioamide to gold centers

Iiris P ¨ a ¨ akk onen , Sirpa J ¨ ¨ a ¨ askel ainen , Igor O. Koshevoy , Pipsa Hirva ¨

^*

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

A R T I C L E I N F O Keywords:

Coordination Thioamide Gold complexes DFT QTAIM

A B S T R A C T

Primary thioamide, 2-pyridinethioamide reacts with gold compounds Na[AuCl4], [AuCl(tetrahydrothiophene)], AuBr3 and AuI to [AuCl2(2-pyridinecarboxamide)] (1), [AuCl(2-pyridinethioamide)]3 (2), [AuBr(2-pyridinethioamide)]3 (3), and [AuI(2-pyridinethioamide)] (4), respectively. Formation of 1 involves interconversion of thioamide to amide and creation of a N,Ń-auracycle. Compounds 2 and 3 both consist of linear trimeric ad- ducts with aurophilic interactions between the gold atoms. In these compounds, the sulfur donor ligands show monodentate coordination. Similar monomeric unit is present in 4, which in solid state produces dimers via S–S chalcogen interactions. The structures were determined by X-ray crystallography. Computational DFT calculations and topological charge density analysis were utilized to clarify interactions, which determine the structures in solid state.

1. Introduction

Thioamides (RC = SNH₂) are a versatile group of molecules with reactions and applications rich in nuances. Thioamides are above all known as a class of drugs. Thioamides are easily desulfurized via C–S bond cleavage and converted to nitriles, imides or amides by organic oxidants [1–3], metals [4–6] or iodine [7]. Therefore, they have been utilized as sulfide precursors in organic heterocyclic chemistry or e.g. in the development of solar cells [8]. Furthermore, the oxidative dimer- ization of thioamides has been found to be a route to substituted heterocyclic compounds in high yields, like the synthesis of dithiazolidines [9–10] for diverse biological and clinical applications as well as building blocks for functional materials. A vast number of reports describing thioamide derivatives, their reactivity and their biological significance has been published.

Thioamides show thione-thiol tautomerism and they are also easily deprotonated. They have different types of donor sites, the hard nitrogen and the soft sulfur, and hence possess possibility to various coordination modes to metal centers. In the solid state, thioamides tend to be strong hydrogen bond donors and moderate acceptors. [11] These properties make thioamide-based molecules practical ligands in transition metal complexes and offer considerable potential as supramolecular synthons in crystal engineering. Even though the transition metal complexes with thioamides are well explored, most of the reactions exploit secondary or

tertiary thioamides. The reactions, too, bring out the multifunctional behavior of the thioamides. Simple additions are typical, but also variable ligand re-formations can take place and, for example, conversion to amide can occur. Some of the complexes formed behave as active anti- cancer metallodrugs, in which the ligand based S → O exchange allows for a potential dual mode activity [12–14].

Among metal complexes bearing primary thioamides as ligands, thioacetamide is the most common and its derivatives have been known for almost a century and structurally characterized complexes e.g. for Ru [15], Co, Ni [16], Cu [17], Ag [18] are known. Among thioamides in general, the studies on primary thioamide complexes of the late transition metals are scarce and only some casual examples are crystallographically characterized.

Aromatic thioamides, such as benzyl, pyrrole [19] and pyridine thioamides serve interesting pathways to innovative architectures.

Integration of primary mono- or ditopic ligands, thiobenzamide or 1,4- dithiobenzamide, with copper halides showed a surprising variety of structures, which ranged from mononuclear molecular complexes to a series of 1D or 2D coordination polymers with variable solvent molecules in the structures. The halide and the solvent guided the reaction routes. The polymers showed variable luminescence and semi- conductive properties. [20].

Mutoh et al. studied the reactions of some rhodium(III) complexes with primary arene thioamides containing various additional

* Corresponding author.

E-mail address: pipsa.hirva@uef.fi (P. Hirva).

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Polyhedron

journal homepage: www.elsevier.com/locate/poly

https://doi.org/10.1016/j.poly.2022.116114 Received 30 June 2022; Accepted 23 August 2022

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substituents at the benzene ring. A large variety of the reactions was observed, and molecular addition products were obtained [21].

2-Substituted pyridine thioamide (thiopicolinamide) offer possibility to cycloauration. Systematic studies between secondary 2-pyridine- thioamides with Au(III) have been performed to explain the effect on the substituents at amide nitrogen. Typically molecular products with the deprotonated N(pyridine),S-chelating ligand were generated. [22]

However, no primary ligands were tested. Despite the lack of structural analysis, the coordination ability of 2-pyridinethioamide towards metals has been utilized in the extraction of metals from the aqueous phases [23], or spectrophotometric determination of metals [24].

4-Pyridinethioamide could be a route to linear chain structures. In reactions with Zn [25] or Co [26–28] salts, the higher tendency to pyridine coordination has been observed. The thioamide group seemed to be inactive towards these metals and mononuclear complexes were formed. The use of other metals or combination of different metals could be a fruitful approach.

The work with thioamides has paved the way to new architectures and the exhibition of luminescent [29], conductive [30] or catalytic [31–32] properties. The study of the interactions in solid state is the basis of crystal engineering. In this work, we aimed to clarify the reactions of 2-pyridinethioamide with gold compounds, since gold has shown the tendency to soft sulfur donors.

2. Results and discussion 2.1. Syntheses and characterization

All the reactions were carried out in metal to ligand ratio of about 1:1 in appropriate solvent according to solubility (acetonitrile or dichloro- methane) at room temperature.

Reaction between Na[AuCl4] and 2-pyridinethioamide led to rapid formation of a red precipitate and successive crystallization gave yellow crystals, which were analyzed crystallographically. The crystallographic details are given in Table S1. The most important bond lengths and bond angles are given in Table S2. The crystalline product proved to be an amide derivative [AuCl2(2-pyridinecarboxamide)] (1), which has earlier been reported to form from 2-pyridinecarboxamide. [33–34] The structure is shown in Fig. 1. It possesses a square planar geometry, where the deprotonated ligand is attached via the pyridine and the amide nitrogen. In solid state, it appears in O^…H–N hydrogen bonded dimers.

The crystal structure verifies that the conversion of the thioamide ligand to carboxamide has taken place. This is not surprising, since this kind of Au(III) catalyzed interconversion in the presence of water is a well- known process. It has been found, that at the first step, the S-amide- gold(III) adduct is formed in stoichiometric amount [35–36]. For comparison, AuCl3 in methanol produced the same product 1.

The elemental analysis, the NMR and IR spectra of the red powder

formed in several reactions give together evidence that a mixture of thioamide and amide derivatives, and possibly some side products are formed, but upon crystallization desulfurization takes place and the yellow crystals of 1 are formed. Comparison of the experimental and calculated spectra are presented in Figs. S2–S4. As a whole, the experimental IR spectrum in Fig. S2 resembles combined simulated spectra of [AuCl2(2-pyridinecarboxamide)] and [AuCl2(2-pyridinethioamide)]

derivatives. The crystalline 1 shows signals, which are in good agree- ment with the earlier observations. [30].

Furthermore, from the NMR spectra of the reaction mixtures, it is obvious that many different types of coordination modes are produced, reflecting the versatile nature of the ligand (Fig. S5). The ¹H spectrum of the reaction precipitate verifies that both products of the pyridinecarboxamide and pyridinethioamide coordinated compounds are present. When the ¹H NMR spectrum was run from the crystalline product 1, it presented the typical peaks of the carboxamide product only.

To study the effect of the oxidation state of gold, we used Au(I) starting material, [AuCl(tetrahydrothiophene)] instead. The tetrahydrothiophene ligand is easily replaced with other donor groups. This indeed resulted in a substitution product found in the solid state as trimeric complex [AuCl(2-pyridinethioamide)]3 (2) with an unchanged thioamide ligand. The structure is given in Fig. 2.

In 2, the neutral ligands are attached to the metal centers via the sulfur atoms. The ligands are situated in alternating orientations on the sides of the Au chain, where the distances between two adjacent metals are 3.0471(5) Å and 3.1068(4) Å. The Au-Au-Au angle is 140.164(8).

The gold atom in the middle has a seesaw geometry and the end metals T-shaped geometry. The solid state structure is further supported by weak S^..S chalcogen interaction of 3. 220(1) Å (sum of the van der Waals radii is 3.60 Å), Au^..Cl 3.2675(8) (vdW 3.41 Å) and N–H^..Cl hydrogen

Fig. 1.The structure of [AuCl2(2-pyridinecarboxamide)] (1). Au-Cl1 2.2524(6) Å, Au-Cl2 2.2876(5) Å, Au-N1 2.033(2) Å, Au-N2 1.994(2) Å, N(2)-H^…O(1) 2.900(3) Å. Thermal ellipsoids are shown at 50% probability level.

Fig. 2.Trimeric structure of [AuCl(2-pyridinethioamide)]3 (2). Selected pa- rameters: Au1-Au2 3.0471(5) Å, Au2-Au3 3.1068(4) Å, Au1-Au2-Au3 140.164 (8)^◦, Au-S 2.2512(6)-2.2663(7) Å, Au-Cl 2.2798(7)-2.2947(7) Å, intermolecular S^…S 3.220(1) Å, N^_H^..Cl 3.270(3) and 3.348(2) Å. The structure of [AuBr(2- pyridinethioamide)]3 (3) is analogous and is given in ESI.

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bonding within the distances 3.270(3) and 3.348(2) Å. The reaction was clean and gave pure 2 as the only product precipitated from the reaction mixture. Similar type of linear phosphine derivatives of gold, [AuXPR3]1-4, where X =Cl, Br, I are known. [37–38] With the phos- phines, a series from monomer to tetramers was observed, but explanation on the length of the metal chain was not clear. To study the effect of chain length, we attempted to computationally optimize more extended structures of [AuCl(2-pyridinethioamide)]n, where n =4–6.

The free optimization at the DFT level led in most cases to adjacent dimeric aggregates with the intermolecular distance of over 7 Å, except when n =5, where a dimer +trimer was formed (Fig. S6). The final reason for this remained unclear, but it could be concluded that the aggregation to larger units than the monomer is favored, when X =Cl.

To study the effect of the halogen, AuBr3 was tested. Also, this reaction proceeded rapidly and yielded the orange product [AuBr(2-pyridinethioamide)]3 (3) after crystallization, which is analogous to 2 (the structure is given in Fig. S1). Slightly surprisingly, Au(III) starting material here gave the same structure as Au(I) in the presence of Cl. The spectroscopic analysis of this reaction indicated its more straightforward propagation to the trimeric adduct compared to the formation of 1 from [AuCl₄]^-. Minor amounts of some side products were spectroscopically observed, so the reaction is not quite as selective as the formation of 2.

To extend the study of the effect of halogens, the reactivity of AuI was tested. The same basic structure [AuI(2-pyridinethioamide] (4) with sulfur bonded ligand was obtained in an uncomplicated reaction (Fig. 3).

However, instead of a trimeric structure, the monomers exist in pairs with S^…S chalcogen bonds of 3.272(1) Å. In the solid state, weak N–H^…I interactions are present.

2.2. Structure and charge density of the building blocks

Comparison of the charge distribution in the separate free ligands 2- pyridinecarboxamide (LO) and 2-pyridinethioamide (LS) was done by calculating the electrostatic potential surfaces (ESP) utilizing the QTAIM method39^,40^,41^,42^,43 The corresponding ESP surfaces are shown in Fig. 4.

Consistently with the larger electronegativity and the hard nature of oxygen compared to soft sulfur, the negative charge is concentrating on the carbonyl oxygen in larger extent (ligand LO). However, careful in- spection of the ESP of the ligand LS shows that the charge density of sulfur contains a more positive region at the C––S bond direction, the σ-hole, which will enable formation of chalcogen bonds [44]. The chalcogen bonding, resembling halogen bonding, can be expected to have notable role in crystal packing.

The possibility of different tautomeric forms of the ligands was studied by optimizing separate ligand models at the DFT level of theory and comparing the total energies of the various possibilities. It was obtained, that the amide and thioamide forms were about 130 (LO) and 100 (LS) kJmol⁻¹ thermodynamically more favorable than the

corresponding iminol or iminothiol tautomers. Furthermore, when the HOMO-LUMO gaps of the single molecules were investigated to estab- lish the chemical stability of the ligands, the carboxamide ligand (LO) was found to have larger gap (5.81 eV vs 4.18 eV) than thioamide (LS), indicating fairly easy thioamide-carboxamide conversion.

2.3. Nature of bonding in the crystals

In the synthesis of complex 1, characterization by IR and NMR spectra led to the conclusion, that several forms of the ligand are present and consequently, both the carboxamide and thioamide derivatives of the Au(III) compound are formed. However, only the carboxamide compound crystallized easily. We sought explanation on this fact by optimizing computationally dimeric, hydrogen-bonded models with both ligands LO and LS and studying their charge density via QTAIM.

The resulting bond paths and the selected BCPs are shown in Fig. 5.

Table 1 lists the properties of the electron density at the selected BCPs (numbering in Fig. 5). The models are completely symmetrical exhib- iting the same properties in both [AuCl2L] units.

In both dimeric models 1A and 1 T, strong bonds between the gold centers and the nitrogens are formed with considerable electron sharing, as can be seen from the value of |V|/G, which is well over the limit of 1, and DI(A,B) approaching 1, which gives the number of shared electrons between atoms A and B. The formation of rather strong hydrogen

Fig. 3. Packing of [AuI(2-pyridinethioamide)] (4). Au1-S1 2.279(1) Å, Au1-I1 2.5449(3) Å, S^..S 3.272(1) Å, N^_H^..I 3.607(4) and 3.658(4) Å.

Fig. 4.Electrostatic potential maps for the pyridine2-amide and 2-pyridinethioamide ligands calculated by QTAIM method. The bond paths and bond critical points (BCPs, green dots) of the ligands are also shown. Color coding of the atoms: C =gray, H =white, N =blue, O =red, S =yellow.

Fig. 5.Bond paths and bond critical points (BCPs, small green dots) for the dimeric model of complex 1A (carboxamide). Similar model was optimized also with thioamide ligand (1T). Color code of the atoms: C =gray, H =white, N = blue, O =red, Cl =green, Au =orange.

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bonding interactions is also obvious from the BCP #6, which shows typical properties of non-covalent interactions such as small electron density ρ, |V|/G ≤1, and very small DI(A,B). At the same time, the C––O (or C––S) and N–H bonds are somewhat weakened compared to the single molecule. On the other hand, while other BCPs show similar values in both amide and thioamide complexes, the strength of the hydrogen bonding interactions is much smaller in the thioamide case and there is smaller electron density at the NH^…S BCP than in the NH^…O BCP, indicating more difficult forming of such dimeric aggregates in the solid state of the thioamide complex. Since it can be expected that the formation of the hydrogen bonding interactions has a major role in crystal packing of the materials, this observation explains why the crystalline material contains solely the amide compound.

The nature of bonding in the trimeric [AuX(2-pyridinethioamide)]3

(X =Cl, Br, I) models was also studied with the QTAIM methods. Table 2 lists the selected properties of the electron density for compound 2 (model 2_3) and compared to the monomeric complex [AuCl(2- pyridinethioamide)] (model 2_1). The corresponding values for compounds 3 and 4 have been presented in Table S3. It should be noted that both the monomeric and trimeric models were computationally fully optimized, which enabled to study the iodine complex in the trimeric form, even though it was not experimentally found. Fig. 6 shows the numbering of the BCPs.

When the monomer units form the trimer in the crystalline state, only small differences can be found in the intramolecular interactions.

The strength of the Au-S and Au-X bonds as well as the C––S bonds re- duces slightly. However, the values presented in Table 2 correspond to the central unit, and the values of the end units are even closer to those in the monomer. The aurophilic interactions showed very small electron density (0.13 eÅ⁻³), substantial electron sharing (|V|/G ~ 1.5),

delocalization index of 0.2, and interaction energy of − 18 kJmol⁻¹. There is also an uneven charge distribution of the gold atoms, so that in the central unit the positive charge of Au2 is somewhat larger than in Au1 or Au3.

The effect of different halogens can be seen in Table S3. Within the monomer models, the nature of the coordinative bonds changes slightly.

The interaction energy at the Au-S and Au-X BCPs decreases in the order Cl >Br >I (Au-S: − 178, − 173, − 164 kJmol⁻¹; Au-X: − 186, − 163, − 104 kJmol⁻¹, respectively). The same trend can be seen in the value of the electron density ρ. The ratio between the potential energy density and the kinetic energy density, |V|/G, shows an increasing trend Cl (1.46) <

Br (1.56) <I (1.68) at the Au-X bond critical point, indicating increasing degree of covalency from Cl to I complex. The delocalization index at the Au-X BCP also increases in the same order.

In the trimeric models, the properties of the electron density follow the same trends for all intramolecular interactions, as could already be seen for the Cl compound in Table 2. The effect of the halogen in the aurophilic interactions is very small, and therefore all models optimized in very similar geometries, even with iodo ligands. However, when the QTAIM atomic charges of the gold ions were calculated, important differences were found depending on the halogen involved. Table S4 lists the charges of the coordinating atoms in different trimeric models. First of all, the positive charge of the gold atoms is larger in the central unit of the trimer, indicating the effect of aurophilic interactions in the charge distribution. Furthermore, the charge q(Au2) decreases in the order Cl (0.183) >Br (0.149) >I (0.035). The small charge of gold ions in the iodo compound could explain the missing aurophilic interactions and the formation of monomeric units in the crystal structure of 4.

In the crystal structure of 2 and 3, two trimers are packed together with weak interactions. We calculated the properties of the interactions with models consisting of two adjacent [AuX(2-pyridinethioamide)]3

motifs cut directly from the experimental crystal structures. The models and the properties of the electron density according to the QTAIM analysis are presented in Fig. S7 and Table S5. In general, the intramolecular interactions within both trimers were unchanged as compared with the single trimeric model. Small differences between the values in the models n_3 and n_6 can be explained via slightly different geometries for the optimized and non-optimized models. The most important weak interactions keeping the two trimers together were the Table 1

Properties of the electron density at the selected bond critical points (see Fig. 5) of models 1A and 1T. ρ =electron density at the BCP, |V|/G =ratio between potential energy density and kinetic energy density, DI(A,B) =delocalization index between atoms A and B, EINT =interaction energy.

BCP# Type ρ _(eÅ^¡³₎ _|V|/G _DI(A,B) _E_INT_(kJmol^¡¹₎ [AuCl2(2-pyridinecarboxamide)]2 (1A)

1 Au-N1 1.019 1.50 0.90 −287

2 Au-N2 0.843 1.37 0.72 −236

3 N1-H 2.089 12.11 0.62 −611

4 C––O 2.658 2.00 1.15 −1754

5 Cl^…H(C) 0.093 0.83 0.06 −11

6 NH^…O 0.204 1.03 0.09 –33

[AuCl2(2-pyridinethioamide)]2 (1 T)

1 Au-N1 1.010 1.49 0.87 −287

2 Au-N2 0.854 1.37 0.73 −241

3 N1-H 2.116 12.23 0.65 −619

4 C––S 1.453 2.07 1.55 −653

5 Cl^…H(C) 0.095 0.83 0.05 −12

6 NH^…S 0.119 0.98 0.04 −15

Table 2

Properties of the electron density at the selected bond critical points (see Fig. 6) of models 2_1 (monomer) and 2_3 (trimer). ρ =electron density at the BCP, |V|/

G =ratio between potential energy density and kinetic energy density, DI(A,B)

=delocalization index between atoms A and B, EINT =interaction energy.

BCP# Type ρ _(eÅ^¡³₎ _|V|/G _DI(A,B) _E_INT_(kJmol^¡¹₎ [AuCl(2-pyridinethioamide)] (2_1)

2 Au-S 0.753 1.53 1.00 −178

3 Au-Cl 0.758 1.46 1.12 −186

4 C––S 1.465 2.29 1.43 −610

[AuCl(2-pyridinethioamide)]3 (2_3)

1 Au1^…Au2 0.132 1.08 0.21 −18

1^′ Au2^…Au3 0.137 1.09 0.22 −18

2 Au2-S 0.747 1.54 0.99 −172

3 Au2-Cl 0.649 1.38 0.90 −155

4 C––S 1.449 2.49 1.37 −556

Fig. 6. Bond paths and bond critical points (BCPs, green dots) for model 2_3.

Color code of the atoms: C =gray, H =white, N =blue, S =yellow, Cl =green, Au =orange.

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S^…S chalcogen bonds, which exhibited typical values for non-covalent interactions with the weak interaction energies ranging from − 26 to –32 kJmol⁻¹.

Packing of iodo complex was also investigated by analyzing the electron density in a more extended model 4_8. The model and the properties of the electron density at the selected BCPs are presented in Fig. S9 and Table S6. Again, the major intermolecular interactions are the weak S^…S chalcogen bonds, which attach the monomeric units together. However, the missing aurophilic interactions are compensated with other types of non-covalent interactions, including π-π interactions.

3. Experimental 3.1. Materials and methods

Commercially available reagents AuBr3 (99 %, Alfa Aesar), Na [AuCl4]^.2H2O (99 %, Sigma Aldrich), AuCl3 (Au 64.4 % min, Alfa Aesar), AuI (99 %, Alfa Aesar) and 2-pyridinethioamide (97 %, Sigma Aldrich) were used without purification. [AuCl(tetrahydrothiophene)]

was prepared according to literature method. [45] The solvents were dried using molecular sieves. Infrared spectra were measured from KBr pellets using Bruker Vertex 70 Fourier transform infrared spectrophotometer in the range of 4000–400 cm⁻¹. The elemental analysis was performed on varioMICRO V1.7. The ¹H NMR spectra were recorded on JEOL 500 MHz spectrometer in d₆-DMSO. The UV–vis spectra were recorded on Perkin Elmer Lambda 900 spectrophotometer in DMSO.

3.2. Crystal structure determination

The crystals of 1–4 were immersed in cryo-oil, mounted in a Nylon loop, and measured at a temperature of 150 K. The X-ray diffraction data were collected on Bruker Kappa Apex II and Smart Apex II diffractom- eters using Mo Kα radiation (λ =0.71073 Å). The APEX2 [46] program package was used for cell refinements and data reductions. The structures were solved by direct methods using the SHELXS-2018 [47] program with the WinGX [48] graphical user interface. A numerical absorption correction (SADABS) [49] was applied to all data. Structural refinements were carried out using SHELXL-2018. [47] The N–H hydrogen atom in 1 was located from the difference Fourier map and constrained to ride on their parent atoms, with C–H =0.95 Å and N–H

=0.88 Å, Uiso =1.2 Ueq (parent atom). All other hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C–H =0.95–0.98 Å and Uiso =1.2–1.5 Ueq (parent atom). The crystallographic details are summarized in Table S1.

3.3. Computational details

All calculations were performed by applying Gaussian 09 software package [50]. The optimized geometry and simulated infrared spectra with no scaling for small molecular models of the structures were obtained by PBE0 functional [51] with 6-31G(d) basis set for C, N, Cl, Br, and H atoms, 6-311G(d) for I atoms, and def2-TZVPPD basis set [52] for Au atoms.

To obtain the electronic properties of the compounds we performed topological charge density analysis with the QTAIM [39–43] (Quantum Theory of Atoms in Molecules) method, which allowed us to access the nature of the bonding via calculating different properties of the electron density at the bond critical points (BCPs). The analysis was done with the AIMALL program [53] using the wavefunction obtained from the DFT calculations.

3.4. Synthetic procedures

Synthesis of [AuCl2(2-pyridinecarboxamide)] (1): Na[AuCl4]^. 2H2O (145,6 mg, 0,366 mmol) and 2-pyridinethiomide (51.1 mg, 0.370 mmol) were dissolved in MeCN (2.0 ml +2.0 ml). The solutions were

combined and immediately a red precipitate was formed. The solution was stirred for one hour, filtered and washed with MeCN. Yield 42.2 mg, 29.6 %. Orange crystals were obtained from tetrahydrofuran solution at +4˚C. IR (KBr): ν(C––O) 1653 cm⁻¹. Calc. for AuCl2C6H5N2O C% 18.53;

H% 1.30; N% 7.20. Calc. for AuCl₂C₆H₅N₂S C% 17.79; H% 1.24; N%

6.92; S% 7.92. Found. C% 15.75; H% 1.49; N% 6.11; S% 8.31 (precipitate). ¹H NMR: 9.18, 8.84, 8.48, 8.00, 7.91 ppm. λ^max265, 325 nm.

Synthesis of [AuCl(2-pyridinethioamide)]3 (2): [AuCl(tetrahydrothiophene)] (116,6 mg, 0,404 mmol) and 2-pyridinethiomide (49.6 mg, 0.359 mmol) were dissolved in CH2Cl2 (2.0 +2.0 ml). The solutions were combined. Immediately a purple precipitate was formed. The solution was stirred for one hour, filtered and washed with CH2Cl2. Yield 21,1 mg, 22.8 %. Orange crystals were obtained from thf at +4˚C. Calc.

for Au3Cl3S3N6C18H18 C% 19.44; H% 1.63; N% 7.56; S% 8.65. Found. C

% 15.75; H% 1.50; N% 6.12; S% 8.31 (precipitate). ¹H NMR: 9.83, 8.65, 8.34, 8.00, 7.65 ppm. λ^max278, 329 nm.

Synthesis of [AuBr(2-pyridinethioamide)]3 (3): AuBr3 (199.5 mg, 0,457 mmol) and 2-pyridinethiomide (49.9 mg, 0.361 mmol) were dissolved in MeCN (2.0 ml +2.0 ml). The solutions were combined.

Immediately an orange precipitate was formed. The solution was stirred for two days, filtered and washed with MeCN. Yield mg. 15.8 mg, 10.9

%. Orange crystals were obtained from thf at + 4˚C. Calc.

Au3Br3S3N6C18H18 C% 17.36; H% 1.46; N% 6.75; S% 7.73 % Found. C%

15.93; H% 1.36; N% 5.98; S% 12.12 (precipitate). ¹H NMR: 9.84(2H), 8.42, 8.08, 7.79, 7.41 ppm. λ^max275, 315, 325 nm.

Synthesis of [AuI(2-pyridinethioamide)] (4): AuI (119.8 mg, 0.370 mmol) was suspended in 4.0 ml of MeCN. A solution of 2-pyridinethiomide (52.3 mg, 0.379 mmol) in MeCN (2.0) was added to the suspension. The mixture was stirred for 30 min and filtered. The precipitate was washed with 2.0 ml of MeCN, which was combined to the solution. The solution was evaporated to dryness to give dark red product. Yield 72.1 mg, 17.3 %. Orange crystals were obtained from CH2Cl2 at +4˚C. Calc. for AuIC6H6N2S C% 15.60; H% 1.31; N% 6.06; S%

6.94. Found. C% 18.12; H% 1.62; N% 6.98; S% 7.07. ¹H NMR: 9.83, 10.51; 8.65, 8.36, 8.00, 7.65 ppm. λ^max273, 329, 659 nm.

4. Conclusions

In this paper, we present reactions of 2-pyridinethioamide with gold salts. The ligand can act as a monodentate sulfur donor, but also interconversion to amide is possible. The versatile nature of the ligand and the possibility to aurophilic interactions can lead to various monomeric, dimeric or trimeric units, which can further aggregate via weak interactions in the solid state. The electronic properties of different halogens Cl, Br, and I were found to affect the preferred form of the compounds. According to the QTAIM analysis, the S^…S chalcogen bonds as well as the N–H^…O or the N–H^…S hydrogen bonds can be suggested to be the major interactions determining the solid state packing of the molecules.

CRediT authorship contribution statement

Iiris P¨aakk¨ onen: Investigation, Formal analysis, Writing ¨ – original draft. Sirpa J¨aaskel¨ ¨ainen: Investigation, Formal analysis, Writing – original draft. Igor O. Koshevoy: Investigation, Validation, Writing – review & editing. Pipsa Hirva: Investigation, Formal analysis, Writing – original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Data availability

Data will be made available on request.

Acknowledgements

We acknowledge grants of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-in- fras-2016072533).

Appendix A. Supplementary data

CCDC 2169975-2169978 contains the supplementary crystallographic data for 1–4. These data can be obtained free of charge via http s://eur03.safelinks.protection.outlook.com/?url=http%3A%2F%

2Fwww.ccdc.cam.ac.uk%2Fconts%2Fretrieving.html&data=05%

7C01%7C%7C98ef10550d0343f1b99108da65930bca%7C87879f2e 73044bf2baf263e7f83f3c34%7C0%7C0%7C637933979498677695%

7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAw

MDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%

7C3000%7C%7C%7C&sdata=eoX1J%2F7SRZCm8VtXtR04YYXIz31%

2BDvLt32r0RUnamGQ%3D&reserved=0, or from the Cambridge Crys- tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:

(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplemen- tary data to this article can be found online at https://doi.org/10.1016 /j.poly.2022.116114.

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