Benzothiazolethione complexes of coinage metals: from mononuclear complexes to clusters and polymers

Solid State Sciences 97 (2019) 105980

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Benzothiazolethione complexes of coinage metals: from mononuclear complexes to clusters and polymers

T

Sirpa Jääskeläinena,∗, Laura Koskinena, Matti Haukkab, Pipsa Hirvaa a b

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland

A R T I C LE I N FO

A B S T R A C T

Keywords: Gold Copper Benzothiazolethione DFT QTAIM

The reactions of 2(3H)-benzothiazolethione (Hbtt) with [AuCl(tetrahydrothiophene)] and CuBr2 were studied, and found to yield a tetranuclear cluster compound [Au(btt)]4 [1] and a polymeric structure [CuBr(bttbtt)]n.nTHF (2). Crystallographic and spectroscopic methods were used for the characterization. In 1, the monoanionic ligand acted as a bidentate bridging N,S-donor giving a molecular cluster structure of an asymmetric coordination isomer. In the formation of 2, the ligand was dimerized by forming a S–S bond after deprotonation, and coordination via nitrogen donors to metal atoms took place leading to a polymeric structure. To clarify the diversity of reactions of Hbtt with coinage metals, and to verify the experimental characterization, DFT studies were performed. The nature of the coordination of the Hbtt ligand was further investigated via topological charge density analysis by QTAIM methods.

1. Introduction

tendency to coordinate with the nitrogen donors and thereby can form a large variety of structures, ranging from mononuclear complexes to ring structures and further to catena compounds and 2- and 3-dimensional structures. In addition, thousands of copper compounds with variable organosulfur ligands are known with richness of structural features. On the other hand, in copper complexes of Hbtt, the metal in oxidation state I and coordination through sulfur appear to be preferred, but utilization of nitrogen donor site is also possible. In [Cu2(μ2-Hbtt) (Hbtt)2I2] the ligand acts as a sulfur donor [6]. Often the structures contain auxiliary phosphine ligands, and a series of mono [7–10] and dinuclear [11–13] complexes with either bridging or terminal S-ligands is known. When Cu(II) was let to react with Hbtt, reduction of the metal with simultaneous dimerization of the deprotonated ligand, and further S atom insertion to the nascent S–S bond, occurred. A mononuclear complex with the ligand utilizing both sulfur and nitrogen in coordination was formed [14]. Yue et al. have published cluster structures [Cu(btt)]6 and [Cu(btt)]4 with μ-S,N-btt, obtained from solvothermal synthesis. The nuclearity was determined by the cooling rate of the reaction mixture. Both clusters have symmetrically coordinated ligands, each thione group bridges two metals and the nitrogen atom coordinates to an adjacent third metal. Thus each metal has two-coordinated sulfur atoms and one nitrogen from three btt groups. The compounds showed luminescent properties associated to the metallophilic interactions [15,16].

Aromatic nitrogen heterocyclic thiones (e.g. imidazole thiones, thiazolethiones, pyridine thiones) have diverse coordination ability with metals [1,2]. The combination of soft and hard donor sites, the occurrence of the thione-thiol tautomerism and the acidic character enrich their behavior and enable their rich coordination chemistry varying from molecular level to self-assembled structures. Due to the potential applications e.g. in biochemistry, pharmacology, photochemistry and catalysis, the thiones with coinage metals (Cu, Ag and Au) are notably more studied than the other metal complexes. The formation of functional materials is supported by inter- and intramolecular interactions, such as metallophilic interactions[3]. 2(3H)-benzothiazolethione (Hbtt; known also as 2-mercaptobenzothiazole in other tautomeric form), is widely used to construct self-assembled metal thione complexes due to its rigid backbone, aromatic delocalized electron system and rather wide S–C–N bite angle, which enhance its usability. The ligand appears in its compounds either in the original Hbbt form or as a monoanionic btt. It has been used in synthesis as it is, or it has been generated from dimeric 2,2′-dithiobis (benzothiazole) by S–S bond cleavage [4]. Vice versa, dimerization of Hbtt by constructing a S–S bond or by some other in situ ligand transformations have been observed [5]. With imidazole-containing ligands, copper generally has a great

∗

Corresponding author. E-mail address: sirpa.jaaskelainen@uef.fi (S. Jääskeläinen).

https://doi.org/10.1016/j.solidstatesciences.2019.105980 Received 25 April 2019; Received in revised form 12 August 2019; Accepted 14 August 2019 Available online 15 August 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.

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The btt ligands are deprotonated and bridge together the gold atoms and, surprisingly, adopt either N–Au–N, S–Au–S or S–Au–N coordination mode instead of each gold atom having the same coordination environment, which could be anticipated to represent the lowest energy structure. The coordinating bond lengths are Au(1)–N(1) 2.036(6) Å, Au(3)–S(3) 2.288(2) Å, Au(2)–N(2) 2.078(6) Å and Au(2)–S(1) 2.272(2) Å. The four gold atoms form a distorted square as intramolecular aurophilic interactions connect adjacent gold atoms with Au(1)–Au(2) 2.9852(5) Å and Au(2)–Au(3) 2.9869(5) Å and further between the opposite gold atoms Au(2)-Au(2#1) 3.3027(6) Å (#1 -x +1,y,-z+3/2). The sum of the van der Waals radii of two gold atoms is 3.32 Å. No aurophilic interaction is formed between Au(1) and Au(3), as pointed out by the long distance 4.9757(6) Å. The cluster structure can also be considered as a 16-membered twisted ring comprising the gold atoms, all N and S donor atoms, and one C atom of each ligand. Thus in 1 the coordination to the metals is more complicated than in earlier reported symmetric [Cu(btt)]4, [Cu(btt)]6 and [Ag(btt)]6.15,16,21 Most important bond lengths and angles: Au(1)–Au(2) 2.9852(5), Au(1)–Au(3) 4.9757(6), Au(2)–Au(2)#1 3.3027(6), Au(2)–Au(3) 2.9869(5), Au(1)–N(1) 2.036(6), Au(2)–N(2) 2.078(6), Au(2)–S(1) 2.272(2), Au(3)–S(3) 2.288(2) Å, S(3)-Au(3)-S(3)#1 169.70(11), S (1)–Au(2) #1-N(2)#1 170.6(2), N(1)-Au(1)-N(1)#1 176.1(4), Au(1)-Au (2)-Au(3) 112.842(1), Au(2)-Au(3)-Au(2)#1 67.13(1)°, Au(1)–H(6) 2.9495(3), Au(2)–H(13) 2.8864(3) Å (Symmetry transformations used to generate equivalent atoms: #1 -x+1,y,-z+3/2). The 1H NMR spectrum of 1 in solution showed broad overlapping signals and was therefore inconclusive on the preferred structure. NMR does not generally represent very well the solid state structures for higher nuclearity clusters and polymers, since in solution they are rather in metal-ligand fragments than in molecular forms. Also the presence of multiple coordination isomers cannot fully be excluded, but 1 was the only product, which crystallized in the present reaction conditions. The IR spectrum in potassium bromide showed that in coordination the strong absorption of the ligand at 1497 cm−1 (combination δ(N–H), ν(C–N), δ(N–C–S)) disappeared, which is an indication of the ligand deprotonation. The observation was confirmed by computational simulation of the IR frequencies (shown in supplementary information, Fig. S1). In addition, the comparison of the spectra of separate Hbtt ligand and the coordinated complex [Au(btt)]4 showed that stretching frequency of the C]S double bond shifted to smaller wavenumbers because of the modification of the bond index upon coordination. It should be noted, that the Au–N and Au–S IR frequencies were simulated to be less than 200 cm−1, which is beyond the range of the experimental spectra, which prevented obtaining additional information on the coordination.

With silver, Hbtt reacts preferably via sulfur to give, for example, mono- and dinuclear phosphine complexes [AgX(Hbtt)(PPh3)2] (X = Cl, Br [17], I [18]), [Ag(Hbtt)2(PPh3)2]+ [19] and [Ag2(μCl)2(Hbtt)2(PPh3)2] [20]. Recently, a catena compound comprised by bridging iodo ligands and terminal Hbtt ligands was reported.4 A hexanuclear silver complex [Ag(btt)]6, which is held together by six bridging btt ligands, was prepared in solvothermal synthesis. The coordination type of the ligands is identical with the [Cu(btt)]4 and [Cu (btt)]6 clusters. Also this silver cluster, like the copper ones, showed luminescent properties originated from metal-centered interactions [21]. Furthermore, a mixed metal Ag–Rh complex, where the thione is coordinated to both metals and the nitrogen atom to Rh, has been reported [22,23] During past few years, btt-borate complexes of Cu and Ag have been structurally, photophysically and structurally studied [24,25]. Only a few gold complexes with Hbtt (or btt) are known in the literature. Au(I) prefers the coordination via sulfur. When Au(III) starting material is used, the reaction often involve reduction of the metal center, but a simple attachment of the ligand is also possible [26–28]. The Hbbt compound as a ligand offers variable coordination types and gives rise to interesting structures owing to tendency to self-assembly. Complexes from mono- and dinuclearity to closed shell clusters are known. Further possibility to different coordination isomers, like in the dinuclear complexes the head-to-head and the head-to-tail forms [29–33], diversify the chemistry. Reaction conditions often affect the course of the reaction. Obviously, various metal-ligand fragments may be present in the reaction solution. The final solid-state assembly, especially in the high nuclearity products, is regulated by competitive intra- and intermolecular interactions. In this work, we studied the reaction of 2(3H)-benzothiazolethione (Hbtt) with [AuCl(tetrahydrotiophene)] ([AuCl(tht)]) and CuBr2. To understand the preference to the divergent products, we used computational methods to analyze the electron density of molecular models for the crystallized compounds, and compared the nature of the metalmetal and metal-ligand bonds as well as other possible intra- or intermolecular interactions affecting the stability of the structures. In addition, spectroscopic data was calculated in order to verify and explain the experimental characterization of the compounds. 2. Results and discussion 2.1. Synthesis and characterization of [Au(btt)]4 [1] The reaction of Hbbt with [AuCl(tht)] yielded a tetranuclear complex [Au(benzothiazolethione)]4, [Au(btt)]4 (1). The structure is given in Fig. 1 and the crystallographic details in Table 4.

Fig. 1. Structure (a) and schematic presentation (b) of [Au(benzothiazolethione)]4 (1). 2

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Scheme 1. The four possible coordination isomers. Isomer b represents the obtained crystal structure. Energies correspond to the relative total energies referenced to the lowest energy structure c.

In order to investigate the possibility for different coordination isomers, we optimized the structures of four molecular models representing different orientations of the btt ligands (Scheme 1). The optimized structures are shown as supporting information in Fig. S2. The relative total energies of the isomers a-d are presented in Scheme 1. The four isomers represent different head-to-tail or head-to-head coordination modes, of which the isomer c with Au(1) and Au(3) exhibiting S,S coordination and Au(2) and Au(2)#1 N,N coordination has the lowest total energy. This head-to-head coordination environment leads to the most symmetrical metallophilic interactions and also to the same Au–Au distances for all gold atoms (Fig. S2, Table S1). However, the diagonal distance Au(2)–Au(2)#1, where weak metallophilic interaction was suggested, is clearly the longest in isomer c, which can be suggested to decrease its stability compared to the other isomers. As a consequence, the difference in energy is rather small among isomers ad, less than 11 kJ mol-1, and it is likely that they would coexist in solution, even though only b was found to crystallize in the solid state. The final solid state structure would depend on subtle changes in the reaction and crystallization conditions. It should be noted, that only the diagonal distances Au(2)–Au(2)#1 and Au(1)–Au(3) vary within the isomers (Table S1), and the other Au–Au distances are nearly the same, indicating different distortion of the square-like metal core. The largest distortion (the shortest Au(2)–Au(2)#1 distance and the longest Au (1)–Au(3) distance) was found in the optimized structure of isomer b, in agreement with the experimental distorted structure. In comparison, most often the head-to-tail coordination of the bridging ligands has been found clearly more favourable than head-to-head, as was previously obtained for isomers of, for example, the dimeric “paddle wheel” rhodium compounds with ortho-metalated thienyl phosphine ligands [34]. The QTAIM analysis of the structural models for four-nuclear gold clusters revealed the nature of the intramolecular interactions present between the different gold atoms. The results indicate typical weak aurophilic interactions between Au(1)–Au(2), Au(2)–Au(3), Au(3)–Au (2)#1, and Au(2)–Au(2)#1 (see Fig. S3, which presents the bond paths and BCPs for isomers a-d), but not between Au(1)–Au(3), which has the largest interatomic distance. Table 1 lists the selected properties of the electron density at the Au–Au bond critical points (BCPs) for different coordination isomers a-d. The charge of the gold atoms is strongly influenced by the local coordination environment, as can be seen from the AIM charge values shown in Table 2. The largest positive charge is obtained for the N,N coordinated gold atoms, whereas the S,S coordinated gold atoms exhibit the smallest positive charge. Otherwise, there is only a small variation on the charges of gold atoms having the same local environment. However, even if the electronic nature of the gold atoms is quite different between the isomers, the aurophilic interactions show very similar nature in all Au–Au BCPs regardless of the surroundings. The electron density ρ, the ratio of the potential energy density and the kinetic energy density |V|/G, and the interaction energies EINT indicate, that there exists a rather strong (around 30 kJ mol-1) attractive interaction between the four gold atoms forming the square-like metal core,

Table 1 Selected properties of the electron density at the Au–Au bond critical points (BCPs) for different coordination isomers of 1, according to the QTAIM analysis. Properties

BCP

a

b

c

d

ρ (eÅ−3)

Au(1)–Au(2) Au(2)–Au(3) Au(3)–Au(2)#1 Au(2)#1-Au(1) Au(2)–Au(2)#1 Au(1)–Au(3) Au(1)–Au(2) Au(2)–Au(3) Au(3)–Au(2)#1 Au(2)#1-Au(1) Au(2)–Au(2)#1 Au(1)–Au(3) Au(1)–Au(2) Au(2)–Au(3) Au(3)–Au(2)#1 Au(2)#1-Au(1) Au(2)–Au(2)#1 Au(1)–Au(3) Au(1) Au(2) Au(3) Au(2)#1

0.202 0.204 0.202 0.204 0.083 – 1.15 1.14 1.15 1.14 0.97 – −31.5 −32.0 −31.5 −32.0 −9.5 – 0.219 0.234 0.219 0.233

0.195 0.199 0.199 0.195 0.115 – 1.14 1.14 1.14 1.14 1.05 – −29.9 −30.9 −30.9 −29.9 −14.7 – 0.359 0.239 0.104 0.240

0.199 0.199 0.199 0.199 0.069 – 1.14 1.14 1.14 1.14 0.92 – −32.4 −31.2 −31.7 −29.3 −10.1 – 0.095 0.379 0.095 0.379

0.207 0.201 0.203 0.192 0.087 – 1.15 1.14 1.15 1.14 0.98 – −32.4 −31.2 −31.7 −29.3 −10.1 – 0.215 0.238 0.100 0.378

|V|/G

EINT (kJmol−1)

q(Au)

Table 2 Examples of the intra- and intermolecular interactions in the four-unit model {[Cu2Br2(btt-btt)4]}4 of 2, according to the QTAIM analysis. BCP

ρ (eÅ−3)

|V|/G

EINT (kJmol−1)

Cu(1)–N(1) Cu(1)–N(2) Cu(1)–Br(1) Cu(1)–Br(2) S–S(intra) S⋯S S⋯S S⋯S Br⋯H π…π

0.484 0.509 0.342 0.374 0.984 0.103 0.065 0.039 0.058 0.036

1.14 1.15 1.18 1.21 2.49 0.88 0.78 0.71 0.78 0.77

−147.1 −158.8 −72.8 −82.4 −175.5 −13.1 −7.4 −3.7 −6.1 −3.2

and only one weaker diagonal interaction causing distortions to the square-like structure. The |V|/G > 1 suggests, that the main metallophilic interactions are not purely electrostatic, but exhibit some covalent nature, again rather typical for stronger metallophilic interactions. The amount of electron density at the BCPs reflects the geometrical differences rather than the variation in the coordination environment of the metals. Therefore it can be concluded, that the aurophilic interactions originate from the support of the bridging ligands, which facilitate close contact of the Au atoms. The fact that the less symmetrical isomer b was the only experimentally obtained structure in crystalline state was further investigated by analyzing the intermolecular interactions in a more extended model via QTAIM method. The model included four [Au(btt)]4 units with 3

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dimeric model [Cu2Br2(btt-(H)btt)4] (Fig. S5a), two of the four ligands broke into monomeric units, which were not coordinated to copper atoms. Only with full deprotonation of both coordinated and non-coordinated nitrogen atoms of the ligands, the optimization led to the same structure than in experimentally obtained 2. The optimized structure is shown in Fig. S5b. It should be noted, that also in the optimization of the separate btt-btt ligand, obtaining the dimeric structure required full deprotonation of all nitrogen atoms. In the freely optimized structure of the dimeric molecular model of 2, the torsions of the ligand “tails” are not restricted by the surroundings as they are in the solid state packing of the molecules. Because of this additional freedom, the fully optimized single molecule model does not necessarily give accurate description of the intermolecular interactions present only in the solid state. Therefore, we cut a more extended model containing four adjacent molecules from the experimental crystal structure, and performed a topological charge density analysis according to the QTAIM method. The nature of the main intraand intermolecular interactions is presented in Table 2. The complete picture of the bond paths and bond critical points is presented in Fig. S6. Again, the Cu–N bond forming the polymeric structure has a typical nature of the metal-ligand bond: relatively small electron density at the Cu–N BCPs, value of |V|/G indicating partial sharing of electrons, and large interaction energies around −150 kJ mol-1, showing strong attractive interaction. The bridging bromides attach with slightly less strength, but slightly larger degree of covalency. The intradimer S–S bond is a true covalent bond, as can be seen from the |V|/G value over 2. Most interestingly, the organization of the polymeric network via all the dimeric btt-btt ligands enables many intermolecular S⋯S weak interactions, which show varying strength depending on the distance of the sulfur atoms. On the other hand, even though the energies are small (from −4 to −13 kJ mol-1 in the examples presented in Table 3), the large amount of them makes the S⋯S interactions the main supporting feature between the separate molecules in the crystal structure of 2.

experimental geometry, shown in Fig. S4. The main intermolecular interactions are formed between the aromatic system of the ligands and the sulfur and gold atoms of the adjacent cluster. However, the intermolecular interactions are rather weak, mostly less than 5 kJ mol-1 (Table S2), and the more likely explanation for the favourable structure for crystallization of isomer b could be the most effective intramolecular overlap of the π systems of the ligands creating stronger intramolecular π … π interactions. Furthermore, the additional intramolecular S⋯S interactions between adjacent bridging ligands stabilize a suitable geometry for crystallization. Nevertheless, the existence of other coordination isomers cannot be ruled out in solution because of the similar stability of separate clusters a-d. Very weak agostic interactions between benzyl hydrogens and gold atoms with a distance of Au(1) … H(6) 2.9495(3) and Au(2) … H(13) 2.8864(3) Å, were observed via the QTAIM analysis (Table S2). Similar, but stronger C–H⋯Au interactions were found in our previous study of the reactions between AuCl(tht) and methyl substituted mbtt.26 The preference of aurophilic interactions is probably based on the steric factors of the complex controlled by the sterically constrained ligands. The formation of Au–Au interactions further weakens the formation of agostic interactions as the electron density needed for the C–H⋯Au interaction is used in the aurophilic interaction. 2.2. Synthesis and characterization of [CuBr(btt-btt)]n [2] The reaction between CuBr2 and Hbtt gave polymeric [CuBr(2,2′dithiobis(benzothiazole)]n. nTHF, [CuBr(btt-btt)]n . n THF (2) (Fig. 2). The ligand is dimerized in the well-known reaction of the deprotonated thiol 2 Cu2+ + 2 RS- ⇌ 2 Cu+ + RSSR with concomitant reduction of copper. The dimerized ligand is coordinated solely by the imidazole nitrogens to Cu(I) centers, which are bridged by two bromo ligands. The Cu–N bond lengths are 2.070(2) and 2.048(2) Å, in accordance with the Au–N distances found in 1. The Cu–Cu distance is 2.943 Å showing no metallophilic interaction (the sum of the van der Waals radii of the metals is 2.80 Å). The S–S bond in the dimerized ligand is 2.051 Å. The environment of each copper atom is slightly distorted tetrahedral with N–Cu–N 107.92° and S–Cu–S 106.44°. The mutual S⋯S distances of S1 – S4 vary between 3.072 and 3.191 Å, indicating weak intramolecular S⋯S interactions (vdW radii 3.6 Å). The torsion angles SCSS are 8.80° and 2.68° and the torsion angle CSSC 100.62° To synthesize the polymeric complex 2, strictly anhydrous reaction conditions had to be used. THF solvent molecule can act as proton acceptor and subsequent deprotonation of the ligand takes place. The presence of water inhibits THF acting as a base and deprotonation of thiazole and thus formation of 2. Support for this hypothesis was obtained from theoretical calculations. When the full DFT optimization was performed in the presence of partially protonated ligands in a

2.3. Optical properties Experimental UV–Vis spectra showed maximum absorption for the copper complex 2 at 434 and 464 nm, which are considerable higher than the value obtained for gold complex 1 (319 nm). To analyze the excitations, TD-DFT simulated spectra for molecular models [Au(btt)]4 and [Cu2Br2(btt-btt)4] were compared to the spectra of the dimeric bttbtt ligand and the protonated Hbtt (tautomer with N–H). Table 3 shows the main excitations and their composition, the simulated spectra are presented in Fig. S7. The lowest energy excitation is shown at much larger wavelength in copper compound than in the Au4 cluster, which is in agreement with

Fig. 2. Left: The asymmetric unit of [CuBr(2,2′-dithiobis(benzothiazole)]n. n THF (2). Right: Packing of 2 along the crystallographic a-axis. Most important bond lengths and angles: Br(1)–Cu(1)#1 2.4542(4), Br(1)–Cu(1) 2.4986(4), Cu(1)–N(1) 2.070(2), Cu(1)–N(2) 2.048(2), Cu(1)#1-Br(1)-Cu(1) 72.909(13) Å, N(2)-Cu(1)-N (1)108.07(8)° (Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y+1,-z+1). The solvent THF is omitted for clarity. 4

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Table 3 The most important excitations in the TD-DFT simulated spectra (in DMSO) for molecular models [Au(btt)]4 [1] and [Cu2Br2(btt-btt)4] [2], and the separate ligands btt-btt and Hbtt. λ = wavelength (experimental values in parenthesis), f = oscillator strength. Complex

λ (nm)

f

interpretation

1

348 326 (319) 390 (464) 368 (434) 321 290 261

0.0103 0.2667 0.0479 0.0319 0.1240 0.5373 0.5065

HOMO= > LUMO(79%); HOMO-1= > LUMO(10%) HOMO-2= > LUMO(33%); HOMO= > LUMO+2(25%) HOMO= > LUMO+1(74%); HOMO-3= > LUMO(10%) HOMO-2= > LUMO+1(42%); HOMO-3= > LUMO(30%) HOMO-4= > LUMO+3(39%); HOMO-8= > LUMO(16%) HOMO= > LUMO(98%) HOMO= > LUMO+3(43%); HOMO-1= > LUMO+2(30%)

2

Hbtt btt-btt

Fig. 3. Frontier molecular orbitals involved in the lowest energy excitations in compounds 1 and 2. Hydrogens have been omitted for clarity.

compounds 1 and 2. Despite the differences in the lowest energy excitations, the main signals show similar features for both the Cu dimer and the Au cluster. These signals constitute of typical MLCT (or LL) excitations between HOMO-n and LUMO + n, and are therefore red-shifted compared to the intraligand transitions of the separate ligand molecules. Examples of the MOs involved in the excitations are shown in the supporting information (Figs. S8–S12).

the experimental measurements, although the actual wavelengths are underestimated for [Cu2Br2(btt-btt)4]. Because of the less symmetrical structure, the copper complex exhibits more excitations with complicated mixture of orbitals than the gold cluster, which shows a more even distribution of MOs. The lowest energy excitation in [Cu2Br2(bttbtt)4] originates mainly from the excitation from HOMO to LUMO+1, where the HOMO is formed from a combination of Cu(d) orbitals and the Br(p) orbitals, and the LUMO+1 is expanded mainly over the S–S bond of the btt-btt ligands (Fig. 3). Another strong excitation at 368 nm originates from MOs, whose appearance is very similar to HOMO and LUMO+1. A full presentation of the orbitals involved in the excitations is given in Figs. S8–S12. On the other hand, in the gold cluster the HOMO = > LUMO excitation at 348 nm is only seen as tailing of the more intensive higher energy signals. In this case, the HOMO again forms from the combination of the metal d orbitals and p orbitals of the coordinating sulfur or nitrogen atoms. The largest portion of gold atom d orbital contribution in the highest occupied orbitals is concentrating around Au(3), which has S–Au–S environment, while Au(1) with N–Au–N environment shows the weakest metal contribution. However, in LUMO or (LUMO + n) orbitals of 1 there is a 10–30% contribution from the metal d orbitals, which is not seen in the LUMOs of compound 2. This involvement of metal d-orbitals destabilizes the LUMO, resulting in the larger energy of excitation in 1 compared to complex 2. Fig. 3 presents the frontier orbitals involved in the lowest energy excitations for both

3. Experimental 3.1. Materials and methods H[AuCl4].3H2O (Alfa Aesar, 99,99%) and tetrahydrothiophene (Aldrich, 99%) for the synthesis of [AuCl(tetrahydrothiophene)] were commercially available and used as received. CuBr2 (J.T.Baker), and 2(3H)-benzothiazolethione (Hbtt) (AlfaAesar 97%) were also commercially available. [AuCl(tht)] was prepared as described in literature.[35] All solvents used for the synthesis of 1 were dried with molecular sieves. The synthesis of 2 was performed under nitrogen, the solid starting materials were dried under vacuum for 30 min and the THF solvent was distilled from sodium benzophenone. The elemental analysis was determined with varioMICRO V1.7 instrument. Bruker Avance 400 MHz spectrometer was used for the NMR measurements. IR spectra were 5

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the optimized gold-gold distances were only slightly overestimated (less than 3%) when compared to the experimental ones, therefore the selected DFT method was able to accurately describe also the aurophilic interactions. Solvent effects of the DMSO solvent were calculated with the conductor-like polarized continuum model (CPCM) [45] for the gas phase optimized structures.

measured in KBr with Shimadzu FTIR-8400S and UV-VIS spectra in DMSO with a PerkinElmer Lambda 900 UV/VIS/NIR spectrometer. Due to the poor solubility the compound 1, measurement of the 1H NMR was conducted by making the reaction in d-THF and measuring the spectrum from the reaction mixture. 3.2. X-ray structure determinations

3.4. Synthesis and analysis of [Au(btt)]4 (1) The crystals were immersed in perfluoropolyether cryo-oil, mounted in a nylon loop, and measured at a temperature of 120 K. The X-ray diffraction data were collected on a Bruker Kappa Apex II Duo diffractometer using Mo Kα radiation (λ = 0.71073 Å). The Apex 2 program package [36]was used for the cell refinements and data reductions. The structures were solved by direct methods using the SHELXS97 software [37]. A semi-empirical absorption correction (SADABS) [38] was applied to all data. Structural refinements were carried out using the SHELXL-97.37 The solvent of crystallization (THF) in 2 was disordered over two sites with occupancy ratio of 0.65/0.35. The O–C and C–C distances were restrained to be similar in within each disordered molecules. Both in 1 and 2 the hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C–H = 0.95–0.99 Å and Uiso = 1.2 Ueq (parent atom). The crystallographic details are summarized in Table 4.

Hbbt (10.0 mg, 0.060 mmol) was dissolved in MeOH (1.0 ml). AuCl (tht) (19.3 mg, 0.060 mmol) was dissolved in CH2Cl2 (1.0 ml) and added to the ligand solution. The mixture was stirred at room temperature for 1 h yielding a white precipitate. Yield 8.5 mg, 39%. 1H NMR (THF): δ 7.80 (Ar, 1H); overlapping signals 7.56; 7.53; 7.52; 7.50 (Ar, 2H); 7.42 (Ar, 1H) ppm. IR (KBr): 1449 (s), 1385, 1244, 1087, 1034, 1013 (s) cm−1. UV–Vis (DMSO): 319 nm. Calc. for C28H16Au4N4S8 C% 23.15; H% 1.11; N% 3.86; S% 17.66. Found. C% 23.31; H% 1.24; N% 3.84; S% 17.33. Crystals for x-ray diffraction: Hbbt (10.0 mg, 0.060 mmol) was dissolved in THF (2.0 ml) and AuCl(tht) (19.3 mg, 0.060 mmol) was added to the ligand solution. The mixture was stirred in room temperature for 15 min. Crystallization at room temperature by CH2Cl2 diffusion gave light yellow crystals in a few days.

3.3. Computational details

3.5. Synthesis and analysis of [CuBr(btt-btt)2]n . n THF [2]

All models were calculated with the Gaussian09 program package [39] at the DFT level of theory with a hybrid density functional PBE0 [40,41]. The basis set comprised quasi-relativistic effective core potential basis set def2-TZVPPD [42] for metal atoms and the bridging Br atoms in compound 2. All other atoms were described with standard allelectron basis set 6-31G(d,p) to facilitate calculation of larger systems. To obtain the electronic properties of the complexes, we performed topological charge density analysis with the QTAIM (Quantum Theory of Atoms in Molecules) [43] 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 [44] using the wavefunctions obtained from the DFT calculations. TD-DFT calculations were employed to further compare the electronic properties of the complexes. The UV-VIS spectra were simulated with fully optimized separate molecular models. It should be noted, that

CuBr2 (61.0 mg, 0.27 mmol) was suspended in THF (8.0 ml) and Hbtt (90.0 mg, 0.54 mmol) was dissolved in THF (2.0 ml). After 60 min stirring the solutions were combined and left at room temperature. In 20 h an orange precipitate of product with some green impurity of the starting material was formed (47.7 mg). The precipitate was filtered and the solution was left for further crystallization. Next day orange pure crystals suitable for x-ray measurement were formed (19.6 mg). The total yield of the pure product (64.4 mg, 43.2%) was calculated on the basis of elemental analysis. 1H NMR (C2D6SO): δ 8.08 (d, 1H), 7.95 (d, 1H), 7.54 (m, 1H), 7.45 (m, 1H). IR (KBr): 1451, 1417, 1311, 1010 cm−1. UV–Vis(DMSO) 434 and 464 nm. Calc. for C18H16BrCuN2OS4 C% 39.45; H% 2.94; N% 5.11; S% 23.40. Found. C% 39.18; H% 3.07; N% 5.16; S% 23.99%. 4. Conclusions The results confirm that the Hbtt ligand reacts very differently with Au(I) or Cu(II) salts. With Au ions, separate tetranuclear clusters are formed in which each gold atom have various coordination environments (N–Au–N, S–Au–S, or N–Au–S coordination). When comparing the DFT optimized molecular models, energy difference between the isomers was found to be small enough (less than 11 kJ mol-1) to allow existence of all isomers, even though only isomer b was found to crystallize experimentally. In the crystalline state, only very weak intermolecular interactions were found according to the QTAIM analysis, which did not explain the preference of a certain isomer in the solid state. Computational results indicate, that subtle changes in the reaction and crystallization conditions determine the final solid state structure. Topological analysis of a more extended model of the crystalline structure revealed, along with rather strong aurophilic interactions, additional intramolecular π … π and S⋯S interactions between adjacent bridging ligands, which are able to stabilize a suitable geometry for crystallization. On the other hand, copper formed dimeric structures with bridging bromides, and if the reaction was performed in completely anhydrous conditions, the ligand also dimerized via deprotonation. The deprotonation of the Hbtt ligands was also verified with DFT optimizations. Computational analysis of the electron density in a more extended {[Cu2Br2(btt-btt)4]}4 model showed several intra- and intermolecular interactions, mainly forming between the sulfur atoms, which were

Table 4 Crystal data.

empirical formula fw temp (K) λ(Å) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalc (Mg/m3) μ(Mo Kα) (mm−1) No. reflns. Unique reflns. GOOF (F2) Rint R1a (I ≥ 2σ) wR2b (I ≥ 2σ) a b

1

2

C28H16Au4N4S8 1452.79 120(2) 0.71073 Monoclinic C2/c 20.7004(7) 12.6222(5) 11.8147(4) 105.278(2) 2977.90(18) 4 3.240 20.238 15511 4224 0.986 0.0425 0.0314 0.0646

C18H16BrCuN2OS4 548.02 120(2) 0.71073 Monoclinic P21/n 12.3405(4) 9.8209(4) 17.0023(6) 105.640(2) 1984.30(12) 4 1.834 3.547 22225 5778 1.034 0.0241 0.0312 0.0811

R1 = Σ||Fo| – |Fc||/Σ|Fo|. wR2 = [Σ[w(Fo2 – Fc2)2]/Σ[w(Fo2)2]]1/2. 6

Solid State Sciences 97 (2019) 105980

S. Jääskeläinen, et al.

found to restrict the torsional freedom of the ligands, and to enable formation of a polymeric network between different dimers. Differences in the optical properties of the Au and Cu complexes were studied by simulating the absorption spectra via TD-DFT. In agreement with the experimental findings, the copper complex exhibited much larger wavelength in the lowest energy excitations than the corresponding gold cluster. This could be explained by considerable contribution of metal d-orbitals in the LUMO of the [Au(btt)]4 molecular model, which destabilized the LUMO and resulted to larger energy. The LUMOs of the copper complex did not show similar contribution of the Cu(d) orbitals. However, the main signals in the UV–Vis spectra were found to be rather similar for both the Cu dimer and the Au cluster, as they constituted of typical MLCT excitations.

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