Z. Naturforsch. 2020; 75(3)b: 239–249 Alexander Kossmann, Rayko Ehnert, Andrea Preuß, Natalia Rüffer, Marcus Korb, Steffen Schulze, Christoph Tegenkamp, Frank Köster and Heinrich Lang* The di(thiourea)gold(I) complex [Au{S=C(NH2)2}2] [SO3Me] as a precursor for the convenient preparation of gold nanoparticles https://doi.org/10.1515/znb-2019-0213 addition of reducing agents. TEM, electron diffraction, Received December 2, 2019; accepted January 31, 2020 and UV/Vis spectroscopy studies were carried out. Au NPs of size 15 ± 4 nm were formed, showing the characteristic Abstract: The synthesis of [Au{S=C(NH ) } ][SO Me] (1) 2 2 2 3 surface plasmon resonance at 528 nm. (a) by the anodic oxidation of gold metal in an anolyte of thiourea and methansulfonic acid and (b) by the reaction Keywords: bis(thiourea) gold complex; electrolysis; nano- of Au(OH)3 with an aqueous solution of methanesulfonic particles; solid-state structure; thermal behavior. acid in the presence of thiourea is reported. The structure of 1 in the solid state has been determined by single-crys- tal X-ray diffraction showing a linear S–Au–S unit with the 1 Introduction thiourea ligands in a leaflet structure folded by 113.2(3)°. The cation of complex 1 is a dimer, based on short S · · · C Diverse techniques exist for applying very thin gold coat- interactions between two adjacent mononuclear cations. ings to solid surfaces, mainly on metals, most often on The thermal decomposition behavior of 1 was studied by copper, silver and nickel [1–4]. Most commonly, chemical TG and TG-MS confirming that it decomposes under inert and electrochemical gold plating is used commercially, gas or oxygen atmosphere in four steps in the temperature i.e. in electronics industry, to provide a corrosion resistant range of 200–650°C. Initial decomposition starts with the conductive film [5]. Examples include electrical connec- release and fragmentation of one of the thiourea ligands, tors, printed circuit boards etc. In gold plating chemis- followed by the anion degradation. Powder X-ray diffrac- try, soluble gold chloride and gold cyanide complexes tion studies specified the formation of gold metal. Based are commonly used [6]. The most important gold plating on this observation, complex 1 was used as precursor for chemical is K[Au(CN)2] [6]. However, gold cyanide com- the formation of gold nanoparticles (Au NPs) in 1-hexa- plexes are toxic [7] and hence it is desirable to develop and decylamine (c = 4.0 mol L−1) at T = 330°C without any use environment friendly precursors. Recently, great efforts have been made to prepare group 11 metal layers based on predefined nanoparticles *Corresponding author: Heinrich Lang, Technische Universität (=NPs), as they have discrete particle sizes between 1 and Chemnitz, Faculty of Natural Sciences, Institute of Chemistry, 100 nm diameter [8]. Nanoparticles have unique physico- Inorganic Chemistry, D-09107 Chemnitz, Germany, chemical properties, which are affected by the particle Phone: +49 (0)371 531 21210, Fax: +49 (0)371 531 21219, E-mail: [email protected] size, shape, size distribution and particle-to-particle Alexander Kossmann, Andrea Preuß and Natalia Rüffer: Technische interaction [9–16]. For example, group 11 metal NPs allow Universität Chemnitz, Faculty of Natural Sciences, Institute of the design of smart materials including nano-devices Chemistry, Inorganic Chemistry, D-09107 Chemnitz, Germany [17–23], owing to their unique optical [24–36], electrical Rayko Ehnert: Hochschule Mittweida, Faculty of Applied Computer [37–45], magnetic [46–51], catalytic [52–63], or biological Sciences and Biosciences, Biotechnology and Chemistry, [64–71] properties. Two synthetic methodologies for metal Technikumplatz 17, D-09648 Mittweida, Germany Marcus Korb: The University of Western Australia, Faculty of Science, colloid generation are predominantly used, which are the School of Molecular Sciences, Perth, WA 6009, Australia top-down and bottom-up approach [72]. The latter pro- Steffen Schulze and Christoph Tegenkamp: Technische Universität cedure enables the straightforward synthesis of defined Chemnitz, Faculty of Natural Sciences, Institute of Physics, Solid gold nanoparticles (Au NPs) by, for example, chemical Surface Analysis, Reichenhainer Straße 70, D-09126 Chemnitz, reduction of a gold source [73], decomposition of metal- Germany Frank Köster: Hochschule Mittweida, Faculty of Engineering organic compounds [74–77] or applying electrochemical Sciences, Process Engineering/Surface Engineering, [78] as well as photochemical [79] methods. Turkevich Technikumplatz 17, D-09648 Mittweida, Germany and coworkers prepared gold colloids in aqueous media 240 A. Kossmann et al.: The di(thiourea)gold(I) complex [Au{S=C(NH2)2}2][SO3Me] as a precursor by the reduction of chloroauric acid with sodium citrate π-electron density in the C–N bond [85–87]. The Me group [80], whilst the Brust-Schiffrin method reduces chloro- of the counter ion is observed as a singlet at δ = 2.41 ppm. auric acid by sodium borohydride in the presence of an In the 13C{1H} NMR spectrum a characteristic reso- alkanethiol [81]. Furthermore, the Möller group prepared nance signal at δ = 175.3 ppm is observed for the C=S Au NPs using sodium borohydride or hydrazine as reduc- carbon atom, shifted up-field by 8.5 ppm as compared to ing agent in micelles [82]. These methodologies need the non-complexed thiourea [86]. Complexation endorses the use of reducing agents and the presence of stabilizing sur- decrease of the C=S bond order, resulting in the formation factants [80–82]. of a partial CN double bond character [85, 86]. Herein we report on the efficient synthesis of the IR spectroscopy was used to investigate the nature of gold(I) coordination complex [Au{S=C(NH2)2}2][SO3Me] the ligand-to-metal coordination of 1. Therein, the pres- by using either electrolysis or a wet chemical process. Its ence of a non-coordinated mesylate anion was evidenced −1 chemical and physical properties are described and its use by the appearance of two bands at 1193 cm (νas(SO3)) and −1 as a precursor for Au NPs is discussed. 1058 cm (νas(SO3)) [88]. The ν(NH) absorptions of the thio- −1 urea ligands at 3100–3400 cm and the δ(NH2) vibrations at 1600 cm−1 are representative for this family of complexes [89]. The respective asymmetric ν(CN) mode at 1469 cm−1 2 Results and discussion is shifted to higher frequencies by 26 cm−1 as compared to non-coordinated thiourea. The ν(C=S) vibration of 1 is The title complex [Au{S=C(NH ) } ][SO Me] (1) could 2 2 2 3 shifted from 725 to 710 cm−1 upon coordination, confirm- be prepared by either electrolysis or by the reaction of ing the dative binding of the thiourea ligands to gold(I) Au(OH) with thiourea in aqueous solution in the presence 3 [90]. Further proof of thiourea coordination is provided of methanesulfonic acid at ambient temperature, whereby by the appearance of the asymmetric ν(C=S) bands at 1432 thiourea reduces Au(III) to Au(I) and is itself oxidized to and 1385 cm−1 [89, 90]. formamidine disulfide [83, 84] (Scheme 1). Electrolysis was performed in a cell where the half chambers were separated by an ion-selective membrane, 2.1 Solid-state structure containing as anolyte an aqueous solution of methanesul- fonic acid and thiourea. The oxidation of the gold anode The molecular structure of 1 in the solid state has been occurs by applying a potential of smaller than 7.0 V. determined by single-crystal X-ray diffraction (Fig. 1). After appropriate work-up, complex 1 could be iso- Selected bond distances (Å) and angles (deg) are summa- lated as a colorless air stable solid in a yield of 85% (elec- rized in the caption of Fig. 1. Suitable single crystals were trolysis) or 98% (wet synthetic procedure). Complex 1 is obtained by slow cooling of a concentrated ethanol solu- soluble in common polar organic solvents and water. tion from 60°C to ambient temperature. The gold(I) complex 1 has been characterized by 1H Complex 1 crystallizes in the monoclinic space group and 13C{1H} NMR and IR spectroscopy, high resolution C2/c with one formula unit in the asymmetric unit. The ESI-TOF mass spectrometry, elemental analysis and sin- gold(I) ion is coordinated by two thiourea ligands (Au1–S1 gle-crystal X-ray diffraction. The thermal behavior of 1 2.2774(14) Å and Au1–S2 2.2727(14) Å) in a linear fashion was studied by TG (thermogravimetry), TG-MS (TG-cou- (S1–Au1–S2 179.50(5)°), similar to other bis(thiourea) gold pled mass spectrometry) and DSC (Differential Scanning complexes, e.g. [Au{S=C(NH2)2}2][Br] and [Au{S=C(NH2)2}2] Calorimetry). [Cl] [91, 92]. 1 The H NMR spectrum of 1 shows for the NH2 units The C=S entity is rotated by 106.17(18) / 107.31(19)° two signals at δ = 8.18 and 8.47 ppm, which are shifted (Au1–S1/2–C1/2) towards the linear S1–Au1–S2 unit downfield by 1.3 ppm if compared to the free thiourea (179.50(5)°), which also agrees with previous findings ligand. The de-shielding is caused by the coordination (see above) [91, 92]. The coordination of the C=S moiety of the thione unit to Au+ resulting in an increase of the increases the C=S bond length from, e.g. 1.688 Å [93] for Scheme 1: Synthesis of complex 1. (i) HCl-HNO3 (3:1, v/v). (ii) NaOH. (iii) MeSO3H, S=C(NH2)2. A. Kossmann et al.: The di(thiourea)gold(I) complex [Au{S=C(NH2)2}2][SO3Me] as a precursor 241 2+ Fig.