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Communication

Organogold(III) Complexes with Chelating Thiourea-Type Ligands

by
Suelen Ferreira Sucena
,
Adelheid Hagenbach
and
Ulrich Abram
*
Institute of Chemistry and Biochemistry, Freie Universität Berlin, Fabeckstr. 34/36, 14195 Berlin, Germany
*
Author to whom correspondence should be addressed.
Chemistry 2025, 7(6), 174; https://doi.org/10.3390/chemistry7060174
Submission received: 3 October 2025 / Revised: 24 October 2025 / Accepted: 29 October 2025 / Published: 3 November 2025
(This article belongs to the Special Issue Celebrating the 50th Anniversary of Professor Valentine Ananikov)
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Abstract

The gold(III) starting material [Au(damp-κC1,N)Cl2] (Hdamp = 2-(dimethylaminomethyl)benzene) reacts with the thiourea-type ligands 3,3-diethyl-1-benzoylthiourea (HL1) or N-(3,3-diethylamino-thiocarbonyl)-N′-(2-hydroxyphenyl)benzamidine (H2L2) under formation of the gold(III) cations [Au(damp-κC1,N)(L1-κS,O)]+ (1) and [Au(Hdamp-κC1)(L2-κS,N,O)]+ (2). The products have been isolated in crystalline form as their PF6− salts and studied by X-ray diffraction and spectroscopic methods. The preservation of the gold(III) oxidation state and the square-planar coordination spheres in the products is most probably due to the formation of chelate rings by the incoming ligands and the presence of the Au–C bond to the phenyl rings of the damp− or Hdamp ligands.

1. Introduction

Reactions of the common gold(III) starting material [AuCl4]− with thiourea- or thione-type ligands frequently result in a reduction in the transition metal and the formation of gold(I) products [1]. This is not completely unexpected with regard to the redox potential of Au(III) and the ready formation of disulfide-like compounds. However, some exceptions are known, which are mainly related to complexes containing aromatic thiones [2,3,4,5], the presence of gold–carbon bonds [5,6], and/or the stabilization of the coordination number 4 by the formation of chelate rings [5,7]. The latter finding seems to be restricted to ligand systems, in which the chelate rings are supported by the formation of π-systems, while potential aliphatic chelators such as N,N′-diisobutyloxycarbonyl-N″,N‴-(1,3-propylene)bis(thiourea) [8] seem not to be able to establish the corresponding chelate rings. Instead, they act as reductant and form linear gold(I) complexes exclusively using their sulfur atoms. A reduction in gold(III) and the formation of oxidized sulfur-containing compounds such as sulfate ions or thiadiazolium cations have also been observed during reactions of tetrachloridoaurate with benzoylthioureas (e.g., HL1 of Figure 1) or thiocarbamoylbenzamidines (e.g., HL0 of Figure 1)) [9,10,11,12,13].
Contrariwise, the isolation of gold(III) was successful when powerful chelators such as tridentate thiosemicarbazides were applied, which extended π-systems upon coordination to Au(III) [14]. Interestingly, in such cases, the chelating capacity of related aliphatic ligand systems such as H2L0′ of Figure 1 also seems to be insufficient for the stabilization of the square-planar coordination sphere of gold(III) [8,15].
A larger number of stable Au(III) complexes are available through a combination of the above-mentioned organometallic approach with chelating ligands [16,17,18]. Of particular interest are those with coordinated 2-(dimethylaminomethyl)phenyl (damp−) ligands [5,8,19,20,21,22,23,24]. The resulting complexes are of interest as potential anti-cancer or anti-parasital drugs [21,23,24]. In the present communication, we report the reactions of [Au(damp-κC1,N)Cl2] with the bidentate benzoylthiourea derivative HL1 and the potentially tridentate thiocarbamoyl benzamidine H2L2. Both ligands are depicted in Figure 1.

2. Materials and Methods

[Au(damp-κC1,N)Cl2] [25], HL1 [26] and H2L2 [27] were prepared following the procedures in the literature. Solvents were dried using conventional methods.
IR spectra were measured as KBr pellets on a Shimadzu IR Affinity-1 spectrometer (Shimadzu, Kyoto, Japan). Elemental analysis of carbon, hydrogen, nitrogen, and sulfur were determined using a Heraeus vario EL elemental analyzer (Elementar, Langensebold, Germany). The NMR spectra were recorded on JEOL 400 MHz spectrometers (JEOL, Kyoto, Japan). ESI TOF mass spectra were measured with an Agilent 6210 ESI TOF (Agilent Technologies, Santa Clara, CA, USA) in CH2Cl2/MeOH (1:1) mixtures. The intensities for the X-ray determinations were collected on a Bruker D8 Venture (BRUKER, Billerica, MA, USA) instrument with Mo/Kα radiation. Absorption correction was carried out with the APEX 3 software suite [28]. Structure solution and refinement were performed with the SHELX programs [29,30] included in OLEX2, version 1.5 [31]. Hydrogen atoms were calculated for idealized positions and treated with the ‘riding model’ option of SHELXL. Details are given in the Supplementary Materials. The representations of the molecular structures were achieved using the program MERCURY, version 2024, 2.0 [32].
Synthesis of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)). [Au(damp-κC1,N)Cl2] (120 mg, 0.3 mmol) dissolved in 3 mL of dry THF was added dropwise to a stirred solution of HL1 (71 mg, 0.3 mmol) in 2 mL of dry THF. Three drops of Et3N were added. The color of the solution immediately turned yellow and a colorless solid of (HNEt3)Cl precipitated. The suspension was stirred for 2 h at room temperature and filtered. The solvent was removed in vacuum and the residue was dissolved in 3 mL of EtOH. (NBu4)PF6 (116 mg, 0.3 mmol) was added and the suspension was stirred for 20 min, which resulted in the formation of a pale-yellow precipitate. The product was filtered off, washed with EtOH and recrystallized from a mixture of CH2Cl2/MeOH. Yield: 159 mg (75%). Elemental analysis: Calcd. for [C21H27AuN3OS](PF6): C, 35.5; H, 3.8; N, 5.9; S, 4.5%. Found: C, 35.3; H, 3.2; N, 5.9; S, 4.7%. IR (KBr, cm−1): 3070 (w), 2980 (w), 2940 (w), 1510 (s), 1448 (m), 1408 (s), 1356 (m), 1254 (m), 1200 (w), 1076 (w), 839 (s), 752 (m), 715 (m), 557 (s). 1H NMR (CD2Cl2, ppm): 8.21–8.06 (m, 2H, Ph); 7.61 (t, J = 7.3 Hz, 2H, Ph); 7.55–7.46 (m, 2H, Ph); 7.40–7.15 (m, 4H, Ph); 4.51 (s, 2H, NCH2); 3.95 (m, 4H, CH2); 3.31 (s, 6H, CH3); 1.43 (t, J = 7.1 Hz, 3H, CH3); 1.34 (t, J = 7.1 Hz, 3H, CH3). 13C{1H} NMR (CD2Cl2, ppm): 169.4 (C=S); 161.5 (C=O); 144.8 (C-Au), 135.5 (Ph-CH2), 134.0 (Ph-damp); 132.6 (Ph); 129.0 (Ph); 127.9 (Ph); 127.5 (Ph), 124.4 (Ph-damp); 71.6 (CH2-N(CH3)2); 50.6 ((CH3)2-N); 47.6, 47.2 (CH2); 11.9 (CH3) 11.7 (CH3). ESI+ MS (m/z): 566.1731, 100% [M]+ (Calcd. 566.1541).
Synthesis of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)). [Au(damp-κC1,N)Cl2] (80 mg, 0.2 mmol) was suspended in 1 mL of MeOH and H2L2 (64 mg, 0.2 mmol) dissolved in 4 mL of MeOH was added dropwise. After one hour of stirring at room temperature, KPF6 (38 mg, 0.2 mmol) was added, giving a dark-yellow solution, which was filtered via celite. The resulting solution was left to evaporate slowly, which resulted in the formation of orange red crystals. Yield: 83 mg (52%). Elemental analysis: Calcd. for [C27H32AuN4OS](PF6): C, 37.9; H, 4.4; N, 6.4; S, 3.9%. Found: C, 37.9; H, 4.2; N, 6.1; S, 3.5%. IR (KBr, cm−1): 3302 (m), 3064 (w), 2980 (w), 2937 (w), 2877 (w), 2847 (w), 1610 (m), 1560 (s), 1529 (s), 1458 (m), 1384 (m), 1363 (m), 1254 (m), 1141 (m), 1028 (w), 981 (w), 960 (w), 943 (w), 839 (s), 763 (m), 710 (m), 557 (s). 1H NMR (CDCl3, ppm): 8.66 (br, 1H, NH+); 8.17–8.0 (br, 1H, Ph); 7.62–7.41 (m, 2H, Ph); 7.41–7.27 (m, 3H, Ph); 7.22–6.99 (m, 5H, Ph); 6.88–6.70 (m, 2H, Ph); 4.15 (s, 2H, NCH2); 3.98 (q, J = 7.12 Hz, 2H, CH2); 3.80 (q, J = 7.1, 2H, CH2); 3.48 (s, 3H, NCH3); 2.79 (s, 3H, NCH3); 1.42 (m, 6H, CH3). 13C{1H} NMR (CDCl3, ppm): 173.0 (C=S); 157.7 (N-C=N); 147.8 (C-O), 146.9 (Ph), 145.4 (C-CH2); 133.3 (Ph-N), 132.0 (Ph); 130.5 (Ph), 129.7 (2C (Ph)); 129.3 (C-Au); 127.9 (2C (Ph)); 127.3 (Ph); 125.1 (2C (Ph)); 124.9 (Ph); 121.5 (Ph); 120.9 (Ph); 116.3 (Ph); 74.2 (CH2-NH(CH3)2); 52.2 ((CH3)2-N); 48.9 (CH2); 47.3 (CH2); 13.5 (CH3); 12.6 (CH3). ESI+ MS (m/z): 657.1934, 4% [M]+ (Calcd. 657.1963); 524.1056, 15% [Au(H2L2)]+ (Calcd. 524.1071); 326.1328, 50% [HL2]+ (Calcd. 326.1327); 255.0592, 40% [AuSCN]+ (Calcd. 254.9417); 196.0765, 100% [HO(C6H4)NC(C6H5)]+ (Calcd. 196.765), 134.0973, 50% [Hdamp]+ (Calcd. 134.0970).

3. Results

The square-planar gold(III) chelate [Au(damp-κC1,N)] has been proven as a suitable starting material for the synthesis of organometallic gold(III) complexes. The two chloride ligands are readily replaced and the damp− ligand commonly remains coordinated, either as C1,N chelate or monodentate C1-bonded [33,34,35,36,37,38,39,40,41,42,43,44,45,46]. Such coordination modes are also observed in the products obtained during reactions of [Au(damp-κC1,N)] with HL1 and H2L2 (Scheme 1).
A yellow, crystalline solid of the composition [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)) was formed during a reaction of [Au(damp-κC1,N)] with HL1 and (NBu4)PF6. Addition of NEt3 as a supporting base promotes the deprotonation of the benzoylthiourea and the formation of a chelate. It is recommended to perform such reactions in dry THF since the formed side-product (HNEt3)Cl is almost insoluble in this solvent and can readily be removed by filtration. Single crystals of 1(PF6) were isolated in pure form from the remaining solution. Deprotonation of the benzoylthiourea is evident by the absence of the νN-H band of the ligand in the spectrum of the complex. Chelate formation is suggested by the IR spectrum of the product. It displays a strong bathochromic shift in the νC=O stretch, which is found at 1510 cm−1 for 1(PF6) and corresponds to a shift in more than 100 cm−1 compared with the spectrum of HL1. This is a strong indicator for the formation of an extended π-system, as has also been found in numerous chelate complexes containing the same ligand [26,47,48,49,50,51,52]. The ESI+ mass spectrum of 1(PF6) depicts a peak at m/z = 566.1731 as the most intense signal of the entire spectrum. It corresponds to the parent signal of 1+ and fits with the calculated value of 566.1541. Further support of the structure of 1(PF6) is given by its 1H and 13C{1H} NMR spectra. A signal splitting is observed for the -NEt2 groups due to a hindered rotation around the adjacent C-NEt2 bonds, which is typical for such compounds and has also been observed for the uncoordinated ligand and other metal complexes [53,54,55]. The coordination of a chelate-bonded damp− ligand is supported by the presence of two singlet signals at 3.31 and 4.51 ppm.
The reaction between of [Au(damp-κC1,N)] and H2L2 could be performed in methanol without the addition of a supporting base. After treating the reaction mixture with KPF6, an orange-yellow solid of the composition [Au(Hdamp-κC1)(L2-S,N,O)](PF6) (2(PF6)) could be isolated. Spectroscopic studies strongly suggest the cleavage of the Au–N bond of the starting material and the protonation of the released -CH2N(CH3)2 unit by the detection of an νN-H band at 2605 cm−1 and a related NH signal in the 1H NMR spectrum at 8.7 ppm. Consequently, the incoming thiocarbamoyl benzamidine H2L2 is doubly deprotonated in the 2+ cation and coordinates as the tridentate ligand. Such a bonding situation is the same as that observed earlier for some rhenium, technetium, or ruthenium complexes [27,56,57,58].
Single crystals of both compounds suitable for X-ray diffraction have been grown from CH2Cl2/MeOH solutions. Figure 2 depicts ellipsoid plots of the complex cations of [Au(damp-κC1,N)(L1-S,O)](PF6) and [Au(Hdamp-κC1)(L2-S,N,O)](PF6). They confirm the structures, which have been derived from the spectroscopic information. Selected bond lengths and angles are summarized in Table 1.
The bonding situation found for the monoanionic, bidentate O,S-chelator in the complex cation 1+ confirms the presence of an extended π-system, which has already been concluded from the IR data. A considerable C–N bond length equalization is visible, which also includes the bond to the exocyclic nitrogen atom N12. The C–S bond of 1.759(4) Å is slightly longer than those found in comparable thiourea complexes [59]. Surprisingly, the amide C11-O11 bond in the complex is shorter (1.183(5) Å) than in the uncoordinated benzoylthiourea (1.219(2) Å) [60]. This suggests an unusual bonding situation and partially clashes with the observed bathochromic shift in the νC=O stretches observed in the IR spectra of the compounds. However, we attribute this finding to uncertainty, which appeared during the modeling of the structure due to a crystallographic data set of relatively low quality. The phenyl ring is bonded in trans position to O11. The coordination sphere of the gold atom is almost ideally square-planar (RMS deviation of 0.0327 for a plane including the atoms Au, S, C21, O11 and N21) with an practically neglectable maximum deviation of 0.036 Å due to the sulfur atom. Planarity is also found for the six-membered chelate ring including the benzoylthioureato ligand, while the five-membered ring of the damp− ligand shows clear deviations.
A planar coordination sphere is also observed for the gold atom in the [Au(Hdamp-κC1)(L2-κS,N,O)]+ cation (RMS deviation of 0.0148 for the plane including the atoms Au, S, C21, N1 and O1, maximum deviation for C21 of 0.0181Å). The results of the crystallographic study confirm the conclusions drawn from the spectroscopic studies, including the Au–N21 bond cleavage during the ligand exchange procedure and the tridentate coordination of {L2}2−. Apparently, the chelating effect of the incoming tridentate S,N,O ligand energetically outweighs the retention of the C1,N coordination of the damp− ligand. Similarly to the situation in a number of gold complexes with monodentate damp ligands [5,19,20,21,22,23,24], the released dimethylamino residue is protonated and establishes a hydrogen bond, which probably contributes to the stabilization of the complex cation.

4. Conclusions

Preservation of square-planar gold(III) complexes with thiourea-type ligands is possible for organometallic compounds containing chelating {Au(damp-κC1,N)}2+ or monodentate {Au(Hdamp-κC1)}3+ units. The stabilization of the high oxidation state of the transition metal is supported by the formation of chelate rings by the incoming ligands.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry7060174/s1, Figure S1: Ellipsoid representation of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)). The thermal ellipsoids are set at a 50% probability level. Hydrogen atoms bonded to carbon atoms are omitted for clarity; Figure S2: Ellipsoid representation of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)). The thermal ellipsoids are set at a 30% probability level. Hydrogen atoms bonded to carbon atoms are omitted for clarity; Figure S3: IR spectrum of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)); Figure S4: 1H NMR spectrum of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)) in CD2Cl2; Figure S5: 13C NMR spectrum of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)) in CD2Cl2; Figure S6: ESI+ mass spectrum of [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)) in a CH2Cl2/MeOH (1:1) mixture; Figure S7: IR spectrum of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)); Figure S8: 1H NMR spectrum of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)) in CDCl3; Figure S9: 13C NMR spectrum of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)) in CDCl3; Figure S10: ESI+ mass spectrum of [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)) in a CH2Cl2/MeOH (1:1) mixture. Table S1: Crystallographic data and data collection parameters; Table S2: Bond lengths (Å) in [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)); Table S3: Bond angles (°) in [Au(damp-κC1,N)(L1-κS,O)](PF6) (1(PF6)); Table S4: Bond lengths (Å) in [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)); Table S5: Bond angles (°) in [Au(Hdamp-κC1)(L2-κS,N,O)](PF6) (2(PF6)).

Author Contributions

Conceptualization, S.F.S. and U.A.; methodology, S.F.S. and A.H.; validation, S.F.S., A.H. and U.A.; formal analysis, S.F.S. and A.H.; investigation, S.F.S., A.H. and U.A.; resources, U.A.; data curation, S.F.S., A.H. and U.A.; writing—original draft preparation, U.A.; writing—review and editing, S.F.S., A.H. and U.A.; visualization, U.A.; supervision, U.A.; project administration, U.A.; funding acquisition, S.F.S. and U.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by CNPq (Brazil, PhD scholarship to S.F.S.) and DAAD (Germany).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We would like to acknowledge the assistance of the Core Facility BioSupraMol supported by the DFG.

Conflicts of Interest

The authors declare no conflicts of interest.

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Chemistry 07 00174 g001
Figure 1. Thiourea-type ligands relevant for the present paper.
Figure 1. Thiourea-type ligands relevant for the present paper.
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Scheme 1. Reactions of [Au(damp-κC1,N)Cl2] with HL1 and H2L2.
Scheme 1. Reactions of [Au(damp-κC1,N)Cl2] with HL1 and H2L2.
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Figure 2. Ellipsoid representations of the complex cations of (a) [Au(damp-κC1,N)(L1-κS,O)](PF6) and (b) [Au(Hdamp-κC1)(L2-κS,N,O)](PF6). Thermal ellipsoids represent 50% probability. Hydrogen atoms bonded to carbon atoms are omitted for clarity.
Figure 2. Ellipsoid representations of the complex cations of (a) [Au(damp-κC1,N)(L1-κS,O)](PF6) and (b) [Au(Hdamp-κC1)(L2-κS,N,O)](PF6). Thermal ellipsoids represent 50% probability. Hydrogen atoms bonded to carbon atoms are omitted for clarity.
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Table 1. Selected bond lengths (Å) and angles (°) in the [Au(damp-κC1,N)(L1-κS,O)]+ and [Au(Hdamp-κC1)(L2-κS,N,O)]+ cations.
Table 1. Selected bond lengths (Å) and angles (°) in the [Au(damp-κC1,N)(L1-κS,O)]+ and [Au(Hdamp-κC1)(L2-κS,N,O)]+ cations.
[Au(damp-κC1,N)(L1-κS,O)]+
Au–S2.266(1)Au–O112.110(3)Au–C212.005(4)Au–N212.115(3)
O11–C111.183(5)C11–N111.360(5)N11–C121.349(5)C12–N121.317(5)
C12–S1.759(4)S–Au–O1194.00(8)S–Au–C2191.2(1)S–Au–N21173.5(1)
O11–Au–C21174.7(2)O11–Au–N2192.0(1)C21–Au–N2182.9(2)Au–O11–C11129.0(3)
O11–C11–N11131.2(4)C11–N11–C12126.8(4)N11–C12–N12116.2(4)N11–C12–S128.6(3)
C12–S–Au106.3(1)Au–C21–C26113.0(3)Au–N21–C27105.8(3)
[Au(Hdamp-κC1)(L2-κS,N,O)]+
Au–S2.269(2)Au–N12.050(5)Au–O12.031(4)Au–C212.041(6)
O1–C361.364(7)C31–N11.437(7)N1–C111.331(7)C11–N111.338(8)
N11–C121.343(8)C12–S1.745(6)S–Au–N198.1(1)S–Au–O1179.3(1)
S–Au–C2186.9(2)N1–Au–O182.6(2)N1–Au–C21174.7(2)O1–Au–C2192.4(2)
Au–O1–C36111.2(3)O1–C36–C31119.9(5)C36–C31–N1114.9(5)C31–N1–Au110.5(4)
C31–N1–N11125.2(5)Au–N1–C11110.5(4)N1–C11–N11125.3(5)C11–N11–C12130.0(5)
N11–C12–N12115.4(5)N11–C12–S127.4(5)C12–S–Au103.3(5)Au–C21–C26121.6(4)
N21–H211.00O1∙∙∙H211.77O1∙∙∙H21–N21150.2N21…O12.688(6)
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Sucena, S.F.; Hagenbach, A.; Abram, U. Organogold(III) Complexes with Chelating Thiourea-Type Ligands. Chemistry 2025, 7, 174. https://doi.org/10.3390/chemistry7060174

AMA Style

Sucena SF, Hagenbach A, Abram U. Organogold(III) Complexes with Chelating Thiourea-Type Ligands. Chemistry. 2025; 7(6):174. https://doi.org/10.3390/chemistry7060174

Chicago/Turabian Style

Sucena, Suelen Ferreira, Adelheid Hagenbach, and Ulrich Abram. 2025. "Organogold(III) Complexes with Chelating Thiourea-Type Ligands" Chemistry 7, no. 6: 174. https://doi.org/10.3390/chemistry7060174

APA Style

Sucena, S. F., Hagenbach, A., & Abram, U. (2025). Organogold(III) Complexes with Chelating Thiourea-Type Ligands. Chemistry, 7(6), 174. https://doi.org/10.3390/chemistry7060174

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