## 3 Results and Discussion
### Structure Elucidation of the Au Complex by NMR
conditions of the synthesis of the Au complex, a possible desulfurization of 2,4-dithiouracil could happen, but also the nitrogen atoms of 2,4-dithiouracil and 2-thiouracil could be deprotonated in the reaction mixture, thus stimulating the conversion of these ligands into some of their tautomeric forms during their complexation with Au as described in a previous study [28]. The signal at 14.13 ppm with an area of 1.00 showed that there is possibly a tautomeric form of 2,4-dithiouracil with one deprotonated nitrogen that could be coordinated with Au as the chemical shift 14.13 ppm most probably corresponded to the signal of the NH-3 proton. The 2,4-dithiouracil would additionally be coordinated to Au by the sulfur atom that is adjacent to the coordinated nitrogen (Figure 1). In support of this hypothesis, the chemical shifts of the carbon signals in the HSQC correlations (7.10 ppm-116.00 ppm) and (7.76 ppm-140.73 ppm) are close to those signals of the carbons (C-5 and C-6) in the structure of the uncoordinated 2,4-dithiouracil (see Table 3). Therefore, the signals at 7.10 ppm, 7.76 ppm, 116.00 ppm, and 140.73 ppm could, respectively, be assigned to the protons (H-5 and H-6) and carbons (C-5 and C-6) of the coordinated 2,4-dithiouracil. The two signals left at each of the following chemical shifts (7.39 ppm and 8.31 ppm), in addition to the corresponding HSQC correlations (7.39 ppm-115.50 ppm) and (8.31-156.48 ppm), possibly indicated the presence of tautomeric forms of 2-thiouracil of the same kind in the Au complex containing deprotonated nitrogen atoms. In such case, these tautomeric forms could be coordinated to Au by the sulfur bound to the carbon at the second position, C-2, and its adjacent nitrogen atoms (see Figure 1). The potential coordination binding sites for the coordinated 2-thiouracil and 2,4-dithiouracil in the Au complex are shown on Figure 1, where the free uncoordinated uracil that was probably produced by desulfurization of the 2,4-dithiouracil ligand could take a place  
Figure 1: Possible structures of the Au complex with 2,4-DTu, 2-Tu, and U.  
in the outer sphere of the complex. Also, it is possible for the free uncoordinated 2-thiouracil and 2,4-dithiouracil ligands to participate in the outer sphere of the Au complex similarly to uracil (Table 4). This structure is similar to that described in our previous paper with 6-methyl-2-thiouracil [42], as this type of bidentate coordination mode is observed in other metal complexes of Cu(II), Cd(II), Hg(II), Rh(III), Ir(III), Pd(II), Pt(II), Pt(IV) [12; 19; 27; 28]. Additionally, the proton solid-state NMR spectrum showed broad peaks at 5.37 ppm and 7.53 ppm, which could be an additional indication for the protons H-5 and H-6 in uracil, thus supporting the potential presence of uracil in the Au complex. The signals of the protons H-5 and H-6 in 2-thiouracil and 2,4-dithiouracil probably cannot be clearly observed in the proton solid-state NMR spectrum because they could also be a part of the broad peaks at 5.37 ppm and 7.53 ppm. The broad signal at 13.21 ppm could involve the signals of the NH protons in the uncoordinated uracil, 2-thiouracil, or 2,4-dithiouracil that could participate as ligands in the outer sphere of the Au complex (Table 2), as it could also contain the signal for the NH proton of the coordinated 2,4-dithiouracil (Figure 1). The broad peak at 3.05 ppm possibly indicated the presence of DMSO-h\({}_{6}\) and H\({}_{2}\)O in the Au complex. The solid-state CP MAS (Figure 2A), and CPPI MAS (Figure 2B) NMR spectra of the Au complex showed signals at ca 189.7 ppm and 170.4 ppm, thus confirming the presence of 2,4-dithiouracil in the inner coordination sphere of the Au. The signals at 116.9 ppm and 121.3 ppm would probably correspond to the carbons C-5 in the coordinated 2-thiouracil, whereas the signal at 157.9 ppm could be for the carbons C-6, respectively.