The tiosemicarbazone HL was synthesized by refluxing equimolar amounts of thiosemicarbazide and β-cyclocetol in methanol according to the experimental protocol described in  (Scheme 1). The structure of the ligand was established by IR, 1H-NMR, 13C-NMR and mass spectroscopy data.
For the preparation of complexes 1–7 were used the metal salts Cu(OAc)2·(H2O), CuSO4·5H2O, Cu(NO3)2·3 H2O, Cu(ClO4)2·6 H2O, CuCl2·2H2O, Pd(OAc)2 and K2(PtCl4) which were refluxed with stirring in a hot clear solution of the ligand in methanol (1:1 and 1:2 metal–ligand ratio). All complexes are microcrystalline solids, which are stable in air. The melting point values are greater than 220 °C. The complexes are insoluble in organic solvents such as methanol, ethanol, chloroform and acetone, but soluble in DMF and DMSO.
The proposed molecular formulas for thiosemicarbazone HL and complexes 1–7 (Figure 1) are in agreement with the stoichiometries obtained based on analytical data (see Experimental). The molar conductance values of the soluble complexes in DMF (8–15 ohm−1∙cm2∙mol−1) showed that all complexes are non-electrolytes .
2.1.1. 1H-NMR and 13C-NMR Spectra
The 1H-NMR (DMSO-d6) spectra of diamagnetic Pd(II) and Pt(II) complexes showed the disappearance of the signal due to the =N-NH proton, indicating the deprotonation of this group, while the signals of the other NH (NH2) protons were observed at 10.42 and 10.44 ppm, respectively. The signal due to the OH proton remained unchanged as observed in the free ligand. In the 13C-NMR (DMSO-d6) spectra of the two complexes was observed that imine C=N carbon atoms signal is shifted downfield and appeared at 173.274 and 175.208 ppm, respectively. This information indicates the coordination of the metal center to the azomethine nitrogen and the sulfur in the thiol form and the NH2 group is not coordinated to the metal ion .
2.1.2. Infrared Spectra and Coordination Mode
The thiosemicarbazone group can exhibit thione-thiol tautomerism, because it contains a thioamide functional group. The presence of the band υ(N2′-H) at 3214 cm−1 and the absence of the one from 2570 cm−1 specific to υ(SH), indicates the ligand’s existence in thione form in a solid state . In all IR spectra of the complexes the band at 3214 cm−1 is shifted to lower wavenumbers by 38 cm−1 and the presence of new band at 1617–1631 cm−1 (except for 2), due to the υ(N2′=C) group, indicates the C=S thioenolisation followed by deprotonation and complexation with the metal ions. The band at 3360 attributed to the υ(N4′-H) stretching mode in the spectra of the ligand did not present any considerable change in the complexes and thus ruled out the possibility of metal ion bonding in this area . Also, in the spectrum of ligand HL two bands appear at 1276, 815 cm−1 assigned to the υ(C=S) + υ(C=N) frequencies, respectively, and the υ(C=S) (tioamide IV) frequency. In the complexes’ spectra these bands are shifted towards lower frequencies indicating the coordination of the thione/thiol sulphur [35,36]. On the other hand, in the spectra of all complexes (except 2) the emergence of a new band in the 581–628 cm−1 range, due to υ(C-S) vibration is also a sign of sulphur coordination in the thiol form.
The presence of υas(COO−) and υs(COO−) absorption bands at 1648, 1533 cm−1 and 1565, 1473 cm−1, respectively, in the IR spectra of complexes 1 and 6 suggests that bidentate behavior of the acetate group .
Three bands, which appeared at 1211, 1118, 1089 cm−1 in the IR spectrum of complex 2 are assignable to ʋ3 of the bidentate coordinated sulfato group. Further, the presence of the medium bands (ʋ1) at 931 cm−1 confirm the bidentate behaviour of the sulfato group .
The nitrate complex 3 has two bands at 1404 and 1210 cm−1 corresponding to υ5 and υ1, with a separation of 194 cm−1, and a medium band at 934 cm−1 assigned to υ2 of the nitrate group. The separation υ5–υ1 can be used as a criterion to distinguish between the degree of covalence of the nitrate coordination. These values indicate the presence of a terminal monodentate nitrate group .
The perchlorate complex 4 shows a band at 1089 cm−1 split into three components and a strong band at 628 cm−1. Splitting of this band indicates a bidentate coordination of the perchlorate group .
The frequency at 3436 cm−1 in HL, is assigned to the stretching vibration of the OH group of the thiosemicarbazone nucleus. All the complexes (except 7) exhibit υ(OH) and γ(H2O) bands in the 3445–3453 and 1132–1143 cm−1 regions, which are indicative of coordinated water .
2.1.3. Electronic Spectra and Magnetic Studies
The electronic spectrum in the polycrystalline state of ligand HL showed the maximum intraligand absorption: 34,820 and 30,900 cm−1, assigned to π→π* and n→π* transitions, corresponding to azomethine and thioamide groups of the ligand . These bands are shifted after complexation and new bands appeared in the 28,250–27,340 cm−1 region and were attributed to S→Cu(II) charge transfer bands.
The electronic spectra for complexes 1, 2, 4 and 5 show d–d bands in the regions 12,550–13,570 and at 20,000 cm−1 (Figure S1) which may be assigned to → and →
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