Received 22 January 2016; accepted 19 July 2016; published 22 July 2016
Thiourea is the analogue compound to urea and has a considerably wide range of applications. The properties of urea and thiourea differ significantly because of the difference in electronegativity between sulfur and oxygen  . Thiourea derivatives are selective analytical reagents, especially for the determination of transition metal in complexes  . The biological activity of thiourea complexes has been successfully screened for various biological actions. On the other hand some thiourea derivatives have been used in commercial fungicides  -  . Substituted thioureas have recently gained much interest in the synthesis of wide variety of biologically active compounds   . The complexing capacity of thiourea derivatives has been reported in several papers   . The metal complexes of thiourea are neutral and their colors vary with the nature of the metal ions. These chelating agents have been remarkable ones for analytical chemistry  . Also, thiourea coordinates to metal ions as neutral ligands, monoanions and dianions   . Our goal in this paper is to prepare novel metal complexes via chemical and tribochemical methods. The isolated complexes were characterized by chemical and spectral measurements. Finally a comparative study has been described and discussed.
N-(O-hydoxyphenyl)-N'-phenylthiourea (L) was synthesized by refluxing equivalent amounts of 2-aminophenol (10.9 g) dissolved in EtOH and phenyl isothiocyanate (12 ml) on hot plate for 2 h. The white product (m.p.; 138˚C; yield: 93%) was obtained by cooling and characterized by chemical and spectral methods. The ligand (L) is insoluble in most organic solvents but easily soluble in DMF and DMSO.
2.1. Preparation of Metal Complexes
Preparation of Cu2+ and Co2+ Complexes by Chemical Method
Two complexes derived from Cu2+ and Co2+ ions with the general formulae, [Cu(L)2(EtOH2)]Cl2 and [Co(L)2- Cl2]Cl, were synthesized by the direct reaction of CuCl2・2H2O (1.7 g, 0.01 mol) and/or CoCl2・6H2O (2.8 g, 0.01 mol) dissolved in EtOH (50 ml) with N-(O-hydoxyphenyl)-N'-phenylthiourea (4.8 g, 0.01 mol) dissolved in absolute EtOH (25 ml). The reaction mixtures were refluxed on a water bath for ~3 h. The two complexes were filtered off, washed several times with absolute EtOH followed by dry diethyl ether and finally dried in a vacuum desiccator over anhydrous CaCl2. The results of elemental analyses and some physical properties are shown in Table 1.
2.2. Physical Measurements
The contents of copper and cobalt were carried using complexometric titration and xylenol orange as an indicator  . Elemental analyses contents (C, H and N) were determined at the Microanalytical Unit, Center of King Fahad Institute at Jeddah, Saudi Arabia. Molar conductivities measurements were carried out using Tacussel model CD 75. The chloride contents were determined as AgCl  . The IR spectra in the 200 - 4000 cm−1 range were recorded in KBr on a Mattson 5000 FTIR Spectrometer. The electronic spectra of the Cu+, Cu2+, Co2+ and Co3+ complexes were recorded in Nujol mull in the range (200 - 900 nm) using Unicam spectrometer model
Table 1. Physical and chemical analysis of L and its complexes.
UV2. 1H-NMR spectra of the free ligand and some of its metal complexes (Cu+ and Co3+) were recorded on Jeol-90Q Fourier transform (200 MHz) in CDCl3 and 1H-NMR Spectrometer (400 MHz) in d6-DMSO at King Saud University at Riyadh, Saudi Arabia. Magnetic moments were determined using a Sherwood balance at room temperature (25˚C) with Hg[Co(NSC)4] as a calibrate. The diamagnetic corrections for the ligand and the metal atoms were computed using Pascal’s constants  .
3. Results and Discussion
The results of the isolated solid complexes are listed in Table 1. All the isolated metal complexes are colored, stable against light and insoluble in most common organic solvents but easily soluble in DMF and DMSO. The molar conductivities for the chloride complexes, [Cu(L)EtOH)2]Cl2 and [Co(L)2Cl2]Cl, in DMSO at 25˚C are 66.3 and 49.0 Ω−1∙cm2∙mol−1 indicating 1:2 and 1:1 electrolyte, respectively. The electrolytic nature of these complexes  , except the two iodide complexes of the general formulae, [Cu(L)I・H2O]・1/2H2O and [Co(L)- I3・EtOH]・3EtOH are non-conducting in DMSO. Also, the results show that the metal complexes have comparatively low melting points (146˚C - 260˚C) suggesting a weak bonding between the metal ions and L. The structure of the ligand (L) is shown in Figure 1.
The IR spectrum of L in KBr (Figure 2) shows several bands at 3530, 3220, 3156 and 2948 cm−1 assigned to ν (OH; free), ν (NH; free), ν (NH; hydrogen-bonded) and ν (OH; hydrogen-bonded) vibrations, respectively. The bands observed at 1594, 1267 and 765 cm−1 are assigned to ν (C=C) vibration while the latter two bands are assigned to ν (CS) vibrations, respectively. The observation of broad weak bands in the 1940 - 1700 and 2750 - 2500 cm−1 region suggests the presence of intra-molecular hydrogen bonding of the types O-H…N and/or N-H…O   . The results suggest that the first type of the hydrogen bonding (O-H…N) is more likely occurred.
Figure 1. Structure of the ligand (L).
Figure 2. IR spectrum of L in KBr.
The 1H-NMR spectrum of L in d6-DMSO (Figure 3) displays four signals with equal intensities at 9.98, 9.82, 9.12 and 8.1, relative to TMS. The first two signals are assigned to the OH (hydrogen-bonded) and OH (free) protons, respectively. The results are taken as strong evidence for the existence of hydrogen bonding and confirm the results from IR spectrum. The latter two signals are attributed to the protons of NH attached to (NH-CS) and (NH-Ph), respectively. The multiplet signals in the (7.57 - 6.79) ppm region are assigned to the protons of the two phenyl rings attached to the thiourea moiety.
A comparison of the IR spectra of L with Cu2+ and Co2+ complexes allows us to determine the mode of bonding. The IR spectra of the two complexes obtained by chemical method with the general formulae, [Cu(L)- EtOH)2]Cl2 (Figure 4) and [Co(L)2Cl2]Cl (Figure 5), indicate that the L behaves in a bidentate manner and coordinates via the OH and NH groups forming five-member ring around the metal ions (Figure 6 and Figure 7). The negative shifts of these two bands to lower wave numbers show the involvement of both the OH and NH groups in bonding. In comparing the IR data of L with the complexes we observed that the CS group remains more or less at the same position excluding the participation of this group in bonding.
The electronic spectra of the complexes of the Cu2+ and Co3+ with the general formulae, [Cu(L)(EtOH)2]Cl2 (Figure 8) and [Co(L)2Cl2]Cl (Figure 9), were carried out in Nujol mull. The spectra of the first Cu2+ complex show a band at 14,368 cm−1 for the former Cu2+ complex which is attributed to 2Eg → 2T2g transition  in an distorted-octahedral geometry around the Cu2+ ion. The value of magnetic moment is 1.8 B.M. is taken as additional evidence for the existence of distorted-octahedral geometry around the Cu2+ ion. On the other hand, the diamagnetic nature of the Co2+ complex suggests the oxidation of Co2+ to Co3+ and the complex has a d6-configura- tion (low-spin). The electronic spectrum of [Co(L)2Cl2]Cl in Nujol mull shows three bands in the 15,700,
Figure 3. 1H-NMR spectrum of L in d6-DMSO.
Figure 4. IR spectrum of [Cu(L)EtOH)2]Cl2 in nujol mull.
Figure 5. IR spectrum of [Co(L)2Cl2]Cl in nujol mull.
19,840 and 21,740 - 28,900 cm−1 regions attributed to the 1A1g → 3T2g, 1A1g → 1T1g and 1A1g → 1T2g transitions, respectively, in a low-spin Co3+ system. The observation of these three bands may also suggest that the complexes have trans-configuration   .
The Cu+ and Co3+ complexes synthesized by tribochemical reaction are diamagnetic in nature. Also, the absent of any d-d transition bands in case of the Cu+ complex synthesized is taken as evidence for the reduction of Cu2+ to Cu+. Figure 10 illustrates the structure of the iodide complex [Cu(L)I・H2O]・1/2H2O. The mechanism of
Figure 6. Structure of [Cu(L)(EtOH)2]Cl2.
Figure 7. Structure of [Co(L)2Cl2]Cl.
Figure 8. The electronic spectrum of [Cu(L)(EtOH)2]Cl2 in nujol mull.
Figure 9. The electronic spectrum of [Co(L)2Cl2]Cl in nujol mull.
Figure 10. Structure of [Cu(L)I・H2O]・1/2H2O.
reduction of Cu2+ to Cu+ as well as the substitution of chloride by iodide ions during grinding and extraction by solvent is reported in our earlier work  .
The IR spectrum of the Co3+ complex with the general formula, [Co(L)I3(EtOH)]・3EtOH (Figure 11), shows that the ligand behaves in a bidentate manner via the OH of the phenolic group and NH groups as discussed above. Also, the mechanism of oxidation of Co2+ to Co3+ as well as the substitution of chloride by iodide ions during grinding and extraction by solvent is reported earlier by Mostafa et al.  . The reduction process is observed in case of the reduction of Cu2+ to Cu+ while the oxidation process in case of Co2+ to Co3+ complexes. Both types are diamagnetic in nature, which explains the nature of the isolated complexes.
The 1H-NMR spectra of the Cu+ and Co3+ complexes in d6-DMSO (Figure 12 and Figure 13) with the general formulae, [Cu(L)I・H2O]・1/2H2O and [Co(L)I3(EtOH)]・3EtOH, are more less the same except that the former shows the signals of the OH proton due to the water molecule at 6.5 ppm. The latter complex exhibits signals at 13.7, 4.1 and 3.2 ppm assigned to the protons of OH, CH2 and CH3 of EtOH molecule, respectively.
Figure 11. Structure of [Co(L)I3(EtOH)]・3EtOH.
Figure 12. 1H-NMR spectrum of [Co(L)I3(EtOH)]・3EtOH in d6-DMSO.
In continuation of our earlier work on tribochemical reactions and the role of metal ions as well as the ligand used in reduction of Cu2+and oxidation of Co2+, we extend our work to include N-(O-hydoxyphenyl)-N'-phe- nylthiourea (L) with Cu+, Cu2+, Co2+ and Co3+ by chemical and tribochemical reactions. The ligand coordinates
Figure 13. 1H-NMR spectrum of [Co(L)I3(EtOH)]・3EtOH in d6-DMSO.
in a bidentate manner towards the metal ions. The results indicate that the substitution of the chloride by iodide ions has occurred in Cu2+ and Co2+ complexes. Also, the reduction of Cu2+ to Cu+ and the oxidation of Co2+ to Co3+ have occurred and been confirmed by chemical, spectral and magnetic measurements.
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