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 [1] . Thiourea derivatives are selective analytical reagents, especially for the determination of transition metal in complexes [2] . 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 [3] - [6] . Substituted thioureas have recently gained much interest in the synthesis of wide variety of biologically active compounds [7] [8] . The complexing capacity of thiourea derivatives has been reported in several papers [9] [10] . 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 [11] . Also, thiourea coordinates to metal ions as neutral ligands, monoanions and dianions [12] [13] . 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.

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 [16] , 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 [17] [18] . The results suggest that the first type of the hydrogen bonding (O-H…N) is more likely occurred.

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 ²E_g → ²T_2g transition [19] 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,

19,840 and 21,740 - 28,900 cm⁻¹ regions attributed to the ¹A_1g → ³T_2g, ¹A_1g → ¹T_1g and ¹A_1g → ¹T_2g transitions, respectively, in a low-spin Co³⁺ system. The observation of these three bands may also suggest that the complexes have trans-configuration [20] [21] .

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

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 [22] .

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. [22] . 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.

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

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.

Sawsan Mohamed Al-Ashqar, (2016) Comparative Studies between Chemical and Tribochemical Reactions of Some Metal Complexes Derived from N-(O-Hydoxyphenyl)-N'-phenylthiourea (L). Open Journal of Inorganic Chemistry,06,195-204. doi: 10.4236/ojic.2016.63015