All chemicals were purchased from Merck and used without further purification. FT-IR spectra were recorded in the frequency range of 4000 - 400 cm⁻¹ with use of a Perkin−Elmer RXI spectrometer using KBr disks at room temperature. Elemental analysis was carried out using a Perkin-Elmer 2400(II) CHN analyzer. ¹H NMR and ¹³C NMR spectra were recorded with a Bruker Avance 300 spectrometer using DMSO-d⁶ as solvent. UV-Vis spectra were recorded with a Perkin-Elmer Lambda 25 spectrophotometer, using two matched 10-mm quartz cells.

Crystallographic data for (1) were collected with use of a Bruker Apex DUO diffractometer, Cu K_α radiation and a Cryosystems low temperature apparatus. The crystal was twinned, and the data reduction was carried out using two components. The twin law was -1 0 0 0 -1 0 -1 0 1.

Data reduction and cell refinement were carried with SAINT [36] . An absorption correction was applied with use of the program TWINABS [36] . The programs used to solve and refine the structure were SHELXS [37] and SHELXL-2014/7 [38] , respectively. The hydrogen atom H4 on the OH group was not located in a difference Fourier map. Its position was estimated based on geometric considerations and its similarity to the position of H1. During refinement, restraints were applied such that O-H was 0.84 Å. and the C-O-H-N grouping was flat. The remaining, C-H hydrogen atoms were included as riding on their parent carbon atoms. All hydrogen U_iso values were set to 20% more than the U_eq of their parent atoms.

Data for compound (2) were collected at the Advanced Light Source, Lawrence Berkeley National Lab, California using an Oxford Cryostream low temperature device. The radiation employed was 0.77490 Å and the diffraction apparatus was a Bruker D8 with a PHOTON 100 CMOS detector. This structure was also twinned, with a twin law -1 0 -0.292 0 -1 0 0 0 1. Software programs utilized were the same as for (1). Crystal data and refinement details are reported in Table 1.

The required starting material, 2-amino-4-phenylthiazole, was prepared from acetophenone, iodine, and thiourea through the procedure developed by Dodson [35] and Woodbridge [39] . In a general procedure, to a mixture consisting of 0.2 mole of acetophenone and 0.4 mole of thiourea, 0.2 mole of iodine was slowly added in small portions. After this stage, the reaction mixture was heated overnight on a steam-bath. It was then diluted with cold distilled water and made alkaline with sodium hydroxide. The precipitated 2-amino-4-phenylthiazole was separated and crystallized from an ethanol/H₂O mixture. Properties: Yield: 75%, M.p. = 144˚C - 146˚C, IR (KBr): ῡ(cm⁻¹): 3434(s), 3249(m), 3150(m), 3115(m), 1598(s), 1532(s), 1518(s), 1483(m), 1441(m), 1340(s), 1330(s), 1306(m), 1201(m), 1039(s), 1022(s), 912(s), 845(s), 772(s), 714(s), 667(m), 567(w), 472(m). ¹H NMR (300 MHz, DMSO-d₆) δ7.77 - 7.80 (d, 2H, J = 7.3 Hz, ArH), 7.32 - 7.37 (t, 2H, J = 7.3 Hz, ArH), 7.21 - 7.25 (t, 1H, J = 7.3 Hz, ArH), 7.08 (s, 2H, NH₂), 6.98 (s, 1H, ArH). Elemental Anal. calc. for C₉H₈N₂S: C, 61.34; H, 4.58; N, 15.90. Found: C, 61.02; H, 4.47; N, 15.60%.

2-Amino-4-phenylthiazole (3.6 g) and 1.52 g of thiourea were dissolved in 50 mL mixture of warm ethanol and distilled water. A portion of 7.6 g of iodine was then added to this solution with stirring. After the addition was completed, the resultant red-brown mixture was refluxed for three hours and then poured into cooled distilled water. The mixture was extracted with dichloromethane (3 × 50 mL). The filtrate was concentrated and the crude product purified by column chromatography (silica gel, CH₂Cl₂/n-hexane: 2/3 V/V). The fraction containing the desired bis-[2-amino-4-phenyl-5-thiazolyl] disulfide was then made alkaline with sodium hydroxide solution. A yellow precipitate was formed which was further recrystallized from water/acetic acid to give a yellow solid. Properties: Yield: 25%, M.p = 180˚C - 182˚C. FT-IR (KBr): ῡ(cm⁻¹): 3420(s), 3282(s), 3192(s), 2360(br), 1682(s), 1628(s), 1514(s), 1470(s), 1438(m), 1355(m), 1334(m), 1277(s), 1179(w), 890(m), 771(s), 696(s). ¹H NMR (300 MHz, DMSO-d₆) δ7.76 - 7.79 (d, 4H, J = 7.3 Hz, ArH), 7.69 - 7.74 (t, 4H, J = 7.3 Hz, ArH), 7.30 -

7.35 (t, 2H, J = 7.3 Hz, ArH), 7.60 (s, 4H, NH₂). ¹³C NMR, (d⁶-DMSO) δ, ppm: 109.9, 128.4, 128.8, 129.8, 134.7, 159.4 and 171.9. Elemental Anal. calc. for C₁₈H₁₄N₄S₄: C, 52.15; H, 3.40; N, 13.51. Found: C, 51.95; H, 3.27; N, 13.37%.

This substrate was prepared according to the method described in [40] . Properties: Yield: 63%, Yellow solid; M.p. = 120˚C - 122˚C. FT-IR (KBr): ῡ(cm⁻¹): 3069(m), 2886(w), 1665(s), 1627(m), 1579(m), 1509(m), 1473(s), 1343(s), 1287(s), 1181(w), 929(m), 914(m), 771(m), 718(s), 633(s), 462(w). ¹H NMR (300 MHz, DMSO-d₆,) δ10.2 (s, 1H, aldehyde-H), 8.34 (s, 1H, ArH), 8.25 - 8.28 (d, 1H, J = 8.1 Hz, ArH), 7.12 - 7.16 (d, 1H, J = 8.1 Hz, ArH), 8.01 (s, 1H, OH). Elemental Anal. calc. for C₇H₅NO₄: C, 50.31; H, 3.02; N, 8.38. Found: C, 50.15; H, 3.17; N, 8.28%.

To a solution of 5-nitro-salicylaldehyde (0.34 g, 2 mmol) in absolute ethanol (20 mL) was added bis-[2-amino- 4-phenyl-5-thiazolyl] disulfide (0.41 g, 1 mmol) in ethanol. After the addition, the solution was refluxed overnight. The obtained orange precipitation was filtered, washed with ethanol and dried in vacuum. The orange plate-like crystals of the compound suitable for X-ray analysis were obtained by slow evaporation of the solvent within several weeks. Properties: Yield: 83%, M.p = 225˚C - 227˚C. FT-IR (KBr): ῡ(cm⁻¹): 3429(br), 3064(w), 1929(m), 1601(s), 1561(s), 1520(w), 1476(s), 1460(s), 1435(s), 1345(s), 1296(m), 1166(s), 1096(s), 809(m), 767(s), 693(s), 654(m), 631(m). ¹H NMR (300 MHz, DMSO-d₆) δ8.76 (s, 2H, ArH), 8.65 - 8.68 (d, 2H, J = 8.1 Hz, ArH), 8.20 (s, 2H, imine-H), 8.06 (s, 2H, OH), 7.57 - 7.60 (d, 4H, J = 7.3 Hz, ArH), 7.32 - 7.36 (d, 2H, J = 8.1 Hz, ArH), 7.19 - 7.24 (t, 4H, J = 7.3 Hz, ArH), 7.00 - 7.05 (t, 2H, J = 7.3 Hz, ArH). ¹³C NMR, (d⁶-DMSO) δ, ppm: 107.3, 118.5, 122.2, 124.4, 127.7, 128.8, 130.8, 129.5, 133.7, 139.8, 157.9, 165.7 and 170.7. Elemental Anal. calc. for C₃₂H₂₀N₆O₆S₄: C, 53.92; H, 2.83; N, 11.79. Found: C, 53.69; H, 2.78; N, 11.69%.

A copious amount of CuCl₂・2H₂O salt (1 mmol) was dissolved in methanol. Then, an alkaline solution of the Schiff base ligand (1) (1 mmol) in methanol was slowly added. Subsequently, reflux of the solution was continued overnight. A black precipitate was obtained from the reaction mixture. At the end, the precipitate was filtered and washed several times with methanol and then dried in vacuum. Properties: This solid decompsed at 230˚C. FT-IR (KBr): υ(cm⁻¹): 3445(br), 3059(w), 2924(w), 1605(s), 1543(s), 1497(m), 1460(s), 1439(m), 1381(m), 1322(s), 1181(m), 1162(m), 1100(s), 837(m), 770(w), 694(s), 660(m), 515(w). Elemental Anal. calc. for CuC₃₂H₁₉N₆O₆S₄Cl: C, 47.40; H, 2.36; N, 10.37. Found: C, 47.21; H, 2.38; N, 10.65%.

This complex was prepared in a manner as described above with this difference that Co(NO₃)₂・6H₂O (1 mmol) salt was used instead. After refluxing overnight, the black crystals were filtered off, washed with methanol and dried in vacuum. Properties: This solid decomposed at 265˚C. FT-IR (KBr): υ(cm⁻¹): 3350(br), 3059(w), 1646(m), 1602(s), 1543(s), 1479(s), 1460(s), 1439(s), 1382(m), 1319(s), 1179(s), 1130(w), 1100(s), 951(m), 838(m), 771(m), 756(m), 694(s), 656(m), 505(m). Elemental Anal. calc. for CoC₃₂H₁₈N₆O₆S₄: C, 49.93; H, 2.36; N, 10.92. Found: C, 49.21; H, 2.41; N, 10.89%. To produce single crystals of this complex, the precipitate was dissolved in DMSO and the mixture kept in air to allow the solvent to evaporate. After several weeks the orange plate-like crystals were obtained by slow evaporation of the solvent. However, the X-ray analysis revealed that an unexpected Co(II) complex had been formed in which the imine bond of the Schiff base ligand had been hydrolyzed in DMSO and the product was found to be a Co(II) complex in which 5-nitro-salicylaldehyde and two DMSO molecules were coordinated to the central atom in a slightly distorted octahedral fashion.

The condensation reaction between bis-[2-amino-4-pheny1-5-thiazolyl] disulfide and 5-nitro-salicylaldehyde in a 1:2 molar ratio in methanol afforded the desired Schiff base (1), and its molecular structure was further investigated by single crystal X-ray diffraction. The molecular structure, together with the crystallographic numbering scheme of (1) are shown at Figure 1, with thermal ellipsoids drawn at the 50% probability level. As shown, the molecule possesses a disulfide (−S−S−) bond and adopts a cis configuration with respect to this bond. The C10−S2−S3 and C11−S3−S2 bond angles are 101.4 (3)˚ and 100.4 (3)˚ which indicate the tetrahedral environments around each sulfur atom with slightly deviations. This small deviation from ideal tetrahedral angle results from the repulsion between lone pairs on the sulfur atoms. The dihedral angle of C10−S2−S3−C11 is −58.5 (4)˚ which indicates that it is twisted around the disulfide bond. In comparison to the average S−S bond length reported for similar structures (2.02 ± 0.03 Å) [41] - [44] , the S−S bond in this present Schiff base is somewhat elongated at 2.112 (3) Å (see Table 2). For example, Schroth et al., have synthesized two compounds with S−S

Symmetry code (#1): −x + 1, −y + 1, −z + 1.

bond distances of 2.061 Å and 2.067 Å [43] . Also in a disulfide-linked Schiff base prepared by Donzelli et al., a value of 2.064 (2) Å is reported for a similar bond [45] . The C10−S2 and C11−S3 bond distances are 1.742 (8) Å and 1.757 (7) Å, and the lengths of two C7−N2 and C14−N5 imine bonds are 1.281 (11) Å and 1.296 (10) Å. Also, the four C22−C21−C9−N1, C26−C21−C9−C10, C28−C27−C12−N4 and C32−C27−C12−C11 dihedral angles are −145.5 (7)˚, −148.8 (8)˚, 31.7 (10)˚ and 34.2 (12)˚, respectively, indicate that pendant phenyl groups and thiazole rings are not located in the same planes. Furthermore, the C8−S1−C10 and C11−S4−C13 angles are smaller than that of 2-amino-4-phenylthiazole hydrobromide monohydrate [88.8 (4)˚ and 88.1 (4) vs. 90.17˚] [46] . The exocyclic, [C7−N1, C8−N1, C13−N4 and C14−N4], and heterocyclic, [C8−N2, C9−N2, C12−N3 and C13−N3] bond distances are listed in Table 2, in which are in excellent agreement with Fehlmann’s data [47] . It was expected that the C−N bond distances would be dissimilar as a consequence of different bond order between these atoms. The C9−C21 and C12−C27 bond distances in (1) are 1.480 (12) Å and 1.481 (12) Å. Compared with the value of 1.506 Å found in 2-amino-4-phenylthiazole hydrobromide monohydrate; there is a greater double-bond character of the corresponding bonds in (1). Also, the inter-ring distances of C9−C10 and C11−C12 are 1.375 (11) Å and 1.363 (12) Å. The angles of 88.8 (4)˚ and 88.1 (4)˚ at the heterocyclic sulfur atoms [C8−S1−C10 and C11−S4−C13] are common for substituted thiazole molecules. As shown in Table 3, the structure of (1) contains two distinct intramolecular hydrogen bond between the hydrogen atom of the phenolic group and the nitrogen atom of imine fragment, O1−H1・・・N2 and O4−H4・・・N5 with D・・・A distances of 2.636 (10) Å and 2.576 (9) Å.

As mentioned, we started our investigation to synthesize a complex of Co(II) with the potentially tetradentate N₂O₂ Schiff base (1). However, recrystallization in DMSO in the presence of trace water from the cobalt salt, caused complete hydrolysis of the Schiff base. Subsequent coordination of the resultant 5-nitro-salicylaldehyde with Co(II) ion occurs with the formation of the [Co(NO₂-sal)₂(DMSO)₂] complex (Scheme 1). It is well known that the characteristic bond of the Schiff bases, C=N, has reversible nature which allows by hydrolysis, obtaining the initial corresponding aldehyde and amine compounds [48] [49] . It seems that the presence of the metal cations accelerates the hydrolysis of the imine bond regenerating the starting materials. X-ray single crystal diffraction study reveals that the crystal structure of the reaction product consists of neutral octahedral units shown in Figure 2. The Co(II) complex lies on center of inversion and the octahedral environment of Co(II) ion is formed by the four oxygen atoms of 5-nitro-salicylaldehyde moiety and two molecules of DMSO in the apical positions.

The 5-nitro-salicylaldehyde ligands coordinate in a typical mode through the phenolate and carbonyl oxygen atoms, forming six-membered chelate rings with the bite angles of 88.40 (7)˚. The Co−O bond lengths formed by phenolate and carbonyl oxygen atoms [Co1−O3 = 2.0124 (17) Å and Co1−O4 = 2.1015 (18) Å] are shorter than those of Co−O bonds from DMSO molecules [Co1−O5 = 2.1234 (18) Å].

Numerous complexes are known in which 5-nitro-salicylaldehyde ligand has coordinated to the central atom. Papadopoulos et al., have reported syntheses and X-ray structures of a few Co(II) complexes [50] in which the metal cations were bonded to two deprotonated 5-nitro-salicycaldehyde ligands and one phenanthroline or neocuproine ligand to fulfill its octahedral coordination sphere. In our sample, each substituted salicylaldehyde

Symmetry codes: (#1) 1 − x, −y, 1 − z; (#2) x, 1/2 − y, 1/2 + z; (#3) 1 − x, 1 − y, −z.

ligand was coordinated through the deprotonated phenolic oxygen atom and through the carbonyl oxygen. The reported Co−O bond distances range from 2.0202 (2) Å to 2.140 (1) Å and are in good agreement with those found in our structure. In addition, Tangoulis et al., has recently published a work in which single chain magnets of Co(II) are formed by using 5-nitro-salicycaldehyde as ligand [51] .

The main characteristic of the FT-IR spectrum of Schiff base (1), is disappearance of the absorption bands of -NH₂ (3420 cm⁻¹) and carbonyl (1665 cm⁻¹) fragments from starting materials i.e., bis-[2-amino-4-pheny1-5- thiazolyl] disulfide and 5-nitro-salicycaldehyde and in return, appearance of the stretching of azomethine group, υ(C=N), at 1561cm⁻¹ which changes upon coordination of the ligand to the metal atoms. The stretching of OH phenolic group appears at 3429 cm⁻¹. In addition, the band near 1345 cm⁻¹ is assigned to υ(C−O) stretching of the phenolic group. Although the absorption of υ(C=C) band does not change from the free ligand to the Co(II) and Cu(II) complexes, the downfield shift of the υ(C=N) vibration (1543 cm⁻¹) suggests that the nitrogen atom of imine group participates in coordination to the Co(II) and Cu(II) atoms, respectively.

In the ¹H NMR spectra of 2-amino-4-phenylthiazole, the signal at 7.21 ppm is assigned to the amino groups. The protons of the pendant phenyl ring are also observed at 7.23 - 7.80 ppm and the hydrogen atom of thiazole ring appears at 6.98 ppm. In the ¹H NMR spectrum of the bis-[2-amino-4-pheny1-5-thiazolyl] disulfide, the signal at 7.69 ppm is assigned to the amino groups and protons of the aromatic ring are also observed at 7.23 - 7.80 ppm. In 5-nitro-salicylaldehyde, the signal at δ 11.96 ppm is assigned to the proton of the OH group and the signal for CHO fragments appears at 10.22 ppm. In (1), the broad signal at δ 12.52 ppm is assigned to the proton of the OH group. It was observed that this signal completely disappears when adding D₂O. The single proton of -CH=N has chemical shift at δ 9.18 ppm and signals around δ 7.00 - 8.80 ppm are attributed to the protons of aromatic rings. In the ¹³C NMR spectrum, the carbon atoms of -CH=N and C−O groups show resonances at 170.7 and 157.9 ppm, respectively. The remaining carbon atoms of the aromatic rings have signals that range from 107.3 to 139.8 ppm. As both complexes are paramagnetic and exhibit broad ¹H NMR signals, no further analysis can be performed.

The electronic absorption spectral bands of the Schiff base, Cu(II) and Co(II) complexes were recorded over the range 200 - 600 nm in DMSO. The electronic spectrum of the Schiff base ligand shows two absorption bands at 372 and 426 nm. The short-wave band may be attributed to electron transitions in the aromatic rings (intra ligand (IL) n−π*), and the longer wavelength band at 426 nm is assigned to the π−π* transition in the azomethine chromospheres in the Schiff base ligand [52] . The Cu(II) complex displays absorption bands at 364 nm and 420 nm, respectively. Also, the Co(II) complex displays absorption bands at 365 nm and 429 nm.

In this study, we also report further investigation of the Cu(II) and Co(II) complexes. It is important to note that the method used in this research has some advantages over the Jobs analysis. In fact, a limited number of samples (10 samples for instance) with different volume ratios should be prepared in Jobs analysis. However, in the UV-Vis titration the desired amount of metal (5 ´ 10⁻³ M) is added to the ligand solution (5.0 ´ 10⁻⁵ M) in one cell and then this addition continues up to 30 times. The absorbance spectrum of the mixture is recorded after each addition. Therefore, the numbers of obtained points are higher in this method and the accuracy would definitely be better. In a typical procedure, 2.0 mL of ligand solution in methanol is placed in the spectrophotometer cell and the absorbance of the solution is measured. Then an appropriate amount of CuCl₂ or Co(NO₃)₂ in methanol is added in a stepwise manner using a 2-ml Hamilton syringe. The absorbance spectrum of the solution is recorded after each addition. The Cu(II) and Co(II) solutions were continually added until the desired metal to ligand mole ratio was achieved. The electronic absorption spectra of the Schiff base ligand (1) in the presence of increasing concentration of CuCl₂ and Co(NO₃)₂ in MeOH at room temperature is shown in Figure 3 and Figure 4, respectively. The resulting absorbance against [L]/[M²⁺] (M²⁺: Cu(II), Co(II)) mole ratio plot is shown in the

inset of each figure. As mentioned, the injection point at ligand-to-metal molar ratio of about1 indicates the formation of ML compound with mole ratio of 1:1.