# Synthesis, Characterization, and Magnetic and Thermal Studies on Some Metal(II) Thiophenyl Schiff Base Complexes

Authors: Aderoju Amoke Osowole
Journal: International Journal of Inorganic Chemistry
 (2011)

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## Abstract

4-(Thiophen-3-yl)-aniline undergoes condensation witho-vanillin to form an ONS donor Schiff base, 2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol, which forms complexes of the type [ML2]xH2O (where M = Mn, Co, Ni, Cu, Zn, Pd). These complexes are characterized by elemental analysis, 1H nmr, electronic, mass, and IR spectroscopies and conductance measurements. The electronic, IR and CHN data are supportive of a 4-coordinate tetrahedral geometry for Mn(II), Co(II), Ni(II), and Zn(II) complexes and square-planar geometry for Cu(II) and Pd(II) complexes, with the chromophores N2O2. The magnetic data reveals that the complexes are magnetically dilute and mononuclear with exception of the Cu(II) complex, which exhibits some anti-ferromagnetisms. The complexes are air-stable solids, and none is an electrolyte in nitro methane.


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## Body

## 1. Introduction

My group has in the last five years been actively involved in the synthesis, characterization, and thermal and biological properties of various Schiff bases and their M(II) chelates (M  =  VO, Mn, Co, Ni, Cu, Zn, Pd), with the objectives of deriving Schiff base chelates which can  be used as precursors in metal-organic chemical vapor depositions (MOCVD) and  those with goodin vitro antimicrobial activities as surface cleaning agents [1–5]. Thiophenyl Schiff bases are particularly interesting because of their wide range of activities such as anticancer activity as shown by benzyl-N-[1-(thiophenyl-3-yl)ethylidene] hydrazine carbodithioate [6], antibacterial and antifungal activities typified by thiophenyl-azetidinones, -cephalexins and -vinyl anilines [7–16] as well as optical exhibited by thiophene-2-aldazine [17]. Other activities include structural activity exemplified by the hexacarbonyldiiron complex of N-(2-thienylmethylidene)aniline, which is composed of two thienyl moieties derived from the original thienyl imine and coupled together by a C–C bond [18]. Furthermore, Palladium imine complexes of 2-thiophenecarboxaldehyde are used as catalysts in the Suzuki cross-coupling of aryl bromides with phenyl boric acid [19]. Extensive literature reviews show that no studies are reported on the Schiff base derived from o-vanillin and 4-(thiophene-3-yl)-aniline and its metal(II) chelates [6–19]. Thus, the objective of this work is to synthesize, characterize, and investigate the magnetic and thermal properties of the Schiff base, 2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol and its Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Pd(II) complexes. These metal complexes and its ligands are new, being reported here for the first time.


## 2. Experimental Details

### 2.1. Materials and Physical Measurements

Reagent gradeo-vanillin, 4-(thiophene-3-yl)-aniline, manganese(II) nitrate dehydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, copper(II) nitrate hexahydrate, zinc(II) nitrate hexahydrate, and palladium(II) chloride are purchased from BDH and Aldrich chemicals and are used as received. Solvents are dried and distilled before use according to standard procedures. Melting points (uncorrected) are determined using the Stuart scientific melting point SMP1 machine, and conductivities of 10−3 M solutions of the complexes are measured in nitromethane at 25°C using a MC-1, Mark V conductivity meter with a cell constant of 1.0. The solid reflectance spectra are recorded on a Perkin-Elmer λ20 spectrophotometer while infrared spectra are measured as KBr discs on a Perkin-Elmer FTIR paragon 1000 spectrometer in the range 4000–400 cm−1. The elemental analyses C, H, and N are recorded on GmbH VarioEl analyser, and manganese, cobalt, nickel, copper, zinc, and palladium are determined titrimetrically and by atomic absorption spectroscopy [20]. The 1H nmr spectra are recorded on a 300 MHz Oxford Varian NMR instrument in CDCl3 at 295 K. 1H chemical shifts are referenced to the residual signals of the protons of CDCl3 and are quoted in ppm.  Magnetic susceptibilities are measured on Johnson Matthey magnetic susceptibility balance, and diamagnetic corrections are calculated using Pascal’s constants [21]. Thermogravimetric analyses are done in static air, using a T6 Linseis thermal analyser with a heating rate of 10°C/min in the range 30–700°C. The MALDI-TOF mass and atomic absorption spectra are obtained using a  Bruker Daltonic Reflex  TOF spectrometer with graphite  as  matrix  and Perkin Elmer Analyst 200 coupled to Winlab 32 software assembly, respectively.


### 2.2. Preparation of the Schiff Base (2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol)

A  20 mL solution  of 8.71 mmol (1.33 g)o-vanillin in absolute ethanol is added dropwise to a stirring solution of  8.71 mmol (1.53 g) of 4-(thiophene-3-yl)-aniline  in 30 mL of absolute ethanol. The resulting orange-colored solution is refluxed for 4 h after addition of 4 drops of acetic acid.  The orange product formed on cooling to room temperature is filtered and recrystallized from ethanol. The yield of the title compound is 1.88 g (70%). 1H nmr (ppm) δ 11.13 (s, 1H, OH), 9.93 (s, 1H, HCN), 8.70 (1H, s, C2HS), 7.35–7.43 (m, 3H, C6H3); 7.03–7.24 (m, 4H, C6H4); 6.91–7.01 (m, 2H, C2H2S); 3.96 (s, 3H, OCH3).


### 2.3. Preparation of the Metal(II) Complexes

The various complexes are prepared by gradual addition of 0.35 mmol (0.06–0.11 g) M(NO3)2·6H2O (M  =  Mn, Co, Ni, Cu, Zn) neat to a stirring  0.7 mmol (0.22 g) of the ligand in 30 mL of  absolute ethanol. The resulting solutions are then buffered with 0.7 mmol (0.10 mL) of triethylamine and refluxed for 6 h during which the products formed.  The precipitated solids are filtered, washed with ethanol, and dried over anhydrous calcium chloride. The yields are 0.12 g (50%), 0.17 g (70%), 0.17 g (70%), 0.17 g (70%),  and 0.14 g (60%), respectively.The Pd(II) complex is prepared using a similar method. 0.36 mmol (0.064 g) of  Pd(II) chloride, in 10 mL of absolute ethanol, is added dropwise to a stirring  solution of 0.72 mmol (0.23 g) of  the ligand in 30 mL of  absolute ethanol. The resulting solution is then buffered with 0.72 mmol (0.11 mL) of triethylamine and refluxed for 6 h during which the product is formed.  The product is filtered, washed with ethanol, and dried over anhydrous calcium chloride. The yield is 0.18 g (70%).Proton nmr measurements are done only for the diamagnetic Zn(II) and Pd(II) complexes.  Zn(II) complex:1H nmr (ppm) 8.70 (s, 1H, HCN), 8.34 (1H, s, C2HS), 7.28–7.43 (m, 3H, C6H3); 7.01–7.10 (m, 4H, C6H4); 6.63–6.94 (m, 2H, C2H2S); 3.94 (s, 3H, OCH3).Pd(II) complex:1H nmr (ppm) 8.67 (s, 1H, HCN), 8.30 (1H, s, C2HS), 7.48–7.66 (m, 3H, C6H3); 7.00–7.47 (m, 4H, C6H4); 6.88–6.98 (m, 2H, C2H2S); 3.94 (s, 3H, OCH3).


## 2.1. Materials and Physical Measurements

Reagent gradeo-vanillin, 4-(thiophene-3-yl)-aniline, manganese(II) nitrate dehydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, copper(II) nitrate hexahydrate, zinc(II) nitrate hexahydrate, and palladium(II) chloride are purchased from BDH and Aldrich chemicals and are used as received. Solvents are dried and distilled before use according to standard procedures. Melting points (uncorrected) are determined using the Stuart scientific melting point SMP1 machine, and conductivities of 10−3 M solutions of the complexes are measured in nitromethane at 25°C using a MC-1, Mark V conductivity meter with a cell constant of 1.0. The solid reflectance spectra are recorded on a Perkin-Elmer λ20 spectrophotometer while infrared spectra are measured as KBr discs on a Perkin-Elmer FTIR paragon 1000 spectrometer in the range 4000–400 cm−1. The elemental analyses C, H, and N are recorded on GmbH VarioEl analyser, and manganese, cobalt, nickel, copper, zinc, and palladium are determined titrimetrically and by atomic absorption spectroscopy [20]. The 1H nmr spectra are recorded on a 300 MHz Oxford Varian NMR instrument in CDCl3 at 295 K. 1H chemical shifts are referenced to the residual signals of the protons of CDCl3 and are quoted in ppm.  Magnetic susceptibilities are measured on Johnson Matthey magnetic susceptibility balance, and diamagnetic corrections are calculated using Pascal’s constants [21]. Thermogravimetric analyses are done in static air, using a T6 Linseis thermal analyser with a heating rate of 10°C/min in the range 30–700°C. The MALDI-TOF mass and atomic absorption spectra are obtained using a  Bruker Daltonic Reflex  TOF spectrometer with graphite  as  matrix  and Perkin Elmer Analyst 200 coupled to Winlab 32 software assembly, respectively.


## 2.2. Preparation of the Schiff Base (2-methoxy-6-[(4-thiophene-3-yl-phenylimino)-methyl]-phenol)

A  20 mL solution  of 8.71 mmol (1.33 g)o-vanillin in absolute ethanol is added dropwise to a stirring solution of  8.71 mmol (1.53 g) of 4-(thiophene-3-yl)-aniline  in 30 mL of absolute ethanol. The resulting orange-colored solution is refluxed for 4 h after addition of 4 drops of acetic acid.  The orange product formed on cooling to room temperature is filtered and recrystallized from ethanol. The yield of the title compound is 1.88 g (70%). 1H nmr (ppm) δ 11.13 (s, 1H, OH), 9.93 (s, 1H, HCN), 8.70 (1H, s, C2HS), 7.35–7.43 (m, 3H, C6H3); 7.03–7.24 (m, 4H, C6H4); 6.91–7.01 (m, 2H, C2H2S); 3.96 (s, 3H, OCH3).


## 2.3. Preparation of the Metal(II) Complexes

The various complexes are prepared by gradual addition of 0.35 mmol (0.06–0.11 g) M(NO3)2·6H2O (M  =  Mn, Co, Ni, Cu, Zn) neat to a stirring  0.7 mmol (0.22 g) of the ligand in 30 mL of  absolute ethanol. The resulting solutions are then buffered with 0.7 mmol (0.10 mL) of triethylamine and refluxed for 6 h during which the products formed.  The precipitated solids are filtered, washed with ethanol, and dried over anhydrous calcium chloride. The yields are 0.12 g (50%), 0.17 g (70%), 0.17 g (70%), 0.17 g (70%),  and 0.14 g (60%), respectively.The Pd(II) complex is prepared using a similar method. 0.36 mmol (0.064 g) of  Pd(II) chloride, in 10 mL of absolute ethanol, is added dropwise to a stirring  solution of 0.72 mmol (0.23 g) of  the ligand in 30 mL of  absolute ethanol. The resulting solution is then buffered with 0.72 mmol (0.11 mL) of triethylamine and refluxed for 6 h during which the product is formed.  The product is filtered, washed with ethanol, and dried over anhydrous calcium chloride. The yield is 0.18 g (70%).Proton nmr measurements are done only for the diamagnetic Zn(II) and Pd(II) complexes.  Zn(II) complex:1H nmr (ppm) 8.70 (s, 1H, HCN), 8.34 (1H, s, C2HS), 7.28–7.43 (m, 3H, C6H3); 7.01–7.10 (m, 4H, C6H4); 6.63–6.94 (m, 2H, C2H2S); 3.94 (s, 3H, OCH3).Pd(II) complex:1H nmr (ppm) 8.67 (s, 1H, HCN), 8.30 (1H, s, C2HS), 7.48–7.66 (m, 3H, C6H3); 7.00–7.47 (m, 4H, C6H4); 6.88–6.98 (m, 2H, C2H2S); 3.94 (s, 3H, OCH3).


## 3. Results and Discussion

The  equations for the formation of the complexes are(1)M(NO3)2+2HL⟶[ML2]+2HNO3
(where  M  =  Zn(II), Cu(II), Mn(II), Co(II), Ni(II))(2)PdCl2+2HL⟶[PdL2]+2HCl.All complexes adopt [ML2] stoichiometry, except Mn(II), Co(II) and  Ni(II) complexes that form as [ML2]xH2O, where x=1 and 0.5, respectively. Proposed structures for the ligand and the Cu(II) complex are shown in Figure 1.  The formation of this ligand is confirmed by microanalysis and 1H nmr.  The colors, melting points, and room temperature magnetic moments (μeff) of the compounds are presented in Table 1. Attempts to isolate suitable crystals for single X-ray structural determination have not been successful so far.Table 1
Analytical data for the ligand and its complexes.
Compound (empirical formula)Formula massColorm/z (100%)μeff% YieldΛm*M.p (°C)Analysis (calculated)% C% H% N% MHL (C18H15NO2S)309.38Orange309—80—150–15169.82(69.88)4.53 (4.89)4.35 (4.53)—[MnL2]H2O (MnC36H30N2S2O5)689.73green6715.705010.0338–34062.54 (62.69)4.32 (4.38)4.02 (4.06)8.02 (7.97)[CoL2]H2O (CoC36H30N2S2O5)693.71Brick Red6754.337020.0290–29262.36 (62.33)3.64 (4.36)4.06 (4.04)8.48 (8.50)[NiL2]1/2H2O (NiC36H30N2S2O4.5)684.48Yellow6753.107015.0193–19563.07 (63.17)3.86 (4.27)4.17 (4.09)8.55 (8.58)[CuL2]2(Cu2C72H56N4S4O8)680.31Brown6801.56709.0205–20764.14 (63.56)4.06 (4.15)4.01 (4.12)9.35 (9.34)[ZnL2](ZnC36H28N2S2O4)681.76Yellow681D6017.0316–31863.13 (63.42)3.84 (4.14)3.81 (4.11)9.52 (9.53)[PdL2](PdC36H28N2S2O4)723.18green720D7012.0148–15060.37 (59.79)4.32 (3.90)3.86 (3.87)14.70 (14.72)*Ω−1 cm2 mol−1, D: diamagnetic.Proposed structures for the ligand (a) and its Cu(II) complex (b).
(a)(b)


### 3.1. Infrared Spectra

The relevant infrared bands of the compounds are presented in Table2. The broadband at 3360 cm−1 in the ligand, which is conspicuously absent in the spectra of the metal(II) Schiff base complexes, is assigned as vOH stretching frequency, and  it confirms involvement of the phenol O in chelation. It is broad due to intramolecular hydrogen bonding, usually very strong in Schiff bases [1].  The new broadband at 3500 cm−1 in the spectra of Co(II), Ni(II), and Mn(II) complexes is assigned to the vOH frequency of crystallization  H2O. The uncoordinated C=N stretching vibrations are observed as three bands between 1614–1521 cm−1 in the ligand [2–5] and 1639–1503 cm−1 in the metal complexes with exceptions of the Cu(II) and Pd(II) complex which have two bands. The bathochromic/hypsochromic shifts of these bands in the complexes are attributed to the involvement of N atom of C=N in coordination to the metal ions. Moreover, it has been documented that  square planar  Pd(II) and Cu(II) Schiff base complexes do exhibit geometric isomerism [22], with the trans isomer showing  two vC=N bands and the cis isomer a lone vC=N band. The spectra of the Cu(II) and Pd(II) complexes in this work show two vC=N bands and are consequently in the trans-isomeric form.  The  vPh/C–O  and δC–H  vibrations of the ligand are observed at 1461–1364 and  972 cm−1, respectively. These suffer bathochromic shifts to 1298–1177 and 896–720 cm−1 in the Schiff base complexes due to the coordination of the phenol oxgen atom and  pseudoaromatic nature of the chelates [5, 6].  The observation of new bands at 480–405 and 581–542 cm−1due to v(M–O) and v(M–N) [11, 14, 22] is further evidence of coordination.Table 2
Relevant infrared and electronic spectral data of the ligand and its complexes.
CompoundνOHν(C=N)  +  ν(C=C)νPh/C–OδC–Hν(M–N)ν(M–O)Electronic transitions (kK)HL3360b1614s 1593s 1521s1461s     1364s972s——33.50, 36.25, 40.20[MnL2]H2O3500b1612s 1589s 1551s1298s      1194s848s  720s580m 561s450m 420s15.00,  23.11, 30.26, 33.45, 40.20[CoL2]H2O3500b1609s 1557s 1503s1231m    1180m840s 777m575s 537m475m 415m10.82, 18.20, 32.0, 37.40, 41.0[NiL2]1/2H2O3500b1599s 1585s 1537s1233s      1192s853s 780m581s   564s460s  453s14.29, 20.21, 30.72, 36.25, 40.40[CuL2]—1587s 1538s1237m    1193m856s 795s573s   557s465s   410s14.09, 21.60, 30.92, 34.35, 41.20[ZnL2]—1609s 1582s 1504s1231m    1197m896s 731m579s 562m480m 405m28.20, 35.00, 42.00[PdL2]—1592s 1540s1240s       1177s843m 783m562m 542m450m 420m18.20, 26.32, 30.25, 39.20, 43.00b:  broad, m:  medium, s:  strong.,  1 kK:  1000 cm−1.


### 3.2. Electronic Spectra and Magnetic Moments

The electronic spectral data for the complexes are presented in Table2. The Mn(II) Schiff base complex shows two absorption bands at 15.00 and 23.11 kK, respectively, consistent with a four-coordinate, tetrahedral geometry and  are assigned to 6A1  →  4E1 (ν1) and 6A1  →  4A1 (ν2)   transitions [23].  A room temperature moment of 5.92 B.M is usually observed for the Mn(II) compounds, regardless of stereochemistry  because the ground term is an 6A1, and thus, orbital contribution is nil. This Mn(II) complex has a moment of 5.70 B.M. complementary of tetrahedral geometry [1].The cobalt(II) Schiff base complex gives two absorption bands at 10.82 and 18.20 kK,  respectively, typical of a four-coordinate tetrahedral geometry and is assigned to4A2  →  4T1(P) (v2) and  4A2  →  4T1(P) (v3). The transition 4A2  →  4T2 (v1) in the range 5–7 kK is not observed as usual since it lies in the infrared region  [10].  This geometry is corroborated by a moment of 4.33 B.M [11].Nickel(II) complexes are known to exhibit complicated equilibria between coordination  numbers six (octahedral) to four (square planar/tetrahedral) [24].  The Ni(II) Schiff base complex exhibits two absorption bands at 14.29 and 20.21 kK typical of a 4-coordinate tetrahedral geometry, assigned   to 3T1(F)  →  3T2, (ν2) and  3T1(F)  →  3A2, (ν3) transitions. Its moment of 3.10 B.M is complimentary of tetrahedral geometry, since moments of 3.1–3.5 B.M. are reported for distorted tetrahedral complexes [12].The copper(II) complex displays two bands at 14.09 and 21.60 kK,  assigned to2B1g  →  2A1g and  2B1g  →  2E1g transitions of 4-coordinate, square planar geometry [13]. A moment of 1.9–2.2 B.M. is usually observed for mononuclear copper(II) complexes, regardless of stereochemistry [14]. A magnetic moment of 1.56 B.M. is observed for this complex, indicative of the presence of some anti-ferromagnetic interactions, operating through Cu–Cu interactions [25]. However, this could not be probed further due to lack of facilities for variable temperature magnetic measurements and nonsuitable crystal for single X-ray diffraction measurement (Figure 1).The Zn(II) complex expectedly shows only charge transfer transition from M→L andπ-π* transitions, as no d-d  transition is expected  at 28.20, 35.00, and 42.0 kK, respectively. This complex is diamagnetic, confirming its tetrahedral geometry [12–14].The Pd(II) complex shows absorption bands at 18.20  and 26.32 kK, typical of square planar geometry and is assigned to1A1g → 1B1g and 1A1g → 1E2g transitions. This complex is expectedly diamagnetic [26].


### 3.3.1H nmr Spectra

The ligand shows the phenolic proton as a singlet atδ 11.13 ppm (s, 1H, OH), the imine (s, 1H, HCN), and 2-thiophenyl (s, H, C2HS) protons resonate as singlets at 9.93 and 8.70 ppm, respectively. The o-vanillin (m, 3H, C6H3) and phenyl (aniline) protons (m, 4H, C6H4) are both observed as multiplets at δ 7.35–7.43 and 7.03–7.24 ppm, respectively.  The  two protons of thiophenyl ring at 4 and 5 positions come up as a multiplet at 6.91–7.01 ppm (m, 2H, –C2H2S), and  the methoxy protons  are observed as a singlet at 3.96 ppm (s, 3H, OCH3).In the Zn(II) complex  spectrum, the phenolic proton disappears, an indication  of coordination of the phenol oxygen to the Zn(II) ion, and other protons are all upfield shifted in comparison  to the ligand.  The imine hydrogen resonates as a singlet at 8.70 ppm (s, 1H, HCN) while the 2-thiophenyl proton is seen at 8.34 ppm  as a singlet (s, H, C2HS).Theo-vanillin protons  resonate as a multiplet at δ 7.28–7.43 ppm (m, 3H, C6H3), and the phenyl(aniline) protons are seen as  multiplets at 7.01–7.10 ppm  (m, 4H, C6H4). The  two protons of thiophenyl ring at 4 and 5 positions resonate as a multiplet at 6.63–6.94 ppm (m, 2H, –C2H2S), and the methoxy protons  are seen as a singlet at 3.94 ppm (s, 3H, OCH3). This shift shows deshielding, a consequence of coordination of the imine nitrogen atom [27].The Pd(II) complex spectrum  reveals the absence of the phenolic proton. This is indicative of involvement of the phenol oxygen in coordination to the Pd(II) ion.  The other protons are all upfield shifted in comparison to the ligand with exceptions of  theo-vanillin and phenyl(aniline) protons which are downfield shifted. The imine hydrogen (s, 1H, HCN) and 2-thiophenyl (s, H, C2HS) protons resonate as singlets at 8.67 and 8.30 ppm,  respectively. The o-vanillin (m, 3H, C6H3), phenyl(aniline) (m, 4H, C6H4),  and two protons of thiophenyl ring (m, 2H, –C2H2S) at 4 and 5 positions are all seen as  multiplets  at δ 7.48–7.66, 7.00–7.47, and 6.88–6.98 ppm,  respectively. The methoxy protons resonate as a singlet at 3.94 ppm (s, 3H, OCH3). These shifts are indicative of coordination of the imine nitrogen atom to the Pd(II) ion [26].


### 3.4. Mass Spectroscopy and Thermal Studies

The mass spectra of ligand and the complexes showed peaks attributed to the molecular ions m/z at 309 [L]+; 671 [MnL2-2H]+; 675 [CoL2-2H]+; 675 [NiL2-2H]+; 680 [CuL2-2H]+; 681 [ZnL2-2H]+ and 720 [PdL2], respectively, and are presented in Table 1.The thermal degradation of the ligand and complexes is presented in Table3. The ligand, HL, decomposes  in three steps. First, the loss of the fragment C2H2 and  0.5 mol N2 at 30–220°C, with mass losses of (obs.  =  12.96%, calc.  =  12.93%). The next step involves the loss of the organic fraction, C11H8O2S, with mass losses of (obs.  =  66.32%, calc.  =  65.94%) at 220–420°C. The final step is  the loss of the fragment C5H5, with mass losses of (obs.  =  22.78%, calc.  =  21.01%) at 420–700°C.Table 3
Thermal data for the ligand and its complexes.
CompoundTemperature range (°C)TG weight loss (%)AssignmentsCalc.FoundHL (C18H15NSO2)30–22012.9312.96C2H2  +  0.5N2220–42065.9466.32C11H8O2S420–70022.7821.01C5H5[MnL2]H2O (MnC36H30N2S2O5)30–2009.579.40H2O  +  1.5O2210–45044.0744.45C20H18NS450–70022.0422.53C8H10OSMn (residue)[CoL2]H2O (CoC36H30N2S2O5)30–2004.904.77CH4  +  H2O200–40020.7621.45C6H8S2400–70057.0857.50C24H16O4N2Co (residue)[NiL2]1/2H2O (NiC36H30N2S2O4.5)30–24010.6710.560.5H2O  +  SO2240–42023.0822.73C10H10N2420–70055.8155.51C25H18SO2Ni (residue)[CuL2]  (CuC36H28N2S2O4)30–20017.9317.86C8H12N200–40033.8133.63C13H10O2S400–70038.8142.00C15H6NO2SCu (residue)[ZnL2]  (ZnC36H28N2S2O4)30–2605.005.17H2S260–44027.8727.80C11H10OS440–70052.2252.21C22H16N2O3Zn (residue)[PdL2]  (PdC36H28N2S2O4)30–2103.873.50C2H4210–40040.7941.33C18H15SO2400–70033.3233.42C13H9N2SOPdO (residue)The Mn(II) complex decomposes  in three phases. The first phase corresponds to the loss of 1.5 moles of O2 and H2O between 30–200°C with mass losses of (obs.  =  9.40%, calc.  =  9.57%). The second phase is from 210 to 450°C and is attributed to the loss of the organic moiety C20H18NS with mass losses of (obs.  =  44.37%, calc.  =  44.07%). The final phase shows the loss of the organic moiety, C8H10OS, at 450–700°C with mass losses of (obs.  =  22.53%, calc.  =  22.33%) leaving Mn as the final product, and the fragment C8N is lost as 8CO2 and 0.5N2.The decomposition of the Co(II) complex also occurred in three steps. The first step is due to the loss of a mole of water and CH4 at 30–200°C, with mass losses of (obs.  =  4.77%, calc.  =  4.90%). The successive decomposition occurs within  a temperature range of 200–400°C and is attributed to the loss of the organic moiety C6H8S2 with mass losses of (obs.  =  21.45%, calc.  =  20.76%). The last step involves the loss of the organic moiety, C24H16N2O4, at 400–700°C with mass losses of (obs.  =  57.50%, calc.  =  57.08%). The final product is Co, and the C5 fragment is lost as 5CO2.The TGA curve of the Ni(II) complex reveals a three-step decomposition. The first is the loss of 0.5 mole of water and SO2 at 30–240°C, with mass losses of (obs.  =  10.56%, calc.  =  10.67%). The second step ranges from 240 to 420°C and is assigned to the loss of the organic moiety, C10H10N2 with mass losses (obs.  =  22.73%, calc.  =  23.08%). The final step is within a temperature range of 420–700°C and is attributed to the loss of the organic moiety C25H18SO2 (obs.  =  55.51%, calc.  =  55.81%). The remaining fraction is Ni residue, and the fragment CH  is lost as  CO2  +  0.5H2.Cu(II) complex decomposes  in three steps. The first step is attributed to the loss of the fragment C8H12N, with mass losses of (obs.  =  17.86%, calc.  =  17.89%) at 30–200°C. The second step ranges from 200 to 400°C and is attributed to the loss of the fragment C13H10O2S, with mass losses of (obs.  =  33.63%, calc.  =  33.74%). The final step is from 400 to 700°C corresponding to the loss of the organic moiety C15H4O2NS, with mass losses of (obs.  =  42.0%, calc.  =  38.72%). The remaining residue is Cu.The Zn(II) complex decomposes in three steps. Step one is between 30–260°C, which indicates the loss of H2S, with mass losses of (obs.  =  5.17%, calc.  =  5.00%). The second step  involves the loss of the organic moiety C11H10OS, from 260 to 440°C, with mass losses of (obs.  =  27.80%, calc.  =  27.87%). The final step is attributed to the loss of the organic moiety, C22H16N2O3, at 440–700°C, with  mass  losses of (obs.  =  52.21%, calc.  =  52.22%), leaving behind the Zn residue, and  the C3 fragment is lost as 3CO2.The Pd(II) complex  also decomposes in three phases. The first phase is between 30–210°C and  is attributed  to loss of  C2H4, with mass losses of (obs.  =  3.50%, calc.  =  3.87%). The second phase involves  the loss of the organic moiety, C18H15O2S at 210–400°C, with mass losses of (obs.  =  41.33%, calc.  =  40.79%) while the final stage involves the loss of the organic moiety, C13H9N2SO, from 400 to 700°C, with mass losses of (obs.  =  33.42%, calc.  =  33.32%),  leaving behind  PdO residue, and the fragment C3 is lost as 3CO2.In all cases with the exceptions of the ligand and the Cu(II) complex, the decomposition pattern showed the loss of carbon fragments which got oxidized to CO2, and hydrogen or nitrogen which were lost as gases. Thus, the decomposition pattern corroborates the proposed formulation of the complex.


### 3.5. Conductance

The molar conductances of the complexes in nitromethane are below 20.0 ohm−1 cm2 mol−1 confirming their covalent nature.  A value in the range 75–90 ohm−1 cm2 mol−1 is expected for a 1 : 1 electrolyte [12].


## 3.1. Infrared Spectra

The relevant infrared bands of the compounds are presented in Table2. The broadband at 3360 cm−1 in the ligand, which is conspicuously absent in the spectra of the metal(II) Schiff base complexes, is assigned as vOH stretching frequency, and  it confirms involvement of the phenol O in chelation. It is broad due to intramolecular hydrogen bonding, usually very strong in Schiff bases [1].  The new broadband at 3500 cm−1 in the spectra of Co(II), Ni(II), and Mn(II) complexes is assigned to the vOH frequency of crystallization  H2O. The uncoordinated C=N stretching vibrations are observed as three bands between 1614–1521 cm−1 in the ligand [2–5] and 1639–1503 cm−1 in the metal complexes with exceptions of the Cu(II) and Pd(II) complex which have two bands. The bathochromic/hypsochromic shifts of these bands in the complexes are attributed to the involvement of N atom of C=N in coordination to the metal ions. Moreover, it has been documented that  square planar  Pd(II) and Cu(II) Schiff base complexes do exhibit geometric isomerism [22], with the trans isomer showing  two vC=N bands and the cis isomer a lone vC=N band. The spectra of the Cu(II) and Pd(II) complexes in this work show two vC=N bands and are consequently in the trans-isomeric form.  The  vPh/C–O  and δC–H  vibrations of the ligand are observed at 1461–1364 and  972 cm−1, respectively. These suffer bathochromic shifts to 1298–1177 and 896–720 cm−1 in the Schiff base complexes due to the coordination of the phenol oxgen atom and  pseudoaromatic nature of the chelates [5, 6].  The observation of new bands at 480–405 and 581–542 cm−1due to v(M–O) and v(M–N) [11, 14, 22] is further evidence of coordination.Table 2
Relevant infrared and electronic spectral data of the ligand and its complexes.
CompoundνOHν(C=N)  +  ν(C=C)νPh/C–OδC–Hν(M–N)ν(M–O)Electronic transitions (kK)HL3360b1614s 1593s 1521s1461s     1364s972s——33.50, 36.25, 40.20[MnL2]H2O3500b1612s 1589s 1551s1298s      1194s848s  720s580m 561s450m 420s15.00,  23.11, 30.26, 33.45, 40.20[CoL2]H2O3500b1609s 1557s 1503s1231m    1180m840s 777m575s 537m475m 415m10.82, 18.20, 32.0, 37.40, 41.0[NiL2]1/2H2O3500b1599s 1585s 1537s1233s      1192s853s 780m581s   564s460s  453s14.29, 20.21, 30.72, 36.25, 40.40[CuL2]—1587s 1538s1237m    1193m856s 795s573s   557s465s   410s14.09, 21.60, 30.92, 34.35, 41.20[ZnL2]—1609s 1582s 1504s1231m    1197m896s 731m579s 562m480m 405m28.20, 35.00, 42.00[PdL2]—1592s 1540s1240s       1177s843m 783m562m 542m450m 420m18.20, 26.32, 30.25, 39.20, 43.00b:  broad, m:  medium, s:  strong.,  1 kK:  1000 cm−1.


## 3.2. Electronic Spectra and Magnetic Moments

The electronic spectral data for the complexes are presented in Table2. The Mn(II) Schiff base complex shows two absorption bands at 15.00 and 23.11 kK, respectively, consistent with a four-coordinate, tetrahedral geometry and  are assigned to 6A1  →  4E1 (ν1) and 6A1  →  4A1 (ν2)   transitions [23].  A room temperature moment of 5.92 B.M is usually observed for the Mn(II) compounds, regardless of stereochemistry  because the ground term is an 6A1, and thus, orbital contribution is nil. This Mn(II) complex has a moment of 5.70 B.M. complementary of tetrahedral geometry [1].The cobalt(II) Schiff base complex gives two absorption bands at 10.82 and 18.20 kK,  respectively, typical of a four-coordinate tetrahedral geometry and is assigned to4A2  →  4T1(P) (v2) and  4A2  →  4T1(P) (v3). The transition 4A2  →  4T2 (v1) in the range 5–7 kK is not observed as usual since it lies in the infrared region  [10].  This geometry is corroborated by a moment of 4.33 B.M [11].Nickel(II) complexes are known to exhibit complicated equilibria between coordination  numbers six (octahedral) to four (square planar/tetrahedral) [24].  The Ni(II) Schiff base complex exhibits two absorption bands at 14.29 and 20.21 kK typical of a 4-coordinate tetrahedral geometry, assigned   to 3T1(F)  →  3T2, (ν2) and  3T1(F)  →  3A2, (ν3) transitions. Its moment of 3.10 B.M is complimentary of tetrahedral geometry, since moments of 3.1–3.5 B.M. are reported for distorted tetrahedral complexes [12].The copper(II) complex displays two bands at 14.09 and 21.60 kK,  assigned to2B1g  →  2A1g and  2B1g  →  2E1g transitions of 4-coordinate, square planar geometry [13]. A moment of 1.9–2.2 B.M. is usually observed for mononuclear copper(II) complexes, regardless of stereochemistry [14]. A magnetic moment of 1.56 B.M. is observed for this complex, indicative of the presence of some anti-ferromagnetic interactions, operating through Cu–Cu interactions [25]. However, this could not be probed further due to lack of facilities for variable temperature magnetic measurements and nonsuitable crystal for single X-ray diffraction measurement (Figure 1).The Zn(II) complex expectedly shows only charge transfer transition from M→L andπ-π* transitions, as no d-d  transition is expected  at 28.20, 35.00, and 42.0 kK, respectively. This complex is diamagnetic, confirming its tetrahedral geometry [12–14].The Pd(II) complex shows absorption bands at 18.20  and 26.32 kK, typical of square planar geometry and is assigned to1A1g → 1B1g and 1A1g → 1E2g transitions. This complex is expectedly diamagnetic [26].


## 3.3.1H nmr Spectra

The ligand shows the phenolic proton as a singlet atδ 11.13 ppm (s, 1H, OH), the imine (s, 1H, HCN), and 2-thiophenyl (s, H, C2HS) protons resonate as singlets at 9.93 and 8.70 ppm, respectively. The o-vanillin (m, 3H, C6H3) and phenyl (aniline) protons (m, 4H, C6H4) are both observed as multiplets at δ 7.35–7.43 and 7.03–7.24 ppm, respectively.  The  two protons of thiophenyl ring at 4 and 5 positions come up as a multiplet at 6.91–7.01 ppm (m, 2H, –C2H2S), and  the methoxy protons  are observed as a singlet at 3.96 ppm (s, 3H, OCH3).In the Zn(II) complex  spectrum, the phenolic proton disappears, an indication  of coordination of the phenol oxygen to the Zn(II) ion, and other protons are all upfield shifted in comparison  to the ligand.  The imine hydrogen resonates as a singlet at 8.70 ppm (s, 1H, HCN) while the 2-thiophenyl proton is seen at 8.34 ppm  as a singlet (s, H, C2HS).Theo-vanillin protons  resonate as a multiplet at δ 7.28–7.43 ppm (m, 3H, C6H3), and the phenyl(aniline) protons are seen as  multiplets at 7.01–7.10 ppm  (m, 4H, C6H4). The  two protons of thiophenyl ring at 4 and 5 positions resonate as a multiplet at 6.63–6.94 ppm (m, 2H, –C2H2S), and the methoxy protons  are seen as a singlet at 3.94 ppm (s, 3H, OCH3). This shift shows deshielding, a consequence of coordination of the imine nitrogen atom [27].The Pd(II) complex spectrum  reveals the absence of the phenolic proton. This is indicative of involvement of the phenol oxygen in coordination to the Pd(II) ion.  The other protons are all upfield shifted in comparison to the ligand with exceptions of  theo-vanillin and phenyl(aniline) protons which are downfield shifted. The imine hydrogen (s, 1H, HCN) and 2-thiophenyl (s, H, C2HS) protons resonate as singlets at 8.67 and 8.30 ppm,  respectively. The o-vanillin (m, 3H, C6H3), phenyl(aniline) (m, 4H, C6H4),  and two protons of thiophenyl ring (m, 2H, –C2H2S) at 4 and 5 positions are all seen as  multiplets  at δ 7.48–7.66, 7.00–7.47, and 6.88–6.98 ppm,  respectively. The methoxy protons resonate as a singlet at 3.94 ppm (s, 3H, OCH3). These shifts are indicative of coordination of the imine nitrogen atom to the Pd(II) ion [26].


## 3.4. Mass Spectroscopy and Thermal Studies

The mass spectra of ligand and the complexes showed peaks attributed to the molecular ions m/z at 309 [L]+; 671 [MnL2-2H]+; 675 [CoL2-2H]+; 675 [NiL2-2H]+; 680 [CuL2-2H]+; 681 [ZnL2-2H]+ and 720 [PdL2], respectively, and are presented in Table 1.The thermal degradation of the ligand and complexes is presented in Table3. The ligand, HL, decomposes  in three steps. First, the loss of the fragment C2H2 and  0.5 mol N2 at 30–220°C, with mass losses of (obs.  =  12.96%, calc.  =  12.93%). The next step involves the loss of the organic fraction, C11H8O2S, with mass losses of (obs.  =  66.32%, calc.  =  65.94%) at 220–420°C. The final step is  the loss of the fragment C5H5, with mass losses of (obs.  =  22.78%, calc.  =  21.01%) at 420–700°C.Table 3
Thermal data for the ligand and its complexes.
CompoundTemperature range (°C)TG weight loss (%)AssignmentsCalc.FoundHL (C18H15NSO2)30–22012.9312.96C2H2  +  0.5N2220–42065.9466.32C11H8O2S420–70022.7821.01C5H5[MnL2]H2O (MnC36H30N2S2O5)30–2009.579.40H2O  +  1.5O2210–45044.0744.45C20H18NS450–70022.0422.53C8H10OSMn (residue)[CoL2]H2O (CoC36H30N2S2O5)30–2004.904.77CH4  +  H2O200–40020.7621.45C6H8S2400–70057.0857.50C24H16O4N2Co (residue)[NiL2]1/2H2O (NiC36H30N2S2O4.5)30–24010.6710.560.5H2O  +  SO2240–42023.0822.73C10H10N2420–70055.8155.51C25H18SO2Ni (residue)[CuL2]  (CuC36H28N2S2O4)30–20017.9317.86C8H12N200–40033.8133.63C13H10O2S400–70038.8142.00C15H6NO2SCu (residue)[ZnL2]  (ZnC36H28N2S2O4)30–2605.005.17H2S260–44027.8727.80C11H10OS440–70052.2252.21C22H16N2O3Zn (residue)[PdL2]  (PdC36H28N2S2O4)30–2103.873.50C2H4210–40040.7941.33C18H15SO2400–70033.3233.42C13H9N2SOPdO (residue)The Mn(II) complex decomposes  in three phases. The first phase corresponds to the loss of 1.5 moles of O2 and H2O between 30–200°C with mass losses of (obs.  =  9.40%, calc.  =  9.57%). The second phase is from 210 to 450°C and is attributed to the loss of the organic moiety C20H18NS with mass losses of (obs.  =  44.37%, calc.  =  44.07%). The final phase shows the loss of the organic moiety, C8H10OS, at 450–700°C with mass losses of (obs.  =  22.53%, calc.  =  22.33%) leaving Mn as the final product, and the fragment C8N is lost as 8CO2 and 0.5N2.The decomposition of the Co(II) complex also occurred in three steps. The first step is due to the loss of a mole of water and CH4 at 30–200°C, with mass losses of (obs.  =  4.77%, calc.  =  4.90%). The successive decomposition occurs within  a temperature range of 200–400°C and is attributed to the loss of the organic moiety C6H8S2 with mass losses of (obs.  =  21.45%, calc.  =  20.76%). The last step involves the loss of the organic moiety, C24H16N2O4, at 400–700°C with mass losses of (obs.  =  57.50%, calc.  =  57.08%). The final product is Co, and the C5 fragment is lost as 5CO2.The TGA curve of the Ni(II) complex reveals a three-step decomposition. The first is the loss of 0.5 mole of water and SO2 at 30–240°C, with mass losses of (obs.  =  10.56%, calc.  =  10.67%). The second step ranges from 240 to 420°C and is assigned to the loss of the organic moiety, C10H10N2 with mass losses (obs.  =  22.73%, calc.  =  23.08%). The final step is within a temperature range of 420–700°C and is attributed to the loss of the organic moiety C25H18SO2 (obs.  =  55.51%, calc.  =  55.81%). The remaining fraction is Ni residue, and the fragment CH  is lost as  CO2  +  0.5H2.Cu(II) complex decomposes  in three steps. The first step is attributed to the loss of the fragment C8H12N, with mass losses of (obs.  =  17.86%, calc.  =  17.89%) at 30–200°C. The second step ranges from 200 to 400°C and is attributed to the loss of the fragment C13H10O2S, with mass losses of (obs.  =  33.63%, calc.  =  33.74%). The final step is from 400 to 700°C corresponding to the loss of the organic moiety C15H4O2NS, with mass losses of (obs.  =  42.0%, calc.  =  38.72%). The remaining residue is Cu.The Zn(II) complex decomposes in three steps. Step one is between 30–260°C, which indicates the loss of H2S, with mass losses of (obs.  =  5.17%, calc.  =  5.00%). The second step  involves the loss of the organic moiety C11H10OS, from 260 to 440°C, with mass losses of (obs.  =  27.80%, calc.  =  27.87%). The final step is attributed to the loss of the organic moiety, C22H16N2O3, at 440–700°C, with  mass  losses of (obs.  =  52.21%, calc.  =  52.22%), leaving behind the Zn residue, and  the C3 fragment is lost as 3CO2.The Pd(II) complex  also decomposes in three phases. The first phase is between 30–210°C and  is attributed  to loss of  C2H4, with mass losses of (obs.  =  3.50%, calc.  =  3.87%). The second phase involves  the loss of the organic moiety, C18H15O2S at 210–400°C, with mass losses of (obs.  =  41.33%, calc.  =  40.79%) while the final stage involves the loss of the organic moiety, C13H9N2SO, from 400 to 700°C, with mass losses of (obs.  =  33.42%, calc.  =  33.32%),  leaving behind  PdO residue, and the fragment C3 is lost as 3CO2.In all cases with the exceptions of the ligand and the Cu(II) complex, the decomposition pattern showed the loss of carbon fragments which got oxidized to CO2, and hydrogen or nitrogen which were lost as gases. Thus, the decomposition pattern corroborates the proposed formulation of the complex.


## 3.5. Conductance

The molar conductances of the complexes in nitromethane are below 20.0 ohm−1 cm2 mol−1 confirming their covalent nature.  A value in the range 75–90 ohm−1 cm2 mol−1 is expected for a 1 : 1 electrolyte [12].


## 4. Conclusion

The Schiff-base ligand coordinates to the Mn(II), Ni(II), Co(II), Cu(II), Pd(II), and Zn(II) ions in a tetradentate manner using the N2O2  chromophores. The assignment of a 4-coordinate square-planar  geometry to Cu(II) and Pd(II)  complexes and tetrahedral geometry to Mn(II), Ni(II), Co(II), and Zn(II) complexes is corroborated by elemental analysis, thermal, magnetic,  and electronic spectral measurements. The Cu(II) and Pd(II) complexes exhibit geometric isomerism and are in the  trans-isomeric form as confirmed by their infrared spectra. Furthermore, the Cu(II) complex exhibits some anti-ferromagnetic interactions, operating through a dimeric structure while the other complexes are mononuclear.



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*Source: 650186-2011-02-08.xml*