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Synthesis and spectroscopic studies of CoII, NiII, FeIII and ThIV complexes derived from 2,2′-dihydroxy-3,3′-di(carboxymethyl)-1,1′-binaphthyl
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Spectrochimica Acta Part A 71 (2008) 133–139
Synthesis and spectroscopic studies of CoII, NiII, FeIII and ThIV complexes derived from 2,2-dihydroxy-3, 3-di(carboxymethyl)-1,1-binaphthyl A.I. Hanafy a , A.K.T. Maki a , K. El-Mankhaly a , M.M. Mostafa b,∗ a
Chemistry Department, Faculty of Science, Al-Azher University, Cairo, Egypt b Chemistry Department, Faculty of Science, Mansoura University, Mansoura, P.O. Box 15, Egypt Received 21 June 2007; accepted 18 October 2007
Abstract The synthesis of 2,2 -dihydroxy-3,3 -di(carboxymethyl)-1,1 -binaphthyl (H2 L) and its novel metal complexes with CoII , NiII , FeIII and ThIV salts are reported. The ligand and its metal complexes have been characterized on the basis of analytical, conductance, spectral (IR, UV–vis, 1 H NMR, mass) and magnetic susceptibility measurements. The M¨ossbauer spectrum of the FeIII complex indicates a low-spin octahedral geometry around the FeIII ion. The IR and 1 H NMR spectral data show that the ligand behaves in a dibasic bidentate fashion coordinating to two metal atoms through the two deprotonated naphthyl OH groups and acts in a dibasic tetradentate manner using both carbonyl oxygen’s and the deprotonated naphthyl OH groups coordinating to two metal ions. Thermal studies (TGA, DTA) confirm the presence of solvents either inside or outside the coordination sphere and support the mechanism of the decomposition process. The value of [α]20 D for the ligand has been determined in DMSO. © 2007 Elsevier B.V. All rights reserved. Keywords: Spectroscopic studies; Chiral ligands; 2,2 -Dihydroxy-3,3 -di(carboxymethyl)-1,1 -Binaphthyl; M¨ossbauer of FeIII complex
1. Introduction 1,1 -Binaphthol and its derivatives have been used successfully as chiral ligands in asymmetric transformations and catalysets [1]. Several efforts have been done to obtain these ligands in enantiopure forms. These efforts focused on the optical resolutions of racemic compounds [2] and oxidative asymmetric homocoupling of a chiral 2-naphthols. Oxidative coupling of 2naphthol to binaphthol utilizing chiral VOIV complexes has been reported with reasonably high enantioselectivities [3]. Another area of intensive study is the oxidative coupling of 3-hydroxy2-naphthoate ester catalyzed by copper–amine complexes. The ester group is essential for oxidative coupling of 3-hydroxy-2naphthoate ester catalyzed by copper–amine complexes through a bidentated chelation to the copper catalyst CuI diamine com-
∗
Corresponding author. E-mail address: [email protected] (M.M. Mostafa).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.10.056
plex has also been used for asymmetric oxidative coupling of methyl 3-hydroxy-2-naphthoate to the corresponding binaphthol derivative [4]. Binaphthyl and its derivatives have been used for synthesis of a new class of optically pure dendrimers containing chiral binaphthyl and binaphthyl-based oligomer cores [5]. Although 2,2 -disubstituted derivatives of 1,1 -binaphthyl have been used in organic synthesis as chirality inducers [6–9], no work that includes 2,2 -dihydroxy-3,3 -di(carboxymethyl)1,1 -binaphthyl (H2 L) as ligand has been reported earlier. The ligand (H2 L; structure 1) has been synthesized as reported in the literature [7]. Also, in continuation of our earlier work [8,9] and in view of the importance of this ligand, we report herein the synthesis and characterization of CoII , NiII , FeIII and ThIV complexes by conventional chemical and physical methods. The possible modes of chelation are discussed on the basis of spectral (UV–vis, IR, 1 H NMR, mass, M¨ossbauer) and magnetic measurements. Moreover, thermal measurements (TGA, DTA) have been used to shed more light on the structures of the isolated solid complexes.
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several times with EtOH (50 ml) and finally dried in a desiccator over silica gel. [Ni2 (L)2 (H2 O)4 ]• 2H2 O
2. Experimental The purity of the ligand was checked by its melting point (278–280 ◦ C), elemental analyses [Calcd. value for C24 H18 O6 (402.408): C, 71.6; H, 4.5%. Found: C, 71.4; H, 4.5%] and molecular weight (Calcd. 402, MS m/z: 402 (M+ )). 2.1. Preparation of metal complexes [Co2 (L)2 (H2 O)2 (EtOH)2 ]• H2 O A solution of cobalt(II) acetate tetrahydrate (0.5 g, 0.02 M) in 1,1 -dichloroethane (25 ml) was added to an EtOH solution (20 ml) of H2 L (0.4 g, 0.01 M) with stirring and the yellow solution of the reaction mixture was heated under reflux on a water bath for 1 h. Solid anhydrous sodium acetate (0.2 g) was added to adjust the pH of the reaction mixture to ca. 6.0 and the solution was then heated for 1/2 h at 90 ◦ C. A yellow precipitate of the CoII complex was obtained on adding water (25 ml). The product (yield, 1.06 g) was filtered, washed thoroughly with hot EtOH (30 ml) and finally dried in a desiccator over P4 O10 . [Co(HL)2 (MeOH)]• H2 O A solution of H2 L (0.4 g, 0.01 M) in 1,1 -dichloroethane (10 ml) was added to an MeOH solution (15 ml) of cobalt acetate tetrahydrate (0.5 g, 0.02 M). The solution became red in color the mixture was heated under reflux on a water bath and then concentrated to half of its volume followed by adding 0.2 g of anhydrous sodium acetate to adjust the pH of the solution to pH ≈6.0. At the end of reflux the color of the solution becomes deep red. On adding water (25 ml) a deep-red oil was formed which changes to a yellow powder on adding absolute MeOH (20 ml). The product (yield, 0.91 g) was filtered, washed with absolute MeOH, dried in an oven at 80 ◦ C and finally stored in a desiccator over silica gel. [Co(HL)2 (MeOH)] This yellow complex was obtained from the previous complex [Co(HL)2 (MeOH)]·H2 O, by heating in an oven at 120 ◦ C for 2 h. [Fe(HL)(L)(H2 O)2 ]• EtOH Solid anhydrous ferric chloride (0.5 g, 0.02 M) was added to a solution (25 ml) of H2 L (0.62 g, 0.02 M) in pentanol (30 ml) and the reaction mixture became green. The solution was then heated under reflux for 1 h and then concentrated to half of its volume. Upon addition of EtOH (25 ml) a pale brown precipitate (yield, 1.32 g) was obtained. The product was filtered off, washed
A MeOH solution (25 ml) of nickel acetate tetrahydrate (0.5 g, 0.02 M) was heated on a water bath and H2 L (0.4 g, 0.01 M) in pentanol (20 ml) was added. The pH of the solution reaches 5.0 without adding sodium acetate. The reaction mixture was subsequently heated under reflux on a water bath for 3 h. The yellow product precipitated upon heating (yield, 1.02 g) was filtered, washed with absolute EtOH (50 ml) and finally dried in a desiccator over silica gel. [Th(L)(NO3 )2 (H2 O)2 ]• 3H2 O The ligand (0.35 g, 0.01 M) was dissolved in pentanol (20 ml) and then solid thoriumIV nitrate pentahydrate (0.5 g, 0.01 M) was added and the mixture was heated at reflux for 2 h. The reaction mixture was concentrated to half of its volume and then absolute MeOH (25 ml) was added. The yellow precipitate formed upon addition of MeOH (yield, 0.65 g) was filtered, washed with diethyl ether (30 ml) and finally dried in a desiccator over silica gel. 2.2. Physical measurements NiII , CoII , FeIII and ThIV ions of the complexes were determined by complexometric titration using xylenol orange as indicator [10,11]. Carbon and hydrogen contents were determined at the Microanalytical Unit, Cairo University, Egypt. IR spectra of the solid complexes were recorded in KBr while the IR spectra of H2 L were recorded in KBr, CHCl3 and Nujol mull on a Mattson 5000 FTIR spectrometer. The electronic spectra of the solid complexes were recorded in DMSO, except for the CoII complex [Co2 L2 (H2 O)2 (EtOH)2 ]·H2 O, which was measured in Nujol. The electronic spectra of H2 L were determined in EtOH, CCl4 and C6 H6 on a Unicam model UV2 spectrometer. Magnetic susceptibilities were determined using a Johnson Matthey balance at room temperature (25 ◦ C) with Hg[Co(NCS)4 ] as the standard. The diamagnetic correction for the ligand and the metal atoms were computed using Pascal’s constants [12]. Thermal analysis measurements (TGA, DTA) were recorded on a model TGA-50H Shimadzu thermogravimetric analyser using 20 mg samples. The flow rate of nitrogen gas and heating rate was 20 ml/min and 10 ◦ C/min, respectively. 1 H NMR spectra of the ligand were recorded on Joel-90Q Fourier Transform (200 MHz) and Jeol FX 90Q (90 MHz) spectrometers in d6 DMSO. The mass spectra of the ligand and its metal complexes were recorded on a Shimadzu GC-S-QP 1000 EX spectrometer using a direct inlet system at Cairo University. The molar conductivities of the FeIII and ThIV complexes in DMSO were carried out using a TDS model 72 conductivity bridge at AlAzhar University. The specific optical rotation, [α]20 D , for the ligand as well as the FeIII and ThIV complexes was measured with a model Bellingham, Stanley Limited polarimeter at AlAzhar University. Finally, the M¨ossbauer spectrum of the FeIII complex was recorded at a constant acceleration spectrometer with laser calibration at Al-Azhar University. The M¨ossbauer
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source was 57 Co diffused in a Pd matrix, with an initial activity of 20 mci. 3. Results and discussion The reactions of H2 L with some transition metal salts may be represented by the following equations: 2Co(OAc)2 · 4H2 O + 2H2 L 1,1 −Dichloroethane,EtOH
−→
Reflux,1 h,pH=6
[Co2 L2 (H2 O)2 (EtoH)2 ] · H2 O +4AcOH
Pentanol,EtOH
FeCl3 + 2H2 L
−→
Reflux,1 h
Th(NO3 )4 · 5H2 O + H2 L
−→
Reflux,1 h
◦ The value of the specific rotation ([α]20 D = +3.5 , c 0.1 mg/ml in DMSO) of the free ligand suggests that the naphthyl groups are slightly rotated around the 1,1 -position. All the metal complexes were obtained at pH below 7.0 (pH 2–6). The role of the pH during complex formation is essential i.e., the CoII and NiII complexes were obtained at pH values of 6 and 5, respectively, while the FeIII and ThIV complexes are easily formed in acidic solutions (pH 2.2–4). Also, the role of the anion attached to the metal ion has a great effect on the complex formation. The CoII and NiII complexes could not be isolated using the chlorides while the complexes were formed at once by using the corresponding acetates.
3.1. IR and 1 H NMR spectral measurements
[Fe(HL)(L)(H2 O)2 · EtOH + 3HCl Pentanol
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[Th(L)(NO3 )2 (H2 O)4 ] · ×H2 O + 2HNO3
This study indicates that the ligand (H2 L) forms very stable complexes of the general formulae [Co2 L2 (H2 O)2 (EtOH)2 ]·H2 O, [Co(HL)2 (MeOH)]·H2 O, [Co(HL)2 (MeOH)], [Ni2 L2 (H2 O)4 ]·2H2 O, [Fe(L)(HL)(H2 O)2 ]·EtOH and [Th(L) (NO3 )2 (H2 O)4 ]·H2 O. The high melting points (>300 ◦ C) of all complexes, except the FeIII complex (m.p. 275 ◦ C), as well as their insolubility in H2 O and common organic solvents suggest the polymeric nature of all complexes. Also, all of the solid complexes are insoluble in DMF and DMSO except those of ThIV and FeIII which are freely soluble in these solvents. All attempts to isolate crystalline compounds to study the molecular structure by X-ray technique failed. The molar conductivities of the ThIV and FeIII complexes in DMSO at 25 ◦ C are 28 and 0 −1 cm2 mol−1 ,respectively, indicating non-electrolytic character [13]. The high yields of the CoII and NiII complexes reflect the importance of this ligand as analytical reagent for the determination of these ions by gravimetric technique. All of the isolated solid complexes are pale yellow, powder-like salts except the FeIII complex which is pale brown (Table 1).
The IR spectrum of H2 L in KBr shows a strong band at 1680 cm−1 together with a broad medium-intensity band centered at 3180 cm−1 and assigned to (C O) and (OH) vibrations, respectively. Also, the observation of broad but weak bands in the 2100–1800 cm−1 region suggests that intramolecular hydrogen bonding is the operating in the free ligand as shown in structure 2 (Fig. 1).
It is quite clear that the existence of two groups containing lone pairs of electrons in the ortho position will cause the elec-
Table 1 Analytical and physical data of the metal complexes derived from H2 L Compound
[Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O C52 H50 Co2 O17 (1064.8) [Co(HL)2 MeOH]·H2 O C49 H40 CoO14 (911.8) [Co(HL)2 MeOH] C49 H38 Co2 O13 (893.8) [Fe(HL)(L)(H2 O)2 ]·EtOH C50 H43 FeO15 (939.7) [Ni2 (L)2 (H2 O)4 ]·2H2 O C48 H43 Ni2 O18 (1025.3) [Th(L)(NO3 )2 (H2 O)2 ]·3H2 O C24 H26 N2 O17 Th (846.5)
Color
M.p. (◦ C)
Yield %
Yellow
>300
99.0
Yellow
>300
98.5
Yellow
>300
98.0
Pale Brown Yellow
275
91.0
>300
99.0
Yellow
>300
88.0
H2 L = C24 H18 O6 , HL = C24 H17 O6 , L = C24 H16 O6 .
μeff
Found (Calcd.) % C
H
M
(B.M.)
58.7 (58.7) 64.4 (64.5) 65.6 (65.8) 64.0 (63.9) 55.9 (56.2) 33.9 (34.1)
4.1 (4.7) 4.6 (4.4) 4.1 (4.3) 4.4 (4.5) 4.1 (4.3) 2.5 (3.1)
11.6 (11.1) 6.6 (6.5) 6.6 (6.6) 5.3 (5.2) 11.8 (11.4) 27.6 (27.4)
2.85 4.60 4.55
1.87 2.77 Diamagnetic
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Fig. 1. Structure of the ligand (H2 L).
trostatic repulsion between the non-bonding electrons present on the oxygen atoms of the hydroxyl and carbonyl oxygen groups. This behaviour makes the carbonyl oxygen to go out of the plane and spatially be far from the proton of the hydroxyl group. Also, it prevents intramolecular hydrogen bonding between the OH and C O groups (Fig. 2b.). The formation of two intramolecular hydrogen bonds between the two naphthyl OH groups (Fig. 2a) requires a small rotation around the 1-1 -position which helps to form two intramolecular hydrogen bonds. This rotation is ◦ supported on the basis of the [α]20 D value (+3.5 ) in DMSO. It is interesting to point out that the observation of a very strong band at higher wavenumber (1680 cm−1 ) assigned to the (C O) vibration suggests that the carbonyl oxygen is not taking part in hydrogen bonding and, consequently, Fig. 2b is ruled out. Moreover, the IR spectrum of H2 L in CHCl3 shows a shift of the OH band to higher wavenumber (3240 cm−1 ) while the carbonyl band remains unchanged (1680 cm−1 ), indicating the partial dissolution of the intramolecular hydrogen bonding between the two OH groups. Finally, two bands are observed at 1322 and 1217 cm−1 in the spectrum of the free ligand (H2 L). The former band is a mixture of δ(OH) + ν(C O(H) in which δ(OH) is dominant while the latter band is a mixture of ν(C O(H) + δ(OH) in which ν(C O) is dominant [14,15]. Doubtless, the observation of only one band for both the carbonyl and OH groups confirms the symmetry of H2 L (Figs. 3–6). The 1 H NMR spectrum of H2 L in d6 -DMSO exhibits two signals at 4.05 and 10.4 ppm, assigned to the protons of the methyl ester and OH groups, respectively. The latter signal disappears upon deuteration. The multiplet signals in the 7.0–8.8 ppm region are assigned to the protons of the two binaphthyl rings and the peak areas are equivalent to 10 protons. In order to determine the mode of chelation, the IR spectra of the free ligand (H2 L) and its metal complexes were compared. The results show that H2 L behaves as a bidentate ligand through
Fig. 2. Types of intramolecular hydrogen bonding in H2 L.
Fig. 3. Suggested structure of [Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O.
Fig. 4. Suggested structure of [Fe(HL)(L)(H2 O)2 ]·EtOH.
Fig. 5. Suggested structure of [Co(HL)2 (MeOH)]·H2 O.
the two naphthyl OH groups with displacement of two protons as shown in structure 3. This structure is supported by the following evidence. The ν(OH) bands of the complexes, especially the CoII complex are absent whereas the position of the carbonyl oxygen band remains unchanged. A band at 530 cm−1 is assigned to the ν(Co O) [16] vibration. Also, the observation of a new band at 3440 cm−1 suggests the existence of solvent molecules around the CoII ion.
Fig. 6. Suggested structure of [Ni2 (L)2 (H2 O)4 ]·2H2 O.
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The high melting point (>300 ◦ C) and the insolubility of the complex in all organic solvents supports the polymeric nature of this complex. The IR spectrum of the FeIII complex in KBr shows two bands at 3488 and 3175 cm−1 assigned to ν(OH) (H2 O) and ν(OH) (naphthyl) vibrations, respectively. The small shift of the naphthyl OH group to lower wavenumber indicates the involvement of this group in bonding (4). Also, the position of the carbonyl oxygen remains unchanged (1681 cm−1 ) indicating that this group does not take part in coordination. The band observed at 550 cm−1 is assigned to the ν(Fe O) [12] vibration. CoII
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H2 L also behaves as binegative tetradentate ligand towards two nickel ions coordinating through the carbonyl oxygen and the deprotonated naphthyl OH groups which is supported by the following evidence. The OH band disappeared together and the carbonyl oxygen frequency is shifted to lower wavenumbers (10 cm−1 ) with a decrease of its intensity. The appearance of new bands at 3460 and 450 cm−1 assigned to ν(OH), (H2 O) and ν(Ni O) [12] vibrations, respectively. The existence of hydrogen bonding between the coordinated H2 O molecules and the naphthyl OH and/or the carbonyl oxygen groups is suggested by the observation of a broad weak band centered at 3192 cm−1 . The IR spectrum of the ThIV complex [Th(L)(NO3 )2 (H2 O)2 ]·3H2 O, in KBr is the same as that of the NiII complex except that the ratio of M:L for the ThIV complex is 1:1 while it is 2:2 for the NiII complex. The changes observed in the spectrum of this nitrato complex show new bands at 1350 (ν4 ), 830 (ν6 ), 1447 (ν1 ), 1760 (ν2 + ν3 ) and 1730 (ν2 + ν5 ) cm−1 which are consistent with the bidentate nature of the nitrate group [18]. The difference (30 cm−1 ) between the two bands at 1760 and 1730 cm−1 is taken as additional evidence for the bidentate nature of the nitrate group [19]. Finally, in the spectra of all complexes, we observed shift of the band at 1322 cm−1 , due to δ(OH) + ν(C O), to higher wavenumbers while the band at 1217 cm−1 , due to ν(C O) + δ(OH), is shifted to lower wavenumbers. 3.2. Magnetic and electronic spectral studies
The 1 H NMR spectrum of the FeIII complex in d6 -DMSO shows two signals at 4.2 and 11.0 ppm and assigned to the protons of Me and OH, respectively. The multiplet signals in the 7.6–9.2 ppm range are assigned to the protons of the naphthyl groups. The ratio of the protons of naphthyl:methyl:OH is 20:12:1 and indicates that three OH groups are deprotonated while only one OH group remains protonated. Also, the broad signal at 4.5 ppm is assigned to the protons of H2 O molecules. The OH signal disappears upon deuteration. The values of the isomeric shift (0.3212) and quadrupole splitting (0.7032) from the M¨ossbauer spectrum are consistent with a low-spin octahedral geometry for the Fe(III) ion [17]. The thermograms of the FeIII complex show five peaks at 180, 280, 350, 440 and 510 ◦ C. The first two endothermic peaks are assigned to the removal of EtOH and H2 O molecules while the other three exothermic peaks are mainly due to decarboxylation, dehydroxylation and complete decomposition with formation of Fe2 O3 .
The analytical and magnetic data of the solid complexes are presented in Table 1. The UV spectrum of H2 L in EtOH shows six bands at 33,445 (290 l cm−1 mol−1 ), 34,600 (390 l cm−1 mol−1 ), 35,840 (560 l cm−1 mol−1 ), 40,985 (9400 l cm−1 mol−1 ), 41,670 (9600 l cm−1 mol−1 ) and 44,640 cm−1 (13,200 l cm−1 mol−1 ) assignable to B, K1 , K2 , E2 , E1 and n → σ* transitions [5,20], respectively. Also, a weak band observed at 26,315 cm−1 (85 l cm−1 mol−1 ) is assigned to the n → π* transition for the two carbonyl groups. Again, the observation of only one carbonyl group at 26,315 cm−1 suggests that the two carbonyl groups are symmetric as confirmed previously by the IR spectra. On the other hand, the spectra of H2 L in non-polar solvents (CCl4 , C6 H6 ) show a red-shift of the n → π* transition while showing a blue-shift of the π → π* band. This means that the decreasing polarity of the solvent will be accompanied by shifting of the n → π* and n → σ* bands to longer wavelength while the π → π* band undergoes a shift to shorter wavelength. These results are quite consistent with results published in the literature [16]. The observed molecular weight (M+ ) of H2 L in the mass spectrum was found to be 402 in agreement with the molecular weight of the ligand. The electronic spectrum of the CoII complex [Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O, in Nujol shows a broad band centered at 22,040 cm−1 which is tentatively assigned to the 2 A1g → 2 B2g transition in a square-planar configuration [21–23]. The band observed at 26,740 cm−1 is assigned to a charge-transfer transition. Also, the value of the magnetic
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moment (2.85 B.M.) is additional evidence for a square-planar geometry (3) around the CoII ion [17,22]. The magnetic moments of the other two CoII complexes [Co(HL)2 (MeOH)]·H2 O and [Co(HL)2 (MeOH)], are 4.60 and 4.55 B.M., respectively, indicating a tetrahedral, octahedral or square-pyramidal configuration around the CoII ion. The electronic spectra of the two complexes in DMSO and/or Nujol are more or less the same and show several bands at 23,040, 17,270, 16,810 and 15,750 cm−1 . The spectra as well as the values of magnetic moments indicate a high-spin octahedral or square-pyramidal structure around the CoII ion. The comparatively high ε values (120–160 l cm−1 mol−1 ) for the two complexes rule out the existence of an octahedral structure and support a square-pyramidal structure around the CoII ions [17,23] as shown in structure 5.
B.M.) for each NiII ion is in good agreement with the octahedral geometry [19] around the NiII ion (6).
3.3. Thermal studies
On the other hand, μeff (1.87 B.M.) of the FeIII complex [Fe(HL)(L)(H2 O)2 ]·EtOH, suggests a low-spin octahedral structure around the FeIII ion [20]. The electronic spectrum in DMSO exhibits a weak band at 15,770 cm−1 together with a shoulder at 21,550 cm−1 assignable to the 2 T2 → 2 A2 transition [24]. The bands observed at 25,510, 28,900 and 31,950 cm−1 are assigned to π → t2g , charge-transfer and ligand transitions, respectively. All the results are consistent with low-spin octahedral FeIII (4). The formation of low-spin FeIII complexes with hard donors such as oxygen is explained on the basis that the resulting complex has a tetragonal structure and, consequently, the bond lengths between the four oxygen donors and FeIII in the xy direction are very short which causes a large splitting of the d-orbitals. Also, the strong covalent bonding between the oxygen donors and FeIII ion suggests the large Δ value (crystal field stabilization energy) which helps in the formation of the low-spin configuration. The electronic spectrum of the nickel complex [Ni2 (L)2 (H2 O)4 ]·2H2 O, shows two bands at 28,010 and 17,544 cm−1 assigned to 3 A2g → 3 T1g (P) and 3 A2g → 3 T1g (F) transitions, respectively. The third band assigned to 3A → 3T 2g 2g was calculated [25] and found to be at 11,200 cm−1 . The bands observed at 22,700 and 15,300 cm−1 are assigned to 3 A2g → 1 T1g and 3 A2g → 1 Eg transitions, respectively. Also, the values of B and β were calculated [26] and found to be 797 and 0.77 cm−1 , respectively. Moreover, the ν2 /ν1 ratio (1.57) gives further support for the octahedral geometry [27]. Finally, the value of the magnetic moment (2.77
Thermal measurements of H2 L were conducted by means of TG, TGA and DTA. The thermograms show three peaks at 280, 426 and 515 ◦ C attributed to melting point, decarboxylation and dehydroxylation, respectively. Thermal measurements of [Co(L)2 (H2 O)2 (EtOH)2 ]·H2 O show four peaks at 200 (13.88%), 350 (32.7%), 440 (30.1%) and 520 ◦ C (16.5%) assignable to the removal of solvents (EtOH, H2 O), decarboxylation, dehydroxylation processes and complete decomposition with formation of Co2 O3 . The difference between the other two CoII complexes [Co(HL)2 (MeOH)]·H2 O and [Co(HL)2 (MeOH)], is the existence of only one water molecule outside the coordination sphere in the former complex. On heating this complex, the water molecule disappeared at 120 ◦ C and the complex converts to the second CoII complex [Co(HL)2 MeOH]. This assumption is based on the results of thermogravimetric analysis. The thermograms for the two CoII complexes show four peaks but the former complex [Co(HL)2 MeOH]·H2 O, exhibits an additional peak at 120 ◦ C corresponding to the removal of water of hydration. 3.4. Mass spectroscopic studies The observed M+ value (1065) of the CoII complex [Co2 (L+ )2 (H2 O)2 (EtOH)2 ]·H2 O, coincides with the calculated value (1064.8). References [1] (a) C. Bolm, J. Hildebrand, K. Muniz, N. Hermanns, Angew. Chem. Int. Ed. 40 (2001) 3284; (b) P. Kocovsky, S. Vyskocil, M. Smrcina, Chem. Rev. 103 (2003) 3213. [2] (a) M. Periasamy, L. Venkatraman, J. Thomas, J. Org. Chem. 62 (1997) 4302; (b) F. Toda, K. Tanaka, Chem. Commun. (1997) 1087. [3] (a) C. Chu, B. Uang, Tetrahedron: Asymmetry 14 (2003) 53; (b) J. Gao, J. Reibenspies, A. Martell, Angew. Chem. Int. Ed. 42 (2003) 6008. [4] K. Kim, D. Lee, Y. Lee, D. Ko, D. Ha, Tetrahedron 60 (2004) 9037.
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[17] S.J. Lippard, J.M. Berg, Principles of Bioinorganic Chemistry, Mill Valley, Califorinia, 1994. [18] M. Choca, J.R. Ferraro, K. Nakamoto, J. Chem. Soc. Dalton Trans. 20 (1972) 2297. [19] A.B.P. Lever, E. Mantovani, B.S. Ramasulamy, Can. J. Chem. 49 (1971) 1957. [20] C.N.R. Rao, Ultraviolet and Visible Spectroscopy. Chemical Applications, Plenum Press, New York, 1975. [21] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1968. [22] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fourth ed., Interscience, 1980. [23] R. Morassi, I. Bertini, L. Coord, Chem. Revs. 11 (1973) 343. [24] B.N. Figgis, J. Lewis, F.E. Mabbs, G.A. Webb, J. Chem. Soc. 4 (1966) 422. [25] A.B.P. Lever, J. Chem. Educ. 45 (1968) 711. [26] A.E. Undehill, D.E. Billing, Nature 210 (1966) 834. [27] L. Sacconi, Trans. Metal Chem. 4 (1968) 199.
Spectrochimica Acta Part A 71 (2008) 133–139
Synthesis and spectroscopic studies of CoII, NiII, FeIII and ThIV complexes derived from 2,2-dihydroxy-3, 3-di(carboxymethyl)-1,1-binaphthyl A.I. Hanafy a , A.K.T. Maki a , K. El-Mankhaly a , M.M. Mostafa b,∗ a
Chemistry Department, Faculty of Science, Al-Azher University, Cairo, Egypt b Chemistry Department, Faculty of Science, Mansoura University, Mansoura, P.O. Box 15, Egypt Received 21 June 2007; accepted 18 October 2007
Abstract The synthesis of 2,2 -dihydroxy-3,3 -di(carboxymethyl)-1,1 -binaphthyl (H2 L) and its novel metal complexes with CoII , NiII , FeIII and ThIV salts are reported. The ligand and its metal complexes have been characterized on the basis of analytical, conductance, spectral (IR, UV–vis, 1 H NMR, mass) and magnetic susceptibility measurements. The M¨ossbauer spectrum of the FeIII complex indicates a low-spin octahedral geometry around the FeIII ion. The IR and 1 H NMR spectral data show that the ligand behaves in a dibasic bidentate fashion coordinating to two metal atoms through the two deprotonated naphthyl OH groups and acts in a dibasic tetradentate manner using both carbonyl oxygen’s and the deprotonated naphthyl OH groups coordinating to two metal ions. Thermal studies (TGA, DTA) confirm the presence of solvents either inside or outside the coordination sphere and support the mechanism of the decomposition process. The value of [α]20 D for the ligand has been determined in DMSO. © 2007 Elsevier B.V. All rights reserved. Keywords: Spectroscopic studies; Chiral ligands; 2,2 -Dihydroxy-3,3 -di(carboxymethyl)-1,1 -Binaphthyl; M¨ossbauer of FeIII complex
1. Introduction 1,1 -Binaphthol and its derivatives have been used successfully as chiral ligands in asymmetric transformations and catalysets [1]. Several efforts have been done to obtain these ligands in enantiopure forms. These efforts focused on the optical resolutions of racemic compounds [2] and oxidative asymmetric homocoupling of a chiral 2-naphthols. Oxidative coupling of 2naphthol to binaphthol utilizing chiral VOIV complexes has been reported with reasonably high enantioselectivities [3]. Another area of intensive study is the oxidative coupling of 3-hydroxy2-naphthoate ester catalyzed by copper–amine complexes. The ester group is essential for oxidative coupling of 3-hydroxy-2naphthoate ester catalyzed by copper–amine complexes through a bidentated chelation to the copper catalyst CuI diamine com-
∗
Corresponding author. E-mail address: [email protected] (M.M. Mostafa).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.10.056
plex has also been used for asymmetric oxidative coupling of methyl 3-hydroxy-2-naphthoate to the corresponding binaphthol derivative [4]. Binaphthyl and its derivatives have been used for synthesis of a new class of optically pure dendrimers containing chiral binaphthyl and binaphthyl-based oligomer cores [5]. Although 2,2 -disubstituted derivatives of 1,1 -binaphthyl have been used in organic synthesis as chirality inducers [6–9], no work that includes 2,2 -dihydroxy-3,3 -di(carboxymethyl)1,1 -binaphthyl (H2 L) as ligand has been reported earlier. The ligand (H2 L; structure 1) has been synthesized as reported in the literature [7]. Also, in continuation of our earlier work [8,9] and in view of the importance of this ligand, we report herein the synthesis and characterization of CoII , NiII , FeIII and ThIV complexes by conventional chemical and physical methods. The possible modes of chelation are discussed on the basis of spectral (UV–vis, IR, 1 H NMR, mass, M¨ossbauer) and magnetic measurements. Moreover, thermal measurements (TGA, DTA) have been used to shed more light on the structures of the isolated solid complexes.
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several times with EtOH (50 ml) and finally dried in a desiccator over silica gel. [Ni2 (L)2 (H2 O)4 ]• 2H2 O
2. Experimental The purity of the ligand was checked by its melting point (278–280 ◦ C), elemental analyses [Calcd. value for C24 H18 O6 (402.408): C, 71.6; H, 4.5%. Found: C, 71.4; H, 4.5%] and molecular weight (Calcd. 402, MS m/z: 402 (M+ )). 2.1. Preparation of metal complexes [Co2 (L)2 (H2 O)2 (EtOH)2 ]• H2 O A solution of cobalt(II) acetate tetrahydrate (0.5 g, 0.02 M) in 1,1 -dichloroethane (25 ml) was added to an EtOH solution (20 ml) of H2 L (0.4 g, 0.01 M) with stirring and the yellow solution of the reaction mixture was heated under reflux on a water bath for 1 h. Solid anhydrous sodium acetate (0.2 g) was added to adjust the pH of the reaction mixture to ca. 6.0 and the solution was then heated for 1/2 h at 90 ◦ C. A yellow precipitate of the CoII complex was obtained on adding water (25 ml). The product (yield, 1.06 g) was filtered, washed thoroughly with hot EtOH (30 ml) and finally dried in a desiccator over P4 O10 . [Co(HL)2 (MeOH)]• H2 O A solution of H2 L (0.4 g, 0.01 M) in 1,1 -dichloroethane (10 ml) was added to an MeOH solution (15 ml) of cobalt acetate tetrahydrate (0.5 g, 0.02 M). The solution became red in color the mixture was heated under reflux on a water bath and then concentrated to half of its volume followed by adding 0.2 g of anhydrous sodium acetate to adjust the pH of the solution to pH ≈6.0. At the end of reflux the color of the solution becomes deep red. On adding water (25 ml) a deep-red oil was formed which changes to a yellow powder on adding absolute MeOH (20 ml). The product (yield, 0.91 g) was filtered, washed with absolute MeOH, dried in an oven at 80 ◦ C and finally stored in a desiccator over silica gel. [Co(HL)2 (MeOH)] This yellow complex was obtained from the previous complex [Co(HL)2 (MeOH)]·H2 O, by heating in an oven at 120 ◦ C for 2 h. [Fe(HL)(L)(H2 O)2 ]• EtOH Solid anhydrous ferric chloride (0.5 g, 0.02 M) was added to a solution (25 ml) of H2 L (0.62 g, 0.02 M) in pentanol (30 ml) and the reaction mixture became green. The solution was then heated under reflux for 1 h and then concentrated to half of its volume. Upon addition of EtOH (25 ml) a pale brown precipitate (yield, 1.32 g) was obtained. The product was filtered off, washed
A MeOH solution (25 ml) of nickel acetate tetrahydrate (0.5 g, 0.02 M) was heated on a water bath and H2 L (0.4 g, 0.01 M) in pentanol (20 ml) was added. The pH of the solution reaches 5.0 without adding sodium acetate. The reaction mixture was subsequently heated under reflux on a water bath for 3 h. The yellow product precipitated upon heating (yield, 1.02 g) was filtered, washed with absolute EtOH (50 ml) and finally dried in a desiccator over silica gel. [Th(L)(NO3 )2 (H2 O)2 ]• 3H2 O The ligand (0.35 g, 0.01 M) was dissolved in pentanol (20 ml) and then solid thoriumIV nitrate pentahydrate (0.5 g, 0.01 M) was added and the mixture was heated at reflux for 2 h. The reaction mixture was concentrated to half of its volume and then absolute MeOH (25 ml) was added. The yellow precipitate formed upon addition of MeOH (yield, 0.65 g) was filtered, washed with diethyl ether (30 ml) and finally dried in a desiccator over silica gel. 2.2. Physical measurements NiII , CoII , FeIII and ThIV ions of the complexes were determined by complexometric titration using xylenol orange as indicator [10,11]. Carbon and hydrogen contents were determined at the Microanalytical Unit, Cairo University, Egypt. IR spectra of the solid complexes were recorded in KBr while the IR spectra of H2 L were recorded in KBr, CHCl3 and Nujol mull on a Mattson 5000 FTIR spectrometer. The electronic spectra of the solid complexes were recorded in DMSO, except for the CoII complex [Co2 L2 (H2 O)2 (EtOH)2 ]·H2 O, which was measured in Nujol. The electronic spectra of H2 L were determined in EtOH, CCl4 and C6 H6 on a Unicam model UV2 spectrometer. Magnetic susceptibilities were determined using a Johnson Matthey balance at room temperature (25 ◦ C) with Hg[Co(NCS)4 ] as the standard. The diamagnetic correction for the ligand and the metal atoms were computed using Pascal’s constants [12]. Thermal analysis measurements (TGA, DTA) were recorded on a model TGA-50H Shimadzu thermogravimetric analyser using 20 mg samples. The flow rate of nitrogen gas and heating rate was 20 ml/min and 10 ◦ C/min, respectively. 1 H NMR spectra of the ligand were recorded on Joel-90Q Fourier Transform (200 MHz) and Jeol FX 90Q (90 MHz) spectrometers in d6 DMSO. The mass spectra of the ligand and its metal complexes were recorded on a Shimadzu GC-S-QP 1000 EX spectrometer using a direct inlet system at Cairo University. The molar conductivities of the FeIII and ThIV complexes in DMSO were carried out using a TDS model 72 conductivity bridge at AlAzhar University. The specific optical rotation, [α]20 D , for the ligand as well as the FeIII and ThIV complexes was measured with a model Bellingham, Stanley Limited polarimeter at AlAzhar University. Finally, the M¨ossbauer spectrum of the FeIII complex was recorded at a constant acceleration spectrometer with laser calibration at Al-Azhar University. The M¨ossbauer
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source was 57 Co diffused in a Pd matrix, with an initial activity of 20 mci. 3. Results and discussion The reactions of H2 L with some transition metal salts may be represented by the following equations: 2Co(OAc)2 · 4H2 O + 2H2 L 1,1 −Dichloroethane,EtOH
−→
Reflux,1 h,pH=6
[Co2 L2 (H2 O)2 (EtoH)2 ] · H2 O +4AcOH
Pentanol,EtOH
FeCl3 + 2H2 L
−→
Reflux,1 h
Th(NO3 )4 · 5H2 O + H2 L
−→
Reflux,1 h
◦ The value of the specific rotation ([α]20 D = +3.5 , c 0.1 mg/ml in DMSO) of the free ligand suggests that the naphthyl groups are slightly rotated around the 1,1 -position. All the metal complexes were obtained at pH below 7.0 (pH 2–6). The role of the pH during complex formation is essential i.e., the CoII and NiII complexes were obtained at pH values of 6 and 5, respectively, while the FeIII and ThIV complexes are easily formed in acidic solutions (pH 2.2–4). Also, the role of the anion attached to the metal ion has a great effect on the complex formation. The CoII and NiII complexes could not be isolated using the chlorides while the complexes were formed at once by using the corresponding acetates.
3.1. IR and 1 H NMR spectral measurements
[Fe(HL)(L)(H2 O)2 · EtOH + 3HCl Pentanol
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[Th(L)(NO3 )2 (H2 O)4 ] · ×H2 O + 2HNO3
This study indicates that the ligand (H2 L) forms very stable complexes of the general formulae [Co2 L2 (H2 O)2 (EtOH)2 ]·H2 O, [Co(HL)2 (MeOH)]·H2 O, [Co(HL)2 (MeOH)], [Ni2 L2 (H2 O)4 ]·2H2 O, [Fe(L)(HL)(H2 O)2 ]·EtOH and [Th(L) (NO3 )2 (H2 O)4 ]·H2 O. The high melting points (>300 ◦ C) of all complexes, except the FeIII complex (m.p. 275 ◦ C), as well as their insolubility in H2 O and common organic solvents suggest the polymeric nature of all complexes. Also, all of the solid complexes are insoluble in DMF and DMSO except those of ThIV and FeIII which are freely soluble in these solvents. All attempts to isolate crystalline compounds to study the molecular structure by X-ray technique failed. The molar conductivities of the ThIV and FeIII complexes in DMSO at 25 ◦ C are 28 and 0 −1 cm2 mol−1 ,respectively, indicating non-electrolytic character [13]. The high yields of the CoII and NiII complexes reflect the importance of this ligand as analytical reagent for the determination of these ions by gravimetric technique. All of the isolated solid complexes are pale yellow, powder-like salts except the FeIII complex which is pale brown (Table 1).
The IR spectrum of H2 L in KBr shows a strong band at 1680 cm−1 together with a broad medium-intensity band centered at 3180 cm−1 and assigned to (C O) and (OH) vibrations, respectively. Also, the observation of broad but weak bands in the 2100–1800 cm−1 region suggests that intramolecular hydrogen bonding is the operating in the free ligand as shown in structure 2 (Fig. 1).
It is quite clear that the existence of two groups containing lone pairs of electrons in the ortho position will cause the elec-
Table 1 Analytical and physical data of the metal complexes derived from H2 L Compound
[Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O C52 H50 Co2 O17 (1064.8) [Co(HL)2 MeOH]·H2 O C49 H40 CoO14 (911.8) [Co(HL)2 MeOH] C49 H38 Co2 O13 (893.8) [Fe(HL)(L)(H2 O)2 ]·EtOH C50 H43 FeO15 (939.7) [Ni2 (L)2 (H2 O)4 ]·2H2 O C48 H43 Ni2 O18 (1025.3) [Th(L)(NO3 )2 (H2 O)2 ]·3H2 O C24 H26 N2 O17 Th (846.5)
Color
M.p. (◦ C)
Yield %
Yellow
>300
99.0
Yellow
>300
98.5
Yellow
>300
98.0
Pale Brown Yellow
275
91.0
>300
99.0
Yellow
>300
88.0
H2 L = C24 H18 O6 , HL = C24 H17 O6 , L = C24 H16 O6 .
μeff
Found (Calcd.) % C
H
M
(B.M.)
58.7 (58.7) 64.4 (64.5) 65.6 (65.8) 64.0 (63.9) 55.9 (56.2) 33.9 (34.1)
4.1 (4.7) 4.6 (4.4) 4.1 (4.3) 4.4 (4.5) 4.1 (4.3) 2.5 (3.1)
11.6 (11.1) 6.6 (6.5) 6.6 (6.6) 5.3 (5.2) 11.8 (11.4) 27.6 (27.4)
2.85 4.60 4.55
1.87 2.77 Diamagnetic
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Fig. 1. Structure of the ligand (H2 L).
trostatic repulsion between the non-bonding electrons present on the oxygen atoms of the hydroxyl and carbonyl oxygen groups. This behaviour makes the carbonyl oxygen to go out of the plane and spatially be far from the proton of the hydroxyl group. Also, it prevents intramolecular hydrogen bonding between the OH and C O groups (Fig. 2b.). The formation of two intramolecular hydrogen bonds between the two naphthyl OH groups (Fig. 2a) requires a small rotation around the 1-1 -position which helps to form two intramolecular hydrogen bonds. This rotation is ◦ supported on the basis of the [α]20 D value (+3.5 ) in DMSO. It is interesting to point out that the observation of a very strong band at higher wavenumber (1680 cm−1 ) assigned to the (C O) vibration suggests that the carbonyl oxygen is not taking part in hydrogen bonding and, consequently, Fig. 2b is ruled out. Moreover, the IR spectrum of H2 L in CHCl3 shows a shift of the OH band to higher wavenumber (3240 cm−1 ) while the carbonyl band remains unchanged (1680 cm−1 ), indicating the partial dissolution of the intramolecular hydrogen bonding between the two OH groups. Finally, two bands are observed at 1322 and 1217 cm−1 in the spectrum of the free ligand (H2 L). The former band is a mixture of δ(OH) + ν(C O(H) in which δ(OH) is dominant while the latter band is a mixture of ν(C O(H) + δ(OH) in which ν(C O) is dominant [14,15]. Doubtless, the observation of only one band for both the carbonyl and OH groups confirms the symmetry of H2 L (Figs. 3–6). The 1 H NMR spectrum of H2 L in d6 -DMSO exhibits two signals at 4.05 and 10.4 ppm, assigned to the protons of the methyl ester and OH groups, respectively. The latter signal disappears upon deuteration. The multiplet signals in the 7.0–8.8 ppm region are assigned to the protons of the two binaphthyl rings and the peak areas are equivalent to 10 protons. In order to determine the mode of chelation, the IR spectra of the free ligand (H2 L) and its metal complexes were compared. The results show that H2 L behaves as a bidentate ligand through
Fig. 2. Types of intramolecular hydrogen bonding in H2 L.
Fig. 3. Suggested structure of [Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O.
Fig. 4. Suggested structure of [Fe(HL)(L)(H2 O)2 ]·EtOH.
Fig. 5. Suggested structure of [Co(HL)2 (MeOH)]·H2 O.
the two naphthyl OH groups with displacement of two protons as shown in structure 3. This structure is supported by the following evidence. The ν(OH) bands of the complexes, especially the CoII complex are absent whereas the position of the carbonyl oxygen band remains unchanged. A band at 530 cm−1 is assigned to the ν(Co O) [16] vibration. Also, the observation of a new band at 3440 cm−1 suggests the existence of solvent molecules around the CoII ion.
Fig. 6. Suggested structure of [Ni2 (L)2 (H2 O)4 ]·2H2 O.
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The high melting point (>300 ◦ C) and the insolubility of the complex in all organic solvents supports the polymeric nature of this complex. The IR spectrum of the FeIII complex in KBr shows two bands at 3488 and 3175 cm−1 assigned to ν(OH) (H2 O) and ν(OH) (naphthyl) vibrations, respectively. The small shift of the naphthyl OH group to lower wavenumber indicates the involvement of this group in bonding (4). Also, the position of the carbonyl oxygen remains unchanged (1681 cm−1 ) indicating that this group does not take part in coordination. The band observed at 550 cm−1 is assigned to the ν(Fe O) [12] vibration. CoII
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H2 L also behaves as binegative tetradentate ligand towards two nickel ions coordinating through the carbonyl oxygen and the deprotonated naphthyl OH groups which is supported by the following evidence. The OH band disappeared together and the carbonyl oxygen frequency is shifted to lower wavenumbers (10 cm−1 ) with a decrease of its intensity. The appearance of new bands at 3460 and 450 cm−1 assigned to ν(OH), (H2 O) and ν(Ni O) [12] vibrations, respectively. The existence of hydrogen bonding between the coordinated H2 O molecules and the naphthyl OH and/or the carbonyl oxygen groups is suggested by the observation of a broad weak band centered at 3192 cm−1 . The IR spectrum of the ThIV complex [Th(L)(NO3 )2 (H2 O)2 ]·3H2 O, in KBr is the same as that of the NiII complex except that the ratio of M:L for the ThIV complex is 1:1 while it is 2:2 for the NiII complex. The changes observed in the spectrum of this nitrato complex show new bands at 1350 (ν4 ), 830 (ν6 ), 1447 (ν1 ), 1760 (ν2 + ν3 ) and 1730 (ν2 + ν5 ) cm−1 which are consistent with the bidentate nature of the nitrate group [18]. The difference (30 cm−1 ) between the two bands at 1760 and 1730 cm−1 is taken as additional evidence for the bidentate nature of the nitrate group [19]. Finally, in the spectra of all complexes, we observed shift of the band at 1322 cm−1 , due to δ(OH) + ν(C O), to higher wavenumbers while the band at 1217 cm−1 , due to ν(C O) + δ(OH), is shifted to lower wavenumbers. 3.2. Magnetic and electronic spectral studies
The 1 H NMR spectrum of the FeIII complex in d6 -DMSO shows two signals at 4.2 and 11.0 ppm and assigned to the protons of Me and OH, respectively. The multiplet signals in the 7.6–9.2 ppm range are assigned to the protons of the naphthyl groups. The ratio of the protons of naphthyl:methyl:OH is 20:12:1 and indicates that three OH groups are deprotonated while only one OH group remains protonated. Also, the broad signal at 4.5 ppm is assigned to the protons of H2 O molecules. The OH signal disappears upon deuteration. The values of the isomeric shift (0.3212) and quadrupole splitting (0.7032) from the M¨ossbauer spectrum are consistent with a low-spin octahedral geometry for the Fe(III) ion [17]. The thermograms of the FeIII complex show five peaks at 180, 280, 350, 440 and 510 ◦ C. The first two endothermic peaks are assigned to the removal of EtOH and H2 O molecules while the other three exothermic peaks are mainly due to decarboxylation, dehydroxylation and complete decomposition with formation of Fe2 O3 .
The analytical and magnetic data of the solid complexes are presented in Table 1. The UV spectrum of H2 L in EtOH shows six bands at 33,445 (290 l cm−1 mol−1 ), 34,600 (390 l cm−1 mol−1 ), 35,840 (560 l cm−1 mol−1 ), 40,985 (9400 l cm−1 mol−1 ), 41,670 (9600 l cm−1 mol−1 ) and 44,640 cm−1 (13,200 l cm−1 mol−1 ) assignable to B, K1 , K2 , E2 , E1 and n → σ* transitions [5,20], respectively. Also, a weak band observed at 26,315 cm−1 (85 l cm−1 mol−1 ) is assigned to the n → π* transition for the two carbonyl groups. Again, the observation of only one carbonyl group at 26,315 cm−1 suggests that the two carbonyl groups are symmetric as confirmed previously by the IR spectra. On the other hand, the spectra of H2 L in non-polar solvents (CCl4 , C6 H6 ) show a red-shift of the n → π* transition while showing a blue-shift of the π → π* band. This means that the decreasing polarity of the solvent will be accompanied by shifting of the n → π* and n → σ* bands to longer wavelength while the π → π* band undergoes a shift to shorter wavelength. These results are quite consistent with results published in the literature [16]. The observed molecular weight (M+ ) of H2 L in the mass spectrum was found to be 402 in agreement with the molecular weight of the ligand. The electronic spectrum of the CoII complex [Co2 (L)2 (H2 O)2 (EtOH)2 ]·H2 O, in Nujol shows a broad band centered at 22,040 cm−1 which is tentatively assigned to the 2 A1g → 2 B2g transition in a square-planar configuration [21–23]. The band observed at 26,740 cm−1 is assigned to a charge-transfer transition. Also, the value of the magnetic
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moment (2.85 B.M.) is additional evidence for a square-planar geometry (3) around the CoII ion [17,22]. The magnetic moments of the other two CoII complexes [Co(HL)2 (MeOH)]·H2 O and [Co(HL)2 (MeOH)], are 4.60 and 4.55 B.M., respectively, indicating a tetrahedral, octahedral or square-pyramidal configuration around the CoII ion. The electronic spectra of the two complexes in DMSO and/or Nujol are more or less the same and show several bands at 23,040, 17,270, 16,810 and 15,750 cm−1 . The spectra as well as the values of magnetic moments indicate a high-spin octahedral or square-pyramidal structure around the CoII ion. The comparatively high ε values (120–160 l cm−1 mol−1 ) for the two complexes rule out the existence of an octahedral structure and support a square-pyramidal structure around the CoII ions [17,23] as shown in structure 5.
B.M.) for each NiII ion is in good agreement with the octahedral geometry [19] around the NiII ion (6).
3.3. Thermal studies
On the other hand, μeff (1.87 B.M.) of the FeIII complex [Fe(HL)(L)(H2 O)2 ]·EtOH, suggests a low-spin octahedral structure around the FeIII ion [20]. The electronic spectrum in DMSO exhibits a weak band at 15,770 cm−1 together with a shoulder at 21,550 cm−1 assignable to the 2 T2 → 2 A2 transition [24]. The bands observed at 25,510, 28,900 and 31,950 cm−1 are assigned to π → t2g , charge-transfer and ligand transitions, respectively. All the results are consistent with low-spin octahedral FeIII (4). The formation of low-spin FeIII complexes with hard donors such as oxygen is explained on the basis that the resulting complex has a tetragonal structure and, consequently, the bond lengths between the four oxygen donors and FeIII in the xy direction are very short which causes a large splitting of the d-orbitals. Also, the strong covalent bonding between the oxygen donors and FeIII ion suggests the large Δ value (crystal field stabilization energy) which helps in the formation of the low-spin configuration. The electronic spectrum of the nickel complex [Ni2 (L)2 (H2 O)4 ]·2H2 O, shows two bands at 28,010 and 17,544 cm−1 assigned to 3 A2g → 3 T1g (P) and 3 A2g → 3 T1g (F) transitions, respectively. The third band assigned to 3A → 3T 2g 2g was calculated [25] and found to be at 11,200 cm−1 . The bands observed at 22,700 and 15,300 cm−1 are assigned to 3 A2g → 1 T1g and 3 A2g → 1 Eg transitions, respectively. Also, the values of B and β were calculated [26] and found to be 797 and 0.77 cm−1 , respectively. Moreover, the ν2 /ν1 ratio (1.57) gives further support for the octahedral geometry [27]. Finally, the value of the magnetic moment (2.77
Thermal measurements of H2 L were conducted by means of TG, TGA and DTA. The thermograms show three peaks at 280, 426 and 515 ◦ C attributed to melting point, decarboxylation and dehydroxylation, respectively. Thermal measurements of [Co(L)2 (H2 O)2 (EtOH)2 ]·H2 O show four peaks at 200 (13.88%), 350 (32.7%), 440 (30.1%) and 520 ◦ C (16.5%) assignable to the removal of solvents (EtOH, H2 O), decarboxylation, dehydroxylation processes and complete decomposition with formation of Co2 O3 . The difference between the other two CoII complexes [Co(HL)2 (MeOH)]·H2 O and [Co(HL)2 (MeOH)], is the existence of only one water molecule outside the coordination sphere in the former complex. On heating this complex, the water molecule disappeared at 120 ◦ C and the complex converts to the second CoII complex [Co(HL)2 MeOH]. This assumption is based on the results of thermogravimetric analysis. The thermograms for the two CoII complexes show four peaks but the former complex [Co(HL)2 MeOH]·H2 O, exhibits an additional peak at 120 ◦ C corresponding to the removal of water of hydration. 3.4. Mass spectroscopic studies The observed M+ value (1065) of the CoII complex [Co2 (L+ )2 (H2 O)2 (EtOH)2 ]·H2 O, coincides with the calculated value (1064.8). References [1] (a) C. Bolm, J. Hildebrand, K. Muniz, N. Hermanns, Angew. Chem. Int. Ed. 40 (2001) 3284; (b) P. Kocovsky, S. Vyskocil, M. Smrcina, Chem. Rev. 103 (2003) 3213. [2] (a) M. Periasamy, L. Venkatraman, J. Thomas, J. Org. Chem. 62 (1997) 4302; (b) F. Toda, K. Tanaka, Chem. Commun. (1997) 1087. [3] (a) C. Chu, B. Uang, Tetrahedron: Asymmetry 14 (2003) 53; (b) J. Gao, J. Reibenspies, A. Martell, Angew. Chem. Int. Ed. 42 (2003) 6008. [4] K. Kim, D. Lee, Y. Lee, D. Ko, D. Ha, Tetrahedron 60 (2004) 9037.
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