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EPR and Spectroscopic Studies of S-Methyl-N-salicylidenehydrazinecarbothioatophenanthrolineoxovanadium(IV) as Model Compound for Vanadium Bound to Nitrogen and Sulfur Heteroatoms Jimmy S. Hwang* and M. O. Hamad Al-Turabi† King Fahd University of Petroleum & Minerals, Department of Chemistry, Dhahran 31261, Saudi Arabia
Laila El-Sayed and H. A. M. Al-Gwidi‡ Department of Chemistry, Faculty of Science, Alexandria University, Alexandria 21321, Egypt Received June 2, 1999. Revised Manuscript Received October 21, 1999
Synthesis and EPR characterization of a vanadyl complex salicylaldehyde Schiff base in methylene chloride is described. The vanadyl complex synthesized is S-methyl-N-salicylidenehydrazinecarbothioatophenanthrolineoxovanadium(IV). The magnetic parameters for this system are determined from computer simulation of the rigid limit EPR spectrum at 77 K and analysis of the isotropic spectrum at room temperature at X-band frequency. A good correlation is found between g| and A| of the system studied when compared with other model compounds of vanadyl complexes in which possible combinations of four ligands may include VO(N4), VO(N2SO), VO(N2S2), VO(NS3), and VO(S4). This information could be useful in characterizing non-porphyrin species of vanadium in asphaltenes.
Introduction EPR spectroscopy can be applied to reveal the environment of the vanadyl ion and the nature of the ligand types in petroleum. This information will be significant for characterizing the vanadyl compounds present in heavy crude oil.1,2 Recently, we applied EPR and IR data to study a model compound in which the coordination of the four donor atoms in the vanadyl complex consists of ligands with half nitrogen and half sulfur (N2S2).3 The hyperfine and g-tensor values were determined, using the method of Wilson and Kivelson,4 from the rigid limit spectrum using second-order perturbation theory, and from A0 and g0 determined from the isotropic liquid spectrum at room temperature, respectively. A linear relationship3 was found between g| and A| for vanadyl complexes with equatorial ligand field of the type VO(N4), VO(N2S2) and VO(S4). In this paper, we report the synthesis and EPR study of a model compound with equatorial ligand fields of the type N2SO. In this study, we would like to improve the precision and accuracy of * Corresponding author. † Present Address: KFUPM Research Institute, P.O. Box 626, Dhahran 31261, Saudi Arabia. ‡ Present Address: Girl’s College of Education, Chemistry Department, Riyadh, Saudi Arabia. (1) Yen, T. F. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, 1975; pp 167-181. (2) Boucher, L. J.; Tynan, E. C.; Yen, T. F. In Electron Spin Resonance of Metal Complexes; Yen, T. F., Ed.; Plenum: New York, 1969; p 111. (3) Hwang, J. S.; Al-Turabi, M. O. H.; El-Sayed, L. Energy Fuels 1994, 8, 793. (4) Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44, 154.
the magnetic tensor parameters by carrying out a computer simulation of the rigid limit spectra of vanadyl compounds and compare these results with the experimental rigid limit spectra. One of the objectives of this study is to see if the linear relationship between g| and A| is also obeyed for our model compound. A second objective is to find a correlation between g| and A| of the two systems studied and compare with other model compounds of vanadyl complexes with equatorial ligands containing nitrogen, oxygen, and sulfur heteroatoms. Experimental Section (1) Preparation of the Organic Ligand. Hydrazine-Smethylcarbodithioate, NH2NHCSSCH3, was prepared from the reaction of ammonium salt NH2NHCSSNH4 with CH3I using the method previously described.5 The corresponding salicylaldehyde Schiff base was prepared by the condensation of the methyl ester with an equimolar amount of salicylaldehyde in absolute ethanol as described previously.6 (2) Preparation of methyl-N-salicylidenehydrazinecarbodithioatophenanthrolineoxovanadium(IV). To a hot solution of methyl-N-salicylidenehydrazine carbodithioate (0.01 mol) in methanol (70 mL) were added an aqueous solution of VOSO4‚3H2O (0.01 mol) and a hot solution of 1,10-phenanthroline (0.01 mol) in methanol (20 mL). The stoichiometric amount of sodium acetate (0.01 mol) dissolved in methanol (30 mL) is added to the resulting brownish green solution. A shiny brownish red precipitate is formed on cooling. (5) Audrieth, L. B.; Scott, E. S.; Kippur, P. S. J. Org. Chem. Acta 1953, 19, 733. (6) Ali, M. Akbar; Livingstone, S. E.; Philips. D. J. Inorg. Chim. Acta 1973, 7 (2), 179.
10.1021/ef9901084 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/29/1999
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It was filtered and washed with water, then dried under vacuum over P2O5. The elemental analysis was as follows: {Found: C, 53.6; H, 3.4; N, 11.8; V, 10.8%. Calculated for VC21H16N4O2S2: C, 53.3; H, 3.4; N, 11.8; V, 10.8%}. (3) Physical Measurements: The IR spectra were recorded using a Perkin-Elmer model FT-IR 1720 spectrophotometer. The electronic spectra were made using a PYE UNICAM SP 8-100 spectrophotometer. The magnetic moment of the solid sample was measured at room temperature using a JME magnetic susceptibility balance. (4) EPR Measurements. The instruments used to record EPR spectra were as follows: (i) Bruker ER-200D-SRC spectrometer. Bruker ESP 300E data system, Bruker ER 4111 variable temperature unit, Bruker ER-035 NMR gaussmeter and HP model 5432-A frequency counter. (ii) Pope Scientific vacuum system for degassing the sample tubes. Samples in the concentration range (5 ( 3) × 10-4 M were prepared in methylene chloride (spectroscopic grade purchased from Merck). The sample of the above concentration was transferred into a 2 mm i.d. Pyrex tube with a fine dropper. The sample was degassed by a freeze-pump-thaw procedure several times for thorough degassing and sealed under liquid nitrogen by using a torch.
Results and Discussion The reaction of methyl-N-salicylidenehydrazinecarbodithioate(VOL) and 1,10-phenanthroline with vanadyl sulfate trihydrate in a 1:1:1 molar ratio afforded a complex (Figure 1) whose analytical data correspond to the molecular formula VOL(NN) where L refers to the doubly deprotonated salicylaldehyde Schiff base and NN to 1,10-phenanthroline. The complex is soluble in chloroform and methylene chloride, and slightly soluble in toluene. The infrared spectra of the salicylaldehyde Schiff base and the VOL(NN) complex have been recorded in KBr disks. The infrared spectra of the Schiff base exhibit two bands at 3130 and 3000 cm-1, respectively, due to ν(N-H) stretching mode and hydrogenbonded ν(O-H) vibration. This band disappears completely from the IR spectrum of VOL(NN). The IR spectrum of the Schiff base also shows a strong band at 1645 cm-1 attributable to ν(CdN) and a series of bands at ca. 1110, 1050, 1030, 970, and 950 cm-1 associated with N-CSSCH3 residue.7 The IR spectrum of VOL(NN) reveals an appreciable shift of ν(CdN) to 1596 cm-1 and the disappearance of the 1110 cm-1 band. This band has been attributed to δ(N-H) deformation coupled with ν(CdS) stretching modes.8,9 The spectrum exhibits also a strong absorption at 955 cm-1 due to ν(VdO) stretching frequency. These IR spectral results indicate that, although salicylaldehyde Schiff base exists in the thion form in the solid state, it acts as a dinegative tridentate ligand in VOL(NN), coordination taking place via the azomethene nitrogen and the deprotonated thiol sulfur and phenolic oxygen. The electronic spectrum of VOL(NN) in chlorofrom displayed an unsymmetric broad absorption band centered at 740 nm. This absorption can tentatively be assigned to Band I (dxy f dxz, dyz) and Band II (dxy f (7) Iskander, M. F.; El-Sayed, L. J. Inorg. Nucl. Chem. 1971, 33, 4253. (8) Ali, M. Akbar; Livingstone, S. E.; Philips, D. J. Inorg. Chim. Acta 1971, 5 (1), 119. (9) Battistoni, C.; Giuliani, A. M.; Paparazzo, E.; Tarli, F. J. Chem. Soc., Dalton Trans. 1984, 1293.
Hwang et al.
Figure 1. S-Methyl-N-salicylidenehydrazinecarbodithioatophenanthroline oxovanadium(IV). Table 1. Isotropic Magnetic Parameters compound
solvent
g0
A0(G)
VO(N2SO)
CH2Cl2
1.9774
92.20
dx2-y2) in terms of the energy level scheme proposed by Ballhausen and Gray.10 Band III (dxy f dz2) has not been detected. The room-temperature magnetic moment (1.73 µB) of VOL(NN) is equal to the spin only value of 1.73 µB when the orbital contribution is completely quenched. Determination of the Magnetic Parameters. I. Isotropic Magnetic Parameters. The same procedure as outlined in [3] was followed. In brief, the roomtemperature EPR spectrum, with DPPH as an internal standard, was recorded at 9 GHz for VO(N2SO) compound in methylene chloride and is shown in Figure 2. Each spectrum consists of eight lines arising from the interaction of a single unpaired electron (S ) 1/2) with the quenched orbital angular momentum of the vanadium nucleus of spin I ) 7/2. The isotropic hyperfine constants A0 and g0 were obtained from BM for line M and B-M for line -M by eqs 1 and 24
A0 ) g 0 - gs ) gs
{
-g0β0(BM - B-M) 2Mp
}
Bs - (BM + B-M)/2 (BM + B-M)/2
(1)
-
2A20p2[I(I + 1) - M2] gsβ20(BM + B-M)2
(2)
where p is the reduced Planck’s constant, ω0 is the microwave frequency in rad/s, β0 is the Bohr magneton, and gs and Bs are the isotropic g value and the resonant value of the magnetic field (in gauss), respectively, for the standard of known g-value. A0 and g0 values were determined for each pair of M and -M lines and averaged over all pairs for the VO(N2SO) system and are given in Table 1. II. Anisotropic Magnetic Parameters. The rigid limit spectra of VO(N2SO) in methylene chloride at 9 GHz is shown in Figure 3. The anisotropic tensors were first determined from the rigid limit spectrum using secondorder perturbation theory, as Wilson and Kivelson have done for vanadyl acetylacetonate.4 The values of gx, gy, gz, and Ax, Ay, Az were calculated from the position of (10) Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962, 1, 111.
Model Compound for V Bound to N and S Heteroatoms
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Figure 2. X-band spectrum of VO(N2SO) in CH2Cl2 at room temperature with DPPH as internal standard.
the resonant lines in the glassy spectrum for bis(Smethyl-3-isopropylidenehydrazinecarbodithioato)oxovanadium(IV).3 In this work, the magnetic parameters derived from the glassy spectrum were used as a first approximation in the computer simulation employing the general method of Lefebvre and Maruani11 adapted to oxovandium(IV) systems. The simulation was carried out using Microsoft Power Station (Version 4.0) on PC. Rather than feeding g and A tensor values each time into the computer, a small subroutine was written to vary the g and A tensor values with predetermined increments giving up to 1000 output files. The simulation employs Simpson’s numerical integration over θ in 45 intervals and φ in 25 intervals. The calculated g and A parameters for VO(N2SO) system are listed in Table 2. III. Crystal Field Approximation. According to Balhausen and Gray,10 the energy level scheme of oxovanadium complexes can be considered as a strong tetragonal compression of an octahedral field. The electronic energy level scheme is depicted in Figure 4. When dxy is the singly occupied orbital, the equations for g| and g⊥ are
g| ) ge -
8λ Ex2 - y2 - Exy
(3)
2λ Exz - Exy
(4)
g⊥ ) ge -
Thus for D4h and C4v symmetry the g values sequence is ge > g⊥ > g|. When there is lower symmetry due to more distortions
g x ) ge -
2λ Exz - Exy
(5)
g y ) ge -
2λ Eyz - Exy
(6)
with ge > gx > gy > gz. (11) Lefebvre, R.; Maruani, J. J. Chem. Phys. 1965, 42, 1480.
Gauss
Figure 3. Rigid limit spectrum of VO(N2SO) in CH2Cl2 at 77 K, experimental (solid) and simulation (dashed).
Figure 4. Splitting of the vanadium d levels with C4v symmetry. Table 2. Magnetic Parameters Determined from Rigid Limit Spectrum compound
VO(N2SO)
solvent gx gy gz g0 Ax(G) Ay(G) Az(G) A0(G)
CH2Cl2 1.9915 1.9835 1.9573 1.9774 54.76 54.76 166.62 92.05
For distorted tetrahedral structure, gz ) ge > gx > gy. As can be seen from Table 2, ge > gx > gy > gz which supports a five-coordinated distorted square pyramidal structure or distorted octahedral field. For the VO(N2SO) system, all three transitions are showing up as a broad peak centered at 740 nm, which is in agreement with the obtained magnetic parameters. There is a small distortion from axial symmetry which can be seen in the difference between gx and gy. This is
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Figure 6. Correlation between g| and A| for a variety of square pyramidal vanadyl(IV) complexes; (++) ref 23, †† this work. (The data for VO(O4), VO(N2O2), VO(N4), and VO(S2O2) were reprinted with permission from Plenum Press, cf. ref 2.)
Figure 5. Correlation between A0 and g0 for a variety of square pyramidal vanadyl(IV) complexes; (*) ref 2, (**) ref 20, (+) ref 21, † ref 22, (++) ref 23, (††) this work.
attributed to the presence of 1,10-phenanthroline ligand which is rigid, decreasing the possibility of the distortion. The fact that Ax ) Ay reflects axial symmetry of the electron density in the equatorial plane of the VO(N2SO) system. This might be due to the fact that both S and O are negatively charged with the general π-bonding order of the ligand donor atoms, S g O.2 As most of the x- and y-components in the rigid limit spectrum are overlapping, the error in Ax and Ay was approximately (0.5 G. Application to Non-Porphyrin Vanadium Species in Asphaltenes. Yen1 found a good linear relationship between A0 and g0 (Figure 5) for VO2+ complexes for ligands made of varying compositions. The correlation between g0 and A0 values is quite good for a variety of square pyramidal vanadyl(IV) complexes with equatorial ligand fields of the type VO(O4), VO(N2O2), VO(S2O2), and VO(N4). Holyk13,14 has shown that the correlation between g| and A| is perhaps more useful because g|| depends directly on the in-plane ligand field and the range of the values for g| and A| is about twice that of g0 and A0. We found that there is a linear relationship between g| and A| for vanadyl complexes with equatorial ligands of the type VO(N4), VO(N2S2), and VO(S4). In this work, we found that the linear relationship between g| and A| is obeyed for the VO(N2SO) model system as can be seen in Figure 6 that the VO(N2SO) system is falling between VO(N2S2) and VO(N2O2) systems. Interestingly, the Holyk relation is found to be valid for vanadium systems containing nitrogen, sulfur, and oxygen ligand types. This would make it more useful as potential techniques for petroleum characterization. The chemistry and structure of the non-porphyrin species of vanadium in asphaltenes are poorly understood and few studies have dealt with the characterization of the vanadyl compounds present. If one uses sequential elution solvent chromatography to obtain (12) Atherton, N. M. Electron Spin Resonance; Ellis Horwood Ltd.: Chichester, England, 1973; p 198. (13) Holyk, N. H. M.S. Thesis, University of New Hampshire, Durham, 1979. (14) Chasteen, N. D. In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1989; Vol. 3, Chapter 2, pp 53-119.
fractions for asphaltenes based on functionality, then one can use the correlation diagram between g| and A| to determine more explicitly the nature of the square planar complexes responsible for the characteristic spectrum, particularly with regard to nitrogen and sulfur heteroatoms. Sequential elution solvent chromatography has been successfully applied to complex bitumous materials such as solvent-refined coal15 and asphalts.16 The practical application of this information is process design. Demetalation of non-porphyrin species may proceed differently than for porphyrin species.17 The first step in hydrodemetalation of porphyrin model compounds is hydrogenation possibly at the pyrole ring.18,19 No equivalent studies have been done on nonporphyrin model compounds which may react by a totally different mechanism. The possibility of different mechanism of hydrometalation as a function of metal coordination sites could affect catalyst or process design for heavy feeds. Conclusion EPR study was carried out for the VO(N2SO) model compound in methylene chloride at room temperature and at 9 GHz microwave frequency. Rigid limit spectrum was taken at 77 K and analyzed by computer simulation to yield the magnetic parameters. For our system,
g e > gx > gy > gz which gives support to a distorted octahedral structure, which is distorted to the extent that it could be resembling a distorted square pyramidal structure. This is consistent with observed electronic spectra of the system and is in support of the energy level scheme of Balhausen and Gray. The g values of this model compound show that there is little distortion from axial symmetry (gx ≈ gy). (15) Farcasiu, M. Fuel 1977, 56, 9. (16) Dark, W. A. J. Chromatogr. Sci. 1978, 16, 289. (17) Rankel, L. S.; Rollman, L. D. Fuel 1983, 62, 44. (18) Kameyema, H.; Yamada, M.; Amano, A. J. Jpn. Petrol. Inst. 1981, 24, 317. (19) Agawral, R.; Wei, J. Ind. Eng. Chem. Process Res. Dev. 1984, 23, 505. (20) Assour, J. M. J. Chem. Phys. 1965, 43, 2477. (21) Bozis, R. A.; McCormick, B. J. Inorg. Chem. 1970, 9, 1514. (22) Dickson, F. E.; Kunesh, C. J.; McGinnis, E. L.; Petrakis, L. Anal. Chem. 1972, 44, 978. (23) Atherton, N. M.; Locke, J.; McCleverty, J. A. Chem. Ind. 1965, 29, 1300.
Model Compound for V Bound to N and S Heteroatoms
We found a good correlation between g| and A| values for N2S2, N2SO, and N2O2 ligand types and all agreed well with the other ligand types studied by Holyk. We observed that there is a linear relationship between g| and A| for ligands made of varying compositions and there is a good correlation for varying complexes with equatorial ligands of the type VO(N4), VO(N2S2), VO(N2-
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SO), and VO(S4). This could to be useful as potential techniques for petroleum characterization. Acknowledgment. This research was supported by King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia. EF9901084
