Energy & Fuels 1994,8, 793-797
793
EPR and Spectroscopic Studies of
Bis(S-methyl-3-isopropylidenehydrazinecarbodithioato) oxovanadium(1V) as Model Compound for Vanadium Bound to Nitrogen and Sulfur Heteroatoms Jimmy S. Hwang* and M. 0. Hamad Al-Turabi Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Laila El-Sayed Department of Chemistry, Faculty of Science, Alexandria University, Alexandria 21321, Egypt Received July 19, 1993. Revised Manuscript Received February 1, 1994"
The synthesis and spectroscopic and EPR characterization of a vanadyl complex with acetone Schiff base, bis(S-methyl-3-isopropylidenehydrazinecarbodithioato)oxovanadium(IV) ,are described. The magnetic moment of 1.75 pg at room temperature is close to the spin only magnetic moment (1.73p g ) . The experimental values ofgll= 1.960andg, = 1.988and the normal occurence of v(V=O) vibration suggesta monomericfive-coordinate square pyramical structure of the complex. A correlation was found between gll and All of the title compound when compared with other model compounds of the vanadyl complexes in which possible combinations of four ligands may include all nitrogen (N4) and all sulfur (S4).This information could be useful in characterizing aqueous pyridine cuts of asphaltenes with molecular weights less than 400 found in some heavy crude oils.
Introduction
Experimental Section
The interest in characterizing the vanadyl compounds present in heavy crude oi1,l aside from the obvious geochemical interest, resides in the fact that a novel removal method of vanadium for process and environmental reasons will ultimately rely on knowing the vanadyl compound composition of these complexes in petroleum matrices. EPR spectroscopy can be applied to reveal the environment of thevanadyl ion and thenature of the ligand types. EPR is a powerful spectroscopic technique for the identification of vanadium(1V) environments in petro1eum.l It is capable of revealing the environment of the vanadyl complex, the nature of the ligand types, and the distortion of the complex and association with other systems. This information will be significant for determining the overall structure of metal-containing compounds in heavy crude oil. In this paper, we report the synthesis and EPR study of a model compound in which the coordination of the four donor atoms in the vanadyl complex consist of half nitrogen and half sulfur (N2S2). The model compound chosen for this study is bis(S-methyl3-isopropylidenehydrazinecarbodithioato)oxovanadium(IV), VO[(CH3)2C=NNCSSCHJ2. EPR studies using vanadyl complex as spin probes have been performed in petroleum,2liquid and frozen media,37* poly(viny1 alcohol) gels,s liquid and human ser~transferrin~ as well as many other biological systems.10
1. Preparation of the Organic Ligand. Hydrazine-Smethylcarbodithioate, NHzNHCSSCHs, was prepared using the method previouslydescribed." The correspondingacetoneSchiff base was obtained by refluxing for l/Z h the methyl ester with excess acetone in ethanoic solution.12 2. Preparation of Bis(Bmethyl-34sopropylidenehydrazinecarbodithioata)oxovanadium(IV). A solution of vanadyl chloride (0.01mol) in absolute ethanol (5mL) was added to a hot solution of the acetone Schiff base (0.02 mol) in absolute ethanol (30 mL). The reaction mixture was boiled under reflux for about 6 h and then left to cool to room temperature. Shining black crystals precipitated out and were filtered and dried under vacuum. The same reaction was carried out in the presenceof anhydrous sodium acetate (0.02 mol). The reaction conditionsare not critical except for the addition of sodiumacetate which shouldnot exceed the stoichiometric amount as the vanadyl ion is not stable in neutral or alkaline medium. The complex precipitated out after reflux for 1 h together with a brown amorphous residue. In the absence of sodium acetate, the complex is formed without any byproducts but it needed a longer reflux. The sodium acetate enhancesthe reaction but in this casethe complexis contaminated
0
Abstract published in Advance ACS Abstracts, March 15, 1994.
(1) Yen, T. F. In The Role of Trace Metals in Petroleum; Yen, T. F.,
Ed.; Ann Arbor Science: Ann Arbor, MI, 1975,pp 167-181. (2)Garrett, B. B.; Gulick, W. M. Jr. J. Chem. SOC., Faraday Trans. 1 1983, 79, 1733. (3) Hwang, J. S.; Rahman, M. H. Chem. Phys. Lett. 1992,199,286and references therein. (4)Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44,154.
0887-0624/94/2508-0793$04.50/0
(5)Willigen, H. Van J. Phys. Chem. 1983,87, 3366. (6) Bruno, G. V.; Harrington, J. K.; Eastman, M. P. J. Phys. Chem. 1978,82, 2582. (7) (a) Luckhurst, G. R. InLiquid Crystals and Plastic Crystals; Gray, G. W., and Windsor, P. A., E&.; Ellis Horwood: New York, 1974, Vol. 2,Chapter 7,pp 144-191. (b) Luckhurst, G. R. In ESR Relaxation in Liquids; Muus, L. T., and Atkins, P. W., Eds.; Plenum Press: New York, 1972, Chapter XV, pp 411-442. (8)Kaplan, J. I.; Gelerinter, E.; Fryburg, G. C. Mol. Cryst. Liquid Cryst. 1973,23, 69. (9)White, K.W.; Chasteen, N. D. J. Phys. Chem. 1979,83, 279. (10)Smith, I. C. P.; Swartz, H. M.;Bolton, J. R.; Borg,D. C. Biological Applications of Electron Spin Resonance; J. Wiley: New York, 1972. (11)Audrieth, L.B.;Scott, E. S.; Kippur, P. S. J.Org. Chem. 1953,19, 733. (12)El-Sayed, L.;Iskander, M. F.; El-Toukhy, A. J.Znorg.Nucl. Chem. 1974,36, 1739.
0 1994 American Chemical Society
Hwang et al.
794 Energy & Fuels, Vol. 8, No. 3, 1994
with a brown residue and needed recrystallizationfrom benzene. The precipitate was filtered and recrystallized from benzene. No precautions were taken concerning dry atmosphere or removal of HC1 produced in the reaction. [Found: C, 30.5; H, 4.1; N, 14.2; V, 10.8%. Calcd for VO(CE,HBNZS~)Z: C, 30.8; H, 4.65; N, 14.4; V, 10.5%.1 3. EPR Measurements. The instruments used to record EPR spectra were as follows: (i) Varian E-109 EPR spectrometer, Varian E-935 data acquisition system interfaced with HP-9835 computer for data processing.
(ii) Bruker ER-SOOD-SRCspectrometer, Bruker ER140 data system, and CMD-96 hard disk from Control Data Corp. (iii) Pope Scientific vacuum system for degassing the sample tubes. Samples in the concentration range (3-5) X 1W M were prepared in toluene (spectroscopicgrade purchased from Merck). The sample of the above concentration was transferred into a 2 mm i.d. Pyrex sample tube with a fine dropper. The sample was degassed by freeze-pump-thaw procedure several times for thorough degassing and sealed under liquid nitrogen by using a
dxl
I
b.
Figure 1. Splitting of the vanadium d levels with Cc symmetry according to Ballhausen and Gray.16 I
torch.
Results and Discussion The reaction of the acetone Schiff base (CH3)2C=NNHCSSCH3 with vanadyl chloride in a 2:l molar ratio afforded the bis-VOL2 complex where L refers to the deprotonated Schiff base. The complex is soluble in noncoordinating solvents such as benzene and chloroform giving a green solution and dissolves in pyridine to yield a brown solution which soon turns turbid. The infrared (IR) spectroscopies of both the acetone Schiff base and the vanadyl complex were performed in potassium bromide disk. The IR spectrum of the Schiff base shows a moderately intense well-definedband at 3200 cm-1 due to u(N-H) stretching mode, an azomethine linkage u(C=N-) at 1650cm-l, and aseries of strong bands at 1200, 1040, 970, and 860 cm-' associated with the N-CSSCH3 residue.13 Comparison of the IR spectrum of VOL2 with the parent ligand reveals a lack of any absorption band due to N-H stretch besides a shift to u(C=N) to a lower frequency, 1590cm-l and disappearance of the 1040-cm-1band. This band has been attributed to 6(N-H) deformation coupled with u(C=S) stretching vibration modes.14J5 The IR spectrum of VOL2 exhibits also a strong absorption at 985 cm-1 due to u(V=O) stretching frequency. These IR spectral results indicate that, although the acetone Schiff base exists in the thione form in the solid state, it acts as a mononegative bidentate ligand in VOL2, coordination to VO2+ taking place via the azomethine nitrogen and the deprotonated thiol sulfur atoms. The electronic spectrum of VO [(CH3)2C=NNCSSCHsI 2 in benzene exhibits a low-intensity broad asymmetric band which shows two maxima at approximately 668 and 612 nm on top of the broad band envelope. A sharper and more intense band is located at 520 nm. The energy level diagram (Figure 1)of Ballhausen16typical for d1transition metal ion in compressed octahedral or square pyramidal complexes can be used as a basis for the assignment of the (13) Iskander, M.F.;El-Sayed, L. J. Inorg. Nucl. Chem. 1971,33, 4253. (14) Ali, M.Akbar; Livingstone, S. E.; Phillips, D. J. Inorg. Chim. Acta 1971,5(1), 119. (15) Battistoni, C . ; Giuliani, A. M.; Paparazzo, E.; Tarli, F. J. Chem. SOC.,Dalton Trans. 1984,1293. (16) Ballhausen, C.J.; Gray, H. B. Inorg. Chem. 1962,1, 111.
Figure 2. X-band (9.487 GHz) EPR spectrum of VO[(CH&C=NNCSSCH& in toluene at room temperature with DPPH as internal standard.
- - -
d-d bands. The broad low-intensity band can be attributed to bzg e,' (dxy d,,, dxz)transition (band I) while the sharp, more intense band at 520 nm (band 11) is assigned to bzs bl: (d, dXz.p)transition. I t should be pointed out that band I shows some sign of structure on top of the broad band that may indicate a partial lifting d,z) has of the degeneracy of the ex*level. Band I11 (d, not been detected. The room temperature magnetic moment (1.75 p ~ (where p~ = bohr magneton) of VO[(CH3)2C=NNCSSCH312 is close to the spin only magnetic moment (1.73 p g ) as the orbital contribution is almost completely quenched. On the basis of normal magnetic moment and normal occurrence of u(V=O) vibration, a monomericfivecoordinate square pyramidal structure is suggested for the complex, ruling out both dimerization through sulfur bridges and V-0-V=O stacking. Determination of the Magnetic Parameters. I. Isotropic Magnetic Parameters. The spectrum of VO[(CH3)C=NNCSSCH& in toluene at 9.487 GHz was taken at room temperature with DPPH as internal standard and is shown in Figure 2. I t consists of an eightline spectrum arising from the interaction of a single unpaired electron (S = l/2) with the quenched orbital angular momentum of vanadium nucleus of spin (I = '/2). The second-order effect can be seen from the unequal separation of the hyperfine components in Figure 2. The peak-to-peak heights of the eight lines are also unequal because of incomplete motional averaging of the anisotropic Zeeman and hyperfine interactions through the molecular reorientational motion. This type of line shape has been observed for vanadyl acetylacetonate in moderately viscous s~lution.~J' The isotropic hyperfine constants, A0 and go, were obtained from BM for line M and B-M for line -M by
-
(17) Hwang, J.;Kivelson, D.; Plachy, W. J.Chem. Phys. 1972,58,1763.
)
Energy & Fuels, Vol. 8, No. 3, 1994 795
Vanadyl Compounds in Crude Oil
200 G
Figure 3. Rigid limit X-band(9.278 GHz) spectrum VO[(CH&C=NNCSSCH& in toluene at 77 K. The spacings are I = 39.45 G; I1 = 157.84 G I11 = 162.19 G ; IV = 34.44 G; V = 66.38 G. as follows:
Table 1. Isotropic A0 and go for VO[ (CHa)&=NNCSSCHs]r in Toluene
M 712 512 312 112
Ao (G) 86.98
87.09 87.02 87.45 87.14f 0.21
'
go
1.9790 1.9790 1.9790 1.9791 1.9790 f O.OOO1
The value of g, was found to be 1.9603. Combining the values of A, and g, obtained from rigid limit spectrum with those of Aoand go obtained from the liquid spectra, the values of (A, + A,) and (g, + g,) were obtained as follows: A, A, = 3A0 - A, = 101.40G (5)
+ g, + g,
and
= 3g0- g, = 3.9767 (6) The values obtained in eqs 3-6 are first-order calculations
where h is the reduced Planck's constant, wo is the microwave frequency in rad/s, 80 is the Bohr magneton, and gEand B, are the isotropic g value and the resonant value of the magnetic field (in gauss), respectively, for the standard of known g value. For our system, the standard used is diphenylpicrylhydrazyl (DPPH), for which the g, value is 2.0037. A0 and go were determined for each pair of M and -M lines and averaged over all pairs. The results are given in Table 1. II. Anisotropic Magnetic Parameters. The rigid limit spectrum VO[(CH&C=NNCSSCH& in toluene was taken at 77 K at a microwave frequency of 9.279 GHz and is shown in Figure 3. The hyperfine and g-tensor values were determined, using the method of Wilson and Kivelson? from the rigid limit spectrum using secondorder perturbation theory, and from A0 andgo determined from the isotropic liquid spectrum at room temperature, respectively. Spacings I1 and 111, from rigid limit spectrum (Figure 3), were used to determine the first approximation to A,, A,(G)
I' *I1 = 160.02 G
=:
+
2
(3)
From the resonant lines in the glass spectrum, Figure 3, which corresponds to HI(z , the value ofg, was calculated
which should be further refined to second order in the following calculations. Also, from the resonant lines in the glass spectrum, Figure 3, which corresponds to H (1 Y, the value of g, was obtained as follows: g, =
wo(BM - B-M) - A$4(BM
B ~ ( -BB-,~)/ ~ ~h Furthermore, spacing I is:
+ B-M)
(7)
+ (7/2)(A, - A,) (8) h where B (in gauss) is the magnetic field calculated from the microwave frequency (in Hz) and go. Substituting A, andg, from eqs 5 and 6 into eq 8, the following expression was obtained: I (in Hz) =
I (in Hz) =
- gzh
+ (7/2)(3A0 - A, - 2A,) (9)
Equations 7 and 9 were solved to give A, and g, values. Then A, and g, values were also obtained using eqs 5 and 6. In fact, the rigid limit lines for H 11 z , x , and y were governed by
Hwang et al.
796 Energy & Fuels, Vol. 8, No. 3, 1994 Table 2. Magnetic Parameters for VO[(CH&C=Nh'CSSCH& in Toluene Determined from Riaid Limit Spectrum at 77 K ~
gs 1.9945
gY
gz
1.9829
1.9596
Ax(G) 48.76
Ay(G) 54.42
158.23
4
80 0 -
.'
VOS4"
70.01
respectively. The above equations were used to verify the calculated magnetic parameters of the g and A tensors to second order. The calculated g and A values for VO[(CH&C=NNCSSCHJ2 are listed in Table 2. The isotropic and anisotropic g and A parameters are related by the equations A0
(All + 2A,)/3
(13)
where
Neglecting second-order terms,gll and g l in C4" symmetry are related to the optical transition energies through the equations (17)
g, = g e - ~ 2 * ~ c r * ~ t / ~ E x r , y z (18) where ge = 2.0023 is the free electron g value, P z * ~ /31*~, , and cr*2 are the unpaired electron metal orbital populations in the Ixy), 1x2 - y2),and Ixz,yz) orbitals, respectively, and is the one-electron spin-orbit coupling constant of vanadium. These equations' ignore overlap of the ligand and metal orbitals and assume that the metal-based orbitals are the sole contributors to theg-shift. Equations 17 and 18 show that gil and g, should be proportional to the reciprocal of h E , 2 , 2 and hExz,yr,respectively. The midpoint between the two maxima at approximately 668 and 612 nm corresponds to a value of 6.40 X 10" cm for l/hExzyr while the intense band at 520 nm corresponds to a value of 5.20 X 10-5 cm for 1/hEXz.,2. The experimental values of gll = 1.960, and g, = 1.988 or (911 < g, < g,) are in agreement with the optical data and in support of the energy level scheme of Ballhausen and Gray16for the 12.4" symmetry, cf. Figure 1. From the absence of any further splitting of the resonance lines in the rigid limit spectrum (Figure 3) and the constancy (within experimental error) of the go's and Ao's values for different A4 and -A4pairs, as shown in Table 1, it is concluded that there is no V-V interactions of VO[(CHd2C=NNCSSCHJ2 in solution,thus ruling out both dimerization through sulfur bridges and V=O--V=O stacking. This is in accordance with the magnetic and spectral results.
Vanadyl Compounds in Crude Oil
I . 940
1.930
.
120
1
130
Energy & Fuels, Vol. 8, No. 3, 1994 797
1
I40
4
160
I 50
I70
180
190
A , ~(lo-'cm-')
Figure 5. Correlation between gll and All values for a variety of square pyramidal vanadyl(1V)complexes with equatorial ligand fields of the type VO(O,), VO(N202), VO(S202),VO(Nd), VO(N2S2), and VO(S4). [The data for VO(Od),VO(N202), VO(N,), and vO(Sz0~) were reprinted with permission from Plenum Pre~s.2~1 (++) Atherton, N. M.; Locke, J.; McCleverty, J. A. Chem. Ind. 1965,29, 1300. (tt)This work.
directly on the in-plane ligand field and (2) the range of values for gll and All is about twice that of go and Ao. The correlation between gll and All is plotted in Figure 5. One sees that there is a linear relationship between gll and Ail for V02+complexes for ligands made of varying compositions, and the correlation is good for the vanadyl(1V) complexes with equatorial ligand field of the type VO(N4),VO(NzSz),and VO(S4). In the future we are planning to study model compounds with equatorial ligand fields of the type VO(N3S) and VO(NS3) to see if the linear relationship between gil and All is also obeyed for these model compounds. 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. In a study of vanadyl compounds in heavy crude petroleum from Boscan, Cerro Negro, Wilmington, and Prudhoe Bay,23it was found that vanadyl compounds associated with the large molecular weight asphaltenes can be extracted with aqueous pyridine (21) Holyk, N. H. M.S. Thesis, University of New Hampshire,Durham, 1979. (22) 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.
and these compounds have apparent molecular weights greater than 2000 actually have molecular weights less than 400. This strongly implies that a considerable percentage (60-80 % ) of vanadyl non-porphyrin compounds are complexed to the large molecular weight asphaltenic component of the heavy crude petroleum and could be removed by solvent selection extraction techniques. If one uses sequential elution solvent chromatography to obtain fractions for asphaltenes based on functionality, then one can use the correlation diagram betweengll and All to determine more explicitly the nature of the square planar complexes responsible for the characteristic spectrum, particularly with regard to nitrogen and sulfur heteroatoms.
Acknowledgment. We thank King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia, for support of this work, and Mr. M. M. Saleem for recording the rigid limit spectrum. (23) Reynolds, J. G.;Biggs, W. R.; Fetzer, J. C.; Gallegos, E.; Fish, R. H.; Komlenic, J. J.; Wines, B. K. In Characterization of Heavy Crude Oils and Petroleum Residues; Symposium International, Lyons, France, June 25; Technip, Paris, 1984, pp 153-157.
