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Catalytic desulfurization reactions using triple bond dimolybdenum and ditungsten organometallic complexes
Journal
of Molecular Catalysis, 58 (1990) 355-361
365
CATALYTIC DESULFURI ZATION REACTIONS USING TRIPLE BOND DIMOLYRDENUM AND DITUNGSTEN ORGANOMETALLIC COMPLEXES WILHELM
KEIM, YUTONG
ZHU
Institute of Technical Chemistry and Petrochemistry Technical University Aachen, Worringer Weg 1, D-51OOAachen (F.R.G.) EBEBHARDT
HERDTWECK
and WOLFGANG
A. HERBMANN
Institute of Inorganic Chemistry, Technical University Munich, Lichtenbergstmsse 4, D-8046 Garching (F,R.G. ) Ubceived
March 22,1989;
accepted April 11,1989)
Desulfurization reactions using triply bonded dimolybdenum and ditungsten organometallic complexes are described. Esters of thioglycolic acid and related compounds could be desulfurized in high selectivities and conversions. A bimetallic mechanism, in which HSCH&OOR is added to the M.=M triple bond thus forming a six-membered ring intermediate, is proposed.
Introduction Desulfurization reactions are of great technical importance, and they comprise the largest industrial application of transition metal catalysis. Hydrodesulfurization (HDS) is practized on huge scale in the petroleum industry to improve refinery products 11,21. Bimetallic catalysts such as two industry ‘workhorses’ in hydroCo/Mo/Al,03 and Ni/Mo/Ala03 -the processing of petroleum-based feedstocks - are predominantly employed, but tungsten-containing catalysts are also frequently used. Moreover, desulfurization reactions are often applied in the synthesis of fine chemicals to abstract sulfur from selected hydrocarbon molecules. Despite a tremendous number of studies attempting to elucidate the detailed steps of desulfurization reactions, many key questions about the mechanism remain [31. Polynuclear transition metal complexes have been extensively investigated in the past few years as models for catalysis at metal surfaces [4,5]. Dinuclear complexes, the simplest models of clusters, are also very attractive [6]. For instance, reactions according to eqn. (1) could serve as models for addition of molecules to metal-metal multiple bonds thus, mimicking surfaces for chemisorption. A L,M=ML,
B
+ AB-+ L&I-ML,
0304~5102/90/$3.50
0 Elsevier Sequoia/printed
in The Netherlands
356
With this in mind, we have investigated desulfurization reactions using triply bonded dimolybdenum and ditungsten organometallic complexes in the pursuit of three goals: (a) synthesis of fine chemicals via desulfurization; (b) introduction of bi(multi)metallic complexes in catalysis; (c) further understanding of desulfurization based on reactions with organometallic model compounds.
Experimental
All procedures were carried out in an inert atmosphere. The complexes C~~MO.,JCO)~,Cp,Wz(CO), and their analogs were prepared by published procedures [ 7,81. (R =CH,, CH&H,;M =Mo or W) (1) The complex of (0.5 g) CpZM,(CO), (M= MO,W) and 0.5 mol HSCHzCOzR (R = CH3, C2H5) were heated for 20 h at 120 “C. Upon cooling dark crystals separated which were washed with n-pentane and recrystallized from CHzClz. Table 1 summarizes the spectroscopic data, elemental analyses and yields. The structure of complex 1 was also confirmed by X-ray analysis. The catalytic experiments were conducted in a 75 ml Hastelloy B2 steel autoclave. The experimental data are given in Table 2 and Fig. 1. The reaction products were analyzed by a Siemens Sichromat 3 (50 m Carbowax). LMCp(@)(@CH2C02R)12
Results
and discussion
A variety of sulfur-containing compounds, such as n-octylmercaptan, thiophenol and esters of thioglycolic acid, were reacted at 170 “C and 20 bar hydrogen pressure with catalytic amounts of Cp,Mo,(CO),. The results were surprising: whereas n-octylmercaptan and thiophenol hardly reacted, TABLE 1 Spectroscopic Complex
data, elemental analyses and yields of complexes [MCp(rS)(rSCH,CO,R)l.
M
R
1
MO
Me
3
MO
Et
4
W
Me
IR (cm-l)
1720,125O 810 1725, 1250 810 1750,125O 805
‘HNMR a-ppm
6.35, 3.91 2.10, 1.05 6.35, 3.40 2.21
MS M+
Analysis
(%)
Yield (%I
c (CaIC.)
H (cak.)
596
32.1 (32.2)
3.32 (3.36)
49
624
34.5 (34.6)
3.68 (3.64)
42
772
25.6 (24.9)
3.01 (2.62)
35
357
TABLE 2 Desulfurization of HS(CHz),COX compounds with CpzMoz(CO),” Reactions studied
Conv. (a)
HS-CHz-COOCzH, HS-CHz-COOCH, HS-CHs-C-CH, 8 HS--CHz-C-N(CH,), d HS-C,H,o-NH, HS-CH,--CH,-C-OCH, 8 HS-CHz-CHz-OH
Select. (I)
-
CHs-COOCzH, CHs-COOCHs CH,--C-CH,
97 53 96
100 91 78
-
A CH,-C--N(CH,),
100
100
-
a aniline CH,-CHz-C-OCH,
49 2
100 100
-
8 CHs-CHz-OH
5
so
a170oc; 20h; pressure Hz [CpMo~CO),l,:substrate = 1:40.
20 bar;
solvent
toluene : methanol
1:l
(20 ml);
HSCH2C02C2H5 could be desulfurized quantitatively to the corresponding ethyl acetate (selectivities >95%). These results prompted us to investigate a broader scope of thioglycolate-related compounds, elucidated in Table 2. Good results for desulfurization were obtained with HSCH,CO,R (R = CH3, C2H5), HSCH2COCH3, HSCH&ON(CH& and HSC6H4-o-NH2. Surprisingly, HS(CH2)&02CH3 and HS(CH,),OH hardly reacted. It is noteworthy that under identical conditions HSCH2COOCH3 gave poorer results than the corresponding ethyl ester. Comparable conversions and selectivities could be reached by changing the electron density of Figure 1 exhibits the results obtained by introducing Cp,Mo,(COL.
90
4 --
::y:::::: ::::>:f:: v.-.*.-.*
80 -70 -60 -50 -40 -30 -20 -, 0 -O-
:::::::::::
..*.*.*.*.* :.:.:.:.I: ..... ::::y$:: :::::::::::: ::y$:::: :.:.:.:.:.: ..... 5*.~.v.~. :::::$::::: :.:.>>>: :.:.:.:.:.:. ':y::::::: :.:.:.:.:.:. ...... ::y::::::: .:.:.:.:.:. .:.:.:.:.>: :,:.:.:*:i :.:.:.:.:.:. ...*.*.*.*. :';:i:# ::::y::;: .:.:.:.:.:. .:.:.:.:.:.: y::::::::: :::::::::::: ::::y::::: *.*.*.*.*.*. :::::::::::: y::::::::: ::y::$:: ::::::::::: :::::::::::: y::::::::: . ..I....... *.*.*.*.*.* :.:.:.:.:.:. ...... :::::::::::: Cp$b*lCO)~
lCpMe)*MoZICO)*
Cp*Mc$Co), cpzwztco~,
Fig. 1. Desulfurization of HSCHzCOzCH, Complex: HSCHzCOzCH, = 1: 40; 170 “C; 20 h; pressure H, = 20 bar; toluene (10 ml); methanol (10 ml).
Cp(C5H5), CpMe(C,H&H,) and Cp*(C&,(CH&) as ligands. With selectivities between 90-lOO%, the complex [Cp,*Mo(C0)J2 performed best. Instead of CpzMo,(CO), the corresponding tungsten complex also proved active (Fig. 1). However, selectivities and conversions are poorer. Even the use of Cp*W,(CO), did not improve the catalytic properties. During the catalytic runs, precipitations of dark solids were observed. These precipitates were catalytically inactive for further desulfurizations. Obviously ‘catalytic death’ in desulfurization occurred when these complexes were formed. To gain further insight into the nature of these precipitates, stoichiometric reactions between Cp,Mo,(CO), and HSCH.$0&H3 were carried out according to eqn. (2). Cp$vlo2K0)4
+ HSCH2C02CH3 -B - co
(2)
Complex 1 (see experimental part) could be identified spectroscopically as a ,&- and @CH&O&H3-bridged compound. Interestingly, all four CO ligands have been displaced in this reaction. Thiolate or sulfide ligands are widely used to bridge bimetallic complexes [6, g-131. Their structures are not always unequivocal from IR and NMR data. Therefore an X-ray analysis of complex 1 was carried out,* which confirmed the structure in eqn. (2). The molecule is located around a crystallographic center of symmetry (* in Fig. 2). Attributable to a molybdenum double bond [143, the ‘Mo-MO’ distance amounts to 259.0 (~1) pm. While the Cp-Mo-MO” angle computes to 180.01(l)“, the cyclopentadienyl rings lie in planes perpendicular to the Mo=Mo axis. and 4 complexes 3 The CP~MO~(~~S)~(CISCH~C~~C,H,), Cp,W2(~S)2(~SCH2COZCH3)2 could also be isolated in a stoichiometric reaction according to eqn. (2). The spectral identification of these complexes is given in the experimental part. Spectral comparison of complexes 1, 3 and 4 with those of the precipitates obtained during the corresponding catalytic runs (Table 2) showed agreement. *Small reddish brown columns; triclinic space group Pl (Int. Tab. No. 2); a =771.5(l), b = 804.7(l), c = 9960(2)pm, (Y= 84.32(2)“, @ = -69.45(2)“, y = 80.78(2)“; V = 572 x lo6 pm3; fi = 624.5 a.m.u.; 2 = 1; Fooo= 312; p(ca1c.j = 1.813 gem-s; CAD-4 (Enraf-Nonius, MoK, I= 71.073 pm, graphite monochromator); T = 24 + 1 “C; o-scans (2.0”~ 0 < 25.0); scan width (1.15 + 0.3Otan 8)” + 25% for background determination; t(max) = 90 s; 3989 measured reflections (+h, +K, +I); 1977 independent reflections (NO) with 1 > 0.0 and 127 parameters (NV) full matrix least-squares refined; R = Z(I IF,1- IF,1))@ IF,1= 0.027; R, = [Zw(lF,( - IFc()2/rw JFo1211’2= 0.032 GCF= [Pw(~F,I-_IF,I)~/(NO - NV)lm = 3.922 with w = l/a’(F,,. Structure solution according to Patterson method and difference Fourier techniques. No absorption correction (p = 14.4cm-‘). The hydrogen atoms are calculated but not refined. Anomalous dispersion taken into account; shift/err:
359
Fig. 2. ORTEP drawing of complex 1 (50% level, hydrogen6 omitted). Selected bond lengths (pm) end angles (“) in the MO, S-core; Mo-MO” 259.0(<1), Mo-Sl 249.0(l), MoSl” 247.4(l), Mo-S2 235.7(l), MoS2” 235.2(l); Mo-Sl-MO” 62.92(l), Mo-S2-MO” 66,74(l), Sl-MoSl” 117.08(l), S2-Mo-82” 113.26(l).
HS CH2CO$H3
-2 Scheme 1. Proposed reaction mechanism for desulfurization of methyl tbioglycolates.
Mechanistic
considerations
Scheme 1 elucidates a reaction proposal for the desulfurization of HSCH&OOCH3 with CpMoz(CO),. In this proposal HSCH&0&H3 reacts with the Mo=Mo triple bond yielding a six-membered ring complex of type 2, which is the result of an oxidative-type addition of the HS-moiety with one of the molybdenum atoms. The second molybdenum coordinates to the oxygen atom of methyl thioglycolate. Upon reaction of complex 2 with hydrogen, intermediates of type 5 (eqn. (3)) are formed which can eliminate HzS and methyl acetate, thus closing the catalytic cycle of Scheme 1. H
H
H
&I! / ly’ S ’
-
“CppMop”
+
H2S
+
CH3C02CH3
(3)
// CH2-C-0CH3
Catalytic deactivation (catalytic death) occurs when the sulfur atom is not removed via H,S but bridges the two MO atoms as in complex 1. The formation of @bridged complexes could be prevented by applying higher hydrogen pressure. Indeed, better turnover numbers were obtained with 40 bars hydrogen pressure [ 151. These results may indicate why hydrogen pressures up to 200 bar are needed in HDS reactions. The existence of two catalytic sites has often been proposed in the literature to account for hydrotreating reactions [16,171. The successful results in desulfurizing HS&H,-o-NH2 to aniline, HSCH,COCH, to acetone and HSCHzCON(CH& to dimethylacetylamine can also be explained with the proposed mechanism. Also the failure to desulfurize HS(CH&C0zCH3 is understandable, because a less stable seven-membered ring intermediate of type 2 must be formed.
Acknowledgement
We wish to thank the Bundesminister fur Forschung und Technologie for support of this work.
References H. Tops*, B. S. Clausen and Nan-Yu, Znd. Eng. Chem. F~ndum., 25 (1986) 25. 0. Weisser and S. Landa, Sulphide Catalysts, their Properties and Applications, Pergamon Press, Oxford, 1973. B. C. Gates, I. R. Katzer and G. C. A. Schuit, Chemistry ofCatalytic Processes, McGraw-Hill, New York 1979. E. L. Muetterties and M. J. Krause, Angew. Chem., Znt. Ed. En&., 22 (1983) 135.
361 5 B. C. Gates, L. Guczi and H. Kniizinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986. 6 M. H. Chisholm, Reactiuity of Metal-Metal Bonds, ACS Symposium Series 155, Washington, D.C., 1981. 7 M. D. Curtis, N. A. Fotinos, L. Messerle and A. P. Sattelberger, Zrwrg. CZrem.,22 (1988) 1559. 8 R. B. King and M. Z. Iquba, J. Organometall. Chem. 171 (1979) 53. 9 F. A. Cotton and R. A. Walton, Multiple Bonds Between Metal Atoms, Wiley, New York, 1982. 10 L. D. Tanner, R. C. Haltiwanger and M. R. Dubois, Znorg. C&m., 27 (1988) 1741. 11 J. Wachter, J. CoorcZ.Chem., 15 (1987) 219. 12 M. El Khahfa, M. Gueguen, R. Mercier, F. Y. Petillon, J.-Y. Saillard and J. Talarmin, Organometallics, 8 (1989) 140. 13 M. H. Chisholm, J. F. Coming and J. C. Huffman, Znorg. Chem., 21 (1982) 286. 14 M. R. Dubois, M. C. Van Derveer, D. L. Dubois, R. C. Haltiwanger and W. K. Miiller, J. Am. C&m. Sot., 102 (1980) 7456. 15 Yutong Zhu, Thesis, RWTH Aachen, 1989. 16 H. Kwart, J. Katzer and J. Horgan, J. Phys. Chem., 86 (1982, 2641. 17 G. Muralidhar, F. E. Massoth and J. Shabtai, Amer. Chem. Sot. Div. Pet. C&m. Prepr., 27 (1982) 722. 18 R. E. Schmidt, M. Birhahn, W. Massa, P. Kiprof and E. Herdtweck, STRUX-II, Programmsystem zur Verarbeitung von Roentgendaten, Universitat Marburg (1980) und TU Miichen (1985,1987), F.R.G. Further details ofthe crystal structure determination can be obtained from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen 2 (F.R.G.), by quoting the depository number CSD 000000, the name of the authors and the journal citation.
of Molecular Catalysis, 58 (1990) 355-361
365
CATALYTIC DESULFURI ZATION REACTIONS USING TRIPLE BOND DIMOLYRDENUM AND DITUNGSTEN ORGANOMETALLIC COMPLEXES WILHELM
KEIM, YUTONG
ZHU
Institute of Technical Chemistry and Petrochemistry Technical University Aachen, Worringer Weg 1, D-51OOAachen (F.R.G.) EBEBHARDT
HERDTWECK
and WOLFGANG
A. HERBMANN
Institute of Inorganic Chemistry, Technical University Munich, Lichtenbergstmsse 4, D-8046 Garching (F,R.G. ) Ubceived
March 22,1989;
accepted April 11,1989)
Desulfurization reactions using triply bonded dimolybdenum and ditungsten organometallic complexes are described. Esters of thioglycolic acid and related compounds could be desulfurized in high selectivities and conversions. A bimetallic mechanism, in which HSCH&OOR is added to the M.=M triple bond thus forming a six-membered ring intermediate, is proposed.
Introduction Desulfurization reactions are of great technical importance, and they comprise the largest industrial application of transition metal catalysis. Hydrodesulfurization (HDS) is practized on huge scale in the petroleum industry to improve refinery products 11,21. Bimetallic catalysts such as two industry ‘workhorses’ in hydroCo/Mo/Al,03 and Ni/Mo/Ala03 -the processing of petroleum-based feedstocks - are predominantly employed, but tungsten-containing catalysts are also frequently used. Moreover, desulfurization reactions are often applied in the synthesis of fine chemicals to abstract sulfur from selected hydrocarbon molecules. Despite a tremendous number of studies attempting to elucidate the detailed steps of desulfurization reactions, many key questions about the mechanism remain [31. Polynuclear transition metal complexes have been extensively investigated in the past few years as models for catalysis at metal surfaces [4,5]. Dinuclear complexes, the simplest models of clusters, are also very attractive [6]. For instance, reactions according to eqn. (1) could serve as models for addition of molecules to metal-metal multiple bonds thus, mimicking surfaces for chemisorption. A L,M=ML,
B
+ AB-+ L&I-ML,
0304~5102/90/$3.50
0 Elsevier Sequoia/printed
in The Netherlands
356
With this in mind, we have investigated desulfurization reactions using triply bonded dimolybdenum and ditungsten organometallic complexes in the pursuit of three goals: (a) synthesis of fine chemicals via desulfurization; (b) introduction of bi(multi)metallic complexes in catalysis; (c) further understanding of desulfurization based on reactions with organometallic model compounds.
Experimental
All procedures were carried out in an inert atmosphere. The complexes C~~MO.,JCO)~,Cp,Wz(CO), and their analogs were prepared by published procedures [ 7,81. (R =CH,, CH&H,;M =Mo or W) (1) The complex of (0.5 g) CpZM,(CO), (M= MO,W) and 0.5 mol HSCHzCOzR (R = CH3, C2H5) were heated for 20 h at 120 “C. Upon cooling dark crystals separated which were washed with n-pentane and recrystallized from CHzClz. Table 1 summarizes the spectroscopic data, elemental analyses and yields. The structure of complex 1 was also confirmed by X-ray analysis. The catalytic experiments were conducted in a 75 ml Hastelloy B2 steel autoclave. The experimental data are given in Table 2 and Fig. 1. The reaction products were analyzed by a Siemens Sichromat 3 (50 m Carbowax). LMCp(@)(@CH2C02R)12
Results
and discussion
A variety of sulfur-containing compounds, such as n-octylmercaptan, thiophenol and esters of thioglycolic acid, were reacted at 170 “C and 20 bar hydrogen pressure with catalytic amounts of Cp,Mo,(CO),. The results were surprising: whereas n-octylmercaptan and thiophenol hardly reacted, TABLE 1 Spectroscopic Complex
data, elemental analyses and yields of complexes [MCp(rS)(rSCH,CO,R)l.
M
R
1
MO
Me
3
MO
Et
4
W
Me
IR (cm-l)
1720,125O 810 1725, 1250 810 1750,125O 805
‘HNMR a-ppm
6.35, 3.91 2.10, 1.05 6.35, 3.40 2.21
MS M+
Analysis
(%)
Yield (%I
c (CaIC.)
H (cak.)
596
32.1 (32.2)
3.32 (3.36)
49
624
34.5 (34.6)
3.68 (3.64)
42
772
25.6 (24.9)
3.01 (2.62)
35
357
TABLE 2 Desulfurization of HS(CHz),COX compounds with CpzMoz(CO),” Reactions studied
Conv. (a)
HS-CHz-COOCzH, HS-CHz-COOCH, HS-CHs-C-CH, 8 HS--CHz-C-N(CH,), d HS-C,H,o-NH, HS-CH,--CH,-C-OCH, 8 HS-CHz-CHz-OH
Select. (I)
-
CHs-COOCzH, CHs-COOCHs CH,--C-CH,
97 53 96
100 91 78
-
A CH,-C--N(CH,),
100
100
-
a aniline CH,-CHz-C-OCH,
49 2
100 100
-
8 CHs-CHz-OH
5
so
a170oc; 20h; pressure Hz [CpMo~CO),l,:substrate = 1:40.
20 bar;
solvent
toluene : methanol
1:l
(20 ml);
HSCH2C02C2H5 could be desulfurized quantitatively to the corresponding ethyl acetate (selectivities >95%). These results prompted us to investigate a broader scope of thioglycolate-related compounds, elucidated in Table 2. Good results for desulfurization were obtained with HSCH,CO,R (R = CH3, C2H5), HSCH2COCH3, HSCH&ON(CH& and HSC6H4-o-NH2. Surprisingly, HS(CH2)&02CH3 and HS(CH,),OH hardly reacted. It is noteworthy that under identical conditions HSCH2COOCH3 gave poorer results than the corresponding ethyl ester. Comparable conversions and selectivities could be reached by changing the electron density of Figure 1 exhibits the results obtained by introducing Cp,Mo,(COL.
90
4 --
::y:::::: ::::>:f:: v.-.*.-.*
80 -70 -60 -50 -40 -30 -20 -, 0 -O-
:::::::::::
..*.*.*.*.* :.:.:.:.I: ..... ::::y$:: :::::::::::: ::y$:::: :.:.:.:.:.: ..... 5*.~.v.~. :::::$::::: :.:.>>>: :.:.:.:.:.:. ':y::::::: :.:.:.:.:.:. ...... ::y::::::: .:.:.:.:.:. .:.:.:.:.>: :,:.:.:*:i :.:.:.:.:.:. ...*.*.*.*. :';:i:# ::::y::;: .:.:.:.:.:. .:.:.:.:.:.: y::::::::: :::::::::::: ::::y::::: *.*.*.*.*.*. :::::::::::: y::::::::: ::y::$:: ::::::::::: :::::::::::: y::::::::: . ..I....... *.*.*.*.*.* :.:.:.:.:.:. ...... :::::::::::: Cp$b*lCO)~
lCpMe)*MoZICO)*
Cp*Mc$Co), cpzwztco~,
Fig. 1. Desulfurization of HSCHzCOzCH, Complex: HSCHzCOzCH, = 1: 40; 170 “C; 20 h; pressure H, = 20 bar; toluene (10 ml); methanol (10 ml).
Cp(C5H5), CpMe(C,H&H,) and Cp*(C&,(CH&) as ligands. With selectivities between 90-lOO%, the complex [Cp,*Mo(C0)J2 performed best. Instead of CpzMo,(CO), the corresponding tungsten complex also proved active (Fig. 1). However, selectivities and conversions are poorer. Even the use of Cp*W,(CO), did not improve the catalytic properties. During the catalytic runs, precipitations of dark solids were observed. These precipitates were catalytically inactive for further desulfurizations. Obviously ‘catalytic death’ in desulfurization occurred when these complexes were formed. To gain further insight into the nature of these precipitates, stoichiometric reactions between Cp,Mo,(CO), and HSCH.$0&H3 were carried out according to eqn. (2). Cp$vlo2K0)4
+ HSCH2C02CH3 -B - co
(2)
Complex 1 (see experimental part) could be identified spectroscopically as a ,&- and @CH&O&H3-bridged compound. Interestingly, all four CO ligands have been displaced in this reaction. Thiolate or sulfide ligands are widely used to bridge bimetallic complexes [6, g-131. Their structures are not always unequivocal from IR and NMR data. Therefore an X-ray analysis of complex 1 was carried out,* which confirmed the structure in eqn. (2). The molecule is located around a crystallographic center of symmetry (* in Fig. 2). Attributable to a molybdenum double bond [143, the ‘Mo-MO’ distance amounts to 259.0 (~1) pm. While the Cp-Mo-MO” angle computes to 180.01(l)“, the cyclopentadienyl rings lie in planes perpendicular to the Mo=Mo axis. and 4 complexes 3 The CP~MO~(~~S)~(CISCH~C~~C,H,), Cp,W2(~S)2(~SCH2COZCH3)2 could also be isolated in a stoichiometric reaction according to eqn. (2). The spectral identification of these complexes is given in the experimental part. Spectral comparison of complexes 1, 3 and 4 with those of the precipitates obtained during the corresponding catalytic runs (Table 2) showed agreement. *Small reddish brown columns; triclinic space group Pl (Int. Tab. No. 2); a =771.5(l), b = 804.7(l), c = 9960(2)pm, (Y= 84.32(2)“, @ = -69.45(2)“, y = 80.78(2)“; V = 572 x lo6 pm3; fi = 624.5 a.m.u.; 2 = 1; Fooo= 312; p(ca1c.j = 1.813 gem-s; CAD-4 (Enraf-Nonius, MoK, I= 71.073 pm, graphite monochromator); T = 24 + 1 “C; o-scans (2.0”~ 0 < 25.0); scan width (1.15 + 0.3Otan 8)” + 25% for background determination; t(max) = 90 s; 3989 measured reflections (+h, +K, +I); 1977 independent reflections (NO) with 1 > 0.0 and 127 parameters (NV) full matrix least-squares refined; R = Z(I IF,1- IF,1))@ IF,1= 0.027; R, = [Zw(lF,( - IFc()2/rw JFo1211’2= 0.032 GCF= [Pw(~F,I-_IF,I)~/(NO - NV)lm = 3.922 with w = l/a’(F,,. Structure solution according to Patterson method and difference Fourier techniques. No absorption correction (p = 14.4cm-‘). The hydrogen atoms are calculated but not refined. Anomalous dispersion taken into account; shift/err:
359
Fig. 2. ORTEP drawing of complex 1 (50% level, hydrogen6 omitted). Selected bond lengths (pm) end angles (“) in the MO, S-core; Mo-MO” 259.0(<1), Mo-Sl 249.0(l), MoSl” 247.4(l), Mo-S2 235.7(l), MoS2” 235.2(l); Mo-Sl-MO” 62.92(l), Mo-S2-MO” 66,74(l), Sl-MoSl” 117.08(l), S2-Mo-82” 113.26(l).
HS CH2CO$H3
-2 Scheme 1. Proposed reaction mechanism for desulfurization of methyl tbioglycolates.
Mechanistic
considerations
Scheme 1 elucidates a reaction proposal for the desulfurization of HSCH&OOCH3 with CpMoz(CO),. In this proposal HSCH&0&H3 reacts with the Mo=Mo triple bond yielding a six-membered ring complex of type 2, which is the result of an oxidative-type addition of the HS-moiety with one of the molybdenum atoms. The second molybdenum coordinates to the oxygen atom of methyl thioglycolate. Upon reaction of complex 2 with hydrogen, intermediates of type 5 (eqn. (3)) are formed which can eliminate HzS and methyl acetate, thus closing the catalytic cycle of Scheme 1. H
H
H
&I! / ly’ S ’
-
“CppMop”
+
H2S
+
CH3C02CH3
(3)
// CH2-C-0CH3
Catalytic deactivation (catalytic death) occurs when the sulfur atom is not removed via H,S but bridges the two MO atoms as in complex 1. The formation of @bridged complexes could be prevented by applying higher hydrogen pressure. Indeed, better turnover numbers were obtained with 40 bars hydrogen pressure [ 151. These results may indicate why hydrogen pressures up to 200 bar are needed in HDS reactions. The existence of two catalytic sites has often been proposed in the literature to account for hydrotreating reactions [16,171. The successful results in desulfurizing HS&H,-o-NH2 to aniline, HSCH,COCH, to acetone and HSCHzCON(CH& to dimethylacetylamine can also be explained with the proposed mechanism. Also the failure to desulfurize HS(CH&C0zCH3 is understandable, because a less stable seven-membered ring intermediate of type 2 must be formed.
Acknowledgement
We wish to thank the Bundesminister fur Forschung und Technologie for support of this work.
References H. Tops*, B. S. Clausen and Nan-Yu, Znd. Eng. Chem. F~ndum., 25 (1986) 25. 0. Weisser and S. Landa, Sulphide Catalysts, their Properties and Applications, Pergamon Press, Oxford, 1973. B. C. Gates, I. R. Katzer and G. C. A. Schuit, Chemistry ofCatalytic Processes, McGraw-Hill, New York 1979. E. L. Muetterties and M. J. Krause, Angew. Chem., Znt. Ed. En&., 22 (1983) 135.
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