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Hydrodesulfurization of Thiophene and Benzothiophene to Butane and Ethylbenzene by a Homogeneous Iridium Complex

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Hydrodesulfurization of Thiophene and Benzothiophene to Butane and Ethylbenzene by a Homogeneous Iridium Complex 1912 Organometallics 1997, 16, 1912-1919 Hydrodesulfurization of Thiophene and Benzothiophene to Butane and Ethylbenzene by a Homogeneous Iridium Complex David A. Vicic and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York 14627 Received December 3, 1996X 5 Reaction of the dimer [Cp*IrHCl]2 (Cp* ) η -C5Me5) in benzene solution with either thiophene or benzothiophene at 90 °C in the presence of H2 gives the hydrogenolysis products [Cp*IrCl]2(µ-H)(µ-SC4H9)(1) and [Cp*IrCl]2(µ-H)[µ-S(C6H4)CH2CH3](2), respectively, in high yields. Upon further thermolysis under H2, the completely desulfurized products, butane and ethylbenzene, can be made. Complexes 1 and 2 were characterized by single-crystal X-ray diffraction. In the absence of H2, reaction of [Cp*Ir HCl]2 with thiophene gives an additional trinuclear product [Cp*IrCl]3(H)(SC4H6)(3), which was also structurally character- ized. Introduction Previously in our lab the dimeric species [Cp*IrH3]2 was observed to cleave both C-S bonds of thiophene in The hydroprocessing of crude oil is one of the largest the presence of a hydrogen acceptor (eq 1).4a Reaction scale chemical processes carried out in industry today. In this process, heteroatom impurities such as thio- phenes, mercaptans, and quinolines are removed, mak- ing the oil amenable to further refining. Removal of the sulfur compounds, in particular, decreases the contribu- tions to acid rain production upon fuel combustion and is also valuable in preventing catalyst poisoning both in the refinement process and in automobiles.1 Because of both the environmental and economic rewards that can be achieved through the hydrodesulfurization (HDS) of this butadiene complex with hydrogen yielded the process, recent research has focused on trying to better desulfurized product n-butane. The ability of the dimer- understand the mechanism of HDS in hopes of design- ic species to completely remove a sulfur atom from ing more efficient catalysts.2 thiophene led to our interest in finding a similar The difficulty in modeling HDS in solution lies greatly compound which did not require a hydrogen acceptor in the ability of a given system to cleave both carbon- to generate the active species and could also desulfurize sulfur bonds of various thiophenes. Mimicking the the more refractory thiophenes such as benzothiophene. industrial process would require that both carbon- This led us to explore the reactivity of the bis(µ-hydrido)- sulfur bonds be cleaved in a dihydrogen environment, bis[chloro(pentamethylcyclopentadienyl)]iridium com- affording the hydrogenated organic products plus ad- plex, first synthesized by Gill and Maitlis,8 toward sorbed sulfur or H2S. While many ring-opening oxida- thiophenic molecules. tive additions of one of the C-S bonds of thiophene have been reported,3 fewer examples of complete desulfur- ization to butanes, butenes, or metal alkyls have been Results and Discussion reported.4 In addition, there have been few reports of hydrodesulfurization of benzothiophene to ethylbenzene Heating a solution of [Cp*IrHCl]2 in benzene (90 °C, by well-defined homogeneous model systems.5-7 In this 3 h) with an excess of thiophene in the presence of 1 work is presented a homogeneous model system which atm of H2 afforded [Cp*IrCl]2(µ-H)(µ-SC4H9)(1) (eq 2) can desulfurize thiophene and benzothiophene to butane and ethylbenzene, respectively, in the presence of H2. (4) Most examples come from polynuclear organometallic systems: (a) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1994, 116, 198-203. X Abstract published in Advance ACS Abstracts, April 1, 1997. (b) Arce, A. J.; Arrojo, P.; Deeming, A. J.; De Sanctis, Y. J. Chem. Soc., (1) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Dalton Trans. 1992, 2423-2424. (c) Riaz, U.; Curnow, O. J.; Curtis, Catalysis; Springer-Verlag: Berlin, 1996. M. D. J. Am. Chem. Soc. 1991, 113, 1416-1417. (d) Chen, J.; Daniels, (2) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387-394. Rauchfuss, L. M.; Angelici, R. J. J. Am. Chem. Soc. 1991, 113, 2544-2552. T. B. Prog. Inorg. Chem. 1991, 39, 259-329. Bianchini, C.; Meli, A. J. (5) Ogilvy, A. E.; Draganjac, M.; Rauchfuss, T. B.; Wilson, S. R. Chem. Soc., Dalton Trans. 1996, 801-814. Organometallics 1988, 7, 1171-1177. (3) See, for example: (a) Selnau, H. E.; Merola, J. S. Organometallics (6) Desulfurization of benzothiophene to ethylbenzene was reported 1993, 12, 1583-1591. (b) Garcia, J. J.; Mann, B. E.; Adams, H.; Bailey, to occur by Bianchini et al., but was found to be due to the presence of N. A.; Maitlis, P. M. J. Am. Chem. Soc. 1995, 117, 2179-2186. (c) Buys, heterogeneous particles. See: Bianchini, C.; Herrera, V.; Jimenez, M. I. E.; Field, L. D.; Hambley, R. W.; McQueen, A. E. D. J. Chem. Soc., V.; Meli, A.; Sa´nchez-Delgado, R.; Vizza, F. J. Am. Chem. Soc. 1995, Chem. Commun. 1994, 557-558. (d) Paneque, M.; Taboada, S.; 117, 8567-8575. Carmona, E. Organometallics 1996, 15, 2678-2679. (e) Jones, W. D.; (7) Bianchini, C.; Jime´nez, M. V.; Mealli, C.; Meli, A.; Moneti, S.; Dong, L. J. Am. Chem. Soc. 1991, 113, 559-564. (f) Chen, J.; Angelici, Patinec, V.; Vizza, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1706- R. J. J. Am. Chem. Soc. 1990, 112, 199-204. (g) Bianchini, C.; Meli, 1707. A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Herrer, V.; Sanchez-Delgado, (8) Gill, D. S.; Maitlis, P. M. J. Organomet. Chem. 1975, 87, 359- R. J. Am. Chem. Soc. 1993, 115, 2731-2742. 364. S0276-7333(96)01010-2 CCC: $14.00 © 1997 American Chemical Society Hydrodesulfurization of Thiophene and Benzothiophene Organometallics, Vol. 16, No. 9, 1997 1913 Figure 1. ORTEP drawing of [Cp*IrCl]2(µ-H)(µ-SC4H9). Ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted from the Cp* ligand for clarity. Figure 2. ORTEP drawing of [Cp*IrCl]2(µ-H)(µ-S(C6H4)- Table 1. Selected Bond Lengths (Å) and Angles C2H5). Ellipsoids are shown at the 30% probability level. (deg) for 1 Hydrogen atoms on the ligands have been omitted for S(1)-C(21) 1.799(12) Ir(2)-Cl(2) 2.404(3) clarity. S(1)-Ir(2) 2.335(2) C(21)-C(22) 1.53(2) S(1)-Ir(1) 2.337(2) C(22)-C(23) 1.53(2) There are two singlets representing the magnetically Ir(1)-Cl(1) 2.419(2) C(23)-C(24) 1.55(2) inequivalent Cp* methyl groups. The R-CH hydrogen Ir(1)-Ir(2) 2.9041(4) 2 atoms of the butanethiolate moiety are diastereotopic, C(21)-S(1)-Ir(2) 112.5(4) S(1)-Ir(2)-Ir(1) 51.60(6) appearing as multiplets at δ 3.36 and 2.05. A COSY C(21)-S(1)-Ir(1) 111.3(4) Cl(2)-Ir(2)-Ir(1) 90.11(7) spectrum shows coupling of the R-protons to broad Ir(2)-S(1)-Ir(1) 76.86(7) C(22)-C(21)-S(1) 110.0(8) S(1)-Ir(1)-Cl(1) 84.89(9) C(23)-C(22)-C(21) 112.7(10) multiplets in the region δ 1.5-1.9, which in turn couple S(1)-Ir(1)-Ir(2) 51.54(6) C(22)-C(23)-C(24) 111.0(12) to the terminal methyl group at δ 1.02. A hydride Cl(1)-Ir(1)-Ir(2) 90.51(6) resonance is seen in the 1H NMR spectrum at δ -15.54 in C6D6, similar to that seen in the spectrum of the 9 as a red crystalline solid in 80% isolated yield after isoelectronic complex [{Cp*Ir}2(µ-H)(µ-Cl)Cl2], which purification. Under relatively mild conditions the has a hydride resonance at -13.73 in C6D6. Further- more, the absence of an IR band in the 1800-2100 cm-1 range is consistent with a bridging hydride.8 The complex [Cp*IrHCl]2 was found to undergo a similar reaction with benzothiophene and H2 (90 °C, 2 h), yielding the red crystalline species [Cp*IrCl]2(µ-H)- (µ-S(C6H4)CH2CH3)(2) in 85% isolated yield after purification (eq 3). Selective cleavage of the vinylic C-S thiophene molecule was ring opened and completely hydrogenated, having been transformed into a bridging n-butanethiolate ligand. No intermediates were ob- served while monitoring this reaction by 1H NMR spectroscopy. As is evident from the crystal structure of 1 (Figure 1), the Cp* and chlorine ligands are trans and the sulfur atom is pyramidal, rendering the complex with C 1 bond is observed. Again, no intermediates were ob- symmetry. Crystallographic data for 1 are listed in served while monitoring the reaction by 1H NMR Tables 1 and 2. Carbon-carbon bond lengths of the spectroscopy. The X-ray crystal structure is shown in butanethiolate moiety confirm that hydrogenation of Figure 2, and the corresponding crystallographic data thiophene is complete, with distances of 1.53(2), 1.53- are given in Tables 2 and 3. The bridging hydride (2), and 1.55(2) Å for the C -C ,C -C , and C - 21 22 22 23 23 ligand was located and refined. The C symmetry of C bonds, respectively, thus establishing their single- 1 24 complex 2 due to sulfur pyramidalization is similar to bond character. The bridging hydride ligand was located and refined. The solid state structure is con- (9) White, C.; Oliver, A. J.; Maitlis, P. M. J. Chem. Soc., Dalton sistent with the 1H NMR data of the complex in solution.
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