HYDRODESULFURIZATION (HDS) MODEL SYSTEMS - OPENING, HYDROGENATION, AND HYDRODESULFURIZATION OF DIBENZOTHIOPHENE (DBT) AT IRIDIUM - FIRST CASE OF CATALYTIC HDS OF DBT IN HOMOGENEOUS PHASE

The kinetic selectivity for C-H vs C-S activation of dibenzothiophene (DBT) by the [(triphos)IrH] fragment has been observed upon either thermolysis of(triphos)Ir(H)(2)(C2H5) in THF in the temperature range from 70 to 160 degrees C or dehydrohalogenation of (triphos)Ir(H)(2)Cl with t-BuLi at room temperature [triphos = MeC(CH(2)PPh(2))(3)]. C-H bond cleavage already occurs at 20 degrees C to give as many as three isomeric DBTyl complexes of the formula (triphos)Ir(H)(2)(DBTyl). The kinetic preference follows the order 3-DBTyl > 4-DBTyl greater than or equal to 2-DBTyl, while the thermodynamic stability is in the order 4-DBTyl > 3-DBTyl > 2-DBTyl. Both C-H insertion and C-S insertion occur in the temperature range from 120 to 160 degrees C. Above the latter temperature, C-S insertion prevails over C-H insertion, and the complex (triphos)IrH(eta(2)-C,S-DBT) (5) is generated quantitatively. By reaction with H-2 (THF, 170 degrees C, 30 atm of H-2, 4 h), 5 is converted to a 31:69 mixture of the 2-phenylthiophenolate dihydride (triphos)Ir(H)(2)(SC12H9) (7) and the trihydride (triphos)Ir(H)(3) (8) while free 2-phenylthiophenol, DBT, and biphenyl + H2S are evolved in a relative ratio of 48:42:10. In the presence of an excess of DBT, the reaction is catalytic and converts 10 mol of DBT/mol of 5 in 24 h to both hydrogenation (60%) and desulfurization (40%) products. A rationale of the catalysis cycle is discussed in the light of the results of a study involving the use of isolated compounds in a variety of independent reactions. In accord with previous studies, the thiolate complex 7 is proposed as the intermediate species that undergoes desulfurization by action of H-2.