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)IrHl fragment has been observed upon either thermolysis of (triphos)Ir(HMC2Hd in THF in the temperature range from 70 to 160 "C or dehydrohalogenation of (triphos)Ir- (H)2C1 with t-BuLi at room temperature [triphos = MeC(CH2PPhM C-H bond cleavage already occurs at 20 "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-DBTy1 I 2-DBTy1, while the thermodynamic stability is in the order 4-DBTyl > 3-DBTy1 > 2-DBTyl. Both C-H insertion and C-S insertion occur in the temperature range from 120 to 160 "C. Above the latter temperature, C-S insertion prevails over C-H insertion, and the complex ( triphos)IrH(q2-C,S-DBT) (5) is generated quantitatively. By reaction with Hz (THF, 170 "C, 30 atm of H2, 4 h), 5 is converted to a 31:69 mixture of the 2-phenylthiophenolate dihydride (triphos)Ir(H)2(SClzHd (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 H2.