OPENING, DESULFURIZATION, AND HYDROGENATION OF THIOPHENE AT IRIDIUM - AN EXPERIMENTAL-STUDY IN A HOMOGENEOUS PHASE

The eta4-benzene complex [(triphos)Ir(C6H6)]BPh4 reacts with thiophene to give the iridathiabenzene complex [(triphos)Ir(eta2-C,S-C4H4S)]BPh4 (1) [triphos = MeC(CH2PPh2)3]. Compound 1 is selectively converted to the butadienethiolate complex [(triphos)Ir(eta3-SCH=CHCH=CH2)](2) by reaction with LiHBEt3 via the (thiapentadienyl)-hydride kinetic intermediate[(triphos)IrH(eta2-C,S-C4H4S)](3). Compound 2 is straightforwardly obtained by reaction of [(triphos)Ir(H)2(C2H5)] with thiophene. This reaction produces also the (2-thienyl)dihydride [(triphos)Ir(H)2-(2-C4H3S)] (4) through a parallel C-H bond activation path. The thienyl complex is not a kinetic intermediate for the opening reaction of thiophene. Compound 2 reacts with HBF4.OEt2 in the presence of CO yielding the thiocrotonaldehyde complex [(triphos)Ir(CO){eta4-S=CHCH--CH(Me)}]BF4 (5) and with PhSH to give the allylthioaldehyde derivative [(triphos)Ir(SPh)(eta4-S=CHCH2CH=CH2)] (6). The latter compound is stable in the solid state, but re-forms 2 and PhSH in solution unless an excess of thiophenol is added. Stirring 2 with a 5-fold excess of HCl produces the trichloride [(triphos)IrCl3], H2S, CH2=CHCH=CH2, CH2=CHCH2CH2SH, and CH3CH2-CH=CHSH. Methylation of 2 with Mel, followed by NaBPh4 addition, gives the methyl buta-1,3-dienyl thioether complex [(triphos)Ir{eta3-SMe)CH=CHCH=CH2}]BPh4-0.5EtOH (8) which has been characterized by a single-crystal X-ray analysis. Compound 8 crystallizes in the space group p2(1)/n. The coordination geometry around the iridium center is a distorted octahedron. The phosphorus atoms of triphos occupy three fac positions of the coordination polyhedron. The coordination of the metal fragment is completed by the thioether ligand which uses the sulfur atom and the two carbon atoms of the distal olefinic moiety to bind the metal. Compound 8 reacts with gaseous HCl converting to [(triphos)IrCl3] and evolving CH2=CHCH2CH2SMe and CH3CH2CH=CHSMe. Treatment of 8 with THF.BH3, followed by ROH addition (R = Me, Et), gives [(triphos)IrH(SMe)(ROH)]BPh4 (R = Me, 9; Et, 10) and buta-1,3-diene. All reactions have been carried out in tetrahydrofuran.