HDS model systems. 2,3-Dihydrothiophene as an intermediate in the homogeneous hydrogenation of thiophene to tetrahydrothiophene at iridium

Article abstract of DOI:10.1021/om00014a050

Thermolysis of the bis(thiophene) (T) dihydride [(PPh₃)₂Ir(H)₂(η¹-SC ₄H₄)₂]PF₆ (1) in 1,2-dichloroethane at 80°C gives the thioallyl complex [(PPh₃)₂IrH(η⁴-SC₄H ₅)]PF₆ (2) by a stereospecific endo migration of one of the coordinated hydrides. Reaction of the thioallyl complex with H₂ under mild conditions yields the bis(tetrahydrothiophene) (THT) dihydride [(PPh₃)₂Ir(H)₂(η¹-SC ₄H₈)₂]PF₆ (3) plus the pentahydride [{(PPh₃)₂IrH}₂(μ²-H) ₃]PF₆ (4); if the hydrogenation is carried out in the presence of excess T, complex 3 is obtained exclusively. Interaction of 2 with CO at room temperature produces the two novel isomeric derivatives [(PPh₃)₂Ir(H)(CO)(η²-C ₅,S-SC₄H₅)]PF₆ (7) and [(PPh₃)₂Ir(H)(CO)(η²-C ₃,S-SC₄H₅)]PF₆ (8) in a 5:1 ratio, which do not interconvert from -50 to +80°C; longer reaction times (24 h, 25°C) or higher temperatures (3 h, 50°C) result in quantitative conversion of the complex to [(PPh₃)₂-Ir(CO)₃]PF₆ with concomitant liberation of 2,5-dihydrothiophene (2,5-DHT) and 2,3-DHT in a 5:1 ratio. Reaction of 2 with 2,5-DHT under H₂ at 80°C produces the new bis(2,5-DHT) dihydride complex [(PPh₃)₂Ir(H)₂(η¹-2,5-S-SC ₄H₆)₂]PF₆ (5), whereas the analogous reaction with 2,3-DHT results in hydrogenation to yield the bis(THT) dihydride derivative 3. Complex 5 and its 2,3-DHT analog [(PPh₃)₂Ir(H)₂(η¹-2,3-SC ₄H₆)₂]PF₆ (6) may also be prepared by displacement of coordinated acetone in [(PPh₃)₂Ir(H)₂(Me₂CO) ₂]PF₆ (5) by the appropriate ligand. Complex 5 does not react with H₂ at 80°C, whereas 6 is readily transformed into 3 under analogous conditions. Interaction of 2 with syngas results in the production of 7 and 8, together with free 2,5-DHT and 2,3-DHT, plus small amounts of [(PPh₃)₂Ir(H₂)(CO)(η¹-SC ₄H₈)]PF₆ (10) and [(PPh₃)₂Ir(H)₂(CO)₂]PF₆ (11). These combined results clearly establish that only the 2,3-DHT isomer is an intermediate in the homogeneous hydrogenation of T to THT (via the isolated thioallyl (2) and hydrido-η²-C,S (8) intermediates) with this Ir system, a result of relevance in connection with the HDS mechanisms. The THT complex 10 crystallizes in monoclinic space group P2₁ (No. 4) with a = 9.563(4) A?, b = 23.201(4) A?, c = 11.221(2) A?, β = 97.87° (2), Z = 2, and V = 2466.16 A?³. The structure of the complex cation consists of a distorted octahedron with mutually cis-hydrides, cis-THT, and CO and trans-PPh₃ ligands.