HDS MODEL SYSTEMS - 2,3-DIHYDROTHIOPHENE AS AN INTERMEDIATE IN THE HOMOGENEOUS HYDROGENATION OF THIOPHENE TO TETRAHYDROTHIOPHENE AT IRIDIUM

Thermolysis of the bis(thiophene) (T) dihydride L(PPh3)2Ir(H)2(eta1-SC4H4)2]PF6 (1) in 1,2-dichloroethane at 80-degrees-C gives the thioallyl complex [(PPh3)2IrH(eta4-SC4H5)]PF6 (2) by a stereospecific endo migration of one of the coordinated hydrides. Reaction of the thioallyl complex with H-2 under mild conditions yields the bis(tetrahydrothiophene) (THT) dihydride [(PPh3)2Ir(H)2(eta1-SC4H8)2]PF6 (3) plus the pentahydride [{(PPh3)2IrH}2(mu2-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)(eta2-C5,S-SC4H5)]PF6 (7) and [(PPh3)2Ir(H)(CO)(eta2-C3,S-SC4H5)]PF6 (8) in a 5:1 ratio, which do not interconvert from -50 to +80-degrees-C; longer reaction times (24 h, 25-degrees-C) or higher temperatures (3 h, 50-degrees-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 H-2 at 80-degrees-C produces the new bis(2,5-DHT) dihydride complex [(PPh3)2Ir(H)2(eta1-2,5-SC4H6)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(eta1-2,3-SC4H6)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 H-2 at 80-degrees-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(H)2(CO)(eta1-SC4H8)]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-eta2-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 P2(1) (No. 4) with a = 9.563(4) angstrom, b = 23.201(4) angstrom, c = 11.221(2) angstrom, beta = 97.870 (2), Z = 2, and V = 2466.16 A3. The structure of the complex cation consists of a distorted octahedron with mutually cis-hydrides, cis-THT, and CO and trans-PPh3 ligands.