Reactivity of the Bis(dihydrogen) Complex [RuH₂(η²-H₂)₂(PCy₃)₂] toward S-Heteroaromatic Compounds. Catalytic Hydrogenation of Thiophene

Room-temperature stoichiometric reaction of the bis(dihydrogen) complex [RuH2(η2-H2)2(PCy3)2] (1) with thiophene leads to the formation of a new complex that has been isolated and characterized as an η4-thioallyl complex [RuH(η4(S,C)-SC4H5)(PCy3)2] (2). This complex easily regenerates 1 upon treatment with dihydrogen and can be successfully used as a catalyst precursor in thiophene hydrogenation to 2,3,4,5-tetrahydrothiophene (THT). The reaction of 1 with 2-acetylthiophene leads to a regioselective 1,5-C−S bond splitting with formal hydrogenation of two double CC bonds and coordination of a new 2-hexen-2-olato-3-thiolato ligand in an η2(O,S) mode to form [RuH2{η2(O,S)-C6H10OS}(PCy3)2] (3). The new complex 3 has been characterized by 1H, 31P, and 13C NMR studies including 1H DPFGSE TOCSY, 2D-1H−1H{31P} COSY DQF, and the correlated 13C−1H HMQC LR spectra. The solid state molecular structure of 3 has been unequivocally determined by single-crystal X-ray structure analysis. The bis(dihydrogen) complex 1 is an effective catalyst precursor for the homogeneous hydrogenation of thiophene (T) to 2,3,4,5-tetrahydrothiophene (THT), 2-methylthiophene (2-MeT) to 2-methyltetrahydrothiophene (2-MeTHT), 2-acetylthiophene (2-AcT) to 1-(2-thienyl)ethanol (1-(2-Tyl)E), 2-thiophenecarboxaldehyde (2-TA) to 2-thiophenemethanol (2-TM), and benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]thiophene (DHBT) under mild conditions (80 °C, 3 bar H2). Dibenzo[b,d]thiophene (DBT) is not reduced under these conditions due to the formation of the S-coordinated dihydrogen complex [RuH2(η2-H2){η1(S)-C12H8S}(PCy3)2] (4).