Equilibria of the thiametallacycles with tris(triethylphosphine)platinum(0) and dibenzothiophene, benzothiophene, or thiophene: The hydrodesulfurization reaction

Article abstract of DOI:10.1021/ja00113a007

The thiaplatinacycles, [(PtSC₁₂H₈)(PEt₃)₂], 1, [(PtSC₈H₆)(PEt₃)₂], 2, and [(PtSC₄H₄)(PEt₃)₂], 3, in which Pt(PEt₃)₂ has inserted into one C-S bond of dibenzothiophene (DBT), benzothiophene (BT), and thiophene (T), respectively, are formed by the reversible reaction of tris(triethylphosphine)platinum(0) with the thiophene. The structure of complex [(PtSC₈H₆)(PEt₃)₂] (2) was confirmed by an X-ray determination that showed a square-planar Pt(II) bound to two cis PEt₃ ligands, and to the S and the CH=, in a six-membered Pt-S-C-C-C-C ring [Pt-S(1), 2.315(5) ?; Pt-C(4), 2.035(19) ?; Pt-P(1), 2.301(5) ?, trans to S(1); and Pt-P(2), 2.386(5) ?, trans to C(4); P(1)-Pt-P(2), 97.6(2)°; S(1)-Pt-P(2), 87.0(2)°; S(1)-Pt-C(4), 88.6(6)°; P(1)-Pt-C(4), 86.8(6)°]. The adducts 1 and 3, derived from DBT and T, have been shown to have similar structures spectroscopically. The free thiophenes and Pt(PEt₃)₃ are regenerated on heating complexes 1-3 with Et₃P; [(PtSC_aH_b)(PEt₃)₂] + Et₃P ? [Pt(Et₃P)₃] + SC_aH_b. K_eqm = [Pt(Et₃P)3][SC_aH_b]/[PtSC_aH _b)(PEt₃)₂][Et₃P] and is 10 for 1 (DBT) and 3 (T) and 1 for 2 (BT) at 100 °C. The complexes 1-3 undergo a variety of cleavage reactions; thus, the Pt-C bond in 1 is cleaved by HCl to give 2-phenylthiophenol and [Pt(PEt₃)₂Cl₂], and all three complexes are degraded by hydride reagents to give the completely desulfurized hydrocarbons: biphenyl from 1, styrene and ethylbenzene from 2, and butadiene and butenes from 3. When the reactions are carried out with Et₃SiH as reducer, the complex [Pt(SH)(H)(PEt₃)₂] is identified among the products, together with [Pt(Et₃Si)(H)(PEt₃)₂], The former is a coproduct of the desulfurization, while the latter arises from addition of Et₃SiH to [Pt(PEt₃)₂] (from reversal of the formation of [(PtSC_aH_b))(PEt₃)₂]) and is accompanied by the formation of the free thiophene. Under comparable conditions, the amount of desulfurization decreases in the order BT complex 2 (73%) > DBT complex 1 (50%) > T complex 3 (ca. 4%). These hydride reactions show the novel feature that the C-S bond is cleaved as well as the Pt-C bond; possible mechanisms for the platinum and for the known nickel- and hydride-promoted hydrodesulfurization (HDS) reactions are discussed.