EQUILIBRIA OF THE THIAMETALLACYCLES WITH TRIS(TRIETHYLPHOSPHINE)PLATINUM(0) AND DIBENZOTHIOPHENE, BENZOTHIOPHENE, OR THIOPHENE - THE HYDRODESULFURIZATION REACTION

The thiaplatinacycles, [(PtSC12H8)(PEt(3))(2)], 1, [(PtSC8H6)(PEt(3))(2)], 2, and [(PtSC4H4)(PEt(3))(2)], 3, in which Pt(PEt(3))(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(O) with the thiophene. The structure of complex [(PtSC8H6)(PEt(3))(2)] (2) was confirmed by an X-ray determination that showed a square-planar Pt(II) bound to two cis PEt(3) 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) Angstrom; Pt-C(4), 2.035(19) Angstrom; Pt-P(1), 2.301(5) Angstrom, trans to S(1); and Pt-P(2), 2.386(5) Angstrom, trans to C(4); P(1)-Pt-P(2), 97.6(2)degrees; S(1)-Pt-P(2), 87.0(2)degrees; S(1)-Pt-C(4), 88.6(6)degrees; P(1)-Pt-C(4), 86.8(6)degrees]. The adducts 1 and 3, derived from DBT and T, have been shown to have similar structures spectroscopically. The free thiophenes and Pt(PEt(3))(3) are regenerated on heating complexes 1-3 with Et(3)P; [(PtSCaHb)(PEt(3))(2)] + Et(3)P reversible arrow [Pt(Et(3)P)(3)] + SCaHb. K-eqm = [Pt(Et(3)P)(3)] [SCaHb]/[PtSCaHb)(PEt(3))(2)] [Et(3)P] and is 10 for 1 (DBT) and 3 (T) and 1 for 2 (BT) at 100 degrees 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(PEt(3))(2)Cl-2], 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 Et(3)SiH as reducer, the complex [Pt(SH)(H)(PEt(3))(2)] is identified among the products, together with [Pt(Et(3)Si)(H)(PEt(3))(2)]. The former is a coproduct of the desulfurization, while the latter arises from addition of Et(3)SiH to [Pt(PEt(3))(2)] (from reversal of the formation of [(PtSCaHb)(PEt(3))(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.