Arene vs Thiophene Reduction in the System (C6R6)Ru(C4R4S)2+ and the Protonation of η4-Thiophene Ligands

Shifang Luo, Thomas B. Rauchfuss, Scott R. Wilson

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Abstract

Reaction of [(C6Me6)RuCl2]2 with 4 equiv of AgOTf (OTf is OSO2CF3) followed by the addition of various thiophenes gave the salts [(η6-C6Me6)Ru(η5-C4R4S)](OTf)2 (1a-c) where C4R4S is thiophene (1a), 2,5-dimethylthiophene (1b), and 2,3,4,5-tetramethylthiophene (1c). Cobaltocene reduction of 1b,c produced yellow-orange (C6Me6)Ru(C4R4S) (2b,c). 1H and 13C NMR spectroscopic studies (20 to −70 °C for 2b) showed that these neutral complexes are described as (η6-C6Me6)Ru(η4-C4R4S). Electrochemical reduction of 1c occurs in two one-electron steps at −442 and −607 mV vs Ag/AgCl. These potentials are close to those for [Ru(η5-C4Me4S)2]2+ but 400 mV more anodic than those for [Ru(η6-C6Me6)2]2+, indicating that the thiophene ligand is a better electron acceptor than the arene. Treatment of 2c with Mo(CO)5(THF) afforded (η6-C6Me6)Ru[(η41-C4Me4S)Mo(CO)5]. Complexes 2b, c were protonated with NH4PF6 affording [(η6-C6Me6)Ru(η4-C4HR4S)]PF6 (3b,c). While (η6-C6Me6)Ru(η4-C4H4S) (2a) appears to be unstable at room temperature, in situ protonation at −78 °C afforded [(η6-C6Me6)Ru(η4-C4H5S)]PF6 (3a) which was isolable. 1H NMR spectra of 3a-c, assigned using homonuclear decoupling and 2-dimensional 1H–1H COSY NMR methods, indicate that protonation occurs at the carbon α to sulfur. Treatment of 3a with ND4PF6 gave only one stereoisomer of [(η6-C6Me6)Ru(η4-C4H4DS)]PF6 (3a-d1). The X-ray crystallographic study of 3b confirms that the protonation has created an sp3 carbon center with an equatorial hydrogen. The metal coordination sphere consists of an allyl, a thioether, and hexamethylbenzene. Thus protonation reengages the metal-sulfur bond in the conversion of [(η6-C6Me6)Ru(η4-C4R4S) to [(η6-C6Me6)Ru(η4-C4R4S-2-H)]+. It is proposed that the hydrogen arrives at the 2-carbon position via the intermediacy of a metal hydride, followed by an agostic complex. [(η6-C6Me6)Ru(η4-2,5-Me2C4H2S-2-H)]PF6 (3b) crystallizes in the space group P21/c with a = 14.111 (4) Å, b = 9.381 (2) Å, c= 15.820 (3) Å, β = 90.12 (2)°. Refinement of 3102 reflections converged to a final R of 0.039 (Rw = 0.049). The C(sp3)-S bond in 3b is 1.91 (2) Å, 0.1 Å longer than a normal C(sp3)-S bond. Protonation of (η5-C5Me5)Rh(η4-C4Me4S) with NH4PF6 afforded [(η5-C5Me5)Rh(η4-C4Me4S-2-H)]PF6, which is spectroscopically analogous to 3a-c.

Original languageEnglish (US)
Pages (from-to)8515-8520
Number of pages6
JournalJournal of the American Chemical Society
Volume114
Issue number22
DOIs
StatePublished - Oct 1 1992

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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