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

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[(η⁴:η¹-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 (R_w = 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.