ARENE VS THIOPHENE REDUCTION IN THE SYSTEM (C6R6)RU(C4R4S)2+ AND THE PROTONATION OF ETA-4-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 [(eta6-C6Me6)Ru(eta5-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). H-1 and C-13 NMR spectroscopic studies (20 to -70-degrees-C for 2b) showed that these neutral complexes are described as (eta6-C6Me6)Ru(eta4-C4R4S). Electrochemical reduction of lc occurs in two one-electron steps at -442 and -607 mV vs Ag/AgCl. These potentials are close to those for [Ru(eta5-C4Me4S)2]2+ but 400 mV more anodic than those for [Ru(eta6-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 (eta-C6Me6)Ru{(eta4:eta1-C4Me4S)Mo(CO)5]. Complexes 2b, c were protonated with NH4PF6 affording [(eta6-C6Me6)Ru(eta4-C4HR4S)]PF6 (3b,c). While (eta6-C6Me6)Ru(eta4-C4H4S) (2a) appears to be unstable at room temperature, in situ protonation at -78-degrees-C afforded [(eta6-C6Me6)Ru(eta4-C4H5S)]PF6 (3a) which was isolable. H-1 NMR spectra of 3a-c, assigned using homonuclear decoupling and 2-dimensional H-1-H-1 COSY NMR methods, indicate that protonation occurs at the carbon alpha to sulfur. Treatment of 3a with ND4PF6 gave only one stereoisomer of [(eta6-C6Me6)Ru(eta4-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 [(eta6-C6Me6)Ru(eta4-C4R4S) to [(eta6-C6Me6)Ru(eta4-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. [(eta6-C6Me6)Ru(eta4-2,5-Me2C4H2S-2-H)]PF6 (3b) crystallizes in the space group P2(1)/c with a = 14.111 (4) angstrom, b = 9.381 (2) angstrom, c = 15.820 (3) angstrom, beta = 90.12 (2)-degrees. 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) angstrom, 0.1 angstrom longer than a normal C(sp3)-S bond. Protonation of (eta5-C5Me5)Rh(eta4-C4Me4S) with NH4PF6 afforded [(eta5-C5Me5)Rh(eta4-C4Me4S-2-H)]PF6, which is spectroscopically analogous to 3a-c.