C-H bond cleavage in thiophenes by [P(CH2CH2PPh2)(3)Ru]. UV flash kinetic spectroscopy discloses the ruthenium-thiophene adduct which precedes C-H insertion

Photolysis of [(PP3)RuH2] (PP3 = P(CH2CH2PPh2)(3)) (1) in THF at 23 degrees C under an inert atmosphere (N-2, Ar, or He) in the presence of thiophene (T) or ethyl 2-thiophenecarboxylate (2-CO2EtT) gives the (hydride)2-thienyl complexes [(PP3)Ru(H)(2-Tyl)] (5) (Tyl = C4H3S) and [(PP3)Ru(H)(2-CO(2)EtTyl)] (7) (CO(2)EtTyl = C4H2(CO2Et)S), respectively. The C-H insertion products 5 and 7 are also selectively obtained by the thermal reaction of the Ru(0) dinitrogen complex [(PP3)Ru(N-2)] (4) in THF with T or 2-CO2EtT. Complexes 5 and 7 are both photochemically and thermally stable. Under comparable conditions, the photolysis of the Os derivative [(PP3)OsH2] in THF in the presence of T exclusively yields the C-H insertion product [(PP3)Os(H)(2-Tyl)]. Photolysis of 1 at 23 degrees C under nitrogen and in the presence of 2,5-dimethylthiophene (2,5-Me2T) gives the complex [(PP3)Ru(eta(1)-S-2,5-Me2T)] (8) containing an S-bound thiophene molecule. Complex 8 is also obtained by reaction of isolated 4 with 2,5-Me2(T). The Ru(0) transient, [(PP3)Ru], generated by laser flash photolysis in either cyclohexane or THF solution, has been detected and characterized by UV-vis spectroscopy. [(PP3)Ru] reacts with T (k(2) = (1.4 +/- 0.2) x 10(6) dm(3) mol(-1) s(-1)), yielding an adduct [(PP3)Ru(T)]. In cyclohexane, [(PP3)Ru(T)] decays to the C-H insertion product 5 with first-order kinetics (k = 20 +/- 1 s(-1) at 296 K). In THF, the 16 e(-) Ru(0) transient forms with THF a short-lived adduct, probably bound through oxygen, [(PP3)Ru(THF)], which reacts with T to give [(PP3)Ru(T)] (k(obs) = 680 s(-1), [T] = 0.3 mol dm(-3) under Ar). This T adduct decays to [(PP3)Ru(2-Tyl)H] with first-order kinetics (k = 18 +/- 1 s(-1)). Similar measurements with 2,5-Me2T yield a rate constant for [(PP3)Ru] + 2,5-Me2T in cyclohexane of (1.0 +/- 0.3) x 10(6) dm(3) mol(-1) s(-1). The resulting [(PP3)Ru(2,5-Me2T)] is kinetically stable. Flash photolysis of [(PP3)OsH2] in cyclohexane with added T yields rate constants for [(PP3)0s] + T and for isomerization of [(PP3)Os(T)] of (3.7 +/- 0.1) x 10(5) dm(3) mol(-1) s(-1) and 8 +/- 1 s(-1), respectively.