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 de grees C under an inert atmosphere (N-2, Ar, or He) in the presence of thiophene (T) or ethyl 2-thiophenecarboxylate (2-CO2EtT) gives the (hy dride)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 photochemicall y 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-dimethylthi ophene (2,5-Me2T) gives the complex [(PP3)Ru(eta(1)-S-2,5-Me2T)] (8) c ontaining 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 so lution, 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)] deca ys 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 TH F 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] wit h 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 cyclohexan e 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 f or 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.