C−H Bond Cleavage in Thiophenes by [P(CH2CH2PPh2)3Ru]. UV Flash Kinetic Spectroscopy Discloses the Ruthenium−Thiophene Adduct Which Precedes C−H Insertion

Photolysis of [(PP 3 )RuH 2 ] (PP 3 = P(CH 2 CH 2 PPh 2 ) 3 ) ( 1 ) in THF at 23 °C under an inert atmosphere (N 2, Ar, or He) in the presence of thiophene ( T ) or ethyl 2-thiophenecarboxylate (2-CO 2 Et T ) gives the (hydride)2-thienyl complexes [(PP 3 )Ru(H)(2-Tyl)] ( 5 ) (Tyl = C 4 H 3 S) and [(PP 3 )Ru(H)(2-CO 2 EtTyl)] ( 7 ) (CO 2 EtTyl = C 4 H 2 (CO 2 Et)S), respectively. The C−H insertion products 5 and 7 are also selectively obtained by the thermal reaction of the Ru(0) dinitrogen complex [(PP 3 )Ru(N 2 )] ( 4 ) in THF with T or 2-CO 2 Et T . Complexes 5 and 7 are both photochemically and thermally stable. Under comparable conditions, the photolysis of the Os derivative [(PP 3 )OsH 2 ] in THF in the presence of T exclusively yields the C−H insertion product [(PP 3 )Os(H)(2-Tyl)]. Photolysis of 1 at 23 °C under nitrogen and in the presence of 2,5-dimethylthiophene (2,5-Me 2 T ) gives the complex [(PP 3 )Ru(η 1 - S -2,5-Me 2 T )] ( 8 ) containing an S -bound thiophene molecule. Complex 8 is also obtained by reaction of isolated 4 with 2,5-Me 2 T . The Ru(0) transient, [(PP 3 )Ru], generated by laser flash photolysis in either cyclohexane or THF solution, has been detected and characterized by UV−vis spectroscopy. [(PP 3 )Ru] reacts with T ( k 2 = (1.4 ± 0.2) × 10 6 dm 3 mol - 1 s - 1 ), yielding an adduct [(PP 3 )Ru( T )]. In cyclohexane, [(PP 3 )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, [(PP 3 )Ru(THF)], which reacts with T to give [(PP 3 )Ru( T )] ( k obs = 680 s - 1, [ T ] = 0.3 mol dm - 3 under Ar). This T adduct decays to [(PP 3 )Ru(2-Tyl)H] with first-order kinetics ( k = 18 ± 1 s - 1 ). Similar measurements with 2,5-Me 2 T yield a rate constant for [(PP 3 )Ru] + 2,5-Me 2 T in cyclohexane of (1.0 ± 0.3) × 10 6 dm 3 mol - 1 s - 1 . The resulting [(PP 3 )Ru(2,5-Me 2 T )] is kinetically stable. Flash photolysis of [(PP 3 )OsH 2 ] in cyclohexane with added T yields rate constants for [(PP 3 )Os] + T and for isomerization of [(PP 3 )Os( T )] of (3.7 ± 0.1) × 10 5 dm 3 mol - 1 s - 1 and 8 ± 1 s - 1, respectively.