Photochemistry of M(PP3)H2 (M = Ru, Os; PP3 = P(CH2CH2PPh2)3): Preparative, NMR, and Time-Resolved Studies

Abstract

Photochemical reaction of Ru(PP3)H2 (PP3 = P(CH2CH2PPh2)3) in THF under a rigorously inert atmosphere yields the cyclometalated complex Ru[(Ph2PCH2CH2)2P(CH2CH2PPhC6H4)]H. The latter is converted back to Ru(PP3)H2 under H2 and reacts even with traces of N2 to yield Ru(PP3)(N2). The dinitrogen complex may be synthesized directly by a number of methods. NMR spectroscopy shows that photolysis of Ru(PP3)H2 under C2H4 and CO yields Ru(PP3)(C2H4) and Ru(PP3)(CO), respectively. Photolysis of Ru(PP3)H2 with HSiEt3 in THF yields Ru(PP3)(SiEt3)H, while photolysis in mixtures of THF and benzene at low temperature yields Ru(PP3)(Ph)H. The latter is also generated by reduction of Ru(PP3)Cl2 in the presence of benzene. Os(PP3)(Ph)H is formed either by photolysis of Os(PP3)H2 or by reduction of Os(PP3)Cl2 in the presence of benzene. Irradiation of Os(PP3)H2 in THF or THF/hexane mixtures initially yields the THF C−H activation product, Os(PP3)(2-C4H7O)H. This complex is also generated by reduction of Os(PP3)Cl2 with sodium naphthalenide under N2 in the presence of THF. Os(PP3)(2-C4H7O)H is converted to the cyclometalated complex, Os[(Ph2PCH2CH2)2P(CH2CH2PPhC6H4)]H, on irradiation in THF and to Os(PP3)(Ph)H on irradiation in benzene. Reaction of Os(PP3)H2 with CH3OTf (Tf = triflate) yields Os(PP3)(OTf)H, which is converted to the labile Os(PP3)(CH3)H on reaction with methyllithium. Laser flash photolysis of Ru(PP3)H2 in cyclohexane (laser wavelength 308 nm) yields transient Ru(PP3) with an absorption maximum at 395 nm. The transient reacts with H2, C6H6, HSiEt3, CO, N2, C2H4, and THF with little discrimination; the second-order rate constants for these reactions lie in the range 5 × 105−2 × 106 dm3 mol-1 s-1 at 295 K. Kinetic isotope effects have been determined for the reaction with benzene and THF, as 1.5 (0.2) and 1.1 (0.2), respectively. Activation parameters for reaction of Ru(PP3) are as follows: with HSiEt3 ΔH⧧ = 35 (2) kJ mol-1, ΔS⧧ = −18 (6) J K-1 mol-1; with C6H6 ΔH⧧ = 39 (4) kJ mol-1, ΔS⧧ ≈ 0 J K-1 mol-1. The reaction with THF yields a short-lived adduct, probably bound through oxygen, which is rapidly converted to the cyclometalated product. Laser flash photolysis of Os(PP3)H2 generates transient Os(PP3) (λmax = 390 nm). The transient kinetics of Os(PP3) are substantially different from its ruthenium analogue. It reacts with alkanes and shows different behavior toward THF but is unaffected by addition of H2. Rate constants in the range 6 × 104−6 × 105 dm3 mol-1 s-1 (295 K) are presented for reaction with C6H6, THF, HSiEt3, CO, C2H4, N2, and several alkanes. Kinetic isotope effects have been determined for the reactions with methylcyclohexane and benzene as 5.6 (1.5) and 0.6 (0.1), respectively. The rate constants for reaction with alkanes rise in the order, methylcyclohexane < pentane < heptane < methane. The rate constants for reaction with methane and benzene are insignificantly different. Following reaction of Os(PP3) with THF to form Os(PP3)(THF), C−H insertion occurs with a first-order rate constant of 4.2 (8) × 103 s-1 with kH/kD = 2.6 (0.4). The activation parameters for reaction of Os(PP3) with substrates are as follows: with pentane ΔH⧧ = 27 (1) kJ mol-1, ΔS⧧ = −59 (4) J K-1 mol-1; with HSiEt3 ΔH⧧ = 31 (5) kJ mol-1, ΔS⧧ = −27 (12) J K-1 mol-1; with C6H6 ΔH⧧ = 38 (3) kJ mol-1, ΔS⧧ = −7 (9) J K-1 mol-1.