A Kinetic Study on the Substitution for Acetonitrile at the trans-to-μ-Oxido Sites in a Bis(μ-acetato)(μ-oxido) diruthenium(III) Dipositive Complex: Dissociative- Associative Transition of the Activation Mode for the Substitution of Pyridine Derivatives

摘要

A new diruthenium(III) complex [Ru_2~(III) (μ-O)(μ-CH_3CO_2)_2- (bpy)_2(CH_3CN)_2](PF_6)_2 [1(PF_6)_2, bpy = 2,2'-bipyridine] has been synthesized, and the dynamic behaviour of CH_3CN at the trans-to-μ-oxido sites were investigated by ~1H NMR and UV/Vis spectroscopy at 0-60 ℃ under 0.101-147 MPa in CH_3CN. The solvent exchange in CD_3CN at 5 ℃ gave the rate constant k = 2.43×10~(-3) s~(-1) and the activation parameters ΔH~? = 124 kJmol~(-1) and ΔS~? = +150 JK~(-1)mol~(-1), which point to a dissociative mechanism (I_d or D). The ligand substitution of pyridine (py) in CD_3CN at 30 ℃, yielding [Ru_2~(III) (μ-O)(μ-CH_3CO_2)_2(bpy)_2(py)_2](PF_6)_2 [2a(PF_6)_2], gave a rate constant per site of k = 1.56×10~(-2) M~(-1) s~(-1), and the activation parameters ΔH~? = 81 kJmol~(-1) and ΔS~? = ~(-1)3 JK~(-1)mol~(-1). The substitution of various pyridine derivatives (Rpy) for CH_3CN in 1(PF_6)_2 exhibited varying activation parameters: ΔH? ranged from 66 to 92kJmol~(-1), ΔS~? ranged from -51 to +22 JK~(-1)mol~(-1), and ΔV~? (for Rpy with R = H, 3-Ac, 4-NH_2, 4-CN, 3-CN) ranged from +1.3 to +3.2 cm~3mol~(-1). These results suggest an interchange (I) mechanism for the substitution of Rpy, for which its strong electron-donating ability is responsible. A compensation effect is seen between ΔH~? and ΔS~?, and both have linear relationships with the Hammett parameters. The substituent R electronically influences the strength of the interaction of Rpy with Ru center in the transition state to shift the activation mode to a dissociative (I_d) or an associative (I_a) interchange.