Me₃NO存在下Ir₄（CO）₁₁L（L=CO,PPh₃）的CO取代反应动力学研究

A Kinetic Study of CO Substitution of Ir4_CO₁₁ L（L=CO, PPh₃） in the Presence of Me₃NO

刊用FT-IR和UV技术跟踪反应进程,研究了在Me3NO存在下Ir4（CO）12和Ir4（CO）11PPh3分别在CHCl3—C2H5OH和CHCl3溶剂中取代羰基反应的动力学与机理。结果表明反应遵循二级速率定律:r=k2[Me3NO][配合物]。该速率方程与缔合机理相一致。将Ir4（CO）11L和Os3（CO）11L（L=CO,PPh3）体系的动力学结果相比较,着重讨论了桥基因素对反应活性的影响。

Our previous work had shown the significance of bridging CO in the reactioities of M4(CO)12 clusters. In order to examine further the importance of CO bridging concept, in this paper the kinetics for the reactions of Ir4(CO)11 L ( L = CO, PPh3) with triphenylphosphine (PPh3 ) and triphenylphosphite (P(OPh)3) in the presence of catalytic promoter, Me3NO, is reported. The rates of reactions have been shown to be second order: first order in metal complex concentration, first order in Me3NO concentration, and zero order in entering ligand concentration. This result indicates an associative pathway for the reactions, supported by experimental values of the activaton parameters which are low for ΔH≠and negative for ΔS≠. The associative mechanism similar to what was proposed earlier on such reactions has been suggested. The kinetic results of Ir4(CO)11L were compared with those of Os3(CO)11L(L=CO, PPh3), and the comparative results show that Os3(CO)12and Ir4(CO)12 react at about the same rate, whereas Ir4(CO)11PPh3 reacts 15 times faster than does Os3(CO)11PPh3. Since neither Ir4(CO)12nor Os3(CO)12 has bridging COs in both solid state and solution, it is expected the tendency of forming bridging CO in the reaction transition state for the two clusters may not differ singnificantly. This may account for the similarity of reactivities for Ir4(CO)12 and Os3(CO)12. As for Ir4 (CO)11PPh3 and Os3(CO)11PPh3, the former has three bridging COs and the latter does not have bridging COs in the ground state. Since the bridging CO groups in Ir4 (CO)11PPh3 may stabilize the transition state by delocalizing the increased electron density imposed on the clusters by the attacking nucleophile, greater stablilty of the transition state results in a rate enhancement.