Mechanismic Investigation on the Cleavage of Phosphate Monoester Catalyzed by Unsymmetrical Macrocyclic Dinuclear Complexes: The Selection of Metal Centers and the Intrinsic Flexibility of the Ligand

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• 作者：Xuepeng Zhang ; Yajie Zhu ; Xiaowei Zheng ; David Lee Phillips ; Cunyuan Zhao
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：April 7, 2014
• 年：2014
• 卷：53
• 期：7
• 页码：3354-3361
• 全文大小：491K
• 年卷期：v.53,no.7(April 7, 2014)
• ISSN：1520-510X

文摘

The hydrolysis mechanisms of phosphor-monoester monoanions NPP^{鈥?/sup> (p-nitrophenyl phosphate) catalyzed by unsymmetrical bivalent dinuclear complexes are explored using DFT calculations in this report. Four basic catalyst鈥搒ubstrate binding modes are proposed, and two optional compartments for the location of the nucleophile-coordinated metal center are also considered. Five plausible mechanisms are examined in this computational study. Mechanisms 1, 2, and 3 employ an unsymmetrical dizinc complex. All three mechanisms are based on concerted S_N2 addition鈥搒ubstitution pathways. Mechanism 1, which involves more electronegative oxygen atoms attached to the imine nitrogen atoms in the nucleophile-coordinated compartment, was found to be more competitive compared to the other two mechanisms. Mechanisms 4 and 5 are based on consideration of the substitution of the bivalent metal centers and the intrinsic flexibility of the ligand. Both mechanisms 4 and 5 are based on stepwise S_N2-type reactions. Magnesium ions with hard base properties and more available coordination sites were found to be good candidates as a substitute in the MII dinuclear phosphatases. The reaction energy barriers for the more distorted complexes are lower than those of the less distorted complexes. The proper intermediate distance and a functional second coordination sphere lead to significant catalytic power in the reactions studied. More importantly, the mechanistic differences between the concerted and the stepwise pathways suggest that a better nucleophile with more available coordination sites (from either the metal centers or a functional second coordination sphere) favors concerted mechanisms for the reactions of interest. The results reported in the paper are consistent with and provide a reasonable interpretation for experimental observations in the literature. More importantly, our present results provide some practical suggestions for the selection of the metal centers and how to approach the design of a catalyst.}