Development and Kinetic Analysis of Homogeneous and Heterogeneous Transition Metal Catalysts for the Cleavage of Phosphate Esters in Methanol
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Described here are detailed kinetic studies probing the structural elements which are crucial for the catalytic activity of dinuclear Zn(II) complexes towards phosphate diester cleavage. First, two sets of dinuclear Zn(II) complexes (a member with and without a bridging oxyanion linker group) were synthesized and their ability to promote the cyclization of 2-hydroxypropyl-p-nitrophenyl phosphate, a common model for RNA, was compared. Kinetic studies indicated that the complexes without the oxyanion linker were more active in promoting the cyclization in methanol under pH controlled conditions at 25 degrees. Quantitative energetics analysis shows that the rate reduction is attributable to a decrease in the second-order rate constant for the cyclization reaction, which adds 3.7 and 6.5 kcal/mol of activation energy to the respective reactions mediated by the complex with the oxyanion linker. Secondly, we have investigated a series of dinuclear Zn(II) complexes that incorporate various substituents including hydrophobic and hydrogen-bonding ones. Analysis of the data at the pH optimum for each reaction indicates that the presence of the H-bonding groups and alkyl groups provides similar increases (at least an order of magnitude) of the kcat terms over the unfunctionalized complex. There is also no clear trend that H-bonding groups or the alkyl groups provide stronger binding to the substrate than the parent complex. We also describe here the preparation and kinetic analysis of a series of solid supported transition metal catalysts for the cleavage of P=O chemical warfare simulants and P=S pesticides. We report a kinetic study of a 1,10-phenanthroline:Zn(II) complex immobilized on macroporous polystyrene which is capable of accelerating the cleavage of G-agent and V-agent simulants in methanol at neutral and ambient temperature by up to one hundred thousand-fold. The materials are recoverable and can be recycled at least ten times. We have also devised a methodology for simple immobilization of an ortho-palladated dimethylbenzylamine complex on macroporous polystyrene and amorphous silica gel. We report the catalyst preparation and a detailed kinetic study of their catalysis of the methanolysis of five P=S pesticides at neutral and ambient temperature. The polymeric catalysts give over billion-fold acceleration compared to the uncatalyzed background reaction at the same pH.