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Mechanism and Transition State Structure of Aryl Methylphosphonate Esters Doubly Coordinated to a Dinuclear Cobalt(III) Center

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posted on 09.09.2009, 00:00 by Guoqiang Feng, Eric A. Tanifum, Harry Adams, Alvan C. Hengge, Nicholas H. Williams
Reactivities of five phosphonate esters each coordinated to a dinuclear Co(III) complex were investigated ([Co2(tacn)2(OH)2{O2P(Me)OAr}]3+; tacn = 1,4,7-triazacyclononane; substituent = m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); p-Cl (1d); unsubstituted (1e)). Hydrolysis of the phosphonate esters in 1a to 1e is specific base catalyzed and takes place by intramolecular oxide attack on the bridging phosphonate. These data define a Brønsted βlg of −1.12, considerably more negative than that of the hydrolysis of the uncomplexed phosphonates (−0.69). For 1b, the kinetic isotope effects in the leaving group are 18klg = 1.0228 and 15k = 1.0014, at the nonbridging phosphoryl oxygens 18knonbridge = 0.9954, and at the nucleophilic oxygen18knuc = 1.0105. The KIEs and the βlg data point to a transition state for the alkaline hydrolysis of 1b that is similar to that of a phosphate monoester complex with the same leaving group, rather than the isoelectronic diester complex. The data from these model systems parallel the observation that in protein phosphatase-1, which has an active site that resembles the structures of these complexes, the catalyzed hydrolysis of aryl methylphosphonates and aryl phosphates are much more similar to one another than the uncomplexed hydrolysis reactions of the two substrates.