REACTIVITY OF COORDINATED PHOSPHATE-ESTERS - PENTAAMMINECOBALT(III) COMPLEXES

The reactivity of two phosphate ester complexes designed to test the efficacy of different modes of activation of phosphate esters by metal ions has been investigated. Ethyl 4-nitrophenyl phosphate coordinated to the pentaamminecobalt(III) moiety liberates nitrophenolate in basic solution 106-fold faster than the free phosphodiester. The reaction proceeds via attack of coordinated amido ion to yield a four-membered N,O chelate phosphoramidate ethyl ester. The four-membered chelate does not display the enhanced reactivity of the five-membered-ring cyclic ethylene phosphates but decays with cobalt-ligand bond rupture and yields finally free ethyl phosphoramidate. The aminolysis is accompanied by some loss of ethyl 4-nitrophenyl phosphate by the SN1(CB) mechanism. The binuclear complex (μ-nitrophenyl phosphato)decaamminedicobalt(4+) undergoes aminolysis in basic aqueous media, also by intramolecular attack of coordinated amido ion. The reaction proceeds some 102-fold faster than the analogous aminolysis of the mononuclear complex, (4-nitrophenyl phosphato)pentaamminecobalt(1+). The reaction is also accompanied by some SN1(CB) loss of the intact ligand; in this case, the ligand is the mononuclear complex. This study illuminates some of the modes by which metal ions can enhance the reactivity of phosphate esters. In agreement with other studies, the electrostatic and inductive effects are estimated to contribute ~ 102-fold to the rate enhancement, while the intramolecularity of the reaction is responsible for the remainder of the observed rate enhancement. © 1990, American Chemical Society. All rights reserved.