Reactivity of Phosphate Diesters Doubly Coordinated to a Dinuclear Cobalt(III) Complex: Dependence of the Reactivity on the Basicity of the Leaving Group

Abstract

Reactivities of eight phosphate diesters each coordinated to a dinuclear Co(III) complex were investigated ([Co2(tacn)2(OH)2{O2P(OR)2}]3+; tacn = 1,4,7-triazacyclononane). Four of the complexes were coordinated by substituted phenyl methyl phosphates (substituent m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); unsubstituted (1d)) and two by substituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsubstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phosphate (4)) were also investigated. Hydrolysis of the phosphate diesters in 1a to 1d takes place by intramolecular oxide attack on the bridging phosphate while hydrolysis of 3 principally takes place by intermolecular hydroxide attack on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by intramolecular transesterification. Dimethyl phosphate dissociates from 4 without any observable cleavage of the diester. These results can be understood in terms of the More O'Ferrall−Jencks energy diagram, and they indicate that there will be considerable cooperativity between the oxide activation and leaving group activation.