REACTIVITY OF PHOSPHATE DIESTERS DOUBLY COORDINATED TO A DINUCLEAR COBALT(III) COMPLEX - DEPENDENCE OF THE REACTIVITY ON THE BASICITY OF THE LEAVING GROUP

Reactivities of eight phosphate diesters each coordinated to a dinucle ar Co(III) complex were investigated ([Co-2(tacn)(2)(OH)(2){O2P(OR)(2) }](3+); tacn = 1,4,7-triazacyclononane). Four of the complexes were co ordinated by substituted phenyl methyl phosphates (substituent m-F, F- NO2 (la); p-NO2 (Ib); m-NO2 (Ic); unsubstituted (Id)) and two by subst ituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsu bstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phos phate (4)) were also investigated. Hydrolysis of the phosphate diester s in la to Id takes place by intramolecular oxide attack on the bridgi ng phosphate while hydrolysis of 3 principally takes place by intermol ecular hydroxide attack: on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by int ramolecular 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 t hey indicate that there will be considerable cooperativity between the oxide activation and leaving group activation.