ALKALINE-HYDROLYSIS OF [CO(CYCLEN)(O2CO)](+) AND DIRECT OBSERVATION OF DEPROTONATED S(N)1CB INTERMEDIATES

A stopped-flow/rapid-scan study of the hydrolysis of the carbonate che late [Co(cyclen) (O2CO)](+) in alkaline solution ([OH-] = 0.05 to 1.0 M, 25.0 degrees C, I = 1.0 M (NaClO4)) has shown that the reaction fol lows consecutive first order steps, the first (t(1/2) = 66 ms at [OH-] = 1.0 M) corresponding to the OH- promoted reversible ring-opening of the chelate and the second (t(1/2) = 4.1 s at [OH-] = 1.0 M) to irrev ersible loss of CO32- from the cis- [Co(cyclen)(OH)(OCO2)] product. Re sults from O-18 tracer studies show that both steps occur via Co-O bon d cleavage. Deprotonation of amine centres in the cyclen ligand is imp ortant in both processes, with significant concentrations of the amido complex [Co(cyclen-H)(O2CO)] being generated initially, and with cis- [Co(cyclen-H)(OH)(OCO2)](-) also being directly observed subsequently. The experimentally determined rate laws are interpreted in terms of S (N)1CB mechanisms for the ring-opening and hydrolysis reactions, with the doubly deprotonated intermediate cis-[Co(cyclen-2H)(OH)(OCO2)](2-) being an important contributor to the latter.