The kinetics and mechanism of aquation and base hydrolysis of the cis-[Co(cylen)Cl-2](+) cation (cyclen equals 1,4,7,10-tetraazacyclododecane)

The aquation of cis-[Co(cyclen)Cl-2](+) (cyclen = 1,4,7,10-tetraazacyclododecane) has been studied over a range of temperatures with 0.1 mol dm(-3) HNO3 as solvent. At 25 degrees C, k(aq) = 4.5 x 10(-3) s(-1) with Delta H-double dagger = 78 kJ mol(-1) and Delta S-298(double dagger) = -21 J K-1 mol(-1). Base hydrolysis of cis-[Co(cyclen)Cl-2](+) is extremely rapid with k(OH) = 2.1 x 10(7) dm(3) mol(-1) s(-1) at 25 degrees C and I = 0.1 mol dm(-3). This is the largest rate constant so far reported for the base hydrolysis of a cis-dichloro-complex of a saturated macrocycle. The activation parameters, Delta H-double dagger = 53 kJ mol(-1) and Delta S-298(double dagger) = 73 J K-1 mol(-1) are consistent with a mechanism in which deprotonation of the substrate is rate-determining. This conclusion is confirmed by the observation of general base catalysis by formate ion, The Bronsted beta value for the reaction is ra, 0.72 and S(N)1(CB) and E2 mechanisms are considered to account for the kinetic results. Base hydrolysis of cis-[Co(cyclen)(OH)Cl](+) has also been studied in the pH range 6.5 to 8.7. The value of k(OH) = 3.8 x 10(2) dm(3) mol(-1) s(-1) at 25 degrees C and I = 0.1 mol dm(-3) with Delta H-double dagger = 110 kJ mol(-1) and Delta S-298(double dagger) = 171 J K-1 mol(-1) are consistent with an S(N)1(CB) mechanism.