THE KINETICS AND MECHANISM OF AQUATION AND BASE HYDROLYSIS OF THE CIS-[CO(CYLEN)CL-2](-TETRAAZACYCLODODECANE)() CATION (CYCLEN EQUALS 1,4,7,10)

The aquation of cis-[Co(cyclen)Cl-2](+) (cyclen = 1,4,7,10-tetraazacyc lododecane) 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) w ith Delta H-double dagger = 78 kJ mol(-1) and Delta S-298(double dagge r) = -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 deg rees C and I = 0.1 mol dm(-3). This is the largest rate constant so fa r reported for the base hydrolysis of a cis-dichloro-complex of a satu rated macrocycle. The activation parameters, Delta H-double dagger = 5 3 kJ mol(-1) and Delta S-298(double dagger) = 73 J K-1 mol(-1) are con sistent with a mechanism in which deprotonation of the substrate is ra te-determining. This conclusion is confirmed by the observation of gen eral base catalysis by formate ion, The Bronsted beta value for the re action is ra, 0.72 and S(N)1(CB) and E2 mechanisms are considered to a ccount 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 mo l dm(-3) with Delta H-double dagger = 110 kJ mol(-1) and Delta S-298(d ouble dagger) = 171 J K-1 mol(-1) are consistent with an S(N)1(CB) mec hanism.