THE KINETICS OF THE ACID-CATALYZED DECARBOXYLATION OF CIS-[CO([15]ANEN(4))CO3](-TETRA-AZAPENTADECANE)() ([15]ANEN(4) = 1,4,8,12)

The decarboxylation of [Co([15]aneN(4))CO3](+) in nitric acid solution s has been studied in detail over a temperature range at I = 1.0 mol d m(-3). The reaction is biphasic, the initial reaction is the carbonate ring opening step, which is followed by very rapid decarboxylation of the monodentate carbonate intermediate. The second step is the isomer isation of cis-[Co([15]aneN(4))CO3](+) to the trans-isomer. At constan t hydrogen ion concentration, values of k(obs) for the carbonate ring opening reaction are given by k(obs) = k(o) + k(H)[H+], where k(o) rel ates to the spontaneous water hydrolysis and k(H) to the acid-catalyse d process. The temperature dependence of k(H) gives Delta H dagger = 7 1 +/- 2 J K-1 mol(-1) and Delta S(298)dagger = - 66 +/- 6 J K-1 mol(-1 ). The solvent deuterium isotope effect for the acid-catalysed decarbo xylation k(D2O)/k(H2O) is 2.35, which is consistent with a rapid pre-e quilibrium protonation of the carbonate ring, followed by rate-determi ning ring opening. The mechanisms of these reactions are considered in detail. At 25 degrees C, the rate constant for the isomerisation reac tion is 4.9 x 10(-6) s(-1) and is independent of the acidity in the ra nge [H+] = 0.05 to 0.5 mol dm(-3) at I = 1.0 mol dm(-3). (C) 1998 Else vier Science Ltd. All rights reserved.