Kinetics and mechanism of carboxylate complex formation of macrocyclic chromium(III) complexes

The kinetics and the thermodynamics of the equilibration between the cis chromium(III) complex of the macrocyclic rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane ligand and acetic acid/acetate has been investigated at a range of temperatures in 1 M (H/Na) (OH/Br) and at a range of acidities from strongly acidic to strongly basic solution. The system is characterized by an unusually large formation constant for the monoacetato complex, about 105M−1 at 25 °C, which makes acetate a better ligand for the macrocyclic chromium(III) complex than for the proton. The kinetic behaviour of the acetatoaqua- and the diacetato-complex is markedly different. This is rationalized in terms of the differences in basicity of the two complexes, resulting from the differences in intra-molecular hydrogen bonding patterns, which stabilizes the un-protonated acetatoaqua-complex and the protonated form of the diacetato-complex.