Polyazamacrocycles. Part 7. Kinetics of aquation of trans-dichloro and trans-chloro (nitro) complexes of cobalt (III) with tetra-aza macrocycles

Abstract

Kinetic studies on the aquation of trans-[CoLCl₂]⁺ and trans-[CoL(NO₂)⁺](L = tetra-aza macrocycle) complexes show large variation in rates. The dependence of aquation rates on conformational strain energy is true only for a limited number of compounds. The rate enhancements can be rationalised in terms of the ability of the macrocycle to fold across a fold axis. The large-ringed macrocyclic complexes trans-[CoL⁶Cl₂]⁺(L⁶= 1,4,8,11 -tetra-azacyclohexadecane) and trans-[CoL⁷Cl₂]⁺(L⁷= 1,4,8,11 -tetra-azacycloheptadecane) aquate at much lower rates due to curtailed folding ability. The N ⋯ N bite distances also suggest a lower aquation rate for these complexes. The complexes of 2,5,5,7,9,12,12,14-octamethyl- and 5,7,12,14-tetraethyl-5,12-dimethyl-1,4,8,11 -tetra-azacyclotetradecane aquate with low rates lying in a narrow range. No specific trends can be identified in these complexes. These studies also show that very large aquation rate enhancements in cobalt (III) complexes of 14-membered macrocycles are possible only if gem-dialkyl groups are present.