Metal Complexes with Macrocyclic Ligands. XV. The Complexation Kinetics of Open Chain and Cyclic Tetraazaligands with Ni2+ in DMSO and DMF

The complexation kinetics of 2,6,9, 13‐tetraazatetradecane (1), 1,4,8, 11‐tetraazacyclotetradecane (2) and N,N′,N″,N'‐tetramethyl‐1,4,8, 11‐tetraazacyclotetradecane (3) with Ni2+ were studied by the stopped‐flow technique in DMSO and DMF. The biomecular rate constants kLNi (Table 2) follow in both solvents the order 1≳2>3. The similar complexation rates of 1 and 2 in their unprotonated form indicate that for both the open chain and the cyclic ligand the same mechanism holds. By comparison with the solvent exchange the rate determining step of the complexation is the dissociation of the first solvent molecule in the outer‐sphere complex. The lower reactivity of 3 is probably due to steric effects. In the case of 2 a second step in the complexation process was observed and explained by a rearrangement of the ligand already coordinated to the metal ion.