Metal complexes with macrocyclic ligands. Part XX. Influence of coordinated ligands onto the complexation rate of Ni²⁺ with 1,4,8,11‐tetraazacyclotetradecane

Abstract

SummaryThe kinetics of the reaction between 1,4,8,1l-tetraazacyclotetradecane (Cy) and Ni" in the presence of a series of ligands L = fluoride, acetate, glycolate, oxalate, malonate, succinate, methanetriacetate, 1,3,5-~yclohexanetriacetate, tricarballylate, picolinate, glycinate, iminodiacetate, nitrilotriacetate, N,N-ethylenediiminodiacetate, ammonia, pyridine, ethylenediamine, 1 ,?propanediamine and diethylenetriamine were studied by pH-static and spectrophotometric methods at 25" and Z = 0.5. By analysis of the log k/log [L] and/or log k/pH profiles the resolved bimolecular rate constants ky$i (Table 3 ) were determined using a non-linear least-square fitting procedure. Practically for all systems the rate constant k&, describing the reaction between the 1:l Ni2+ complex and the monoprotonated form of the macrocycle, was obtained. In some cases, however, also k&Z and kpy were found. Since the experimental conditions were choosen so that NIL was mainly formed, the reactivity of NiL, was generally not measurable. The effect of the number of coordinated donor groups in NiL and of the charge of NiL on k,N$ is discussed. Both effects seem to indicate that for the reaction between NiL and CyH+ first bond formation is not the rate-determining step.Introduction. -The kinetics of metal-ion complexation by tetraazamacrocycles has been studied under variation of the ring size [2] [3], the degree of substitution [4] and protonation. In general, one has to differentiate between protonated and non-protonated species of the macrocycle, when complexation rates are discussed. So it was found that in strongly alkaline solution Cu(0H); and Cu(OH):-react with open-chain and cyclic tetraamines at similar rates, indicating that the same mechanism is involved [5]. Studies in organic solvents also show that the complexation of Ni2+ with openchain and cyclic N,-ligands is governed by the solvent-exchange rate independently of the nature of the ligand [6]. This was also found for a series of N,O,-macrocycles in MeOH [7] and for S,-macrocyles in H20/EtOH mixtures [8]. I13 contrast to this, mono-(CyH+; Cy = 1,4,8,1 I-tetraazacyclotetradecane) and diprotonated (CyHP) tetraazama-