Ring-size effects of macrocyclic ligands on complex formation: the copper(II) complex of the 15-membered cyclic tetra-amine 1,4,8,12-tetra-azacyclopentadecane
- 1 January 1976
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 22,p. 2341-2345
- https://doi.org/10.1039/dt9760002341
Abstract
The thermodynamics and kinetics of complex formation between CuII and 1,4,8,12-tetra-azacyclopentadecane (L4) have been studied polarographically. The stability constant of 1024.4 ± 0.2(I= 0.20 mol dm–3, 25 °C) for the 1 : l complex containing 5,6,6,6-membered chelates is 107- and 103-fold greater than those for complexes of the corresponding open-chain tetra-amines 4,8.diazaundecane-1.11-diamine (L5) containing 6,6,6-chelates and 4,7-diazadecane-1,10-diamine (L5) containing 5,6,6-chelates, respectively. Depending on the choice of the reference complexes, either the ΔH term (cf the L5 complex) or the ΔS term (cf, the L6 complex) makes a major contribution to the macrocyclic effects. A significant effectof the macrocyclic ring expansion is a facile protonation of the complexed ligand in acetate buffer solution, KCu(HL) H=[CuL2+][H+]/[Cu(HL)3+]=(3.0 ± 0.2)× 10–6,ol dm–3. The rate law for the formation of the complex of L4 in acetate buffer is of the form d[Cu(HL)3+]/dt=k5-[Cu(O2CMe)+][HL+], where k5=(4.0 ± 0.5)× 108 dm3 mol–1s–1 at 25 °C and the activation parameters are ΔH‡= 13.7 ± 0.3 kcal mol–1 and ΔS‡= 26.7 ± 1.0 cal K–1 mol–1. The larger formation and dissociation rate constants suggest that the 15-membered macrocycle is more flexible relative to the smaller analogues.Keywords
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