Equilibria and kinetics of complex formation between zinc(II), lead(II), and cadmium(II), and 12-, 13-, 14-, and 15-membered macrocyclic tetra-amines
- 1 January 1977
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 22,p. 2269-2276
- https://doi.org/10.1039/dt9770002269
Abstract
Polarographic methods have been used to determine the equilibria and kinetics of reaction of ZnII, PbII, and CdII with 12- to 15-membered macrocyclic tetra-amines including 1,4,7,10-tetra-azacyclododecane (L1), 1,4,7,10-tetra-azacyclotridecane (L2), 1,4,8,11-tetra-azacyclotetradecane (L3), and 1,4,8,12-tetra-azacyciopentadecane (L4) in acetate buffer solutions. Unlike the copper(II) system. the stability constants of the zinc complexes hardly change with macrocyclic ring size : log KZnL= 16.2, 15.6, 15.5, and 15.0 for L1, L2, L3, and L4, respectively (I0.20 mol dm–3 and 25 °C). The much larger CdII and PbII form complexes only with L1: log KCdL 14.3 and log KPbL, 15.9. The 103–105 times greater stabilities of the macrocyclic complexes compared with the related complexes of linear terra-amines are all due to favourable entropy changes, regardless of the metal-ion size. The rate law for complex formation in acetate buffers is d[ML]/dt=kH[M(O2CMe)+][HL+]+k2H[M(O2CMe)+][H2L2+] for all the metal ions. A comparison with reactions of CuII and L1–L4 shows that the values of kH parallel the well established rates of water exchange of the aquametal ions.Keywords
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