Solution chemistry of macrocycles. Part 4. Thermodynamics of protonation and complexation of several N2S2 macrocycles
- 1 January 1985
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 6,p. 1169-1172
- https://doi.org/10.1039/dt9850001169
Abstract
The protonation of six N2S2 macrocycles L1—L6 and the open-chain ligand 2,13-dithia-6,9-diazatetradecane (L7) as well as their complexation with Cu2+ have been studied by pH titrations, spectrophotometric titrations, and calorimetry at 25 °C and I= 0.5 mol dm–3(KNO3). The first protonation constant, pKH,1, of these ligands is in the range of slightly acidified secondary aliphatic ammonium ions, whereas the second pKH,2 is essentially determined by electrostatic factors. So the difference pKH,1– pKH,2 is larger for the cis than for the trans derivatives and decreases with ring size for both cis and trans compounds. The ligands give mononuclear species [ML]2+ with stabilities of 1010.6–1016 dm3 mol–1 for Cu2+ and 107.8–109.1 dm3 mol–1 for Ni2+. In addition for the 12-membered ligands (L1 and L2) and the open-chain ligand L7 partially hydrolysed species [CuL(OH)]+ are also observed. The Cu2+ complex of 1,4-dithia-8,11-diazacycIotetradecane is more stable than that of the open-chain ligand L7. This extra stability (macrocyclic effect) is due to both favourable enthalpic and entropic terms.Keywords
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