Drahos, B., Pniok, M., Havlickova, J., Kotek, J., Cisarova, I., Hermann, P., Lukes, I., Toth, E.
Dalton Transactions (2011) 40 (39) 10131-10146 - doi : 10.1039/C1DT10543D
A new class of macrocyclic ligands based on 1-oxa-4,7-diazacyclononane was synthesized and their Mn2+ complexes were investigated with respect to stability and relaxation properties. Each ligand has two pendant arms involving carboxylic (H2L1 - 1-oxa-4,7-diazacyclononane-4,7-diacetic acid), phosphonic (H4L2 - 1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphonic acid)), phosphinic (H2L3 - 1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphinic acid)) or phenylphosphinic (H2L4 - 1-oxa-4,7-diazacyclononane-4,7-bis[methylene(phenyl)phosphinic acid]) acid moieties. H2L3 and H2L4 were synthesized for the first time. The crystal structure of the Mn2+ complex with H2L4 confirmed a coordination number of 6 for Mn2+. The protonation constants of all ligands and the stability constants of their complexes with Mn2+ and some biologically or biomedically relevant metal ions were determined by potentiometry. The protonation sequence of H2L3 was followed by 1H and 31P NMR titration and the second protonation step was attributed to the second macrocyclic nitrogen atom. The potentiometric data revealed a relatively low thermodynamic stability of the Mn2+ complexes with all ligands investigated. For H2L3 and H2L4, full Mn2+ complexation cannot be achieved even with 100% ligand excess. The transmetallation of MnL1 and MnL2 with Zn2+ was too fast to be followed at pH 6. Variable temperature 1H NMRD and 17O NMR measurements have been performed on MnL1 and MnL2 to provide information on water exchange and rotational dynamics. The 17O chemical shifts indicate hydration equilibrium between mono- and bishydrated species for MnL1, while MnL2 is monohydrated. The water exchange is considerably faster on MnL1 (kex298 = 1.2 [times] 109 s-1) than on MnL2 (kex298 = 1.2 [times] 107 s-1). Small endogenous anions (phosphate, carbonate, citrate) do not replace the coordinated water in either of the complexes, but they induce their slow decomposition. All Mn2+ complexes are stable toward air-oxidation.