Zinc(II) complexation with bis(1,4,7-triazacyclononane) ligands containing polymethylene linker groups. An equilibrium and structural study

Cooperativity between Metal Ions in the Cleavage of Phosphate Diesters and RNA by Dinuclear Zn(II) Catalysts

“…Zompa and co-workers used pH-potentiometric titrations to study the solution chemistry of a series of Zn(II) complexes of ligands containing two L1 macrocycles linked by alkyl groups with two to eight carbons . They found that the ligand L3 is able to form stable dinuclear complexes with Zn(II), consistent with our NMR studies.…”

“…Zn(II) Binding to Dinuclear Ligands. The macrocyclic ligands L2OH and L3 strongly bind two Zn(II) ions in solution as determined previously in studies using pH−potentiometric titrations. , The large binding constants of these ligands lead to 97 and 98% of the Zn(II) being bound in dinuclear complexes in solutions containing 10.0 mM Zn(NO 3 ) 2 and 5.0 mM L3 or L2OH, respectively, at pH 6.53. Here we use NMR spectroscopy to further study the structure of the different metal complexes postulated in these titrations and, for cases where the potentiometric data is not available, to confirm that two Zn(II) ions bind to the ligand under conditions similar to those in the kinetics experiments (vide infra).…”

“…This result is supported by the X-ray crystal structure obtained for the complex Zn 2 (L2O) and the results of pH-potentiometric titrations . These data indicate that the ligand L2OH is able to form both stable mononuclear and dinuclear Zn(II) complexes with L2OH, a result that differs from that of the related ligand 1,3-bis(1,4,7-triaza-1-cyclononyl)-propane, which only forms a very stable mononuclear sandwich complex with Zn(II) . This distinct behavior is likely due to the presence of the alcohol group in the linker moiety that favors the opening of the stable sandwich complex to form the dinuclear Zn(II) complex when the ratio of Zn(NO 3 ) 2 to ligand is increased to 2.0.…”

“…In addition, both the 1 H and 13 C NMR spectra of this complex suggest a highly symmetric structure with two equivalent L1 units. This NMR solution data, together with structural data for complexes of related bis(L1) ligands, suggests that the Zn(L2OH) complex has a sandwich structure with both L1 units binding a single Zn(II) ion to form a six-coordinate complex. , Such a structural motif, where the alcohol group is not coordinated to the metal ion, has been found for other potentially dinucleating systems under conditions of equimolar amounts of metal ion and ligand. , An alternate structure we cannot rule out is a dimeric complex with each Zn(II) bound to two L1 units from different ligands. Such a dimeric complex of the form Zn 2 (L2OH) 2 could give rise to the NMR spectra we observe.…”

“…However, such dimeric complexes are generally observed for alkyl linkers with four, five, or six methylene units. Ligands containing two L1 units linked by a propyl group form the intramolecular metal ion sandwich structure. ,, For the dinuclear complex Zn 2 (L2O), the linker is relatively rigid as suggested by nuclear Overhauser effect experiments. All the linker protons exhibit a marked upfield shift compared to the free ligand (Figure ).…”