Metal Ion-Controlled Self-Assembly Using Pyrimidine Hydrazone Molecular Strands with Terminal Hydroxymethyl Groups: A Comparison of Pb(II) and Zn(II) Complexes

Metal complexation studies were performed with the ditopic pyrimidine–hydrazone (pym–hyz) strand 6-hydroxymethylpyridine-2-carboxaldehyde (2-methyl-pyrimidine-4,6-diyl)bis(1-methylhydrazone) (1) and Pb(ClO4)2·3H2O, Pb(SO3CF3)2·H2O, Zn(SO3CF3)2, and Zn(BF4)2 to examine the ability of 1 to form various supramolecular architectures. X-ray crystallographic and NMR studies showed that coordination of the Pb(II) salts with 1 on a 2:1 metal/ligand ratio in CH3CN and CH3NO2 resulted in the linear complexes [Pb21(ClO4)4] (2), [Pb21(ClO4)3(H2O)]ClO4 (3), and [Pb21(SO3CF3)3(H2O)]SO3CF3 (4). Two unusually distorted [2 × 2] grid complexes, [Pb1(ClO4)]4(ClO4)4 (5) and [Pb1(ClO4)]4(ClO4)4·4CH3NO2 (6), were formed by reacting Pb(ClO4)2·6H2O and 1 on a 1:1 metal/ligand ratio in CH3CN and CH3NO2. These grids formed despite coordination of the hydroxymethyl arms due to the large, flexible coordination sphere of the Pb(II) ions. A [2 × 2] grid complex was formed in solution by reacting Pb(SO3CF3)2·H2O and 1 on a 1:1 metal/ligand ratio in CH3CN as shown by 1H NMR, microanalysis, and ESMS. Reacting the Zn(II) salts with 1 on a 2:1 metal/ligand ratio gave the linear complexes [Zn21(H2O)4](SO3CF3)4·C2H5O (7) and [Zn21(BF4)(H2O)2(CH3CN)](BF4)3·H2O (8). 1H NMR studies showed the Zn(II) and Pb(II) ions in these linear complexes were labile undergoing metal ion exchange. All of the complexes exhibited pym–hyz linkages in their cisoid conformation and binding between the hydroxymethyl arms and the metal ions. No complexes were isolated from reacting either of the Zn(II) salts with 1 on a 1:1 metal/ligand ratio, due to the smaller size of the Zn(II) coordination sphere as compared to the much larger Pb(II) ions.