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)center dot 3H(2)O, Pb(SO3CF3)(2)center dot 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 [Pb(2)1(ClO4)(4)] (2), [Pb(2)1(ClO4)(3)-(H2O)]ClO4 (3), and [Pb(2)1(SO3CF3)(3)(H2O)]SO3CF3 (4). Two unusually distorted [2 x 2] grid complexes, [Pb1(ClO4)](4)(ClO4)(4) (5) and [Pb1(ClO4)](4)(ClO4)(4)center dot 4CH(3)NO(2) (6), were formed by reacting Pb-(ClO4)(2)center dot 6H(2)O 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 x 2] grid complex was formed in solution by reacting Pb(SO3CF3)(2)center dot. H2O and 1 on a 1:1 metal/ligand ratio in CH3CN as shown by H-1 NMR, microanalysis, and ESMS. Reacting the Zn(II) salts with 1 on a 2:1 metal/ligand ratio gave the linear complexes [Zn(2)1(H2O)(4)](SO3CF3)(4)center dot C2H5O (7) and [Zn(2)1(BF4)(H2O)(2)(CH3CN)](BF4)(3)center dot H2O (8). H-1 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(11) 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.