Effects of the Central Lanthanide Ion Crystal Radius on the 15-MCCuII(N)pheHA-5 Structure

Abstract

Twenty crystal structures of the Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complex, where pheHA = phenylalanine hydroxamic acid and where Ln(III) = Y(III) and La(III)-Tm(III), except Pm(III), with the nitrate and/or hydroxide anion are used to assess the effect of the central metal ion on the metallacrown structure. Each Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complex is amphiphilic with a hydrophobic side consisting of the phenyl groups of the pheHA ligand and a side without the aromatic residues. Three general structures are observed for the Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complexes. In the Type 1 structures, the central metal ion does not bind a nitrate anion on the metallacrown's hydrophobic face, and two adjacent metallacrowns dimerize through their phenyl groups producing a hydrophobic compartment. In the Type 2 structures, the central metal ion binds a nitrate in a bidentate fashion on the hydrophobic face. There are two distinct types of Type 2 metallacrowns, designated A and B. Type 2A metallacrowns have a water molecule bound to the central metal ion on the hydrophilic face, while Type 2B metallacrowns have a monodentate nitrate ion bound on the hydrophilic face to the central metal ion. The Type 2 metallacrowns also dimerize via the phenyl groups to form a hydrophobic compartment. In Type 3 structures, the central metal ion binds a nitrate in a bidentate fashion on the hydrophobic side, but instead of forming dimers, the metallacrowns pack in a helical arrangement to give either P or M one-dimensional helices. Regardless of the type of metallacrown, the overall trend observed is that as the Ln(III) ion crystal radius increases, the metallacrown cavity radius also increases while the metallacrown becomes more planar. This conclusion is demonstrated by a decrease in the oxime oxygen distances to the oxime oxygen mean plane and a decrease in the ring Cu(II) distances to the Cu(II) mean plane as the metallacrown cavity radius increases and the lanthanide crystal radius increases. In addition, a decrease in the O(oxime)-Cu(II)-N(oxime)-O(oxime) torsion (dihedral) angles is also observed as the metallacrown cavity radius and the lanthanide crystal radius both increase. These observations help explain the thermodynamic preferences for Ln(III) ions within this class of metallacrowns and may be used to design compartments capable of binding guests in different orientations within chiral, soft solids.