Using the Structural Versatility of Lanthanide Metallacrowns to Tune Anion Recognition, Self-Assembly, and Luminescence Properties.

Abstract

Natural and artificial supramolecular systems utilize weak, reversible interactions to control the organization of matter and facilitate separations, catalysis, and materials design. Metallacrowns are metal dense macrocycles assembled through dative bonds. This thesis explores how structural changes to the building blocks can enhance the solid state organization, anion recognition, structure, and physical properties of the metallacrown. A systematic crystallographic study was performed to assess how the side chain and central metal influences the size and selectivity of dimeric Ln(III)[15-MC-5] compartments. Phenyl side chains with 0-2 methylene spacers generate dimeric compartments ranging from 9.7 to 15.2 Å in height that selectively encapsulate 4.3 to 9.2 Å long dicarboxylate guests. Up to five carboxylate guests can be encapsulated based on considerations of side chain sterics and the coordination number of the central metal. Solution studies were performed with a novel voltammetric binding assay. Chiral Gd(III)[15-MCCu(II),L-pheHA-5]Cl3 metallocavitands bind alpha-amino acid analogs with affinity constants ranging from 70 +/- 7 to 3260 +/- 70 M-1 in 50% aqueous methanol for N-acetyl-R-phenylalanine and S-mandelate, respectively. Increasing the methylene spacers in the phenyl side chain from 0 to 2 increases the binding constant for benzoate from 800 +/- 100 to 3000 +/- 300 M-1 in 40% aqueous acetonitrile. Mandelate enantioselectivity is enhanced (KS/KR up to 2.2 +/- 0.6) by changing the phenyl side chain. Optically transparent lanthanide metallacrowns with Zn(II) ring metals were assembled for their luminescence properties. Six distinct metallacrown motifs were assembled from picoline hydroxamic acid, Zn(II), Ln(III) through a strict solvent dependence and structurally characterized: Zn(II)[12-MC-4], Ln(III)[12-MC-4], Ln(III)[15-MC-5], Ln(III)[12-MC-4]2, Ln(III)[12-MC-4]2[24-MC-8], and Ln(III)2Zn¬9. The Yb(III)[12-MC-4]2[24-MC-8] is luminescent in methanol, emitting at 980 nm upon excitation at 325 nm with a quantum yield of 0.89 % and lifetime of 14 microseconds. This complex is the brightest near-infrared emitting lanthanide coordination complex in protic solvents due to reduced vibrational quenching through the removal of C-H oscillators from the proximity of the lanthanide ion by the metallacrown. These results reveal that the unique topology and structural versatility of metallacrowns create great promise for chiral separations, catalysis, and molecular materials.