Enhanced Guest Affinity and Enantioselectivity through Variation of the Gd3+[15-Metallacrown-5] Side Chain

Supramolecular hosts that bind guests reversibly are investigated for potential catalysis and separations applications. Chiral Ln(3+)[15-Metallacrown-5] metallocavitands bind carboxylate guests in hydrophobic cavities generated by their ligand side chains. A thermodynamic study on Gd3+[15-metallacrown-5] hosts with ligands bearing phenyl side chains containing 0, 1, and 2 methylene spacers (1-pgHA, 1-pheRA, 1-hpheHA, respectively) is presented to quantitatively assess how guest affinity and chiral selectivity can be enhanced through changes to the ligand side chain. Guest binding affinity was measured with cyclic voltammetry using ferrocene carboxylate as a redox probe. K-a values between ferrocene carboxylate and 1-pgHA and 1-pheHA were 4800 +/- 400 M-1 and 4400 +/- 700 M-1, respectively. Significantly stronger binding affinity of 12100 +/- 700 M-1 was measured with 1-hpheHA, a result of the longer side-chains more completely encapsulating the guest. A similar trend was observed with benzoate. The side chain also influenced enantioselectivity, as K-S/K-R values of up to 2.2 +/- 0.6 were measured. The side chain dependent guest binding supports the development of highly selective Ln(3+)[15-Metallacrown-S] hosts for use in catalysis and separations through careful ligand design.