Beta-diketonates are bidentate O-donor ligands that are commonly found in lanthanides and transition metal complexes. However, the ability of β-diketonates to complex with neutral anion receptors remains unknown. Cyanostar (CNstar) is a five-fold symmetric macrocyclic ionophore that binds anions using C-H hydrogen bond donors. CNstar demonstrates size-based selectivity for anion complexation. CNstar-anion complexes are defined by the ratio of CNstar macrocycles to anions, and complexes with high affinity anions favor a 2:1 sandwich complex that doubles the number of C-H hydrogen bond donors and can be stabilized by pi-stacking. Larger anions such as PF₆^- and ClO₄^- are preferred because they size-match well with the CNstar cavity. Interestingly, these anions are considered weakly or non-coordinating anions on account of their lack of coordination to metal cations. In contrast, β-diketonates bind to transition metal and lanthanide cations. Therefore, we will investigate their coordination to this neutral CH-hydrogen bonding CNstar macrocycle. The use of β-diketonates for CNstar complexation expands the scope of anion binding studies to investigate how unique characteristics of β-diketonates such as size, resonance-stabilized partial negative charges, and substituent effects alter complex formation. Using cation metathesis, we prepared tetrabutylammonium-β-diketonate salts. These initial experiments involved synthesizing and purifying β-diketonates that are soluble in solvents compatible with CNstar. Following synthesis, the salts were characterized using infrared spectroscopy (IR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Once identity and purity of the β-diketonate salts were confirmed, ¹H NMR and UV-Vis titrations were performed with each anion to CNstar. Competition titrations were also performed with other anions to compare the relative strength of binding of β-diketonates to CNstar.

Kati Young

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