Syntheses and structures of tetrakis(1-methyluracilato)palladium complexes capturing alkali metal ions. A new type of metallo-podand

Four tetrakis(1-methyluracilato)palladium complexes containing alkali metal ions, [Li2Pd(1-MeU−)4] (1), [Na2Pd(1-MeU−)4] (2), [K2Pd(1-MeU−)4] (3), and [Cs2Pd(1-MeU−)4] (4), have been prepared from M2PdCl4 (M=Li+, Na+, K+, and Cs+) and 4 equiv. of 1-methyluracil (1-MeUH). The X-ray crystal structure analyses of complexes 2, 3 and 4 have revealed that each palladium atom is coordinated in a square-planar geometry with four 1-MeUH-derived N3-deprotonated imidato groups, upright-oriented with angles of 60–70° from the basal coordination plane. Interestingly, the carbonyl groups of the four imidato rings form two cavities above and below the Pd(II) atom, and two alkali metal ions are incorporated with PdM distances of 3.007(4) and 3.137(3) Å for 2, 3.432(3) and 3.594(3) Å for 3, and 3.746(1) and 3.999(1) Å for 4, respectively. The cavity sizes of the three complexes are tunable according to the ionic radii of the alkali metal ions. 1H NMR spectra of complexes 1–4 indicate that all proton signals of the 1-MeU− moiety are shifted up-field relative to those of free 1-MeU−. The magnitude of the up-field shifts, 1<2<3<4, corresponds well to the order of the ionic potentials of alkali metal ions. These up-field shifts are reduced by coordination of 5-fluoro-1-methyluracil in place of 1-MeUH, indicating that the electron-donating character of 1-MeU− to Pd(II) is decreased by substitution with an electron-withdrawing fluoro group. These observations suggest that the up-field shifts may be rationalized in terms of π-back donation from Pd(II) dπ-orbital to imidato π*-orbital and the ionic potentials of alkali metals, which have also been investigated on the basis of the DFT calculation. On the basis of these characteristic features, the [M2Pd(1-MeU)4] complexes may be classified as a new type of metallo-podand that captures alkali metal ions.