Comparison of 4d- and 4f-Metal Chemistry through Spectroscopic Analysis of 1,10-Phenanthroline Coordination Compounds in Solution and Embedded Polystyrene Beads

Abstract

The chemistry of 4d and 4f metals was investigated at the undergraduate level in an effort to incorporate f-element chemistry in the curriculum. This was accomplished through microwave-assisted synthesis of 1,10-phenanthroline (phen) coordination compounds [Eu(phen)3](PF6)3 and [Ru(phen)3](PF6)2, and embedding the coordination compounds in polystyrene (PS) beads. Through a combination of 1-D/2-D spectroscopic techniques, the metal-phen coordination compounds in solution and in the solid state were probed. It was validated that the strong interaction between the phen ligand and Ru2+ is due to the diffuse nature of the 4d orbitals. The interaction resulted in a low energy MLCT band, which provides opportunities for excitation at lower energies using 4d metals. In contrast, the phen ligand and core Eu3+ 4f orbitals exhibited a weak interaction. This was supported by variable-temperature NMR (VT-NMR) measurements, which revealed relatively well-separated proton resonances with minimal differences in chemical shifts for the phen ligand at 25 °C and the [Eu(phen)3](PF6)3 compound at −42 °C, indicating a weak Eu–N interaction. The weak interaction resulted in the formation of an aqua-containing complex in solution evidenced by emission lifetime measurements. Despite the weak interaction, selective excitation (via ligand π–π* versus direct f–f) of the Eu3+ compound allowed for color tuning, leading to the generation of a cool white light. Electron probe microanalysis (EPMA) of the metal-embedded PS beads indicated relatively monodispersed distribution of the metal complexes in the polymer.