Complexes of Nucleophiles with Rare Earth Chelates. I. Gas Chromatographic Studies1a of Lanthanide Nuclear Magnetic Resonance Shift Reagents

The present study reports the retention of many different ethers, ketones, alcohols, esters, olefins, and alkanes by gas-chromatographic columns with liquid phases composed of solutions of tris-β-diketonate rare earth chelates in squalane. The dependence of the interaction on the nature of the organic solute, the nature of the chelate ligand, the radius of the metal ion, and the degree of polymerization was investigated. The more nucleophilic organic compounds, for example, tetrahydrofuran (THF), were found to undergo much stronger interaction with the metal chelates than the less basic solutes. The Er(III) chelates of fluorinated β-diketonates (for example, the new ligand 3-trifluoroacetyl-d-camphorate anion, facam) underwent a much greater interaction with the nucleophiles than similar nonfluorinated Er(III) compounds. The retention time of THF increases exponentially with the inverse of the metal ionic radius in the facam chelates. The data indicate that this effect is due to a decrease in the extent of polymerization of the chelates with decreasing radius. In squalane, over the concentration range studied, the polymeric form of Tb(facam)₃ appears to be a dimer; molecular weight determinations support this conclusion. The regularities observed in the gc data can be used to predict the efficacy of various rare earth chelates as nmr shift reagents. Since the structure of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium(III) is trigonal prismatic in the solid state, a reaction scheme is postulated in which a fused monocapped trigonal-prismatic dimer dissociates in solution to yield a monomer with a trigonal-prismatic structure. It is this monomer to which a donor becomes bonded to form a monocapped trigonal-prismatic adduct in both the gc and nmr studies.