Speciation of organic-soluble europium(iii) α1-Wells–Dawson complexes

Abstract

In this contribution, we provide a comprehensive understanding of the speciation of the Eu(III) complex of the lacunary Wells–Dawson isomer, α1-[P₂W₁₇O₆₁]¹⁰⁻ in organic media. The Wells–Dawson polyoxometalate, α1-[P₂W₁₇O₆₁]¹⁰⁻ (abbreviated as α1) forms well-defined complexes with europium(III) (and other lanthanide(III)) ions in aqueous solution of predominantly 1 : 1 stoichiometries. The 8-coordinate Eu(III) ion is bound to 4 basic terminal oxygens (O_α1) and four water molecules (O_H2O) that complete the coordination sphere. Tetra-n-butylammonium (TBA) cations are employed to render the [(H₂O)₄Eu(α1-P₂W₁₇O₆₁)]^7 − (Eu-α1) complex soluble in acetonitrile. Europium(III) provides the unique opportunity to employ luminescence spectroscopy and multinuclear NMR to probe the coordination environment. We interrogate the innermost coordination sphere of the Eu(III) ion in acetonitrile solution and in MeCN/H₂O mixtures. We provide evidence toward the fractional displacement and coordination of acetonitrile within the TBA salts, that is consistent with recent EXAFS data. ³¹P NMR and Stern–Volmer quenching studies suggest that dimerization to the 2 : 2 species is negligible in acetonitrile and MeCN–H₂O mixtures. The decreasing transition energy in the excitation spectroscopy of the TBA-Eu-α1 analog upon dilution is consistent with a nephelauxetic effect, which is attributed to a slight increase in covalency upon replacement of water with acetonitrile. Determination of the number of bound waters (q) is also consistent with acetonitrile–water exchange. The reactivity of the 1 : 1 TBA-Eu-α1 with heterocyclic aromatic amines (1,10-phenanthroline, phen, and 2,2′ bipyridine, bipy) in MeCN was probed by titrations monitoring the Eu(III) emission upon sensitization by the “antenna ligands”. Binding constants for the products 1 : 1 TBA_x−yH_y[(Phen)(H₂O)₂Eu(α1-P₂W₁₇O₆₁)] and 1 : 2 TBA_x−yH_y[(Phen)₂Eu(α1-P₂W₁₇O₆₁)] (denoted 1 : 1 TBA-Eu-α1:phen and 1 : 2 TBA-Eu-α1:phen, respectively), were determined: logK₁: 7.05 ± 0.04 and logK₂: 4.63 ± 0.10. These are reasonably strong formation constants for Ln phenanthroline complexes. In comparison the bipyridine complexes are much weaker. Excitation spectroscopy reveals that the coordination environment about the Eu(III) center is consistent with the ternary 1 : 1 TBA-Ln-α1:phen or 1 : 2 TBA-Ln-α1:phen complexes. Multinuclear NMR spectroscopy shows significant chemical shift changes at 1 : 1 and 1 : 2 stoichiometries and the chemical shift of bound water tracks with the titration to validate expulsion of the H₂O upon coordination of phenanthroline.