Formation of 1 ∶ 1 and 2 ∶ 2 complexes of Ce(iii) with the heteropolytungstate anion α2-[P2W17O61]10−, and their interaction with proline. The structure of [Ce2(P2W17O61)2(H2O)8]14−

Abstract

The “1 ∶ 1” complex of Ce³⁺ and α₂-[P₂W₁₇O₆₁]¹⁰⁻ is formed in an equimolar solution of Ce(NO₃)₃ and K₁₀[P₂W₁₇O₆₁], but attempts to isolate a solid yield salts of the 1 ∶ 2 complexes first described by Peacock and Weakley in 1971. With a 3-fold excess of Ce³⁺ however, a crystalline ammonium salt (anion 1) could be isolated in 90% yield based on tungstate. Structural analysis of these crystals (monoclinic, C2/c; a = 46.1124(6); b = 12.7451(1); c = 26.4624(2) Å, β = 93.570(1)°, Z = 4; 18856 independent reflections, R1 = 0.0799; wR2 = 0.1594) revealed a head-to-head dimeric anion with each 9-coordinate Ce(III) cation attached to four oxygens that surround the lacuna of one tungstophosphate anion, a terminal O(W) of the second tungstophosphate, and four water molecules. Phosphorus-31 NMR spectra of solutions of 1 revealed two sets of resonances, A and B, the relative intensities of which could be fitted to an equilibrium 2A B, with K = 1.46 ± 0.04 M⁻¹ at 23 °C. Further NMR analysis suggests a scheme in which 1, which is in rapid exchange with the “monomer” (2), is also in slow exchange with a labile associate of Ce³⁺ and the 1 ∶ 2 complex. The exchange process between 1 and 2 is responsible for the non-observance of one of the expected 9 lines in the tungsten-183 NMR spectrum and an exchange-broadened P-NMR line. Addition of proline to form a ternary complex [Ce(P₂W₁₇O₆₁)(proline)(aq)]⁷⁻(K_form = 4.5 ± 0.1 M⁻¹ from ¹H-NMR) restores the missing W-NMR resonance.