Cerium acetylacetonates—new aspects, including the lamellar clathrate [Ce(acac)4]·10H2O

Reactions of CeCl3·7H2O and Ce(NO3)3·6H2O with Naacac or NH4acac in aqueous solution at 21 and 45 °C yielded the trihydrate [Ce(acac)3(H2O)2]·H2O and the dihydrate [Ce(acac)3(H2O)2], respectively, whereas similar treatment of (NH4)2[Ce(NO3)6] gave the trihydrate at both temperatures. Desiccation of the hydrates over silica gel left the dihydrate unchanged, whereas the trihydrate underwent decomposition rather than dehydration. Aerial oxidation of [Ce(acac)3(H2O)2] in CH2Cl2 and toluene yielded α-[Ce(acac)4] and β-[Ce(acac)4], respectively, the structure of the former being re-determined with improved precision. Careful treatment of aqueous (NH4)4[Ce(SO4)4] and Hacac (initially pH 1–2) with aqueous ammonia to pH 5 precipitated hydrated [Ce(acac)4], from which [Ce(acac)4]·10H2O was isolated as unstable, light-sensitive single crystals, and the structure was determined. The complex is a laminar clathrate containing layers of Ce(acac)4 molecules sandwiched between extensive hydrogen-bonded layers of water molecules which do not interact with the metal. Electrochemical experiments confirmed the unstable nature of hydrated CeIII(acac)3, while the reduction of [Ce(acac)4] yielded well-defined cyclic voltammograms in acetonitrile and acetone, corresponding to a quasi-reversible process. For the [CeIV(acac)4]/[CeIII(acac)4]−redox couple, a calculated reversible potential of 0.22±0.02 V versus SHE was obtained in acetone or acetonitrile (0.1 M Bu4NPF6) at both gold and glassy carbon electrodes. This potential is consistent with the ease of both oxidation and reduction of cerium acetylacetonate complexes as found in the synthetic studies.