Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates.

The homoleptic pyrazolate complexes [CeIII 4 (Me2 pz)12 ] and [CeIV (Me2 pz)4 ]2 quantitatively insert CO2 to give [CeIII 4 (Me2 pz⋅CO2 )12 ] and [CeIV (Me2 pz⋅CO2 )4 ], respectively (Me2 pz=3,5-dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO2 per [CeIV (Me2 pz)4 ] complex could be inserted to give trimetallic [Ce3 (Me2 pz)9 (Me2 pz⋅CO2 )3 (thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO2 under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co-catalyst tetra-n-butylammonium bromide (TBAB).