Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

The homoleptic pyrazolate complexes [Ce(III) (4)(Me(2)pz)(12)] and [Ce(IV)(Me(2)pz)(4)](2) quantitatively insert CO(2) to give [Ce(III) (4)(Me(2)pz⋅CO(2))(12)] and [Ce(IV)(Me(2)pz⋅CO(2))(4)], respectively (Me(2)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 CO(2) per [Ce(IV)(Me(2)pz)(4)] complex could be inserted to give trimetallic [Ce(3)(Me(2)pz)(9)(Me(2)pz⋅CO(2))(3)(thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO(2) under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co‐catalyst tetra‐n‐butylammonium bromide (TBAB).