Cerium(III) Azolate Promoted CO₂ Insertion

A series of sandwich cerium azolates (pyrazolates, triazolates, tetrazolates) has been synthesized via salt-metathesis (cerous precursor: [Cp*₂CeCl₂K(thf)]_n; Cp* = C₅Me₅) and protonolysis protocols (cerous precursors: Cp*₂Ce[N(SiHMe₂)₂] or Cp*₂Ce[N(SiMe₃)₂]) and their carboxophilicity has been probed. The sandwich and half-sandwich complexes Cp*₂Ce(pz^Me,Me) and Cp*Ce(pz^Me,Me)₂(thf)₂ show exhaustive CO₂ insertion into the pyrazolato moieties, which leads to bis(carbamato)-bridged dimeric complexes. Complex Cp*₂Ce(pz^Ph,Ph), featuring the sterically more demanding and less nucleophilic diphenylpyrazolato ligand inserts only one molecule CO₂ per two metal centers, forming the asymmetrically bridged complex Cp*₂Ce(μ-pz^Ph,Ph·CO₂)CeCp*₂(pz^Ph,Ph). The triazolato derivatives Cp*₂Ce(tz^Me,Me)(dmap) and Cp*₂Ce(tz^Ph,Ph) indicate CO₂ insertion only for the DMAP-free complex. Utilizing the nitrogen-richer 5-phenyltetrazole resulted in the formation of the trimer [Cp*₂Ce(tet^Ph)]₃, which does not insert CO₂ at ambient temperature. The absorption and electrochemical properties of the compounds were investigated and their catalytic activity in the cycloaddition of CO₂ and propylene oxide was examined.