Pyrazolates advance cerium chemistry: a CeIII/CeIV redox equilibrium with benzoquinone

Abstract

Two stable cerium(IV) 3,5-dialkylpyrazolate complexes are presented, namely dimeric [Ce(Me₂pz)₄]₂ (Me₂pz = 3,5-dimethylpyrazolate) and monomeric Ce(tBu₂pz)₄ (tBu₂pz = 3,5-di-tert-butylpyrazolate) along with their trivalent counterparts [Ce(Me₂pz)₃] and [Ce(tBu₂pz)₃]₂. All complexes were obtained from protonolysis reactions employing the silylamide precursors Ce[N(SiHMe₂)₂]₄ and Ce[N(SiMe₃)₂]₃. Treatment of homoleptic Ce^IV and Ce^III Me₂pz complexes with 1,4-hydroquinone (H₂hq) or 1,4-benzoquinone (bq), respectively, ultimately gave the same trimetallic Ce^III species via a cerium redox equilibrium. The Ce^III complex Ce₃(Me₂pz)₅(pchd)₂(L) (pchd = 1,4-bis(3,5-dimethylpyrazol-1-yl)cyclohex-2,5-diene-1,4-diolato; L = Me₂pzH or (thf)₂) results from a di-1,4-pyrazolyl attack on pre-coordinated bq. The reduction of bq by [Ce(Me₂pz)₃(thf)]₂, and re-oxidation by the resulting Ce^IV species was supported by UV-vis spectroscopic investigations. Comparisons with the redox-innocent complexes [Ln(Me₂pz)₃(thf)]₂ (Ln = La and Pr) revealed far less selective reactions with bq, giving hexametallic and octametallic rare-earth metal side products containing 2-Me₂pz substituted hq ligands.