Cerium(III/IV) Formamidinate Chemistry, and a Stable Cerium(IV) Diolate

Four new cerium(III) formamidinate complexes comprising [Ce(p-TolForm)(3)], [Ce(DFForm)(3)(thf)(2)], [Ce(DFForm)(3)], and [Ce(EtForm)(3)] were synthesized by protonolysis reactions using [Ce{N(SiMe3)(2)}(3)] and formamidines of varying functionality, namely N,N-bis(4-methylphenyl)formamidine (p-TolFormH), N,N-bis(2,6-difluorophenyl)formamidine (DFFormH), and the sterically more demanding N,N-bis(2,6-diethylphenyl)formamidine (EtFormH). The bimetallic cerium lithium complex [LiCe(DFForm)(4)] was synthesized by treating a mixture of [Ce{N(SiHMe2)(2)}(3)(thf)(2)] and [Li{N(SiHMe2)(2)}] with four equivalents of DFFormH in toluene. Oxidation of the trivalent cerium(III) formamidinate complexes by trityl chloride (Ph3CCl) caused dramatic color changes, although the cerium(IV) species appeared transient and reformed cerium(III) complexes and N-trityl-N,N-diarylformamidines shortly after oxidation. The first structurally characterized homoleptic cerium(IV) formamidinate complex [Ce(p-TolForm)(4)] was obtained through a protonolysis reaction between [Ce{N(SiHMe2)(2)}(4)] and four equivalents of p-TolFormH. [Ce{N(SiHMe2)(2)}(4)] was also treated with DFFormH and EtFormH, but the resulting cerium(IV) complexes decomposed before isolation was possible. The new cerium(IV) silylamide complex [Ce{N(SiMe3)(2)}(3)(bda)(0.5)](2) (bda=1,4-benzenediolato) was synthesized by treatment of [Ce{N(SiMe3)(2)}(3)] with half an equivalent of 1,4-benzoquinone, and showed remarkable resistance towards protonolysis or reduction.