Cerium(III/IV) formamidinate chemistry, and a stable cerium(IV) diolate

Article abstract of DOI:10.1002/chem.201304582

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(SiHMe ₂)₂}₃(thf)₂] and [Li{N(SiHMe ₂)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. Figure ate: The reaction of ate complex [Li(thf)Ce{N(SiHMe ₂)₂}₄ with C₂Cl₆ gives straightforward access to the homoleptic complex [Ce{N(SiHMe₂) ₂}₄], which engages in protonolysis reactions (see scheme) with formamidines to afford the first Ce^IV formamidinate species [Ce(p-TolForm)₄] (p-TolForm=N,N′-bis(4-methylphenyl) formamidinato).