Redox-enhanced hemilability of a tris(tert-butoxy)siloxy ligand at cerium

Abstract

The stabilizing effect of a tris(tert-butoxy)siloxy ligand on cerium(IV) is revealed by electrochemical and computation methods as well as by targeted redox chemistry. Ceric homoleptic complex Ce[OSi(OtBu)₃]₄ was obtained by the reaction of [Et₄N]₂[CeCl₆] with NaOSi(OtBu)₃ at ambient temperature in acetonitrile, while cerous ion-separated complex [Ce{OSi(OtBu)₃}₄][K(2.2.2-crypt)] was readily synthesized from [Ce{OSi(OtBu)₃}₄K] and cryptand. The solid-state structures of monocerium complexes Ce[OSi(OtBu)₃]₄ and Ce[OSi(OtBu)₃]₄(THF) show 5- and 6-coordinate Ce^IV centers (one κ²-bonded siloxy ligand), while complex [Ce{OSi(OtBu)₃}₄][K(2.2.2-crypt)] exhibits a 4-coordinate Ce^III center (all-terminal siloxy coordination). A comparative electrochemical study of Ce[OSi(OtBu)₃]₄ and [Ce{OSi(OtBu)₃}₄][K(2.2.2-crypt)] suggests a redox-modulated molecular rearrangement process, featuring oxidation-state dependent formation and release of a Ce⋯OtBu coordination. While the overall stabilization of Ce^IV by the siloxy ligand is evident, significant extra stabilization is gained if the siloxy ligand coordinates in a chelating fashion, which is further supported by DFT calculations. Natural bond orbital (NBO) analysis indicates an enhanced donation of the siloxy ligand electron density into the unfilled Ce^IV 6s, 4f, and 5d orbitals. Ce^IV to Ce^III reduction readily occurs when homoleptic complex Ce[OSi(OtBu)₃]₄ is treated with cobaltocene, affording the separated ion pair [Ce{OSi(OtBu)₃}₄][CoCp₂], featuring exclusive terminal siloxy bonding in the solid-state, similar to that detected for [Ce{OSi(OtBu)₃}₄][K(2.2.2-crypt)].