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

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 CeIV centers (one κ2-bonded siloxy ligand), while complex [Ce{OSi(OtBu)3}4][K(2.2.2-crypt)] exhibits a 4-coordinate CeIII 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 CeOtBu coordination. While the overall stabilization of CeIV 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 CeIV 6s, 4f, and 5d orbitals. CeIV to CeIII 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)].