Dicerium letterbox-shaped tetraphenolates: f-block complexes designed for two-electron chemistry

Rare examples of molecular, dinuclear Ce^III and Pr^III complexes with robust Ln-coordination are accessible by use of the tetraphenolate pTP as a supporting, chelating O-donor ligand platform, pTP = [{2-(OC₆H₂R₂-2,4)₂CH}-C₆H₄-1,4]⁴⁻ that favours the higher formal oxidation states accessible to rare earths. Two classes of complexes have been made from the platforms; one metallacyclic 2 + 2 [Ln₂(pTP)₂] framework with a rigid, letterbox-shaped geometry and [Ln(aryloxide)₄] core, and one more flexible [(LnX)₂(pTP)] with one rare earth ion at either end of the platform. The Ln^III letterbox complexes have two K⁺ counter-cations, one of which sits inside the letterbox, binding the two central arenes of the platform sufficiently strongly that it cannot be displaced by solvent molecules (THF and pyridine) or crown ethers. Oxidation of the Ce^III lettterboxes is facile and forms the unusual neutral molecular (Ce^IV)₂ letterbox in which the Ce^IV reduction potential is −1.83 V vs. Fc/Fc⁺. The electronic structure of the Ce(III/IV) complexes was investigated using HERFD-XAS (high energy resolution fluorescence detection X-ray absorption spectroscopy).