Synthesis and Properties of Stable Oxo-Bridged Dinuclear Pr ^IV Complexes

The development of the molecular chemistry of lanthanides in the +4 oxidation state has encountered significant challenges due to their limited solution stability. Here, we demonstrate that combining siloxide ligands with oxo ligands leads to highly stable Pr IV complexes. We report that the Pr III /Pr III oxo-bridged complex, [K(tol)K 4 {Pr III (OSiMe 3 ) 5 }{Pr III (OSiMe 3 ) 4 }(μ-O)], 4, can be easily prepared by controlled hydrolysis of the pentakis–siloxide complex [K 2 Pr III (OSiMe 3 ) 5 ] 2, 2, followed by deprotonation. Oxidation of complex 4 with thianthrene tetrafluoroborate (thiaBF 4 ) in the presence of KOSiMe 3 afforded the Pr III /Pr IV mixed valent complex [K(tol)K 4 {Pr III/IV (OSiMe 3 ) 5 } 2 (μ-O)] 5 that showed localized valence with a very short Pr IV ═O bond, consistent with multiple bonding interactions. DFT studies further confirmed the presence of a polarized Pr IV ═O multiple bond along with a very weak ionic Pr III –O interaction. Further oxidation of complex 5 led to the isolation of the Pr IV /Pr IV complex [K 4 {Pr IV (OSiMe 3 ) 5 } 2 (μ-O)], 6 . Complex 6 shows remarkable solution stability at room temperature over a long period of time. Interestingly, the EPR spectra and magnetic properties of these molecular Pr IV complexes closely resemble those of Pr IV ions in perovskite oxide lattices. We anticipate the presented strategy will pave the way for the development of new Ln IV chemistry.