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 PrIV complexes. We report that the PrIII/PrIII oxo-bridged complex, [K(tol)K4{PrIII(OSiMe3)5}{PrIII(OSiMe3)4}(μ-O)], 4, can be easily prepared by controlled hydrolysis of the pentakis–siloxide complex [K2PrIII(OSiMe3)5]2, 2, followed by deprotonation. Oxidation of complex 4 with thianthrene tetrafluoroborate (thiaBF4) in the presence of KOSiMe3 afforded the PrIII/PrIV mixed valent complex [K(tol)K4{PrIII/IV(OSiMe3)5}2(μ-O)] 5 that showed localized valence with a very short PrIV═O bond, consistent with multiple bonding interactions. DFT studies further confirmed the presence of a polarized PrIV═O multiple bond along with a very weak ionic PrIII–O interaction. Further oxidation of complex 5 led to the isolation of the PrIV/PrIV complex [K4{PrIV(OSiMe3)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 PrIV complexes closely resemble those of PrIV ions in perovskite oxide lattices. We anticipate the presented strategy will pave the way for the development of new LnIV chemistry.