PCET-driven Reactivity of Neptunyl(VI) Yields Oxo-bridged Np(V) and Np(IV) Species

A reaction sequence based upon the principles of proton-coupled electron transfer (PCET) has been used to access two unconventional oxo-deficient polynuclear complexes of neptunium (Np). The complexes featuring mono-μ2-oxo motifs assemble under ambient atmosphere upon dissolution of neptunyl(VI) diacetate dihydrate (NpO2(OAc)2(H2O)2·HOAc) in methanol followed by addition of a supporting pentadentate ligand (LNM); one complex is a mixed-valent [NpV,NpIV,NpV ] trimer with two bridging μ2-oxos and the other is a [NpV,NpV ] dimer featuring a single μ2-oxo. The outer Np centers are also capped with terminal oxo ligands. Spectroscopic and spectrokinetic findings show that intermediate [NpV O2(OAc)]n species form prior to metal chelation by LNM; electrolysis experiments demonstrate that production of Np(V) gives rise to asynchronous proton transfer that does not occur otherwise (in the Np(VI) state) as well as condensation (loss of H2O) and formation of the polynuclear complexes. The oxo-deficient nature of these products is attributable to the reduction/condensation reaction sequence of PCET. Consequently, PCET reactivity appears poised to complement more established techniques for interconverting actinide oxidation states, a prospect with considerable applications in fuel recycling for low-carbon nuclear energy.