PCET-Driven Reactivity of Neptunyl(VI) Yields Oxo-Bridged Np(V) and Np(IV) Species.

Two unconventional polynuclear complexes of neptunium (Np) featuring mono- μ 2 ${\mu _2 }$ -oxo motifs have been accessed by proton-coupled electron transfer (PCET) reactivity involving the dissolution of neptunyl(VI) diacetate dihydrate ( NpO 2 ( OAc ) 2 ( H 2 O ) 2 · ${{\rm{NpO}}_{\rm{2}} {\rm{(OAc)}}_{\rm{2}} {\rm{(H}}_{\rm{2}} {\rm{O)}}_{\rm{2}} \cdot }$ HOAc) in methanol followed by addition of a pentadentate Schiff-base ligand. One complex is a mixed-valent [Np V ${^V }$ ,Np I V ${^{IV} }$ ,Np V ${^V }$ ] trimer with two bridging μ 2 ${\mu _2 }$ -oxos and the other is a [Np V ${^V }$ ,Np V ${^V }$ ] dimer featuring a single μ 2 ${\mu _2 }$ -oxo. In both complexes the outer Np centers are capped with terminal oxo ligands. Spectroscopic and spectrokinetic studies aimed at elucidating mechanistic details of complex formation in this system show that intermediate multinuclear [Np V O 2 ${^V {\rm{O}}_2 }$ (OAc)] n ${_n }$ species form prior to metal chelation by the ligand; 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 with loss of H 2 O ${{\rm{H}}_{\rm{2}} {\rm{O}}}$ and formation of the polynuclear complexes. We attribute the oxo-deficient nature of these products, with respect to conventional actinyl ([AnO 2 ${_2 }$ ] m + ${^{m + } }$ ) species, to the reduction/condensation reaction sequence of PCET.