Transient Triamidoamine Neptunium(V)-Mono(Imido) Complexes: C-H Activations and Hydrogen Atom Transfer Driven by...

Metal-mono(imido) linkages have been known for seven decades, and they are found in transition metal, main group, lanthanide, thorium, and uranium complexes. However, transuranium-mono(imido) complexes remain unknown in any scenario. Here, we present evidence for transient neptunium(V)-mono(imido) complexes. Treatment of [Np^III(Tren^TIPS)] (1, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-) with N₃R (R = SiMe₃; 1-adamantyl, Ad) results in N₂ evolution and dark purple solutions consistent with the formation of [Np^V(Tren^TIPS)(NR)] (3NpNR). However, solutions of 3NpNR rapidly turn orange, where for R = SiMe₃ the isolated 1:1 products are [Np^IV(Tren^TIPS){N(H)SiMe₃}] (4a) and [Np^IV(Tren^TIPS-2H){N(H)SiMe₃}] (4b, Tren^TIPS-2H = {N(CH₂CH₂NSiPrⁱ₃)₂(NCH₂CH₂NSiPrⁱ₂C[Me]=CH₂)}^3-). The latter contains a dehydrogenated-Prⁱ vinyl functionality accounting for the source of the two amido H atoms. The reaction for R = Ad proceeds similarly, but only [Np^IV(Tren^TIPS){N(H)Ad}] (5a) could be unequivocally confirmed, though its isolation suggests generality of the imido-to-amido functional group transformation. Complexes 4a/4b exhibit slow relaxation of their magnetization, adding to the small number of transuranium single ion magnets. Experimental and computational analysis suggests that the amido products are formed by C-H activation and two sequential hydrogen atom transfer reactions involving a three-step proton-coupled electron-transfer sequence of H^• radical abstraction, electron transfer, then another H^• radical abstraction step. In contrast to transient 3NpNR, the 5f² uranium(IV)-imido complex [K(2.2.2-cryptand)][U^IV(Tren^TIPS)(NSiMe₃)] (8UNSiMe₃) is robust, even in boiling THF, suggesting the transience of 5f² 3NpNR is not due to the 5fⁿ-count but the increased effective nuclear charge of neptunium vs uranium. This work highlights divergence of uranium- and neptunium-imido stabilities, emphasizing that the latter is an inherently challenging synthetic target.