Transient Triamidoamine Neptunium(V)–Mono(Imido) Complexes: C–H Activations and Hydrogen Atom Transfer Driven by Effective Nuclear Charge

: 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 [NpIII(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) with N3R (R = SiMe3; 1-adamantyl, Ad) results in N2 evolution and dark purple solutions consistent with the formation of [NpV(TrenTIPS)(NR)] (3NpNR). However, solutions of 3NpNR rapidly turn orange, where for R = SiMe3 the isolated 1:1 products are [NpIV(TrenTIPS){N(H)SiMe3}] (4a) and [NpIV(TrenTIPS-2H){N(H)SiMe3}] (4b, TrenTIPS-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 [NpIV(TrenTIPS){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)][UIV(TrenTIPS)(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.