Carbene Complexes of Neptunium.

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and -heterocyclic carbene-neptunium(III) complexes that exhibit polarized-covalent σπ multiple and dative σ single transuranium-carbon bond interactions, respectively. The reaction of [NpI(THF)] with [Rb(BIPMH)] (BIPMH = {HC(PPhNSiMe)}) affords [(BIPMH)Np(I)(THF)] () in situ, and subsequent treatment with the -heterocyclic carbene {C(NMeCMe)} (I) allows isolation of [(BIPMH)Np(I)(I)] (). Separate treatment of in situ prepared with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM)Np(I)(DME)] (, BIPM = {C(PPhNSiMe)}). Analogously, addition of benzyl potassium and I to gives [(BIPM)Np(I)(I)] (). The synthesis of - was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of - provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these Np═C bonds are more covalent than U═C, Ce═C, and Pm═C congeners but comparable to analogous U═C bonds in terms of bond orders and total metal contributions to the M═C bonds. A preliminary assessment of Np═C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.