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 N-heterocyclic carbene–neptunium(III) complexes that exhibit polarized-covalent σ²π² multiple and dative σ² single transuranium-carbon bond interactions, respectively. The reaction of [NpᴵᴵᴵI₃(THF)₄] with [Rb(BIPMᵀᴹSH)] (BIPMᵀᴹSH = {HC(PPh₂NSiMe₃)₂}¹–) affords [(BIPMᵀᴹSH)Npᴵᴵᴵ(I)₂(THF)] (3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe)₂} (Iᴹᵉ⁴) allows isolation of [(BIPMᵀᴹSH)Npᴵᴵᴵ(I)₂(Iᴹᵉ⁴)] (4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPMᵀᴹS)Npᴵᴵᴵ(I)(DME)] (5Np, BIPMᵀᴹS = {C(PPh₂NSiMe₃)₂}²–). Analogously, addition of benzyl potassium and Iᴹᵉ⁴ to 4Np gives [(BIPMᵀᴹS)Npᴵᴵᴵ(I)(Iᴹᵉ⁴)₂] (6Np). The synthesis of 3Np–6Np 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 4Np–6Np 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 Npᴵᴵᴵ═C bonds are more covalent than Uᴵᴵᴵ═C, Ceᴵᴵᴵ═C, and Pmᴵᴵᴵ═C congeners but comparable to analogous Uᴵⱽ═C bonds in terms of bond orders and total metal contributions to the M═C bonds. A preliminary assessment of Npᴵᴵᴵ═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.