Carbene Complexes of Neptunium

Abstract

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 σ ²π ² multiple and dative σ ² single transuranium-carbon bond interactions, respectively. The reaction of [Np ^IIII ₃(THF) ₄] with [Rb(BIPM ^TMSH)] (BIPM ^TMSH = {HC(PPh ₂NSiMe ₃) ₂} ^1–) affords [(BIPM ^TMSH)Np ^III(I) ₂(THF)] ( 3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe) ₂} (I ^Me4) allows isolation of [(BIPM ^TMSH)Np ^III(I) ₂(I ^Me4)] ( 4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM ^TMS)Np ^III(I)(DME)] ( 5Np, BIPM ^TMS = {C(PPh ₂NSiMe ₃) ₂} ^2–). Analogously, addition of benzyl potassium and I ^Me4 to 4Np gives [(BIPM ^TMS)Np ^III(I)(I ^Me4) 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 Np ^III=C bonds are more covalent than U ^III=C, Ce ^III=C, and Pm ^III=C congeners but comparable to analogous U ^IV=C bonds in terms of bond orders and total metal contributions to the M=C bonds. A preliminary assessment of Np ^III=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.