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 <i>N</i>-heterocyclic carbene-neptunium(III) complexes that exhibit polarized-covalent σ²π² multiple and dative σ² single transuranium-carbon bond interactions, respectively. The reaction of [Np^IIII₃(THF)₄] with [Rb(BIPM^TMSH)] (BIPM^TMSH = {HC(PPh₂NSiMe₃)₂}^1-) affords [(BIPM^TMSH)Np^III(I)₂(THF)] (<b>3Np</b>) in situ, and subsequent treatment with the <i>N</i>-heterocyclic carbene {C(NMeCMe)₂} (I^Me4) allows isolation of [(BIPM^TMSH)Np^III(I)₂(I^Me4)] (<b>4Np</b>). Separate treatment of in situ prepared <b>3Np</b> with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM^TMS)Np^III(I)(DME)] (<b>5Np</b>, BIPM^TMS = {C(PPh₂NSiMe₃)₂}^2-). Analogously, addition of benzyl potassium and I^Me4 to <b>4Np</b> gives [(BIPM^TMS)Np^III(I)(I^Me4)₂] (<b>6Np</b>). The synthesis of <b>3Np</b>-<b>6Np</b> 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 <b>4Np</b>-<b>6Np</b> 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.