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 σ 2 π 2 multiple and dative σ 2 single transuranium-carbon bond interactions, respectively. The reaction of [Np III I 3 (THF) 4 ] with [Rb(BIPM TMS H)] (BIPM TMS H = {HC(PPh 2 NSiMe 3 ) 2 } 1- ) affords [(BIPM TMS H)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 TMS H)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 2 NSiMe 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.