Compared to the extensive investigations of d-block metalligand multiple bonding and reactivity, [1] the corresponding field of f-block chemistry is underdeveloped. [2,3] For uranium, imido and oxo complexes dominate, yet there is a paucity of uranium carbenes that do not derive from neutral free carbenes. [3] The first uranium carbenes, [U(h 5 -C 5 H 5 ) 3 (CHPMe 2 R)] (R = Ph, I; Me, II), were reported by Gilje et al. [4] Uranium carbenes have been detected in matrix isolation experiments, [5] and implicated in reactions of ketones with UCl 4 / Li(Hg). [6] Recently, Ephritikhine et al. reported a range of uranium carbenes, exemplified by [U{C(PPh 2 S) 2 }(BH 4 ) 2 -(THF) 2 ], [7] and, as part of a program studying f-block carbenes, [8] we reported the homoleptic uranium carbene [U{C(PPh 2 NMes) 2 } 2 ] (1, Mes = 2,4,6-trimethylphenyl). [9] To date, all uranium carbenes with U À C multiple bonds incorporate uranium(IV). Higher-valence analogues are notable for their absence, which contrasts to the dominance of high-oxidation-state uranium oxo and imido complexes. [3,10] However, pentavalent uranium chemistry has been revitalized recently, [10] and encouraged by this and the absence of any uranium(V) carbenes we targeted a pentavalent uranium carbene by an oxidation strategy. Herein, we report the facile synthesis, structure, and reactivity of the first pentavalent uranium carbene, which permits direct comparisons between U V =C and U IV =C bonds for the first time.
Complex 1 was prepared from a disproportionation reaction between [UI 3 (thf) 4 ] and [Li 4 {C(PPh 2 NMes) 2 } 2 ]. [9] We therefore employed [UCl 4 (thf) 3 ] and treated it with [Li 4 {C(PPh 2 NSiMe 3 ) 2 } 2 ] [11] in toluene/Et 2 O (Scheme 1). The uranium(IV) carbene [U{C(PPh 2 NSiMe 3 ) 2 }(Cl)(m-Cl) 2 Li-(thf) 2 ] (2) was isolated as yellow plates in 62 % yield following workup and recrystallization from THF. [12] The molecular structure of 2, as determined by X-ray crystallography, is shown in Figure 1 a with selected bond