Nature of the Arsonium‐Ylide Ph₃As=CH₂ and a Uranium(IV) Arsonium–Carbene Complex

Abstract

Treatment of [Ph3EMe][I] with [Na{N(SiMe3)2}] affords the ylides [Ph3E=CH2] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium‐ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long‐proposed hypothesis of increasing pyramidalisation of the ylide‐carbon, highlighting the increasing dominance of E+−C− dipolar resonance form (sp3‐C) over the E=C ene π‐bonded form (sp2‐C), as group 15 is descended. The uranium(IV)–cyclometallate complex [U{N(CH2CH2NSiPri3)2(CH2CH2SiPri2CH(Me)CH2)}] reacts with 1As and 1P by α‐proton abstraction to give [U(TrenTIPS)(CHEPh3)] (TrenTIPS=N(CH2CH2NSiPri3)3; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium‐carbene complex. The short U−C distances and obtuse U‐C‐E angles suggest significant U=C double bond character. A shorter U−C distance is found for 2As than 2P, consistent with increased uranium‐ and reduced pnictonium‐stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.

Reductive Coupling of Xylyl Isocyanide Mediated by Low-Valent Uranium

“…A ctinide carbene complexes have received considerable attention in the past decade, 1−9 due to their unique modes of reactivity and interesting electronic structures. 10−13 Recent advancements in this area include a U(IV) silylphosphino carbene, [U(BIPM TMS ){C(SiMe 3 )(PPh 2 )}-(DMAP) 2 ] (BIPM TMS = C(PPh 2 NSiMe 3 ) 2 ), and a U(IV) arsenido-stabilized carbene, [U(Tren TIPS )(CHAsPh 3 )] (Tren TIPS = N(CH 2 CH 2 NSi i Pr 3 ) 3 ), both reported by Liddle and co-workers, 4,5,14 and a U(IV) carbodiphosphorane, [UCl 4 {C(PPh 3 ) 2 }], reported by Zhu and co-workers, which features a formal U−C "double dative bond". 15 Despite this progress, though, the structural and electronic diversity of actinide carbenes is still relatively limited.…”

Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR2)3}An(CCCPh2)]− (An = U, Th; R = SiMe3)

“… 13 Similarly, the uranium( iv ) arsonium carbene complex [U(Tren TIPS )(CHAsPh 3 )] (Tren TIPS = N(CH 2 CH 2 NSiPr i 3 ) 3 ) features stabilization by an As( v ) substituent. 14 These heteroatom substituents help dissipate the negative charge at the carbene carbon caused by the weak An C π-bond, which itself results from the energetic mismatch between actinide and carbon valence orbitals combined with the relatively small r max of the 5f orbitals. 14 Without these substituents, the An C bond would likely be too reactive to isolate.…”

Polarised covalent thorium(iv)– and uranium(iv)–silicon bonds

“…8,43 A notable feature of these magnetic profiles is the increased low temperature magnetic moment from 1-U to 2-U (0.46 vs. 0.77 m B ), which we attribute to a ligand field effect. 44 The addition of ligands to free 5f ions contributes to the loss of the (2J + 1)-fold ground state degeneracy, imparting significant mixing of excited states into the ''free ion'' ground state. This effect is dictated by the symmetry, orientation, and strength of the ligand field, resulting in variations in the energies between the ground and lowest excited states, impacting on variable temperature magnetic behaviour.…”