Why do U react like that? Reaction of 2-Li-C6H4CH2NMe2 with [MCl4(DME)n] (M=Th, n=2; M=U, n=0) results in the formation of a thorium aryl complex, [Th(2-C6H4CH2NMe2)4] or a uranium benzyne complex, [Li][U(2,3-C6H3CH2NMe2)(2-C6H4CH2NMe2)3]. A DFT analysis suggests that the formation of a benzyne complex with U but not with Th is a kinetic and not thermodynamic effect.

We report herein the synthesis of the first structurally characterized homoleptic actinide aryl complexes, [Li(DME) 3 ] 2 [Th(C 6 H 5 ) 6 ] (1) and [Li(THF)(12-crown-4)] 2 [Th(C 6 H 5 ) 6 ] (2), which feature an anion possessing a regular octahedral (1) or a severely distorted octahedral (2) geometry.The solid-state structure of 2 suggests the presence of pseudo-agostic ortho C-HÁ Á ÁTh interactions, which arise from r(C-H) -Th(5f) donation. The non-octahedral structure is also favoured in solution at low temperatures.High-valent homoleptic transition metal complexes of the type [MR 6 ] q (R = alkyl, aryl; q = 0, À1, À2) have proven to be incredibly useful for exploring the nature of the M-C bond. [1][2][3][4][5] In particular, the observation of a trigonal prismatic structure for these complexes, as opposed to the more common octahedral geometry, is considered strong evidence for the involvement of the d orbitals in the metal-carbon bonds. 6 ]. 12,13 The latter was oxidized in situ to U(CH 2 SiMe 3 ) 6 , which was characterized using 13 C and 1 H NMR spectroscopies, accompanied by relativistic DFT calculations of NMR chemical shifts. 16 In each uranium example, an octahedral geometry was observed, which is perhaps not surprising, as significant f-orbital participation in bonding tends to favor an octahedral geometry. 17 In contrast, d orbital involvement in Th-C bonding is expected to be more significant than f orbital participation, 18,19 and therefore the observation of a trigonal prismatic structure should be more likely for this element relative to the other actinides. 10 Given this consideration, we endeavored to synthesize and structurally characterize the homoleptic actinide-aryl complex, [Th(C 6 H 5 ) 6 ] 2À , by reacting ThCl 4 with an excess of phenyl lithium.Addition of 8 equiv. of PhLi to a THF solution of ThCl 4 (DME) 2 results in the formation of a white solid and a dark amber solution. Isolation of this amber material, followed by recrystallization from concentrated DME, afforded [Li(DME) 3 ] 2 [Th(C 6 H 5 ) 6 ] (1) as a pale yellow crystalline material in 56% yield (Scheme 1). Complex 1 is thermally unstable at room temperature (RT), both as a solid and in solution, but is stable for up to 2 weeks as a solid at À25 1C. It is insoluble in hexane, aromatic solvents, and diethyl ether, but very soluble in THF and DME. Its 1 H NMR spectrum in THF-d 8 at RT reveals resonances at 7.61 ppm, 7.00 ppm and 6.79 ppm in the ratio of 2 : 2 : 1, corresponding to the ortho, meta, and para protons of the phenyl ring, respectively. In addition, there is a single resonance in its 7 Li{ 1 H} NMR spectrum at 2.60 ppm. The 13 C{ 1 H} NMR spectrum in THF-d 8 exhibits a key fingerprint resonance at 220.5 ppm, which corresponds to the ipso-carbon, as also confirmed by our relativistic DFT calculations of the NMR chemical shifts (cf. Table S2 in the ESI †). This resonance is shifted notably downfield from the corresponding 13 C-ipso shifts in related Ti, Zr, and Hf aryl complexes (which typically appear between 183 an...

Reaction of UO 2 (dbm) 2 (THF) (dbm ¼ OC(Ph)CHC(Ph)O) with 1 equiv. of R 3 SiH (R ¼ Ph, Et), in the presence of B(C 6 F 5) 3 , results in the formation of U(OB{C 6 F 5 } 3)(OSiR 3)(dbm) 2 (THF) (R ¼ Ph, 1; Et, 2), which were isolated as red-orange crystalline solids in good yields. Interestingly, the addition of 1 equiv. of H(dbm) to 2 results in protonation of the-OSiEt 3 ligand and formation of U(OB{C 6 F 5 } 3)(dbm) 3 (4) in 33% yield, along with formation of HOSiEt 3. Furthermore, addition of HOSiEt 3 and 1 equiv. of THF to 4, results in the formation 2, revealing that this process is reversible. The two-step conversion of UO 2 (dbm) 2 (THF) to 4 represents a rare example of controlled uranyl oxo ligand cleavage at ambient temperature and pressure. Comparison of diffraction and density functional theory data for 4 suggests the presence of the inverse trans influence, with a very shallow potential energy well for distortion along the trans U-O bond.

Reaction of [UO 2 Cl 2 (THF) 2 ] 2 with 2 equiv of H N4 ( H N4 = 2,11-diaza [3,3](2,6) pyridinophane) or Me N4 ( Me N4 = N,3]

Reaction of [U VI O 2 (dppmo) 2 (OTf)][OTf] (dppmo = Ph 2 P(O)CH 2 P(O)Ph 2 ) with 4 equiv of