Synthesis, structure and bonding of hexaphenyl thorium(iv): observation of a non-octahedral structure

Abstract

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...

Homoleptic Aryl Complexes of Uranium (IV)

“…Compared to complexes 1 and 2,complex 3 is significantly less distorted from octahedral symmetry as it lacks lithium counterion interaction with any of the aryl rings.L ow temperature X-ray experiments resulted in ad isorder over two positions of the one crystallographically unique aryl ring. Unfortunately,d ue to the limitation of the crystallographic À3(S 6 )symmetry and having just one unique ligand, the exact distortion from octahedral of the six-coordinate complex is difficult to define.U sing the distorted thorium hexaphenyl complex reported by Hayton and co-workers, [14] the geometry was assigned as that observed in Figure 3A.T he increased thermal stability of 3 (stable at 0 8 8Cfor several hours) enabled X-ray experiments at elevated temperatures (243 K) to be performed to evaluate the effects of temperature on the structural distortion of the complex. Ad ecrease in distortion was observed at elevated experimental temperatures where the dianion is ordered with respect to the crystallographic À3( S 6 )p osition and an early octahedral geometry is observed ( Figure 3B).…”

“…It is of interest to note,h owever,t hat Hayton and co-workers have recently reported two homoleptic thorium aryl-ate complexes,e ach with unique geometries about the actinide metal center (Figure 1). [14] We were motivated to re-explore this longstanding challenge in the preparation of homoleptic uranium-aryl complexes with non-sterically encumbered aryl ligands utilizing our low temperature synthetic capabilities previously successfully applied to similar synthetic challenges in homoleptic iron-aryl chemistry. [15] Herein, we report the first homoleptic six-coordinate uranate (IV) complexes containing only uranium-carbon s-bonds between auranium center and phenyl, p-tolyl and p-Cl-phenyl ligands and, combined with spectroscopic and theoretical studies,d evelop insight into electronic structure and bonding in these unique complexes.…”

“…This type of agostic interaction has also been observed by Hayton et al in the previously reported distorted six-coordinate thorium phenyl-ate species. [14] Noting the structural similarities of complexes 1 and 2, isolation of an analogous [U(Ar) 6 ] 2À complex containing an electron-poor aryl ring system, p-chlorophenyl, was also targeted. Drop-wise addition of 6equiv of p-chlorophenyllithium (p-Cl-PhLi)t o[ UCl 4 (1,4-dioxane)] 2 in Et 2 O/THF at À80 8 8Cr esulted in an immediate color change from yellow/ orange to very dark red, nearing black.…”

Homoleptic Aryl Complexes of Uranium (IV)

“…This type of agostic interaction has also been observed by Hayton et al. in the previously reported distorted six‐coordinate thorium phenyl‐ate species …”

“…Despite this important advancement, homoleptic uranium–aryl complexes beyond the +3 oxidation state and containing non‐sterically encumbering substituents remain an unsolved synthetic problem. It is of interest to note, however, that Hayton and co‐workers have recently reported two homoleptic thorium aryl‐ate complexes, each with unique geometries about the actinide metal center (Figure ) …”