Synthesis and Characterization of an Oxo-Centered Homotrimetallic Uranium(IV)–Cyclobutadienyl Dianion Complex

Abstract

Reaction of [Li 2 {C 4 (SiMe 3 ) 4 }(THF) 2 ] (1) with [U(η 5 -C 5 Me 5 )I 2 (THF)] (2) produced the oxo-centered homotrimetallic uranium−pentamethylcyclopentadienyl complex [{U(η 5 -C 5 Me 5 )(μ-I) 2 } 3 {μ 3 -O}{Li(THF) 3 } 0.5 ] 2 [Li(THF) 4 ] (3) as the only isolable product in a very low yield. In contrast, reaction of 2 with [Mg{C 4 (SiMe 3 ) 4 }(THF) 3 ] (4) produced the oxo-centered homotrimetallic uranium(IV)−cyclobutadienyl complex [{U(C 4 [SiMe 3 ] 4 )(μ-I) 2 } 3 {μ 3 -O}][Mg(THF) 6 ] (5). The solid state structure of 5 reveals average U−C and U−C 4 centroid distances of 2.574( 7) and 2.355(7) Å, respectively, and displacements of the silicon atoms from the C 4 plane ranging from 0.478(13) to 0.6528(12) Å. Variable-temperature magnetic susceptibility measurements on powdered 5 confirm the exclusively uranium(IV) formulation with no evidence found for any uranium•••uranium magnetic coupling. Quantum chemical calculations suggest polarized uranium−cyclobutadienyl bonding interactions but underscore the essentially exclusive π-bonding nature of these linkages with no δ-bonding component. This π-bonding also accounts for the displacements of the silyl groups from the C 4 plane, which maximizes U−C 4 orbital overlap. The compounds reported here have been variously characterized by single-crystal X-ray diffraction, ATR-IR spectroscopy, elemental analysis, SQUID magnetometry, and quantum chemical calculations.

Heteroleptic actinocenes: a thorium(iv)–cyclobutadienyl–cyclooctatetraenyl–di-potassium-cyclooctatetraenyl complex

“…The average cyclobutadienyl C 4 -ring C-C distances in 1 are identical at 1.481(8) A, and statistically indistinguishable from the analogous metric for A (1.488(17) A). [8][9][10] The displacement of the cyclobutadienyl silyl substituents in 1, out of the plane of the C 4 -ring away from the respective thorium centres, is notable due to their asymmetry. Opposing silyl groups are displaced by similar distances to each other, but each pair is quite different to the other.…”

“…6,7 Very recently, the uranium(IV)-cyclobutadienyl half sandwich pianostool complex [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Li(THF) 4 ] (A) was reported, 8 and shortly aer that four uranium(IV)-cyclobutadienyl complexes [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Na(12-crown-4)], 9 [U{C 4 -(SiMe 3 ) 4 }(BH 4 ) 2 (m-BH 4 ){K(THF) 2 }] 2 , 9 [U(BH 4 ){C 4 (SiMe 3 ) 4 }{k 3 -C 4 H(SiMe 3 ) 3 -k-(CH 2 SiMe 2 )}][Na( t BuOMe) 3.6 (THF) 0.4 ], 9 and [{U(C 4 [SiMe 3 ] 4 )(m-I) 2 } 3 {m 3 -O}][Mg(THF) 6 ] were disclosed. 10 These six uranium-cyclobutadienyl complexes constitute all actinidecyclobutadienyl chemistry to date, in stark contrast to the plethora of reported f-element h n -carbocyclic C 5-9 complexes. 11 Thus, a thorium-cyclobutadienyl complex of any kind is conspicuous by its absence, but realising such a target would provide comparisons between uranium and thorium and provide further insight into actinide-cyclobutadienyl bonding.…”

The Emergence of Actinide Cyclobutadienyl Chemistry

“…The average U-C distance in 9 is 2.574(7) Å;; statistically indistinguishable from that measured for halfsandwich complex 5 (2.513(17) Å), despite the differing uranium coordination environments in the two complexes. [22,31] As found for all crystallographically characterised uraniumtetra(trimethylsilyl)cyclobutadienyl complexes, in 9 the cyclobutadienyl silyl substituents bend out of the plane of the C4ring, away from the coordinated uranium centre. These displacements range from 0.478(13) to 0.652(12) Å, which is a broader range than found for 5.…”

“…[32] However, it was found that reaction of [U(η 5 -C5Me5)I2(THF)] with [Mg{C4(SiMe3)4}(THF)2] (1Mg) in benzene led to the formation of the oxo-centred homotrimetallic uranium(IV)-cyclobutadienyl complex [{U(C4[SiMe3]4)(μ-I)2}3{μ3-O}][Mg(THF)6] (9) in 19% crystalline yield (Scheme 5). [24,31] The reasons for the divergent reactivities displayed by 1Li and 1Mg towards [U(η 5 -C5Me5)I2(THF)] are unclear, but are perhaps a consequence of the differing Lewis acidities of the two s-block metals. As determined by X-ray diffraction, complex 9 features three uranium(IV) ions in a trigonal planar arrangement around a central μ3-oxo ligand, with two bridging μ-iodide ligands between each pair of uranium centres.…”

“…As determined by X-ray diffraction, complex 9 features three uranium(IV) ions in a trigonal planar arrangement around a central μ3-oxo ligand, with two bridging μ-iodide ligands between each pair of uranium centres. [31] Therefore, all three uranium centres in 9 adopt pseudo-octahedral half-sandwich geometries, with each coordinated by four iodide ligands, the oxo ligand and an η 4cyclobutadienyl ligand. The average U-C distance in 9 is 2.574(7) Å;; statistically indistinguishable from that measured for halfsandwich complex 5 (2.513(17) Å), despite the differing uranium coordination environments in the two complexes.…”