Actinide arene-metalates: ion pairing effects on the electronic structure of unsupported uranium–arenide sandwich complexes

Addition of [UI(2)(THF)(3)(μ-OMe)](2)·THF (2·THF) to THF solutions containing 6 equiv. of K[C(14)H(10)] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF)(2)](2)[U(η(6)-C(14)H(10))(η(4)-C(14)H(10))(μ-OMe)](2)·4THF (118C6·4THF) and {[K(THF)(3)][U(η(6)-C(14)H(10))(η(4)-C(14)H(10))(μ-OMe)]}(2) (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U(iv) formal assignments for the metal centres in both 118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μ(B)) is significantly lower than that of 118C6·4THF (4.40 μ(B)) at 300 K. Furthermore, the XANES data shows the U L(III)-edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η(6)-C(14)H(10))(η(4)-C(14)H(10))(μ-OMe)](2)}(2−) (1*) shows highly polarized uranium–arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide–arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.