Chatt reaction methods were employed to synthesize the first well characterized actinide-arene sandwich complexes. Namely, addition of [UI<sub>2</sub>(THF)<sub>3</sub>(μ-OMe)]<sub>2</sub>⸱THF (<b>2⸱THF</b>) to THF solutions containing 6 equiv. of K[C<sub>14</sub>H<sub>10</sub>] generates the dimeric complexes [K(18-crown-6)(THF)<sub>2</sub>]<sub>2</sub>[U(η<sup>6</sup>-C<sub>14</sub>H<sub>10</sub>)(η<sup>4</sup>-C<sub>14</sub>H<sub>10</sub>)(μ-OMe)]<sub>2</sub>⸱4THF (<b>118C6</b>⸱4THF) and {[K(THF)<sub>3</sub>][U(η<sup>6</sup>-C<sub>14</sub>H<sub>10</sub>)(η<sup>4</sup>-C<sub>14</sub>H<sub>10</sub>)(μ-OMe)]}<sub>2</sub> (<b>1THF</b>) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both <b>118C6</b>⸱4THF and <b>1THF</b> 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 <b>118C6</b>⸱4THF and <b>1THF</b> to be structurally similar, possessing uranium centers sandwiched between anthracene ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arene ion pairing that is seen in <b>1THF</b> but absent in <b>118C6</b>⸱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 centers in both <b>118C6</b>⸱4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of <b>1THF</b> (3.74 µB) is significantly lower than that of <b>118C6</b>⸱4THF (4.40 µB) at 300 K. Furthermore, the XANES data shows the U LIII-edge absorption energy for 1THF to be 0.9 eV higher than that of <b>118C6</b>⸱4THF, suggestive of more oxidized metal centers in the former. Of note, CASSCF calculations on the model complex {[U(η<sup>6</sup>-C<sub>14</sub>H<sub>10</sub>)(η<sup>4</sup>-C<sub>14</sub>H<sub>10</sub>)(μ-OMe)]<sub>2</sub>}<sup>2-</sup> (<b>1*</b>) shows highly polarized uranium-arene 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-arene bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.<br>
Actinide arene-metalates: Ion pairing effects on the electronic structure of unsupported uranium-arenide sandwich complexes