Linear versus Bent Uranium(II) Metallocenes─A Local Vibrational Mode Study

Uranium metallocenes have recently attracted attention driven by their use as catalysts in organometallic synthesis. In addition to bent U(IV) and U(III), an U(II) metallocene [(η⁵-C₅ⁱ Pr₅)₂U] was synthesized with an unusual linear Cp-U-Cp angle. In this work, we investigated 22 U(II) metallocenes, (i) assessing the intrinsic strength of the U-ring interactions in these complexes with a novel bond strength measure based on our local vibrational mode analysis and (ii) systematically exploring what makes these U(II) metallocenes bent. We included relativistic effects through the NESCau Hamiltonian and complemented the local mode analysis with natural bonding orbital (NBO) and quantum theory of atoms in molecules (QTAIM) data. Our study led to the following results: (i) reduction of bulky U-ring ligand substituents does not lead to bent complexes for alkyl substituents (ⁱPr and ⁱBu) in contrast to SiMe₃ ring substituents, which are all bent. (ii) The most bent complexes are [(η⁵-C₅H₄SiMe₃)₂U] (130°) and [η⁵-P₅H₅)₂U] (143°). (iii) Linear complexes showed one hybridized NBO with s/d character, while bent structures were characterized by s/d/f mixing. (iv) We did not observe a correlation between the strength of the U-ring interaction and the amount of the ring-U-ring bend; the strongest interaction was found for [η⁵-Cp)₂U] and the weakest for [η⁵-P₅H₅)₂U]. In conclusion, our results provide a foundation for the design of U(II) metallocenes with specific physicochemical properties and increased reactivity.