Unraveling f-block bonding interactions: novel uranium complexes

In a series of break-through papers, Prof. Steve Liddle and his collaborators report the synthesis, isolation, and characterization of new uranium complexes.

In a sustained, exciting effort to establish and understand novel chemical bonds in f-block elements, they have discovered for the first time an f-block cyclobutadienyl complex (Nat. Commun. 4, 2323), a terminal U-N triple bond (Science 337, 717, Nat. Chem. 5, 482), O=U^(VI)=C(PR₂)₂ (J. Am. Chem. Soc. 134, 10047), and U^(V) carbenes (Angew. Chem. Int. Ed. 50, 2383).

First U(V)-N triple bond (left), later oxidized to the first U(VI)-N triple bond. First U-cyclobutadienyl complex (right).

Relativistic DFT calculations with ADF afforded invaluable insight in the chemical bonds of these latest uranium complexes and how they compare to longer-established U bonds and analogous bonds in d-block complexes. The bonding interactions are routinely studied through bond orders (Mayer, Nalewajski-Mrozek), QTAIM, NBO, orbital populations, and MDC charges and spin densities.

Bonding interactions in uranium complexes
Although the 5f-electrons are predominantly non-bonding, they do participate in U-X multiple bonds as well as carbocyclic π-electron ligands.

Uranium’s oxidation state can strongly affect bond orders as well as the relative participation of 5f and 6d orbitals in the π-bonds. At short U-X distances, the N 2p_z orbital has anti-bonding interactions with 5f and 6d orbitals, pushing the σ-bond higher in energy than the π-bonds for the U-N triple bond. Although the U-N distance shrinks from 1.825 to 1.799 Å upon oxidation from U^(V) to U^(VI), the Mayer bond index is hardly affected (2.91, 2.92).

While there is a strong covalent, δ-backbonding, interaction between the U 5f-orbitals and the central arene ring in inverted sandwich complexes (Nature Chem. 3, 454), the interaction between the two U centers sandwiching cyclobutadienyl are predominantly electrostatic owing to a size mismatch and small orbital overlap.

U-N interactions in a model complex with a terminal U^(VI)-N triple bond (truncated from experimental [U(N)(Tren^TIPS)]; Tren^TIPS = {N(CH₂CH₂NSiⁱPr₃)₃}³⁻, ⁱPr = CH(CH₃)₂)). The σ-bond (HOMO, top right) is higher in energy than the two quasi-degenerate π-bonds (one shown, bottom left).

The Liddle group also investigates single-molecule magnets, catalytic CO activation, and has synthesized and characterized various other interesting U-complexes. Visit their website to learn more.

More interesting U bonds established by the Liddle group. Left to right: the first U-Ga bond; the first U-Re bond; the first U-Ru bond; the first U-Co bond.