Diuranacyclobutadiene: Evidence of Electron Delocalization in the Actinide-Carbon Ring

Extending π electron delocalization to encompass f-block actinides within a carbon-based framework constitutes a fundamental challenge because of the inherently poor spatial overlap between contracted actinide 5f and carbon 2p orbitals. Here we report the synthesis and isolation of a diuranacyclobutadiene complex, which represents the first example of an actinide metallacycle featuring a fully delocalized π system. Single-crystal X-ray diffraction reveals a symmetric, planar U₂C₂ core with four essentially equivalent U-C bonds (averaging 2.23 Å). These bond lengths fall distinctly between typical U-C single (ca. 2.45 Å) and double (ca. 2.01 Å) bonds, providing structural evidence for multicenter π delocalization. Comprehensive theoretical analyses demonstrate that uranium 5f/6d orbital participation enables two symmetry-required phase inversions within the cyclic conjugation pathway, transforming the classical four-π-electron antiaromatic cyclobutadiene framework into a stabilized four-π-electron Möbius aromatic system. This work fundamentally expands the frontier of electron delocalization in metal-carbon rings to encompass the heaviest elements, validating the unique and previously unrealized capacity of actinide orbitals to participate in extended π systems.