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 U 2 C 2 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.