Butadienyl Diiron Complexes: Nonplanar Metalla‐Aromatics Involving σ‐Type Orbital Overlap

Abstract A new class of nonplanar metalla‐aromatics, diiron complexes bridged by a 1,3‐butadienyl dianionic ligand, were synthesized in high yields from dilithio reagents and two equivalents of FeBr 2 . The complexes consist of two antiferromagnetically coupled high‐spin Fe II centers, as revealed by magnetometry, Mössbauer spectroscopy, and DFT calculations. Furthermore, experimental (X‐ray structural analysis) and theoretical analyses (NICS, ICSS, AICD, MOs) suggest that the complexes are aromatic. Remarkably, this nonplanar metalla‐aromaticity is achieved by an uncommon σ‐type overlap between the ligand p and metal d orbitals, in sharp contrast to the intensively studied planar aromatic systems featuring delocalized π‐type bonding. Specifically, the σ‐type interaction between the two Fe 3d xz orbitals and the butadienyl π orbital results in the formation of a six‐electron conjugated system and hence enables the aromatic character.