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 FeBr2. The complexes consist of two antiferromagnetically coupled high-spin Fe(II)centers, as revealed by magnetometry, Mossbauer 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 sigma-type overlap between the ligand p and metal d orbitals, in sharp contrast to the intensively studied planar aromatic systems featuring delocalized pi-type bonding. Specifically, the sigma-type interaction between the two Fe 3d(xz)orbitals and the butadienyl pi orbital results in the formation of a six-electron conjugated system and hence enables the aromatic character.