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

• Published in: Angewandte Chemie International Edition Vol. 59; no. 43; pp. 19048 - 19053
• Main Authors: Yu, Chao, Zhong, Mingdong, Zhang, Yongliang, Wei, Junnian, Ma, Wangyang, Zhang, Wen‐Xiong, Ye, Shengfa, Xi, Zhenfeng
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 19.10.2020; Wiley Subscription Services, Inc
• Edition: International ed. in English
• Subjects: Antiferromagnetism; Aromatic compounds; Aromaticity; butadienyl dianion; Chemistry; Chemistry, Multidisciplinary; diiron complexes; Ligands; Magnetic measurement; metalla-aromatics; Mossbauer spectroscopy; nonplanar aromaticity; Orbitals; Physical Sciences; Reagents; Science & Technology; Structural analysis; σ-type overlap; SUBSTITUTION; butadienyl dianion; ELECTRONS; DELOCALIZATION; REACTIVITY; ANISOTROPY; diiron complexes; nonplanar aromaticity; DIANIONS; metalla-aromatics; CHEMISTRY; sigma-type overlap; MULTIPLE BONDS; METALLABENZENES; INDEPENDENT CHEMICAL-SHIFTS
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202008986

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 FeII 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 3dxz orbitals and the butadienyl π orbital results in the formation of a six‐electron conjugated system and hence enables the aromatic character. A butadienyl bridge and two antiferromagnetically coupled high‐spin FeII ions unite to form a nonplanar metalla‐aromatic molecule. A σ‐type overlap between the two Fe 3dxz orbitals and the π orbital of the butadienyl ligand makes the six‐electron‐conjugated system aromatic.