Demonstration of “Möbius” Aromaticity in Planar Metallacycles

AbstractMöbius aromaticity, predicted by Edgar Heilbronner in 1964, is a stabilizing effect exhibited by 4 n electron fully conjugated cyclic molecules (or transition states) with an odd number of orbital phase inversions. Although it has previously been suggested that this effect might also apply to planar metallacycles in which a transition metal employs a d orbital in δ‐type binding mode, only very few examples of stable twisted molecules composed of main group elements are known. We report herein, the first computationally confirmed 4 n π aromatic planar metallacyclic examples and their building principles. Aromatic stabilization energy (ASE) of a 8 π metalla‐cycloheptatriene [Fe(CH)6H2], with four doubly occupied π orbitals and a HOMA value of +0.80 (cf. benzene=+1.0), an NICS(0) value of −8.5 (benzene=−9.8, NICS=nucleus independent chemical shift), and with one phase inversion, is +27.5 kcal mol−1 (about two‐thirds of the value for benzene). In contrast, an unknown non‐Möbius 1,4‐dimetallabenzene [Fe2(CH)4H4], also with 8 π electrons, and without phase inversions, has an ASE of −4.1 kcal mol−1 and a NICS(0)=+15.6, indicative of antiaromaticity. Aromaticity of the proposed Möbius aromatic metallacycles is confirmed by using magnetic (NICS(0), NICS(1)zz, δ1H) and geometric (HOMA) aromaticity criteria, planarity, and near equalized CC bond lengths, bonding analysis (Wiberg bond indices, NBO, and NLMO analysis). The role of wave function boundary conditions (periodic vs. antiperiodic) in chemistry is further stressed, being equivalent to Zimmerman’s concept of nodal parity for Möbius/Hückel systems.