Computational Investigations of Ligand Fluxionality in Fe3(C8H8)3

Abstract

The recently synthesized tris(μ-1,3,5,7-cyclooctatetraene)triiron, Fe3(C8H8)3, was investigated by DFT computational studies. In the gas phase, the cluster adopts C3h symmetry and pseudo-η3:η5 hapticity to each of the cyclooctatetraene (COT) ligands, which exist in a π-delocalized V conformation with each ligand bridging two metal atoms. A higher symmetry structure, D3h, lies only slightly above the C3h structure and represents a transition state between two energetically identical C3h structures. The computed charge on the iron atoms is near 0 by both the Hirshfeld and Voronoi methods and comports with the compound having a formal oxidation state of 0. In addition, the cluster can adopt a second conformation with the three COT ligands in a π-delocalized tub configuration with a decrease in stability of only ∼3 kcal/mol. The ligands can also rotate about the metal–metal framework with a transition barrier of only ∼1 kcal/mol. Dissociation of a single COT ligand requires ∼57 kcal/mol. The low-energy ligand conformational changes, sliding, and rotation likely make significant contribution to the experimentally observed fluxionality of the ligands. A triplet structure was also examined and found to be nearly isoenergetic with the singlet. The triplet potential energy surface resembles that of the singlet in that a variety of low-energy transformations can contribute to ligand fluxionality.