Structures and Bonding in Hexacarbonyl Diiron Polyenes: Cycloheptatriene and 1,3,5-Cyclooctatriene

Structural preferences of (1,3,5-cyclooctatriene) hexacarbonyl diiron [(C8H10)Fe-2(CO)(6)] and cycloheptatriene hexacarbonyl diiron [(C7H8)Fe-2(CO)(6)] were explored using density functional theory (DFT) computations. DFT computations together with experimental results demonstrated that structure with the [eta(3), (eta(1), eta(2))] mode is the preferred structure in (C8H10)Fe-2(CO)(6), and the [eta(3),eta(3)] mode is preferred in (C7H8)Fe-2(CO)(6). For (C8H10)Fe-2(CO)(6), the conversion between the structures with [eta(3), (eta(1), eta(2))] mode and the [eta(3), eta(3)] mode is prevented by the relatively high activation barrier. (C8H10)Fe-2(CO)(6) is indicated as a fluxional molecule with a Gibbs free energy of activation of 8.5 kcal/mol for its ring flicking process, and an excellent linear correlation (R-2 = 0.9909) for the DFT simulated H-1-NMR spectra was obtained. Results provided here will develop the understanding on the structures of other polyene analogs.