Haptotropic Rearrangement between Two Isomers of ((μ₂:η³:η⁵-guaiazulene)Fe₂(CO)₅ Revisited: Both Thermal and Photochemical Processes Induce the Haptotropic Interconversion

Interconversion between two haptotropic isomers of (μ₂:η³:η⁵-guaiazulene)Fe₂(CO)₅ (1a,b) occurred at 70-105 °C. The thermal equilibrium ratios between la and lb are around 55: 45 in this temperature range, indicating that la is more stable than lb by 0.1 kcal/mol. The activation energies estimated from kinetic studies are ΔG^‡₃₇₃ = 28 ± 1 kcal/mol, ΔH^‡₃₇₃ = 23 ± 2 kcal/mol, and ΔS^‡₃₇₃ = −15 ± 5 cal/(mol·deg) either from la to lb or from lb to la. UV or visible photolysis also induced the interconversion to give a mixture of la,b in a ratio of 35:65. The haptotropic rearrangement of the ruthenium analogues, ((μ₂:η³:η⁵-guaiazulene)- Ru₂(CO)₅ (2a,b), also occurred either thermally or photochemically to give a mixture of isomers in a ratio of 55:45. Photochemical replacement of CO ligands in la,b by PEt₃ gave a mixture of mono- and diphosphine compounds, ((μ₂:η³:η⁵-guaiazulene)Fe₂(PEt₃)(CO)₄ (4) and ((μ₂:η³:η⁵-guaiazulene)Fe₂(PEt₃)₂(CO)₃ (5), the ratio of which was controlled by the amount of added PEts. Mechanistic aspects are discussed on the basis of the reaction profile of the haptotropic interconversion and the ligand substitution.