Thermally Reversible Photochemical Haptotropic Rearrangement of Diiron Carbonyl Complexes Bearing a Bridging Acenaphthylene or Aceanthrylene Ligand

The haptotropic rearrangement of diiron carbonyl species on the conjugate π-ligand in (μ235-acenaphthylene)Fe2(CO)5 (1) and its analogue (μ235-aceanthrylene)Fe2(CO)5 (2) is investigated in both solution and solid states. The diiron complexes 1 and 2 have two isomers, and each isomer is isolated and characterized. Isomers 1-A and 2-A are thermodynamically more stable than 1-B and 2-B, respectively (ΔG°296 = 2.3 kcal/mol for 1; ΔG°296 > 2.3 kcal/mol for 2). The thermodynamically less stable isomers can be prepared photochemically. Interconversions between 1-A and 1-B and between 2-A and 2-B occur both thermally and photochemically. Kinetic studies on the thermal isomerizations in solution provide ΔG298= 23 ± 1 kcal/mol, ΔH298= 23 ± 1 kcal/mol, and ΔS298= 1 ± 4 cal/mol·deg for the reaction of 1-B to 1-A, and ΔG298= 26 ± 1 kcal/mol, ΔH298= 26 ± 1 kcal/mol, and ΔS298= 0 ± 2 cal/mol·deg for the reaction of 2-B to 2-A. Photochemical interconversion at λ = 600 nm provides the isomer ratio of 1-A:1-B = 9:91 at the photostatic state, and 2-A:2-B = 66:34. In both of the isomerization reactions, irradiation with light of shorter wavelength tends to give lower ratios of the thermodynamically less stable isomers. The quantum yields of the isomerization reactions are found to be Φ1-A1-B = 0.30(3), Φ1-B1-A = 0.09(1), Φ2-A2-B = 0.013(2), and Φ2-B2-A = 0.009(1). From these experimental data the reaction mechanisms of the thermal and photochemical haptotropic rearrangements are discussed with the aid of EHMO calculations. A single crystal of 1-B converts to a single crystal of 1-A upon heating. A photolysis (5 min by a 500 W xenon lamp)/thermal treatment (10 min at 100 °C) cycle of a KBr pellet containing 1-A or 1-B results in reversible interconversion between 1-A and 1-B in the solid state, which can be monitored by the IR absorption at ν = 1825 cm-1. The cycle is repeatable for over 10 times without change in the signal intensity. These results are the first to indicate a possible organometallic photochromism in the solid state detectable by the IR light.