om034117i_si_001.cif (72.04 kB)
Thermally Reversible Photochemical Haptotropic Rearrangement of Diiron Carbonyl Complexes Bearing a Bridging Acenaphthylene or Aceanthrylene Ligand
dataset
posted on 2004-02-16, 00:00 authored by Shota Niibayashi, Kouki Matsubara, Masa-aki Haga, Hideo NagashimaThe haptotropic rearrangement of diiron carbonyl species on the conjugate π-ligand in
(μ2,η3:η5-acenaphthylene)Fe2(CO)5 (1) and its analogue (μ2,η3:η5-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 ΔG⧧298= 23 ± 1 kcal/mol, ΔH⧧298= 23 ± 1 kcal/mol, and ΔS⧧298= 1 ± 4 cal/mol·deg
for the reaction of 1-B to 1-A, and ΔG⧧298= 26 ± 1 kcal/mol, ΔH⧧298= 26 ± 1 kcal/mol, and
ΔS⧧298= 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-A→1-B = 0.30(3), Φ1-B→1-A
= 0.09(1), Φ2-A→2-B = 0.013(2), and Φ2-B→2-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.