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Example 2: Photochromic Organometallics

photochromic compounds In collaboration with Ted Burkey's group at the Univeristy of Memphis, we have the studied the linkage isomerization mechanism for the photochromic derivatives of (C5H4R)Mn(CO)3 with chelatable R-functional groups. UV irradiation of the tricarbonyl complex causes CO dissociation. Both pyridyl (2) and carbonyl (1) chelates are observed following irradiation.

isomerization mechanism Based upon the results from PBE density functional theory calculations, the lowest-energy isomerization pathway from 1 (the carbonyl chelate) to 2 (the pyridine chelate) proceeds via three steps: 1 (the lower-energy oxygen-bound linkage isomer with a carbonyl to pyridine σ-trans conformation) to 3 (an η2 π-bound carbonyl intermediate), 3 to 4 (an η2 π -bound pyridyl intermediate), and 4 to 2 (the nitrogen-bound linkage isomer). The computed activation enthalpy is 20.8 kcal/mol (1 to TS-3-4). Confirmation of the calculations were obtained from kinetic experiments where the rates of decay of proton NMR peaks for 1 and recovery of those for 2 were found to be equal and first order in 1. The corresponding experimental activation enthalpies obtained from Eyring plots were in good agreement with the computed activation enthalpy (22.0 ± 0.5 and 20.8 ± 0.2 kcal/mol, respectively). Furthermore, the computational ΔH and ΔG are quite similar in value indicating that TΔS is small (-1.0 kcal/mol at 25 °C). This value is also in good agreement with the small experimental TΔS (1.0 kcal/mol) which was obtained from the experimental activation entropy of 3.5 ± 0.1 eu (calculated using the average experimental activation enthalpy of 21.4 ± 0.8 kcal/mol). A low activation entropy is consistent with a pathway where the side chain never dissociates from the metal as proposed for 1342. The computational results for intermediates 3 with an η2 metal-carbonyl interaction and likewise 4 with an η2 metal-pyridine interaction that does not involve the nitrogen suggest a mechanism where the metal walks along the π bonds of the side chain from one functional group to another instead of complete dissociation from the side chain followed by addition of the pyridyl group. Computational results indicate that dissociation of the side-chain functional group to form a coordinatively-unsaturated 16-electron complex followed by rotation and coordination by the second functional group is higher in energy than the π-bound pathway. A two-step pathway from 1 to 2 was also found: from 1 to 5 (the higher-energy cis conformer of the oxygen-bound linkage isomer) and from 5 to 2. However, this two-step pathway (152) is higher in energy by more than 6 kcal/mol compared to the π-bound pathway.

  • To, Duke, Junker, O'Brien, Ross, Barnes, Webster, Burkey Organometallics, 2008, 27, 289-296.