Photochemistry of chromium arene tricarbonyl complexes with tethered pyridinyl and propenyl groups: Investigations of the effect of ring size on chelate formation, structure, and linkage isomerization


Chromium arene tricarbonyl complexes with tethered pyridinyl groups [Cr{η6-C6H5(CH2) n(2-Py)}(CO)3] (4-6) (2-Py = 2-pyridinyl, n = 1-3, respectively) were synthesized and irradiated to form the chelates [Cr{η6-C6H5(CH2) n(2-Py)-κN}(CO)2] (7-9). Studies examined the effect of ring size and structure on chromophore λmax, stability, and photosensitivity, which are factors important for photochromes based on linkage isomerization of tethered functional groups. The studies also include [Cr{η6-C6H5CH(2-Py)CH2CH=CH 2}(CO)3] (3), which has a bifunctional tether of propenyl and pyridinyl groups, and irradiation produces the linkage isomers [Cr{η6-C6H5(CH(2-Py)CH2CH= CH2)-κN}(CO)2] (1) and [Cr{η6-C 6H5(CH(2-Py)CH2CH=CH2) (η2-CH=CH2)}(CO)2] (2). X-ray crystal structures for 7-9 show that the dihedral angle between the coordinated pyridinyl groups and the phenyl-chromium centroid increases from 1 to 73 (n = 1-3, respectively). The experimental and TDDFT computed optical changes accompanying an increase in the dihedral angle are modest and not monotonic for 7-9 due to structural changes inherent in the chelate rings. An increase in Cr-N bond lengths and decrease in their bond energies were observed experimentally and computationally for the series of 7-9. The quantum yields for formation of the five-, six-, and seven-membered chelate rings during the conversion of 4-6 to 7-9, respectively, were within experimental error for that observed for conversion of 10 [Cr{η6-C6H6}(CO) 3] with free pyridine to 11 [Cr{η6-C6H 6}(C5H5N-κN)(CO)2], indicating that the product-determining step precedes chelation. The enthalpies for chelation of 4-6 to 7-9 were determined independently by photoacoustic calorimetry and DFT computations. The computationally derived mechanism for thermal isomerization of 1 to 2 indicates that the transition state is a dissociative interchange with a free energy of activation of 27.9 kcal mol -1 (1 → 2), a result consistent with an experimentally bistable photochrome. The results indicate which tether properties are important for optimizing photochrome performance. © 2014 American Chemical Society.

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