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)_<i>n</i>(2-Py)}­(CO)₃] (<b>4</b>–<b>6</b>) (2-Py = 2-pyridinyl, <i>n</i> = 1–3, respectively) were synthesized and irradiated to form the chelates [Cr­{η⁶-C₆H₅(CH₂)_<i>n</i>(2-Py)-κ<i>N</i>}­(CO)₂] (<b>7</b>–<b>9</b>). Studies examined the effect of ring size and structure on chromophore λ_max, 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CH₂}­(CO)₃] (<b>3</b>), which has a bifunctional tether of propenyl and pyridinyl groups, and irradiation produces the linkage isomers [Cr­{η⁶-C₆H₅(CH­(2-Py)­CH₂CHCH₂)-κ<i>N</i>}­(CO)₂] (<b>1</b>) and [Cr­{η⁶-C₆H₅(CH­(2-Py)­CH₂CHCH₂)­(η²-CHCH₂)}­(CO)₂] (<b>2</b>). X-ray crystal structures for <b>7</b>–<b>9</b> show that the dihedral angle between the coordinated pyridinyl groups and the phenyl-chromium centroid increases from 1 to 73° (<i>n</i> = 1–3, respectively). The experimental and TDDFT computed optical changes accompanying an increase in the dihedral angle are modest and not monotonic for <b>7</b>–<b>9</b> 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 <b>7</b>–<b>9</b>. The quantum yields for formation of the five-, six-, and seven-membered chelate rings during the conversion of <b>4</b>–<b>6</b> to <b>7</b>–<b>9</b>, respectively, were within experimental error for that observed for conversion of <b>10</b> [Cr­{η⁶-C₆H₆}­(CO)₃] with free pyridine to <b>11</b> [Cr­{η⁶-C₆H₆}­(C₅H₅N-κ<i>N</i>)­(CO)₂], indicating that the product-determining step precedes chelation. The enthalpies for chelation of <b>4</b>–<b>6</b> to <b>7</b>–<b>9</b> were determined independently by photoacoustic calorimetry and DFT computations. The computationally derived mechanism for thermal isomerization of <b>1</b> to <b>2</b> indicates that the transition state is a dissociative interchange with a free energy of activation of 27.9 kcal mol^–1 (<b>1</b> → <b>2</b>), a result consistent with an experimentally bistable photochrome. The results indicate which tether properties are important for optimizing photochrome performance