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CH2}­(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-C6H5(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-C6H6}­(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.