Photosubstitution Reactivity of a Series of Pentacarbonylpyridinotungsten(0) Complexes Having Ligand Field or Charge-Transfer Lowest Excited States

Emission, electronic absorption, and photosubstitution efficiencies are reported for W(CO)₅X (X = 3,4-dimethylpyridine, 4-methylpyridine, pyridine, 3-bromopyridine, 3-acetylpyridine, 3-benzoylpyridine, 3,5-dibromopyridine, 4-cyanopyridine, 4-acetylpyridine, 4-benzoylpyridine, and 4-formylpyridine). The spectral data show that the W ⟶ pyridyl charge transfer (CT) state moves smoothly to the red with more electron-withdrawing substituents. Using W(CO)₅(piperidine) as a comparison, the ligand field (LF) absorptions remain constant in energy for all X used here. Photosubstitution of X occurs in all cases upon irradiation into the lowest energy absorption system, but the quantum efficiency is a dramatic function of whether the lowest energy absorption is LF or W ⟶ X CT. The CT state is virtually unreactive [W(CO)₅(4-formylpyridine) yields substitution of 4-formylpyridine at 514 nm with a quantum efficiency of 0.002], while the LF state gives X substitution with a quantum efficiency of ~0.60 for complexes where the LF state is the lowest energy excited state.