Accelerated Direct Hydroxylation of Aryl Chlorides with Water to Phenols via the Proximity Effect in a Heterogeneous Metallaphotocatalyst

Abstract

The proximity of different active sites in multi-catalytic systems is crucial in influencing the catalytic reactions, that is, to occur or to be accelerated. Here, we reported a heterogeneous metallaphotocatalyst prepared by embedding Ni(II) species in a photosensitive covalent organic framework (COF). A concerted triad catalytic behavior executed by electrons, holes, and Ni species triggered a dramatic catalytic enhancement in the activation of aryl chlorides with water to phenols. It demonstrated a 50-fold increment in its activity when compared to the homogeneous analogy, for example, Ni(BPDA)Br2. Also, it was able to efficiently activate a variety of aryl chlorides to the corresponding phenols with moderate to high yields. Interestingly, it was effective even for base-sensitive substituents or chlorobenzene bearing NH or NH2 groups. On the basis of the detailed experiments, the proximity of the photoactive COF and Ni(II) sites facilitates rapid electron transfer to produce Ni(I) active sites for the oxidative addition. Meanwhile, the photogenerated holes oxidized water to the hydroxyl radical, which then attacked Ni(III) intermediates to complete the catalytic cycle.