Special-selective C–H oxidation of toluene to benzaldehyde by a hybrid polyoxometalate photocatalyst including a rare [P6W48Fe6O180]30– anion

It is a challenge to develop new photocatalysts for high selective oxidation of primary carbon-hydrogen (C–H) bonds with O2 under mild condition. Herein, a novel hybrid K6(H2O)8H24(C26H16N4O4)8[P6W48Fe6O180]·6H2O (FeW–DPNDI) was firstly synthesized by incorporating a rare Fe-substituted trimeric Dawson derivative and photosensitizer N,N'-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) via solvothermal method. In FeW–DPNDI, [P6W48Fe6O180]30– and DPNDI are orderly arranged in the three-dimensional framework by the anion···π interaction and C–H···O hydrogen bonds, that is beneficial to promote the electron transfer among components. FeW–DPNDI is a typical n-type semiconductor with the calculated energy level at 2.22 V vs. NHE of the highest occupied molecular orbital (HOMO) and – 0.57 vs. NHE of the lowest unoccupied molecular orbital (LUMO), which is theoretically feasible for the reduction O2 to O2•– because of the more negative potential of CB than the standard redox potential (E(O2•–/O2) = – 0.33 V vs. NHE). The proton conductivity of FeW–DPNDI is 2.0 × 10–4 S cm−1 at 98% relative humidity and 358 K. FeW–DPNDI exhibits special selectivity for generating benzaldehyde in the photocatalytic oxidation of stubborn C–H bond of toluene under mild condition. Moreover, FeW–DPNDI can be easily isolated from the products and reused. FeW–DPNDI is the first example of the trimmer of Fe-substituted Dawson derivative, but also the first hybrid POM explored in the activating C–H bond of toluene under white light irradiation.