Visible-light photocatalytic selective oxidation of C(sp3)–H bonds by anion–cation dual-metal-site nanoscale localized carbon nitride

Abstract

Selective oxidation of C(sp³)–H bonds to carbonyl groups by abstracting H with a photoinduced highly active oxygen radical is an effective method used to give high value products. Here, we report a heterogeneous photocatalytic alkanes C–H bonds oxidation method under the irradiation of visible light (λ = 425 nm) at ambient temperature using an anion–cation dual-metal-site modulated carbon nitride. The optimized cation (C) of Fe³⁺ or Ni²⁺, with an anion (A) of phosphotungstate (PW₁₂³⁻) constitutes the nanoscale dual-metal-site (DMS). With a Fe–PW₁₂ dual-metal-site as a model (FePW), we demonstrate a A–C DMS nanoscale localized carbon nitride (A–C/g-C₃N₄) exhibiting a highly enhanced photocatalytic activity with a high product yield (86% conversion), selectivity (up to 99%), and a wide functional group tolerance (52 examples). The carbon nitride performs the roles of both the visible light response, and improves the selectivity for the oxidation of C(sp³)–H bonds to carbonyl groups, along with the function of A–C DMS in promoting product yield. Mechanistic studies indicate that this reaction follows a radical pathway catalyzed by a photogenerated electron and hole on A–C/g-C₃N₄ that is mediated by the ^tBuO˙ and ^tBuOO˙ radicals. Notably, a 10 g scale reaction was successfully achieved for alkane photocatalytic oxidation to the corresponding product with a good yield (80% conversion), and high selectivity (95%) under natural sunlight at ambient temperature. In addition, this A–C/g-C₃N₄ photocatalyst is highly robust and can be reused at least six times and the activity is maintained.