Achieving highly efficient carbon radical-mediated cross-coupling reaction in a confined radical microenvironment within a metal-organic framework

Abstract

It has been well-demonstrated that the combination of photosensitive (PS), hydrogen atom transfer (HAT) and single electron transfer (SET) processes can achieve efficient radical-mediated organic synthesis, but such reaction systems are usually homogeneous, requiring additional HAT agents and can only activate one substrate. Here, we constructed two crystalline porous materials, Zr/Hf-NDI, which possess excellent light absorbing capacity and a confined radical microenvironment, making them able to integrate PS, HAT, and SET processes to simultaneously activate two substrates. Thus, as heterogeneous photocatalysts, they exhibited excellent catalytic performance for the carbon radical-mediated cross-coupling reaction between alcohols and o-phenylenediamine (OPD) to synthesize benzimidazoles (yield > 99%). More importantly, they displayed very good substrate compatibility, especially for OPD substrates with electron-withdrawing groups, even surpassing those of noble metal catalysts. In situ characterizations combined with theoretical calculations showed that the high activity of these catalysts arose from: (i) the metal-oxo clusters and radical NDI˙^- ligands can form hydrogen bonding traction activation for the alcohol substrate, and thus facilitate it to generate key intermediate α-carbon radical through a HAT process; (ii) the OPD substrate, acting as an electron donor, forms strong D-A interaction with the NDI ligand and activates the NDI and itself into radicals NDI˙^- and OPD˙⁺, respectively, via an SET process, further promoting the reaction. To the best of our knowledge, this is the best performing crystalline porous catalyst for photocatalytic carbon radical-induced benzimidazole synthesis.

Scheme 1. Schematic illustration of the photocatalytic C–H activation methods. (a) PS, SET and HAT processes occurring in a homogeneous system for C–H activation in previous work. (b) PS, SET and HAT processes occurring in a heterogeneous system for C–H activation in this work.

Fig. 1. Design concept and crystal structure of the photocatalyst. (a) Schematic illustration of the desired photocatalyst with a confined microenvironment and SET and HAT functions for activating OPD and alcohol substrates to form the corresponding radical species for the subsequent cross-coupling reaction. (b) The three-dimensional porous structure of Zr-NDI, which possesses large 1D rectangular channels along the c-axis. (c) The microenvironment of the 1D channel, which possess potential D–A interaction and hydrogen bond binding sites for the capture of electron rich and alcohol substrates.

Fig. 2. Photophysical characterization of Zr-NDI and Zr-NDI˙⁻. (a) UV-Vis diffuse reflection spectrum of H₂NDI, Zr-NDI and Zr-NDI˙⁻. (b) EPR spectra of solid radical Zr-NDI˙⁻. (c) Tauc plot of Zr-NDI and Zr-NDI˙⁻ determined by the Kubelka–Munk formula from the original UV-Vis diffuse reflection spectrum. (d) Mott–Schottky plot measurement for Zr-NDI˙⁻. Inset: energy diagram of the HOMO and LUMO levels of Zr-NDI˙⁻. (e) Transient photocurrent responses of Zr-NDI and Zr-NDI˙⁻ under Xe lamp irradiation. (f) EIS Nyquist plots of Zr-NDI and Zr-NDI˙⁻.

Fig. 3. The photocatalytic performance of Zr-NDI and Zr-NDI˙⁻ for the cross-coupling reaction of OPD and ethanol. (a) Yields of 2MBZ for Zr-NDI, Hf-NDI, Zr-NDI˙⁻ and Hf-NDI˙⁻ under the standard reaction conditions. (b) Time-dependent yields of 2MBZ for Zr-NDI and Zr-NDI˙⁻. (c) Conversion of OPD and selectivity of 2MBZ for various different catalysts under similar reaction conditions. (d) Cycling performance of Zr-NDI˙⁻.

Fig. 4. The characterization of the reaction mechanism. (a) The photocatalytic performance of control experiments over Zr-NDI˙⁻. (b) EPR spectra of Zr-NDI˙⁻ in the presence of DMPO as a trapping agent under light irradiation and in the dark. (c) HRMS spectra of the DMPO adduct to the ˙CH(CH₃)OH intermediate under the standard conditions ([M + H⁺] C₈H₁₈O₂N⁺: 160.2333, found: 160.1329). (d) In situ DRIFTS of the photocatalytic synthesis of 2MBZ using OPD and ethanol as reactants over Zr-NDI˙⁻.

Fig. 5. The DFT calculation and proposed reaction mechanism. (a) Optimized structure of OPD in the Zr-NDI pores. (b) Diagram of the charge density difference between OPD and Zr-NDI. (c) Optimized structure of ethanol in the Zr-NDI pores. (d) Diagram of the charge density difference between ethanol and Zr-NDI. (e) Free energy diagram for cross-coupling of OPD and ethanol to 2MBZ with or without catalyst. (f) Proposed reaction mechanism for photocatalytic 2MBZ synthesis over Zr-NDI.