Framework-guided control of coordination number in metal-organic frameworks for promoting CO2 photoreduction

Abstract

Precise control over the coordination number of catalytic metal centers is essential for tuning reactivity and elucidating structure-activity relationships, but achieving this in heterogeneous catalysts remains challenging. Herein, we demonstrate a framework-guided approach to modulate the coordination number of Fe-porphyrin centers within three MOFs (PCN-222, PCN-223, and NUPF-2) for optimized CO₂ photoreduction. Owing to their distinct topologies, these MOFs enable the selective axial coordination of varying amounts of 4,4′-dipyridylamine (dipya) linkers between neighboring Fe-porphyrin pairs, resulting in Fe centers with different coordination numbers. Among them, PCN-222-dipya_0.97, featuring FeN₅ sites, achieves the highest photocatalytic CO₂-to-HCOOH activity of 184.6 µmol g ⁻¹_cat h⁻¹, which is three times higher than the parent FeN₄-based PCN-222. In contrast, full saturation of both axial sites to form FeN₆ in the other two MOFs suppresses CO₂ reduction activity. Density functional theory calculations reveal that axial coordination alters the electronic structure of the Fe center and lowers the energy barrier of the rate-determining step. This work establishes a framework-based approach to precisely control the coordination environment and elucidate the structure-function relationships in porphyrin-based catalytic systems.