Spatial placement of half-Salen-bound nickel single-atoms on MOFs governs cooperative photocatalytic hydrogen evolution

The integration of single-atom catalysts into metal–organic frameworks (MOFs) offers a promising strategy for coupling molecular catalytic sites with photoactive architectures. While previous studies have focused primarily on the identity and coordination environment of single-atom sites, the role of their spatial placement within photocatalytic MOFs remains poorly understood. Here, we demonstrate how site-specific placement of linker-bound nickel single atoms significantly influences the photocatalytic hydrogen evolution efficiency in MIL-125(Ti)-NH2. Using a post-synthetic Schiff-base strategy, we construct half-Salen coordination motifs that direct Ni(II) incorporation into distinct spatial environments within the framework. Spectroscopic and porosity analyses reveal that salicylaldehydederived sites are preferentially formed within the internal pore network, whereas bulkier aldehydes predominantly modify external surfaces. Despite exhibiting lower degrees of functionalization and lower Ni loading, internally positioned Ni(II) sites deliver the highest photocatalytic activity, reaching 11.2 mmol g-1 h-1 under visible-light irradiation and outperform all other materials in the series. Notably,