Decoding Surface Electron Density–Reactivity Relationship in Ni-Porous Organic Polymer Catalyst for Cyclohexene Oxidation

The development of efficient and sustainable heterogeneous catalysts remains central to the advancement of green oxidation chemistry. Herein, we report a Ni-salphen-derived metalated porous organic polymer (Ni@CAB), synthesized via a simple Friedel-Crafts alkylation strategy that integrates atomically dispersed Ni-N₂O₂ active sites into a robust carbazole-linked framework. A combination of 2D solid-state ¹³C-¹H double cross-polarization (CP) correlation NMR, XPS, synchrotron-based XAS, and electron microscopy techniques confirmed the structural integrity, amorphous porous architecture, and uniform dispersion of Ni centers. Electronic analyses revealed reduced surface electron density at the Ni sites, enhancing their Lewis acidity and catalytic reactivity. Ni@CAB demonstrated exceptional performance in the aerobic allylic oxidation of cyclohexene under ambient conditions, affording complete conversion with high selectivity and remarkable recyclability over multiple cycles without detectable Ni leaching. Complementary DFT calculations unveiled favorable charge transfer and energetically viable pathways consistent with experimental observations. This study establishes surface electron density as a powerful activity descriptor and underscores the promise of rationally engineered metalated porous polymers for sustainable oxidation catalysis.