Bioinspired Cu( <scp>ii</scp> ) complexes with tunable axial donors: unravelling structure–function correlations in phenoxazinone synthase mimics

-phenoxazin-3-one (APX) under physiological conditions without external oxidants, with the catalytic efficiency following the order 2 > 3 > 1. Density functional theory (DFT) and time-dependent DFT (TDDFT) studies corroborated the experimental observations. The optimized geometries closely matched the crystal structures, and the computed spin densities indicated greater Cu-ligand covalency in the N3O complex compared to the N4 and N3S analogues. The combined experimental and computational results highlight how subtle modifications in the primary coordination sphere influence the electronic structure, redox behaviour, and catalytic performance of the bio-inspired copper systems, thus providing a mechanistic rationale for phenoxazinone synthase activity.