Density functional theory (DFT) calculations are performed to investigate recent experimentally studied ring-closing sulfoxide imidation catalyzed by Fe(II)-phthalocyanine (Fe^IIPc). Our results reveal that the ground state of iron phthalocyanine nitrene intermediate (PcFeNR, R = (CH₂)₃(SO)Ph), which is believed to mediate the intramolecular imitation, is triplet state featuring a diradical structure. The formation of PcFeNR is the result of a denitrification process with a calculated high-barrier of 23.4 kcal/mol which is in good agreement with the experimentally observed high reaction temperature of 100 ℃. The generated PcFeNR undergoes a low-barrier intramolecular nucleophilic attack by proximal nitrogen atom on the sulfur accomplishing the cyclization of sulfoxide. This study provides theoretical insights into the mechanism-based design of useful catalysts for nitrene transfer reactions.