Chromium Complex of Macrocyclic Ligands as Precursor for Nitric Oxide Release: A Theoretical Study

Our research on the chromium complex of macrocyclic ligands as a precursor for nitric oxide release makes a significant contribution to the field of chemistry. We conduct a detailed analysis of nitrito chromium complexes, specifically trans‐[M(III)L 1–5 (ONO) 2 ] + , where M=Cr(III) and L 1 ‐L 5 represent different ligands such as L 1 =1,4,8,11‐tetraazacyclotetradecane, L 2 = (5,7‐dimethyl‐6‐benzylcyclam), L 3 = (5,7‐dimethyl‐6‐anthracylcyclam), L 4 = (5,7‐dimethyl‐6‐(p‐hydroxymethylbenzyl)‐1,4, 8,11‐cyclam) and L 5 = (5,7‐dimethyl‐6‐(1¢‐methyl‐4'‐(1”‐carboxymethylpyrene) benzyl)‐1,4,8,11‐tetraazacyclotetradecane). Our objective is to comprehensively understand the mechanism of NO release and identify the key factors influencing NO delivery. The optimized structure of the complexes at spin states S=1/2 or 3/2 indicates a decrease in the Cr(III)−O bond length (1.669–1.671 Å) along with an increase in the Cr(III) O − N O bond length (2.735–2.741 Å), which facilitates the release of NO. Furthermore, there is a significant change in the bond angle (Cr− O −NO), from 120.4° to 116.9°, to S=3/2, thus enlarging the O − N O bond and supporting the β‐cleavage of NO from the complex. The calculated activation energy for the complexes reflects the energy difference between the low‐spin doublet and high‐spin quartet state due to spin crossover (SCO). Moreover, the Natural Transition Orbitals (NTOs) confirm the involvement of a hole‐particle in the excitation. Additionally, TD‐DFT reveals the pendant chromophore's role in generating NO, as the chromophore antenna effectively enhances light absorption.