Electronic tuning of nitric oxide release from manganese nitrosyl complexes by visible light irradiation: enhancement of nitric oxide release efficiency by the nitro-substituted quinoline ligand

Abstract

Manganese nitrosyl {MnNO}⁶ complexes of general formula [Mn(dpaq^R)(NO)]ClO₄ (1^R), where dpaq^R denotes a series of pentadentate monoamido ligands, 2-[N,N-bis(pyridin-2-ylmethyl)]-amino-N′-quinolin-8-yl-acetamido with R = OMe, H, Cl and NO₂ at the 5-position of the quinoline moiety, were prepared. The derivatives 1^R were characterized by ¹H NMR, IR and UV–vis spectrometry as well as by single-crystal X-ray crystallography. The N–O bond and the amido CO bond stretching frequencies, as well as the redox potentials of 1^R derivatives, substantially varied depending on the nature of the substituent group R on the quinoline ring, indicating that the π back-bonding from Mn to NO groups becomes weak as the substituent group R becomes more electron withdrawing. The nitro-substituted derivative 1^NO2 is unique among the series; the tail of its absorption bands extends to the NIR region (up to 700 nm), and the apparent NO releasing rate from 1^NO2 by light irradiation at 650 nm was ca. 4-fold higher than the other derivatives.