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

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 H-1 NMR, IR and UV-vis spectrometry as well as by single-crystal X-ray crystallography. The N-O bond and the amido C=O 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 p 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.