Photoactivity of Mono- and Dicarbonyl Complexes of Ruthenium(II) Bearing an N,N,S-Donor Ligand: Role of Ancillary Ligands on the Capacity of CO Photorelease

One monocarbonyl and one dicarbonyl complex of ruthenium(II), namely, [Ru(Cl)(CO)(qmtpm)(PPh3)]BF4 (2) and [Ru(Cl)(CO)(2)(qmtpm)]ClO4 (3), derived from the tridentate ligand 2-quinoline-N-(2'-methylthiophenyl)methyleneimine (qmtpm) have been synthesized and structurally characterized. The qmtpm ligand binds in a meridional fashion in these carbonyl complexes, and in 3, the two carbon monoxide (CO) ligands are cis to each other. Solutions of 2 in ethanol, chloroform, or acetonitrile rapidly release CO upon illumination with low-power (3-15 mW) light in the 300-450 nm range. Loss of CO from 2 brings about a dramatic color change from yellow to magenta because of the formation of [Ru(Cl)(MeCN)(qmtpm)(PPh3)]BF4 (4). In acetonitrile, photorelease of CO from 3 under 360 nm light occurs in two steps, and the violet photoproduct [Ru(Cl)(MeCN)(2)(qmtpm)](+) upon reaction with Ag+ and PPh3 affords red [Ru(MeCN)(2)(qmtpm)(PPh3)](ClO4)(2) (5). The structure of 5 has also been determined by X-ray crystallography. Reduced myoglobin assay confirms that 2 and 3 act as photoactive CO-releasing molecules (photoCORMs) that deliver 1 and 2 equiv of CO, respectively. The results of density functional theory (DFT) and time-dependent DFT studies confirm that electronic transitions from molecular orbitals with predominantly Ru-CO character to ligand-based pi* orbitals facilitate CO release from these two photoCORMs. Complexes 2-5 have provided an additional opportunity to analyze the roles of the ancillary ligands, namely, PPh3, Cl-, and MeCN, in shifting the positions of the metal-to-ligand charge-transfer bands and the associated sensitivity of the two photoCORMs to different wavelengths of light. Collectively, the results provide helpful hints toward the future design of photoCORMs that release CO upon exposure to visible light.