Search

Two manganese(I) carbonyl complexes derived from 2-(pyridyl)benzothiazole (pbt) and 1,10-phenanthroline (phen) release carbon monoxide (CO) under low-power broad-band visible-light illumination. CO photorelease from [Mn(CO) 3 (pbt)(PTA)]CF 3 SO 3 (1, where PTA = 1,3,5-triaza-7-phosphaadamantane) is accompanied by an emergence of a strong fluorescence around 400 nm from almost nonfluorescent preirradiated 1. However, [Mn(CO) 3 (phen)(PTA)]CF 3 SO 3 (2) showed no such phenomenon upon prolonged illumination under similar experimental conditions. The two analogous rhenium(I) complexes, namely, [Re(CO) 3 (pbt)(PTA)]CF 3 SO 3 (3) and [Re(CO) 3 (phen)(PTA)]CF 3 SO 3 (4), have also been synthesized and characterized to compare their photo properties with the manganese congeners. Complexes 3 and 4 exhibit moderate CO release upon irradiation with low-power UV light. All four complexes are highly soluble in anaerobic/aerobic aqueous media and are also considerably more stable when kept under dark conditions. The inherently luminescent rhenium complex 3 was utilized to demonstrate cellular internalization of these types of compounds by MDA-MB-231 (human breast cancer) cells, while the two biocompatible manganese(I) complexes (1 and 2) have been applied to assess the cell viability of these malignant cells upon CO delivery.

A Re(I) carbonyl complex derived from 2-(2-pyridyl)-benzothiazole (pbt), [Re(H2O)(CO)3(pbt)](CF3SO3) (1), rapidly releases CO under low-power UV illumination. CO photorelease from 1 is accompanied by a change in luminescence from orange to deep blue. These two distinct luminescence signals have been successfully employed to track (a) the entry of the pro-drug 1 into cancer cells and (b) the end of the CO (drug) delivery step within the target.

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 π* 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.

The unusual role of CO as a signaling molecule in several physiological pathways has spurred research in the area of synthesizing new CO-releasing molecules (CORMs) as exogenous CO donors. Auxiliary control on CO delivery can be achieved if CO can be released under the control of light. To synthesize such photoactive CORMs (photoCORMs) with the aid of smart design, a series of manganese carbonyls have been synthesized with ligands that contain extended conjugation and electron-rich donors on their frames. Five such photoCORMs, namely, [Mn(pimq)(CO)(3)(MeCN)]ClO(4) (1, where pimq = (2-phenyliminomethyl)quinoline), [Mn(qmtpm)(CO)(3)(MeCN)]ClO(4) (2, where qmtpm = 2-quinoline-N-(2'-methylthiophenyl) methyleneimine), [Mn(qmtpm)(CO)(3)Br] (3) [Mn(pmtpm)(CO)(3)(MeCN)]ClO(4) (4, where pmtpm = 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine), and [Mn(pmtpm)(CO)(3)Br] (5), have been synthesized and structurally characterized. These designed carbonyls readily release CO upon exposure to light (400-550 nm). The apparent CO release rates and quantum yield values at 509 nm (φ(509)) of the photoCORMs increase steadily with rise in conjugation in the ligand frame and inclusion of a -SMe group. Addition of Br(-) as an ancillary ligand also improves the CO-donating capacity. Results of density functional theory (DFT) and time dependent DFT (TDDFT) studies indicate that low energy metal-to-ligand charge transfer (MLCT) transitions from Mn-CO bonding into ligand-π orbitals lead to reduction of M-CO(π*) back-bonding and loss of CO from these photoCORMs. Inclusion of -SMe and Br(-) in the coordination sphere attenuates the energies of the HOMO and LUMO levels and causes further enhancement of CO photorelease. Collectively, the results of this work demonstrate that new photoCORMs with excellent sensitivity to visible light can be synthesized on the basis of smart design principles.