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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.

Two manganese(i) carbonyl complexes derived from α,α'-diimine ligands with extended conjugated framework namely [MnBr(CO)3(BIAN)] () and [MnBr(CO)3(MIAN)] (), have been synthesized and structurally characterized. Unlike the previously reported photoactive CO-releasing molecules (photoCORMs), these two complexes exhibit unusually high sensitivity toward low power (0.3-10 mW) visible light (λ ≥ 520 nm) even in the solid state and rapidly release carbon monoxide (CO) upon illumination. The role of the ligand frames in such activity has been examined with the help of theoretical calculations. Application of these photoCORMs in delivering high fluxes of CO to biological targets is anticipated.

The therapeutic potential of photoactive CO-releasing molecules (photoCORMs) have called for close examination of the roles of the ligand(s) and the central metal atoms on the overall photochemical labilization of the metal-CO bonds. Along this line, we have synthesized four metal complexes, namely, [MnBr(azpy)(CO)3 ] (1), [Mn(azpy)(CO)3 (PPh3 )]ClO4 (2), [ReBr(azpy)(CO)3 ] (3), and [Re(azpy)(CO)3 (PPh3 )]ClO4 (4), derived from 2-phenylazopyridine. These complexes were characterized by spectroscopic and crystallographic studies. Although both 1 and 3 exhibit strong metal-to-ligand charge-transfer bands in the 500-600 nm region, only 1 photoreleases CO upon illumination with visible light. Results of theoretical studies were used to gain insight into this surprising difference. Strong spin-orbit coupling (prominent in heavy metals) appears to promote intersystem crossing to a triplet state in 3, a step that discourages CO release upon illumination with visible light. Slow release of CO from 2 and 4 also indicates that strong σ-donating ligands, such as Br(-) , accelerate the rate of CO photorelease relative to π-acid ligands, such as PPh3 ., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.