Twelve CO molecules for the price of one. A simple water-soluble organometallic CORM, [Mn(CO)₃(µ₃-OH)]₄

The tetramer [Mn(CO)₃(µ₃-OH)]₄ (1) forms hydrogen bonded adducts with a variety of molecules. Most commonly, these are 1···aromatic hydrocarbon adducts obtained from synthesis and purification, and these adducts are insoluble in water. We hypothesized that the adduct-free tetramer is water-soluble and tested this by developing a gram-scale synthesis of the adduct-free tetramer (1) and measuring its molar solubility, which we determined to be 0.95 ± 0.06 mM at 20 °C. This success allowed us to study the characterization and aqueous properties of 1 for the first time. For example, ambient light exposure causes loss of all twelve CO ligands, accompanied by formation of reactive oxygen species (ROS) and manganese oxides. Since CO solubility in water is 0.93 mM at 20 °C and 1 atm, a solution of 1 effectively increases the aqueous availability of CO twelve-fold from atmospheric pressure CO. This prompted our exploration of 1 as a water-soluble CO releasing molecule (CORM) and its ability to elicit cell antiproliferation was tested with four cell lines. Indeed, 1 induces dose-dependent cell antiproliferation properties, albeit the cytotoxicity of CO cannot be decoupled from manganese oxide or ROS toxicity and the role of light was not explicitly controlled for. This point is magnified because of recent revelations that biological studies of CORMs often neglect these other factors. Nevertheless, 1 is an excellent CORM in water and does not carry ancillary ligands that may interfere with applications that use the released CO molecules.