Spectroscopic and Theoretical Investigation of Water Binding in a Copper–Calcium Complex

Water oxidation catalysis relies critically on the organization of water molecules near reactive centers. Inspired by the Oxygen Evolving Complex in Photosystem II, we developed a copper-calcium model complex to investigate water coordination effects. We synthesized and characterized a [Cu(L-H)(BF₄)] complex featuring a tetradentate N₃O ligand. Upon the addition of calcium hydroxide, the complex transforms into a stable copper-calcium complex with two coordinated water molecules. Detailed characterization by ultraviolet-visible (UV-vis) spectroscopy, electrospray ionization mass spectrometry (ESI-MS), helium-tagging IR photodissociation spectroscopy, and density functional theory (DFT) calculations revealed the structure and hydrogen-bonding network within the complex. The data demonstrate that water molecules are preorganized via calcium coordination and hydrogen bonding to the ligand. Such tight coordination of water molecules in the vicinity of the copper reaction center could facilitate selective O-O bond formation in water oxidation processes by stabilizing reactive intermediates and preventing deleterious side reactions.