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.