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.水氧化催化的核心在于反应中心附近水分子的有序排布。受光系统II（Photosystem II）中的放氧复合体（Oxygen Evolving Complex）启发，我们开发了一种铜-钙模型配合物，以探究水分子配位的作用效应。我们合成并表征了一种带有四齿N3O配体的[Cu(L-H)(BF4)]配合物。加入氢氧化钙后，该配合物会转变为一种包含两个配位水分子的稳定铜-钙配合物。通过紫外-可见（UV–vis）光谱、电喷雾电离质谱（ESI-MS）、氦标记红外光解离光谱以及密度泛函理论（DFT）计算开展的详细表征，揭示了该配合物的结构及其内部的氢键网络。实验数据表明，水分子通过钙配位以及与配体形成氢键实现预组织。这种在铜反应中心附近紧密排布的水分子配位结构，可通过稳定反应中间体并抑制有害副反应，助力水氧化过程中选择性O-O键的形成。