Probing the Hydrogen-Bonded Water Network at the Active Site of a Water Oxidation Catalyst: [Ru(bpy)(tpy)(H₂O)]²⁺·(H₂O)_0–4

The infrared spectra of gas-phase mass-selected [Ru­(bpy)­(tpy)­(H₂O)]²⁺·(H₂O)_0–4 clusters (bpy = 2,2′-bipyridine; tpy = 2,2′:6,2″-terpyridine) in the OH stretching region are reported. These species are formed by bringing the homogeneous water oxidation catalyst [Ru­(bpy)­(tpy)­(H₂O]²⁺ from solution into the gas phase via electrospray ionization (ESI) and reconstructing the water network at the active site by condensing additional water onto the complex in a cryogenic ion trap. Infrared predissociation spectroscopy is used to probe the structure of these clusters via their distinctive OH stretch frequencies, which are sensitive to the shape and strength of the local hydrogen-bonding network. The analysis of the spectra, aided by electronic structure calculations, highlights the formation of strong hydrogen bonds between the aqua ligand and the solvating water molecules in the first solvation shell. These interactions are found to propagate through the subsequent solvation shells and lead to the stabilization of asymmetric solvation motifs. Electronic structure calculations show that these strong hydrogen bonds are promoted by charge transfer from the H atom of the aqua ligand to the Ru–OH₂ bond.