Crystalline Hydrogen Bonding of Water Molecules Confined in a Metal-Organic Framework

Hydrogen bonding (H-bonding) of water molecules confined in nanopores is of particular interest because it is expected to exhibit chemical features different from bulk water due to interaction with the pore walls. Here, we show a crystalline behavior of H-bonded water molecules residing in the nanocages of a paddlewheel metal-organic framework (HKUST-1), using in situ and ex situ synchrotron single-crystal X-ray diffraction (SCXRD) and Raman spectroscopy. The crystalline H-bond is demonstrated by: (i) vibrational chain connectivity linking the paddlewheel Cu–Cu stretch to coordinated H2O and subsequently H-bonded H2O (Cu–Cu…H2O(coordinating)…H2O(H-bonded)), and (ii) spatial ordering of H-bonded water molecules at room temperature, where disorder is typically expected. Additionally, we show substantial distortion of the paddlewheel Cu2+ centers arising with water coordination. We further suggest a dynamic coordination-bond character of the confined H-bond, by which an H-bonded water instantaneously becomes a coordinated ligand at a Cu2+ center after dissociation of a previously coordinated H2O.