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 molecules due to their interaction with the wall lining the pores. Herein, we show a crystalline behavior of H-bonded water molecules residing in the nanocages of a paddlewheel metal-organic framework, providing in situ and ex situ synchrotron single-crystal X-ray diffraction and Raman spectroscopy studies. The crystalline H-bond is demonstrated by proving the vibrational chain connectivity arising between hydrogen bond and paddlewheel Cu-Cu bond in sequentially connected Cu-Cu·····coordinating H₂O·····H-bonded H₂O and by proving the spatial ordering of H-bonded water molecules at room temperature, where they are anticipated to be disordered. Additionally, we show a substantial distortion of the paddlewheel Cu²⁺-centers that arises with water coordination simultaneously. Also, we suggest the dynamic coordination bond character of the H-bond of the confined water, by which an H-bond transitions to a coordination-bond at the Cu²⁺-center instantaneously after dissociating a previously coordinated H₂O.