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

Hydrogen bonding (H-bonding) is a crucial intermolecular force with significant implications across various scientific disciplines, including chemistry, biology, physics, energy science, and environmental science. Understanding H-bond dynamics, particularly in confined environments like nanopores, is challenging due to the altered chemical behavior of water molecules compared to bulk water. While zeolites and carbon nanotubes have been studied, MOFs offer unique advantages due to their tunable porosity, modular topology, and diverse chemical functionalities. MOFs have shown promise in applications like proton conduction, adsorption-based heat pumping, and atmospheric water harvesting, where H-bonding plays a vital role. The dynamics of H-bonds in MOFs are complex, influenced by factors such as pore size and shape, the chemical nature of the pore walls, and the presence of open metal sites (OMSs). OMSs can significantly affect water molecule mobility due to potential H-bonds with coordinating water molecules, leading to a specific H-bond network along the interior walls. Previous research has highlighted the templating effect of H-bonds in MOFs, showcasing the formation of one-dimensional chains or preferential water binding sites depending on humidity and water loading. This study focuses on HKUST-1, a paddlewheel MOF known for its unique structural features and sensitivity of its Cu-Cu stretching vibration to coordinating species. The researchers hypothesize that H-bonds between coordinating and pore-filling water molecules in HKUST-1 will influence the Cu-Cu vibrational mode and lead to site-selective water occupation in the pores even at room temperature.

The literature review extensively covers the importance of hydrogen bonding in various fields and the challenges in understanding its dynamics in confined systems. It cites numerous studies examining H-bonding in zeolites and carbon nanotubes, highlighting the dependence of H-bond dynamics on the chemical environment. The review emphasizes the potential of metal-organic frameworks (MOFs) as structurally well-defined porous materials with tunable physicochemical properties, relevant to proton conduction, adsorption-based heat pumping, and atmospheric water harvesting. The impact of open metal sites (OMSs) on water mobility and H-bond networks within MOF pores is discussed, referencing studies demonstrating templating effects and loading-dependent water structural changes. Finally, the unique properties of HKUST-1, including its paddlewheel structure, two types of large cages and one type of small cage, and Raman-sensitive Cu-Cu stretching vibration, are highlighted as justification for its selection in this research.

The study employed a combination of experimental techniques to investigate the hydrogen bonding behavior of water molecules within HKUST-1. The researchers synthesized HKUST-1 single crystals and prepared samples in various states: activated (Act-HKUST-1, without water molecules), water-coordinating (H₂O[C]-HKUST-1, only coordinating water molecules), and water-filled (H₂O[F]-HKUST-1, both coordinating and pore-filling water molecules). Analogous samples with methanol and ethanol were also prepared. Phase purity was confirmed through ¹H NMR spectroscopy and powder X-ray diffraction (PXRD). Raman spectroscopy was used to analyze the Cu-Cu stretching vibration, which is sensitive to the environment around the Cu²⁺ centers. The correlation between Cu-Cu vibrational band energies and bond lengths was determined through synchrotron single-crystal X-ray diffraction (SCXRD) experiments at 220 K and 298 K. In situ Raman experiments were conducted to monitor spectral changes upon water ingress into Act-HKUST-1 crystals exposed to moist air at room temperature. Simultaneous weight changes were monitored with a microbalance and color changes with an optical microscope. The correlation between Cu-Cu vibrational frequency and length changes was investigated using in situ SCXRD measurements at 298 K. The methodology also involved isotope studies with D₂O and H₂¹⁸O, investigations of temperature effects on vibrational energy, and analyses of EtOH and MeOH-containing HKUST-1 samples.

Key findings include the demonstration of vibrational chain connectivity between hydrogen bonds and the paddlewheel Cu-Cu bond in HKUST-1, using Raman spectroscopy. The researchers observed distinct Raman bands corresponding to different water loading states (activated, water-coordinating, and water-filled). SCXRD analysis revealed that even at room temperature, unbound hydrogen-bonded water molecules exhibited high spatial ordering near coordinating water molecules, exhibiting ice-like spatial ordering. This contradicts the expectations of high disorder in bulk water at room temperature. A substantial distortion of the paddlewheel Cu²⁺ centers was observed after water coordination, evidenced by changes in Cu-Cu bond length and Oвтc-Cu-Oвтc angle. The Cu-Cu bond length varied depending on the state (activated: 2.485 Å; water-coordinating: 2.624 Å; water-filled: 2.613 Å). The study also revealed the dynamic coordination bond character of the hydrogen bonds, where hydrogen-bonded water molecules can transiently coordinate with the Cu²⁺ center after replacing a previously coordinated water molecule. In situ Raman and SCXRD studies at 298 K confirmed the correlation between the Cu-Cu bond length, vibrational frequency, and water uptake. In situ observations showed a ratiometric change in Raman band intensity and a continuous, smooth shift towards higher wavenumbers with increasing water content. The weight increase of Act-HKUST-1 upon water exposure revealed a molar ratio of [H₂O]/[Cu²⁺] close to 5.9. The in situ SCXRD measurements also indicated a high degree of spatial ordering of the water molecules within the framework and demonstrated a correlation between the Cu-Cu length and the amount of water molecules.

The findings address the research question by demonstrating the crystalline nature of hydrogen-bonded water molecules within the HKUST-1 framework at room temperature. The observed vibrational chain connectivity, spatial ordering, and the dynamic coordination bond character of the confined hydrogen bonds highlight the unique behavior of water in the MOF environment. This differs significantly from the disordered nature of bulk water. The distortion of the Cu²⁺ centers and their correlation with water coordination and hydrogen bonding underline the strong interaction between the water molecules and the framework. These results offer a deeper understanding of the H-bond dynamics within MOFs and provide insights into how the H-bond network can affect the framework's properties. This is significant for developing porous materials for applications such as proton conduction and atmospheric water harvesting. The study successfully employs a combined approach of Raman and SCXRD to elucidate intricate correlations between vibrational properties, water structure, and framework geometry.

This study reveals the crystalline behavior of confined water molecules within the HKUST-1 MOF, showcasing vibrational chain connectivity, ice-like spatial ordering at room temperature, and dynamic coordination bond characters. The findings indicate a substantial distortion of Cu²⁺ centers due to water coordination and hydrogen bonding. These observations are significant for understanding fundamental H-bond behavior in materials and developing efficient porous materials for diverse applications, including proton conduction, heat pumping, and atmospheric water harvesting. Further research could explore other MOF structures with varied pore geometries and metal centers to investigate the generality of these findings and their potential for tailored material design.

The study mainly focuses on HKUST-1, limiting the generalizability of the findings to other MOF types. While the in situ measurements provide valuable insights, the process of exposing the crystals to moisture might introduce uncontrolled factors, potentially influencing the results. The exact mechanism of the dynamic coordination bond character of the H-bonds warrants further investigation. The study is limited to specific water loadings and it may be worth investigating different water uptake levels and their effect on the observed phenomena.