Three-Dimensional Phthalocyanine Metal-Catecholates for High Electrochemical Carbon Dioxide Reduction.

The synthesis of a new anionic 3D metal-catecholate framework, termed MOF-1992, is achieved by linking tetratopic cobalt phthalocyanin-2,3,9,10,16,17,23,24-octaol linkers with Fe3(-C2O2-)6(OH2)2 trimers into an extended framework of roc topology. MOF-1992 exhibits sterically accessible Co active sites together with charge transfer properties. Cathodes based on MOF-1992 and carbon black (CB) display a high coverage of electroactive sites (270 nmol cm-2) and a high current density (-16.5 mA cm-2; overpotential, -0.52 V) for the CO2 to CO reduction reaction in water (faradaic efficiency, 80%). Over the 6 h experiment, MOF-1992/CB cathodes reach turnover numbers of 5800 with turnover frequencies of 0.20 s-1 per active site

Multistep Solid-State Organic Synthesis of Carbamate-Linked Covalent Organic Frameworks.

Herein, we demonstrate the first example of a multistep solid-state organic synthesis, in which a new imine-linked two-dimensional covalent organic framework (COF-170, 1) was transformed through three consecutive postsynthetic modifications into porous, crystalline cyclic carbamate and thiocarbamate-linked frameworks. These linkages are previously unreported and inaccessible through de novo synthesis. While not altering the overall connectivity of the framework, these chemical transformations induce significant conformational and structural changes at each step, highlighting the key importance of noncovalent interactions and conformational flexibility to COF crystallinity and porosity. These transformations were assessed using 15N multiCP-MAS NMR spectroscopy, providing the first quantitation of yields in COF postsynthetic modification reactions, as well as of amine defect sites in imine-linked COFs. This multistep COF linkage postsynthetic modification represents a significant step toward bringing the precision of organic solution-phase synthesis to extended solid-state compounds

Disorders and Dynamics of Reticular Materials Studied by Single-Crystal X-ray Diffraction

The work presented here focuses on understanding the complex structural phenomena inside metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs) by single-crystal X-ray diffraction (SXRD). These materials feature high porosity inside crystal structures, making spaces for the displacements and motions of the framework that are rarely observed in conventional crystals formed by close packing of molecules. While most studies solely rely on SXRD to obtain the skeleton of the structures, recent advances in the crystallographic analyses of reticular materials highlight the importance of rigorous interpretation of the disorders and how the observed static disorders might be associated with dynamics. Chapters 2 and 3 describe a framework-assisted crystal structure determination method for complex small molecules, the coordinative alignment method. Guests were incorporated and covalently attached into MOF-520, providing a platform to understand the behaviors of the guests when residing in a void space. Chapter 2 analyzes the stereoselectivity of the method originated from asymmetric coordination bond, which explains why the method can prevent a primary source of disorder that often interferes structural determination of guests. The thorough look at the solvent-induced guest disorder in Chapter 3 provides an insight into solvent-guest interactions that generally exist in porous crystals, and additionally introduces a method for improving the quality of structural solutions by solvent removal. Chapter 4 reports a correction to a previously reported ZIF structure (ZIF-90) after considering merohedral twinning. Rigorous crystallographic studies revealed the origin of merohedral twinning and associated it with a displacive phase transition at elevated temperatures, introducing a new facet to the forms of disorders in ZIFs and an unprecedented cause of displacive phase transition among any other materials. Finally, Chapter 5 describes the design and synthesis of a woven COF, which by design will transfer to a structure of interlocking 2D rings upon post-synthetic demetallation. It aims at a material at the boundary of crystalline and non-crystalline and thus challenging the boundary of crystallographic characterizations

Small-angle X-ray scattering studies of enzymes

Enzyme function requires conformational changes to achieve substrate binding, domain rearrangements, and interactions with partner proteins, but these movements are difficult to observe. Small-angle X-ray scattering (SAXS) is a versatile structural technique that can probe such conformational changes under solution conditions that are physiologically relevant. Although it is generally considered a low-resolution structural technique, when used to study conformational changes as a function of time, ligand binding, or protein interactions, SAXS can provide rich insight into enzyme behavior, including subtle domain movements. In this perspective, we highlight recent uses of SAXS to probe structural enzyme changes upon ligand and partner-protein binding and discuss tools for signal deconvolution of complex protein solutions

Coordinative Alignment in the Pores of MOFs for the Structural Determination of N-, S-, and P-Containing Organic Compounds Including Complex Chiral Molecules

Coordinative alignment of target small molecules onto a chiral metal–organic framework (MOF-520)provides a powerful method to determine the structures of small molecules through single-crystal X-ray diffraction (SXRD). In this work, the structures of 17 molecules with eight new coordinating functionalities and varying size have been determined by this method, four of which are complex molecules being crystallized for the first time. The chirality of the MOF backbone not only enables enantioselective crystallization of chiral small molecules from a racemic mixture but also imposes diastereoselective incorporation upon achiral molecules. Crystallographic studies assisted by density functional theory (DFT) calculations indicate that the stereoselectivity of MOF-520 not exclusively comes from the steric confinement of the chiral pore environment but also from asymmetric chemical bonding of the target molecules with the framework that is able to provide sufficient energy difference between possible coordination configurations

Urea-Linked Covalent Organic Frameworks.

2D covalent organic frameworks (COFs) with flexible urea linkages have been synthesized by condensation of 1,3,5-triformylphloroglucinol (TFP) with 1,4-phenylenediurea (BDU) or 1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea (DMBDU). The resulting COF-117 and COF-118 undergo reversible structural dynamics within their layers, in response to inclusion and removal of guest molecules, emanating from urea C-N bond rotation and interlayer hydrogen-bonding interactions. These compounds are the first urea-linked COFs, serving to expand the scope of reticular chemistry

Ionic Conduction Mechanism and Design of Metal-Organic Framework Based Quasi-Solid-State Electrolytes.

We report the theoretical and experimental investigation of two polyoxometalate-based metal-organic frameworks (MOFs), [(MnMo6)2(TFPM)]imine and [(AlMo6)2(TFPM)]imine, as quasi-solid-state electrolytes. Classical molecular dynamics coupled with quantum chemistry and grand canonical Monte Carlo are utilized to model the corresponding diffusion and ionic conduction in the two materials. Using different approximate levels of ion diffusion behavior, the primary ionic conduction mechanism was identified as solvent-assisted hopping (>77%). Detailed static and dynamic solvation structures were obtained to interpret Li+ motion with high spatial and temporal resolution. A rationally designed noninterpenetrating MOF-688(one-fold) material is proposed to achieve 6-8 times better performance (1.6-1.7 mS cm-1) than the current state-of-the-art (0.19-0.35 mS cm-1)

Urea-Linked Covalent Organic Frameworks.

2D covalent organic frameworks (COFs) with flexible urea linkages have been synthesized by condensation of 1,3,5-triformylphloroglucinol (TFP) with 1,4-phenylenediurea (BDU) or 1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea (DMBDU). The resulting COF-117 and COF-118 undergo reversible structural dynamics within their layers, in response to inclusion and removal of guest molecules, emanating from urea C-N bond rotation and interlayer hydrogen-bonding interactions. These compounds are the first urea-linked COFs, serving to expand the scope of reticular chemistry

Architectural Stabilization of a Gold(III) Catalyst in Metal-Organic Frameworks.

Unimolecular decomposition pathways are challenging to address in transition-metal catalysis and have previously not been suppressed via incorporation into a solid support. Two robust metal-organic frameworks (IRMOF-10 and bio-MOF-100) are used for the architectural stabilization of a structurally well-defined gold(III) catalyst. The inherent rigidity of these materials is utilized to preclude a unimolecular decomposition pathway - reductive elimination. Through this architectural stabilization strategy, decomposition of the incorporated gold(III) catalyst in the metal-organic frameworks is not observed; in contrast, the homogeneous analogue is prone to decomposition in solution. Stabilization of the catalyst in these metal-organic frameworks precludes leaching and enables recyclability, which is crucial for productive heterogeneous catalysis

Highly Active and Stable Single-Atom Cu Catalysts Supported by a Metal–Organic Framework

Single-atom catalysts are often considered as the ultimate design principle for supported catalysts, due to their unique geometric and electronic properties and their highly efficient use of precious materials. Here, we report a single-atom catalyst, Cu/UiO-66, prepared by a covalent attachment of Cu atoms to the defect sites at the zirconium oxide clusters of the metal–organic framework (MOF) UiO-66. Kinetic measurements show this catalyst to be highly active and stable under realistic reaction conditions for two important test reactions, the oxidation of CO at temperatures up to 350 °C, which makes this interesting for application in catalytic converters for cars, and for CO removal via selective oxidation of CO in H2-rich feed gases, where it shows an excellent selectivity of about 100% for CO oxidation. Time-resolved operando spectroscopy measurements indicate that the activity of the catalyst is associated with atomically dispersed, positively charged ionic Cu species. Density functional theory (DFT) calculations in combination with experimental data show that Cu binds to the MOF by –OH/–OH2 ligands capping the defect sites at the Zr oxide clusters