Uranium(IV) and Thorium(IV) Coordination Polymers Based on Tritopic Carboxylic Acids.

Nine new coordination polymers based on U(IV) and Th(IV) were synthesized solvothermally utilizing four different trianionic carboxylates (H3BHTC = biphenyl-3,4',5-tricarboxylic acid, H3NTB = 4,4',4″-nitrilotribenzoic acid, H3BTB = 4,4',4″-benzene-1,3,5-triyl-tris(benzoic acid), H3BTE = 4,4',4″-(1,3,5-benzenetriyltri-2,1-ethynediyl)trisbenzoic acid). The influence of the ligand architecture, the pH, the stoichiometry, the nature of the metal, and the concentration on the structure and dimensionality of the final actinide assembly is discussed. The H3BHTC ligand allowed the synthesis of a cationic three-dimensional (3D) framework [U(BHTC)(DMF)3]I (1), which is the first example of a cationic U(IV) polymer. The H3NTB ligand yielded the 3D neutral polymer [U3(NTB)4] (2) or the two-dimensional (2D) cationic polymer [U(NTB)(NMP)3]I (3), depending on the solvent. When conditions leading to (2) were used with a Th(IV) precursor, the 2D neutral polymer [Th(NTB)(DMF)3Cl] (4) was obtained. The ligand H3BTB allowed the synthesis of two 3D cationic networks [U(BTB)(DMF)2]I (5) and [U(BTB)(DMF)3]I (7) or the neutral 3D analogue [U3(BTB)4] (6), depending on the precursor's oxidation state and the acidity of the reaction mixture. The ligand H3BTE allowed the synthesis of the anionic 3D [(CH3)2NH2][U2(BTE)3] (8) framework featuring large accessible pores, and under the same conditions, an isostructural Th(IV) was also obtained [(CH3)2NH2][Th2(BTE)3] (8-Th). All isolated coordination polymers were characterized by single-crystal X-ray diffraction (SCXRD). The Langmuir surface areas of the U(IV) polymers (2), (7), and (8) increased from 140 to 310 m2/g owing to the increasing size of the linker, with polymer (8) showing a value that is comparable to the highest surface area reported to date. The effect of the postsynthetic solvent substitution was also studied, revealing a crystal-to-crystal transformation of the cationic framework (7) to the neutral framework [U(BTB)(THF)I] (7c).

A Simple, Transition Metal Catalyst-Free Method for the Design of Complex Organic Building Blocks Used to Construct Porous Metal-Organic Frameworks.

The design of metal-organic frameworks (MOFs) having large pore sizes and volumes often requires the use of complex organic ligands, currently synthesized using costly and time-consuming palladium-catalyzed coupling chemistry. Thus, in the present work, a new strategy for ligand design is reported, where piperazine and dihydrophenazine units are used as substitutes for benzene rings, which are the basic building block of most MOF ligands. This chemistry, which is based on simple, nucleophilic aromatic substitution (SN Ar) reactions, is used for the transition metal catalyst-free construction of 21 new, carboxylate-based ligands with varying sizes, shapes, and denticity and 15 linear di- and tetra-nitriles. Moreover, to demonstrate the utility of the ligands as building blocks, 16 new structurally diverse MOFs having surface areas up to 3100 m2 g-1 were also synthesized.

A post-synthetic modification strategy for enhancing Pt adsorption efficiency in MOF/polymer composites.

Growing polymers inside porous metal-organic frameworks (MOFs) can allow incoming guests to access the backbone of otherwise non-porous polymers, boosting the number and/or strength of available adsorption sites inside the porous support. In the present work, we have devised a novel post-synthetic modification (PSM) strategy that allows one to graft metal-chelating functionality onto a polymer backbone while inside MOF pores, enhancing the material's ability to recover Pt(iv) from complex liquids. For this, polydopamine (PDA) was first grown inside of a MOF, known as Fe-BTC (or MIL-100 Fe). Next, a small thiol-containing molecule, 2,3-dimercapto-1-propanol (DIP), was grafted to the PDA via a Michael addition. After the modification of the PDA, the Pt adsorption capacity and selectivity were greatly enhanced, particularly in the low concentration regime, due to the high affinity of the thiols towards Pt. Moreover, the modified composite was found to be highly selective for precious metals (Pt, Pd, and Au) over common base metals found in electronic waste (i.e., Pb, Cu, Ni, and Zn). X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) provided insight into the Pt adsorption/reduction process. Last, the PSM was extended to various thiols to demonstrate the versatility of the chemistry. It is hoped that this work will open pathways for the future design of novel adsorbents that are fine-tuned for the rapid, selective retrieval of high-value and/or critical metals from complex liquids.

3D vs. turbostratic: controlling metal-organic framework dimensionality via N-heterocyclic carbene chemistry.

Using azolium-based ligands for the construction of metal-organic frameworks (MOFs) is a viable strategy to immobilize catalytically active N-heterocyclic carbenes (NHC) or NHC-derived species inside MOF pores. Thus, in the present work, a novel copper MOF referred to as Cu-Sp5-BF4, is constructed using an imidazolinium ligand, H2Sp5-BF4, 1,3-bis(4-carboxyphenyl)-4,5-dihydro-1H-imidazole-3-ium tetrafluoroborate. The resulting framework, which offers large pore apertures, enables the post-synthetic modification of the C2 carbon on the ligand backbone with methoxide units. A combination of X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ssNMR) and electron microscopy (EM), are used to show that the post-synthetic methoxide modification alters the dimensionality of the material, forming a turbostratic phase, an event that further improves the accessibility of the NHC sites promoting a second modification step that is carried out via grafting iridium to the NHC. A combination of X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) methods are used to shed light on the iridium speciation, and the catalytic activity of the Ir-NHC containing MOF is demonstrated using a model reaction, stilbene hydrogenation.

Hybridization of Synthetic Humins with a Metal-Organic Framework for Precious Metal Recovery and Reuse.

The number of synthetic strategies used to functionalize MOFs with polymers is rapidly growing; this stems from the knowledge that non-native polymeric guests can significantly boost MOF performance in a number of desirable applications. The current work presents a scalable and solid-state method for MOF/polymer composite production. This simple method constitutes mixing a MOF powder, namely, Fe-BTC (BTC = 1,3,5-benzenetricarboxylate), with a biomass-derived solid monomer, 5-hydroxymethylfurfural (HMF), and subsequently heating the solids; the latter promotes both solid-state diffusion of HMF into the MOF and the formation of polymeric humin species with a high density of accessible hydroxyl functionality within the MOF pore. The resulting composite, Fe-BTC/humin, was found to selectively extract Ag+ ions from laundry wastewater. Subsequent reduction of the Ag+ species yields a novel catalyst, Fe-BTC/humin/Ag, that is able to drive the organic transformation of cinnamaldehyde in a highly selective manner. Moreover, the catalyst exhibited recyclability up to five cycles, which is in contrast to the Fe-BTC/Ag catalyst without the humin-based polymer. It is envisioned that MOF/polymer composites that are able to selectively extract precious metals from liquid waste streams can be used for the future production of sustainable catalysts; this work was aimed at demonstrating a proof of concept in this regard. Moreover, this study brings more understanding of the impact that MOFs can have on polymer functionalities. Understanding the polymer structure and how it can be manipulated will help us realize the high degree of future potential of this distinct class of composite materials.