ZrIVCoordination Polymers Based on a Naturally Occurring Phenolic Derivative

Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Pyrogallate-Based Metal-Organic Framework with a Two-Dimensional Secondary Building Unit

We report a metal-organic framework (MOF) with a rare two-dimensional (2D) secondary building unit (SBU). The SBU comprises mixed-valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3',4,4',5,5'-hexahydroxybiphenyl, which also define the iron-oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed-valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

A tris-oxovanadium pyrogallate complex: synthesis, structure, and magnetic and electronic properties

With the aim of identifying new cation-phenolate complexes, we herein investigated the reactivity of pyrogallol (H₃pgal) with vanadium salts. A trimetallic anionic complex was identified, and found to be formed under a broad set of reaction conditions. This complex, with the formula V₃O₃(pgal)₃^3-, consists of three oxovanadium(IV) units connected together by three pyrogallate ligands to afford a bowl-shaped species presenting a pseudo 3-fold symmetry axis. Its crystal structure is reported, as well as its characterisation by a broad set of techniques, including powder X-ray diffraction, thermogravimetric analysis, infrared and Raman spectroscopy, and solid state UV-visible diffuse reflectance. Its redox activity both in solution and in the solid state is described, together with its magnetic behavior. Finally, the relevance of this trimetallic unit in the field of phenolic-based biocoatings and Metal Organic Framework (MOF) synthesis is briefly discussed.

Hexahydroxytriphenylene for the synthesis of group 13 MOFs - a new inorganic building unit in a β-cristobalite type structure

Two new, microporous MOFs of framework composition ((CH3)2NH2)2[M3O(HHTP)(HHTP˙)], M = Al3+, Ga3+, H6HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene, are described. Electron diffraction combined with molecular simulations show that these compounds crystallize in the β-cristobalite structure, containing a new type of trinuclear inorganic building unit for MOFs and radical anions.

Supramolecular assemblies of phenolic metalloporphyrins: Structures and electrochemical studies

The reactivity of two phenolic porphyrins bearing respectively catechol and gallol-derived meso substituents (5,10,15,20-tetrakis(3,4-dihydroxyphenyl)porphyrin and 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin) with trivalent metallic ions (Fe, Mn, In) was studied. Six supramolecular compounds were obtained and structurally characterized by single crystal X-ray diffraction. In each compound, the supramolecular assembly was based on the axial coordination of a phenolate function to the metallic ion lying in the porphyrinic core. A great diversity of supramolecular architectures was accessible through such simple arrangements, and objects ranging from dimers to one-dimensional polymers were isolated. Some of these assemblies were further investigated in solution by mass spectrometry and by UV-vis absorption spectroscopy. For the iron-based materials, the redox behavior was studied in solution through cyclic voltammetry experiments in inert conditions and under air.

Acid and Base Resistant Zirconium Polyphenolate-Metalloporphyrin Scaffolds for Efficient CO2 Photoreduction

A series of zirconium polyphenolate-decorated-(metallo)porphyrin metal-organic frameworks (MOFs), ZrPP-n (n = 1, 2), featuring infinite Zr^IV -oxo chains linked via polyphenolate groups on four peripheries of eclipse-arranged porphyrin macrocycles, are successfully constructed through a top-down process from simulation to synthesis. These are the unusual examples of Zr-MOFs (or MOFs in general) based on phenolic porphyrins, instead of commonly known carboxylate-based types. Representative ZrPP-1 not only exhibits strong acid resistance (pH = 1, HCl) but also remains intact even when immersed in saturated NaOH solution (≈20 m), an exceptionally large range of pH resistance among MOFs. The metallation at the porphyrin core gives rise to materials with enhanced sorption and catalytic properties. In particular, ZrPP-1-Co, with precise and uniform distribution of active centers, exhibits not only high CO₂ trapping capability (≈90 cm³ g^-1 at 1 atm, 273 K, among the highest in Zr-MOFs) but also high photocatalytic activity for reduction of CO₂ into CO (≈14 mmol g^-1 h^-1 ) and high selectivity over CH₄ (>96.4%) without any cocatalyst under visible-light irradiation (λ > 420 nm). Given the strong chemical resistance under extreme alkali conditions, these catalysts can be recycled without appreciable loss of activity. The possible mechanism for photocatalytic reduction of CO₂ -to-CO over ZrPP-1-Co is also proposed.

Identification of Zr(iv)-based architectures generated from ligands incorporating the 2,2'-biphenolato unit

The structural identification in solution of the Zr(iv) complexes involving two 2,2-biphenol-based proligands is reported. The proligand L(1)H2 contains one 2,2-biphenol unit whereas L(2)H4 incorporates two 2,2-biphenol units linked by a para-phenylene bridge. Diffusion Ordered Spectroscopy (DOSY) combined with electrospray mass spectrometry analysis and density functional theory (DFT) allowed for determining the molecular structures of such Zr(iv)-based architectures. It is proposed that [Zr(OPr(i))4(HOPr(i))] in the presence of L(1)H2 generates an octahedral complex formulated as [ZrL(1)3H2]. Concerning the self-assembled architecture incorporating the L(2) ligand, the analytical data highlight the formation of an unprecedented neutral Zr(iv) triple-stranded helicate ([Zr2L(2)3H4]). Insight into the geometry of these complexes is obtained via DFT calculations. Remarkably, the helicate structure characterized in solution strongly contrasts with the triple-stranded structure of the complex that crystallizes.

Crystal structure dependent in vitro antioxidant activity of biocompatible calcium gallate MOFs

Two novel 3-D coordination polymers, denoted MIL-155 and MIL-156 (MIL stands for Materials Institute Lavoisier), built up from calcium and the naturally occurring gallic acid (H₄gal), have been hydrothermally synthesized and their crystal structures were determined by single-crystal X-ray diffraction. These solids are based on different inorganic subunits: infinite chains of edge-sharing dimers of CaO₇ polyhedra linked through partially deprotonated gallate ligands (H₂gal^2-) for MIL-155 or [Ca₂(H₂O)(H₂gal)₂]·2H₂O, and ribbon-like inorganic subunits containing both eight-fold or six-fold coordinated Ca^II ions linked through fully deprotonated gallate ligands (gal^4-) for MIL-156 or [Ca₃K₂(H₂O)₂(gal)₂]·nH₂O (n∼ 5). Both solids contain small channels filled with water molecules, with, however no accessible porosity towards N₂ at 77 K. MIL-155 and MIL-156 were proven to be biocompatible, as evidenced by in vitro assays (viability and cell proliferation/death balance). While the high chemical stability of MIL-156 makes it almost bioinert, the progressive degradation of MIL-155 leads to an important protective antioxidant effect, associated with the release of the bioactive gallate ligand.

Titanium coordination compounds: from discrete metal complexes to metal–organic frameworks

Recent advances in titanium based MOFs and relevant titanium molecular compounds will be discussed in this review. Particular attention will be devoted to their promising photoredox properties.

Adaptability of the metal(iii,iv) 1,2,3-trioxobenzene rod secondary building unit for the production of chemically stable and catalytically active MOFs

Metal-1,2,3-trioxobenzene rod secondary building units are robust enough to adapt to cations of various charges.

A Breathing Zirconium Metal-Organic Framework with Reversible Loss of Crystallinity by Correlated Nanodomain Formation

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells.

Zr-based metal–organic frameworks: design, synthesis, structure, and applications

This review summarizes the advances in the study of Zr-based metal–organic frameworks in terms of their design, synthesis, structure, and potential applications.