Recent Advances in Zeolite-like Cluster Organic Frameworks

Saiz P, Reizabal A, Wuttke S, Celaya-Azcoaga L, Valverde A, Fernández de Luis R. A State-of-the-Art of Metal-Organic Frameworks for Chromium Photoreduction vs. Photocatalytic Water Remediation

Hexavalent chromium (Cr(VI)) is a highly mobile cancerogenic and teratogenic heavy metal ion. Among the varied technologies applied today to address chromium water pollution, photocatalysis offers a rapid reduction of Cr(VI) to the less toxic Cr(III). In contrast to classic photocatalysts, Metal-Organic frameworks (MOFs) are porous semiconductors that can couple the Cr(VI) to Cr(III) photoreduction to the chromium species immobilization. In this minireview, we wish to discuss and analyze the state-of-the-art of MOFs for Cr(VI) detoxification and contextualizing it to the most recent advances and strategies of MOFs for photocatalysis purposes. The minireview has been structured in three sections: (i) a detailed discussion of the specific experimental techniques employed to characterize MOF photocatalysts, (ii) a description and identification of the key characteristics of MOFs for Cr(VI) photoreduction, and (iii) an outlook and perspective section in order to identify future trends.

Dual-Ligand-Oriented Design of Noncentrosymmetric Complexes with Nonlinear-Optical Activity

When the N- and O-donor ligands are combined as coligands, two noncentrosymmetric (NCS) complexes of [Ni(p-bdc)(tipa)(H₂O)₂]₂·H₂O (1) and Ni(npdc)(tipa)H₂O (2) [tipa = tris[4-(1H-imidazol-1-yl)phenyl]amine, p-H₂bdc = 1,4-benzenedicarboxylic acid, and H₂npdc = 2,6-naphthalenedicarboxylic acid] were achieved under solvothermal conditions. For both structures, N-donor ligands are responsible for the generation of a layered structure, while the O-donor ligands are hung on the layers and are responsible for enhancing the polarity, giving rise to the NCS structures. Because of the different connection modes between the metal centers and different carboxylate ligands (p-bdc^2- in 1 and npdc^2- in 2), 1 and 2 show some structural differences. The p-bdc^2- ligands in 1 are suspended on the upper and lower sides of the [Ni(tipa)]_n layers, while all of the npdc^2- ligands in 2 hang on one side of the [Ni(tipa)]_n layers and point in the same direction, which makes the two NCS complexes show phase-matchable behavior with different second-harmonic-generation (SHG) responses of about 0.9 and 1.5 times that of KH₂PO₄ (KDP), respectively. Theoretical studies reveal that charge transfers between Ni²⁺ and carboxylate ligands make the dominant contribution to the optical properties. It is expected that a dual-ligand strategy may guide the design of novel superior-performing NCS complexes.

A Heniconuclear {Mn21} Cluster-Based Coordination Polymer with Manganese(II) Linkers Showing High Proton Conductivity

An unprecedented metal-linked cluster-based coordination polymer, composed of heniconuclear {Mn₂₁} clusters and Mn(II) ions as the nodes and linkers, respectively, was self-assembled from a facile aqueous synthesis. The structural analysis reveals that the compound possesses a rare 3D 8-connected hex framework topology. Significantly, the compound demonstrates a high proton conductivity of 1.06 mS cm^-1 at 373 K and 98% RH and exhibits a magnetocaloric effect with a magnetic entropy change of -9.94 J kg^-1 K^-1 at H = 80 kOe and T = 6.0 K.

Designed Syntheses of Three {Ni6PW9}-Based Polyoxometalates, from Isolated Cluster to Cluster-Organic Helical Chain

Three new hexa-Ni-substituted Keggin-type polyoxometalates (POMs), [Ni6(OH)3- (DACH)3(H2O)6(PW9O34)]·31H2O (1), [Ni(DACH)2][Ni6(OH)3(DACH)3(HMIP)2(H2O)2(PW9O34)]·56 H2O (2), and [Ni(DACH)2][Ni6(OH)3(DACH)2(AP)(H2O)5(PW9O34)]·2H2O (3) (DACH = 1,2-Diami- nocyclohexane, MIP = 5-Methylisophthalate, AP = Adipate) were successfully made in the presence of DACH under hydrothermal conditions. 1 is an isolated hexa-Ni-substituted Keggin unit decorated by DACH. In order to further construct POM cluster-organic frameworks (POMCOFs) on the basis of 1, by analyzing the steric hindrances and orientations of the POM units, the rigid HMIP and flexible AP ligands were successively incorporated, and another anionic monomeric POM 2 and the new 1D POM cluster organic chain (POMCOC) 3 were obtained. HMIP ligand still acts as a decorating group on the Ni6 core of 2 but results in the different spatial arrangement of the {Ni6PW9} units. AP ligands in 3 successfully bridge adjacent isolated POM cluster units to 1D POMCOC with left-hand helices. The AP in 3 is the longest aliphatic carboxylic acid ligand in POMs, and the 1D POM cluster-AP helical chain represents the first 1D POMCOC with a helical feature.

Designed assembly of heterometallic zeolite-like framework materials from two different supertetrahedral metal clusters

This work demonstrates a feasible strategy for the synthesis of new zeolite-like framework materials. With the strategy of using two different supertetrahedral clusters as secondary building units, two new heterometallic zeolite-like frameworks with isomeric structures but tunable topologies were first made. Besides, the porous nature and the proton conduction performance of the new materials were further studied.

Designable assembly of atomically precise Al4O4 cubane supported mesoporous heterometallic architectures

Heterometallic cluster-based framework materials are of interest in terms of both their porous structures and multi-metallic reactivity. However, such materials have not yet been extensively investigated because of difficulties in their synthesis and structural characterization. Herein, we reported the designable synthesis of atomically precise heterometallic cluster-based framework compounds and their application as catalysts in aldol reactions. By using the synergistic coordination protocol, we successfully isolated a broad range of compounds with the general formula, [Al4M4O4(L)12(DABCO)2] (L = carboxylates; DABCO = 1,4-diazabicyclo[2.2.2]-octane; M2+ = Co2+, Mn2+, Zn2+, Fe2+, Cd2+). The basic heterometallic building blocks contain unprecedented main-group γ-alumina moieties and surrounding unsaturated transition metal centers. Interestingly, the porosity and interpenetration of these frameworks can be rationally regulated through the unprecedented strategy of increment of the metal radius in addition to general introduction of sterically bulky groups on the ligand. Furthermore, these porous materials are effective catalysts for aldol reactions. This work provides a catalytic molecular model platform with accurate molecular bonding between the supporters and catalytically active metal ions.

Four Inorganic-Organic Hybrid Borates: From 2D Layers to 3D Oxoboron Cluster Organic Frameworks

Four inorganic-organic hybrid borates, K[B₆O₉(OH)(en)]·H₂O (1, en = ethylenediamine), K[B₆O₉(OH)(1,3-dap)]·H₂O (2, 1,3-dap = 1,3-diaminopropane), K[B₆O₉(OH)(1,6-dah)_0.5]·H₂O (3, 1,6-dah = 1,6-diaminlhexane) and [(1,3-dap)Cd@B₅O₈(OH)]·0.5H₂O (4), were made under solvothermal conditions. 1 and 2 are isostructural and feature a 2D layer built by B₆O₉(OH) clusters and modified by en and 1,3-dap via B-N-C linkages. By replacing en and 1,3-dap with longer and more flexible 1,6-dah, a new type of oxoboron cluster organic framework 3 was obtained, which was composed of the same B₆O₉(OH) cluster layers as in 1 and 2 and 1,6-dah linkers. By replacing alkali metal K with transition metal Cd under similar synthetic conditions, another type of oxoboron cluster organic framework 4 was made in which the Cd-centered wheel cluster layers and 1,3-dap linkers were connected via Cd-N-C linkages.

Luminescent cluster-organic frameworks constructed from predesigned supertetrahedral {Ln4Zn6} secondary building units

3d-4f heterometallic supertetrahedral clusters with the formula of Ln₄Zn₆(μ₆-O)L₄(CH₃COO)₆(NO₃)₄(CH₃OH)₄(H₂O)₂ (1-Ln, Ln = Eu, Gd, Tb, H₃L = 2-(hydroxymethyl)-2-(pyridin-4-yl)-1,3-propanediol) have been successfully introduced as stable secondary building units (SBUs) to construct new cluster-organic frameworks with tunable emission, demonstrating a promising strategy for developing new optical materials.

Discovery and Supramolecular Interactions of Neutral Palladium-Oxo Clusters Pd16 and Pd24

We report on the synthesis, structure, and physicochemical characterization of the first three examples of neutral palladium-oxo clusters (POCs). The 16-palladium(II)-oxo cluster [Pd16 O24 (OH)8 ((CH3 )2 As)8 ] (Pd16 ) comprises a cyclic palladium-oxo unit capped by eight dimethylarsinate groups. The chloro-derivative [Pd16 Na2 O26 (OH)3  Cl3  ((CH3 )2  As)8 ] (Pd16 Cl) was also prepared, which forms a highly stable 3D supramolecular lattice via strong intermolecular interactions. The 24-palladium(II)-oxo cluster [Pd24 O44 (OH)8 ((CH3 )2 As)16 ] (Pd24 ) can be considered as a bicapped derivative of Pd16 with a tetra-palladium-oxo unit grafted on either side. The three compounds were fully characterized 1) in the solid state by single-crystal and powder XRD, IR, TGA, and solid-state 1 H and 13 C NMR spectroscopy, 2) in solution by 1 H, 13 C NMR and 1 H DOSY spectroscopic methods, and 3) in the gas phase by electrospray ionization mass spectrometry (ESI-MS).

Construction of Titanium-Based Metal-Organic Frameworks Based on the Ti/Cu Heteronuclear Cluster

Presented here are two titanium-based metal-organic frameworks (Ti-MOFs) based on well-defined [Ti₆Cu₆(μ₃-O)₂(μ₂-O)₉(HSO₄)₂(SO₄)₆], which can be easily obtained from a cheap Ti source and CuSO₄ and exhibited interesting magnetic properties. Furthermore, this clusters can be isolated in pure phase. Numerous uncoordinated sites of SO₄ and labile ligands on the Ti and Cu centers of this cluster make it a good candidate as a secondary building unit to construct various Ti-MOFs in the future.

Two isomeric zeolite-like metal–organic frameworks with mechanically responsive luminescence emission and gas adsorption properties

The two isomeric zeolite-like metal–organic frameworks based on tetrahedral TPO3− ligands and tetrahedrally connected Cd2+ ions were prepared. One of the compounds shows mechanic-responsive luminescence after the crystals were ground for different time.

Combining valproate and bipyridyl ligands to construct a 0D core-shell Zn5(μ3-OH)2 cluster and a 2D layered coordination network with a [Zn3(μ3-OH)]2 SBU

Starting from the proposed zinc carboxylate cluster tetrakis(μ-2-propylpentanoato)dizinc(II), Zn₂(μ₂-valp)₄ (I), of valproic acid, a branched short-chain fatty acid, and bipyridine ligands, two new mixed-ligand coordination compounds, namely, bis(2,2'-bipyridine)di-μ₃-hydroxido-hexakis(μ-2-propylpentanoato)bis(2-propylpentanoato)pentazinc(II), [Zn₅(C₈H₁₅O₂)₈(OH)₂(C₁₀H₈N₂)₂] (II), and poly[[bis(μ-4,4'-bipyridine)di-μ₃-hydroxido-octakis(μ-2-propylpentanoato)bis(2-propylpentanoato)hexazinc(II)] dimethylformamide disolvate], {[Zn₆(C₈H₁₅O₂)₁₀(OH)₂(C₁₀H₈N₂)₂]·2C₃H₇NO}_n (III), were synthesized. Compound II is a core-shell-type zero-dimensional discrete Zn₅(μ₃-OH)₂ metal-organic cluster with Zn ions in double-triangle arrangements that share one Zn ion coincident with an inversion centre. The cluster contains three crystallographically non-equivalent Zn ions exhibiting three different coordination geometries (tetrahedral, square pyramidal and octahedral). The cluster cores are well separated and embedded in a protective shell of the aliphatic branched short chains of valproate. As a result, there is no specific interaction between the discrete clusters. Conversely, compound III, a 2D layered coordination network with a secondary building unit (SBU), is formed by Zn₆(μ₃-OH)₂ clusters exhibiting a chair-like hexagonal arrangement. This SBU is formed from two Zn₃(μ₃-OH) trimers related by inversion symmetry and connected by two syn-anti bridging carboxylate groups. Each SBU is connected by four 4,4'-bipyridine ligands producing a 6³-hcb net topology. 2D coordination layers are sandwiched within layers of dimethylformamide molecules that do not interact strongly with the network due to the hydrophobic protection provided by the valproate ligands.

Adjustment of the performance and stability of isostructural zeolitic tetrazolate-imidazolate frameworks

Presented here are two isostructural SOD-type zeolitic tetrazolate-imidazolate frameworks (ZTIFs), Zn(etz)0.86(mim)1.14 (ZTIF-9, Hetz = 5-ethyltetrazole, Hmim = 2-methylimidazole) and Zn(vtz)0.63(mim)1.37 (ZTIF-10, Hvtz = 5-vinyltetrazole). The adjustment of the ligand ratios within these ZTIFs was realized through changing the substituent groups of tetrazole ligands. Remarkably, ZTIF-9 with a suitable ligand ratio perfectly balances gas uptake and stability, exhibiting 6-fold improvement of C2H2 uptake compared to the prototype ZIF-8 (Zn(mim)2).

Controllable incorporation of 1,2,4-triazolate into cluster-based metal-chalcogenide frameworks

1,2,4-Triazolate is controllably incorporated into cluster-based metal-chalcogenide frameworks by adjusting the coordination competition between organic and inorganic species.

One-dimensional chains based on linear tetranuclear copper(i) units: reversible structural transformation and luminescence change

Two extended metal chains based on linear tetracopper units are reported, in which the bromide complex undergoes reversible structural transformation and luminescence change.

A photoactive copper iodide phosphine-based coordination polymer

Coordination of the polymetallic [Cu₄I₄] center by phenyl-based-diphosphine ligands results in a coordination polymer exhibiting luminescence thermochromic properties.

A new type of composite MOFs based on high-valent Sb(v)-based units and cuprous-halide clusters

An unusual high-valent Sb(v)-based unit [SbL₂]^- (L = 2-(hydroxymethyl)-2-(pyridin-4-yl)-1,3-propanediol) was developed for the first time to combine with various cuprous-halide clusters for the construction of a brand-new class of heterometallic MOFs. This work not only promotes the limited development of Sb-based MOFs, but also provides a practical strategy for developing new types of composite materials.

Interlinking supertetrahedral chalcogenolate clusters with bipyridines to form two-dimensional coordination polymers for photocatalytic degradation of organic dye

Chalcogenolate clusters Cd6Ag4(EPh)16(DMF)3(CH3OH) (E = S, Se) with supertetrahedral structures are isolated. Further interlinking the clusters with organic linker 4,4'-trimethylenedipiperidine in the stepwise assembly approach forms two-dimensional coordination polymers. The clusters and the coordination polymers show tunable band gaps and efficient photocatalytic activities for the degradation of aqueous dye solution. This study demonstrates the great potential of using chalcogenolate clusters and their coordination polymers in photocatalysis applications.

A new cluster-based chalcogenide zeolite analogue with a large inter-cluster bridging angle

A new cluster-based chalcogenide zeolite analogue with a large inter-cluster bridging angle.

Insight into the structure and bonding of copper(i) iodide clusters and a cluster-based coordination polymer

The structure and chemical bonding pattern of selected copper(i) iodide clusters and a cluster-based coordination polymer are investigated using DFT.