Homogeneous oxidative transformations mediated by copper catalyst systems

An octanuclear 3-phenyl-5-(2-pyridyl)pyrazolate/phenylsilsesquioxane complex: synthesis, unique structure, and catalytic activity

The first metallasilsesquioxane bearing pyrazolylpyridine ligands, the Cu8-based complex 1, adopts a cage-like structure with two zigzag-type copper tetramers sandwiched by two cyclic Si5 silsesquioxane ligands. The four 3-phenyl-5-(2-pyridyl)pyrazolate ligands in 1 exhibit dual (chelating and bridging) modes of ligation. Compound 1 is very active in the oxidation of alkanes and alcohols.

Cu12-Methylsilsesquioxane Cage Decorated with Cu(dppe)2 Moieties for Mild Oxidative Functionalization of Alkanes

We report a high nuclear (Cu14) complex synthesized via the self-assembly of copper-methylsilsesquioxane induced by the complexation with 1,2-bis(diphenylphosphino)ethane (dppe). The structure includes two cationic CuI(dppe)2 moieties and an anionic silsesquioxane cage of an unprecedented CuII12 structural type. The Cu12 cage fragment exhibits a unique (i) combination of Si4-cyclic/Si2-acyclic silsesquioxane ligands and (ii) encapsulation of two different chloride and carbonate species. This complex acts as a promising precatalyst in the mild oxidation and carboxylation of light alkanes to produce alkyl hydroperoxides, alcohols, ketones, or carboxylic acids. The present study widens the family of copper-methylsilsesquioxane clusters with prospective use in oxidation catalysis.

Dioxygenase Chemistry in Nucleophilic Aldehyde Deformylations Utilizing Dicopper O2-Derived Peroxide Complexes

The chemistry of copper-dioxygen complexes is relevant to copper enzymes in biology as well as in (ligand)Cu-O2 (or Cu2-O2) species utilized in oxidative transformations. For overall energy considerations, as applicable in chemical synthesis, it is beneficial to have an appropriate atom economy; both O-atoms of O2(g) are transferred to the product(s). However, examples of such dioxygenase-type chemistry are extremely rare or not well documented. Herein, we report on nucleophilic oxidative aldehyde deformylation reactivity by the peroxo-dicopper(II) species [Cu2II(BPMPO-)(O22-)]1+ {BPMPO-H = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} and [Cu2II(XYLO-)(O22-)]1+ (XYLO- = a BPMPO- analogue possessing bis(2-{2-pyridyl}ethyl)amine chelating arms). Their dicopper(I) precursors are dioxygenase catalysts. The O2(g)-derived peroxo-dicopper(II) intermediates react rapidly with aldehydes like 2-phenylpropionaldehyde (2-PPA) and cyclohexanecarboxaldehyde (CCA) in 2-methyltetrahydrofuran at -90 °C. Warming to room temperature (RT) followed by workup results in good yields of formate (HC(O)O-) along with ketones (acetophenone or cyclohexanone). Mechanistic investigation shows that [Cu2II(BPMPO-)(O22-)]1+ species initially reacts reversibly with the aldehydes to form detectable dicopper(II) peroxyhemiacetal intermediates, for which optical titrations provide the Keq (at -90 °C) of 73.6 × 102 M-1 (2-PPA) and 10.4 × 102 M-1 (CCA). In the reaction of [Cu2II(XYLO-)(O22-)]1+ with 2-PPA, product complexes characterized by single-crystal X-ray crystallography are the anticipated dicopper(I) complex, [Cu2I(XYLO-)]1+ plus a mixed-valent Cu(I)Cu(II)-formate species. Formate was further identified and confirmed by 1H NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS) analysis. Using 18O2(g)-isotope labeling the reaction produced a high yield of 18-O incorporated acetophenone as well as formate. The overall results signify that true dioxygenase reactions have occurred, supported by a thorough mechanistic investigation.

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

Generation and Reactivity of μ-1,2-Peroxo CuIICuII and Bis-μ-oxo CuIIICuIII Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide

Tetradentate-N4 ligands stabilize dinuclear {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} species, and CuII complexes of these ligands were reported to catalyze the oxidation of benzene with H2O2. Here, we report {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} intermediates of dinucleating bis(tetradentate-N4) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {CuICuI} precursors with O2 or from {CuIICuII} precursors with H2O2 and NEt3. Both intermediates are not stable even at low temperatures, but they show no electrophilic HAT reactivity with DHA. Catalytic investigations on the hydroxylation of benzene with excess H2O2 between 30 and 50 °C indicate that both radical-based and {Cu2On}-based mechanisms depend strongly on the catalytic conditions. In the presence of a radical scavenger, TONs of ∼920/∼720 have been achieved without/with the 6-methyl group of the ligand. Although {CuII(μ-OH)CuII} reacts with excess H2O2 at -40 °C to {CuII(OOH)}2 species, these are only stable for seconds at 20 °C and cannot account for catalytic oxidations over a period of 24 h at 30-50 °C.

Efficient synthesis of 3-alkyl-2-(-1H-1,2,3-triazolyl)methyl)thio)-2,3-dihydroquinazolin-4(1H)-one derivative via multistep synthesis approach by novel Cu@Py-Oxa@SPION catalyst

In this pared, an efficient method is introduced for the synthesis of 3-alkyl-2-(((4-(2-oxopropyl)-1H-1,2,3-triazol-1-yl)alkyl)thio)-2,3-dihydroquinazolin-4(1H)-one derivatives. These novel products have both 1,2,3-triazole and quinazolinone in their structures. For the synthesis of these products, a novel catalyst is designed, synthesized, and characterized by the immobilization of copper onto modified magnetic iron oxide. The catalyst (denoted: Cu@Py-Oxa@SPION) was characterized by several characterization techniques. In this regard, 16 3-alkyl-2-(((4-(2-oxopropyl)-1H-1,2,3-triazol-1-yl)alkyl)thio)-2,3-dihydroquinazolin-4(1H)-one derivatives were synthesized in high isolated yields (77-86%). As an advantage, the catalyst is highly recoverable and its activity has not decreased after 7 sequential runs. The method is very efficient for the synthesis of the products in high isolated yields under mild reaction conditions in a green solvent. The scope of the method is broad and several examples were successfully synthesized using starting materials with different functional groups.

Expanding the Promiscuity of a Copper-Dependent Oxidase for Enantioselective Cross-Coupling of Indoles

Herein, we disclose the highly enantioselective oxidative cross-coupling of 3-hydroxyindole esters with various nucleophilic partners as catalyzed by copper efflux oxidase. The biocatalytic transformation delivers functionalized 2,2-disubstituted indolin-3-ones with excellent optical purity (90-99 % ee), which exhibited anticancer activity against MCF-7 cell lines, as shown by preliminary biological evaluation. Mechanistic studies and molecular docking results suggest the formation of a phenoxyl radical and enantiocontrol facilitated by a suited enzyme chiral pocket. This study is significant with regard to expanding the catalytic repertoire of natural multicopper oxidases as well as enlarging the synthetic toolbox for sustainable asymmetric oxidative coupling.

Bio-inspired Cu(II) amido-quinoline complexes as catalysts for aromatic C-H bond hydroxylation

Cu(II) complexes supported by tetradentate amido-quinoline acyclic ligands (L1 & L2) have been synthesized, characterized, and employed as catalysts for aromatic C-H hydroxylation using H₂O₂ as an oxidant in the absence of an external base with a high selectivity of around 90% for phenols via the non-radical pathway (TON ≥720). The KIE value, various spectroscopic studies and DFT calculation supported the involvement of Cu(II)-OOH species.

Synergistic Effect of FeII and MnII Ions in Cyano-Bridged Heterometallic Coordination Polymers on Catalytic Selectivity of Benzene Oxygenation to Phenol

A cyano-bridged heterometallic coordination polymer with partial deficiencies of CN^- ligands, [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)], forms open metal sites both on Mn^II and Fe^II ions by liberation of monodentate ligands such as NH₃ and H₂O. [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] exhibits high catalytic activity and selectivity of benzene oxygenation to phenol in the presence of m-chloroperoxybenzoic acid as an oxidant. The postcatalytic spectroscopy of [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and catalysis comparison with a physical mixture of [Mn^II(H₂O)₃]₂[Fe^II(CN)₆] and [Fe(H₂O)_3/2]_4/3[Fe(CN)₆], which has open metal sites on both Mn^II and Fe ions separately, indicated that the high activity resulted from high oxidation ability and phenol adsorption ability of Fe^II and Mn^II ions, respectively.

Biomimetic catalytic aerobic oxidation of C-sp(3)-H bonds under mild conditions using galactose oxidase model compound CuIIL

Developing highly efficient catalytic protocols for C-sp(3)-H bond aerobic oxidation under mild conditions is a long-desired goal of chemists. Inspired by nature, a biomimetic approach for the aerobic oxidation of C-sp(3)-H by galactose oxidase model compound CuIIL and NHPI (N-hydroxyphthalimide) was developed. The CuIIL-NHPI system exhibited excellent performance in the oxidation of C-sp(3)-H bonds to ketones, especially for light alkanes. The biomimetic catalytic protocol had a broad substrate scope. Mechanistic studies revealed that the CuI-radical intermediate species generated from the intramolecular redox process of CuIILH2 was critical for O2 activation. Kinetic experiments showed that the activation of NHPI was the rate-determining step. Furthermore, activation of NHPI in the CuIIL-NHPI system was demonstrated by time-resolved EPR results. The persistent PINO (phthalimide-N-oxyl) radical mechanism for the aerobic oxidation of C-sp(3)-H bond was demonstrated.

Efficient synthesis 1,4-cyclohexanedicarboxaldehyde by an engineered alcohol oxidase

In this study, we selected and engineered a flavin adenine dinucleotide (FAD)-dependent alcohol oxidase (AOX) to produce 1,4-cyclohexanedicarboxaldehyde (CHDA), an initial raw material for spiral compounds, from 1,4-cyclohexanedimethanol (CHDM). First, the structure of alcohol oxidase from Arthrobacter cholorphenolicus (AcCO) was analyzed, and the mechanism of AcCO-catalyzed primary alcohol oxidation was elucidated, demonstrating that the energy barrier of the hydride (H-) transfer (13.4 kcal·mol-1 and 20.4 kcal·mol-1) decreases the catalytic efficiency of the primary alcohol oxidation reaction. Therefore, we designed a protein engineering strategy to adjust the catalytically active conformation to shorten the distance of hydride (H-) transfer and further decreased the core energy barrier. Following this strategy, variant W4 (S101A/H351V/N378S/Q329N) was obtained with 112.5-fold increased catalytic efficiency to produce CHDA compared to that of the wild-type strain. The 3 L scale preparation of CHDA reached a titer up to 29.6 g·L-1 with a 42.2% yield by an Escherichia coli whole-cell catalyst, which demonstrates the potential of this system for industrial application.

A multi-technique approach to unveil the redox behaviour and potentiality of homoleptic CuI complexes based on substituted bipyridine ligands in oxygenation reactions

The effect of differently substituted 2,2'-bipyridine ligands (i.e. 6,6'-dimethyl-2,2'-bipyridine, 5,5'-dimethyl-2,2'-bipyridine, 6,6'-dimethoxy-2,2'-bipyridine and 2,2'-bipyridine) on the reversible oxidation of the resulting Cu^I homoleptic complexes is investigated by means of a multi-technique approach (electronic and vibrational spectroscopies, DFT, electrochemistry). Among the four tested complexes, [Cu^I(6,6'-dimethyl-2,2'-bipyridine)₂] (PF₆) shows a peculiar behavior when oxidized with an organic peroxide (i.e. tert-butyl hydroperoxide, tBuOOH). The simultaneous use of UV-Vis-NIR and Raman spectroscopy methods and cyclovoltammetry, supported by DFT based calculations, allowed identifying (i) the change in the oxidation state of the copper ion and (ii) some peculiar modification in the local structure of the metal leading to the formation of a [Cu^IIOH]⁺ species. The latter, being able to oxidize a model molecule (i.e. cyclohexene) and showing the restoration of the original Cu^I complex and the formation of cyclohexanone, confirms the potential of these simple homoleptic Cu^I complexes as model catalysts for partial oxygenation reactions.

Copper-Catalyzed Monooxygenation of Phenols: Evidence for a Mononuclear Reaction Mechanism

The CuI salts [Cu(CH3 CN)4 ]PF and [Cu(oDFB)2 ]PF with the very weakly coordinating anion Al(OC(CF3 )3 )4- (PF) as well as [Cu(NEt3 )2 ]PF comprising the unique, linear bis-triethylamine complex [Cu(NEt3 )2 ]+ were synthesized and examined as catalysts for the conversion of monophenols to o-quinones. The activities of these CuI salts towards monooxygenation of 2,4-di-tert-butylphenol (DTBP-H) were compared to those of [Cu(CH3 CN)4 ]X salts with "classic" anions (BF4- , OTf- , PF6- ), revealing an anion effect on the activity of the catalyst and a ligand effect on the reaction rate. The reaction is drastically accelerated by employing CuII -semiquinone complexes as catalysts, indicating that formation of a CuII complex precedes the actual catalytic cycle. This result and other experimental observations show that with these systems the oxygenation of monophenols does not follow a dinuclear, but a mononuclear pathway analogous to that of topaquinone cofactor biosynthesis in amine oxidase.

Hydrocarbon Oxidation Depth: H2O2/Cu2Cl4·2DMG/CH3CN System

The oxidation of hydrocarbons of different structures under the same conditions is an important stage in the study of the chemical properties of both the hydrocarbons themselves and the oxidation catalysts. In a 50% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), at 50 °C under the same or similar conditions, we oxidized eleven RH hydrocarbons of different structures: mono-, bi- and tri-cyclic, framework and aromatic. To compare the composition of the oxidation products of these hydrocarbons, we introduced a new quantitative characteristic, “distributive oxidation depth D(O), %” and showed the effectiveness of its application. The adiabatic ionization potentials (AIP) and the vertical ionization potentials (VIP) of the molecules of eleven oxidized and related hydrocarbons were calculated using the DFT method in the B3LYP/TZVPP level of theory for comparison with experimental values and correlation with D(O). The same calculations of AIP were made for the molecules of the oxidant, solvent, DMG, related compounds and products. It is shown that component X, which determines the mechanism of oxidation of hydrocarbons RH with AIP(Exp) ≥ AIP(X) = 8.55 ± 0.03 eV, is a trans-DMG molecule. Firstly theoretically estimated experimental values of AIP(trans-DMG) = 8.53 eV and AIP(cis-DMG) = 8.27 eV.

A high-efficient electrochemical synthesis of a low-nuclearity copper-cluster-based metal–organic framework for the size-selective oxidation of alcohols

A copper-cluster-based micropore MOF, namely H-1e, possessing excellent oxidation activity for alcohols (mini-size) to aldehydes with over 99% selectivity and 99% yield, has been obtained by a highly efficient electrochemical synthesis.

C-centered radical-initiated cyclization by directed C(sp3)–H oxidative functionalization

C(sp3)–H functionalization is attracting constant attention. This review emphasizes C-centered radicals initiated cyclization strategies by directed C(sp3)–H oxidative functionalization since 2012.

A product-controllable aerobic oxidative cleavage of vicinal diols using vanadium-based photocatalyst

A photocatalytic controllable oxidative cleavage of C-C bond is developed with molecular oxygen as the oxidant. Herein, a series of vanadium oxide-based photocatalysts were synthesized and characterized by XPS, PL,...

Time-Dependent Self-Assembly of Copper(II) Coordination Polymers and Tetranuclear Rings: Catalysts for Oxidative Functionalization of Saturated Hydrocarbons

This study describes a time-dependent self-assembly generation of new copper(II) coordination compounds from an aqueous-medium reaction mixture composed of copper(II) nitrate, H₃bes biobuffer (N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid), ammonium hydroxide, and benzenecarboxylic acid, namely, 4-methoxybenzoic (Hfmba) or 4-chlorobenzoic (Hfcba) acid. Two products were isolated from each reaction, namely, 1D coordination polymers [Cu₃(μ₃-OH)₂(μ-fmba)₂(fmba)₂(H₂O)₂]_n (1) or [Cu₂(μ-OH)₂(μ-fcba)₂]_n (2) and discrete tetracopper(II) rings [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fmba)]·2H₂O (3) or [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fcba)]·4H₂O (4), respectively. These four compounds were obtained as microcrystalline air-stable solids and characterized by standard methods, including the single-crystal X-ray diffraction. The structures of 1 and 2 feature distinct types of metal-organic chains driven by the μ₃- or μ-OH^- ligands along with the μ-benzenecarboxylate linkers. The structures of 3 and 4 disclose the chairlike Cu₄ rings assembled from four μ-bridging and chelating aminoalcoholate ligands along with μ-benzenecarboxylate moieties playing a core-stabilizing role. Catalytic activity of 1-4 was investigated in two model reactions, namely, (a) the mild oxidation of saturated hydrocarbons with hydrogen peroxide to form alcohols and ketones and (b) the mild carboxylation of alkanes with carbon monoxide, water, and peroxodisulfate to generate carboxylic acids. Cyclohexane and propane were used as model cyclic and gaseous alkanes, while the substrate scope also included cyclopentane, cycloheptane, and cyclooctane. Different reaction parameters were investigated, including an effect of the acid cocatalyst and various selectivity parameters. The obtained total product yields (up to 34% based on C₃H₈ or up to 47% based on C₆H₁₂) in the carboxylation of propane and cyclohexane are remarkable taking into account an inertness of these saturated hydrocarbons and low reaction temperatures (50-60 °C). Apart from notable catalytic activity, this study showcases a novel time-dependent synthetic strategy for the self-assembly of two different Cu(II) compounds from the same reaction mixture.

Unraveling the Mechanism of Aerobic Alcohol Oxidation by a Cu/pytl-β-Cyclodextrin/TEMPO Catalytic System under Air in Neat Water

The mechanism for the oxidation of p-tolylmethanol to p-tolualdehyde catalyzed by a Cu/pytl-β-cyclodextrin/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxy) catalytic system under air in neat water is fully investigated by density functional theory (DFT). Four possible pathways (paths A → D) are presented. The calculated TOF = 0.67 h^-1 for path A is consistent with the experimental TOF = 1.9 h^-1 but much lower than that for path D (TOF = 1.1 × 10⁵ h^-1). The results demonstrate that path A is the dominant pathway under the optimal experimental conditions, even though path D is more kinetically favorable. This is because the concentration of precatalyst 11 [(pytl-β-CD)Cu^II(OH)] in path D is too low to start path D, so p-tolylmethanol oxidation can only proceed via path A. This finding implies that the relative concentration of precatalysts in a one-pot synthesis experiment plays a vital role in the aerobic alcohol oxidation reaction. Based on this finding, we speculate that the direct use of the presynthesized precatalyst 11 or addition of an appropriate amount of NaOH to the reaction solution, but with the total amount of the base added unchanged, is a good way to improve its catalytic activity. Meanwhile, the solvent water was not found to directly participate in the catalytic active sites for the oxidation of alcohols but rather inhibited it by forming the hydrogen-bonded network.

A 3D MOF based on Adamantoid Tetracopper(II) and Aminophosphine Oxide Cages: Structural Features and Magnetic and Catalytic Properties

This work describes an unexpected generation of a new 3D metal-organic framework (MOF), [Cu₄(μ-Cl)₆(μ₄-O)Cu(OH)₂(μ-PTA═O)₄]_n·2nCl-EtOH·2.5nH₂O, from copper(II) chloride and 1,3,5-triaza-7-phosphaadamantane 7-oxide (PTA═O). The obtained product is composed of diamandoid tetracopper(II) [Cu₄(μ-Cl)₆(μ₄-O)] cages and monocopper(II) [Cu(OH)₂] units that are assembled, via the diamandoid μ-PTA═O linkers, into an intricate 3D net with an nbo topology. Magnetic susceptibility measurements on this MOF in the temperature range of 1.8-300 K reveal a ferromagnetic interaction (J = +20 cm^-1) between the neighboring copper(II) ions. Single-point DFT calculations disclose a strong delocalization of the spin density over the tetranuclear unit. The magnitude of exchange coupling, predicted from the broken-symmetry DFT studies, is in good agreement with the experimental data. This copper(II) compound also acts as an active catalyst for the mild oxidation and carboxylation of alkanes. The present study provides a unique example of an MOF that is assembled from two different types of adamantoid Cu₄ and PTA═O cages, thus contributing to widening a diversity of functional metal-organic frameworks.