NO Disproportionation at a Mononuclear Site-Isolated Fe2+ Center in Fe2+-MOF-5

Rational Design of Catalytic Centers in Crystalline Frameworks

Crystalline frameworks including primarily metal organic frameworks (MOF) and covalent organic frameworks (COF) have received much attention in the field of heterogeneous catalysts recently. Beyond providing large surface area and spatial confinement, these crystalline frameworks can be designed to either directly act as or influence the catalytic sites at molecular level. This approach offers a unique advantage to gain deeper insights of structure-activity correlations in solid materials, leading to new guiding principles for rational design of advanced solid catalysts for potential important applications related to energy and fine chemical synthesis. In this review, recent key progress achieved in designing MOF- and COF-based molecular solid catalysts and the mechanistic understanding of the catalytic centers and associated reaction pathways are summarized. The state-of-the-art rational design of MOF- and COF-based solid catalysts in this review is grouped into seven different areas: (i) metalated linkers, (ii) metalated moieties anchored on linkers, (iii) organic moieties anchored on linkers, (iv) encapsulated single sites in pores, and (v) metal-mode-based active sites in MOFs. Along with this, some attention is paid to theoretical studies about the reaction mechanisms. Finally, technical challenges and possible solutions in applying these catalysts for practical applications are also presented.

Selective fluorescent sensing and photocatalytic properties of three MOFs based on naphthalene-1,4-dicarboxylic acid and 2,4,5-tri(4-pyridyl)-imidazole

Three MOFs display high sensitivity in the detection of NB and Ca²⁺ or Co²⁺ ions as luminescent probes. Moreover, 3 exhibits a relatively good photocatalytic degradation activity of rhodamine B.

Three new Zn-based metal–organic frameworks exhibiting selective fluorescence sensing and photocatalytic activity

Three MOFs with different 2D networks have been crystallised in a one-pot solvothermal reaction. They all display high sensitivity in the detection of ACE and relatively good photocatalytic activity in the degradation of RhB.

Copper(I)/NO(g) Reductive Coupling Producing a trans-Hyponitrite Bridged Dicopper(II) Complex: Redox Reversal Giving Copper(I)/NO(g) Disproportionation

A copper complex, [CuI(tmpa)(MeCN)]+, effectively reductively couples NO(g) at RT in methanol (MeOH), giving a structurally characterized hyponitrito-dicopper(II) adduct. Hydrogen-bonding from MeOH is critical for the hyponitrite complex formation and stabilization. This complex exhibits the reverse redox process in aprotic solvents, giving CuI + NO(g), leading to CuI-mediated NO(g)-disproportionation. The relationship of this chemistry to biological iron and/or copper mediated NO(g) reductive coupling to give N2O(g) is discussed.

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O

One-electron reduction of [Fe(NO)-(N3PyS)]BF₄ (1) leads to the production of the metastable nonheme {FeNO}⁸ complex, [Fe(NO)(N3PyS)] (3). Complex 3 is a rare example of a high-spin (S = 1) {FeNO}⁸ and is the first example, to our knowledge, of a mononuclear nonheme {FeNO}⁸ species that generates N₂O. A second, novel route to 3 involves addition of Piloty's acid, an HNO donor, to an Fe^II precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases.

Metal-organic and covalent organic frameworks as single-site catalysts

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

N2O Formation via Reductive Disproportionation of NO by Mononuclear Copper Complexes: A Mechanistic DFT Study

The mechanism of the copper(I)-mediated reductive disproportionation reaction of NO to form N₂O was investigated for five different 3,5-substituted tris(pyrazolyl)borate copper complexes (CuTp^R₁,R₂) by means of DFT calculations. A thorough search of the potential surface was performed, using the B3LYP functional with the def2-SVP basis set for optimization purposes and def2-TZVP single-point calculations for constructing the potential energy surface for two of these complexes. The results can be condensed into six competing reaction mechanisms, two of which were more closely investigated using full def2-TZVP optimized potential and free energies. The results consistently predict the same mechanism to have the lowest overall barrier. For all five different complexes, this is found to be in good agreement with the experimental reaction barriers. The key intermediate for the transition from the N-bound reactant to the O-bound product contains a stable (NO)₃ unit with one N-Cu and one O-Cu bond, which was not included in the mechanistic considerations reported in the literature. Further analysis of the charge distribution and the spin density demonstrates the formation of a Cu(II)-(N₂O₂^-) intermediate and the electronic influence of the different ligands.

Cyclic gas-phase heterogeneous process in a metal–organic framework involving a nickel nitrosyl complex

The cubic metal–organic framework MFU-4l ([Zn₅Cl₄(BTDD)₃], H₂-BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) featuring large pore apertures can be modified post-synthetically via partial or complete substitution of peripheral metal sites and chloride side-ligands, thus opening a route towards a large variety of functionalized MOFs. In this way, Ni-MFU-4l-nitrite (or Ni-MFU-4l-NO₂) with an analytically determined chemical composition [Zn_2.6Ni_2.4(NO₂)_2.9Cl_1.1(BTDD)₃], containing accessible Ni–NO₂ units, was prepared. Ni-MFU-4l-NO₂ undergoes selective heterogeneous gas-phase reduction by carbon monoxide at 350 °C, leading to formation of Ni–NO units at the peripheral sites of the MFU-4l framework (Ni-MFU-4l-NO). The crystallinity and porosity of the MFU-4l framework are completely retained upon this transformation. The so-formed nickel nitrosyl complex, showing high thermal stability, readily reacts with nitrogen monoxide at room temperature, producing Ni–NO₂ units and dinitrogen monoxide (N₂O). Hence, the reaction of Ni-MFU-4l-NO₂ with CO followed by NO represents a cyclic process with an overall stoichiometry 2NO + CO → N₂O + CO₂, in which the Ni-MFU-4l framework serves as a catalyst. It can be considered as a model process for the removal of highly toxic NO and CO gases, which are converted to non-toxic CO₂ and N₂O. Diffuse reflectance infrared Fourier transform spectroscopic studies show that at least 10 cycles can be repeated. The framework's reactivity drops down by ca. 50% after 10 cycles, which is most likely due to the accumulation of highly reactive NO₂ and N₂O₄ contaminants. Therefore, further investigations on characterizing reaction intermediates should be done in order to improve the catalyst's performance. Our results confirm the potential of MFU-4l frameworks as selective single-site catalysts for heterogeneous gas-phase transformations and provide a motivation for further studies.

Dynamic structural flexibility of Fe-MOF-5 evidenced by 57Fe Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy reveals unexpected dynamics at the Fe sites in Fe-substituted MOF-5, especially in the presence of nitrogen.

NO˙ disproportionation by a {RhNO}9 pincer-type complex

NO˙ disproportionation by the pincer-type complex [Rh(PCP^tBu)(NO)]˙ (1˙) results in the formation of Rh(PCP^tBu)(NO)(NO₂) (2) with coordinated nitrite and quantitative release of N₂O.

Ethane C–H bond activation on the Fe(iv)–oxo species in a Zn-based cluster of metal–organic frameworks: a density functional theory study

The generation of a Fe(iv)–oxo complex and its reactivity for C–H bond activation of ethane have been theoretically unraveled.

Emerging Multifunctional Metal-Organic Framework Materials

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.

Intercalation of Coordinatively Unsaturated Fe(III) Ion within Interpenetrated Metal-Organic Framework MOF-5

Coordinatively unsaturated Fe(III) metal sites were successfully incorporated into the iconic MOF-5 framework. This new structure, Fe(III) -iMOF-5, is the first example of an interpenetrated MOF linked through intercalated metal ions. Structural characterization was performed with single-crystal and powder XRD, followed by extensive analysis by spectroscopic methods and solid-state NMR, which reveals the paramagnetic ion through its interaction with the framework. EPR and Mössbauer spectroscopy confirmed that the intercalated ions were indeed Fe(III) , whereas DFT calculations were employed to ascertain the unique pentacoordinate architecture around the Fe(III) ion. Interestingly, this is also the first crystallographic evidence of pentacoordinate Zn(II) within the MOF-5 SBU. This new MOF structure displays the potential for metal-site addition as a framework connector, thus creating further opportunity for the innovative development of new MOF materials.

A trans-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper-Lewis Acid Complex

The reduction of nitric oxide (NO) to nitrous oxide (N2O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N2O2(2-)), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium-tricopper complex generating a trans-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N2O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N2O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N2O reduction to N2, for an overall (partial) conversion of NO to N2. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate.

Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst

Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal-organic framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homology to molecular tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mole of Ni per hour with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.

Exploiting redox activity in metal–organic frameworks: concepts, trends and perspectives

This feature article highlights latest developments in experimental, theoretical and computational concepts relevant to redox-active metal–organic Frameworks.

Origin of highly active metal–organic framework catalysts: defects? Defects!

This article provides a comprehensive review of the nature of catalytic sites in MOFs.