Unveiling reactive metal sites in a Pd pincer MOF: insights into Lewis acid and pore selective catalysis

Metal-Organic Framework-Supported Mono Bipyridyl-Iron Hydroxyl Catalyst for Selective Benzene Hydroxylation into Phenol

Direct hydroxylation of benzene to phenol is more appealing in the industry for the economic and environmentally friendly phenol synthesis than the conventional cumene process. We have developed a UiO-metal-organic framework (MOF)-supported mono bipyridyl-Iron(II) hydroxyl catalyst [bpy-UiO-Fe(OH)2] for the selective benzene hydroxylation into phenol using H2O2 as the oxidant. The heterogeneous bpy-UiO-Fe(OH)2 catalyst showed high activity and remarkable phenol selectivity of 99%, giving the phenol mass-specific activity up to 1261 mmolPhOHgFe-1 h-1 at 60 °C. Bpy-UiO-Fe(OH)2 is significantly more active and selective than its homogeneous counterpart, bipyridine-Fe(OH)2. This enhanced catalytic activity of bpy-UiO-Fe(OH)2 over its homogeneous control is attributed to the active site isolation of the bpy-Fe(OH)2 moiety by the solid MOF that prevents intermolecular decomposition. Moreover, the exceptional selectivity of bpy-UiO-Fe(OH)2 in benzene to phenol conversion is originated via shape-selective catalysis, where the confined reaction space within the porous UiO-MOF prevents the formation of larger overoxidized products such as hydroquinone or benzoquinone, leading to the formation of only smaller-sized phenol after monohydroxylation of benzene. Spectroscopic and controlled experiments and theoretical calculations elucidated the reaction pathway, in which the in situ generated •OH radical mediated by bpy-UiO-FeII(OH)2 is the key species for benzene hydroxylation. This work underscores the significance of MOF-supported earth-abundant metal catalysts for sustainable production of fine chemicals.

Selective Reduction of Nitro Compounds by Organosilanes Catalyzed by a Zirconium Metal-Organic Framework Supported Salicylaldimine-Cobalt(II) Complex

Reducing nitro compounds to amines is a fundamental reaction in producing valuable chemicals in industry. Herein, the synthesis and characterization of a zirconium metal-organic framework-supported salicylaldimine-cobalt(II) chloride (salim-UiO-CoCl) and its application in catalytic reduction of nitro compounds are reported. Salim-UiO-Co displayed excellent catalytic activity in chemoselective reduction of aromatic and aliphatic nitro compounds to the corresponding amines in the presence of phenylsilane as a reducing agent under mild reaction conditions. Salim-UiO-Co catalyzed nitro reduction had a broad substrate scope with excellent tolerance to diverse functional groups, including easily reducible ones such as aldehyde, keto, nitrile, and alkene. Salim-UiO-Co MOF catalyst could be recycled and reused at least 14 times without noticeable losing activity and selectivity. Density functional theory (DFT) studies along with spectroscopic analysis were employed to get into a comprehensive investigation of the reaction mechanism. This work underscores the significance of MOF-supported single-site base-metal catalysts for the sustainable and cost-effective synthesis of chemical feedstocks and fine chemicals.

A supported pyridylimine-cobalt catalyst for N-formylation of amines using CO2

N-Formylation of amines with CO2 as a cheap and non-toxic C1-feedstock and hydrosilane reducing agent is a practical and environment friendly method to synthesize formamides. This study describes an efficient and chemoselective mono-N-formylation of amines using CO2 and phenylsilane under mild conditions using a porous metal-organic framework (MOF)-supported single-site cobalt catalyst (pyrim-UiO-Co). The pyrim-UiO-Co MOF has a UiO-topology, and its organic linkers bear a pyridylimine ligated Co catalytic moiety. A wide range of aliphatic and aromatic amines are transformed into desired N-formamides in moderate to excellent yields under 1-5 bar CO2. Pyrim-UiO-Co is tolerant to various functional groups and could be recycled and reused at least 10 times. Mechanistic investigation using kinetic, spectroscopic and density functional theory studies suggests that the formylation of benzylamine proceeds sequentially via oxidative addition of PhSiH3 and CO2 insertion, followed by a turn-over limiting reaction with an amine. Our work highlights the importance of MOF-based Earth-abundant metal catalysts for the practical and eco-friendly synthesis of fine chemicals using cheap feedstocks.

Facile immobilization of PNNNP-Pd pincer complexes in MFU-4l-OH and the effects of guest loading on Lewis acid catalytic activity

A palladium diphosphine pincer complex H₃(P^NN^NP-PdI) has been encapsulated in the benzotriazolate metal-organic framework MFU-4l-OH ([Zn₅(OH)₄(btdd)₃], btdd^2- = bis(1,2,3-triazolo)dibenzodioxin), and the resulting materials were investigated as Lewis acid catalysts for cyclization of citronellal to isopulegol. Rapid catalyst immobilization is facilitated by a Brønsted acid-base reaction between the H₃(P^NN^NP-PdI) benzoic acid substituents and Zn-OH groups at the framework nodes. Catalyst loading can be controlled up to a maximum of 0.5 pincer complexes per formula unit [PdI-x, Zn₅(OH)_4-nx(btdd)₃(H_3-nP^NN^NP-PdI)_xx = 0.06-0.5, n ≈ 2.75]. Oxidative ligand exchange was used to replace I^- with weakly coordinating BF₄^- anions at the Pd-I sites, generating the activated PdBF4-x catalysts (x = 0.06, 0.10, 0.18, 0.40). The Lewis acid catalytic activity of the PdBF4-x series decreases with increasing catalyst density as a result of the appearance of mass transport limitations. Initial catalytic rates show that the activity of PdBF4-0.06 approaches the intrinsic activity of a homogeneous P^NN^NP-PdBF₄ catalyst analogue. In addition, PdBF4-0.06 exhibits better catalytic activity than the metallolinker-based MOF Zr-PdBF₄ and was not subject to leaching or catalyst degradation processes observed for the homogeneous analogue.

Bimetallic Metal-Organic Coordination Polymers Facilitated the Selective C-F Cleavage of Polyfluoroarenes

Selective C-F bond cleavage of polyfluoroarenes has attracted tremendous interest due to its promising applications in introducing fluorinated building blocks into organic molecules. However, it remains a challenge to achieve highly site-selective C-F bond cleavage because of the intrinsic inertness of the C-F bond and the difficulty in distinguishing specific C-F bonds on the aromatic ring. Herein we report an efficient nucleophilic aromatic substitution (S_NAr) reaction of polyfluoroarenes with Grignard reagents that employs MnFe-based bimetallic metal-organic coordination polymers (MOCPs) as recyclable and reusable heterogeneous catalysts. Significantly, in this reaction, the prepared MOCP (Mn-Fe) catalyst exhibited excellent activity in selective C-F bond cleavage and afforded a series of functionalized polyfluoroarenes in moderate to excellent yields (up to 96%). This work highlights the potential of MOCP catalysts to serve as a tunable platform in Lewis acid catalysis.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Multifunctional polymers with interpenetrating structures: luminescence sensing, ECL behaviors, selective detection of Fe3+ ion and rapid removal of anionic dyes

Three unprecedented metal–organic frameworks with interspersed structures have been synthesized and structurally characterized.

Organic synthesis of high added value molecules with MOF catalysts

Recent examples of organic synthesis of fine chemicals and pharmaceuticals in confined spaces of MOFs are highlighted and compared with silica-based ordered porous solids, such as zeolites or mesoporous (organo)silica. These heterogeneous catalysts offer the possibility of stabilizing the desired transition states and/or intermediates during organic transformations of functional groups and (C-C/C-N) bond forming steps towards the desired functional high added value molecular scaffolds. A short introduction on zeolites, mesoporous silica and metal-organic frameworks is followed by relevant applications in which confined active sites in the pores promote single or multi-step organic synthesis of industrially relevant molecules. A critical discussion on the catalytic performances of the different types of hybrid inorganic-organic catalysts in the synthesis of O- and N-containing acyclic and heterocyclic molecules has been presented.

Postsynthetic Metal Exchange in a Metal-Organic Framework Assembled from Co(III) Diphosphine Pincer Complexes

A Zr metal-organic framework (MOF) 1-CoCl₃ has been synthesized by solvothermal reaction of ZrCl₄ with a carboxylic acid-functionalized Co^III-P^NN^NP pincer complex H₄(L-CoCl₃) ([L-CoCl₃]^4- = [(2,6-(NHPAr₂)₂C₆H₃)CoCl₃]^4-, Ar = p-C₆H₄CO₂^-). The structure of 1-CoCl₃ has been determined by X-ray powder diffraction and exhibits a csq topology that differs from previously reported ftw-net Zr MOFs assembled from related Pd^II- and Pt^II-P^NN^NP pincer complexes. The Co-P^NN^NP pincer species readily demetallate upon reduction of Co^III to Co^II, allowing for transmetalation with late second and third row transition metals in both the homogeneous complex and 1-CoCl₃. Reaction of 1-CoCl₃ with [Rh(nbd)Cl]₂ (nbd = 2,5-nobornadiene) results in complete Rh/Co metal exchange at the supported diphosphine pincer complexes to generate 1-RhCl, which has been inaccessible by direct solvothermal synthesis. Treating 1-CoCl₃ with PtCl₂(SMe₂)₂ in the presence of the mild reductant NEt₃ resulted in nearly complete Co substitution by Pt. In addition, a mixed metal pincer MOF, 1-PtRh, was generated by sequential substitution of Co with Pt followed by Rh.

The impact of an isoreticular expansion strategy on the performance of iodine catalysts supported in multivariate zirconium and aluminum metal–organic frameworks

A new iodine catalyst was supported in two different MOFs and the catalytic activity for the oxidation of hydroquinones and catechols was evaluated.