Heterogenization of molecular catalysts within porous solids: the case of Ni-catalyzed ethylene oligomerization from zeolites to metal-organic frameworks.

The last decade has seen a tremendous expansion of the field of heterogenized molecular catalysis, especially with the growing interest in metal-organic frameworks and related porous hybrid solids. With successful achievements in the transfer from molecular homogeneous catalysis to heterogenized processes come the necessary discussions on methodologies used and a critical assessment on the advantages of heterogenizing molecular catalysis. Here we use the example of nickel-catalyzed ethylene oligomerization, a reaction of both fundamental and applied interest, to review heterogenization methodologies of well-defined molecular catalysts within porous solids while addressing the biases in the comparison between original molecular systems and heterogenized counterparts.

Palladium-Anchored N-Heterocyclic Carbenes in a Porous Organic Polymer: A Heterogeneous Composite Catalyst for Eco-Friendly C-C Coupling.

Aggregation-induced catalyst deactivation during the reaction in supported metal catalysts prevails as one of the pitfalls toward their practical implementation. Herein, a homogeneously dispersed palladium-coordinated N-heterocyclic carbene (NHC) was strategically integrated inside a microporous hyper-cross-linked polymer via post-synthesis structural modulation. Successful immobilization of spatially isolated Pd (II) units onto the polymer scaffold yielded highly robust heterogeneous catalysts 120-MI@Pd NHC and 120-EI@Pd NHC, respectively. 120-EI@NHC Pd (4.41 wt % Pd) illustrated a remarkable catalytic potency (yield up to >99%) toward the eco-friendly Suzuki-Miyaura coupling (SMC) reaction at room temperature. The superior catalytic efficiency of 120-EI@Pd NHC is further highlighted from its excellent functionality tolerance over 42 substrates bearing electronic diversity and a turnover frequency value reaching up to 4.97 × 103 h-1 at a very low catalyst dosage of 0.04 mol %. Pertaining to heterogenization, the polymer catalyst could be easily reused with intact catalytic efficiency for at least 10 cycles. The catalytic competence of 120-EI@NHC Pd in terms of scope, scalability, and sustainability advocates its proficiency, while processability was achieved by crafting 3D aerogel monoliths. The conceptual feasibility was further investigated by devising a cup-based nano-reactor with gram-scale product isolation over three catalytic cycles.

Imidazolium-Functionalized Chemically Robust Ionic Porous Organic Polymers (iPOPs) toward Toxic Oxo-Pollutants Capture from Water.

Fabricating new and efficient materials aimed at containment of water contamination, in particular removing toxic heavy metal based oxo-anions (e. g. CrO4 2- , TcO4 - ) holds paramount importance. In this work, we report two new highly stable imidazolium based ionic porous organic polymers (iPOPs) decorated with multiple interaction sites along with electrostatics driven adsorptive removal of such oxo-anions from water. Both the iPOPs (namely, iPOP-3 and iPOP-4) exhibited rapid sieving kinetics and very high saturation uptake capacity for CrO4 2- anions (170 and 141 mg g-1 for iPOP-3 and iPOP-4 respectively) and ReO4 - (515.5 and 350.3 mg g-1 for iPOP-3 and iPOP-4 respectively), where ReO4 - anions being the non-radioactive surrogative counterpart of radioactive TcO4 - ions. Noticeably, both iPOPs showed exceptional selectivity towards CrO4 2- and ReO4 - even in presence of several other concurrent anions such as Br- , Cl- , SO4 2- , NO3 - etc. The theoretical binding energy calculations via DFT method further confirmed the preferential interaction sites as well as binding energies of both iPOPs towards CrO4 2- and ReO4 - over all other competing anions which corroborates with the experimental high capacity and selectivity of iPOPs toward such oxo-anions.

Palladium Catalyzed Regioselective Synthesis of Substituted Biaryl Amides through Decarbonylative Arylation of Phthalimides.

The Pd(OAc)2 catalyzed cross-coupling of N-substituted phthalimides with aryl halide provides a single step direct access of a wide range of synthetically appealing ortho-substituted biarylamides in high yields through unique carbonyl (CO) replacement. The reaction proceeds through a ligand-free condition and is well tolerant to the diverse functionality of both imide and halide units. The reaction negates any requirement of organometallic reagent and needs a shorter reaction time and comparatively lower temperature as required for previously reported decarbonylative processes.

Guest-Responsive Metal-Organic Frameworks as Scaffolds for Separation and Sensing Applications.

Metal-organic frameworks (MOFs) have evolved to be next-generation utility materials because of their serviceability in a wide variety of applications. Built from organic ligands with multiple binding sites in conjunction with metal ions/clusters, these materials have found profound advantages over their other congeners in the domain of porous materials. The plethora of applications that these materials encompass has motivated material chemists to develop such novel materials, and the catalogue of MOFs is thus ever-escalating. One key feature that MOFs possess is their responsiveness toward incoming guest molecules, resulting in changes in their physical and chemical properties. Such uniqueness generally arises owing to the influenceable ligands and/or metal units that govern the formation of these ordered architectures. The suitable host-guest interactions play an important role in determining the specific responses of these materials and thus find important applications in sensing, catalysis, separation, conduction, etc. In this Account, we focus on the two most relevant applications based on the host-guest interactions that are carried out in our lab, viz., separation and sensing of small molecules. Separation of liquid-phase aromatic hydrocarbons by less energy-intensive adsorption processes has gained attention recently. Because of their tailored structures and functionalized pore surfaces, MOFs have become vital candidates in molecular separation. Prefunctionalization of MOFs by astute choice of ligands and/or metal centers results in targeted separation processes in which the molecular sieving effect plays a crucial role. In this view, separation of C6 and C8 liquid aromatic hydrocarbons, which are essential feedstock in various chemical industries, is one area of research that requires significant attention because of the gruesome separation techniques adopted in such industries. Also, from the environmental perspective, separation of oil/water mixtures demands significant attention because of the hazards of marine oil spillage. We have achieved successful separation of such by careful impregnation of hydrophobic moieties inside the nanochannels of MOFs, resulting in unprecedented efficiency in oil/water separation. Also, recognition of small molecules using optical methods (fluorescence, UV, etc.) has been extended to achieve sensing of various neutral species and anions that are important from environmental point of view. Incorporation of secondary functional groups has been utilized to sense nitroaromatic compounds (NACs) and other small molecules such as H2S, NO, and aromatic phenols. We have also utilized the postfunctionalization strategy via ion exchange to fabricate MOFs for sensing of environmentally toxic and perilous anionic species such as CN- and oxoanions. Our current endeavors to explore the applicability of MOFs in these two significant areas have widened the scope of research, and attempts to fabricate MOFs for real-time applications are underway.

Self-Assembled, Fluorine-Rich Porous Organic Polymers: A Class of Mechanically Stiff and Hydrophobic Materials.

Fluorous organic building blocks were utilized to develop two self-assembled, hydrophobic, fluorinated porous organic polymers (FPOPs), namely, FPOP-100 and FPOP-101. Comprehensive mechanical analyses of these functionalised triazine network polymers marked the introduction of mechanical stiffness among all porous organic network materials; the recorded stiffnesses are analogous to those of their organic-inorganic hybrid polymer congeners, that is, metal-organic frameworks. Furthermore, this study introduces a new paradigm for the simultaneous installation of mechanical stiffness and high surface hydrophobicity into polymeric organic networks, with the potential for transfer among all porous solids. Control experiments with non-fluorinated congeners underlined the key role of fluorine, in particular, bis-trifluoromethyl functionalization in realizing the dual features of mechanical stiffness and superhydrophobicity.

Selective Recognition of Hg2+ ion in Water by a Functionalized Metal-Organic Framework (MOF) Based Chemodosimeter.

A metal-organic framework (MOF)-based highly selective and sensitive probe (UiO-66@Butyne) for the detection of Hg(II) ion has been developed. To the best our knowledge, this is the foremost example of a chemodosimeter-based approach to sense Hg(II) ion using a MOF-based probe. The chemical stability of UiO-66@Butyne renders the sensitive detection of Hg2+ ion in an aqueous phase. UiO-66@Butyne has been found to be selective for Hg(II) ions even in the presence of other metal ions.

A Post-Synthetically Modified MOF for Selective and Sensitive Aqueous-Phase Detection of Highly Toxic Cyanide Ions.

Selective and sensitive detection of toxic cyanide (CN(-) ) by a post-synthetically altered metal-organic framework (MOF) has been achieved. A post-synthetic modification was employed in the MOF to incorporate the specific recognition site with the CN(-) ion over all other anions, such as Cl(-) , Br(-) , and SCN(-) . The aqueous-phase sensing and very low detection limit, the essential prerequisites for an effective sensory material, have been fulfilled by the MOF. Moreover, the present detection level meets the standard set by the World Health Organization (WHO) for the permissible limit of cyanide concentration in drinking water. The utilization of MOF-based materials as the fluorometric probes for selective and sensitive detection of CN(-) ions has not been explored till now.

Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron-Rich and -Deficient Pore Surface.

A porous covalent triazine framework (CTF) consisting of both an electron-deficient central triazine core and electron-rich aromatic building blocks is reported. Taking advantage of the dual nature of the pore surface, bimodal functionality has been achieved. The electron deficiency in the central core has been utilized to address one of the pertinent problems in chemical industries, namely separation of benzene from its cyclic saturated congener, that is, cyclohexane. Also, by virtue of the electron-rich aromatic rings with Lewis basic sites, aqueous-phase chemical sensing of a nitroaromatic compound of highly explosive nature (2,4,6-trinitrophenol; TNP) has been achieved. The present compound supersedes the performance of previously reported COFs in both the aspects. Notably, this reports the first example of pore-surface engineering leading to bimodal functionality in CTFs.

Hydrogen-Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton-Conducting Materials.

Two porous hydrogen-bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra-high proton conduction values (σ) 0.75× 10(-2)  S cm(-1) and 1.8×10(-2)  S cm(-1) under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic-based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen-bonded porous organic frameworks as solid-state proton conducting materials.

Hydroxy-functionalized hyper-cross-linked ultra-microporous organic polymers for selective CO2 capture at room temperature.

Two hydroxy-functionalized hyper-cross-linked ultra-microporous compounds have been synthesized by Friedel-Crafts alkylation reaction and characterised with different spectroscopic techniques. Both compounds exhibit an efficient carbon dioxide uptake over other gases like N2, H2 and O2 at room temperature. A high isosteric heat of adsorption (Qst) has been obtained for both materials because of strong interactions between polar -OH groups and CO2 molecules.

Selective and sensitive aqueous-phase detection of 2,4,6-trinitrophenol (TNP) by an amine-functionalized metal-organic framework.

Highly selective and sensitive aqueous-phase detection of nitro explosive 2,4,6-trinitrophenol (TNP) by a hydrolytically stable 3D luminescent metal-organic framework is reported. The compound senses TNP exclusively even in the presence of other nitro-compounds, with an unprecedented sensitivity in the MOF regime by means of strategic deployment of its free amine groups. Such an accurate sensing of TNP, widely recognized as a harmful environmental contaminant in water media, establishes this new strategic approach as one of the frontiers to tackle present-day security and health concerns in a real-time scenario.

Coherent fusion of water array and protonated amine in a metal-sulfate-based coordination polymer for proton conduction.

A new function of metal-sulfate-based coordination polymer (CP) for proton conduction was investigated through rational integration of a continuous water array and protonated amine in the coordination space of the CP. The H-bonded arrays of water molecules along with nitrogen-rich aromatic cation (protonated melamine) facilitate proton conduction in the compound under humid conditions. Although several reports of metal-oxalate/phosphate-based CPs showing proton conduction are known, this is the first designed synthesis of a metal-sulfate-based CP bearing water arrays functioning as a solid-state proton conductor.

Selective and Sensitive Fluorescence Turn-On Detection of Cyanide Ions in Water by Post Metallization of a MOF.

Owing to detrimental impact of cyanide ion (CN- ) towards the entire living system as well as its availability in drinking water, it has become very important developing potential sensory materials for the selective and sensitive recognition of CN- ions in water. In the domain of sensory materials, luminescent metal-organic frameworks (LMOFs) have been considered as a promising candidate owing to their unique host-guest interaction, where MOFs can serve as an ideal scaffold for encapsulating relevant guest molecules rendering specific functionality. In this study, a post-synthetically modified MOF (viz., CuCl2 @MOF-867) was applied to recognize cyanide (CN- ) ions in water via "turn-on" response. The bipyridyl functionalities in MOF-867 were used to perform post-synthetic metalation to infiltrate CuCl2 inside porous architecture of the MOF. Moreover, a CuCl2 @MOF-867 based probe demonstrated highly selective and sensitive aqueous phase recognition of CN- ions even in the presence of other interfering anions such as Br- , NO3- , I- , SO42- , OAc- , SCN- , NO2- , etc. The selective binding of CN- ions to the copper-metal center has led to the generation of stable Cu(CN)2 species. This phenomenon has further resulted in a fluorescence turn-on response. The aqueous phase cyanide detection by the rationally modified MOF system exhibited very low limit of detection (0.19 µM), which meets the standardized limit stated by World Health Organization (WHO) that is 1.9 µM.

Hydroxy-Functionalized Hypercrosslinked Polymers (HCPs) as Dual Phase Radioactive Iodine Scavengers: Synergy of Porosity and Functionality.

Large-scale nuclear power plant production of iodine radionuclides (129 I, 131 I) pose huge threat in the events of nuclear disaster. Effective removal of radioiodine from nuclear waste is one of the most critical challenge because of the drawbacks of state-of-the-art adsorbents such as high cost, low uptake capacity and non-recyclability. Herein, two hydroxy-functionalized (-OH) hypercrosslinked polymers (HCPs), namely HCP-91 and HCP-92, have been synthesized and employed towards capture of iodine. High chemical stability along with synergistic harmony of high porosity and functionality of these materials makes them suitable candidates for capture of iodine from both vapor phase and water medium. Moreover, both the HCPs showed superior iodine removal performance from water in terms of fast kinetics and high removal efficiency (2.9 g g-1 and 2.49 g g-1 for HCP-91 and HCP-92 respectively). The role of functionality (-OH groups) and porosity has been established with the help of HCP-91, HCP-92 and non-functionalized biphenyl HCP for the efficient capture of I3 - ions from water. In addition, both HCPs exhibited excellent selectivity and recyclability towards triiodide ions, rendering the potential of these materials towards real-time applications. Lastly, Density functional theoretical studies revealed key insights and corroborate well with the experimental findings.

Magnetic Nanoparticle-Embedded Ionic Microporous Polymer Composite as an Efficient Scavenger of Organic Micropollutants.

A cationic microporous composite polymer (120-TMA@Fe) bearing free exchangeable chloride anions alongside easy magnetic separation was crafted through post-polymerization structure modulation. The precursor polymer 120-Cl was synthesized via an "external cross-linking" strategy in a straightforward one-pot Friedel-Crafts reaction. Subsequently, a cationic network accommodating magnetic Fe3O4 nanoparticles, viz., 120-TMA@Fe was fabricated through chemical modifications. 120-TMA@Fe displayed excellent adsorption proficiency both in terms of rapid kinetics and maximum uptake capacity when screened for a wide range of organic micropollutants of various categories. Amongst the tested pollutants, including anionic dyes, aromatic models, plastic components, and pharmaceuticals, 120-TMA@Fe illustrated exceptional performance in removing all of these model pollutants with adsorption equilibrium reaching within only 5 min. The Langmuir adsorption isotherm model determined the theoretical maximum uptake capacity (qmax,e) of 120-TMA@Fe to be 357 mg g-1 for methyl orange dye, 555 mg g-1 for plasticizer bisphenol A, and 285 mg g-1 for antibiotic ibuprofen. Additionally, 120-TMA@Fe showed unaltered performance upon harsh chemical treatment as well as in complex real-world samples. The potency of 120-TMA@Fe was further supported by its outstanding regeneration performance up to 10 cycles.

A luminescent cationic MOF for bimodal recognition of chromium and arsenic based oxo-anions in water.

Water pollution from heavy metals and their toxic oxo-anionic derivatives such as CrO42-, Cr2O72-, HAsO42-, and HAsO32- has become one of the most critical environmental issues. To address this, herein, we report a new hydrolytically stable luminescent Zn(ii) based cationic metal organic framework (MOF), iMOF-4C, which further successfully exhibited a rare dual "turn off/on" fluorescence response toward Cr(vi), As(v) and As(iii) based oxo-anions respectively in water medium. In addition, iMOF-4C was found to maintain its superior selectivity in the presence of other concurrent anions (e.g. SO42-, Cl-, Br-, ClO4-, NO3-, SCN- and CO32-). More importantly, iMOF-4C exhibited an excellent selective and sensitive luminescence "turn-off" response towards CrO42- and Cr2O72- anions in water medium with the quenching constant (Ksv) values as high as 1.31 × 105 M-1 (CrO42-) and 4.85 × 105 M-1 (Cr2O72-), which are found to be the highest among the values reported in the regime of MOFs. Interestingly, iMOF-4C showed fluorescence "turn-on" response toward HAsO42- and HAsO32- with an enhancement coefficient (Kec) of 1.98 × 104 M-1 and 3.56 × 103 M-1 respectively. The high sensitivity and low detection limits make iMOF-4C more feasible for real-time sensing of such toxic oxo-anions in an aqueous medium. Furthermore, the probable sensing mechanism has been investigated by DFT calculation studies and discussed in detail.

A Water-Stable Cationic Metal-Organic Framework with Hydrophobic Pore Surfaces as an Efficient Scavenger of Oxo-Anion Pollutants from Water.

Water contamination due to heavy metal-based toxic oxo-anions (such as CrO42- and TcO4-) is a critical environmental concern that demands immediate mitigation. Herein, we present an effort to counter this issue by a novel chemically stable cationic metal-organic framework (iMOF-2C) with strategic utilization of a ligand with hydrophobic core, known to facilitate such oxo-anion capture process. Moreover, the compound exhibited very fast sieving kinetics for such oxo-anions and a very high uptake capacity for CrO42- (476.3 mg g-1) and ReO4- (691 mg g-1), while the latter being employed as a surrogate analogue for radioactive TcO4- anions. Notably, the compound showed excellent selectivity even in the presence of other competing anions such as NO3-, Cl-, SO42-, ClO4-. etc.. Furthermore, the compound possesses excellent reusability (up to 10 cycles) and is also employed to a stationary phase ion column to decontaminate the aforementioned oxo-anions from water.

Fluorescent "Turn-on" Sensing Based on Metal-Organic Frameworks (MOFs).

Metal-organic frameworks (MOFs) have evolved as an exciting class of materials in the domain of porous materials. The unique features of these materials arise from the combined properties of metal ions/clusters and organic struts which form the building blocks of these fascinating architectures. Among other multifarious applications, MOFs have shown tremendous applications as sensory materials for a wide variety of species. The signal transduction induced mechanism in these confined nanospaces generate optical output in response to a particular analyte which can be detected by wide variety of detection techniques. Fluorometric methods of sensing is one of widely studied method over past few decades. MOF-based fluorometric detection is a key research theme developed over the past few years. In this review, we give a brief overview of the recent developments of MOFs as "turn-on" sensors for a wide range of analytes (viz. cations, anions, volatile organic compounds (VOCs), etc.).

Base-Resistant Ionic Metal-Organic Framework as a Porous Ion-Exchange Sorbent.

A systematic approach has been employed to obtain a hydrolytically stable cationic metal-organic framework (MOF). The synthesized two-dimensional Ni(II)-centered cationic MOF, having its backbone built from purely neutral N-donor ligand, is found to exhibit uncommon resistance over wide pH range, particularly even under highly alkaline conditions. This report presents a rare case of a porous MOF retaining structural integrity under basic conditions, and an even rarer case of a porous cationic MOF. The features of stability and porosity in this ionic MOF have been harnessed for the function of charge- and size-selective capture of small organic dye through ion-exchange process across a wide pH range.