Ritesh Haldar

Flexible and Rigid Amine-Functionalized Microporous Frameworks Based on Different Secondary Building Units: Supramolecular Isomerism, Selective CO2 Capture, and Catalysis

We report the synthesis, structural characterization, and porous properties of two isomeric supramolecular complexes of ([Cd(NH2 bdc)(bphz)0.5 ]⋅DMF⋅H2 O}n (NH2 bdc=2-aminobenzenedicarboxylic acid, bphz=1,2-bis(4-pyridylmethylene)hydrazine) composed of a mixed-ligand system. The first isomer, with a paddle-wheel-type Cd2 (COO)4 secondary building unit (SBU), is flexible in nature, whereas the other isomer has a rigid framework based on a μ-oxo-bridged Cd2 (μ-OCO)2 SBU. Both frameworks are two-fold interpenetrated and the pore surface is decorated with pendant -NH2 and NN functional groups. Both the frameworks are nonporous to N2 , revealed by the type II adsorption profiles. However, at 195 K, the first isomer shows an unusual double-step hysteretic CO2 adsorption profile, whereas the second isomer shows a typical type I CO2 profile. Moreover, at 195 K, both frameworks show excellent selectivity for CO2 among other gases (N2 , O2 , H2 , and Ar), which has been correlated to the specific interaction of CO2 with the -NH2 and NN functionalized pore surface. DFT calculations for the oxo-bridged isomer unveiled that the -NH2 group is the primary binding site for CO2 . The high heat of CO2 adsorption (ΔHads =37.7 kJ mol(-1) ) in the oxo-bridged isomer is realized by NH2 ⋅⋅⋅CO2 /aromatic π⋅⋅⋅CO2 and cooperative CO2 ⋅⋅⋅CO2 interactions. Further, postsynthetic modification of the -NH2 group into -NHCOCH3 in the second isomer leads to a reduced CO2 uptake with lower binding energy, which establishes the critical role of the -NH2 group for CO2 capture. The presence of basic -NH2 sites in the oxo-bridged isomer was further exploited for efficient catalytic activity in a Knoevenagel condensation reaction.

Antiferromagnetic Porous Metal–Organic Framework Containing Mixed-Valence [MnII4MnIII2(μ4-O)2]10+ Units with Catecholase Activity and Selective Gas Adsorption

A multifunctional porous metal organic framework based on mixed-valence hexa-nuclear [Mn(III)(2)Mn(II)(4)O(2)(pyz)(2)(C(6)H(5)CH(2)COO)(10)] (pyz = pyrazine) units has been synthesized. The complex has been characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, and variable-temperature magnetic measurements. The structural analysis reveals that the bidentate pyz molecules connect each [Mn(6)] unit to its four [Mn(6)] neighbors through the peripheral Mn(II) centers, giving rise to a three-dimensional (3D) distorted diamond-like porous framework. Variable-temperature (2-300 K) magnetic susceptibility measurements show the presence of dominant antiferromagnetic interactions within the discrete [Mn(6)] cluster that have been fitted with a model containing three exchange constants developed for the complex (J(1) = -8.6 cm(-1), J(2) = -3.9 cm(-1), and J(3) = -100.0 cm(-1)). Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, catecholase activity of the complex has been studied; the turn over number is determined to be of 2547 h(-1) in acetonitrile. This porous compound shows remarkable selectivity for adsorption of CO(2) over N(2) that may be correlated with the effect of window flexibility of the pore to the corresponding adsorbate molecules.

Amine‐Responsive Adaptable Nanospaces: Fluorescent Porous Coordination Polymer for Molecular Recognition

Flexible and dynamic porous coordination polymers (PCPs) with well-defined nanospaces composed of chromophoric organic linkers provide a scaffold for encapsulation of versatile guest molecules through noncovalent interactions. PCPs thus provide a potential platform for molecular recognition. Herein, we report a flexible 3D supramolecular framework {[Zn(ndc)(o-phen)]⋅DMF}n (o-phen = 1,10-phenanthroline, ndc = 2,6-napthalenedicarboxylate) with confined nanospaces that can accommodate different electron-donating aromatic amine guests with selective turn-on emission signaling. This system serves as a molecular recognition platform through an emission-readout process. Such unprecedented tunable emission with different amines is attributed to its emissive charge-transfer (CT) complexation with o-phen linkers. In certain cases this CT emission is further amplified by energy transfer from the chromophoric linker unit ndc, as evidenced by single-crystal X-ray structural characterization.

Metal–organic frameworks (MOFs) based on mixed linker systems: structural diversities towards functional materials

Coordination polymers are well known organic–inorganic hybrids which can be of different dimensionalities; 1D, 2D and 3D. Based on the geometry of the inorganic metal ions and coordination mode of the organic linkers/clusters the polymers extend to different dimensions and adopt versatile topologies. Frameworks can be built with one linker or more than one linker (mixed linkers) with assembly of different metal ions. Use of a single linker is a well known and accepted methodology to generate high surface area frameworks such as isoreticular metal–organic frameworks (IRMOFs), zeolitic imidazolate frameworks (ZIFs), HKUST and MILs (Material Lavoisier Laboratory). In this context, MOFs composed of mixed linkers provide greater flexibility in terms of surface area, modifiable pore size and chemical environment. In general, one anionic linker and one neutral linker connect with the metal ion/cluster to generate a mixed linker framework. Depending on the binding mode of the anionic linker, the framework extends in different dimensions; a V-shaped anionic linker would not grow in two dimensions, rather a 1D chain will form. The neutral linkers mostly serve as pillars and further increase the dimensionality. Depending on the length of the linkers, porosity can be achieved and systematic control is possible. The extent of entanglement in a 3D framework can also be tuned by altering the neutral or anionic linker. Moreover the linkers can be functionalized extensively to meet the aimed applications such as gas separation, catalysis, magnetism and molecular sensing. Such modulation over functionality and porosity is not possible with a single linker system. In this highlight we aimed to discuss mixed linkers based framework structures, their versatile topologies and tunable porous properties.

Selective carbon dioxide uptake and crystal-to-crystal transformation: porous 3D framework to 1D chain triggered by conformational change of the spacer

α-Po type 3D porous frameworks, {[M(bpe)2(N(CN)2)]N(CN)2·xH2O}n, (M = Zn(II) (x = 5) (1)/Co(II) (x = 4) (2)) (bpe = 1,2-bis(4-pyridyl)ethane, N(CN)2− = dicyanamide anion) composed of mixed ligand systems have been synthesized and structurally characterized. Upon two-fold interpenetration both 3D frameworks 1 and 2 show a bimodal channel structure; the small channels contain non-coordinated N(CN)2− anions and the bigger channels are occupied by guest water molecules. High framework stability for both compounds was realized by similarity in the PXRD pattern in dehydrated state and even a reversible single-crystal-to-single-crystal transformation for framework 1. Both the frameworks display unprecedented structural transformation from 3D framework to 1D {[M(bpe)(N(CN)2)2]}n (M = Zn(II) (1b)/Co(II) (2b)) coordination chain upon removal of one molecule of bpe and concomitant bridging of non-coordinated N(CN)2− and conformational change (anti to gauche) by another bridging bpe linker. Moreover, the dehydrated solid {[Co(bpe)2(N(CN)2)]N(CN)2}n (2a) exhibits highly selective CO2 uptake relative to a number of adsorbates (H2, N2, O2 and Ar) at 195 K.

Exciplex Formation and Energy Transfer in a Self‐Assembled Metal–Organic Hybrid System

Exciting assemblies: A metal-organic self-assembly of pyrenebutyric acid (PBA), 1,10-phenanthroline (o-phen), and Mg(II) shows solid-state fluorescence originating from a 1:1 PBA-o-phen exciplex. This exciplex fluorescence is sensitized by another residual PBA chromophore through an excited-state energy-transfer process. The solvent polarity modulates the self-assembly and the corresponding exciplex as well as the energy transfer, resulting in tunable emission of the hybrid (see figure).

Dynamic Entangled Porous Framework for Hydrocarbon (C2–C3) Storage, CO2 Capture, and Separation

Storage and separation of small (C1-C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO2 from CH4 is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two-fold entangled dynamic framework {[Zn2 (bdc)2 (bpNDI)]⋅4DMF}n with pore surface carved with polar functional groups and aromatic π clouds is exploited for selective capture of CO2 , C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO2 and C2 H2 uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO2 over CH4 is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C2 H2 , C2 H4 , C2 H6 , and C3 H8 over CH4 at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO2 /CH4 C2 H6 /CH4 and CO2 /N2 separation capability at ambient condition.

Pillared-bilayer porous coordination polymers of Zn(II): enhanced hydrophobicity of pore surface by changing the pillar functionality

Two new isostructural porous coordination polymers of Zn(II) {[Zn2(NH2-bdc)2(4-bpdb)]·(H2O)4}n (1) and {[Zn2(NH2-bdc)2(4-bpdh)]·(H2O)4}n (2) [4-bpdb = 1,4-bis-(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene and NH2-bdc = 5-amino-1,3-benzenedicarboxylate] have been synthesized using a mixed ligand system by solvent diffusion and structurally characterized through single crystal X-ray diffraction, variable temperature powder X-ray diffraction and thermogravimetric analysis. Both the coordination polymers are constructed using linear Schiff base linkers of similar length having N–N base functionalities but the only difference is the presence of methyl groups in adjacent carbon atoms of the N–N group in the 4-bpdh ligand. Single-crystal structure analysis revealed that both compounds 1 and 2 have two-dimensional (2D) pillared-bilayer framework structures containing 1D channels (8.3 × 3.8 A2 for 1 and 8.0 × 1.6 A2 for 2) filled with lattice water molecules. Channel dimensions in 2 decrease due to the presence of methyl groups. The desolvated frameworks of 1 and 2 are rigid which is evidenced by variable temperature PXRD. Both the compounds show type-I CO2 uptake profiles and the differences in CO2 adsorption uptakes have been corroborated to their void space (27.1% for 1 and 17.1% for 2). Desolvated forms of compound 1 exhibit remarkably high water adsorption capacity even at low vapor pressure whereas desolvated forms of compound 2 show very low water vapor uptake, which could be ascribed to the hydrophobic nature of the pore surface of 2.

Two 3D metal–organic frameworks of Cd(II): modulation of structures and porous properties based on linker functionalities

Two new Cd(II) based metal–organic frameworks (MOFs), {[Cd(NH2-bdc)(bpe)]·0.5EtOH}n (1) and {[Cd(NO2-bdc)(azbpy)]·4H2O}n (2) (NH2-bdc = 2-amino terephthalic acid, bpe = 1,2-bis(4-pyridyl)ethane, NO2-bdc = 2-nitro terephthalic acid, azbpy = 4,4′-azobipyridine), have been synthesized by a solvent diffusion technique and structurally characterized. Both the frameworks are constructed based on exo-bidentate pyridyl type linkers of similar length but different functionalities. Compound 1 has a 3D structure in which the –NH2 functional group of NH2-bdc is coordinated to Cd(II) and a 1D ultra-micropore accommodates ethanol guest molecules. The desolvated framework of 1 (1′) is rigid as realized from the PXRD patterns and shows a type-I CO2 uptake profile with a reasonably high isosteric heat of adsorption value. Density functional theory (DFT) calculation shows that aromatic π electrons interact strongly with CO2 and the binding energy is 33.4 kJ mol−1. Compound 2 has a two-fold interpenetrated 3D porous framework structure where pendent –NO2 groups of NO2-bdc are aligned on the pore surface. The desolvated framework (2′) exhibits structural transformation and is nonporous to N2. Smaller and gradual CO2 uptake in 2′ can be attributed to the structural contraction. The solvent (H2O, MeOH and EtOH) vapour adsorption studies suggest that the pore surface of 2′ is hydrophobic in nature.

Crystal Dynamics in Multi‐stimuli‐Responsive Entangled Metal–Organic Frameworks

An understanding of solid-state crystal dynamics or flexibility in metal-organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single-crystallinity. In this article, detailed experimental studies on a twofold entangled MOF (f-MOF-1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f-MOF-1, by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f-MOF-1 a and f-MOF-1 b, could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO2 adsorption profiles. DFT-based calculations provide an insight into the selective and gated adsorption behavior with CO2 of f-MOF-1 b. The gate-opening threshold pressure of CO2 adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (-CH2 -CH2 -) in f-MOF-1 to an azo (-N=N-) functionality in an analogous MOF, f-MOF-2. The modulation of functionality has an indirect influence on the gate-opening pressure owing to the difference in inter-net interaction. The framework of f-MOF-1 is highly responsive toward CO2 gas molecules, and these results are supported by DFT calculations.