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1. Effects of intervalence charge transfer interaction between π-stacked mixed valent tetrathiafulvalene ligands on the electrical conductivity of 3D metal–organic frameworks

   https://doi.org/10.1039/d1sc04338b  

   Zhang, Shiyu ; Panda, Dillip K. ; Yadav, Ashok ; Zhou, Wei ; Saha, Sourav ( October 2021 , Chemical Science)

   null (Ed.)

   Achieving a molecular-level understanding of how the structures and compositions of metal–organic frameworks (MOFs) influence their charge carrier concentration and charge transport mechanism—the two key parameters of electrical conductivity—is essential for the successful development of electrically conducting MOFs, which have recently emerged as one of the most coveted functional materials due to their diverse potential applications in advanced electronics and energy technologies. Herein, we have constructed four new alkali metal (Na, K, Rb, and Cs) frameworks based on an electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand, which formed continuous π-stacks, albeit with different π–π-stacking and S⋯S distances ( d π–π and d S⋯S ). These MOFs also contained different amounts of aerobically oxidized TTFTC˙ + radical cations that were quantified by electron spin resonance (ESR) spectroscopy. Density functional theory calculations and diffuse reflectance spectroscopy demonstrated that depending on the π–π-interaction and TTFTC˙ + population, these MOFs enjoyed varying degrees of TTFTC/TTFTC˙ + intervalence charge transfer (IVCT) interactions, which commensurately affected their electronic and optical band gaps and electrical conductivity. Having the shortest d π–π (3.39 Å) and the largest initial TTFTC˙ + population (∼23%), the oxidized Na-MOF 1-ox displayed the narrowest band gap (1.33 eV) and the highest room temperature electrical conductivity (3.6 × 10 −5 S cm −1 ), whereas owing to its longest d π–π (3.68 Å) and a negligible TTFTC˙ + population, neutral Cs-MOF 4 exhibited the widest band gap (2.15 eV) and the lowest electrical conductivity (1.8 × 10 −7 S cm −1 ). The freshly prepared but not optimally oxidized K-MOF 2 and Rb-MOF 3 initially displayed intermediate band gaps and conductivity, however, upon prolonged aerobic oxidation, which raised the TTFTC˙ + population to saturation levels (∼25 and 10%, respectively), the resulting 2-ox and 3-ox displayed much narrower band gaps (∼1.35 eV) and higher electrical conductivity (6.6 × 10 −5 and 4.7 × 10 −5 S cm −1 , respectively). The computational studies indicated that charge movement in these MOFs occurred predominantly through the π-stacked ligands, while the experimental results displayed the combined effects of π–π-interactions, TTFTC˙ + population, and TTFTC/TTFTC˙ + IVCT interaction on their electronic and optical properties, demonstrating that IVCT interactions between the mixed-valent ligands could be exploited as an effective design strategy to develop electrically conducting MOFs. 

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2. Collision cross‐section analysis of self‐assembled metallomacrocycle isomers and isobars via ion mobility mass spectrometry

   https://doi.org/10.1002/rcm.8717  

   Endres, Kevin J. ; Barthelmes, Kevin ; Winter, Andreas ; Antolovich, Robert ; Schubert, Ulrich S. ; Wesdemiotis, Chrys ( February 2020 , Rapid Communications in Mass Spectrometry)

   Coordinatively driven self‐assembly of transition metal ions and bidentate ligands gives rise to organometallic complexes that usually contain superimposed isobars, isomers, and conformers. In this study, the double dispersion ability of ion mobility mass spectrometry (IM‐MS) was used to provide a comprehensive structural characterization of the self‐assembled supramolecular complexes by their mass and charge, revealed by the MS event, and their shape and collision cross‐section (Ω), revealed by the IM event.

   Self‐assembled complexes were synthesized by reacting a bis(terpyridine) ligand exhibiting a 60^odihedral angle between the two ligating terpyridine sites (T) with divalent Zn, Ni, Cd, or Fe. The products were isolated as (Metal²⁺[T])_n(PF₆)_2nsalts and analyzed using IM‐MS after electrospray ionization (ESI) which produced several charge states from eachn‐mer, depending on the number of PF₆ˉ anions lost upon ESI. Experimental Ω data, derived using IM‐MS, and computational Ω predictions were used to elucidate the size and architecture of the complexes.

   Only macrocyclic dimers, trimers, and tetramers were observed with Cd²⁺, whereas Zn²⁺formed the same plus hexameric complexes. These two metals led to the simplest product distributions and no linear isomers. In sharp contrast, Ni²⁺and Fe²⁺formed all possible ring sizes from dimer to hexamer as well as various linear isomers. The experimental and theoretical Ω data indicated rather planar macrocyclic geometries for the dimers and trimers, twisted 3D architectures for the larger rings, and substantially larger sizes with spiral conformation for the linear congeners. Adding PF₆ˉ to the same complex was found to mainly cause size contraction due to new stabilizing anion–cation interactions.

   Complete structural identification could be accomplished using ESI‐IM‐MS. Our results affirm that self‐assembly with Cd²⁺and Zn²⁺proceeds through reversible equilibria that generate the thermodynamically most stable structures, encompassing exclusively macrocyclic architectures that readily accommodate the 60^oligand used. In contrast, complexation with Ni²⁺and Fe²⁺, which form stronger coordinative bonds, proceeds through kinetic control, leading to more complex mixtures and kinetically trapped less stable architectures, such as macrocyclic pentamers and linear isomers.

    

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3. A generative artificial intelligence framework based on a molecular diffusion model for the design of metal-organic frameworks for carbon capture

   https://doi.org/10.1038/s42004-023-01090-2  

   Park, Hyun ; Yan, Xiaoli ; Zhu, Ruijie ; Huerta, Eliu A. ; Chaudhuri, Santanu ; Cooper, Donny ; Foster, Ian ; Tajkhorshid, Emad ( February 2024 , Communications Chemistry)

   Metal-organic frameworks (MOFs) exhibit great promise for CO₂capture. However, finding the best performing materials poses computational and experimental grand challenges in view of the vast chemical space of potential building blocks. Here, we introduce GHP-MOFassemble, a generative artificial intelligence (AI), high performance framework for the rational and accelerated design of MOFs with high CO₂adsorption capacity and synthesizable linkers. GHP-MOFassemble generates novel linkers, assembled with one of three pre-selected metal nodes (Cu paddlewheel, Zn paddlewheel, Zn tetramer) into MOFs in a primitive cubic topology. GHP-MOFassemble screens and validates AI-generated MOFs for uniqueness, synthesizability, structural validity, uses molecular dynamics simulations to study their stability and chemical consistency, and crystal graph neural networks and Grand Canonical Monte Carlo simulations to quantify their CO₂adsorption capacities. We present the top six AI-generated MOFs with CO₂capacities greater than 2m mol g⁻¹, i.e., higher than 96.9% of structures in the hypothetical MOF dataset.

    

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4. Pt(II)-Coordinated Tricomponent Self-Assemblies of Tetrapyridyl Porphyrin and Dicarboxylate Ligands: Are They 3D Prisms or 2D Bow-Ties?

   https://doi.org/10.1039/D1SC06533E  

   Benavides, Paola A. ; Gordillo, Monica A. ; Yadav, Ashok ; Joaqui-Joaqui, M. Andrey ; Saha, Sourav ( March 2022 , Chemical Science)

   Goldup, S (Ed.)

   Thermodynamically favored simultaneous coordination of Pt(II) corners with aza- and carboxylate ligands yields tricomponent coordination complexes with sophisticated structures and functions, which require careful structural characterization to paint accurate depiction of their structure–function relationships. Previous reports had claimed that heteroleptic coordination of cis-(Et3P)2PtII with tetrapyridyl porphyrins (M'TPP, M' = Zn or H2) and dicarboxylate ligands (XDC) yielded 3D tetragonal prisms containing two horizontal M'TPP faces and four vertical XDC pillars connected by eight Pt(II) corners, even though such structures were not supported by their 1H NMR data. Through extensive X-ray crystallographic and NMR studies, herein, we demonstrate that self-assembly of cis-(Et3P)2PtII, M'TPP, and four different XDC linkers having varied lengths and rigidity actually yields bow-tie (⋈)-shaped 2D [{cis-(Et3P)2Pt}4(M'TPP)(XDC)2]4+ complexes featuring a M'TPP core and two parallel XDC linkers connected by four heteroleptic PtII corners instead of 3D prisms. This happened because (i) irrespective of their length (~7–11 Å) and rigidity, the XDC linkers intramolecularly bridged two adjacent pyridyl-N atoms of a M'TPP core via PtII corners instead of connecting two cofacial M'TPP ligands and (ii) the bow-tie complexes are entropically more favored over prisms. The electron-rich ZnTPP core of a bow-tie complex selectively formed a charge-transfer complex with highly π-acidic 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-heaxacarbonitrile but not with a π-donor like pyrene. Thus, this work not only produced novel M'TPP-based bow-tie complexes and demonstrated their selective π-acid recognition capability, but also underscored the importance of proper structural characterization of supramolecular assemblies to ensure accurate depiction of their structure–property relationships. 

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5. Microfluidic Synthesis of Elastomeric Microparticles: A Case Study in Catalysis of Palladium-Mediated Cross-Coupling

   Bennett, Jeffrey A ; Genzer, Jan ; Abolhasani, Milad ( October 2018 , 2018 AIChE Annual Meeting)

   Metal-mediated cross-coupling reactions offer organic chemists a wide array of stereo- and chemically-selective reactions with broad applications in fine chemical and pharmaceutical synthesis.1 Current batch-based synthesis methods are beginning to be replaced with flow chemistry strategies to take advantage of the improved consistency and process control methods offered by continuous flow systems.2,3 Most cross-coupling chemistries still encounter several issues in flow using homogeneous catalysis, including expensive catalyst recovery and air sensitivity due to the chemical nature of the catalyst ligands.1 To mitigate some of these issues, a ligand-free heterogeneous catalysis reaction was developed using palladium (Pd) loaded into a polymeric network of a silicone elastomer, poly(hydromethylsiloxane) (PHMS), that is not air sensitive and can be used with mild reaction solvents (ethanol and water).4 In this work we present a novel method of producing soft catalytic microparticles using a multiphase flow-focusing microreactor and demonstrate their application for continuous Suzuki-Miyaura cross-coupling reactions. The catalytic microparticles are produced in a coaxial glass capillary-based 3D flow-focusing microreactor. The microreactor consists of two precursors, a cross-linking catalyst in toluene and a mixture of the PHMS polymer and a divinyl cross-linker. The dispersed phase containing the polymer, cross-linker, and cross-linking catalyst is continuously mixed and then formed into microdroplets by the continuous phase of water and surfactant (sodium dodecyl sulfate) introduced in a counter-flow configuration. Elastomeric microdroplets with a diameter ranging between 50 to 300 micron are produced at 25 to 250 Hz with a size polydispersity less than 3% in single stream production. The physicochemical properties of the elastomeric microparticles such as particle swelling/softness can be tuned using the ratio of cross-linker to polymer as well as the ratio of polymer mixture to solvent during the particle formation. Swelling in toluene can be tuned up to 400% of the initial particle volume by reducing the concentration of cross-linker in the mixture and increasing the ratio of polymer to solvent during production.5 After the particles are produced and collected, they are transferred into toluene containing palladium acetate, allowing the particles to incorporate the palladium into the polymer network and then reduce the palladium to Pd0 with the Si-H functionality present on the PHMS backbones. After the reduction, the Pd-loaded particles can be washed and dried for storage or switched into an ethanol/water solution for loading into a micro-packed bed reactor (µ-PBR) for continuous organic synthesis. The in-situ reduction of Pd within the PHMS microparticles was confirmed using energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and focused ion beam-SEM, and TEM techniques. In the next step, we used the developed µ-PBR to conduct continuous organic synthesis of 4-phenyltoluene by Suzuki-Miyaura cross-coupling of 4-iodotoluene and phenylboronic acid using potassium carbonate as the base. Catalyst leaching was determined to only occur at sub ppm concentrations even at high solvent flow rates after 24 h of continuous run using inductively coupled plasma mass spectrometry (ICP-MS). The developed µ-PBR using the elastomeric microparticles is an important initial step towards the development of highly-efficient and green continuous manufacturing technologies in the pharma industry. In addition, the developed elastomeric microparticle synthesis technique can be utilized for the development of a library of other chemically cross-linkable polymer/cross-linker pairs for applications in organic synthesis, targeted drug delivery, cell encapsulation, or biomedical imaging. References 1. Ruiz-Castillo P, Buchwald SL. Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions. Chem Rev. 2016;116(19):12564-12649. 2. Adamo A, Beingessner RL, Behnam M, et al. On-demand continuous-flow production of pharmaceuticals in a compact, reconfigurable system. Science. 2016;352(6281):61 LP-67. 3. Jensen KF. Flow Chemistry — Microreaction Technology Comes of Age. 2017;63(3). 4. Stibingerova I, Voltrova S, Kocova S, Lindale M, Srogl J. Modular Approach to Heterogenous Catalysis. Manipulation of Cross-Coupling Catalyst Activity. Org Lett. 2016;18(2):312-315. 5. Bennett JA, Kristof AJ, Vasudevan V, Genzer J, Srogl J, Abolhasani M. Microfluidic synthesis of elastomeric microparticles: A case study in catalysis of palladium-mediated cross-coupling. AIChE J. 2018;0(0):1-10. 

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