Mircea Dinca

Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange

Co-reporter:Romain J.-C. Dubey, Robert J. Comito, Zhenwei Wu, Guanghui Zhang, Adam J. Rieth, Christopher H. Hendon, Jeffrey T. Miller, and Mircea Dincă

Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12664-12664

Publication Date(Web):August 7, 2017

DOI:10.1021/jacs.7b06841

Molecular catalysts offer tremendous advantages for stereoselective polymerization because their activity and selectivity can be optimized and understood mechanistically using the familiar tools of organometallic chemistry. Yet, this exquisite control over selectivity comes at an operational price that is generally not justifiable for the large-scale manufacture of polyfolefins. In this report, we identify Co-MFU-4l, prepared by cation exchange in a metal–organic framework, as a solid catalyst for the polymerization of 1,3-butadiene with high stereoselectivity (>99% 1,4-cis). To our knowledge, this is the highest stereoselectivity achieved with a heterogeneous catalyst for this transformation. The polymer’s low polydispersity (PDI ≈ 2) and the catalyst’s ready recovery and low leaching indicate that our material is a structurally resilient single-site heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for discrete, tris-pyrazolylborate-like coordination of Co(II). With this information, we identify a soluble cobalt complex that mimics the structure and reactivity of Co-MFU-4l, thus providing a well-defined platform for studying the catalytic mechanism in the solution phase. This work underscores the capacity for small molecule-like tunability and mechanistic tractability available to transition metal catalysis in metal–organic frameworks.

Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites

Co-reporter:Natalia B. Shustova, Anthony F. Cozzolino, Sebastian Reineke, Marc Baldo, and Mircea Dincă

Journal of the American Chemical Society September 11, 2013 Volume 135(Issue 36) pp:13326-13329

Publication Date(Web):August 27, 2013

DOI:10.1021/ja407778a

We show that fluorescent molecules incorporated as ligands in rigid, porous metal–organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC2– = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest–framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.

Signature of Metallic Behavior in the Metal–Organic Frameworks M3(hexaiminobenzene)2 (M = Ni, Cu)

Co-reporter:Jin-Hu Dou, Lei Sun, Yicong Ge, Wenbin Li, Christopher H. Hendon, Ju Li, Sheraz Gul, Junko Yano, Eric A. Stach, and Mircea Dincă

Journal of the American Chemical Society October 4, 2017 Volume 139(Issue 39) pp:13608-13608

Publication Date(Web):September 14, 2017

DOI:10.1021/jacs.7b07234

The two-dimensionally connected metal–organic frameworks (MOFs) Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene) are bulk electrical conductors and exhibit ultraviolet-photoelectron spectroscopy (UPS) signatures expected of metallic solids. Electronic band structure calculations confirm that in both materials the Fermi energy lies in a partially filled delocalized band. Together with additional structural characterization and microscopy data, these results represent the first report of metallic behavior and permanent porosity coexisting within a metal–organic framework.

Pt Electrodes Enable the Formation of μ4-O Centers in MOF-5 from Multiple Oxygen Sources

Co-reporter:Minyuan M. Li and Mircea Dincă

ACS Applied Materials & Interfaces October 4, 2017 Volume 9(Issue 39) pp:33528-33528

Publication Date(Web):February 8, 2017

DOI:10.1021/acsami.6b16821

The μ4-O2– ions in the Zn4O(O2C−)6 secondary building units of Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) electrodeposited under cathodic bias can be sourced from nitrate, water, and molecular oxygen when using platinum gauze as working electrodes. The use of Zn(ClO4)2·6H2O, anhydrous Zn(NO3)2, or anhydrous Zn(CF3SO3)2 as Zn2+ sources under rigorous control of other sources of oxygen, including water and O2, confirm that the source of the μ4-O2– ions can be promiscuous. Although this finding reveals a relatively complicated manifold of electrochemical processes responsible for the crystallization of MOF-5 under cathodic bias, it further highlights the importance of hydroxide intermediates in the formation of the Zn4O(O2C–R) secondary building units in this iconic material and is illustrative of the complicated crystallization mechanisms of metal–organic frameworks in general.Keywords: crystals; electrochemistry; electrodeposition; mechanism of reactions; metal−organic frameworks;

Mechanistic Evidence for Ligand-Centered Electrocatalytic Oxygen Reduction with the Conductive MOF Ni3(hexaiminotriphenylene)2

Co-reporter:Elise M. Miner, Sheraz Gul, Nathan D. Ricke, Ernest Pastor, Junko Yano, Vittal K. Yachandra, Troy Van Voorhis, and Mircea Dincă

ACS Catalysis November 3, 2017 Volume 7(Issue 11) pp:7726-7726

Publication Date(Web):September 29, 2017

DOI:10.1021/acscatal.7b02647

Establishing catalytic structure–function relationships introduces the ability to optimize the catalyst structure for enhanced activity, selectivity, and durability against reaction conditions and prolonged catalysis. Here we present experimental and computational data elucidating the mechanism for the O2 reduction reaction with a conductive nickel-based metal–organic framework (MOF). Elucidation of the O2 reduction electrokinetics, understanding the role of the extended MOF structure in providing catalytic activity, observation of how the redox activity and pKa of the organic ligand influences catalysis, and identification of the catalyst active site yield a detailed O2 reduction mechanism where the ligand, rather than the metal, plays a central role. More generally, familiarization with how the structural and electronic properties of the MOF influence reactivity may provide deeper insight into the mechanisms by which less structurally defined nonplatinum group metal electrocatalysts reduce O2.Keywords: 2D materials; electrocatalysis; metal−organic framework; O2 reduction; porous catalysts;

Selective Dimerization of Propylene with Ni-MFU-4l

Co-reporter:Robert J. Comito, Eric D. Metzger, Zhenwei Wu, Guanghui Zhang, Christopher H. Hendon, Jeffrey T. Miller, and Mircea Dincă

Organometallics May 8, 2017 Volume 36(Issue 9) pp:1681-1681

Publication Date(Web):April 25, 2017

DOI:10.1021/acs.organomet.7b00178

We report the selective dimerization of propylene to branched hexenes using Ni-MFU-4l, a solid catalyst prepared by cation exchange. Analysis of the resulting product distribution demonstrates that the selectivity arises from 2,1-insertion and slow product reinsertion, mechanistic features reproduced by a molecular nickel tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4l by X-ray absorption spectroscopy provides evidence for discrete, tris-pyrazolylborate-like coordination of nickel, underscoring the small-molecule analogy that can be made at metal–organic framework nodes.

Heterogeneous Epoxide Carbonylation by Cooperative Ion-Pair Catalysis in Co(CO)4–-Incorporated Cr-MIL-101

Co-reporter:Hoyoung D. Park, Mircea Dincă, and Yuriy Román-Leshkov

ACS Central Science May 24, 2017 Volume 3(Issue 5) pp:444-444

Publication Date(Web):March 21, 2017

DOI:10.1021/acscentsci.7b00075

Despite the commercial desirability of epoxide carbonylation to β-lactones, the reliance of this process on homogeneous catalysts makes its industrial application challenging. Here we report the preparation and use of a Co(CO)4–-incorporated Cr-MIL-101 (Co(CO)4⊂Cr-MIL-101, Cr-MIL-101 = Cr3O(BDC)3F, H2BDC = 1,4-benzenedicarboxylic acid) heterogeneous catalyst for the ring-expansion carbonylation of epoxides, whose activity, selectivity, and substrate scope are on par with those of the reported homogeneous catalysts. We ascribe the observed performance to the unique cooperativity between the postsynthetically introduced Co(CO)4– and the site-isolated Lewis acidic Cr(III) centers in the metal–organic framework (MOF). The heterogeneous nature of Co(CO)4⊂Cr-MIL-101 allows the first demonstration of gas-phase continuous-flow production of β-lactones from epoxides, attesting to the potential applicability of the heterogeneous epoxide carbonylation strategy.

Reversible Capture and Release of Cl2 and Br2 with a Redox-Active Metal–Organic Framework

Co-reporter:Yuri Tulchinsky, Christopher H. Hendon, Kirill A. Lomachenko, Elisa Borfecchia, Brent C. Melot, Matthew R. Hudson, Jacob D. Tarver, Maciej D. Korzyński, Amanda W. Stubbs, Jacob J. Kagan, Carlo Lamberti, Craig M. Brown, and Mircea Dincă

Journal of the American Chemical Society April 26, 2017 Volume 139(Issue 16) pp:5992-5992

Publication Date(Web):March 28, 2017

DOI:10.1021/jacs.7b02161

Extreme toxicity, corrosiveness, and volatility pose serious challenges for the safe storage and transportation of elemental chlorine and bromine, which play critical roles in the chemical industry. Solid materials capable of forming stable nonvolatile compounds upon reaction with elemental halogens may partially mitigate these challenges by allowing safe halogen release on demand. Here we demonstrate that elemental halogens quantitatively oxidize coordinatively unsaturated Co(II) ions in a robust azolate metal–organic framework (MOF) to produce stable and safe-to-handle Co(III) materials featuring terminal Co(III)–halogen bonds. Thermal treatment of the oxidized MOF causes homolytic cleavage of the Co(III)–halogen bonds, reduction to Co(II), and concomitant release of elemental halogens. The reversible chemical storage and thermal release of elemental halogens occur with no significant losses of structural integrity, as the parent cobaltous MOF retains its crystallinity and porosity even after three oxidation/reduction cycles. These results highlight a material operating via redox mechanism that may find utility in the storage and capture of other noxious and corrosive gases.

Moisture Farming with Metal-Organic Frameworks

Co-reporter:Adam J. Rieth, Mircea Dincă

Chem 2017 Volume 2, Issue 6(Volume 2, Issue 6) pp:

Publication Date(Web):8 June 2017

DOI:10.1016/j.chempr.2017.05.017

In a recent issue of Science, Kim et al. describe a device that captures water vapor from the atmosphere at low relative humidity by using a metal-organic framework as the active sorbent. This first-of-its-kind water harvester can be powered by only solar thermal energy.

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

Co-reporter:C. K. Brozek;A. Ozarowski;S. A. Stoian;M. Dincă

Inorganic Chemistry Frontiers 2017 vol. 4(Issue 5) pp:782-788

Publication Date(Web):2017/05/16

DOI:10.1039/C6QI00584E

Temperature-dependent 57Fe Mössbauer spectra were collected on FexZn4−x(1,4-benzenedicarboxylate)3 (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N2, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N2. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena.

High temperature ferromagnetism in π-conjugated two-dimensional metal–organic frameworks

Co-reporter:Wenbin Li;Lei Sun;Jingshan Qi;Pablo Jarillo-Herrero;Mircea Dincă;Ju Li

Chemical Science (2010-Present) 2017 vol. 8(Issue 4) pp:2859-2867

Publication Date(Web):2017/03/28

DOI:10.1039/C6SC05080H

We use first-principles calculations to show that the square symmetry of two-dimensional (2D) metal–organic frameworks (MOFs) made from octaamino-substituted phthalocyanines and square planar Ni2+ ions, which enable strong conjugation of π electrons, has a critical impact on the magnetic properties of the lattice. In particular, we predict the unexpected emergence of a rare high-temperature ferromagnetic half-metallic ground state in one case. Among charge neutral MOFs made from (2,3,9,10,16,17,23,24)-octaiminophthalocyanine (OIPc) metallated with divalent first-row transition metal ions (M-OIPc; M = Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and connected through square planar Ni-bisphenylenediimine moieties, NiMn-OIPc exhibits a half-metallic and ferromagnetic ground state with a large exchange energy resulting from the unique strong hybridization between the d/π orbitals of Mn, the Pc ring, and the Ni-bisphenylenediimine nodes. Notably, we show that for NiMn-OIPc there is a considerable difference between the ferromagnetic ordering temperature (Tc) predicted by a 2D Ising model, which exceeds 600 K, and a Tc of 170 K predicted by our more realistic Monte Carlo simulation that includes magnetic anisotropy. Critically, our simulations adopt two spin models that incorporate magnetic anisotropy in the form of exchange anisotropy and single-ion anisotropy. We further show that in the bulk, 2D layers of NiMn-OIPc adopt a slipped-parallel stacking configuration, and exhibit interlayer magnetic coupling that is sensitive to the relative in-plane displacement between adjacent layers. These results highlight the critical role of magnetic anisotropy in modeling the properties of 2D magnetic systems. More generally, it demonstrates that strong hybridization between open-shell ions and delocalized aromatic π systems with appropriate symmetry, combined with large magnetic anisotropy, will be an effective design strategy to realize ferromagnetic 2D MOFs with high Tc.

Is iron unique in promoting electrical conductivity in MOFs?

Co-reporter:Lei Sun;Christopher H. Hendon;Sarah S. Park;Yuri Tulchinsky;Ruomeng Wan;Fang Wang;Aron Walsh;Mircea Dincă

Chemical Science (2010-Present) 2017 vol. 8(Issue 6) pp:4450-4457

Publication Date(Web):2017/05/30

DOI:10.1039/C7SC00647K

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M2(DOBDC)(DMF)2 (M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+); H4DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide), M2(DSBDC)(DMF)2 (M = Mn2+, Fe2+; H4DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M2Cl2(BTDD)(DMF)2 (M = Mn2+, Fe2+, Co2+, Ni2+; H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)2 (M = Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe2+ and the Fe3+/2+ mixed valency. These results reveal that incorporating Fe2+ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.

Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l

Co-reporter:Eric D. Metzger, Robert J. Comito, Christopher H. HendonMircea Dincă

Journal of the American Chemical Society 2016 Volume 139(Issue 2) pp:757-762

Publication Date(Web):December 14, 2016

DOI:10.1021/jacs.6b10300

A recently developed metal–organic framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of commercial homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calculations to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.

Measuring and Reporting Electrical Conductivity in Metal–Organic Frameworks: Cd2(TTFTB) as a Case Study

Co-reporter:Lei Sun, Sarah S. Park, Dennis Sheberla, and Mircea Dincă

Journal of the American Chemical Society 2016 Volume 138(Issue 44) pp:14772-14782

Publication Date(Web):October 21, 2016

DOI:10.1021/jacs.6b09345

Electrically conductive metal–organic frameworks (MOFs) are emerging as a subclass of porous materials that can have a transformative effect on electronic and renewable energy devices. Systematic advances in these materials depend critically on the accurate and reproducible characterization of their electrical properties. This is made difficult by the numerous techniques available for electrical measurements and the dependence of metrics on device architecture and numerous external variables. These challenges, common to all types of electronic materials and devices, are especially acute for porous materials, whose high surface area make them even more susceptible to interactions with contaminants in the environment. Here, we use the anisotropic semiconducting framework Cd2(TTFTB) (TTFTB4– = tetrathiafulvalene tetrabenzoate) to benchmark several common methods available for measuring electrical properties in MOFs. We show that factors such as temperature, chemical environment (atmosphere), and illumination conditions affect the quality of the data obtained from these techniques. Consistent results emerge only when these factors are strictly controlled and the morphology and anisotropy of the Cd2(TTFTB) single-crystal devices are taken into account. Most importantly, we show that depending on the technique, device construction, and/or the environment, a variance of 1 or even 2 orders of magnitude is not uncommon for even just one material if external factors are not controlled consistently. Differences in conductivity values of even 2 orders of magnitude should therefore be interpreted with caution, especially between different research groups comparing different compounds. These results allow us to propose a reliable protocol for collecting and reporting electrical properties of MOFs, which should help improve the consistency and comparability of reported electrical properties for this important new class of crystalline porous conductors.

Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework

Co-reporter:Robert J. Comito, Keith J. Fritzsching, Benjamin J. Sundell, Klaus Schmidt-Rohr, and Mircea Dincă

Journal of the American Chemical Society 2016 Volume 138(Issue 32) pp:10232-10237

Publication Date(Web):July 21, 2016

DOI:10.1021/jacs.6b05200

The manufacture of advanced polyolefins has been critically enabled by the development of single-site heterogeneous catalysts. Metal-organic frameworks (MOFs) show great potential as heterogeneous catalysts that may be designed and tuned on the molecular level. In this work, exchange of zinc ions in Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) (MFU-4l) with reactive metals serves to establish a general platform for selective olefin polymerization in a high surface area solid promising for industrial catalysis. Characterization of polyethylene produced by these materials demonstrates both molecular and morphological control. Notably, reactivity approaches single-site catalysis, as evidenced by low polydispersity indices, and good molecular weight control. We further show that these new catalysts copolymerize ethylene and propylene. Uniform growth of the polymer around the catalyst particles provides a mechanism for controlling the polymer morphology, a relevant metric for continuous flow processes.

High and Reversible Ammonia Uptake in Mesoporous Azolate Metal–Organic Frameworks with Open Mn, Co, and Ni Sites

Co-reporter:Adam J. Rieth; Yuri Tulchinsky;Mircea Dincă

Journal of the American Chemical Society 2016 Volume 138(Issue 30) pp:9401-9404

Publication Date(Web):July 15, 2016

DOI:10.1021/jacs.6b05723

A series of new mesoporous metal–organic frameworks (MOFs) made from extended bisbenzenetriazolate linkers exhibit coordinatively unsaturated metal sites that are responsible for high and reversible uptake of ammonia. Isostructural Mn, Co, and Ni materials adsorb 15.47, 12.00, and 12.02 mmol of NH3/g, respectively, at STP. Importantly, these near-record capacities are reversible for at least three cycles. These results demonstrate that azolate MOFs are sufficiently thermally and chemically stable to find uses in recyclable sorption, storage, and potentially separation of chemically challenging and/or corrosive gases, especially when designed to exhibit a high density of open metal sites.

Frontier Orbital Engineering of Metal–Organic Frameworks with Extended Inorganic Connectivity: Porous Alkaline-Earth Oxides

Co-reporter:Christopher H. Hendon, Aron Walsh, and Mircea Dincă

Inorganic Chemistry 2016 Volume 55(Issue 15) pp:7265

Publication Date(Web):June 7, 2016

DOI:10.1021/acs.inorgchem.6b00979

The development of conductive metal–organic frameworks is challenging owing to poor electronic communication between metal clusters and the organic ligands that bridge them. One route to overcoming this bottleneck is to extend the inorganic dimensionality, while using the organic components to provide chemical functionality. Using density functional theory methods, we demonstrate how the properties of the alkaline-earth oxides SrO and BaO are transformed upon formation of porous solids with organic oxygen sources (acetate and trifluoroacetate). The electron affinity is significantly enhanced in the hybrid materials, while the ionization potential can be tuned over a large range with the polarity of the organic moiety. Furthermore, because of their high-vacuum fraction, these materials have dielectric properties suitable for low-κ applications.

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

Co-reporter:Eric D. Metzger, Carl K. Brozek, Robert J. Comito, and Mircea Dincă

ACS Central Science 2016 Volume 2(Issue 3) pp:148

Publication Date(Web):February 19, 2016

DOI:10.1021/acscentsci.6b00012

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.

Elektrisch leitfhige porse Metall-organische Gerstverbindungen

Co-reporter:Lei Sun;Dr. Michael G. Campbell; Mircea Dinc&x103;

Angewandte Chemie 2016 Volume 128( Issue 11) pp:3628-3642

Publication Date(Web):

DOI:10.1002/ange.201506219

Abstract

Aufgrund ihrer strukturellen, chemischen und funktionellen Vielfalt werden Metall-organische Gerüstverbindungen (MOFs) seit etwa zwei Jahrzehnten für energierelevante Anwendungen erforscht. Bis vor kurzem mangelte es dabei aber an Anwendungen, für die ein guter Ladungstransport zusammen mit Porosität und einer großen Oberfläche erforderlich ist. Die meisten MOFs sind elektrische Isolatoren, und erst unlängst wurde für einige Materialien dieser Klasse ausgezeichnete elektrische Leitfähigkeit und hohe Ladungsmobilität gefunden. In diesem Aufsatz geben wir eine Übersicht über synthetische und elektronische Designstrategien für Gerüste mit permanenter Porosität und Ladungstransport über große Distanzen. Ferner diskutieren wir wegweisende Experimente zum elektrischen Ladungstransport und ausgewählte Anwendungen für diese spezielle Unterklasse von MOFs.

Electrically Conductive Porous MetalOrganic Frameworks

Co-reporter:Lei Sun;Dr. Michael G. Campbell; Mircea Dinc&x103;

Angewandte Chemie International Edition 2016 Volume 55( Issue 11) pp:3566-3579

Publication Date(Web):

DOI:10.1002/anie.201506219

Abstract

Owing to their outstanding structural, chemical, and functional diversity, metal–organic frameworks (MOFs) have attracted considerable attention over the last two decades in a variety of energy-related applications. Notably missing among these, until recently, were applications that required good charge transport coexisting with porosity and high surface area. Although most MOFs are electrical insulators, several materials in this class have recently demonstrated excellent electrical conductivity and high charge mobility. Herein we review the synthetic and electronic design strategies that have been employed thus far for producing frameworks with permanent porosity and long-range charge transport properties. In addition, key experiments that have been employed to demonstrate electrical transport, as well as selected applications for this subclass of MOFs, will be discussed.

Cation-Dependent Intrinsic Electrical Conductivity in Isostructural Tetrathiafulvalene-Based Microporous Metal–Organic Frameworks

Co-reporter:Sarah S. Park; Eric R. Hontz; Lei Sun; Christopher H. Hendon; Aron Walsh; Troy Van Voorhis;Mircea Dincă

Journal of the American Chemical Society 2015 Volume 137(Issue 5) pp:1774-1777

Publication Date(Web):January 18, 2015

DOI:10.1021/ja512437u

Isostructural metal–organic frameworks (MOFs) M2(TTFTB) (M = Mn, Co, Zn, and Cd; H4TTFTB = tetrathiafulvalene tetrabenzoate) exhibit a striking correlation between their single-crystal conductivities and the shortest S···S interaction defined by neighboring TTF cores, which inversely correlates with the ionic radius of the metal ions. The larger cations cause a pinching of the S···S contact, which is responsible for better orbital overlap between pz orbitals on neighboring S and C atoms. Density functional theory calculations show that these orbitals are critically involved in the valence band of these materials, such that modulation of the S···S distance has an important effect on band dispersion and, implicitly, on the conductivity. The Cd analogue, with the largest cation and shortest S···S contact, shows the largest electrical conductivity, σ = 2.86 (±0.53) × 10–4 S/cm, which is also among the highest in microporous MOFs. These results describe the first demonstration of tunable intrinsic electrical conductivity in this class of materials and serve as a blueprint for controlling charge transport in MOFs with π-stacked motifs.

Chemiresistive Sensor Arrays from Conductive 2D Metal–Organic Frameworks

Co-reporter:Michael G. Campbell; Sophie F. Liu; Timothy M. Swager;Mircea Dincă

Journal of the American Chemical Society 2015 Volume 137(Issue 43) pp:13780-13783

Publication Date(Web):October 11, 2015

DOI:10.1021/jacs.5b09600

Applications of porous metal–organic frameworks (MOFs) in electronic devices are rare, owing in large part to a lack of MOFs that display electrical conductivity. Here, we describe the use of conductive two-dimensional (2D) MOFs as a new class of materials for chemiresistive sensing of volatile organic compounds (VOCs). We demonstrate that a family of structurally analogous 2D MOFs can be used to construct a cross-reactive sensor array that allows for clear discrimination between different categories of VOCs. Experimental data show that multiple sensing mechanisms are operative with high degrees of orthogonality, establishing that the 2D MOFs used here are mechanistically unique and offer advantages relative to other known chemiresistor materials.

Million-Fold Electrical Conductivity Enhancement in Fe2(DEBDC) versus Mn2(DEBDC) (E = S, O)

Co-reporter:Lei Sun; Christopher H. Hendon; Mikael A. Minier; Aron Walsh;Mircea Dincă

Journal of the American Chemical Society 2015 Volume 137(Issue 19) pp:6164-6167

Publication Date(Web):May 1, 2015

DOI:10.1021/jacs.5b02897

Reaction of FeCl2 and H4DSBDC (2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe2(DSBDC), an analogue of M2(DOBDC) (MOF-74, DOBDC4– = 2,5-dihydroxybenzene-1,4-dicarboxylate). The bulk electrical conductivity values of both Fe2(DSBDC) and Fe2(DOBDC) are ∼6 orders of magnitude higher than those of the Mn2+ analogues, Mn2(DEBDC) (E = O, S). Because the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe2+ β-spin electron. These results provide important insight for the rational design of conductive metal–organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials.

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

Co-reporter:Carl K. Brozek; Jeffrey T. Miller; Sebastian A. Stoian;Mircea Dincă

Journal of the American Chemical Society 2015 Volume 137(Issue 23) pp:7495-7501

Publication Date(Web):May 19, 2015

DOI:10.1021/jacs.5b03761

The weak-field ligand environments at the metal nodes of metal–organic frameworks (MOFs) mimic the electronic environment of metalloenzyme active sites, but little is known about the reactivity of MOF nodes toward small molecules of biological relevance. Here, we report that the ferrous ions in Fe2+-exchanged MOF-5 disproportionate nitric oxide to produce nitrous oxide and a ferric nitrito complex. Although mechanistic studies of N–N bond forming transformations often invoke a hyponitrite species, as in nitric oxide reductase and NOx reduction catalysis, little is known about this intermediate in its monoanionic state. Together with the first report of N–N coupling between NO molecules in a MOF, we present evidence for a species that is consistent with a ferric hyponitrite radical, whose isolation is enabled by the spatial constraints of the MOF matrix.

Synthesis and Electrical Properties of Covalent Organic Frameworks with Heavy Chalcogens

Co-reporter:Selma Duhović and Mircea Dincă

Chemistry of Materials 2015 Volume 27(Issue 16) pp:5487

Publication Date(Web):July 30, 2015

DOI:10.1021/acs.chemmater.5b02358

On the Mechanism of MOF-5 Formation under Cathodic Bias

Co-reporter:Minyuan Li and Mircea Dincă

Chemistry of Materials 2015 Volume 27(Issue 9) pp:3203

Publication Date(Web):April 21, 2015

DOI:10.1021/acs.chemmater.5b00899

Thermodynamic parameters of cation exchange in MOF-5 and MFU-4l

Co-reporter:Carl K. Brozek and Mircea Dincă

Chemical Communications 2015 vol. 51(Issue 59) pp:11780-11782

Publication Date(Web):24 Jun 2015

DOI:10.1039/C5CC04249F

We present a method for approximating thermodynamic parameters , ΔH, and ΔS for the cation exchange process in metal–organic frameworks, as exemplified by Ni2+ exchange into Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) and Co2+ exchange into MOF-5 and Zn5Cl4(bis(1H-1,2,3-triazolo-[4,5-b],[4′,5′-i])dibenzo-[1,4]-dioxin)3 (MFU-4l). For these examples, we find that the cation exchange process is endergonic and that parameters such as solvent and cation identity impact the thermodynamics.

Cu3(hexaiminotriphenylene)2: An Electrically Conductive 2D MetalOrganic Framework for Chemiresistive Sensing

Co-reporter:Dr. Michael G. Campbell;Dr. Dennis Sheberla;Sophie F. Liu; Timothy M. Swager ; Mircea Dinc&x103;

Angewandte Chemie International Edition 2015 Volume 54( Issue 14) pp:4349-4352

Publication Date(Web):

DOI:10.1002/anie.201411854

Abstract

The utility of metal–organic frameworks (MOFs) as functional materials in electronic devices has been limited to date by a lack of MOFs that display high electrical conductivity. Here, we report the synthesis of a new electrically conductive 2D MOF, Cu₃(HITP)₂ (HITP=2,3,6,7,10,11-hexaiminotriphenylene), which displays a bulk conductivity of 0.2 S cm⁻¹ (pellet, two-point-probe). Devices synthesized by simple drop casting of Cu₃(HITP)₂ dispersions function as reversible chemiresistive sensors, capable of detecting sub-ppm levels of ammonia vapor. Comparison with the isostructural 2D MOF Ni₃(HITP)₂ shows that the copper sites are critical for ammonia sensing, indicating that rational design/synthesis can be used to tune the functional properties of conductive MOFs.

Cu3(hexaiminotriphenylene)2: An Electrically Conductive 2D MetalOrganic Framework for Chemiresistive Sensing

Co-reporter:Dr. Michael G. Campbell;Dr. Dennis Sheberla;Sophie F. Liu; Timothy M. Swager ; Mircea Dinc&x103;

Angewandte Chemie 2015 Volume 127( Issue 14) pp:4423-4426

Publication Date(Web):

DOI:10.1002/ange.201411854

Abstract

The utility of metal–organic frameworks (MOFs) as functional materials in electronic devices has been limited to date by a lack of MOFs that display high electrical conductivity. Here, we report the synthesis of a new electrically conductive 2D MOF, Cu₃(HITP)₂ (HITP=2,3,6,7,10,11-hexaiminotriphenylene), which displays a bulk conductivity of 0.2 S cm⁻¹ (pellet, two-point-probe). Devices synthesized by simple drop casting of Cu₃(HITP)₂ dispersions function as reversible chemiresistive sensors, capable of detecting sub-ppm levels of ammonia vapor. Comparison with the isostructural 2D MOF Ni₃(HITP)₂ shows that the copper sites are critical for ammonia sensing, indicating that rational design/synthesis can be used to tune the functional properties of conductive MOFs.

Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues

Co-reporter:Carl K. Brozek, Vladimir K. Michaelis, Ta-Chung Ong, Luca Bellarosa, Núria López, Robert G. Griffin, and Mircea Dincă

ACS Central Science 2015 Volume 1(Issue 5) pp:252

Publication Date(Web):July 29, 2015

DOI:10.1021/acscentsci.5b00247

Multinuclear solid-state nuclear magnetic resonance, mass spectrometry, first-principles molecular dynamics simulations, and other complementary evidence reveal that the coordination environment around the Zn2+ ions in MOF-5, one of the most iconic materials among metal–organic frameworks (MOFs), is not rigid. The Zn2+ ions bind solvent molecules, thereby increasing their coordination number, and dynamically dissociate from the framework itself. On average, one ion in each cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized MOF-5 is defined as Zn4O(BDC)3(DMF)x (x = 1–2). Understanding the dynamic behavior of MOF-5 leads to a rational low-temperature cation exchange approach for the synthesis of metastable Zn4–xCoxO(terephthalate)3 (x > 1) materials, which have not been accessible through typical high-temperature solvothermal routes thus far.

Cation exchange at the secondary building units of metal–organic frameworks

Co-reporter:C. K. Brozek and M. Dincă

Chemical Society Reviews 2014 vol. 43(Issue 16) pp:5456-5467

Publication Date(Web):16 May 2014

DOI:10.1039/C4CS00002A

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal–organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.

High Electrical Conductivity in Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, a Semiconducting Metal–Organic Graphene Analogue

Co-reporter:Dennis Sheberla ; Lei Sun ; Martin A. Blood-Forsythe ; Süleyman Er ; Casey R. Wade ; Carl K. Brozek ; Alán Aspuru-Guzik ;Mircea Dincă

Journal of the American Chemical Society 2014 Volume 136(Issue 25) pp:8859-8862

Publication Date(Web):April 21, 2014

DOI:10.1021/ja502765n

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni2+ in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metal–organic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 and 40 S·cm–1, respectively, both records for MOFs and among the best for any coordination polymer.

Ligand Redox Non-innocence in the Stoichiometric Oxidation of Mn2(2,5-dioxidoterephthalate) (Mn-MOF-74)

Co-reporter:Anthony F. Cozzolino ; Carl K. Brozek ; Ryan D. Palmer ; Junko Yano ; Minyuan Li ;Mircea Dincă

Journal of the American Chemical Society 2014 Volume 136(Issue 9) pp:3334-3337

Publication Date(Web):February 17, 2014

DOI:10.1021/ja411808r

Unsaturated metal sites within the nodes of metal–organic frameworks (MOFs) can be interrogated by redox reagents common to small molecule chemistry. We show, for the first time, that an analogue of the iconic M2(2,5-dioxidoterephthalate) (M2DOBDC, MOF-74) class of materials can be stoichiometrically oxidized by one electron per metal center. The reaction of Mn2DOBDC with C6H5ICl2 produces the oxidized material Cl2Mn2DOBDC, which retains crystallinity and porosity. Surprisingly, magnetic measurements, X-ray absorption, and infrared spectroscopic data indicate that the Mn ions maintain a formal oxidation state of +2, suggesting instead the oxidation of the DOBDC4– ligand to the quinone DOBDC2–. These results describe the first example of ligand redox non-innocence in a MOF and a rare instance of stoichiometric electron transfer involving the metal nodes. The methods described herein offer a synthetic toolkit that will be of general use for further explorations of the redox reactivity of MOF nodes.

Selective formation of biphasic thin films of metal–organic frameworks by potential-controlled cathodic electrodeposition

Co-reporter:Minyuan Li and Mircea Dincă

Chemical Science 2014 vol. 5(Issue 1) pp:107-111

Publication Date(Web):05 Sep 2013

DOI:10.1039/C3SC51815A

Cathodic electrodeposition lends itself to the formation of biphasic metal–organic framework thin films at room temperature from single deposition baths using potential bias as the main user input. Depending on the applied potential, we selectively deposit two different phases as either bulk mixtures or bilayer films.

Solvent-Dependent Cation Exchange in MetalOrganic Frameworks

Co-reporter:Carl K. Brozek;Dr. Luca Bellarosa;Tomohiro Soejima;Talia V. Clark; Núria López; Mircea Dinc&x103;

Chemistry - A European Journal 2014 Volume 20( Issue 23) pp:6871-6874

Publication Date(Web):

DOI:10.1002/chem.201402682

Abstract

We investigated which factors govern the critical steps of cation exchange in metal–organic frameworks by studying the effect of various solvents on the insertion of Ni²⁺ into MOF-5 and Co²⁺ into MFU-4l. After plotting the extent of cation insertion versus different solvent parameters, trends emerge that offer insight into the exchange processes for both systems. This approach establishes a method for understanding critical aspects of cation exchange in different MOFs and other materials.

Postsynthetic tuning of hydrophilicity in pyrazolate MOFs to modulate water adsorption properties

Co-reporter:Casey R. Wade, Tachmajal Corrales-Sanchez, Tarun C. Narayan and Mircea Dincă

Energy & Environmental Science 2013 vol. 6(Issue 7) pp:2172-2177

Publication Date(Web):30 May 2013

DOI:10.1039/C3EE40876K

Metal–organic frameworks (MOFs) have attracted interest as adsorbents in water-based adsorption heat pumps owing to their potential for increased water loading capacities and structural and functional tunability versus traditionally used materials such as zeolites and silica. Although pyrazolate-based MOFs exhibit exceptional hydrolytic stability, the water adsorption characteristics of this class of frameworks have remained unexplored in this context. In this report, we describe the modular synthesis of novel dipyrazole ligands containing naphthalenediimide cores functionalized with –H (H2NDI–H), –NHEt (H2NDI–NHEt), or –SEt (H2NDI–SEt) groups. Reaction of these ligands with Zn(NO3)2 afforded an isostructural series of MOFs, Zn(NDI–X), featuring infinite chains of tetrahedral Zn2+ ions bridged by pyrazolate groups and ∼16 Å-wide channels with functionalized naphthalenediimide linkers lining the channel surface. The Type V water adsorption isotherms measured for these materials show water uptake in the 40–50% relative humidity range, suggesting hydrophobic channel interiors. Postsynthetic oxidation of Zn(NDI–SEt) with dimethyldioxirane was used to generate ethyl sulfoxide and ethyl sulfone groups, thereby rendering the channels more hydrophilic, as evidenced by shifts in water uptake to the 30–40% relative humidity range. Such tunability in water adsorption characteristics may find utility in the design of new adsorbents for adsorption-based heat transfer processes. An original MATLAB script, MOF-FIT, which allows for visual modeling of breathing and other structural deformations in MOFs is also presented.

Mn2(2,5-disulfhydrylbenzene-1,4-dicarboxylate): A Microporous Metal–Organic Framework with Infinite (−Mn–S−)∞ Chains and High Intrinsic Charge Mobility

Co-reporter:Lei Sun, Tomoyo Miyakai, Shu Seki, and Mircea Dincă

Journal of the American Chemical Society 2013 Volume 135(Issue 22) pp:8185-8188

Publication Date(Web):May 14, 2013

DOI:10.1021/ja4037516

The reaction of MnCl2 with 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid (H4DSBDC), in which the phenol groups in 2,5-dihydroxybenzene-1,4-dicarboxylic acid (H4DOBDC) have been replaced by thiophenol units, led to the isolation of Mn2(DSBDC), a thiolated analogue of the M2(DOBDC) series of metal–organic frameworks (MOFs). The sulfur atoms participate in infinite one-dimensional Mn–S chains, and Mn2(DSBDC) shows a high surface area and high charge mobility similar to that found in some of the most common organic semiconductors. The synthetic approach to Mn2(DSBDC) and its excellent electronic properties provide a blueprint for a potentially rich area of exploration in microporous conductive MOFs with low-dimensional charge transport pathways.

Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

Co-reporter:Carl K. Brozek ;Mircea Dincă

Journal of the American Chemical Society 2013 Volume 135(Issue 34) pp:12886-12891

Publication Date(Web):July 31, 2013

DOI:10.1021/ja4064475

The metal nodes in metal–organic frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chemical environments at the nodes should facilitate unusual redox chemistry, but the difficulty in synthesizing MOFs with metal ions in reduced oxidation states has precluded such studies. Herein, we demonstrate that MZn3O(O2C−)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V2+ and Ti3+ ions and enable the synthesis of the first MOFs containing these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Additional MOF-5 analogues featuring Cr2+, Cr3+, Mn2+, and Fe2+ at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr2+- converts into Cr3+-substituted MOF-5, while Fe2+-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.

Quantification of Site-Specific Cation Exchange in Metal–Organic Frameworks Using Multi-Wavelength Anomalous X-ray Dispersion

Co-reporter:Carl K. Brozek, Anthony F. Cozzolino, Simon J. Teat, Yu-Sheng Chen, and Mircea Dincă

Chemistry of Materials 2013 Volume 25(Issue 15) pp:2998

Publication Date(Web):July 2, 2013

DOI:10.1021/cm400858d

We employed multiwavelength anomalous X-ray dispersion to determine the relative cation occupation at two crystallographically distinct metal sites in Fe2+-, Cu2+-, and Zn2+-exchanged versions of the microporous metal–organic framework (MOF) known as MnMnBTT (BTT = 1,3,5-benzenetristetrazolate). By exploiting the dispersive differences between Mn, Fe, Cu, and Zn, the extent and location of cation exchange were determined from single crystal X-ray diffraction data sets collected near the K edges of Mn2+ and of the substituting metal, and at a wavelength remote from either edge as a reference. Comparing the anomalous dispersion between these measurements indicated that the extent of Mn2+ replacement depends on the identity of the substituting metal. We contrasted two unique methods to analyze this data with a conventional approach and evaluated their limitations with emphasis on the general application of this method to other heterometallic MOFs, where site-specific metal identification is fundamental to tuning catalytic and physical properties.Keywords: anomalous X-ray dispersion; metal−organic frameworks; postsynthetic ion metathesis;

Facile Deposition of Multicolored Electrochromic MetalOrganic Framework Thin Films

Co-reporter:Dr. Casey R. Wade;Minyuan Li ; Mircea Dinc&x103;

Angewandte Chemie 2013 Volume 125( Issue 50) pp:13619-13623

Publication Date(Web):

DOI:10.1002/ange.201306162

Facile Deposition of Multicolored Electrochromic MetalOrganic Framework Thin Films

Co-reporter:Dr. Casey R. Wade;Minyuan Li ; Mircea Dinc&x103;

Angewandte Chemie International Edition 2013 Volume 52( Issue 50) pp:13377-13381

Publication Date(Web):

DOI:10.1002/anie.201306162

Thiophene-based covalent organic frameworks

Co-reporter:Vladimir K. Michaelis;Mircea Dincă;Robert G. Griffin;Guillaume H. V. Bertrand;Ta-Chung Ong

PNAS 2013 Volume 110 (Issue 13 ) pp:4923-4928

Publication Date(Web):2013-03-26

DOI:10.1073/pnas.1221824110

We report the synthesis and characterization of covalent organic frameworks (COFs) incorporating thiophene-based building blocks. We show that these are amenable to reticular synthesis, and that bent ditopic monomers, such as 2,5-thiophenediboronic acid, are defect-prone building blocks that are susceptible to synthetic variations during COF synthesis. The synthesis and characterization of an unusual charge transfer complex between thieno[3,2-b]thiophene-2,5-diboronic acid and tetracyanoquinodimethane enabled by the unique COF architecture is also presented. Together, these results delineate important synthetic advances toward the implementation of COFs in electronic devices.

Charge Transfer or J-Coupling? Assignment of an Unexpected Red-Shifted Absorption Band in a Naphthalenediimide-Based Metal–Organic Framework

Co-reporter:Brian D. McCarthy, Eric R. Hontz, Shane R. Yost, Troy Van Voorhis, and Mircea Dincă

The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 3) pp:453-458

Publication Date(Web):January 15, 2013

DOI:10.1021/jz302076s

We investigate and assign a previously reported unexpected transition in the metal–organic framework Zn2(NDC)2(DPNI) (1; NDC = 2,6-naphthalenedicarboxylate, DPNI = dipyridyl-naphthalenediimide) that displays linear arrangements of naphthalenediimide ligands. Given the longitudinal transition dipole moment of the DPNI ligands, J-coupling seemed possible. Photophysical measurements revealed a broad, new transition in 1 between 400 and 500 nm. Comparison of the MOF absorption spectra with that of a charge transfer (CT) complex formed by manual grinding of DPNI and H2NDC led to the assignment of the new band in 1 as arising from an interligand CT. Constrained density functional theory utilizing a custom long-range-corrected hybrid functional was employed to determine which ligands were involved in the CT transition. On the basis of relative oscillator strengths, the interligand CT was assigned as principally arising from π-stacked DPNI/NDC dimers rather than the alternative orthogonal pairs within the MOF.Keywords: charge transfer; constrained DFT; J-coupling; Koopman’s theorem; metal−organic frameworks;

High Charge Mobility in a Tetrathiafulvalene-Based Microporous Metal–Organic Framework

Co-reporter:Tarun C. Narayan ; Tomoyo Miyakai ; Shu Seki ;Mircea Dincă

Journal of the American Chemical Society 2012 Volume 134(Issue 31) pp:12932-12935

Publication Date(Web):July 24, 2012

DOI:10.1021/ja3059827

The tetratopic ligand tetrathiafulvalene-tetrabenzoate (H4TTFTB) is used to synthesize Zn2(TTFTB), a new metal–organic framework that contains columnar stacks of tetrathiafulvalene and benzoate-lined infinite one-dimensional channels. The new MOF remains porous upon desolvation and exhibits charge mobility commensurate with some of the best organic semiconductors, confirmed by flash-photolysis-time-resolved microwave conductivity measurements. Zn2(TTFTB) represents the first example of a permanently porous MOF with high charge mobility and may inspire further exploration of the electronic properties of these materials.

Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal–Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission

Co-reporter:Natalia B. Shustova ; Ta-Chung Ong ; Anthony F. Cozzolino ; Vladimir K. Michaelis ; Robert G. Griffin ;Mircea Dincă

Journal of the American Chemical Society 2012 Volume 134(Issue 36) pp:15061-15070

Publication Date(Web):August 13, 2012

DOI:10.1021/ja306042w

Molecules that exhibit emission in the solid state, especially those known as aggregation-induced emission (AIE) chromophores, have found applications in areas as varied as light-emitting diodes and biological sensors. Despite numerous studies, the mechanism of fluorescence quenching in AIE chromophores is still not completely understood. To this end, much interest has focused on understanding the low-frequency vibrational dynamics of prototypical systems, such as tetraphenylethylene (TPE), in the hope that such studies would provide more general principles toward the design of new sensors and electronic materials. We hereby show that a perdeuterated TPE-based metal–organic framework (MOF) serves as an excellent platform for studying the low-energy vibrational modes of AIE-type chromophores. In particular, we use solid-state 2H and 13C NMR experiments to investigate the phenyl ring dynamics of TPE cores that are coordinatively trapped inside a MOF and find a phenyl ring flipping energy barrier of 43(6) kJ/mol. DFT calculations are then used to deconvolute the electronic and steric contributions to this flipping barrier. Finally, we couple the NMR and DFT studies with variable-temperature X-ray diffraction experiments to propose that both the ethylenic C═C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter. To conclude, we use these findings to propose a set of design criteria for the development of tunable turn-on porous sensors constructed from AIE-type molecules, particularly as applied to the design of new multifunctional MOFs.

Conformational Locking by Design: Relating Strain Energy with Luminescence and Stability in Rigid Metal–Organic Frameworks

Co-reporter:Natalia B. Shustova ; Anthony F. Cozzolino ;Mircea Dincă

Journal of the American Chemical Society 2012 Volume 134(Issue 48) pp:19596-19599

Publication Date(Web):November 19, 2012

DOI:10.1021/ja3103154

Minimization of the torsional barrier for phenyl ring flipping in a metal–organic framework (MOF) based on the new ethynyl-extended octacarboxylate ligand H8TDPEPE leads to a fluorescent material with a near-dark state. Immobilization of the ligand in the rigid structure also unexpectedly causes significant strain. We used DFT calculations to estimate the ligand strain energies in our and all other topologically related materials and correlated these with empirical structural descriptors to derive general rules for trapping molecules in high-energy conformations within MOFs. These studies portend possible applications of MOFs for studying fundamental concepts related to conformational locking and its effects on molecular reactivity and chromophore photophysics.

Lattice-imposed geometry in metal–organic frameworks: lacunary Zn4O clusters in MOF-5 serve as tripodal chelating ligands for Ni2+

Co-reporter:Carl K. Brozek and Mircea Dincă

Chemical Science 2012 vol. 3(Issue 6) pp:2110-2113

Publication Date(Web):04 Apr 2012

DOI:10.1039/C2SC20306E

The inorganic clusters in metal–organic frameworks can be used to trap metal ions in coordination geometries that are difficult to achieve in molecular chemistry. We illustrate this concept by using the well-known basic carboxylate clusters in Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) as tripodal chelating ligands that enforce an unusual pseudo-tetrahedral oxygen ligand field around Ni2+. The new Ni-based MOF-5 analogue is characterized by porosity measurements and a suite of electronic structure spectroscopies. Classical ligand field analysis of the Ni2+ ion isolated in MOF-5 classifies the Zn3O(carboxylate)6 “tripodal ligand” as an unusual, stronger field ligand than halides and other oxygen donor ligands. These results may inspire the widespread usage of MOFs as chelating ligands for stabilizing site-isolated metal ions in future reactivity and electronic structure studies.

Investigation of the synthesis, activation, and isosteric heats of CO2 adsorption of the isostructural series of metal–organic frameworks M3(BTC)2 (M = Cr, Fe, Ni, Cu, Mo, Ru)

Co-reporter:Casey R. Wade and Mircea Dincă

Dalton Transactions 2012 vol. 41(Issue 26) pp:7931-7938

Publication Date(Web):26 Apr 2012

DOI:10.1039/C2DT30372H

The synthesis, activation, and heats of CO2 adsorption for the known members of the M3(BTC)2 (HKUST-1) isostructural series (M = Cr, Fe, Ni, Zn, Ni, Cu, Mo) were investigated to gain insight into the impact of CO2–metal interactions for CO2 storage/separation applications. With the use of modified syntheses and activation procedures, improved BET surface areas were obtained for M = Ni, Mo, and Ru. The zero-coverage isosteric heats of CO2 adsorption were measured for the Cu, Cr, Ni, Mo, and Ru analogues and gave values consistent with those reported for MOFs containing coordinatively unsaturated metal sites, but lower than for amine functionalized materials. Notably, the Ni and Ru congeners exhibited the highest CO2 affinities in the studied series. These behaviors were attributed to the presence of residual guest molecules in the case of Ni3(BTC)2(Me2NH)2(H2O) and the increased charge of the dimetal secondary building unit in [Ru3(BTC)2][BTC]0.5.

Reductive Electrosynthesis of Crystalline Metal–Organic Frameworks

Co-reporter:Minyuan Li ;Mircea Dincă

Journal of the American Chemical Society 2011 Volume 133(Issue 33) pp:12926-12929

Publication Date(Web):July 26, 2011

DOI:10.1021/ja2041546

Electroreduction of oxoanions affords hydroxide equivalents that induce selective deposition of crystalline metal–organic frameworks (MOFs) on conductive surfaces. The method is illustrated by cathodic electrodeposition of Zn4O(BDC)3 (MOF-5; BDC = 1,4-benzenedicarboxylate), which is deposited at room temperature in only 15 min under cathodic potential. Although many crystalline phases are known in the Zn2+/BDC2– system, MOF-5 is the only observed crystalline MOF phase under these conditions. This fast and mild method of synthesizing MOFs is amenable to direct surface functionalization and could impact applications requiring conformal coatings of microporous MOFs, such as gas separation membranes and electrochemical sensors.

Turn-On Fluorescence in Tetraphenylethylene-Based Metal–Organic Frameworks: An Alternative to Aggregation-Induced Emission

Co-reporter:Natalia B. Shustova ; Brian D. McCarthy ;Mircea Dincă

Journal of the American Chemical Society 2011 Volume 133(Issue 50) pp:20126-20129

Publication Date(Web):November 10, 2011

DOI:10.1021/ja209327q

Coordinative immobilization of functionalized tetraphenylethylene within rigid porous metal–organic frameworks (MOFs) turns on fluorescence in the typically non-emissive tetraphenylethylene core. The matrix coordination-induced emission effect (MCIE) is complementary to aggregation-induced emission. Despite the large interchromophore distances imposed by coordination to metal ions, a carboxylate analogue of tetraphenylethylene anchored by Zn2+ and Cd2+ ions inside MOFs shows fluorescence lifetimes in line with those of close-packed molecular aggregates. Turn-on fluorescence by coordinative ligation in a porous matrix is a powerful approach that may lead to new materials made from chromophores with molecular rotors. The potential utility of MCIE toward building new sensing materials is demonstrated by tuning the fluorescence response of the porous MOFs as a function of adsorbed small analytes.

Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites

Co-reporter:Natalia B. Shustova ; Anthony F. Cozzolino ; Sebastian Reineke ; Marc Baldo ;Mircea Dincă

Journal of the American Chemical Society () pp:

Publication Date(Web):August 27, 2013

DOI:10.1021/ja407778a

We show that fluorescent molecules incorporated as ligands in rigid, porous metal–organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC2– = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest–framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.

Lattice-imposed geometry in metal–organic frameworks: lacunary Zn4O clusters in MOF-5 serve as tripodal chelating ligands for Ni2+

Co-reporter:Carl K. Brozek and Mircea Dincă

Chemical Science (2010-Present) 2012 - vol. 3(Issue 6) pp:NaN2113-2113

Publication Date(Web):2012/04/04

DOI:10.1039/C2SC20306E

The inorganic clusters in metal–organic frameworks can be used to trap metal ions in coordination geometries that are difficult to achieve in molecular chemistry. We illustrate this concept by using the well-known basic carboxylate clusters in Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) as tripodal chelating ligands that enforce an unusual pseudo-tetrahedral oxygen ligand field around Ni2+. The new Ni-based MOF-5 analogue is characterized by porosity measurements and a suite of electronic structure spectroscopies. Classical ligand field analysis of the Ni2+ ion isolated in MOF-5 classifies the Zn3O(carboxylate)6 “tripodal ligand” as an unusual, stronger field ligand than halides and other oxygen donor ligands. These results may inspire the widespread usage of MOFs as chelating ligands for stabilizing site-isolated metal ions in future reactivity and electronic structure studies.

Cation exchange at the secondary building units of metal–organic frameworks

Co-reporter:C. K. Brozek and M. Dincă

Chemical Society Reviews 2014 - vol. 43(Issue 16) pp:NaN5467-5467

Publication Date(Web):2014/05/16

DOI:10.1039/C4CS00002A

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal–organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.

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

Co-reporter:C. K. Brozek, A. Ozarowski, S. A. Stoian and M. Dincă

Inorganic Chemistry Frontiers 2017 - vol. 4(Issue 5) pp:NaN788-788

Publication Date(Web):2017/01/16

DOI:10.1039/C6QI00584E

Temperature-dependent 57Fe Mössbauer spectra were collected on FexZn4−x(1,4-benzenedicarboxylate)3 (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N2, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N2. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena.

Is iron unique in promoting electrical conductivity in MOFs?

Co-reporter:Lei Sun, Christopher H. Hendon, Sarah S. Park, Yuri Tulchinsky, Ruomeng Wan, Fang Wang, Aron Walsh and Mircea Dincă

Chemical Science (2010-Present) 2017 - vol. 8(Issue 6) pp:NaN4457-4457

Publication Date(Web):2017/04/20

DOI:10.1039/C7SC00647K

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M2(DOBDC)(DMF)2 (M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+); H4DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide), M2(DSBDC)(DMF)2 (M = Mn2+, Fe2+; H4DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M2Cl2(BTDD)(DMF)2 (M = Mn2+, Fe2+, Co2+, Ni2+; H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)2 (M = Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe2+ and the Fe3+/2+ mixed valency. These results reveal that incorporating Fe2+ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.

High temperature ferromagnetism in π-conjugated two-dimensional metal–organic frameworks

Co-reporter:Wenbin Li, Lei Sun, Jingshan Qi, Pablo Jarillo-Herrero, Mircea Dincă and Ju Li

Chemical Science (2010-Present) 2017 - vol. 8(Issue 4) pp:

Publication Date(Web):

DOI:10.1039/C6SC05080H

Thermodynamic parameters of cation exchange in MOF-5 and MFU-4l

Co-reporter:Carl K. Brozek and Mircea Dincă

Chemical Communications 2015 - vol. 51(Issue 59) pp:NaN11782-11782

Publication Date(Web):2015/06/24

DOI:10.1039/C5CC04249F

We present a method for approximating thermodynamic parameters , ΔH, and ΔS for the cation exchange process in metal–organic frameworks, as exemplified by Ni2+ exchange into Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) and Co2+ exchange into MOF-5 and Zn5Cl4(bis(1H-1,2,3-triazolo-[4,5-b],[4′,5′-i])dibenzo-[1,4]-dioxin)3 (MFU-4l). For these examples, we find that the cation exchange process is endergonic and that parameters such as solvent and cation identity impact the thermodynamics.

Selective formation of biphasic thin films of metal–organic frameworks by potential-controlled cathodic electrodeposition

Co-reporter:Minyuan Li and Mircea Dincă

Chemical Science (2010-Present) 2014 - vol. 5(Issue 1) pp:NaN111-111

Publication Date(Web):2013/09/05

DOI:10.1039/C3SC51815A

Cathodic electrodeposition lends itself to the formation of biphasic metal–organic framework thin films at room temperature from single deposition baths using potential bias as the main user input. Depending on the applied potential, we selectively deposit two different phases as either bulk mixtures or bilayer films.

Investigation of the synthesis, activation, and isosteric heats of CO2 adsorption of the isostructural series of metal–organic frameworks M3(BTC)2 (M = Cr, Fe, Ni, Cu, Mo, Ru)

Co-reporter:Casey R. Wade and Mircea Dincă

Dalton Transactions 2012 - vol. 41(Issue 26) pp:NaN7938-7938

Publication Date(Web):2012/04/26

DOI:10.1039/C2DT30372H

The synthesis, activation, and heats of CO2 adsorption for the known members of the M3(BTC)2 (HKUST-1) isostructural series (M = Cr, Fe, Ni, Zn, Ni, Cu, Mo) were investigated to gain insight into the impact of CO2–metal interactions for CO2 storage/separation applications. With the use of modified syntheses and activation procedures, improved BET surface areas were obtained for M = Ni, Mo, and Ru. The zero-coverage isosteric heats of CO2 adsorption were measured for the Cu, Cr, Ni, Mo, and Ru analogues and gave values consistent with those reported for MOFs containing coordinatively unsaturated metal sites, but lower than for amine functionalized materials. Notably, the Ni and Ru congeners exhibited the highest CO2 affinities in the studied series. These behaviors were attributed to the presence of residual guest molecules in the case of Ni3(BTC)2(Me2NH)2(H2O) and the increased charge of the dimetal secondary building unit in [Ru3(BTC)2][BTC]0.5.

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