Crystal engineering of flexible metal organic materials

Crystal engineering is the field of chemistry that studies the design, properties, and application of crystals. An aspect of crystal engineering is the design of coordination networks formed by the rational combination of metal nodes and organic linker ligands. Porous coordination networks, which are also known as porous coordination polymers (PCPs), Metal Organic Materials (MOMs) or Metal Organic Frameworks (MOFs), have captured the interest of researchers worldwide because of their inherent modularity and amenability to crystal engineering principles. PCPs and porous MOMs have been classified into four generations: 1st generation materials collapse on guest removal and lose crystallinity; 2nd generation materials possess a rigid nature upon guest insertion/removal and exhibit a type I isotherm; 3rd generation materials alter their original structure and maintain overall framework connectivity when exposed to external stimuli such as guest incorporation/removal, pressure and heat; 4th generation materials can be fine-tuned via post synthetic modification (PSM), defects or solid solutions. 3 rd generation materials or porous flexible MOMs, have attracted attention owing to  their potential applications in gas storage, separation, drug delivery and catalysis. These flexible MOMs tend to exhibit ‘stepped’ or ‘S-shaped’ isotherm profiles. Herein, we propose classification of such MOMs based on their gas sorption isotherm profiles as follows, i) type F-I (gradual change from open to more open, ii) type F-II (sudden change from open to more open sudden), iii) type F-III (gradual change from closed to open gradual), iv) type F-IV(sudden change from closed to open) and v) type F-V (shape-memory effect). This thesis also examines the three types of network topologies, primitive cubic unit (pcu), diamondiod (dia) and square lattice (sql) networks. The systematic studies we conducted herein offer design principles for future studies of porous flexible coordination networks in terms of understanding their structural transformations and improving the performance of gas storage/separation.  </p

A dynamic and multi-responsive porous flexible metal–organic material

Stimuli responsive materials (SRMs) respond to environmental changes through chemical and/or structural transformations that can be triggered by interactions at solid-gas or solidliquid interfaces, light, pressure or temperature. SRMs span compositions as diverse as organic polymers and porous inorganic solids such as zeolites. Metal–organic materials (MOMs), sustained by metal nodes and organic linker ligands are of special interest as SRMs. SR-MOMs have thus far tended to exhibit only one type of transformation, e.g. breathing, in response to one stimulus, e.g. pressure change. We report [Zn2(4,4′-biphenyldicarboxylate) 2(4,4′-bis(4-pyridyl)biphenyl)]n, an SR-MOM, which exhibits six distinct phases and four types of structural transformation in response to various stimuli. The observed structural transformations, breathing, structural isomerism, shape memory effect, and change in the level of interpenetration, are previously known individually but have not yet been reported to exist collectively in the same compound. The multi-dynamic nature of this SR-MOM is mainly characterised by using in-situ techniques

Tuning the gate-opening pressure in a switching pcu coordination network, X-pcu-5-Zn, by pillar ligand substitution

Coordination networks that reversibly switch between closed and open phases are of topical interest since their stepped isotherms can offer higher working capacities for gas‐storage applications than the related rigid porous coordination networks. To be of practical utility, the pressures at which switching occurs, the gate‐opening and gate‐closing pressures, must lie between the storage and delivery pressures. Here we study the effect of linker substitution to fine‐tune gate‐opening and gate‐closing pressure. Specifically, three variants of a previously reported pcu‐topology MOF, X‐pcu‐5‐Zn, have been prepared: X‐pcu‐6‐Zn, 6=1,2‐bis(4‐pyridyl)ethane (bpe), X‐pcu‐7‐Zn, 7=1,2‐bis(4‐pyridyl)acetylene (bpa), and X‐pcu‐8‐Zn, 8=4,4′‐azopyridine (apy). Each exhibited switching isotherms but at different gate‐opening pressures. The N2, CO2, C2H2, and C2H4 adsorption isotherms consistently indicated that the most flexible dipyridyl organic linker, 6, afforded lower gate‐opening and gate‐closing pressures. This simple design principle enables a rational control of the switching behavior in adsorbent materials

Enhanced Stability toward Humidity in a Family of Hybrid Ultramicroporous Materials Incorporating Cr₂O₇^2– Pillars

Dichromate (Cr₂O₇^2–) pillared <b>pcu</b> hybrid ultramicroporous materials, while previously shown to exhibit benchmark selectivity for small polarizable gases, sometimes suffer from poor stability when exposed to moisture, which could limit their potential application in gas separation systems. In attempting to improve their stability toward humidity, we have crystal engineered two new families of <b>DICRO-L-M-i</b> materials of formula [M­(L)₂(Cr₂O₇)]_<i>n</i> (M = Ni²⁺, Co²⁺; L = <b>5</b>: 1,4-bis­(4-pyridyl)­xylene; <b>6</b>: 1,4-bis­(4-pyridyl)­durene). Evaluating these materials in combination with a previously reported analogue, <b>DICRO-4-Ni-i</b>, in terms of their stability toward humidity has revealed a relationship between increasing the number of methyl groups on the dipyridyl organic linkers and a greater stability toward humidity

Metal cation substitution can tune CO2, H2O and CH4 switching pressure in transiently porous coordination networks

Compared to rigid physisorbents, switching coordination networks that reversibly transform between closed (non-porous) and open (porous) phases offer promise for gas/vapour storage and separation owing to their improved working capacity and desirable thermal management properties. We recently introduced a coordination network, X-dmp-1-Co, which exhibits switching enabled by transient porosity. The resulting “open” phases are generated at threshold pressures even though they are conventionally non-porous. Herein, we report that X-dmp-1-Co is the parent member of a family of transiently porous coordination networks [X-dmp-1-M] (M = Co, Zn and Cd) and that each exhibits transient porosity but switching events occur at different threshold pressures for CO2 (0.8, 2.1 and 15 mbar, for Co, Zn and Cd, respectively, at 195 K), H2O (10, 70 and 75% RH, for Co, Zn and Cd, respectively, at 300 K) and CH4 (</p

Flexible coordination network exhibiting water vapor−induced reversible switching between closed and open phases

That physisorbents can reduce the energy footprint of water vapor capture and release has attracted interest because of potential applications such as moisture harvesting, dehumidification, and heat pumps. In this context, sorbents exhibiting an S-shaped single-step water sorption isotherm are desirable, most of which are structurally rigid sorbents that undergo pore-filling at low relative humidity (RH), ideally below 30% RH. Here, we report that a new flexible one-dimensional (1D) coordination network, [Cu(HQS)(TMBP)] (H2HQS = 8-hydroxyquinoline-5-sulfonic acid and TMBP = 4,4′-trimethylenedipyridine), exhibits at least five phases: two as-synthesized open phases, α ⊃ H2O and β ⊃ MeOH; an activated closed phase (γ); CO2 (δ ⊃ CO2) and C2H2 (ϵ ⊃ C2H2) loaded phases. The γ phase underwent a reversible structural transformation to α ⊃ H2O with a stepped sorption profile (Type F-IV) when exposed to water vapor at 100 cycles and only mild heating (<323 K) is required for regeneration. Unexpectedly, the kinetics of loading and unloading of [Cu(HQS)(TMBP)] compares favorably with well-studied rigid water sorbents such as Al-fumarate, MOF-303, and CAU-10-H. Furthermore, a polymer composite of [Cu(HQS)(TMBP)] was prepared and its water sorption retained its stepped profile and uptake capacity over multiple cycles.</p

A square lattice topology coordination network that exhibits highly selective C2H2/CO2 separation performance

C2H2/CO2 separation is an industrially important process that remains challenging because of the similar physicochemical properties of C2H2 and CO2. We herein report that the new square lattice (sql) coordination network [Cu (bipy‐xylene)2(NO3)2]n, sql‐16‐Cu‐NO3, 16 = bipy‐xylene = 4,4′‐(2,5‐dimethyl‐ 1,4‐phenylene)dipyridine, exists in at least three forms, as‐synthesised (α), activated (α′) and hydrated (β). The activated phase, sql‐16‐Cu‐NO3‐α′, is an ultra microporous material that exhibits high selectivity towards C2H2 over CO2 as revealed by dynamic gas breakthrough experiments (1:1, C2H2/CO2) that afforded 99.87% pure CO2 in the effluent stream. The separation selectivity at 298 K and 1 bar, 78, is the third best value yet reported for C2H2 selective physisorbents whereas the mid‐loading performance sets a new benchmark. The performance of sql‐16‐Cu‐NO3‐α′ is attributed to a new type of C2H2 binding site in which CH···ONO2 interactions enable moderately strong sorbent‐sorbate binding (Qst (C2H2) = 38.6 kJ/mol) at low loading. Conversely, weak CO2 binding (Qst (CO2) = 25.6 kJ/mol) at low loading means that (ΔQst)AC [Qst (C2H2)–Qst (CO2)] is 13 kJ/mol at low coverage and 11.4 kJ/mol at mid‐loading. Analysis of in situ powder X‐ray diffraction and modelling experiments provide insight into the sorption properties and high C2H2/CO2 separation performance of sql‐16‐Cu‐NO3‐α′

A coordination network that reversibly switches between two nonporous polymorphs and a high surface area porous phase

We report a 2-fold interpenetrated primitive cubic (pcu) network X-pcu-5-Zn, [Zn2(DMTDC)2(dpe)] (H2DMTDC = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid, dpe = 1,2-di(4-pyridyl)ethylene), that exhibits reversible switching between an as-synthesized “open” phase, X-pcu-5-Zn-α, and two nonporous or “closed” polymorphs, X-pcu-5-Zn-β and X-pcu-5-Zn-γ. There are two unusual features of X-pcu-5-Zn. The first relates to its sorption properties, which reveal that the α form exhibits high CO2 uptake (ca. 255 cm3/g at 195 K) via reversible closed-to-open switching (type F-IV isotherm) of the type desirable for gas and vapor storage; there are only three other reports of porous materials that combine these two features. Second, we could only isolate the β form by activation of the CO2 loaded α form and it persists through multiple CO2 adsorption/desorption cycles. We are unaware of a new polymorph having been isolated in such a manner. That the observed phase changes of X-pcu-5-Zn-α occur in single-crystal-to-single-crystal fashion enabled structural characterization of the three forms; γ is a coordination isomer of α and β, both of which are based upon “paddlewheel” clusters

Recyclable switching between nonporous and porous phases of a square lattice (sql) topology coordination network

A nonporous square lattice (sql) coordination network [Co(bipy)2(NCS)2]n (sql-1-Co-NCS) exhibits recyclable switching induced by CO2. The sorption isotherms are stepped with moderate hysteresis, temperature controlled and saturation uptake is fixed. Such switching, which has rarely been observed, offers the promise of exceptional working capacity for gas storage

Supporting Information for "Comparing the Structures and Photophysical Properties of Two Charge Transfer Co-crystals"

There are two main folders in this directory, named NpeTCNB and NpeTCNQ.Each folder of these two has three folders named Ground_State_Calculations, TDDFT_Calculations, and PBC_Calculation. These folders has the structures that are reported in the manuscript.The naming convention used is as follows:1) The conformers name start with Str and with a number, ex: Str1, Str2, etc...2) The functional is also included in the conformer name, ex: Str1-B3LYP, Str1-CAM-B3LYP3) The basis sets are labeled as:a) B0 for 6-31G(d,p)b) B1 for 6-31+G(d,p)c) B2 for 6-311+G(d,p)4) When empirical dispersion is used, the file name has "GD3" in the name, ex: Str1-CAM-B3LYP-B0-GD3.log</p