Leveraging Diffusion Kinetics to Reverse Propane/Propylene Adsorption in Zeolitic Imidazolate Framework-8.

The separation challenge posed by propylene/propane mixtures arises from their nearly identical molecular sizes and physicochemical properties. Metal-organic frameworks (MOFs) have demonstrated potential in addressing this challenge through the precision tailoring of pore sizes and surface chemistry. However, introducing modifications at the molecular level remains a considerable hurdle. This work presents an approach to reversibly tune the propylene/propane adsorption preference in zeolitic imidazolate framework-8 (ZIF-8) by manipulating the particle size and gas flow rate. Systematically increasing the ZIF-8 crystals from 9 to 224 µm restricts propane diffusion, thereby reversing its preferential adsorption over propylene. Furthermore, raising the gas flow rate of mixed propylene/propane shifts the rate-determining breakthrough step from thermodynamic equilibrium to kinetics, again reversing the adsorption preference in a particular ZIF-8 sample. We propose "dynamic selectivity (Sd(t))" as a concept that incorporates both thermodynamic and kinetic factors to elucidate these unexpected findings. Moreover, the driving force equation, grounded on the concept of Sd(t), has improved the precision and stability of the computational simulation for fixed-bed adsorption processes. This work underscores the potential of diffusion-based modulation, implemented through manageable external changes, as a viable strategy to optimize separation performance in porous adsorbent materials.

Customizing Metal-Organic Frameworks by Lego-Brick Strategy for One-Step Purification of Ethylene from a Quaternary Gas Mixture.

One-step purification of ethylene (C2 H4 ) from a quaternary gas mixture of C2 H6 /C2 H4 /C2 H2 /CO2 by adsorption is a promising separation process, yet developing adsorbents that synergistically capture various gas impurities remains challenging. Herein, a Lego-brick strategy is proposed to customize pore chemistry in a unified framework material. The ethane-selective MOF platform is further modified with customized binding sites to specifically adsorb acetylene and carbon dioxide, thus one-step purification of C2 H4 with high productivity of polymer-grade product (134 mol kg-1 ) is achieved on the assembly of porous coordination polymer-2,5-furandicarboxylic acid (PCP-FDCA) and PCP-5-aminoisophthalic acid (IPA-NH2 ). Computational studies verify that the low-polarity surface of this MOFs-based platform provides a delicate environment for C2 H6 recognition, and the specific binding sites (FDCA and IPA-NH2 ) exhibit favorable trapping of C2 H2 and CO2 via CHδ+ ···Oδ- and Cδ+ ···Nδ- electrostatic interactions, respectively. The proposed Lego-brick strategy to customize binding sites within the MOFs structure provides new ideas for the design of adsorbents for compounded separation tasks.

Modulating the Structures and Magnetic Properties of Dy(III) Single-Molecule Magnets through Acid-Base Regulation.

Chemical modulation on the structures and physical properties of the coordination complexes is of great interest for the preparation of new functional materials. By changing the acidity or basicity of the reaction medium, the deprotonation degree of a multidentate ligand with multiple active protons, H4daps (H4daps = N',N'″-((1E,1'E)-pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(2-hydroxybenzohydrazide)), can be regulated on purpose. With this ligand of different deprotonation and charges, three new DyIII complexes ([Dy(H3daps)(CH3COO)2(EtOH)]·CH3COOH (1Dy), [Dy2(H2daps)2(EtOH)2(H2O)2(MeOH)2](CF3SO3)2·(H2O)2 (2Dy), and [Dy3(H1daps)2(H2daps)(µ3-OH)(EtOH)(H2O)] (3Dy)) of different nuclearities (mono-, di-, and trinuclear for 1Dy to 3Dy, respectively) have been synthesized and characterized structurally and magnetically. Analyses on the related bond lengths and resulting hydrogen bond modes in the complexes provide the details of the deprotonation position and the charge of the ligands, which can be in the form of H3daps-, H2daps2-, and H1daps3-. Interestingly, the more deprotonated ligand can act as a bridging ligand between the DyIII centers using the phenol and/or carbonyl oxygen atoms, which leads to the multinuclear structures. Magnetic studies on these complexes revealed that complex 1Dy is a field-induced single-molecule magnet (SMM), while complexes 2Dy and 3Dy show SMM behavior under a zero dc field.

Macrocycle supported dimetallic lanthanide complexes with slow magnetic relaxation in Dy2 analogues.

Six dimetallic lanthanide complexes, [Ln2(L')(acac)4] (1Dy-3Gd) (Ln = Dy (1Dy), Tb (2Tb) and Gd (3Gd)) and [Ln2(L')(tfac)4] (4Dy-6Gd) (Ln = Dy (4Dy), Tb (5Tb) and Gd (6Gd)) (H2L' = 1,9-dichloro-3,7,11,15-tetraaza-1,9(1,3)-dibenzenacyclohexadecaphane-2,10-diene-1,9-diol), have been synthesized by the reaction of lanthanide nitrates with the HL ligand in the presence of acetylacetonate (acac) (or trifluoroacetylacetonate (tfac) and triethylamine (HL = 4-chloro-2,6-bis(-((3-((3-(dimethylamino)propyl)amino)propyl)imino)methyl)phenol). Ln-Assisted modification of the Schiff base HL occurred and led to the formation of a new macrocyclic ligand (H2L'). X-ray crystallographic analysis revealed that the LnIII ions of complexes 1Dy-6Gd are all eight-coordinated in a square antiprismatic geometry with D4d local symmetry. Magnetic measurements of these complexes revealed that 1Dy and 4Dy show single-molecule magnet behaviour with energy barriers of 66.7 and 79.0 K, respectively, under a zero direct magnetic field. The orientations of the magnetic axes and crystal field parameters were obtained from theoretical calculations and an electrostatic model. The magneto-structural correlations of SMMs 1Dy and 4Dy are further discussed in detail.

Reversible On-Off Switching of the Hysteretic Spin Crossover in a Cobalt(II) Complex via Crystal to Crystal Transformation.

Reversible controlling and switching of magnetic bistability remains relatively difficult. Here, reversible on-off switching of a hysteretic spin transition in a CoII complex via a single-crystal to single-crystal (SC-SC) transformation during dehydration and rehydration was reported. Upon dehydration, a switching from a basically low spin state to an abrupt and hysteretic spin crossover (SCO) with broad hysteresis loops was achieved. Hysteretic and anisotropic crystal lattice expansion or contraction in the spin transition temperature range was also observed in the dehydrated complex. The magneto-structural relationship in this system was established on the basis of detailed structure analyses on both the hydrated and dehydrated examples over a wide range of temperatures. The elimination of guest internal pressure, the tuning of the supramolecular interactions, and the strong electron-lattice coupling should be responsible for the hysteretic SCO in the dehydrated complex.

A family of lanthanide complexes with a bis-tridentate nitronyl nitroxide radical: syntheses, structures and magnetic properties.

Eleven new lanthanide complexes based on a bis-tridentate nitronyl nitroxide radical NIT-Pm2Py (2-(4,6-di(pyridin-2-yl)pyrimidin-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide), namely (NIT-Pm2Py)Ln(hfac)3 (Ln = Gd (1Gd), Tb (2Tb), Dy (3Dy), Ho (4Ho), Er (5Er), Yb (6Yb)), [(NIT-Pm2Py)Ln2(hfac)6]·xH2O (Ln = Gd (7Gd), Tb (8Tb), Ho (10Ho), x = 0.5 for 7Gd and 1 for 8Tb and 10Ho) and (NIT-Pm2Py)Ln2(hfac)6 (Ln = Dy (9Dy), Er (11Er)) were prepared and characterized. These complexes can be selectively prepared by controlling the reaction ratio of Ln(hfac)3·2H2O to the radical ligand NIT-Pm2Py. Single crystal X-ray crystallographic analyses confirmed that 1Gd-6Yb are isostructural 2p-4f LnIII-radical complexes, in which the NIT-Pm2Py radical acts as a terminal tridentate ligand chelating to one LnIII ion. On the other hand, 7Gd-11Er are isostructural 4f-2p-4f LnIII-radical-LnIII complexes with the NIT-Pm2Py acting as a bridging ligand between two LnIII ions. 7Gd-11Er represent a rare family of complexes showing the NIT bridged 4f-2p-4f three-spin motif. Alternating-current (ac) magnetic susceptibility investigations revealed that complex 6Yb exhibits field-induced frequency dependence, suggesting a possible field-induced single-molecule magnet behavior. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these complexes indicates weak antiferromagnetic coupling between the LnIII and NIT radical.

Two three-dimensional [MoIII(CN)7]4--based magnets showing new topologies and ferrimagnetic ordering below 80 K.

The rational design and synthesis of heptacyanomolybdate-based magnets remain a challenge due to the complexity of this system. Here, we reported the crystal structures and magnetic properties of two three-dimensional (3D) frameworks prepared from the self-assembly of the [MoIII(CN)7]4- unit with MnII ions in the presence of different amide ligands, namely Mn2(DMF)(H2O)2[Mo(CN)7]·H2O·CH3OH (1) and Mn2(DEF)(H2O)[Mo(CN)7] (2) (DMF = N,N'-dimethylformamide and DEF = N,N'-diethylformamide). Single-crystal structure determinations showed that compound 1 crystallizes in the triclinic space group Pi, while 2 crystallizes in the monoclinic space group P21/n. The difference in the structures of 1 and 2 is the coordination mode of the amide molecules: while the DMF molecules in 1 are only terminal ligands, the DEF molecules in 2 act as bridging ligands between two MnII centers. Although their space groups and local coordination environments of the metal centers are of some difference, both compounds have similar extended 3D frameworks where the spin centers are bridged by both the CN- and µ2-O bridges. They both have a three-nodal 4, 4, 7-connecting topological net with the vertex symbol of {43·53}{44·52}{47·54·66·74} for 1, and {43·53}{44·52}{47·54·67·73} for 2, respectively. Magnetic measurements revealed that both 1 and 2 exhibit ferrimagnetic ordering below 80 K together with another anomaly at about 45 K probably owing to spin reorientation. Besides, spin frustration and non-linear alignment of the magnetic moments are also possible due to competitive antiferromagnetic interactions between the spin carriers. These compounds expanded the family of MnII-[MoIII(CN)7]4- magnets with high magnetic ordering temperatures.

Spin crossover in hydrogen-bonded frameworks of FeII complexes with organodisulfonate anions.

We present here the syntheses, crystal structures, and thermal and magnetic properties of a series of mononuclear Fe2+ spin crossover (SCO) complexes of the formula [Fe(bamp)2]·Anion·Solv (Anion = NDS2-, Solv = 2H2O, 1NDS; Anion = BPDS2-, Solv = 4.4H2O, 2BPDS; Anion = ABDS2-, Solv = Et2O and H2O, 3ABDS; Anion = DNDS2-, Solv = MeCN, 4DNDS; bamp = 2,6-pyridinedimethanamine, H2NDS = 1,5-naphthalenedisulphonic acid, H2BPDS = 4,4'-biphenyldisulphonic acid, H2ABDS = 4,4'-azobenzenedisulfonic acid, H2DNDS = 4,4'-dinitrostilbene-2,2'-disulfonic acid). The structures and SCO properties of these complexes can be finely modified by organodisulfonate couteranions. Single-crystal X-ray analyses revealed that all these compounds are hydrogen-bonded three-dimensional frameworks constructed from the charge-assisted hydrogen bonds between bamp donors, organodisulfonate acceptors, and/or crystallized solvent molecules. SCO behavior was observed in all four complexes and has been evidenced by detailed structural and magnetic investigations. While 1NDS exhibits a sharp cooperative SCO transition with a transition temperature T1/2 of 247 K, 2BPDS, 3ABDS, and 4DNDS undergo more gradual SCO transitions with T1/2 values of 176, 171 and 158 K, respectively. Magneto-structural relationship studies revealed that the tunable SCO properties, including the trend of the transition temperatures and the cooperativity of the SCO transition, are mainly attributable to the size of the organodisulfonate anions. This study shows that in order to exhibit cooperative SCO properties, "efficient" hydrogen bonds directly connecting the SCO centers, rather than those between the SCO centers and the innocent neighboring groups, are preferred.

Syntheses and magnetic properties of a bis-tridentate nitronyl nitroxide radical and its metal complexes.

The first bis-tridentate nitronyl nitroxide mono-radical was synthesized. From this ligand and a bis-bidentate nitronyl nitroxide ligand, metal complexes of MnII and CoII ions were also synthesized and characterized. Field-induced slow magnetic relaxation was observed in the radical-bridged CoII complex.

Heterometallic MIILnIII (M = Co/Zn; Ln = Dy/Y) Complexes with Pentagonal Bipyramidal 3d Centers: Syntheses, Structures, and Magnetic Properties.

We herein reported the syntheses, structures, and magnetic properties of three dinuclear heterometallic MIILnIII complexes, namely, [MIILnIII(H2L)(CH3OH)2(NO3)2](NO3)·S (M = Co, Ln = Dy, S = MeOH (1CoDy); M = Zn, Ln = Dy, S = MeOH (2ZnDy); M = Co, Ln = Y, S = MeNO2 (3CoY), H4L = 2,6-diacetylpyridine bis[2-(semicarbazono) propionylhydrazone]. Synthesized from the predesigned multidentate ligand H4L, which has two different coordination pockets (smaller N3O2 and larger N2O4 pockets) suitable for either a 3d or a 4f metal center, all these complexes have very similar structures, where the MII centers possess a pentagonal bipyramidal (PBP) geometry and the LnIII sites have a tetradecahedron geometry. Magnetic measurements on these compounds revealed the existence of weak ferromagnetic coupling between the Co2+ and Dy3+ centers and the field-induced slow magnetic relaxation of all three complexes. Furthermore, theoretical calculation on all these complexes indicates that although the change of the diamagnetic Zn2+ ion to the paramagnetic Co2+ ion only slightly modifies the local magnetic anisotropy of the Dy3+ ion, the weak Co-Dy magnetic interaction decreases the energy barrier. These compounds are the first systematic results of a heterometallic 3d-4f single-molecule magnet containing predesigned PBP 3d metal ions.

Syntheses, structures, and magnetic properties of three new MnII-[MoIII(CN)7]4- molecular magnets.

By reaction of K4[MoIII(CN)7]·2H2O, Mn(ClO4)2·6H2O and bidentate chelating ligands, three new cyano-bridged compounds, namely Mn2(3-pypz)(H2O)(CH3CN)[Mo(CN)7] (1), Mn2(1-pypz)(H2O)(CH3CN)[Mo(CN)7] (2) and Mn2(pyim)(H2O)(CH3CN)[Mo(CN)7] (3) (3-pypz = 2-(1H-pyrazol-3-yl)pyridine, 1-pypz = 2-(1H-pyrazol-1-yl)pyridine, pyim = 2-(1H-imidazol-2-yl)pyridine), have been synthesized and characterized structurally and magnetically. Single crystal X-ray analyses revealed that although the chelating ligands are different, compounds 1 to 3 are isomorphous and crystallize in the same monoclinic space group C2/m. Connected by the bridging cyano groups, one crystallographically unique [Mo(CN)7]4- unit and three crystallographically unique MnII ions of different coordination environments form similar three-dimensional frameworks, which have a four-nodal 3,4,4,7-connecting topological net with a vertex symbol of {43}{44·62}2{410·611}. Magnetic measurements revealed that compounds 1-3 display long-range magnetic ordering with critical temperatures of 64, 66 and 62 K, respectively. These compounds are rare examples of a small number of chelating co-ligand coordinated [Mo(CN)7]4--based magnetic materials. Specifically, the bidentate chelating ligands were successfully introduced into the heptacyanomolybdate system for the first time.

Single-ion magnetism in seven-coordinate YbIII complexes with distorted D5h coordination geometry.

Two seven-coordinate compounds with pentagonal bipyramidal YbIII centers, namely, [Yb(H3Bmshp)(DMF)2Cl2]·DMF·1.5H2O (1) and [Yb(H3Bmshp)(DMF)2Cl2]·H4Bmshp (2) (H4Bmshp = (2,6-bis[(3-methoxysalicylidene)hydrazinecarbonyl]-pyridine)) were synthesized by changing the molar ratio of reactants in DMF. The structures of compounds 1 and 2 are very similar, except for the existence of different lattice molecules: one DMF and one and a half water molecules in 1, and one neutral uncoordinated ligand in 2. The coordination geometries of both the pentagonal bipyramidal YbIII centers (YbCl2N1O4) in compounds 1 and 2 are also very similar with only slight differences. Magnetic data analyses revealed that the subtle structure variations result in remarkable different slow magnetic relaxation properties of compounds 1 and 2. To further understand their magnetic behaviors, ab initio calculations were performed for both compounds 1 and 2. The calculated results indicate that the magnetic anisotropies of compounds 1 and 2 are significantly different: easy-plane magnetic anisotropy for 1 and easy-axis magnetic anisotropy for 2. To the best of our knowledge, these compounds are the first YbIII-based SIMs of pentagonal bipyramidal geometry.

Reversible On-Off Switching of a Single-Molecule Magnet via a Crystal-to-Crystal Chemical Transformation.

Dynamic molecular crystals are of high interest due to their potential applications. Herein we report the reversible on-off switching of single-molecule magnet (SMM) behavior in a [Mo(CN)7]4- based molecular compound. Upon dehydration and rehydration, the trinuclear Mn2Mo molecule [Mn(L)(H2O)]2[Mo(CN)7]·2H2O (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear Mn4Mo2 compound [Mn(L)(H2O)]2[Mn(L)]2[Mo(CN)7]2 (2). This structural transformation involves the breaking and reforming of coordination bonds which leads to significant changes in the color and magnetic properties. Compound 1 is an SMM with an energy barrier of 44.9 cm-1, whereas 2 behaves as a simple paramagnet despite its higher ground state spin value. The distortion of the pentagonal bipyramidal geometry of [Mo(CN)7]4- in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.

Two-dimensional frameworks formed by pentagonal bipyramidal cobalt(ii) ions and hexacyanometallates: antiferromagnetic ordering, metamagnetism and slow magnetic relaxation.

We herein report the syntheses, structures, and magnetic properties of two isostructural two-dimensional (2D) coordination polymers based on a pentagonal bipyramidal CoII unit [Co(TODA)]2+ and two hexacyanometallates, namely [MIII(CN)6]2[CoII(TODA)]3·9H2O (M = Cr (1), Co (2), TODA = 1,4,10-trioxa-7,13-diazacyclopentadecane). Structure analyses show that both complexes have 2D honeycomb structures where the [Co(TODA)]2+ units are bridged by the [MIII(CN)6]3- groups through three cyano groups in the facial positions. Magnetic investigation reveals ferromagnetic coupling between the CrIII and CoII centres through cyanides in 1. Due to the antiferromagnetic interaction between the layers, compound 1 exhibits an antiferromagnetic ordering below 11.4 K, and shows a metamagnetic phase transition under an external dc field. Due to the disorder of the TODA ligands, compound 1 shows a spin glass behavior, which leads to slow magnetic relaxation in 1. A butterfly-shaped hysteresis loop at 1.8 K can be observed with a coercive field of 720 Oe, which is quite large for cyano-bridged Cr-Co molecular magnets. For compound 2 containing the diamagnetic [CoIII(CN)6]3- unit, field-induced slow magnetic relaxation was also verified, which makes compound 2 a rare example of an SIM assembled in a 2D network. An easy-plane magnetic anisotropy with a positive D value (29.9 cm-1 by PHI and 26.5 cm-1 by Anisofit2.0) was deduced for hepta-coordinated CoII centers. These results show the efficiency of the strategy of combining cyanometallates and pentagonal bipyramidal precursors for novel molecular magnetic materials.

Room-temperature switching of magnetic hysteresis by reversible single-crystal-to-single-crystal solvent exchange in imidazole-inspired Fe(ii) complexes.

The recent upsurge in molecular magnetism reflects its application in the areas of sensors and molecular switches. Thermal hysteresis is crucial to the molecular bistability and information storage, a wide hysteresis near room temperature is expected to be of practical sense for the molecular compound. In this work, spin crossover iron(ii) complexes [Fe(Liq)2](BF4)2·(CH3CH2)2O (1-Et2O) and [Fe(Liq)2](BF4)2·3H2O (1-3H2O) were prepared and structurally and magnetically analysed. The single-crystal-to-single-crystal (SCSC) solvation transformation and the influence on the crystal structures and magnetic hysteresis were investigated in an etherification-hydration cycle. At room temperature, X-ray diffraction experiments indicated a transformation from one crystal (1-Et2O, P21212) to another crystal (1-3H2O, P212121) upon humidity exposure and reversible recovery of its crystallinity upon exposure to ether vapor. The etherified phase 1-Et2O exhibits room temperature spin crossover (T1/2 = 305 K) but negligible thermal hysteresis, however the hydrated phase 1-3H2O exhibits the apparent hysteresis loop (T1/2↑ = 346 K, T1/2↓ = 326 K) which expands to room temperature. This effect is associated with the change of intermolecular cooperativity in the etherification-hydration recyclability.

Metallogrid Single-Molecule Magnet: Solvent-Induced Nuclearity Transformation and Magnetic Hysteresis at 16 K.

Structural assembly and reversible transformation between a metallogrid Dy4 SMM (2) and its fragment Dy2 (1) were established in the different solvent media. The zero-field magnetization relaxation was slowed for dysprosium metallogrid (2) with relaxation barrier of Ueff = 61.3 K when compared to Dy2 (1). Both magnetic dilution and application of a moderate magnetic field suppress ground-state quantum tunneling of magnetization and result in an enhanced Ueff of 119.9 and 96.7 K for 2, respectively. Interestingly, the lanthanide metallogrid complex (2) exhibits magnetic hysteresis loop even up to 16 K at a given field sweep rate of 500 Oe/s.

Redox Modulation of Spin Crossover within a Cobalt Metallogrid.

A self-assembled cobalt molecular grid of a pyrazine-bridged bis-tridentate ligand (L(R)), where R = H (1), CH3 (2), and Br (3), was prepared and structurally characterized. Depending on the electronic effects of the substituents on the ligand, the redox of the metal center was systematically modulated, and the magnetic behavior from essentially high-spin Co(II) in 3 versus completely diamagnetic Co(III) in 1 to Co(II) spin-crossover in 2 can be achieved.