Bao-Ying Wang

Turning on the flexibility of isoreticular porous coordination frameworks for drastically tunable framework breathing and thermal expansion

To study the potential flexibility of the (3,9)-connected xmz frameworks, a series of isoreticular metal carboxylate frameworks [M3(μ3-OH)(L)3], namely MCF-18(L,M), were constructed by 3-connected, tripodal pyridyl-dicarboxylate ligands (H2L1 = pyridine-3,5-dicarboxylic acid; H2L2 = 4,4′-(pyridine-3,5-diyl)dibenzoic acid; H2L3 = 2,6-di-p-carboxyphenyl-4,4′-bipyridine) and 9-connected, tricapped trigonal-prismatic M3(μ3-OH)(O2CR)6(py)3 (M = Fe, Co, Ni; py = pyridyl group) clusters. Powder and single-crystal X-ray diffraction studies showed that, while other isoreticular analogs do not show framework flexibility, the newly designed material MCF-18(L3,Ni) can drastically swell 70–105% in volume and 75–121% in length upon inclusion of different guests, the latter of which is the highest reported value to date. Comparison study showed that the nearly uniaxial framework breathing is generated by the special regulation effect of the xmz topology, but can only be activated by a ligand with a suitable shape. Moreover, the thermal expansion profile of MCF-18(L3,Ni) can be drastically tuned by guest, showing extremely large thermal expansion coefficients up to 430 × 10−6 K−1. The guest-included crystals also show water-like thermal expansion behaviors depending on the cooling rate, which is unanticipated for intrinsic crystalline materials.

Chemical/Physical Pressure Tunable Spin-Transition Temperature and Hysteresis in a Two-Step Spin Crossover Porous Coordination Framework

A two-dimensional (2D) square-grid type porous coordination polymer [Fe(bdpt)(2)]·guest (1·g, Hbdpt = 3-(5-bromo-2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole) with isolated small cavities was designed and constructed as a spin-crossover (SCO) material based on octahedral Fe(II)N(6) units and an all-nitrogen ligand. Three guest-inclusion forms were successfully prepared for 1·g (1·EtOH for g = ethanol, 1·MeOH for g = methanol, 1 for g = Null), in which the guest molecules interact with the framework as hydrogen-bonding donors. Magnetic susceptibility measurements showed that 1·g exhibited two-step SCO behavior with different transition temperatures (1·EtOH < 1·MeOH < 1) and hysteresis widths (1·EtOH > 1·MeOH > 1 ≈ 0). Such guest modulation of two-step spin crossover temperature and hysteresis without changing two-step state in a porous coordination framework is unprecedented. X-ray single-crystal structural analyses revealed that all two-step SCO processes were accompanied with interesting symmetry-breaking phase transitions from space group of P2(1)/n for all high-spin Fe(II), to P1 for ordered half high-spin and half low-spin Fe(II), and back to P2(1)/n for all low-spin Fe(II) again by lowering temperature. The different SCO behaviors of 1·g were elucidated by the steric mechanism and guest-host hydrogen-bonding interactions. The SCO behavior of 1·g can be also controlled by external physical pressure.

A zigzag DyIII4 cluster exhibiting single-molecule magnet, ferroelectric and white-light emitting properties

A novel multifunctional complex [Dy4(OH)2(bpt)4(NO3)6(EtOH)2]·EtOH (1, Hbpt = 3,5-bis(pyridin-2-yl)-1,2,4-trizolate) was designed, based on the self-assembly under hydrothermal conditions, and it displays properties of single-molecule magnets (SMMs), white-light emission and ferroelectricity. Studies of static and dynamic magnetism reveal the SMMs behaviour of 1 with energy barrier Ueff = 43.22(8) K. Furthermore, the strategy of a dichromatic mixture of blue (bpt−) and yellow (Dy3+) emissive colours was utilized to obtain white-light emission through modulation of the excitation wavelength. This strategy has been worked up to create lanthanide-based fluorescent molecular magnets recently. Moreover, ferroelectricity, a “plus” feature, was added to the cluster as a result of the polar point group C2v.

Visualizing the distinctly different crystal-to-crystal structural dynamism and sorption behavior of interpenetration-direction isomeric coordination networks

We show here that the interpenetration direction can be more important than the interpenetration number for the porosity, stability, framework flexibility and sorption behaviors of porous coordination frameworks. Solvothermal reactions of Zn(NO3)2 and a shape-asymmetric ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) in different solvents/templates gave three isomeric frameworks [Zn(Hmpba)2] (1, 2, 3) possessing the same polar dia networks and 4-fold interpenetration. However, their polar nets adopt parallel, orthogonal, and anti-parallel arrangements, respectively, giving very similar voids but totally different pore shapes for 1 and 2, and a nonporous structure for 3. Thermogravimetry, powder X-ray diffraction, and sorption analyses revealed remarkably different framework flexibilities and multi-step gas sorption behavior, in which 1 selectively adsorbs CO2 over N2, 2 adsorbs both CO2 and N2, while 3 adsorbs neither CO2 nor N2. Although these compounds cannot retain their single-crystallinity after structural transformations and their complicated structures hinder conventional structural characterization techniques, we successfully combined first-principle calculations and computational simulations to solve the crystal structures of their activated phases, and developed a dynamic computational simulation method combining sorption simulated annealing, molecular mechanics and grand canonical Monte Carlo modelling to perfectly simulate the CO2 adsorption/desorption isotherms and visualize the accompanying continuous/abrupt structural transformations, demonstrating the important role of interpenetration-direction isomerism in functional porous materials.

Unprecedented binodal (7,9)-connected network based on distinct tricobalt(II) clusters: structure, topology and cooperative magnetism

A new Co(II)-based coordination polymer, [{Co3(ina)7.5(μ3–O)}{Co3(ina)2.5(μ3–OH)(μ–OH)(C2H5OH)(H2O)2}3]NO3·3C2H5OH·24H2O (1, also denoted as MCF-39), has been synthesized from the solvothermal reaction of cobalt(II) nitrate and the ligand isonicotinic acid (Hina) with the presence of hmq (2-(hydroxymethyl)-quinolin-8-ol) as a necessary additive. The crystal structure of 1 consists of the 7- and 9-connected trinuclear clusters, and exhibits the first highly-connected binodal network with (34·49·56·62)3(36·415·515) topology. Magnetic studies show that the spin canting and spin-glass-like behaviours were found in 1, and the cooperative effect among intra- and inter-trimer could greatly influence the bulk magnetic behaviour of the three-dimensional (3D) framework.

Magnetic variation induced by structural transformation from coordination chains to layers upon dehydration

The reaction of CoBr(2), 1,2-di(4H-1,2,4-triazol-4-yl)diazene (bta) and KSCN yielded a one-dimensional coordination polymer [Co(SCN)(2)(bta)(H(2)O)(2)] with water molecules coordinated to the metal ions. After dehydration at 100 °C, the compound transformed into a layered coordination polymer [Co(SCN)(2)(bta)], whose structure was determined by powder X-ray diffraction based on the single-crystal structure of another layered coordination polymer [Cd(SCN)(2)(bta)]. Interestingly, a magnetic variation from a simple paramagnet to an antiferromagnetic ordered phase of a single-chain-magnet that exhibits both metamagnetic behaviour and slow magnetic relaxation was observed upon the dehydration process.

Importing spontaneous polarization into a Heisenberg ferromagnet for a potential single-phase multiferroic

A novel two-dimensional Heisenberg ferromagnet with a flexible organic layer, [C6H5(CH2)4NH3]2[CuCl4], undergoes ferroelastic, ferroelectric and ferromagnetic transitions at 337, 143 and 7.8 K, respectively. The ferromagnet was characterized by variable-temperature thermal measurements, in situ single-crystal X-ray structure analyses, anisotropic dielectric measurements, and pyroelectric and magnetic analyses. Differing from the other layered perovskite-like compounds, this compound revealed an extraordinary distorted polar structure with five times the cell volume at its polar phase because of the flexibility of the organic layer. Hence, this presents an approach to introduce spontaneous electric polarization into the Heisenberg ferromagnet for single-phase multiferroics. Moreover, due to the cooperation between the frozen [C6H5(CH2)4NH3]+ cations and the tilting of the CuCl6 octahedra, this potential multiferroic compound simultaneously exhibits dielectric and magnetic bistabilities during both the ferroelastic and ferroelectric transitions.

Two new polar coordination polymers with diamond networks: interpenetration and thermal phase transition

Two new polar coordination polymers, [Cu(Hmpc)2] (1) and [Cu2(mpc)2(DMA)] (2), were synthesized by solvothermal reactions of a ligand 3,5-dimethyl-1H-pyrazole-4-carboxylic acid (H2mpc) with Cu(NO3)2. Single-crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic, polar space group Fdd2. Each square-planar coordinated Cu2+ ion is linked to four adjacent Cu2+ ions by mono-deprotonated Hmpc− ligands, resulting in a severely distorted 4-connected diamond network, inside which the methyl groups of the ligands are densely packed. In 2, non-centrosymmetric binuclear Cu2+ units serve as 4-connecting nodes, forming a non-porous polar structure consisting of 2-fold-interpenetrated diamond networks. A thermal structural phase transition was detected in 2 at around 323 K. At 298 K, 2 crystallizes in the tetragonal chiral space group P41 (2α), with a = 10.9931 (6) A, c = 18.0850(9) A, and Z = 4, whereas at a higher temperature (438 K) it is transferred to the tetragonal non-centrosymmetric space group P42nm (2β), with a = 10.9857(5) A, c = 9.1031(9) A, and Z = 2. By repeating in situ single-crystal X-ray diffraction, this phase transition was found to be reversible between a non-twinning non-centrosymmetric crystal (2β) and a racemic-twinning chiral crystal (2α). The symmetry of the diamond network was increased from C1 to D2d in the heating process, so that two fold interpenetrated networks become strictly parallel along the c-axis at about 323 K, and the c-axis was halved in 2β. At the same time, 2-fold disordered coordinated DMA molecules in 2α were found to be 8-fold disordered in 2β. According to aizu notation, 2 can be classified as 4mmF4 species, which are potentially secondary ferroic.

Thermal-induced reversible ferroelastic phase transition in a new bromethyl-substituted molecular rotor

A new bromethyl-substituted molecular rotor, [Cu(dabcoCH2Br)(H2O)Br3] (dabcoCH2Br+=1-(2-bromethyl)-1,4-diazoniabicyclo[2.2.2]octane cation), which belongs to a family of halomethyl-substituted molecular rotors, was synthesized and structurally characterized. The reversible phase transition at ca. 250 K was well established for this molecular rotor by thermal analyses, variable-temperature X-ray diffraction, and variable temperature dielectric measurements. The order-disorder transformation of the rotator part (dabco moiety) causes ferroelastic phase transition with an Aizu notation of mmmF2/m from high-temperature orthorhombic phase (Pbnm) to low-temperature monoclinic phase (P21/n). More important, in reference to the density functional theory calculations and structural analyses, the key factors to tune the phase transition behaviors are discussed in detail for this family of halomethyl-substituted molecular rotors.

Two magnetic Δ-chain-based Mn(II) and Co(II) coordination polymers with mixed carboxylate–phosphinate and μ3-OH− bridges

The hydrothermal reaction of Mn(II) or Co(II) ions with 2-carboxyethyl(phenyl)phosphinic acid (H2L) afforded Mn(II) and Co(II) carboxylate–phosphinates containing μ3-OH− as a co-ligand, namely [M3(L)2(OH)2] (M = Mn (1) or Co (2)). Such two compounds feature a new layered structure in which Δ-type chains built from alternating corner- and edge-sharing M3(μ3-OH) triangles are further connected via the Y-shaped “–(CH2)2–C(–O–)–” spacers. Magnetic studies reveal that there are dominant antiferromagnetic interactions between the metal ions. In 1, the complicated magnetic couplings in the Δ-type chains result in spin competition, displaying spin-glass behaviors. In 2, spin fluctuation behavior was observed and the critical field at 2 K is 25 kOe.