Wei-Xiong Zhang

A Highly Connected Porous Coordination Polymer with Unusual Channel Structure and Sorption Properties

Making connections: A hydroxy-centered trinuclear nickel cluster has been employed to construct a highly connected, highly symmetric framework with a uninodal nine-connected topology. An array of triakis tetrahedra leads to a biporous intersecting-channel system (see picture).

Switchable Guest Molecular Dynamics in a Perovskite-Like Coordination Polymer toward Sensitive Thermoresponsive Dielectric Materials†

A new perovskite-like coordination polymer [(CH3)2NH2][Cd(N3)3] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH3)2NH2](+) guest accompanied by a synergistic deformation of the [Cd(N3)3](-) framework. The unusual two-staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host-guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.

Controlling guest conformation for efficient purification of butadiene

The weaker adsorption of 1,3-butadiene in a metal-organic framework enables its separation from other C4 hydrocarbons. Selecting against cis conformers Before 1,3-butadiene can be used to make polymers, it must be separated from similar hydrocarbons in an energy-intensive distillation process. Liao et al. show that a zinc metal—organic framework can accommodate the cis isomer of 1,3-butadiene. It binds less tightly than butane and butene because its π-bond conjugation is broken. They used this preferential desorption to separate 1,3-butadiene with ≥99.5% purity under ambient conditions. Science, this issue p. 1193 Conventional adsorbents preferentially adsorb the small, high-polarity, and unsaturated 1,3-butadiene molecule over the other C4 hydrocarbons from which it must be separated. We show from single-crystal x-ray diffraction and computational simulation that a hydrophilic metal-organic framework, [Zn2(btm)2], where H2btm is bis(5-methyl-1H-1,2,4-triazol-3-yl)methane, has quasi-discrete pores that can induce conformational changes in the flexible guest molecules, weakening 1,3-butadiene adsorption through a large bending energy penalty. In a breakthrough operation at ambient temperature and pressure, this guest conformation–controlling adsorbent eluted 1,3-butadiene first, then butane, butene, and isobutene. Thus, 1,3-butadiene can be efficiently purified (≥99.5%) while avoiding high-temperature conditions that can lead to its undesirable polymerization.

Efficient purification of ethene by an ethane-trapping metal-organic framework.

Separating ethene (C2H4) from ethane (C2H6) is of paramount importance and difficulty. Here we show that C2H4 can be efficiently purified by trapping the inert C2H6 in a judiciously designed metal-organic framework. Under ambient conditions, passing a typical cracked gas mixture (15:1 C2H4/C2H6) through 1 litre of this C2H6 selective adsorbent directly produces 56 litres of C2H4 with 99.95%+ purity (required by the C2H4 polymerization reactor) at the outlet, with a single breakthrough operation, while other C2H6 selective materials can only produce ca. ⩽ litre, and conventional C2H4 selective adsorbents require at least four adsorption–desorption cycles to achieve the same C2H4 purity. Single-crystal X-ray diffraction and computational simulation studies showed that the exceptional C2H6 selectivity arises from the proper positioning of multiple electronegative and electropositive functional groups on the ultramicroporous pore surface, which form multiple C–H···N hydrogen bonds with C2H6 instead of the more polar competitor C2H4.

Porous manganese(II) 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazolate frameworks: rational self-assembly, supramolecular isomerism, solid-state transformation, and sorption properties.

Reactions of 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole (Hdpt24) with Mn(OAc)(2) under different conditions give two mononuclear complexes, [Mn(dpt24)(2)(MeOH)(2)] (1) and [Mn(dpt24)(2)(H(2)O)(2)] x 6 H(2)O (2), and three isomeric two-dimensional (2D) coordination polymers alpha-[Mn(dpt24)(2)] (3a), beta-[Mn(dpt24)(2)] x g (3b x g, g = DMF and H(2)O), and gamma-[Mn(dpt24)(2)] x g (3c x g, g = toluene and MeOH). Their structures were characterized by single-crystal and powder X-ray diffractions. In these compounds, four coordination sites of each octahedrally coordinated Mn(II) ion are chelated by two dpt24 ligands in the trans and/or cis configurations. While the two remaining coordination sites are occupied by solvent molecules in 1 and 2, they are occupied by pyridyl nitrogens from neighboring Mn(dpt24)(2) units in 3, forming 4-connected 2D (4,4) networks. The Mn(II) ions in both 3a and 3b are uniquely chelated by dpt24 in the trans or cis configurations, respectively, but Mn(dpt24)(2) in 3c possesses both the trans and cis configurations. The packing fashions of these (4,4) layers in the three isomers of 3 are also different, in which 3a has a close packing structure, while 3b exhibits unique one-dimensional (1D) channels and 3c exhibits two distinct types of 1D channels. As revealed by powder X-ray diffractions, crystals of 1 and 2 can reversibly transform to each other when in contact with the corresponding solvent vapor (H(2)O/MeOH). The gas and vapor sorption studies for porous 3b revealed interesting sorption behaviors. Nitrogen adsorption for 3b was observed at 195 K rather than 77 K, demonstrating the temperature-controlled framework flexibility. It also exhibited high selectivity and storage capacity for carbon dioxide over methane and nitrogen at room temperature. Moreover, 3b also demonstrated potential to separate organic chemicals with similar boiling points, such as benzene and cyclohexane, via pressure swing adsorption process.

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.

Coexistence of spin frustration and long-range magnetic ordering in a triangular CoII3(μ3-OH)-based two-dimensional compound

A two-dimensional compound [Co3(mu3-OH)2(1,2-chdc)2]infinity (1,2-chdc = trans-1,2-cyclohexane-dicarboxylate) comprising triangular arrays of Co(II)3(mu3-OH) affording a Kagomé-like lattice exhibits the coexistence of spin frustration and long-range magnetic ordering.

Remarkably high-temperature spin transition exhibited by new 2D metal–organic frameworks

Highly stable two-dimensional metal–organic frameworks, 2∞[FeII(L)2] (HL = 3-(2-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (1) and 3-(3-methyl-2-pyridyl)-5-(3-pyridyl)-1,2,4-triazole (2), display well defined two-step spin crossover properties at remarkably high temperatures, namely, Tc1 = 329 K, Tc2 = 501 K for 1 and Tc1 = 351 K, Tc2 = 520 K for 2, which are the highest critical temperatures reported so far.

Modular and Stepwise Synthesis of a Hybrid Metal–Organic Framework for Efficient Electrocatalytic Oxygen Evolution

The paddle-wheel type cluster Co2(RCOO)4(LT)2 (R = substituent group, LT = terminal ligand), possessing unusual metal coordination geometry compared with other cobalt compounds, may display high catalytic activity but is highly unstable especially in water. Here, we show that with judicious considerations of the host/guest geometries and modular synthetic strategies, the labile dicobalt clusters can be immobilized and stabilized in a metal-organic framework (MOF) as coordinative guests. The Fe(na)4(LT) fragment in the MOF [{Fe3(μ3-O)(bdc)3}4{Fe(na)4(LT)}3] (H2bdc = 1,4-benzenedicaboxylic acid, Hna = nicotinic acid) can be removed to give [{Fe3(μ3-O)(bdc)3}4] with a unique framework connectivity possessing suitable distribution of open metal sites for binding the dicobalt cluster in the form of Co2(na)4(LT)2. After two-step, single-crystal to single-crystal, postsynthetic modifications, a thermal-, water-, and alkaline-stable MOF [{Fe3(μ3-O)(bdc)3}4{Co2(na)4(LT)2}3] containing the desired dicobalt cluster was obtained, giving extraordinarily high electrocatalytic oxygen evolution activity in water at pH = 13 with overpotential as low as 225 mV at 10.0 mA cm-2.

Spin-Frustrated Complex, [FeIIFeIII(trans-1,4-cyclohexanedicarboxylate)1.5]∞: Interplay between Single-Chain Magnetic Behavior and Magnetic Ordering

A three-dimensional mixed-valent iron(II,III) trans-1,4-cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [Fe(II)Fe(III)(mu(4)-O)(1,4-chdc)(1.5)](infinity), 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH(2) and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mossbauer spectroscopy over a wide range of temperatures. Single-crystal X-ray diffraction studies of 1 at 90(2), 293(2), and 473(2) K reveal a tetrahedral [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) mixed-spin-chain structure with no change in the P1 space group but with subtle changes in the Fe-O and Fe...Fe distances with increasing temperature. These changes are associated with the electron delocalization observed by Mossbauer spectroscopy above 225 K. Magnetic studies reveal three different magnetic regimes in 1 between 2 and 320 K. Above 36 K 1 is a one-dimensional ferrimagnetic-like complex with frustration arising from competing exchange interactions between the iron(II) and iron(III) ions in the chains. Between 36 and 25 K the interchain interactions are non-negligible and 1 undergoes three-dimensional ordering at 32.16 K but with some residual fluctuations. Below 25 K the residual fluctuations slow and eventually freeze below 15 K; the small net moment of 0.22 mu(B) per mole of 1 observed below 15 K may be attributed to a non-collinear or canted spin structure of the spins of the four iron ions in the [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) chains. Below 32 K the Mossbauer spectra of 1 exhibit sharp sextets for both the iron(III) and iron(II) ions and are consistent with either a static long-range or a short-range magnetic ground state or a slow relaxation between two canted magnetic states that are indistinguishable at the observed spectral resolution. The 85 and 155 K spectra reveal no electron delocalization and correspond solely to fixed valence iron(II) and iron(III). Between 225 and 310 K the spectra reveal the onset of electron delocalization such that, at 295 to 310 K, 25, 25, and 50% of the iron in 1 is present as iron(II), iron(III), and iron(II/III) ions, respectively. The absence of any spectral line broadening associated with this electron delocalization and the coexistence of four doublets between 225 and 310 K indicate that the delocalization occurs through electron tunneling via vibronic coupling. The sudden increase in the tunneling rate beginning above about 260 K and the presence of a cusp in the magnetic susceptibility centered at about 275 K strongly suggest the existence of a charge order/disorder transition whose nature and order are discussed.