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Postsynthetic ion exchange of [Co2(μ-Cl)2(btta)] (MAF-X27-Cl, H2bbta =1H,5H-benzo(1,2-d:4,5-d')bistriazole) possessing open metal sites on its pore surface yields a material [Co2(μ-OH)2(bbta)] (MAF-X27-OH) functionalized by both open metal sites and hydroxide ligands, giving drastically improved electrocatalytic activities for the oxygen evolution reaction (an overpotential of 292 mV at 10.0 mA cm(-2) in 1.0 M KOH solution). Isotope tracing experiments further confirm that the hydroxide ligands are involved in the OER process to provide a low-energy intraframework coupling pathway.

X-Ray single-crystal diffraction has been the most straightforward and important technique in structural determination of crystalline materials for understanding their structure-property relationships. This powerful tool can be used to directly visualize the precise and detailed structural information of porous coordination polymers or metal-organic frameworks at different states, which are unique for their flexible host frameworks compared with conventional adsorbents. With a series of selected recent examples, this review gives a brief overview of single-crystal X-ray diffraction studies and single-crystal to single-crystal transformations of porous coordination polymers under various chemical and physical stimuli such as solvent and gas sorption/desorption/exchange, chemical reaction and temperature change.

Conventional adsorbents preferentially adsorb the small, high-polarity, and unsaturated 1,3-butadiene molecule over the other C hydrocarbons from which it must be separated. We show from single-crystal x-ray diffraction and computational simulation that a hydrophilic metal-organic framework, [Zn(btm)], where Hbtm is bis(5-methyl-1-1,2,4-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.

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.

The porous metal azolate framework [Cu(etz)]infinity (MAF-2, Hetz = 3,5-diethyl-1,2,4-triazole) processes an NbO type cuprous triazolate scaffold and a CsCl type hydrophobic channel system, in which the large cavities are interconnected by small apertures with pendant ethyl groups. Since the ethyl-blocked apertures behave as thermoactivated IRIS stops for the guest molecules, the gas sorption behavior of MAF-2 can be controlled by temperature, in which N2 adsorption was observed at 195 K rather than 77 K. Single-crystal X-ray structural analysis revealed that the [Cu(etz)]infinity host framework is not altered upon N2 inclusion, confirming the occurrence of the so-called " kinetically controlled flexibility". By virtue of the kinetically controlled flexibility and hydrophobic pore surface, MAF-2 can adsorb large amounts of small organic molecules but excludes H2O. As demonstrated by single-crystal X-ray structural analyses, MAF-2 shrinks, expands, or distorts its framework to accommodate the hydrogen-bonded hexamers of MeOH, EtOH, or MeCN, respectively. Moreover, MAF-2 can also separate benzene and cyclohexane efficiently, as its flexible scaffold can distort to a certain degree so that benzene can diffuse through the distorted apertures but cyclohexane cannot. Moreover, the adsorption/desorption of these organic vapors induces reversible, multimode structural transformations.

Coordination polymers have been emerging as a topical research field in crystal engineering, solid-state chemistry, and materials science. Considering the wide occurrence of structural and compositional diversity during self-assembly and crystallization, supramolecular isomerism represents an indication of composition control and structure prediction. Actually, supramolecular isomerism is not just an obstacle or challenge, but also a good opportunity for developing novel materials and a better understanding of self-assembly and crystal growth. This critical review provides an overview of the developing knowledge, in the context of supramolecular isomerism, of the design, synthesis, and properties of coordination polymers (97 references).

One-pot solvothermal treatments of organonitriles, ammonia, and Cu(II) salts yielded Cu(I) and 3,5-disubstituted 1,2,4-triazolates. The organic triazolate components were derived from copper-mediated oxidative cycloaddition of nitriles and ammonia, in which a key intermediate 1,3,5-triazapentadienate was isolated as [Cu(II)(4-pytap)(2)] (4-Hpytap = 2,4-di(4-pyridyl)-1,3,5-triazapentadiene) via controlled solvothermal conditions. This intermediate could also be synthesized by Ni(II)-mediated reactions; however, the final triazoles were obtained only when Cu(II) was employed. Therefore, the reaction mechanism of these reactions was elucidated as follows: nitrile was first attacked by ammonia to form the amidine, which further reacted with another nitrile or self-condensed to yield 1,3,5-triazapentadiene, which was coordinated to two Cu(II) ions in its deprotonated form. A two-electron oxidation of the 1,3,5-triazapentadienate mediated by two Cu(II) ions gave one triazolate and Cu(I) cations. Other in situ ligand reactions, such as C-C bond cleavage and hydrolysis, were also found for the nitriles under these solvothermal conditions. Another remarkable feature of these crystalline Cu(I) triazolates is their simple, typical 3- or 4-connected network topologies. The self-assembly of these nets is presumably controlled by steric hindrance, which is subsequently applied to the rational design of the close-packed 2D networks [Cu(I)(tz)](infinity) and [Ag(I)(tz)](infinity) (Htz = 1,2,4-triazole), as well as the porous 3D network [Cu(I)(etz)](infinity) (Hetz = 3,5-diethyl-1,2,4-triazole). The interesting photoluminescence properties of these coinage d(10) metal complexes were also investigated.