Hao-Long Zhou

Geometry analysis and systematic synthesis of highly porous isoreticular frameworks with a unique topology

Porous coordination polymers are well known for their easily tailored framework structures and corresponding properties. Although systematic modulations of pore sizes of binary prototypes have gained great success, simultaneous adjustment of both pore size and shape of ternary prototypes remains unexplored, owing to the difficulty in controlling the self-assembly of multiple molecular building blocks. Here we show that simple geometry analysis can be used to estimate the influence of the linker lengths and length ratios on the synthesis/construction difficulties and framework stabilities of a highly symmetric, ternary prototype composed of a typical trinuclear metal cluster and two types of bridging carboxylate ligands. As predicted, systematic syntheses with 5×5 ligand combinations produced 13 highly porous isoreticular frameworks, which show not only systematic adjustment of pore volumes (0.49–2.04 cm3 g−1) and sizes (7.8–13.0 Å; 5.2–12.0 Å; 7.4–17.4 Å), but also anisotropic modulation of the pore shapes.

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.

Direct visualization of a guest-triggered crystal deformation based on a flexible ultramicroporous framework

Host-guest composites may exhibit abnormal and/or controllable physical properties that are unavailable for traditional solids. However, it is still very difficult to control or visualize the occupancy and motion of the guest. Here we report a flexible ultramicroporous coordination polymer showing exceptional guest-responsive thermal-expansion properties. The vacant crystal exhibits constant and huge thermal expansion over a wide temperature range not only in vacuum but also in air, as its ultramicroporous channel excludes air adsorption even at 77 K. More interestingly, as demonstrated by single-crystal X-ray crystallography, molecular dynamic simulations and solid-state nuclear magnetic resonance, it selectively responds to the molecular rearrangement of N,N-dimethylformamide, leading to conformation reversion of the flexible ligand, which transfers these actions to deform the whole crystal lattice. These results illustrate that combination of ultramicroporous channel and flexible pore surface could be an effective strategy for the utilization of external physical and chemical stimuli.

Supramolecular-jack-like guest in ultramicroporous crystal for exceptional thermal expansion behaviour.

The dynamic behaviours of host frameworks and guest molecules have received much attention for their great relevance with smart materials, but little has been developed to control or understand the host–guest interplay. Here we show that the confined guest can utilize not only molecular static effects but also bulk dynamic properties to control the host dynamics. By virtue of the three-dimensional hinge-like framework and quasi-discrete ultramicropores, a flexible porous coordination polymer exhibits not only drastic guest-modulation effect of the thermal expansion magnitude (up to 422 × 10−6 K−1) and even the anisotropy but also records positive/negative thermal expansion coefficients of +482/−218 × 10−6 K−1. Moreover, single-crystal X-ray diffraction analyses demonstrate that the jack-like motion of the guest supramolecular dimers, being analogous to the anisotropic thermal expansion of bulk van der Waals solids, is crucial for changing the flexibility mode and thermal expansion behaviour of the crystal.

Buffering additive effect in the formation of metal–carboxylate frameworks with slightly different linear M3(RCOO)6 clusters

By treating M(NO3)2 or MCl2 (M = Mn, Fe, Co and Ni) with 2,6-naphthalene dicarboxylic acid (ndcH2), a series of metal–carboxylate frameworks have been solvothermally synthesized under the same route with different buffering anions, which feature two kinds of frameworks based on slightly different linear M3(OOCR)6 secondary building units: [M3(ndc)3(DMF)4]n (M = MnII, FeII and CoII) or [M3(ndc)3(DMF)2(Me2NH)2]n (M = CoII and NiII). Consequently, the two cobalt compounds exhibit different magnetic behaviours.

Grafting alkylamine in UiO-66 by charge-assisted coordination bonds for carbon dioxide capture from high-humidity flue gas

Ethanolamine (Hea) was grafted onto the pore surface of a metal–organic framework [Zr6O4(OH)4(bdc)6] (UiO-66, H2bdc = benzene-1,4-dicarboxylic acid) by a two-step post-synthetic treatment, in which UiO-66 was first dehydrated to form [Zr6O6(bdc)6], and then reacted with Hea to form [Zr6O4(OH)2(ea)2(bdc)6] (UiO-66-EA). CO2 adsorption measurements showed that UiO-66-EA exhibits a CO2 adsorption enthalpy of 66 kJ mol−1, which is much higher than that of UiO-66 (30 kJ mol−1). Solid-state 13C NMR spectra demonstrated that carbamate species are formed by the reaction of CO2 with the amine groups in UiO-66-EA. Single-component gas adsorption measurements indicated a CO2/N2 selectivity of 365 for UiO-66-EA (15 : 75 CO2/N2 at 298 K), which is about 20 times higher than the value for UiO-66 (16). Breakthrough experiments under simulated flue gases conditions (10 : 90 CO2/N2 at 313 K) showed that the N2 purification capacity of UiO-66-EA is ca. 18 times that of UiO-66. More importantly, the breakthrough performance of UiO-66-EA can be completely maintained even at high relative humidity (82%).

Mixed-Lanthanide Porous Coordination Polymers Showing Range-Tunable Ratiometric Luminescence for O2 Sensing

Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for self-referenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eux), were synthesized by solvothermal reactions. Single-crystal X-ray diffraction revealed that MCF-53(Tb/Eux) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF-53(Tb/Eux) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.

Structural diversity of coordination polymers controlled by the metal ion as the sole reaction variable

Three new coordination polymers based on 4,4′-(ethyne-1,2-diyl)dibenzoic acid (H2edba) and 2,2′-bipyridine (bpy) ligands, namely, [Zn(edba)(bpy)]·xGuest (1), [Cu(edba)(bpy)]·xGuest (2), and [Co3(edba)3(bpy)2]·3DMF·H2O (3) (DMF = N,N-dimethylformamide), were synthesized under the same solvothermal conditions except for the use of different metal ions. Single-crystal X-ray analysis indicates that with the different coordination habits of Zn2+, Cu2+, and Co2+ ions, the resulting compounds feature coordination structures with different dimensionalities, namely, a 1D zigzag chain based on single-ion nodes in 1, a 2D sql layer based on dinuclear 4-connected nodes in 2, and a 3D sxb framework based on trinuclear 6-connected nodes in 3. In addition, after the removal of guest molecules, the 2D framework of 2 and the 3D framework of 3 are stable enough for CO2 sorption with Langmuir surface areas of 440 cm2 g−1 and 542 cm3 g−1, respectively. The present study serves as an example, in which the metal ion acts as the sole reaction variable, demonstrating the significance of the metal ion in the structural diversity of coordination polymers.

Tuning oxygen-sensing behaviour of a porous coordination framework by a guest fluorophore

Tris(8-hydroxyquinolinato)aluminium (AlQ3), a classic fluorescent molecule insensitive to oxygen, is encapsulated as a guest fluorophore into a highly porous coordination framework [Zn4O(bpz)2(bdc)] (MAF-X10, H2bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole, H2bdc = 1,4-benzenedicarboxylic acid) to obtain a highly luminescent host–guest hybrid material AlQ3@MAF-X10 combining the advantages of excellent fluorescence of AlQ3, high oxygen permeability of MAF-X10, and the resonance energy transfer between AlQ3 and MAF-X10. Photoluminescence studies showed that AlQ3@MAF-X10 exhibits yellowish green fluorescence with a Stokes shift (9606 cm−1) as large as those of phosphorescent noble metal complexes, long lifetime (37.4 ns) and a considerable oxygen permeability (4.4 × 10−9 mol cm−1 s−1 bar−1), giving rise to high oxygen sensitivity (88.5% quenched at 1 bar, i.e., I0/I100 = 8.7, KSV = 7.6 bar−1) with fast response and good reversibility.

Guest-containing supramolecular isomers of silver(I) 3,5-dialkyl-1,2,4-triazolates: syntheses, structures, and structural transformation behaviours

An in-depth synthetic study for potential new supramolecular isomers of silver(I) 3,5-dialkyl-1,2,4-triazolates was carried out through liquid diffusion and rapid solution mixing of [Ag(NH3)2]OH and the related triazoles. Apart from the literature reported close-packing complexes of [Ag(dmtz)] (1a, Hdmtz = 3,5-dimethyl-1,2,4-triazole), [Ag(detz)] (2a, Hdetz = 3,5-diethyl-1,2,4-triazole), [Ag(dptz)] (3a, Hdptz = 3,5-dipropyl-1,2,4-triazole) and [Ag(diptz)] (4a, Hdiptz = 3,5-diisopropyl-1,2,4-triazole), four guest-containing isomers [Ag5(detz)5]·C6H6 (2b), [Ag4(dptz)4]·0.5C6H6 (3b), [Ag(diptz)]·0.5H2O (4b) and [Ag(diptz)]·C6H6 (4c) were discovered. Single-crystal X-ray diffraction studies revealed that 2b, 3b and 4b are complicated coordination networks with small discrete cavities, while 4c possesses an nbo-a framework topology with large discrete cavities embedding a unique supramolecular hexamer of benzene guest molecules with an orthogonal arrangement. Single-crystal and powder X-ray diffraction as well as thermogravimetry studies showed that 2b and 3b can undergo reversible temperature- and guest-induced single-crystal-to-single-crystal structural transformations, respectively, in which the conformation changes of the flexible alkyl groups play important roles. On the other hand, single crystals of 4c can transform into single crystals of another new isomer [Ag8(diptz)8]·C6H6·CH3OH (4d) with a completely different network connectivity, by immersing in methanol.