Hui Min Wen

Emerging Multifunctional Metal–Organic Framework Materials

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.

Porous Metal–Organic Frameworks for Gas Storage and Separation: What, How, and Why?

Metal-organic frameworks (MOFs) have been emerging as promising multifunctional materials and have shown particularly useful applications for gas storage and separation. We have briefly outlined the early development of this very active research field to provide us a clear picture on what are MOFs and how the research endeavor has been initiated and explored. Following that, we have demonstrated why MOFs are so unique for gas storage and separation: high porosities, tunable framework structures, and immobilized functional sites to fully make use of pore space for gas storage, to optimize their sieving effects, and to differentiate their interactions with gas molecules. Finally, we have provided a perspective on further development of porous MOFs for gas storage and separation.

Porous metal–organic frameworks with Lewis basic nitrogen sites for high-capacity methane storage

The use of porous materials to store/deliver natural gas (mostly methane) in vehicles requires large amounts of methane being stored per unit volume. In this work, we report several porous metal–organic frameworks (MOFs) with NOTT-101 type structures, containing Lewis basic nitrogen sites through the incorporation of pyridine, pyridazine, and pyrimidine groups into the organic linkers. They exhibit significantly higher total volumetric methane storage capacities (∼249–257 cm3 (STP) cm−3 at room temperature (RT) and 65 bar) than NOTT-101a (here the MOF abbreviation with “a” at the end represents the fully activated MOF). The most significant enhancement was observed on UTSA-76a with functional pyrimidine groups (237 cm3 (STP) cm−3 in NOTT-101a vs. 257 cm3 (STP) cm−3 in UTSA-76a). Several multivariate (MTV) MOFs constructed from two types of organic linkers (pyrimidine-functionalized and unfunctionalized) also show systematically improved methane storage capacities with increasing percentage of functionalized organic linkers. The immobilized functional groups have nearly no effect on the methane uptakes at 5 bar but significantly improve the methane storage capacities at 65 bar, so the reported MOFs exhibit excellent methane storage working capacities of ∼188–197 cm3 (STP) cm−3.

Electrical conductance study on 1,3-butadiyne-linked dinuclear ruthenium(II) complexes within single molecule break junctions

Single-molecule conductance of three sulphur-functionalized organometallic wires with two ruthenium(II) centres spaced by 1,3-butadiyne was firstly investigated using an electrochemically assisted-mechanically controllable break junction (EC-MCBJ) approach. It is demonstrated that single-molecular conductance of these diruthenium(II) incorporated systems is significantly higher than oligo(phenylene-ethynylene) (OPE) having comparable lengths and exhibits weaker length dependence. The conductance improvement in these diruthenium(II) molecules is ascribable to the better energy match of the Fermi level of gold electrodes with the HOMO that is mainly resident on the Ru–CC–CC–Ru backbone. Furthermore, modulation of molecular conductance is achieved by changing the length and π-conjugated system of the chelating 2,2′:6′,2′′-terpyridyl ligand.

An Ideal Molecular Sieve for Acetylene Removal from Ethylene with Record Selectivity and Productivity

Realization of ideal molecular sieves, in which the larger gas molecules are completely blocked without sacrificing high adsorption capacities of the preferred smaller gas molecules, can significantly reduce energy costs for gas separation and purification and thus facilitate a possible technological transformation from the traditional energy-intensive cryogenic distillation to the energy-efficient, adsorbent-based separation and purification in the future. Although extensive research endeavors are pursued to target ideal molecular sieves among diverse porous materials, over the past several decades, ideal molecular sieves for the separation and purification of light hydrocarbons are rarely realized. Herein, an ideal porous material, SIFSIX-14-Cu-i (also termed as UTSA-200), is reported with ultrafine tuning of pore size (3.4 Å) to effectively block ethylene (C2 H4 ) molecules but to take up a record-high amount of acetylene (C2 H2 , 58 cm3 cm-3 under 0.01 bar and 298 K). The material therefore sets up new benchmarks for both the adsorption capacity and selectivity, and thus provides a record purification capacity for the removal of trace C2 H2 from C2 H4 with 1.18 mmol g-1 C2 H2 uptake capacity from a 1/99 C2 H2 /C2 H4 mixture to produce 99.9999% pure C2 H4 (much higher than the acceptable purity of 99.996% for polymer-grade C2 H4 ), as demonstrated by experimental breakthrough curves.

Efficient photo-driven hydrogen evolution by binuclear nickel catalysts of different coordination in noble-metal-free systems.

Exploration of the complex Ni2(MBD)4 (MBD = 2-mercaptobenzimidazole) (C1) having different coordinated Ni atoms as a photocatalyst for hydrogen evolution is made. For comparison, the bimetallic Ni2(MBT)4 (MBT = 2-mercaptobenthiazole) (C2) complex with the same coordinated Ni atoms was synthesized. Both of the complexes have been successfully constructed for photo-induced hydrogen production using organic dyes as photosensitizers and triethanolamine (TEOA) as the effective electron donor by visible light (>400 nm) in acetonitrile-water solution. The time-dependence of H2 generation and DFT computational studies demonstrate that the complex C1 is more active than C2 for H2 evolution. The mechanisms of photocatalytic hydrogen generation for C1 and C2 involve different protonation sites resulting from the differences between the two structures.

High acetylene/ethylene separation in a microporous zinc(II) metal–organic framework with low binding energy

A novel zinc(II) metal-organic framework UTSA-67a with narrow one-dimensional (1D) pore channels and inner cages of moderate size has been developed for highly selective separation of C2H2/C2H4 mixtures at room temperature.

Syntheses, structures, and magnetic properties of a family of tetra-, hexa-, and nonanuclear Mn/Ni heterometallic clusters

A family of Mn(III)/Ni(II) heterometallic clusters, [Mn(III)(4)Ni(II)(5)(OH)(4)(hmcH)(4)(pao)(8)Cl(2)]·5DMF (1·5DMF), [Mn(III)(3)Ni(II)(6)(N(3))(2)(pao)(10)(hmcH)(2)(OH)(4)]Br·2MeOH·9H(2)O (2·2MeOH·9H(2)O), [Mn(III)Ni(II)(5)(N(3))(4)(pao)(6)(paoH)(2)(OH)(2)](ClO(4))·MeOH·3H(2)O (3·MeOH·3H(2)O), and [Mn(III)(2)Ni(II)(2)(hmcH)(2)(pao)(4)(OMe)(2)(MeOH)(2)]·2H(2)O·6MeOH (4·2H(2)O·6MeOH) [paoH = pyridine-2-aldoxime, hmcH(3) = 2, 6-Bis(hydroxymethyl)-p-cresol], has been prepared by reactions of Mn(II) salts with [Ni(paoH)(2)Cl(2)], hmcH(3), and NEt(3) in the presence or absence of NaN(3) and characterized. Complex 1 has a Mn(III)(4)Ni(II)(5) topology which can be described as two corner-sharing [Mn(2)Ni(2)O(2)] butterfly units bridged to an outer Mn atom and a Ni atom through alkoxide groups. Complex 2 has a Mn(III)(3)Ni(II)(6) topology that is similar to that of 1 but with two corner-sharing [Mn(2)Ni(2)O(2)] units of 1 replaced with [Mn(3)NiO(2)] and [MnNi(3)O(2)] units as well as the outer Mn atom of 1 substituted by a Ni atom. 1 and 2 represent the largest 3d heterometal/oxime clusters and the biggest Mn(III)Ni(II) clusters discovered to date. Complex 3 possesses a [MnNi(5)(μ-N(3))(2)(μ-OH)(2)](9+) core, whose topology is observed for the first time in a discrete molecule. Careful examination of the structures of 1-3 indicates that the Mn/Ni ratios of the complexes are likely associated with the presence of the different coligands hmcH(2-) and/or N(3)(-). Complex 4 has a Mn(III)(2)Ni(II)(2) defective double-cubane topology. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-4. Fitting of the obtained M/(Nμ(B)) vs H/T data gave S = 5, g = 1.94, and D = -0.38 cm(-1) for 1 and S = 3, g = 2.05, and D = -0.86 cm(-1) for 3. The ground state for 2 was determined from ac data, which indicated an S = 5 ground state. For 4, the pairwise exchange interactions were determined by fitting the susceptibility data vs T based on a 3-J model. Complex 1 exhibits out-of-phase ac susceptibility signals, indicating it may be a SMM.

Modulating Stepwise Photochromism in Platinum(II) Complexes with Dual Dithienylethene–Acetylides by a Progressive Red Shift of Ring-Closure Absorption

To modulate stepwise photochromism by shifting ring-closure absorption of the dithienylethene (DTE) moiety, trans-Pt(PEt3)2(C≡C-DTE)2 [C≡C-DTE = L1o (1oo), L2o (2oo), L3o (3oo), and L4o (4oo)] and cis-Pt(PEt3)2(L4o)2 (5oo) with two identical DTE-acetylides were elaborately designed. With the gradual red shift of ring-closure absorption for L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (602 nm), stepwise photochromism is increasingly facilitated in trans-Pt(PEt3)2(C≡C-DTE)2 following 1oo → 2oo → 3oo → 4oo. The conversion percentage of singly ring-closed 2co-4co to dually ring-closed 2cc-4cc at the photostationary state is progressively increased in the order 1cc (0%) → 2cc (18%) → 3cc (67%) → 4cc (100%). Compared with trans-arranged 4oo, stepwise photochromism in the corresponding cis-counterpart 5oo is less pronounced, ascribed to either direct conversion of 5oo to 5cc or rapid conversion of 5co to 5cc. The progressively facile stepwise photocyclization following 2oo → 3oo → 4oo is reasonably interpreted by gradually enhanced transition character involving LUMO+1, which is the only unoccupied frontier orbital responsible for further photocyclization of singly ring-closed 2co-4co.

Octanuclear MnIII6MnIILn (Ln = Gd, Dy and Er) clusters with a novel core topology: syntheses, structures, and magnetic properties

Reactions of [Mn6O2(piv)10(py)2.5(piv)1.5], Ln(NO3)3·6H2O and N-mdeaH2 in MeCN in the presence of Me3SiCl generated a family of octanuclear Mn/Ln complexes [Mn6(III)Mn(II)Ln(N-mdea)3(N-mdeaH)(piv)8O2(OH)3(NO3)(H2O)]·xCH3CN·xH2O [Ln = Gd (1), Dy (2), Er (3), pivH = pivalic acid, N-mdeaH2 = N-methyl diethanolamine]. Each complex possesses a [Mn6(III)Mn(II)Ln(μ3-O)2(μ3-OH)3](16+) core containing two butterfly-like subunits of [Mn3Ln(μ3-OH)2] and [Mn4(μ3-O)2] sharing a common vertex, and an outer Mn atom ligated to one of the subunits through a μ3-OH(-) ligand. The core topology represents a new Mn/Ln core type. The magnetic susceptibility study of 1-3 indicates the presence of dominant antiferromagnetic interactions within the complexes. For complex 1, which contains an isotropic Gd(III) atom, fitting of the obtained M/(NμB) vs. H/T data gave S = 4, g = 1.90, and D = -0.31 cm(-1). The results were further supported by ac data. Complex exhibits out-of-phase ac susceptibility signals, indicating it may be a SMM.