Meng-Yao Chao

Secondary building units (SBUs) in metal–organic frameworks (MOFs) are essential from both a structural and performance perspective. While a variety of SBUs, such as paddlewheel Cu^II ₂, triangular Cr^III ₃, tetrahedral Zn^II ₄, and octahedral Zr^IV ₆ have been extensively studied, the linear trinuclear SBUs (herein denoted as M₃), though frequently encountered, are rarely discussed as a class. A literature survey reveals that M₃ clusters are ubiquitous in discrete molecular entities as well as in MOFs. Unlike most other cluster types, however, they have an unprecedented metal diversity and ligand tolerance. The single-crystals of some M₃-based MOFs are also sufficiently robust upon guest removal and exchange or multi-step post-modifications to enable catalytic mechanism elucidation. Some of these M₃-based SBUs endow MOFs with stability under demanding conditions necessary, for example, in flue gas separation. Herein we review MOFs sustained by this common but under-appreciated class of SBUs and discuss applications of the resulting MOF motifs.

A heterometallic metal−organic framework (MOF) of [Cd₆Ca₄(BTB)₆(HCOO)₂(DEF)₂(H₂O)₁₂]∙DEF∙xSol (1, H₃BTB = benzene-1,3,5-tribenzoic acid; DEF = N,N′-diethylformamide; xSol. = undefined solvates within the pore) was prepared by solvothermal reaction of Cd(NO₃)₂·4H₂O, CaO and H₃BTB in a mixed solvent of DEF/H₂O/HNO₃. The compatibility of these two divalent cations from different blocks of the periodic table results in a solid-state structure consisting of an unusual combination of a discrete V-shaped heptanuclear cluster of [Cd₂Ca]₂Ca′ and an infinite one-dimensional (1D) chain of [Cd₂CaCa′]_n that are orthogonally linked via a corner-shared Ca²⁺ ion (denoted as Ca′), giving rise to an unprecedented branched-chain secondary building unit (SBU). These SBUs propagate via tridentate BTB to yield a three-dimensional (3D) structure featuring a corner-truncated P4₁ helix in MOF 1. This outcome highlights the unique topologies possible via the combination of carefully chosen s- and d-block metal ions with polydentate ligands.

Crystals of a two-dimensional (2D) metal–organic framework (MOF) [Cd₃(BTB)₂(DEF)₄]·2(DEF)_0.5 (1; BTB = benzene-1,3,5-tribenzolate; DEF = N,N′-diethylformamide) immersed in a solution of trans-1,2-bis(4-pyridyl)ethylene (BPEE) yields an interpenetrated three-dimensional (3D) MOF of [Cd₃(BTB)₂(BPEE)(H₂O)₂]·(BPEE)·xSol (2). Crystals of MOF 2, in turn, undergo a cascade conversion when immersed in DEF, yielding [Cd₃(BTB)₂(BPEE)_1.8(DEF)_0.9(H₂O)_0.8]·xSol (3a) over 100 seconds and [Cd₃(BTB)₂(BPEE)₂(DEF)₂]·xSol (4) after one hour, before finally shuttling back to MOF 1 after six hours.

A cationic three‐dimensional (3D) metal‐organic framework (MOF) sustained by an N,N′‐diethylformamide (DEF)‐solvated zigzag‐shaped Cd₅ cluster secondary building unit (SBU), [Et₂NH₂]₂[Cd₅(BTB)₄(DEF)₂] ⋅ 4.75DEF (1 a, H₃BTB=benzene‐1,3,5‐tribenzoic acid) shows flexible framework behavior and undergoes solvate exchange with CHCl₃ to yield [Et₂NH₂]₂[Cd₅(BTB)₄(DEF)₂] ⋅ xCHCl₃ (1 b) accompanied by changes to pore sizes and shapes. Unexpectedly, the DEF solvates coordinated to the central Cd²⁺ could not be replaced by strongly donor pyridyl and dipyridyl ligands. Additionally, more electron‐deficient pyridyls preferentially coordinated to the flanking Cd²⁺ of the Cd₅ SBU, as exemplified by [Et₂NH₂]₂[Cd₅(BTB)₄(DEF)₂(PyCHO)₂] ⋅ xSol (2 a, PyCHO=4‐pyridinealdehyde) and [Et₂NH₂]₂[Cd₅(BTB)₄(DEF)₂(PyAc)₂] ⋅ xSol (2 b, PyAc=4‐acetylpyridine). Density functional theory (DFT) calculations were used to understand these counterintuitive observations.

Ruthenium(II) complexes bearing a tridentate bis(N‐heterocyclic carbene) ligand reacted with iminoiodanes (PhI=NR) resulting in the formation of isolable ruthenium(III)–amido intermediates, which underwent cleavage of a C−N bond of the tridentate ligand and formation of an N‐substituted imine group. The Ru ^III –amido intermediates have been characterized by ¹ H NMR, UV/Vis, ESI‐MS, and X‐ray crystallography. DFT calculations were performed to provide insight into the reaction mechanism.

Doping of Co ²⁺ in the linear Cd ₃ cluster secondary building units lowers the single-crystal to single-crystal conversion reactivity of the resulting metal–organic framework.

Cuboidal [Ni ₄ O ₄ ]-based clusters are systematically studied with the purpose of evaluating the component contribution to the overall nonlinear optical performances.

Large and permanent porosity is the primary concern when designing metal-organic frameworks (MOFs) for specific applications, such as catalysis and drug delivery. In this article, we report a MOF Co₁₁(BTB)₆(NO₃)₄(DEF)₂(H₂O)₁₄ (1, H₃BTB = 1,3,5-tris(4-carboxyphenyl)benzene; DEF = N,N-diethylformamide) via a mixed cluster secondary building unit (SBU) approach. MOF 1 is sustained by a rare combination of a linear trinuclear Co₃ and two types of dinuclear Co₂ SBUs in a 1:2:2 ratio. These SBUs are bridged by BTB ligands to yield a three-dimensional (3D) non-interpenetrated MOF as a result of the less effective packing due to the geometrically contrasting SBUs. The guest-free framework of 1 has an estimated density of 0.469 g cm⁻³ and exhibits a potential solvent accessible void of 69.6% of the total cell volume. The activated sample of 1 exhibits an estimated Brunauer-Emmett-Teller (BET) surface area of 155 m² g⁻¹ and is capable of CO₂ uptake of 58.61 cm³ g⁻¹ (2.63 mmol g⁻¹, 11.6 wt % at standard temperature and pressure) in a reversible manner at 195 K, showcasing its permanent porosity.

The one-pot reaction of Cd(NO₃)₂·4H₂O and 5-(6-(hydroxymethyl)pyridin-3-yl)isophthalic acid (H₂L) in DMF/H₂O (DMF = N,N-dimethylformamide) produced a two-dimensional (2D) metal–organic framework (MOF) of [Cd(L)(H₂O)₂] (A) bearing aqua-bridged Cd centers, accompanied by two three-dimensional (3D) MOFs [Cd(L)(DMF)_0.5] (B) and [Cd(L)] (C). Removing the bridging aqua molecules of A by heating led to the formation of an additional 3D MOF of [Cd(L)] (D) in a single-crystal to single-crystal (SCSC) manner. The search for the preceding compound that could convert to A resulted in the isolation of a 2D MOF [Cd(L)(DMF)] (E) that readily converted to A in water, but with the loss of single crystallinity. Upon excitation at 350 nm, A, D, E, and the ligand H₂L fluoresced at 460 nm, 468 nm, 475 nm, and 411 nm, respectively. The fluorescence of A could be used for the selective detection of Fe³⁺ in water down to 0.58 ppm. This quenching was not affected by the presence of other common metal ions.