The one-pot reaction of Cd(NO)·4HO and 5-(6-(hydroxymethyl)pyridin-3-yl)isophthalic acid (HL) in DMF/HO (DMF = N,N-dimethylformamide) produced a two-dimensional (2D) metal-organic framework (MOF) of [Cd(L)(HO)] (A) bearing aqua-bridged Cd centers, accompanied by two three-dimensional (3D) MOFs [Cd(L)(DMF)] (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 HL fluoresced at 460 nm, 468 nm, 475 nm, and 411 nm, respectively. The fluorescence of A could be used for the selective detection of Fe in water down to 0.58 ppm. This quenching was not affected by the presence of other common metal ions.

The single-crystal to single-crystal (SCSC) conversion of metal–organic frameworks (MOFs) represents a facile route to new MOFs with structures and functionalities that are challenging to obtain by direct synthesis. However, conversion products are often structurally limited for a given precursor. We herein report that a two-dimensional (2D) MOF featuring a linear Cd3 cluster secondary building unit (SBU) converts into one type of three-dimensional (3D) interpenetrated and two types of 3D non-interpenetrated MOFs upon reaction with dipyridyl ligands. One of the interpenetrated 3D MOFs, in turn, undergoes either ligand substitution to give isoreticular interpenetrated MOFs, or ligand addition to give a self-penetrated 3D MOF. This rich SCSC conversion library is made possible by the inclined nature of the Cd3 SBU with respect to the 2D plane of the starting material to create an anisotropic environment around the SBU.

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 two-dimensional (2D) metal-organic framework (MOF) [Cd2.25Co0.75(BTB)2(DEF)4]·2(DEF)0.5 (1a, BTB = benzene-1,3,5-tribenzolate; DEF = N,N'-diethylformamide) featuring linear trimetallic cluster secondary building units (SBUs) and replaceable DEF solvates reacts smoothly with dipyridyl ligands DPS, DPDS, and BIPY (DPS = 4,4'-dipyridyl sulfide; DPDS = 4,4'-dipyridyl disulfide; BIPY = 4,4'-bipyridine), yielding three-dimensional (3D) non-interpenetrated [Cd2.25Co0.75(BTB)2(DPS)2]·xSol (2) and [Cd2.25Co0.75(BTB)2(DPDS)2]·xSol (3), and interpenetrated [Cd2.25Co0.75(BTB)2(BIPY)(H2O)2]·xSol (4). The smooth conversion of 1a to 3 contrasts the drastic single-crystal to single-crystal (SCSC) conversion observed for the homometallic [Cd3(BTB)2(DEF)4]·2(DEF)0.5 (1b), presumably due to the presence of the relatively Lewis acidic Co2+ that lessens the propensity for binding by soft dipyridyl donors. Activated samples of 2-4 showed no obvious absorption of N2 at 77 K, but reversible type I uptake of CO2 at 195 K.

Five stable clusters sharing the cuboidal [Ni4O4] skeleton are subjected to third-order nonlinear optical (NLO) property measurements. Preliminary results suggest that the NLO property is largely defined by the cluster core skeleton and the directly coordinated atoms, with limited contribution from the heavy atoms peripherally attached to the aromatic ligands.