The metal–organic framework [Fe­(ta)2] (Hta = 1H-1,2,3-triazole) containing Fe­(II) ions and 1,2,3-triazolate ligands shows a reversible phase transition while retaining the cubic crystal symmetry and space group Fd m (no. 227). The phase transition between room temperature (RT-[Fe­(ta)2]; a = 16.6315(2) Å, V = 4600.39(8) Å3) and high temperature (HT-[Fe­(ta)2]; a = 17.7566(4) Å, V = 5598.6(1) Å3) phases occurs at a temperature above 290 °C, whereas the phase transition between HT- and RT-[Fe­(ta)2] starts at a temperature below 210 °C. Both [Fe­(ta)2] polymorphs have identical bond topologies, but they differ by a large increase of the unit cell’s volume of 22% for HT-[Fe­(ta)2]. The compounds are characterized by powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Additionally, Mössbauer spectroscopy, magnetic studies, and the electronic structure of both phases are discussed in detail with respect to the spin-crossover transition from the low-spin (RT-[Fe­(ta)2]) to the high-spin phase (HT-[Fe­(ta)2]).

Ligand exchange reactions at the Kuratowski-type secondary building unit in MFU-4l(arge) metal–organic frameworks (MOFs) result in organometallic porous compounds with metal–carbon bonds of the general formula [Zn5L x Cl4–x (BTDD)3] (4 ≥ x > 3; L = methanido, ethanido, n-butanido, tert-butanido, 3,3-dimethyl-1-butyn-1-ido; H2-BTDD = bis­(1H-1,2,3-triazolo­[4,5-b]­[4′,5′-i])­dibenzo­[1,4]­dioxin) and [Zn1.5Co3.5Me3.1Cl0.9(BTDD)3]. The compounds were characterized by FT-IR, EDX spectroscopy, X-ray powder diffraction (XRPD), and argon adsorption measurements. VT-XRPD, TGA, and TG-MS measurements were applied to investigate the thermal and oxidative stability of the organometallic Zn-MFU-4l derivatives. The hydrolytic stability of all compounds was examined, and a conversion of the methanide to hydroxide ligands is observed in the cobalt-containing compound. DRIFTS measurements of the resulting framework with the composition [Zn1.4Co3.6(OH)3.1Cl0.9(BTDD)3] revealed a mechanism of carbon dioxide binding similar to that of carbonic anhydrase.

In comparison to the vast field of porous materials, research into hydrogen-bonded organic frameworks (HOFs)and especially the subclass of metal hydrogen-bonded organic frameworks (M-HOFs), with only very few structures showing permanent porosity thus faris still in its infancy. Herein, Kuratowski coordination units, which are well-known from various complexes and metal–organic frameworks, were applied to advance this field of research with the synthesis and characterization of a novel series of M-HOFs. Synthesis of Kuratowski complexes with the 1H-benzotriazole-5,6-diamine (H-btda) ligand resulted in molecular building blocks, which initially assemble into CFA-20-Cl and CFA-20-Br M-HOFs ((2,6-lutidinium)+[Zn5X4(btda)6X]−·n(DMF); X = Cl, Br; CFA-20 = Coordination Framework Augsburg University-20, DMF = N,N-dimethylformamide). Both frameworks are isostructurally stabilized via a unique hydrogen bond framework between 12 amine functional groups of six btda ligands and a central 12-fold hydrogen bonded halogen anion, which adapts the coordination of an irregular icosahedron. Postsynthetic ligand exchange at the Kuratowski complex of CFA-20-Cl performed with Tp and Tp* ligands resulted in permanently porous CFA-20-Tp and CFA-20-Tp* M-HOFs ([Zn5Y4(btda)6]; Y = Tp (trispyrazolylborate), Tp* (tris­(3,5-dimethyl-1-pyrazolyl)­borate)), which, thus, constitute an important addition to the meager subclass of permanently porous M-HOFs. Crystallization of the soluble complex of CFA-20-Tp* from dimethylsulfoxide (DMSO) was additionally shown to result in a variant CFA-20-Tp*-DMSO M-HOF structure.

With a view on adding to their use in trace gas sensing, we perform a combined experimental and theoretical study of the change of the conductivity of a metal organic framework (iron (1,2,3)-triazolate, Fe­(ta)2) with the uptake of chemically inert gases. To align our first-principles calculations with experimental measurements, we perform an ensemble average over different microscopic arrangements of the gas molecules in the pores of the metal–organic framework (MOF). Up to the experimentally reachable limit of gas uptake, we find a good agreement between both approaches. Thus, we can employ theory to further interpret our experimental results in terms of changes to the parameters of the Bardeen–Shockley band theory, electron–phonon coupling (in the form of the deformation potential), bulk modulus, and carrier effective mass. We find the first of these to be most strongly influenced through the gas uptake. Furthermore, we find the changes to the deformation potential to strongly depend on the individual microscopic arrangements of molecules in the pores of the MOF. This hints at a possible synthetic engineering of the material, e.g., by closing off certain pores, for a stronger, more interpretable electric response upon gas sorption.

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