Mario Wriedt

Reversible switching from antiferro- to ferromagnetic behavior by solvent-mediated, thermally-induced phase transitions in a trimorphic MOF-based magnetic sponge system.

Hydrothermal reactions of copper(II) acetate, tetrazolate-5-carboxylate (tzc), and the neutral N-donor spacer ligand 1,3-di(4-pyridyl)propane (dpp) lead in a single reaction vial to the simultaneous formation of three different single-crystalline solvates [Cu(tzc)(dpp)]n·0.5C6H14·0.5H2O (1), [Cu(tzc)(dpp)]n·4.5H2O (2), and [Cu(tzc)(dpp)]n·1.25C6H14 (3). All three structures were characterized by single crystal X-ray diffraction. None of these solvates can be prepared as phase-pure bulk materials, but reaction conditions similar to those used for single crystal synthesis yield a phase-pure polycrystalline bulk material of an additional forth solvate phase [Cu(tzc)(dpp)]n·2H2O (4). Investigations of its thermal properties by in situ temperature-dependent synchrotron-based powder diffraction experiments have shown interesting phase transitions upon heating in a helium stream. Initially, the precursor dihydrate 4 transforms to an anhydrous phase [Cu(tzc)(dpp)]n (6I) via the intermediate monohydrate phase [Cu(tzc)(dpp)]n·H2O (5). Upon further heating, phase 6I transforms to a new anhydrous polymorph 6II, which transforms upon cooling to a further new phase 6III. Thermogravimetric measurements performed in tandem with differential scanning calorimetry as well as infrared spectroscopic investigations are in agreement with these findings. The de/resolvation behavior is accompanied by a dramatic change in their magnetic properties: The dihydrate phase shows antiferromagnetic exchange interactions, whereas ferromagnetic properties are observed for the trimorphic anhydrate system. This magnetic sponge-like behavior can be reversibly cycled upon de/resolvation of the material.

Low-energy selective capture of carbon dioxide by a pre-designed elastic single-molecule trap

Single-molecule trap: Easy activation of the water-stable metal-organic framework PCN-200 provides a new route to low-energy selective CO(2) capture through stimuli-responsive adsorption behavior. This elastic CO(2) trapping effect was confirmed by single-component and binary gas-adsorption isotherms and crystallographic determination.

Coexistence of Metamagnetism and Slow Relaxation of the Magnetization in a Cobalt Thiocyanate 2D Coordination Network

Recently, strategies for the design of coordination polymers, hybrid compounds, or metal–organic frameworks (MOFs) that show cooperative magnetic phenomena have become of increasing interest. Because of their great potential for possible applications as storage materials or in molecular electronics, 1D materials with a large magnetic anisotropy, slow relaxation of the magnetization M, and a hysteresis of molecular origin, for example, “single-chain magnets” (SCMs) are of special interest. Moreover, for future applications multifunctional materials are needed, in which different physical properties can be tuned or switched as a function of external parameters. These criteria also apply to metamagnetic compounds, which show different magnetic properties below and above a critical field HC. [1c,4] Unfortunately, because of strong interchain interactions most of these compounds show only 3D ordering above HC. [5] Therefore, only a very few metamagnetic coordination compounds have been reported in which slow relaxation of the magnetization is observed. 6] In our research we have developed an alternative method for the synthesis of compounds that show cooperative magnetic interactions. In this approach transition-metal coordination compounds with terminally bound anions and neutral co-ligands are heated leading to a stepwise removal of the co-ligands and the formation of intermediates with bridging anions and modified magnetic interactions. We have found that a large number of different compounds can be prepared by this route and that the dimensionality of the networks can easily be adjusted. In this context we have reported on the directed synthesis of a compound that shows SCM behavior. Such a behavior usually occurs only in 1D coordination networks, but should, in principle, also be observed in 2D networks if the magnetic chains are separated by magnetically inactive ligands. To investigate this possibility, precursor compounds based on cobalt(II) thiocyanate and the bidentate co-ligand 1,2-bis(4-pyridyl)ethylene (bpe) were prepared, and the intermediates formed by thermal decomposition were characterized for their magnetic properties. The reaction of Co(NCS)2 with an excess of bpe leads to the formation of [Co(NCS)2(bpe)(bpe)]n (1). [10] In its crystal structure the cobalt cations are octahedrally coordinated by four bpe ligands and two terminal N-bonded thiocyanato anions (Figure 1, top). The metal cations are linked by the bpe ligands into chains that are further connected by the coligands into layers. This arrangement leads to the formation of cavities in which additional bpe ligands are trapped. In further experiments using slightly different reaction conditions the hydrate [Co(NCS)2(bpe)2(H2O)2] [10] (2) could be obtained, in which the cobalt(II) cations are surrounded by two bpe ligands, two water molecules, and two terminal N-bonded thiocyanato anions in an octahedral coordination environment (Figure 1, bottom). These complexes are linked into layers by O H···N hydrogen bonds. Compounds 1 and 2 represent potential precursors for the preparation of liganddeficient compounds and thus, were investigated by thermoanalytical methods. On heating compound 1, a single mass step is observed, which leads to the formation of [Co(NCS)2(bpe)]n (4). [10] If the hydrate 2 is heated, two mass steps are observed corresponding to the formation of the anhydrate 3 in the first step, which transforms into compound 4 in the second step (see Supporting Information). Based on this information, single crystals of 4 were prepared using hydrothermal conditions. In the crystal structure of 4 the cobalt cations are octahedrally coordinated by two Sand two N-bonded thiocyanato anions as well as two N-bonded bpe ligands. The cations are linked into chains by m-1,3 bridging thiocyanato anions, which are further connected into layers by the bpe ligands (Figure 1, middle). Magnetic measurements on all the compounds show significant differences between the ligand-rich precursors 1– 3 and the ligand-deficient compound 4. In compounds 1–3 the thiocyanato anions are only terminal N-bonded, so that paramagnetic behavior is observed (see Supporting Information). On cooling, decreasing cm T values are observed until at about 25 K from which point increasing cm T values are observed which decrease again at approximately 4 K. A small magnetic exchange through the bpe ligands cannot be completely excluded. In contrast, for 4 a ferromagnetic coupling is observed between neighbored Co centers at HDC = 1 kOe (DC = direct current). Moreover, in the hysteresis curve a step is observed indicating metamagnetic behavior (Figure 2). Magnetic measurements at HDC = 0.1 kOe show antiferromagnetic behavior (Figure 3). Additional field dependent alternating current (AC) measurements using an external static field (HDC = 2 kOe, HAC = 10 Oe) show a transition from antiferromagnetic to ferromagnetic behavior at H>HC [*] Dipl.-Chem. S. W hlert, Dipl.-Chem. J. Boeckmann, Dr. M. Wriedt, Prof. Dr. C. N ther Institut f r Anorganische Chemie Christian-Albrechts-Universit t zu Kiel Max-Eyth-Strasse 2, 24118 Kiel (Germany) E-mail: cnaether@ac.uni-kiel.de

Thermal decomposition reactions as tool for the synthesis of new metal thiocyanate diazine coordination polymers with cooperative magnetic phenomena.

Reaction of nickel thiocyanate with pyrimidine at room temperature leads to the formation of the new ligand-rich 1:2 (1:2 = ratio between metal and ligand) compound [Ni(NCS)(2)(pyrimidine)(2)](n) (2) which is isotypic to [Co(NCS)(2)(pyrimidine)(2)](n) (1) reported recently. In the crystal structure, the Ni(2+) ions are coordinated by four N atoms of pyrimidine ligands, which connect the metal centers into layers, and two N atoms of terminal bonded thiocyanato anions within slightly distorted octahedra. If the synthesis is performed under solvothermal conditions and an excess of the metal thiocyanate is used, single crystals of the ligand-deficient 1:1 compound [Ni(NCS)(2)(pyrimidine)](n) (4) are obtained. Investigations on the synthesis of this compound show that it is always contaminated with large amounts of the corresponding ligand-rich 1:2 compound 2. In the crystal structure, the Ni(2+) ions are coordinated by two N atoms of pyrimidine ligands, which connect the metal centers into chains, and two N atoms as well as two S atoms of mu-1,3 bridged thiocyanato anions, which conntect these chains into layers, within a slightly distorted octahedral geometry. On heating compounds 1 and 2 transform quantitatively into the ligand-deficient 1:1 Ni compound 4 and its isotypic Co compound 3. If nickel and cobalt thiocyanate are reacted with an excess of pyrimidine in a solvent-free reaction, discrete ligand-rich 1:4 complexes of composition [M(NCS)(2)(pyrimidine)(4)] (M = Co 5 and Ni 6) are obtained, which could be determined by single crystal structure analysis. In their crystal structure the metal ions are coordinated by four terminal bonded pyrimidine ligands and two terminal N-bonded thiocyanato anions. For the ligand-rich 1:2 and ligand-deficient 1:1 compounds magnetic measurements were performed, which reveal different magnetic properties: The 1:2 compounds show a ferromagnetic and the 1:1 compounds an antiferromagnetic ordering at lower temperatures.

Metamagnetism and Single-Chain Magnetic Behavior in a Homospin One-Dimensional Iron(II) Coordination Polymer

Reaction of FeCl(2)·4H(2)O with KNCSe and pyridine in ethanol leads to the formation of the discrete complex [Fe(NCSe)(2)(pyridine)(4)] (1) in which the Fe(II) cations are coordinated by two N-terminal-bonded selenocyanato anions and four pyridine co-ligands. Thermal treatment of compound 1 enforces the removal of half of the co-ligands leading to the formation of a ligand-deficient (lacking on neutral co-ligands) intermediate of composition [Fe(NCSe)(2)(pyridine)(2)](n) (2) to which we have found no access in the liquid phase. Compound 2 is obtained only as a microcrystalline powder, but it is isotypic to [Cd(NCSe)(2)(pyridine)(2)](n) and therefore, its structure was determined by Rietveld refinement. In its crystal structure the metal cations are coordinated by two pyridine ligands and four selenocyanato anions and are linked into chains by μ-1,3 bridging anionic ligands. Magnetic measurements on compound 1 show only paramagnetic behavior, whereas for compound 2 an unexpected magnetic behavior is found, which to the best of our knowledge was never observed before for a iron(II) homospin compound. In this compound metamagnetism and single-chain magnetic behavior coexist. The metamagnetic transition between the antiferromagnetically ordered phase and a field-induced ferromagnetic phase of the high-spin iron(II) spin carriers is observed at a transition field H(C) of 1300 Oe and the single-chain magnetic behavior is characterized by a blocking temperature T(B), estimated to be about 5 K.

Metal–Organic Frameworks as Platforms for the Controlled Nanostructuring of Single-Molecule Magnets

The prototypical single-molecule magnet (SMM) molecule [Mn12O12(O2CCH3)16(OH2)4] was incorporated under mild conditions into a highly porous metal-organic framework (MOF) matrix as a proof of principle for controlled nanostructuring of SMMs. Four independent experiments revealed that the SMM clusters were successfully loaded in the MOF pores, namely synchrotron-based powder diffraction, physisorption analysis, and in-depth magnetic and thermal analyses. The results provide incontrovertible evidence that the magnetic composite, SMM@MOF, combines key SMM properties with the functional properties of MOFs. Most importantly, the incorporated SMMs exhibit a significantly enhanced thermal stability with SMM loading advantageously occurring at the periphery of the bulk MOF crystals with only a single SMM molecule isolated in the transverse direction of the pores.

Rational design of bridging selenocyanates by thermal decomposition reactions

The powerful use of thermal decomposition reactions as tool for the directed synthesis of a new selenocyanate is described. This method offers a facile access to a large number of new compounds, in which the metal centers are bridged by small anionic linker ligands and therefore, cooperative magnetic phenomena can be expected.

Synthesis, Crystal Structure, and Magnetic Properties of the Coordination Polymer [Fe(NCS)2(1,2-bis(4-pyridyl)-ethylene)]n Showing a Two Step Metamagnetic Transition

Reaction of iron(II) thiocyanate with an excess of trans-1,2-bis(4-pyridyl)-ethylene (bpe) in acetonitrile at room temperature leads to the formation of [Fe(NCS)(2)(bpe)(2)·(bpe)] (1), which is isotypic to its Co(II) analogue. Using slightly different reaction conditions the literature known compound [Fe(NCS)(2)(bpe)(2)(H(2)O)(2)] (2) was obtained as a phase pure material. Simultaneous differential thermoanalysis and thermogravimetry prove that the hydrate 2 transforms into the anhydrate [Fe(NCS)(2)(bpe)(2)] (3), that decomposes on further heating into the new ligand-deficient 1:1 compound of composition [Fe(NCS)(2)(bpe)](n) (4), which can also be obtained directly by thermal decomposition of 1. Further investigations reveal that 4 can also be prepared under solvothermal conditions, and single crystal structure analysis shows that the iron(II) cations are linked via μ-1,3 bridging thiocyanato anions into chains, that are further connected into layers by the bpe ligands. Magnetic measurements, performed on powder samples, prove that 1 and 2 show only Curie-Weiss behavior, whereas in 4 antiferromagnetic ordering with a Néel temperature of 5.0 K is observed. At T < 4.0 K a two-step metamagnetic transition occurs at applied magnetic fields of 1300 and 1775 Oe. The magnetic properties are discussed and compared with those of related compounds.

High‐Throughput Analytical Model to Evaluate Materials for Temperature Swing Adsorption Processes

In order for any material to be considered in a post-combustion carbon capture technology, it must have high working capacities of CO₂ from flue gas and be regenerable using as little energy as possible. Shown here is an easy to use method to calculate both working capacities and regeneration energies and thereby predict optimal desorption conditions for any material.

The structural diversity and properties of nine new viologen based zwitterionic metal–organic frameworks

The neutral flexible viologen based ligand 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium dibromide (H2LBr2) and its self-assembly with first-row transition metals in an aqueous media leads to the formation of nine new zwitterionic (ZW) MOF materials with the following compositions: {[CuBr(L)]·(OH)·7H2O}n (1); {[M4(L)6(OH2)12]·2Br·3(bdc)·33H2O}n with M = Mn (2), Co (3) and Ni (4), {[M(bdc)(L)1.5]·9H2O}n with M = Cd (5) and Zn (6); {[Cu2(3-pzc)2(L)(OH2)]·5H2O}n (7); {[ZnCl2(L)0.5]·0.33H2O}n (8) and [Pd(HL)(Br)2(NO2)2(OH2)2] (9) (bdc = 1,4-benzenedicarboxylate, pzc = 3-pyrazole carboxylate). These compounds were characterized by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), infrared spectrometry (IR), elemental analyses, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC). Interestingly, when the samples are exposed to UV irradiation, photochromic behavior is observed for the ligand only, whereas the ZW MOFs are found to be photochemically inert. The fundamental structural origin for this photo reactivity is discussed in detail, as well as an in-depth CSD analysis of important intra- and intermolecular parameters of L-based MOFs.