Synthesis, Ab Initio X‑ray Powder Diffraction Crystal Structure, and Magnetic Properties of Mn₃(OH)₂(C₆H₂O₄S)₂ Metal–Organic Framework

A new hydroxythiophenedicarboxylate metal–organic framework based on Mn^II cations has been obtained by an aqueous two-step procedure including hydrothermal treatment. The structure of Mn₃(OH)₂(C₆H₂O₄S)₂ has been determined ab initio from synchrotron X-ray powder diffraction data and consists of infinite inorganic ribbons which are interlinked by 2,5-thiophenedicarboxylate (tdc) molecules (monoclinic, space group <i>P</i>2₁<i>/c</i>, <i>a</i> = 3.4473(1) Å, <i>b</i> = 19.1287(1) Å, <i>c</i> = 11.0069(1) Å, β = 97.48(1)°, <i>V</i> = 719.65(1) ų, and <i>Z</i> = 2). Each ribbon is built of three vertex-sharing chains of edge-sharing MnO₆ octahedrons. These ribbons are bridged together by the carboxylate functions of the tdc molecule to form a pseudo-2D inorganic subnetwork, while this molecule develops in the third dimension to pillar these pseudo-2D layers. An unprecedented hexadentate symmetric bridging mode is adopted by tdc which bridges two chains of a ribbon on one side and two ribbons of a pseudo-2D inorganic subnetwork on the other side. Magnetic measurements suggest that the titled compound is antiferromagnetic below <i>T</i>_N = 17.7 K. Heat capacity measurements confirm the existence of a magnetic phase transition toward a 3D long-range ordered state. These <i>C</i>_P(<i>T</i>) data have also been used for the calculation of the thermal variations of both the adiabatic temperature change Δ<i>T</i>_ad and magnetic entropy change Δ<i>S</i>_m of the material, namely its magnetocaloric effect

Co₄(OH)₂(C₁₀H₁₆O₄)₃ Metal–Organic Framework: Slow Magnetic Relaxation in the Ordered Phase of Magnetic Chains

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co₄(OH)₂(C₁₀H₁₆O₄)₃ metal–organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt­(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χ<i>T</i> product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ_τ/k_B = 67(1) K for the [10–5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded <i>C</i>_P(<i>T</i>) data are used for the calculation of the thermal variations of both the adiabatic temperature change Δ<i>T</i>_ad and magnetic entropy change Δ<i>S</i>_m. The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length

Chemically-Controlled Stacking of Inorganic Subnets in Coordination Networks: Metal–Organic Magnetic Multilayers

Coordination networks (CNs), such as, for instance, metal–organic frameworks (MOFs), can turn into remarkable magnets, with various topologies of spin carriers and unique opportunities of cross-coupling to other functionalities. Alternatively, distinct inorganic subnetworks that are spatially segregated by organic ligands can lead to coexisting magnetic systems in a single bulk material. Here, we present a system of two CNs of general formula Mn­(H₂O)_<i>x</i>(OOC-(C₆H₄)_<i>y</i>-COO). The compound with two water molecules and one aromatic ring (<i>x</i> = 2; <i>y</i> = 1) has a single two-dimensional magnetic subnet, while the material with <i>x</i> = 1.5 and <i>y</i> = 2 shows, additionally, another type of magnetic layer. In analogy to magnetic multilayers that are deposited by physical methods, these materials can be regarded as metal–organic magnetic multilayers (MOMMs), where the stacking of different types of magnetic layers is controlled by the choice of an organic ligand during the chemical synthesis. This work further paves the way toward organic–inorganic nanostructures with functional magnetic properties

Site-Dependent Substitutions in Mixed-Metal Metal–Organic Frameworks: A Case Study and Guidelines for Analogous Systems

Complex architectures are often found among metal–organic framework (MOF) compounds. The mixed-metal approach to this type of material offers an additional degree of structural complexity, and potential tunability of their properties, which remains largely unexplored. We present an in-depth investigation of the crystal chemistry of mixed-metal MOFs based on succinate linkers (C₄H₄O₄) and having the general formula (M′_1–<i>x</i>M″_<i>x</i>)₅(OH)₂(C₄H₄O₄)₄ with M′/M″ = Mn/Co, Fe/Co, and Mn/Fe. The distribution of the metallic elements over three crystallographic sites throughout the different substitutions is finely characterized by resonant contrast diffraction (RCD) experiments corroborated by neutron diffraction (ND) measurements. We observe a size-effect in the filling of the oxygen octahedra, leading to the existence of compounds in which a partial order of the cations over the different metallic sites exists for some compositions of the Co/Mn solid solution. This points out the existence of complex structural phenomena potentially able to influence the physical behavior of such phases and that might, so far, have been overlooked in MOFs. In order to facilitate future studies on mixed-metal MOFs, we consider the possibility of using conventional single-crystal X-ray diffraction (SCXRD) to locate cations of close electronic densities in such cases. The comparison with the results from dedicated measurements based on synchrotron (RCD) and neutron (ND) radiations indicates guidelines for the use of laboratory SCXRD to address mixed-metal MOFs where metal distribution is fundamental to tuning physical properties

From Hydrated Ni₃(OH)₂(C₈H₄O₄)₂(H₂O)₄ to Anhydrous Ni₂(OH)₂(C₈H₄O₄): Impact of Structural Transformations on Magnetic Properties

Dehydration of the hybrid compound [Ni₃(OH)₂(tp)₂(H₂O)₄] (<b>1</b>) upon heating led to the sequential removal of coordinated water molecules to give [Ni₃(OH)₂(tp)₂(H₂O)₂] (<b>2</b>) at <i>T</i>₁ = 433 K and thereafter anhydrous [Ni₂(OH)₂(tp)] (<b>3</b>) at <i>T</i>₂ = 483 K. These two successive structural transformations were thoroughly characterized by powder X-ray diffraction assisted by density functional theory calculations. The crystal structures of the two new compounds <b>2</b> and <b>3</b> were determined. It was shown that at <i>T</i>₁ (433 K) the infinite nickel oxide chains built of the repeating structural unit [Ni₃(μ₃-OH)₂]⁴⁺ in <b>1</b> collapse and lead to infinite porous layers, forming compound <b>2</b>. The second transformation at <i>T</i>₂ (483 K) gave the expected anhydrous compound <b>3</b>, which is isostructural with Co₂(OH)₂(tp). These irreversible transitions directly affect the magnetic behavior of each phase. Hence, <b>1</b> was found to be antiferromagnetic at <i>T</i>_N = 4.11 K, with metamagnetic behavior with a threshold field <i>H</i>_c of ca. 0.6 T. Compound <b>2</b> exhibits canted antiferromagnetism below <i>T</i>_N = 3.19 K, and <b>3</b> is ferromagnetic below <i>T</i>_C = 4.5 K

From Hydrated Ni₃(OH)₂(C₈H₄O₄)₂(H₂O)₄ to Anhydrous Ni₂(OH)₂(C₈H₄O₄): Impact of Structural Transformations on Magnetic Properties

Dehydration of the hybrid compound [Ni₃(OH)₂(tp)₂(H₂O)₄] (<b>1</b>) upon heating led to the sequential removal of coordinated water molecules to give [Ni₃(OH)₂(tp)₂(H₂O)₂] (<b>2</b>) at <i>T</i>₁ = 433 K and thereafter anhydrous [Ni₂(OH)₂(tp)] (<b>3</b>) at <i>T</i>₂ = 483 K. These two successive structural transformations were thoroughly characterized by powder X-ray diffraction assisted by density functional theory calculations. The crystal structures of the two new compounds <b>2</b> and <b>3</b> were determined. It was shown that at <i>T</i>₁ (433 K) the infinite nickel oxide chains built of the repeating structural unit [Ni₃(μ₃-OH)₂]⁴⁺ in <b>1</b> collapse and lead to infinite porous layers, forming compound <b>2</b>. The second transformation at <i>T</i>₂ (483 K) gave the expected anhydrous compound <b>3</b>, which is isostructural with Co₂(OH)₂(tp). These irreversible transitions directly affect the magnetic behavior of each phase. Hence, <b>1</b> was found to be antiferromagnetic at <i>T</i>_N = 4.11 K, with metamagnetic behavior with a threshold field <i>H</i>_c of ca. 0.6 T. Compound <b>2</b> exhibits canted antiferromagnetism below <i>T</i>_N = 3.19 K, and <b>3</b> is ferromagnetic below <i>T</i>_C = 4.5 K