An invitation to molecular magnetism

A high-spin s-triazine linked fluorenyl radical polymer

The syntheses of high-spin organic polymers have been a daunting task due to the highly reactive nature of organic radicals, especially when they are ferromagnetically coupled. In this paper, we report our approach to obtain high-spin organic polymers, in which a reasonably stable fluorenyl radical was employed as the primary radical unit, and s-triazine serves as the connector that facilitates ferromagnetic coupling between them. Initially, the diamagnetic polymer precursor was synthesized by cyclotrimerization of a cyano-monomer. Subsequently, the high-spin polymers were obtained by oxidizing corresponding anionic polymers using O2 (6) or I2 (7). The temperature-dependent magnetic moments, and field-dependent magnetization data obtained from SQUID measurements revealed ferromagnetic couplings between primary radical units, with coupling J = 7.5 cm-1 and 38.6 cm-1. The percentages of primary unit in the radical form are 29%, and 47% for 6 and 7, respectively. Notably, this marks the first reported instance of a high-spin fluorenyl radical polymer exhibiting ferromagnetic coupling.

Realizing Two-Dimensional Supramolecular Arrays of a Spin Molecule via Halogen Bonding

Well-ordered spin arrays are desirable for next-generation molecule-based magnetic devices, yet their synthetic method remains a challenging task. Herein, we demonstrate the realization of two-dimensional supramolecular spin arrays on surfaces via halogen-bonding molecular self-assembly. A bromine-terminated perchlorotriphenylmethyl radical with net carbon spin was synthesized and deposited on Au(111) to achieve two-dimensional supramolecular spin arrays. By taking advantage of the diversity of halogen bonds, five supramolecular spin arrays form and are probed by low-temperature scanning tunneling microscopy at the single-molecule level. First-principles calculations verify that the formation of three distinct types of halogen bonds can be used to tailor supramolecular spin arrays via molecular coverage and annealing temperature. Our work suggests that supramolecular self-assembly can be a promising method to engineer two-dimensional molecular spin arrays.

Integration of Trinuclear Triangle Copper(II) Secondary Building Units in Octacyanidometallates(IV)-Based Frameworks

The design and synthesis of high-dimensional materials based on secondary building blocks (SBUs) play a pivotal role in the further development of functional molecular materials. Herein, the self-assembly of Cu(II) ions, pyrazole (Hpz), and octacyanidometallate(IV) anions in the presence of water produced two new isostructural three-dimensional systems {[Cu₃(μ₃-OH)(μ-pz)₃(H₂O)₃]₂[M(CN)₈]}·nH₂O (M = W, 1, and Mo, 2). 1 and 2 consist of trinuclear triangle copper(II) (TTC) SBUs and octacyanidometallates(IV). At room temperature, both assemblies display strong antiferromagnetic interactions within the TTC entities with an average Cu^II···Cu^II isotropic magnetic coupling constant of about -145 cm^-1. Moreover, a detailed analysis of magnetic data revealed the presence of spin frustration with antisymmetric magnetic exchange-coupling constants of around +32 and +46 cm^-1 for 1 and 2, respectively. Finally, quantum chemical calculations explained their magnetic and optical properties.

Tuning of magnetic properties of the 2D CN-bridged NiII-NbIV framework by incorporation of guest cations of alkali and alkaline earth metals

The reaction between [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) and [Nb(CN)8]4- in concentrated water solutions of different s-block metal salts leads to the formation of 2-dimensional honeycomb-like coordination networks of the formula Mx[Ni(cyclam)]3[Nb(CN)8]2·nH2O (x = 2: M = Li+, Na+; x = 1: M = Mg2+, Ca2+, Sr2+, Ba2+). The CN-bridged Ni-Nb coordination layers are intersected by channels filled with crystallisation water molecules and guest mono- or di-valent metal cations, which compensate the negative charge of the framework. The structural details and crystal symmetry vary between the networks, depending on the arrangement of the water molecules and the intermolecular interactions enforced by the guest cations. All compounds show long range magnetic order arising from superexchange interactions between paramagnetic NiII (s = 1) and NbIV (s = 1/2) centres through CN-bridges within the layers and weaker inter-layer interactions mediated by H-bonds. The ordering temperature as well as the coercive field of the magnetic hysteresis can be tuned by the type of guest cation, with the highest values achieved for Mg2+ and the lowest for Na+.

Magnetocaloric Effect in Cu5-NIPA Molecular Magnet: A Theoretical Study

We calculated the magnetocaloric properties of the molecular nanomagnet Cu5-NIPA, consisting of five spins S = 1 / 2 arranged in two corner-sharing triangles (hourglass-like structure without magnetic frustration). The thermodynamics of the system in question was described using the quantum Heisenberg model solved within the field ensemble (canonical ensemble) using exact numerical diagonalization. The dependence of the magnetic entropy and magnetic specific heat on the temperature and the external magnetic field was investigated. The isothermal entropy change for a wide range of initial and final magnetic fields was discussed. Due to plateau-like behavior of the isothermal entropy change as a function of the temperature, a high degree of tunability of magnetocaloric effect with the initial and final magnetic field was demonstrated.

Humidity-A Powerful Tool to Customize the Physical Properties of Molecular Magnets

Since the dawn of the century, people have been fascinated by the mysterious force that draws two pieces of lodestone (magnetite, Fe₃ O₄ ) to each other as well as by property of stilbite (natural zeolite, NaCa₄ [Si₂₇ Al₉ O₇₂ ]⋅28 H₂ O) to generate huge amounts of steam by rapidly heating the material. Nowadays, we know the first effect is addressed by magnetic attraction whereas the second one is related to the reversible sorption/desorption of water from humid air inside porous materials. For a long time, it was thought that these two, at first glance, were disjunctive properties that cannot be combined into one material. Nevertheless, the scientists have once again proved that there are no impossible things. Based on the discovery of the first molecular magnets, the idea to combine magnetic properties with other functionalities such as porosity gained great consideration by scientists from different research fields. Very soon, we witnessed numerous reports of novel multifunctional materials among which we can distinguish humidity-responsive magnets. In this manuscript, the most outstanding results for such systems working at normal temperature and pressure (NTP) will be presented to motivate in-depth research on this topic.

Humidity-Induced Switching between Two Magnetic and Structural Phases in a CoII -[WV (CN)8 ] Molecular Magnet

The self-assembly of cobalt(II) with purine and octacyanidotungstate(V) results in the formation of the three-dimensional Co₃ [W(CN)₈ ]₂ (purine)₂ ⋅8.5H₂ O (1) coordination polymer. This compound exhibits humidity-induced variation of the number of water molecules of crystallisation leading to a reversible structural phase transition and the alternation of the long-range ferromagnetic ordering temperature from T_C =29 K for the pristine assembly (1) to T_C =49 K for the sample stored in a low-humidity atmosphere (1-deh). This phenomenon can be attributed to a reversible change in the hydrogen-bonding network resulting in the modification of the local geometries of cobalt(II) as well as the cyanido bridges.

Multifunctionality in molecular magnetism

Molecular magnetism draws from the fundamental ideas of structural chemistry and combines them with experimental physics resulting in one of the highest profile current topics, namely molecular materials that exhibit multifunctionality. Recent advances in the design of new generations of multifunctional molecular magnets that retain the functions of the building blocks and exhibit non-trivial magnetic properties at higher temperatures provide promising evidence that they may be useful for the future construction of nanoscale devices. This article is not a complete review but is rather an introduction into thefascinating world of multifunctional solids with magnetism as the leitmotif. We provide a subjective selection and discussion of the most inspiring examples of multifunctional molecular magnets: magnetic sponges, guest-responsive magnets, molecular magnets with ionic conductivity, photomagnets and non-centrosymmetric and chiral magnets.