Single-chain Magnet Research Articles

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2 Co]-Assembled Mixed-Valence Chain.

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although it is currently difficult. Herein, we assembled the asymmetric {Fe2 Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3 ]2 Co(BIT)} ⋅ 2CH3 OH (1) (Tp=hydrotris(pyrazolyl)borate and BIT=3,4-bis-(1H-imidazol-1-yl)thiophene), which showed reversible multi-phase transitions accompanied by photo-switchable single-chain magnet properties and a dielectric anomaly. Variable-temperature X-ray structural studies revealed thermo- and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off single-chain magnet behavior by alternating 946-nm and 532-nm light irradiation. A substantial anomaly in the dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

A single chain magnet based on a 1D polymeric selenite-bridged Mn(iii) complex with an N2O2 donor

A novel molecular magnetic material with the formula of {[Mn2(salen)2(C4H3SSeO2)](ClO4)}n (Complex 1) [salen = N,N′-(ethylene)bis(salicylideneiminato)] was designed, synthesized and characterized structurally and magnetically.

Weak interchain interaction-dominated magnetic responses in water-extended cobalt(ii)-chains: from magnetic ordering to single-chain magnet

Weak intermolecular interaction-dominated interchain magnetic couplings in water-extended cobalt(ii)-chains are found to be highly responsible for the magnetic evolution from magnetic ordering to single-chain magnet behavior.

The rationalized pathway from field-induced slow magnetic relaxation in CoII–WIV chains to single-chain magnetism in isotopological CoII–WV analogues

The change of the oxidation state from WIV to WV in isotopological CoII–[W(CN)8]n− chains leads to the appearence of pronounced single-chain magnet behaviour.

Light actuated single-chain magnet with magnetic coercivity

A cyanide-bridged {Fe2Co}-based coordination polymer was synthesized. It showed photo-induced slow relaxation of magnetization and a coercive field of 400 Oe.

Polyoxometalate-Assisted Assembly of Pearl-Chain-Like Cyanide-Bridged Single-Chain Magnets

The introduction of Keggin-type POMs of [PMo12O40]3- or [SiW12O40]4- as counteranions into the FeIII-MII cyanometalate system afforded three chain complexes: [(Tp*)Fe(CN)3Ni(DMF)4]2{[(Tp*)Fe(CN)3Ni(DMF)3(H2O)]2Ni(DMF)4}[PMo12O40]2·14DMF (1, Tp*= hydridotris(3,5-dimethylpyrazol-1-yl)borate) and {[(Tp*)Fe(CN)3M(DMF)3(H2O)]2M(DMF)4}[SiW12O40]·3DMF (2, M = NiII; 3, M = CoII). Complex 1 contains both discrete cationic [Fe2Ni2]2+ squares and less-studied {Fe2Ni3}n pearl chains, namely 3,2-chains, while 2 and 3 consist of pure 3,2-chains due to the replacement of [PMo12O40]3- with [SiW12O40]4- bearing one more negative charge. Magnetic studies revealed that all of the complexes exhibit single-chain-magnet (SCM) behaviors with the effective thermal barriers of Δτ1/kB = 61.6 K (infinite regime) and Δτ2/kB = 36.5 K (finite regime) for 1, Δτ/kB = 46.9 K for 2 (finite), and Δτ/kB = 30.6 K for 3 (finite). The POM moieties may play a pivotal role for the realization of this promising archetype of favoring SCM property: (1) the highly negatively charged POMs may facilitate the formation of the uncommon highly positive "pearl chain"; (2) the nanosized POMs necessarily led to the good isolation of the chains in the title complexes, and (3) the employment of POMs with different charges may regulate the resultant complexes in both structure and magnetism.

New Materials and Effects in Molecular Nanomagnets

Molecular magnets are a relatively new class of purely organic or metallo-organic materials, showing magnetism even without an external magnetic field. This interdisciplinary field between chemistry and physics has been gaining increased interest since the 1990s. While bulk molecular magnets are usually hard to build because of their molecular structures, low-dimensional molecular magnets are often easier to construct, down to dot-like (zero-dimensional) structures, which are investigated by different scanning probe technologies. On these scales, new effects such as superparamagnetic behavior or coherent switching during magnetization reversal can be recognized. Here, we give an overview of the recent advances in molecular nanomagnets, starting with single-molecule magnets (0D), typically based on Mn12, Fe8, or Mn4, going further to single-chain magnets (1D) and finally higher-dimensional molecular nanomagnets. This review does not aim to give a comprehensive overview of all research fields dealing with molecular nanomagnets, but instead aims at pointing out diverse possible materials and effects in order to stimulate new research in this broad field of nanomagnetism.

Crystal structure, spectral investigation, thermal properties and evaluation of the antimicrobial behavior of a new 2D polymeric Mn(II): (C6H9N2)2[Mn(C2O4)2].H2O

A new 2D polymeric Mn(II) has been prepared by self-assembly of manganese acetate with oxalic acid and 2-amino-4-picolineand elucidated through single-crystal X-ray diffraction. The complex (C6H9N2)2[Mn(C2O4)2].H2O was considered as a single-chain magnet based on the oxalate linker, intercalated by [C6H9N2]2+ organic cations and one crystallization water molecule that are entrapped between the layers. The Mn(II) ion is coordinated in a slightly distorted octahedral environment by six oxygen atoms belonging to three chelating oxalates. Thermal analysis, FTIR and UV–Vis diffuse reflectance spectroscopy from which the optical properties were determined. The described complex gave antibacterial activity especially against gram-positive bacteria tested species, antifungal activity against Candida albicans specie, as well as lysozyme and anti-biofilm activities.

A single-chain magnet based on bis(end-on azido/alkoxo)-bridged linear [MnIII2MnII] repeating units

The synthesis, structural characterization and magnetic studies of the 1D coordination polymer [MnIII2MnII(L)2(N3)2(MeOH)2]n (1) is presented. Compound 1 consists of MnIII-MnII-MnIII strictly linear trinuclear repeating units with bis(end-on azido/alkoxo) bridges between the central MnII ion and each one of the terminal MnIII ions, defining two rhombic units [MnIIIMnII(μ1,1-N3)(μ-OR)]. Each repeating unit is further linked to its neighboring ones through two μ-OR groups, defining rhombic units [MnIII2(μ-OR)2], thus promoting the chain structure of 1. The magnetic studies revealed that 1 displays single-chain magnet behavior (SCM) with energy barrier Δ = 53 ± 1 K. Compound 1 is the first example of SCM based on a mixed-valence linear trinuclear unit [MnIII2MnII(μ1,1-N3)2(μ-OR)2].

Switching the magnetic hysteresis of an [Feii-NC-Wv]-based coordination polymer by photoinduced reversible spin crossover.

Magnetic bistable materials that feature magnetic hysteresis are comparable to elementary binary units and promising for application in switches and memory devices. In this work, we report a material that consists of parallel cyanide-bridged [Feii-Wv] coordination chains linked together through rigid bis(imidazolyl)-benzene ligands and displays multiple magnetic states. The paramagnetic high-spin and diamagnetic low-spin states of the spin-crossover Feii ions can be interconverted by reversible light-induced excited spin state trapping (LIESST) by alternating between light irradiation of 808 and 473 nm. At 1.8 K, under 808-nm-light irradiation, magnetic interactions between the photogenerated paramagnetic high-spin Feii centres and the Wv centres lead to long fragments that exhibit single-chain magnet behaviour, with a wide magnetic hysteresis and a large coercive field of 19 kOe; under a 473 nm light, isolated Feii-Wv fragments behave as single-molecule magnets instead. At 3.3 K, the high-spin form still displays magnetic hysteresis, albeit narrower, whereas the low-spin one does not.

A Family of Lanthanide Hydroxo Carboxylates with 1D Polymeric Topology and Ln4 Butterfly Core Exhibits Switchable Supramolecular Arrangement.

The unique family of coordination polymers [Ln4(OH)2(piv)10(H2O)2]∞ of 11 lanthanides (Ln = La-Er) has been prepared by a simple solution method based on controlled hydrolysis. The ribbon-like polymeric structure consisting of connected tetranuclear clusters and supported by pivalate ligands and a framework of H-bonds has been revealed by single-crystal X-ray diffraction. While the compounds demonstrate similar PXRD patterns and unit cell parameters, the joint single-crystal XRD and pair distribution function data suggest the significant local structure change along the lanthanide series. The compounds exist as two packing polymorphs (α and β) with similar ribbon geometry, but different supramolecular arrangement of the ribbons. Dehydration of either polymorph does not disturb the tetranuclear core but leads to a translational symmetry loss along the ribbon and a transformation of the 3D-ordered crystal into a 2D-ordered mesostructure. Rehydration of the mesostructure leads to the β polymorph (except La and Ce), allowing the deliberate switching between the polymorphs via dehydration-rehydration evidenced by means of powder X-ray diffraction, pair distribution function analysis, and density functional theory calculations. Ab initio calculations reveal significant magnetic anisotropy of Ln3+ ions with ferro- and antiferromagnetic interactions within tetranuclear [Ln4(OH)2(piv)10(H2O)2] species. Magnetic susceptibility measurements demonstrated antiferromagnetic coupling, slow magnetic relaxation for Dy, Ho, and Er complexes, and field-induced single-chain magnetism for the Dy compound.

Asymmetric Coordination Toward a Photoinduced Single-Chain Magnet Showing High Coercivity Values.

The production of photo-switchable molecular nanomagnets with substantial coercivity, which is indispensable for information storage and process applications, is challenging. Introducing photo-responsive spin-crossover units provides a feasible means of controlling the magnetic anisotropy, interactions, and overall nanomagnet properties. Herein, we report a cyanide-bridged chain 1⋅12H2 O ({[(Pz Tp)FeIII (CN)3 ]2 FeII (Pmat)2 }n ⋅12 H2 O) generated by linking the FeII -based spin-crossover unit with the [(Pz Tp)Fe(CN)3 ]- (Pz Tp: tetrakis(pyrazolyl)borate) building block in the presence of asymmetric ditopic ligand Pmat ((4-pyridine-4-yl)methyleneamino-1,2,4-triazole). Structural characterization revealed that the introduction of this asymmetric ligand led to a distorted coordination environment of FeII ions, which were equatorially coordinated by four cyanide N atoms, and apically coordinated by one pyridine N atom and one triazole N atom. Upon 808-nm light irradiation, 1⋅12H2 O underwent photoinduced spin-crossover and exhibited single-chain magnet behavior with a coercive field of up to 1.3 T. This represents a 3d-based photoinduced single-chain magnet exhibiting pronounced hysteresis.

Origin of Magnetic Relaxation Barriers in a Family of Cobalt(II)-Radical Single-Chain Magnets: Density Functional Theory and Ab Initio Calculations.

Density functional theory (DFT) and ab initio calculations were performed to probe the origin of the magnetic relaxation barriers for two finite single-chain magnets (SCMs) featuring a one-dimension chain, Co(hfac)2(R-NapNIT) (R-NapNIT = 2-(2'-(R-)naphthyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, R = MeO (1) or EtO (2)). Our calculations show that the strong intrachain CoII-CoII exchange coupling interactions transmitted by radicals can contribute much more than ionic anisotropy to the height of the reversal barrier of magnetization for the single-chain magnets (SCMs) with |2E| < |4J/3|. In addition, the anisotropic energy barrier ΔA decreases with the decrease of |2E/J| ratio and finally vanishes in the limit of broad domain walls (|2E| < < |4 J/3|). Therefore, the total magnetic relaxation energy barriers of two SCMs mostly originate from the correlation energy barrier Δξ deriving from the indirect ferromagnetic interaction between CoII-CoII transmitted by the strong CoII-radical antiferromagnetic interactions.

Magnetic investigations of monocrystalline [Co(NCS)2(L)2]n: new insights into single-chain relaxations.

A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.

Single-chain magnet behavior in a finite linear hexanuclear molecule.

The careful monitoring of crystallization conditions of a mixture made of a TbIII building block and a substituted nitronyl-nitroxide that typically provides infinite coordination polymers (chains), affords a remarkably stable linear hexanuclear molecule made of six TbIII ions and five NIT radicals. The hexanuclear units are double-bridged by water molecules but ab initio calculations demonstrate that this bridge is inefficient in mediating any magnetic interaction other than a small dipolar antiferromagnetic coupling. Surprisingly the hexanuclears, despite being finite molecules, show a single-chain magnet (SCM) behavior. This results in a magnetic hysteresis at low temperature whose coercive field is almost doubled when compared to the chains. We thus demonstrate that finite linear molecules can display SCM magnetic relaxation, which is a strong asset for molecular data storage purposes because 1D magnetic relaxation is more robust than the relaxation mechanisms observed in single-molecule magnets (SMMs) where under-barrier magnetic relaxation can operate.

A cyanide-bridged Fe-Co pearl-chain-like single-chain magnet containing 4-coordinate cobalt(II) ions.

Treatment of CoCl2·6H2O and tris(pyrazolyl-1-yl)borate tricyanoiron(III) anions at an elevated temperature (55 °C) afforded two less-common pearl-chain-like compounds, {[(TpR)Fe(CN)3CoCl2]2Co(DMF)4}·nDMF (1, TpR = Tp4-Me = hydridotris(4-methylpyrazol-1-yl)borate, n = 1 and 2, TpR = Tp*Me = hydridotris(3,4,5-trimethylpyrazol-1-yl)borate, n = 4.5), in which the 4-coordinate Co(II) ions and [(TpR)FeIII(CN)3]- units are alternately bridged by cyanide groups into squares, which are further linked with the 6-coordinate Co(II) ions into an infinite chain. Interestingly, the magnetic study revealed that 1 exhibits a typical single-chain magnet behaviour with an effective energy barrier of 28.0 K, while surprisingly no Glauber dynamics was observed for 2 despite their very similar structures. The variations of the local coordination environments of the cobalt ions and the cyanide linkages were evidenced, and they may account for the significant difference in their magnetic properties related to the global magnetic anisotropy and magnetic exchange of the chain.

Effective easy-axis anisotropy of the two-sublattice single-chain magnet with twisted easy planes

Effective easy-axis anisotropy of the two-sublattice single-chain magnet with twisted easy planes

Magnetically coupled iron azide chains

Three well-defined iron(II) compounds CztBu(PyriPr)2FeCl (1), CztBu(PyriPr)2FeCl(THF) (2), and CztBu(PyriPr)2FeN3 (3), supported by the NNN pincer ligand CztBu(PyriPr)2− were synthesized and characterized.Compound 3 features a one-dimensional chain structure built by CztBu(PyriPr)2Fe units and bridged by end-to-end azido ligands. This chain material exhibits magnetic hysteresis at low temperatures and non-superimposable zero-field cooled and field cooled magnetization. These properties are consistent with single-chain magnet behavior, likely arising from superexchange coupling of Fe(II) centers via the azide bridges.

Coordinated Molecule-Modulated Magnetic Phase with Metamagnetism in Metal–Organic Frameworks

Metal Organic Frameworks (MOFs) are a very prominent research topic because of their potential uses in applications such as molecular devices, gas storage systems, and especially single chain magnet (SCM) materials. Among these MOFs, those that feature open metal sites (Ni2+) inherently possess spin states that are tunable via organic molecular coordination. They have demonstrated the potential of SCM materials, exhibiting the possibilities for the spin-canting, magnetic phase transition, and field-induced spin slow dynamics behavior.In this paper, we experimentally and theoretically investigated the magnetic properties of a MOF possessing an open metal site, and its derivatives. It exhibits a magnetic ordered state at low temperatures, which can be altered by altering its imidazole (IM) coordination. Our results show that coordinated IM clearly influences the spin state of Ni(II) ions with orbital geometries, resulting in shifts in critical temperature and field. These obtained results imply that the magnetic behavior in MOFs with IM reveals the coexistence of spin-canting, metamagnetism, magnetic phase transitions, and field-induced spin slow dynamics.To the best of our knowledge, this is the first example of a MOF exhibiting the influence of the spin state of Ni(II) ions with orbital geometries via organic molecular coordination, which shows the coexistence of spin-canting, metamagnetism, magnetic phase transition, and field-induced spin slow dynamics.

Next Generation Multifunctional Nano-Science of Advanced Metal Complexes with Quantum Effect and Nonlinearity

AbstractMy research target can be described as “Next Generation Multifunctional Nano-Science of Advanced Metal Complexes with Quantum Effect and Nonlinearity”. My work encompasses four important key areas: (1) inorganic-organic hybrid systems, (2) nano-size and nano-space, (3) bottom-up and self-assembly, and (4) nonlinearity and quantum effect. Among them, nonlinearity and quantum effect are the most important for nano-science of advanced metal complexes. I have been working on these two topics (nonlinearity and quantum effect) simultaneously for more than 40 years. As for quantum effect, I have focused on Haldane gap systems, single-chain magnets (SCMs), single-molecule magnets (SMMs), Kondo resonance on SMMs, photo-switchable SMMs, metallic conducting SMMs, SMMs encapsulated into single-walled carbon nanotube (SWCNT), and metal-organic framework (MOF)-spintronics for spin qubits, for pursuing high-density memory devices and quantum computing. As for nonlinearity, I have focused on quasi-one-dimensional halogen-bridged metal complexes (MX-Chains; M = Pt, Pd, and Ni; X = Cl, Br, and I) with nonlinear excitons such as solitons and polarons, strongly electron-correlated Ni(III) complexes with gigantic third-order optical nonlinearity, and phase transitions and charge fluctuations between Pd(III) averaged states (= Mott insulator) and Pd(II)-Pd(IV) mixed-valence states (= charge density wave states), for pursuing optical communication, optical switching, and optical computing. In this review article, I will describe the above main topics (quantum effect and nonlinearity) according to my research history of more than 40 years, respectively. Finally, I will propose future perspectives for the two topics.