Modelling the properties of magnetic clusters with complex structures: how symmetry can help us

The purpose of this article is to answer the question of how symmetry helps us to investigate and understand the properties of nanoscopic magnetic clusters with complex structures. The systems of choice will be the three types of polyoxometalates (POMs): (1) POMs containing localised spins; (2) reduced mixed-valence (MV) POMs; (3) partially delocalised POMs in which localised and delocalised subunits coexist and interact. The theoretical tools based on various kinds of symmetry are the following: (1) irreducible tensor operator (ITO) approach based on the so-called 'spin-symmetry' and MAGPACK program; (2) group-theoretical assignment of the exchange multiplets based on spin- and point symmetries; (3) group-theoretical classification of the delocalised electronic and electron-vibrational states of MV POMs; (4) general approach (based on spin symmetry) to evaluate the energy levels of large MV clusters and the corresponding MVPACK program; (5) computational approach (employing point symmetry) to solve multidimensional non-adiabatic vibronic problems in the nanoscopic systems realized as VIBPACK software. We made it our goal to avoid a conventional deductive style of presentation. On the contrary, we first consider specially selected complex POMs and then show by what methods and in what way the theoretical problems arising in the description of the properties of these molecules can be properly solved

Spontaneous Magnetization in Heterometallic NiFe-MOF-74 Microporous Magnets by Controlled Iron Doping

We report the direct synthesis of mixed-metal NiFe-MOF-74 solids that display combination of porosity with ferrimagnetic ordering. Compared to the undoped Ni phase, controlled doping with Fe enables to modify intra and interchain magnetic interactions for the onset of spontaneous magnetization at temperatures fixed by the doping level. Synthesis of porous magnets remains somewhat elusive due to the difficulties in isolating foreseeable metal-organic architectures that combine small bridging linkers, for strong magnetic coupling, with polyaromatic connectors responsible for porosity. In turn, we demonstrate that metal doping is better fitted to modify the magnetism of Metal-Organic Frameworks already available simply by suitable choice of their nature and relative ratio in isostructural solid solutions

Vibronic Model for Intercommunication of Localized Spins via Itinerant Electron

Here we propose a vibronic pseudo Jahn-Teller (JT) model for partially delocalized mixed valence (MV) molecules aimed to the description of the magnetic coupling between the localized spins mediated by the delocalized electron. The model involves the following key interactions: electron transfer in the spin-delocalized subsystem which is mimicked by a dimeric unit, coupling of the itinerant electrons with the molecular vibrations and isotropic magnetic exchange between the localized spins and delocalized electron. The pseudo JT vibronic coupling which is considered in the framework of the Piepho, Krausz and Schatz (PKS) model adapted to the case of partially delocalized MV molecules. It is revealed (qualitatively and quantitively) how the vibronic coupling affects the connection of the localized spins via the itinerant electron

Vibronic recovering of functionality of quantum cellular automata based on bidimeric square cells with violated condition of strong Coulomb repulsion

Strong Coulomb repulsion between the two charges in a square planar mixed-valence cell in quantum cellular automata (QCA) allows us to encode the binary information in the two energetically beneficial diagonal distributions of the electronic density. In this article, we pose a question: to what extent is this condition obligatory for the design of the molecular cell? To answer this question, we examine the ability to use a square-planar cell composed of one-electron mixed valence dimers to function in QCA in a general case when the intracell Coulomb interaction U is not supposed to be extremely strong, which means that it is comparable with the characteristic electron transfer energy (violated strong U limit). Using the two-mode vibronic model treated within the semiclassical (adiabatic) and quantum-mechanical approaches, we demonstrate that strong vibronic coupling is able to create a considerable barrier between the two diagonal-type charge configurations, thus ensuring bistability and polarizability of the cells even if the Coulomb barrier is not sufficient. The cases of weak and moderate Coulomb repulsion and strong vibronic coupling are exemplified by consideration of the cation radicals of the two polycyclic derivatives of norbornadiene [C12H12]+ and [C17H16]+ with the terminal C=C chromophores playing the role of redox sites. By using the detailed ab initio data, we reveal the main characteristics of the bi-dimeric cells composed of these molecules and illustrate the pronounced effect of the vibronic recovery clearly manifesting itself in the shape of the cell-cell response function. Revealing such 'vibronic recovery' of strong localization when the strong U limit is violated suggests a way to a significant expansion of the class of molecular systems suitable as QCA cells

The azido ligand: a useful tool in designing chain compounds exhibiting alternating ferro- and antiferro-magnetic interactions

A one-pot reaction of NiII 1, CoII 2, FeII 3 and MnII 4 with 2,2A-bipyridine (bipy) and azide in water leads to [M(bipy)(N3)2]n chains where the metal ion is alternatively bridged by double end-on (EO) and end-to-end (EE) azido bridges; theoretical analysis of the variable-temperature magnetic susceptibility data of 1 and 4 reveals the occurrence of intrachain alternating ferro- (through EO) and antiferro-magnetic (through EE) interactions.Julve Olcina, Miguel, [email protected] ; Lloret Pastor, Francisco, [email protected] ; Clemente Juan, Juan Modesto, [email protected]

Heptanuclear hydroxo-bridged copper cluster of the dicubane-like type: structural and magnetic characterisations of [Cu7(OH)6Cl2(pn)6(H2O)2](C(CN)3)4Cl2 (pn = 1,3-diaminopropane)

A new polynuclear copper(II) complex [Cu7(OH)6Cl2- (pn)6(H2O)2](C(CN)3)4Cl2 with hydroxo-bridging ligands has been prepared; the centrosymmetric cluster cation can be described as two Cu4O3Cl distorted cubane units sharing one copper cation.Clemente Juan, Juan Modesto, [email protected] ; Gomez Garcia, Carlos Jose, [email protected] ; Coronado Miralles, Eugenio, [email protected]

Designing binuclear transition metal complexes: a new example of the versatility of N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6

[Abstract] N,N′-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4·CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(μ-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0–300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S = 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin–orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S = 3/2 ground state is operative.Galicia. Consellería de Innovación, Industria e Comercio; PGIDIT03TAM10301PRMinisterio de Educación y Ciencia; CTQ 2004-0345

Deciphering the Role of Dipolar Interactions in Magnetic Layered Double Hydroxides

Layered double hydroxides (LDHs) exhibit unparalleled anion exchange properties and the ability to be exfoliated into 2D nanosheets, which can be used as a building block to fabricate a wide variety of hybrid functional nanostructured materials. Still, if one wants to use LDHs as a magnetic building blocks in the design of complex architectures, the role played by the dipolar magnetic interactions in these layered materials needs to be understood. In this work, we synthesized and characterized a five-membered CoAl-LDH series with basal spacing ranging from 7.5 to 34 Å. A detailed experimental characterization allows us to conclude that the main factor governing the dipolar interactions between magnetic layers cannot be the interlayer spacing. Supporting theoretical modeling suggests instead a relevant role for spin correlation size, which, in the limit, is related to the lateral dimension of the layer. These results highlight the importance of cation ordering in the magnetic behavior of LDHs, and underpin the differences with homometallic-layered hydroxides.MAT2017-89993-RCTQ2017-89528-PMDM-2015-0538ERC-2014-CoG/ 647301CA15128Layered double hydroxides (LDHs) exhibit unparalleled anion exchange properties and the ability to be exfoliated into 2D nanosheets, which can be used as a building block to fabricate a wide variety of hybrid functional nanostructured materials. Still, if one wants to use LDHs as a magnetic building blocks in the design of complex architectures, the role played by the dipolar magnetic interactions in these layered materials needs to be understood. In this work, we synthesized and characterized a five-membered CoAl-LDH series with basal spacing ranging from 7.5 to 34 Å. A detailed experimental characterization allows us to conclude that the main factor governing the dipolar interactions between magnetic layers cannot be the interlayer spacing. Supporting theoretical modeling suggests instead a relevant role for spin correlation size, which, in the limit, is related to the lateral dimension of the layer. These results highlight the importance of cation ordering in the magnetic behavior of LDHs, and underpin the differences with homometallic-layered hydroxides

A novel high-spin heterometallic Ni12K4 cluster incorporating large Ni–azide circles and an in situ cyanomethylated di-2-pyridyl ketone

Reaction of di-2-pyridyl ketone (dpk) with nickel acetate and azide in the presence of potassium tert-butylate as a catalytic base generates the title compound, which contains the largest [Ni(m1,1-N3)]6 circles in the discrete ferromagnetically-coupled MII–azide cluster family, and shows an unprecedented in situ cyanomethylation of ketone.Clemente Juan, Juan Modesto, [email protected]