Molecular Magnetic Properties Research Articles

Cleavage of the C-H Bond in Bu3MeP+ by Zinc Porphyrin Dianions: Formation of {ZnII(CH2PBu3)(TPyPH)}- Containing Zn-C(ylide) Bond and the (TPyPH)3- Macrocycle Showing Strong NIR Absorption.

Reduction of {ZnII(TPyP)} to the {ZnII(TPyP)}2- dianions (TPyP: tetra(4-pyridyl)porphyrin) in the presence of Bu3MeP+ allows one to observe the C-H bond cleavage in the methyl group of Bu3MeP+ to form (Bu3MeP+){ZnII(CH2PBu3)(TPyPH)}-·0.337C6H5CH3 (1). Salt 1 is the first coordination complex of neutral CH2PBu3 ylide and metalloporphyrin. The released hydrogen atom attacks the meso-carbon atom of TPyP4- forming a TPyPH3- macrocycle related to phlorins. Decreased symmetry of the TPyPH3- allows the observation of a strong NIR absorption. We discuss the molecular structure, optical, and magnetic properties of 1 together with its formation pathways.

Molecular structure, magnetic properties and catalytic activity in selective ethylene dimerization of nickel (II) complexes with bis(3,5-dimethylpyrazol-1-yl)methane

Nickel(II) complexes with neutral bidentate nitrogen-containing ligands are of great interest as promising precatalysts for the selective oligomerization of ethylene. Complexes of compositions 0.75[LNiCl2]2•0.25[LNiClBr]22 and LNiBr23 where L = bis-(3,5-dimethylpyrazol-1-yl)methane have been synthesized and characterized by single crystal X-ray diffraction, IR and 1H NMR spectroscopies and elemental analysis. The 1H NMR spectra of paramagnetic complexes indicate that the structures of these complexes in solution are similar to ones in the solid state. Magnetic susceptibility of the compounds has been investigated and discussed in the context of their structures. Treatment of these complexes with diethylaluminum chloride (DEAC) or ethylaluminum sesquichloride (EASC) generates active catalysts that oligomerize ethylene to yield mostly butenes with activities up to 930 kg(oligomers)·mol[Ni]−1·h−1 (DEAC).

Electrospray Deposition and Magnetic Properties of Prototypical Molecular Magnets

The controlled deposition, characterization and manipulation of single molecule magnets (SMMs) on surfaces is one of the crucial points to be addressed with regard to their possible implementation as functional units in future electronic and spintronic devices. Profound understanding of molecule-molecule and molecule-substrate interactions is required as well as unraveling their effect on the molecular electronic and magnetic properties. Local measurement techniques like scanning tunneling microscopy (STM) together with ensemble-averaging methods like X-ray absorption spectroscopy (XAS) have been proven to set up an appropriate frame to study these materials. The majority of these studies deal with SMMs that exhibit rather simple structures with mostly only one magnetic ion. The situation gets more complicated when it comes to larger polynuclear compounds that can be quite fragile with respect to surface deposition or not easy to organize on surfaces due to their bulky ligand shell. Here, we provide an overview of our results on successful deposition of polynuclear SMMs on functional surfaces by employing the electrospray ion beam deposition method. For two prototypes in the field, Mn12-ac and Fe4H, we obtain highly ordered submonolayers on functional surfaces and elucidate the electronic coupling to the respective substrates using scanning tunneling spectroscopy (STS). New results for Mn12-ac on graphene/Ir(111) and for Fe4H on Au(111) are compared to previous studies on a decoupling graphene layer. X-ray magnetic circular dichroism (XMCD) measurements on submonolayers of uniformly aligned Fe4 molecules on both substrates reveal its robust magnetism, showing magnetic anisotropy values similar to bulk.

Flavanthrone – a new ligand with accessible radical anion and dianion states: preparation of zwitterionic {(Cp2V)2(flavanthrone)} and {(Cp2V)2(chloranil)} complexes

Molecular structure, optical and magnetic properties of the radical anions and dianions of flavanthrone dye are presented. Flavanthrone and chloranil coordinate two vanadocenes forming zwitter-ionic {(Cp2V+)2(Flavanthrone2−)} and {(Cp2V+)2(QCl42−)}.

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.

Ligand Structure Effects on Molecular Assembly and Magnetic Properties of Copper(II) Complexes with 3-Pyridyl-Substituted Nitronyl Nitroxide Derivatives.

Reaction of Cu(hfac)2 with methyl- and bromo-3-pyridyl-substituted nitronyl nitroxides (LR) leads to assemble a diverse set of coordination complexes: mononuclear [Cu(hfac)2L2-Me], binuclear [{Cu(hfac)2}2(H2O)L2-Me], trinuclear [{Cu(hfac)2}3(L6-Br)2], pentanuclear [{Cu(hfac)2}5(L2-Me)2], and [{Cu(hfac)2}5(L2-Me)4], cocrystals [Cu(hfac)2(L2-Br)2]·[Cu(hfac)2(H2O)2] and [Cu(hfac)2(L2-Br)2]·2[Cu(hfac)2H2O], one-dimensional polymers [Cu(hfac)2L2-Br]n and [Cu(hfac)2L6-Br]n, and cyclic dimers [Cu(hfac)2L5-Me]2, [Cu(hfac)2L5-Br]2, and [Cu(hfac)2L6-Me]2. The molecular structures of the obtained complexes are strongly affected by the substituent type and its location in the pyridine heterocycle. Occupation of the second position of the pyridine ring increases the steric hindrance of both imine and nitroxide coordination sites of L2-R, which is favorable for the formation of various conformers and precipitation of complexes with different molecular structures. The pentanuclear [{Cu(hfac)2}5(L2-Me)2] and [{Cu(hfac)2}5(L2-Me)4] complexes do not have prior analogues and are valuable model objects for investigation of the mechanism of formation of various coordination polymers. The arrangement of long Cu–ONO bonds in {CuO6} square bipyramids due to the weakening nitroxide donor site in complexes, based on L2-Me, L2-Br, and L6-Br ligands, results in ferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides. Complexes with substituents that do not considerably affect the coordination ability of ligands (L5-Me, L5-Br, and L6-Me) exhibit strong antiferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides.

Magnetic configurations of open-shell molecules on metals: The case of CuPc and CoPc on silver

For nanostructured interfaces between open-shell molecules and metal surfaces that involve charge transfer upon adsorption, the investigation of molecular magnetic properties is an interesting yet difficult task, because in principle different magnetic configurations with distinct properties can be found. Here, we study the magnetic properties of CuPc-Ag and CoPc-Ag interfaces, which constitute interesting test cases because charge is transferred to the initially open-shell Pc molecules upon adsorption. Using hybrid density functional theory, we examine the stability of the various magnetic configurations occurring at these nanoscale interfaces, as well as for the corresponding gas-phase anions, and compare our findings to those of previous experimental studies. For CuPc-Ag, we identify a high-spin triplet configuration as the most likely configuration at the interface, whereas for CoPc-Ag a quenching of the total magnetic moment is found. Interestingly, such quenching is consistent with two distinctly different interfacial electronic configurations. These important differences in the magnetic properties of CuPc and CoPc on Ag are rationalized by variations in the interaction of their central metal atoms with the substrate. Our work facilitates a deeper understanding of the magnetic configuration and interlinked electronic-structure properties of molecule-metal interfaces. Furthermore, it highlights the necessity of an appropriate choice of methodology in tandem with a detailed evaluation of the different emerging magnetic properties.

Molecular structures, and optical and magnetic properties of free-base tetrapyrazinoporphyrazine in various reduction states

The synthesis, and spectral and structural characterization of the novel radical anion and dianion salts of free-base tetrapyrazinoporphyrazine (H2TPyzPz) as well as studies of their magnetic properties have been described.

Negatively charged singly-bonded dimers of C1-[C70(CF3)10] and bare C70 fullerene

Singly-bonded {C70(CF3)10−}2 and (C70−)2 dimers are obtained as crystalline salts allowing their molecular structure, and optical and magnetic properties to be studied.

Mapping the Magnetic Anisotropy at the Atomic Scale in Dysprosium Single-Molecule Magnets.

The anisotropy of the magnetic properties of molecular magnets is a key descriptor in the search for improved magnets. Herein, it is shown how an analytical approach using single-crystal polarized neutron diffraction (PND) provides direct access to atomic magnetic susceptibility tensors. The technique was applied for the first time to two Dy-based single-molecule magnets and showed clear axial atomic susceptibility for both DyIII ions. For the triclinic system, bulk magnetization methods are not symmetry-restricted, and the experimental magnetic easy axes from both PND, angular-resolved magnetometry (ARM), and theoretical approaches all match reasonably well. ARM curves simulated from the molecular susceptibility tensor determined with PND show strong resemblance with the experimental ones. For the monoclinic compound, comparison can only be made with the theoretically calculated magnetic anisotropy, and in this case PND yields an easy-axis direction that matches that predicted by electrostatic methods. Importantly, this technique allows the determination of all elements of the magnetic susceptibility tensor and not just the easy-axis direction, as is available from electrostatic predictions. Furthermore, it has the capacity to provide each of the anisotropic magnetic susceptibility tensors for all independent magnetic ions in a molecule and thus allows studies on polynuclear complexes and compounds of higher crystalline symmetry than triclinic.

On the development of the exact two-component relativistic method for calculating indirect NMR spin-spin coupling constants

Recently, to obtain the proper basis functions of small component spinors from the basis functions of large component spinors, the restricted magnetic balance (RMB) has been used in relativistic calculations of molecular magnetic properties, instead of the restricted kinetic balance (RKB). In this study, we attempt to develop the RMB-based exact two-component relativistic method, i.e., RMB-based normalized elimination of the small component (RMB-NESC) method for calculating indirect NMR spin-spin coupling constants (SSCCs), as well as the RKB-NESC method. The RMB-NESC SSCC formulas require complicated one-electron integrals, unlike the RKB-NESC SSCC formulas. However, the standard integral evaluation method for the complicated integrals has not yet been established, while we calculate them by using the finite-size nuclear model and partial integration. By comparison with the RKB-NESC and experimental SSCC values, we discuss whether the RMB-NESC results are valid. Finally, we conclude that our calculation for the complicated integrals must have some problems.

Lower Bounds for Gaussian Estrada Index of Graphs

Suppose that G is a graph over n vertices. G has n eigenvalues (of adjacency matrix) represented by λ1,λ2,⋯,λn. The Gaussian Estrada index, denoted by H(G) (Estrada et al., Chaos 27(2017) 023109), can be defined as H(G)=∑i=1ne−λi2. Gaussian Estrada index underlines the eigenvalues close to zero, which plays an important role in chemistry reactions, such as molecular stability and molecular magnetic properties. In a network of particles governed by quantum mechanics, this graph-theoretic index is known to account for the information encoded in the eigenvalues of the Hamiltonian near zero by folding the graph spectrum. In this paper, we establish some new lower bounds for H(G) in terms of the number of vertices, the number of edges, as well as the first Zagreb index.

Molecular Structure, Optical, and Magnetic Properties of Free‐Base Naphthalocyanine Dianions

The reduction of free‐base naphthalocyanine (H2Nc) with sodium fluorenone ketyl and an excess of bis(triphenylphosphoranylidene)ammonium chloride (PPN+)(Cl–) unexpectedly results in the synthesis of the (PPN+)2(H2Nc2–) salt (1). As sodium fluorenone ketyl can generate only H2Nc·– radical anions, the formation of H2Nc2– is most probably realized through the disproportionation of H2Nc·– to H2Nc0 and H2Nc2–. The H2Nc2– dianions in 1 form layers in which π–π stacks of H2Nc2– are formed along the a direction with a short interplanar distance of 3.30 Å and a HOMO–HOMO (HOMO = highest occupied molecular orbiatl) overlap integral of 3.1 × 10–3. The H2Nc2– macrocycle is distorted owing to the formation of alternate short and long C–N(imine) bonds. The distortion of the macrocycle increases with the negative charge on H2Nc owing to the occupation of the lowest unoccupied molecular orbital (LUMO). Calculations indicate that the distortion disappears in the triplet state of H2Nc2– as the occupation of the LUMO and LUMO+1 orbitals averages their effects on the geometry of the macrocycle. The optical spectrum of 1 shows a lowest energy absorption band at λ = 1230 nm, which is attributed to the HOMO → LUMO+1 excitation and a blueshift of the Q‐band of H2Nc. The elecron paramagnetic resonance (EPR) silence of 1 indicates the diamagnetic singlet state of the H2Nc2– dianions. The DFT calculations indicate that the open‐shell broken‐symmetry singlet state is lower in energy than the corresponding closed‐shell singlet and triplet states. The large singlet–triplet energy gap indicates that the corresponding triplet state is thermally inaccessible.

Synthesis and magnetic properties of chiral molecular magnet regulated by terminal coordination groups

The solvothermal reaction of (HL = 2-(5-Pyrazinyl-1H-[1,2,4]triazol-3-yl)-pyrazine) and NiCl2·6H2O and Mn(HCOO)2·2H2O yielded two novel complexes, namely, Ni(L)(Cl)(DMF) (HPU-11) and {Mn(L)(HCOO)}n (HPU-12), which have been studied based on X-ray single crystal diffraction analysis. In HPU-11, two crystallographic independent NiII ions are connected by two L− ligands to generate a centrosymmetric binuclear cluster. By changing the terminal coordination atom Cl as HCOO, chiral magnet HPU-12 was obtained. In HPU-12, adjacent Mn ions are connected by HCOO− bridges giving rise to a one-dimensional chiral chain structure. The magnetic measurements show the presence of antiferromagnetic interactions between the magnetic centers of the two complexes.

AACID: Anisotropy of the Asymmetric Magnetically Induced Current Density Tensor.

Molecular magnetic properties, such as the magnetizability, the magnetic shielding, and the current strength, all stem from the asymmetric current density tensor. The anisotropy of that tensor, previously defined from its symmetric component only, is a scalar field expected to bear useful information on molecular structure. A more general anisotropy has here been defined, considering the full asymmetric tensor and its eigenvalues in the complex plane. Computations on a few prototypical aromatic molecules show that the anisotropy of the full tensor highlights previously missed structural features and also changes in a more regular way with bond length, as compared to the already known anisotropy of the symmetric component of the tensor.

Molecular Structure, Optical and Magnetic Properties of Dianionic Free‐Base Tetrapyrazinoporphyrazine Macrocycle

AbstractFree‐base tetrapyrazinoporphyrazine (H2TPyzPz) has been synthesized and the reduction of this macrocycle by an excess of sodium fluorenone ketyl in the presence of 2.2.2‐cryptand yields {Na(2.2.2‐cryptand)}2(H2TPyzPz)⋅2C6H4Cl2 (1) salt containing H2TPyzPz2‐ dianions. Blue shift of both Soret and Q‐bands is observed in the spectrum of 1 and a new band is observed at 822 nm. Reduction is accompanied by alternation of the C‐Nimine bonds in the macrocycle. The difference between short and long bonds of 0.092 Å is essentially higher than that for free‐base phthalocyanine radical anions (0.040 Å). This difference can be explained by population of LUMO of H2TPyzP by two electrons in singlet ground state. According to the calculations distortion almost disappears in triplet excited state since in this state two additional electrons occupy orbitals which bonding and antibonding properties are interchanged. Salt 1 shows a weak narrow EPR signal indicating diamagnetism of H2TPyzPz2‐. Calculations also show singlet ground state for the dianions whereas the excited triplet state is positioned by 1130 K higher than the ground state and is not populated at room temperature.

Characterization of point defects in monolayer arsenene

Topological defects that are inevitably found in 2D materials can dramatically affect their properties. Using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) method, the structural, thermodynamic, electronic and magnetic properties of six types of typical point defects in arsenene, i.e. the Stone-Wales defect, single and double vacancies and adatoms, were systemically studied. It was found that these defects were all more easily generated in arsenene with lower formation energies than those with graphene and silicene. Stone-Wales defects can be transformed from pristine arsenene by overcoming a barrier of 2.19 eV and single vacancy defects tend to coalesce into double vacancy defects by diffusion. However, a type of adatom defect does not exhibit kinetic stability at room temperature. In addition, SV defects and another type of adatom defect can remarkably affect the electronic and magnetic properties of arsenene, e.g. they can introduce localized states near the Fermi level, as well as a strongly local magnetic moment due to dangling bond and unpaired electron. Furthermore, the simulated scanning tunneling microscopy (STM) and Raman spectroscopy were computed and the types of point defects can be fully characterized by correlating the STM images and Raman spectra to the defective atomistic structures. The results provide significant insights to the effect of defects in arsenene for potential applications, as well as identifications of two helpful tools (STM and Raman spectroscopy) to distinguish the type of defects in arsenene for future experiments.

The effects of protonated heterocyclic cations on the structural and magnetic properties of tetrachlorocuprate(ii) anions; X-ray, magnetochemical and EPR studies

The crystal and molecular structures and magnetic properties of two new complexes (HMepy)2[CuCl4]·2H2O, 2, and (H2Me2ppz)[CuCl4], 3, formed by tetrachlorocuprate(ii) anions and organic heterocyclic cations, have been determined by the analysis of their X-ray diffraction data, magnetic susceptibility behaviour and EPR spectral parameters.

High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium(iii) metallocenium single-molecule magnets.

A series of dysprosium(iii) metallocenium salts, [Dy(CpiPr4R)2][B(C6F5)4] (R = H (1), Me (2), Et (3), iPr (4)), was synthesized by reaction of DyI3 with the corresponding known NaCpiPr4R (R = H, iPr) and novel NaCpiPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt3)2][B(C6F5)4]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy-C distances and larger Cp-Dy-Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy-C distance decreases and the Cp-Dy-Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2-4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature of 62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm-1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s-1. Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature DyIII metallocenium single-molecule magnets.

Orbital-specific electronic interaction in crystalline films of iron phthalocyanine grown on Au(111) probed by angle-resolved photoemission spectroscopy

Orbital-specific electronic interaction as a key to understanding unique molecular electronic and magnetic properties.