Magnetochemistry, Volume 4, Issue 4 (December 2018) – 18 articles

Herein, we report a novel Tb(III) single chain magnet with the chemical formulae [Tb(μ-OH₂)(phen)(μ-OH)(nb)₂]_n by using 4-nitrobenzoic acid (Hnb) and 1,10-phenanthroline (phen) as ligand system. The single-crystal X-ray diffraction reveals that 4-nitrobenzoic acid acts as a monodentate ligand, water and hydroxyl ions are the bridging ligand and the phen serves as a bidentate chelating ligand. The static magnetic susceptibility measurement (from 2 K to 300 K) shows ferromagnetic interaction at very low temperature (below 6 K). The alternating current (AC) susceptibility data of the complex show temperature and frequency dependence under an applied 2000 Oe DC (direct current) field. The phen moiety behaves as an antenna and enables the complex to show the green light fluorescence emission by absorption-energy transfer-emission mechanism. To calculate the exchange interaction, broken symmetry density functional theory (BS-DFT) calculations have been performed on a model compound which also reveals weak ferromagnetic interaction. Ab initio calculations reveals the anisotropic nature (g_z = 15.8, g_y, g_y = 0) of the metal centre and the quasi doublet nature of ground state with small energy gap and that is well separated from the next excited energy state. Full article

Here we show the important role played by the size of the lanthanoid and the solvent used in the final structures of several two-dimensional magnetic coordination polymers with the ligand chlorocyananilato, (C₆O₄(CN)Cl)²⁻. With this aim we have prepared five compounds: [Nd₂(C₆O₄(CN)Cl)₃(DMF)₆] (1) (DMF = dimethylformamide), [Dy₂(C₆O₄(CN)Cl)₃(DMF)₆]·4H₂O (2), [Ho₂(C₆O₄(CN)Cl)₃(DMF)₆]·2H₂O (3), and [Ln₂(C₆O₄(CN)Cl)₃(DMSO)₆] with Ln = Ce (4) and Nd (5) (DMSO = dimethylsulfoxide). These compounds are formed by two dimensional networks with a (6,3)-topology but, depending on the size of the lanthanoid and on the solvent used, show important structural differences, including the size, shape, distortion and content of the cavities as well as the flatness of the layers. The comparison of compounds 1–3 and 4–5 shows the role played by the size of the lanthanoid while keeping constant the solvent, whereas, the comparison of compounds 1 and 5 shows the role of the solvent (DMF vs. DMSO) while keeping constant the lanthanoid. The magnetic properties of all of them show the absence of noticeable magnetic interactions, in agreement with previous results that can be explained by the internal character of the 4f electron and the weak magnetic coupling mediated by these anilato-based ligands. Full article

A new tetranuclear mixed-valence manganese(II/IV) compound [Mn^IIMn^IV₃(μ-Cl)₃(µ₃-O)(L)₃] (1) (where H₃L = (3E)-3-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylideneamino)propane-1,2-diol) has been synthesized and characterized by different physicochemical methods. Single crystal X-ray diffraction analysis reveals that 1 is a tetrahedral cluster consisting of a Mn₄Cl₃O₄ core in which the only Mn(II) ion is joined through three μ₂-O bridges to an equilateral triangle of Mn(IV) ions, which are connected by a μ₃-O and three μ₂-Cl bridges. The redox behavior of 1 was studied by cyclic voltammetry. Variable temperature magnetic susceptibility measurements of 1 revealed predominant antiferromagnetic coupling inside the Mn₄Cl₃O₄ cluster. Full article

An octahedral Co(II) complex with N′-(2-hydroxybenzylidene)acetohydrazide Schiff base ligand [HL] forms a 3D supramolecular assembly supported by non-coordinating ClO₄⁻ ions and H₂O molecules. Individual spin centres are non-interacting and give rise to significant spin-orbit coupling, resulting in field induced slow magnetisation relaxation; which is characteristic of Single Ion Magnet (SIM) behaviour. Full article

With the aim to provide a general protocol to interpret electron paramagnetic resonance (EPR) spectra of paramagnetic copper(II) coordination compounds, density functional theory (DFT) calculations of spin Hamiltonian parameters g and A for fourteen Cu(II) complexes with different charges, donor sets, and geometry were carried out using ORCA software. The performance of eleven functionals was tested, and on the basis of the mean absolute percent deviation (MAPD) and standard deviation (SD), the ranking of the functionals for A_z is: B3LYP > B3PW91 ~ B3P86 > PBE0 > CAM-B3LYP > TPSSh > BH and HLYP > B2PLYP > MPW1PW91 > ω-B97x-D >> M06; and for g_z is: PBE0 > BH and HLYP > B2PLYP > ω-B97x-D > B3PW91~B3LYP~B3P86 > CAM-B3LYP > TPSSh~MPW1PW91 >> M06. With B3LYP the MAPD with respect to $A_z^{\exp tl}$ is 8.6% with a SD of 4.2%, while with PBE0 the MAPD with respect to $g_z^{\exp tl}$ is 2.9% with a SD of 1.1%. The results of the validation confirm the fundamental role of the second order spin-orbit contribution to A_z. The computational procedure was applied to predict the values of g_z and A_z of the adducts formed by Cu(II) with albumin and two fragments of prion protein, 106–126 and 180–193. Full article

The effect of transition metal substitution for Fe and the structural and magnetic properties of Gd₂Fe₁₆Ga_0.5TM_0.5 (TM = Cr, Mn, Co, Ni, Cu, and Zn) compounds were investigated in this study. Rietveld analysis of X-ray data indicates that all the samples crystallize in the hexagonal Th₂Ni₁₇ structure. The lattice parameters a, c, and the unit cell volume show TM ionic radii dependence. Both Ga and TM atoms show preferred site occupancy for 12j and 12k sites. The saturation magnetization at room temperature was observed for Co, Ni, and Cu of 69, 73, and 77 emu/g, respectively, while a minimum value was observed for Zn (62 emu/g) doping in Gd₂Fe₁₆Ga_0.5TM_0.5. The highest Curie temperature of 590 K was observed for Cu doping which is 15 and 5% higher than Gd₂Fe₁₇ and Gd₂Fe₁₆Ga compounds, respectively. The hyperfine parameters viz. hyperfine field and isomer shift show systematic dependence on the TM atomic number. The observed magnetic and Curie temperature behavior in Gd₂Fe₁₆Ga_0.5TM_0.5 is explained on the basis of Fe(3d)-TM(3d) hybridization. The superior Curie temperature and magnetization value of Co-, Ni-, and Cu-doped Gd₂Fe₁₆Ga_0.5TM_0.5 compounds as compared to pure Gd₂Fe₁₇ or Gd₂Fe₁₆Ga makes Gd₂Fe₁₆Ga_0.5TM_0.5 a potential candidate for high-temperature industrial magnet applications. Full article

The iron (II) coordination compound, {[Fe(3,6 pzdc)](H₂O)₂}₂. (1) has been synthesized from a mixture of FeCl₂.4H₂O and pyridazinedicarboxylate (3,6 pzdc). The molecular structure of complex 1 was determined by single crystal X-ray diffraction study. It reveals that the dinuclear structure contains a pyridazine bridge in between the two metal centers. The variable temperature magnetic study results in g = 2.496(8), J = −2.50(8) cm⁻¹, Ɵ = −0.1 K values, by fitting the magnetic data in a simple dinuclear Fe-Fe model which indicates that the major exchange pathway through the N-N bridge. Presence of dense H-bonding interaction leads to supramolecular network formation. Full article

Recently published structural analysis and galvanomagnetic studies of a large number of different bulk and mesoscopic graphite samples of high quality and purity reveal that the common picture assuming graphite samples as a semimetal with a homogeneous carrier density of conduction electrons is misleading. These new studies indicate that the main electrical conduction path occurs within 2D interfaces embedded in semiconducting Bernal and/or rhombohedral stacking regions. This new knowledge incites us to revise experimentally and theoretically the diamagnetism of graphite samples. We found that the c-axis susceptibility of highly pure oriented graphite samples is not really constant, but can vary several tens of percent for bulk samples with thickness $t\gtrsim 30$ μm, whereas by a much larger factor for samples with a smaller thickness. The observed decrease of the susceptibility with sample thickness qualitatively resembles the one reported for the electrical conductivity and indicates that the main part of the c-axis diamagnetic signal is not intrinsic to the ideal graphite structure, but it is due to the highly conducting 2D interfaces. The interpretation of the main diamagnetic signal of graphite agrees with the reported description of its galvanomagnetic properties and provides a hint to understand some magnetic peculiarities of thin graphite samples. Full article

Three trinuclear Ni(II)-Na(I) complexes, [Ni₂(L¹)₂NaCl₃(H₂O)]·H₂O (1), [Ni₂(L²)₂NaCl₃(H₂O)] (2), and [Ni₂(L³)₂NaCl₃(OC₄H₁₀)] (3) have been synthesized using three different NNO donor tridentate reduced Schiff base ligands, HL¹ = 2-[(3-methylamino-propylamino)-methyl]-phenol, HL² = 2-[(3-methylamino-propylamino)-methyl]-4-chloro-phenol, and HL³ = 2-[(3-methylamino-propylamino)-methyl]-6-methoxy-phenol that had been structurally characterized. Among these complexes, 1 and 2 are isostructural in which dinuclearNi(II) units act as metalloligands to bind Na(I) ions via phenoxido and chlorido bridges. The Na(I) atom is five-coordinated, and the Ni(II) atom possesses hexacordinated distorted octahedral geometry. In contrast, in complex 3, two -OMe groups from the dinuclear Ni(II) unit also coordinate to Na(I) to make its geometry heptacordinated pentagonal bipyramidal. The magnetic measurements of complexes 1–3 indicate ferromagnetic interactions between dimeric Ni(II) units with J = 3.97 cm⁻¹, 4.66 cm⁻¹, and 5.50 cm⁻¹ for 1–3, respectively, as is expected from their low phenoxido bridging angles (89.32°, 89.39°, and 87.32° for 1–3, respectively). The J values have been calculated by broken symmetry DFT method and found to be in good agreement with the experimental values. Full article

In this review on advanced biomolecular EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR regarding the measurement of interactions and dynamics of large molecules embedded in fluid-solution or solid-state environments. Our focus is on the characterization of protein structure, dynamics and interactions, using sophisticated EPR spectroscopy methods. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy to new horizons reaching millimeter and sub-millimeter wavelengths and 15 T Zeeman fields. Expanding traditional applications to paramagnetic systems, spin-labeling of biomolecules has become a mainstream multifrequency approach in EPR spectroscopy. In the high-frequency/high-field EPR region, sub-micromolar concentrations of nitroxide spin-labeled molecules are now sufficient to characterize reaction intermediates of complex biomolecular processes. This offers promising analytical applications in biochemistry and molecular biology where sample material is often difficult to prepare in sufficient concentration for NMR characterization. For multifrequency EPR experiments on frozen solutions typical sample volumes are of the order of 250 μL (S-band), 150 μL (X-band), 10 μL (Q-band) and 1 μL (W-band). These are orders of magnitude smaller than the sample volumes required for modern liquid- or solid-state NMR spectroscopy. An important additional advantage of EPR over NMR is the ability to detect and characterize even short-lived paramagnetic reaction intermediates (down to a lifetime of a few ns). Electron–nuclear and electron–electron double-resonance techniques such as electron–nuclear double resonance (ENDOR), ELDOR-detected NMR, PELDOR (DEER) further improve the spectroscopic selectivity for the various magnetic interactions and their evolution in the frequency and time domains. PELDOR techniques applied to frozen-solution samples of doubly spin-labeled proteins allow for molecular distance measurements ranging up to about 100 Å. For disordered frozen-solution samples high-field EPR spectroscopy allows greatly improved orientational selection of the molecules within the laboratory axes reference system by means of the anisotropic electron Zeeman interaction. Single-crystal resolution is approached at the canonical g-tensor orientations—even for molecules with very small g-anisotropies. Unique structural, functional, and dynamic information about molecular systems is thus revealed that can hardly be obtained by other analytical techniques. On the other hand, the limitation to systems with unpaired electrons means that EPR is less widely used than NMR. However, this limitation also means that EPR offers greater specificity, since ordinary chemical solvents and matrices do not give rise to EPR in contrast to NMR spectra. Thus, multifrequency EPR spectroscopy plays an important role in better understanding paramagnetic species such as organic and inorganic radicals, transition metal complexes as found in many catalysts or metalloenzymes, transient species such as light-generated spin-correlated radical pairs and triplets occurring in protein complexes of photosynthetic reaction centers, electron-transfer relays, etc. Special attention is drawn to high-field EPR experiments on photosynthetic reaction centers embedded in specific sugar matrices that enable organisms to survive extreme dryness and heat stress by adopting an anhydrobiotic state. After a more general overview on methods and applications of advanced multifrequency EPR spectroscopy, a few representative examples are reviewed to some detail in two Case Studies: (I) High-field ELDOR-detected NMR (EDNMR) as a general method for electron–nuclear hyperfine spectroscopy of nitroxide radical and transition metal containing systems; (II) High-field ENDOR and EDNMR studies of the Oxygen Evolving Complex (OEC) in Photosystem II, which performs water oxidation in photosynthesis, i.e., the light-driven splitting of water into its elemental constituents, which is one of the most important chemical reactions on Earth. Full article

The assembly properties of three known spin crossover iron(III) complexes 1–3, at the air–water interface, are reported. All three complexes are amphiphiles, each bearing a pair of C_n alkyl chains on the polyamino Schiff base sal₂trien ligand (n = 6, 12, or 18). Complex 1 is water-soluble but complexes 2 and 3 form Langmuir films, and attempts were made to transfer the film of the C₁₈ complex 3 to a glass surface. The nature of the assembly of more concentrated solutions of 3 in water was investigated by light scattering, cryo-SEM (scanning electron microscopy), and TEM (transmission electron microscopy), all of which indicated nanoparticle formation. Lyophilization of the assembly of complex 3 in water yielded a powder with a markedly different magnetic profile from the powder recovered from the initial synthesis, notably, the spin crossover was almost completely quenched, and the thermal behavior was predominantly low spin, suggesting that nanoparticle formation traps the system in one spin state. Full article

A new {Dy₅} cluster compound has been synthesized and structurally characterized from the initial use of the Schiff base ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH₂) in coordination chemistry. The 1:1 reaction between Dy(hpd)₃∙2H₂O and nacpH₂, in a solvent mixture comprising CH₂Cl₂ and MeOH, afforded orange crystals of [Dy₅(OH)₂(hpd)₃(nacp)₅(MeOH)₅] (1) in 70% yield, where hpd⁻ is the anion of 3,5-heptadione. The {Dy₅} complex can be described as two vertical {Dy₃(μ₃-OH)}⁸⁺ triangles sharing a common vertex; such a metal topology is unprecedented in 4f-metal cluster chemistry. Direct current (dc) magnetic susceptibility studies revealed the presence of some weak ferromagnetic exchange interactions between the five Dy^III ions at low temperatures. Alternating current (ac) magnetic susceptibility measurements at zero applied dc field showed that complex 1∙3MeOH∙CH₂Cl₂ exhibits temperature- and frequency-dependent out-of-phase signals below ~20 K, characteristics of a single-molecule magnet (SMM). The resulting relaxation times were used to construct an Arrhenius-type plot and determine an effective energy barrier, U_eff, of 100 K for the magnetization reversal. The application of a small dc field of 200 Oe resulted in the surpassing of the quantum tunneling process and subsequently the increase of the U_eff to a value of 170 K. The reported results are part of a long-term program aiming at the preparation of structurally and magnetically interesting lanthanide complexes bearing various Schiff base chelating/bridging ligands. Full article

A systematic study on the self-assembled solution system of human serum albumin (HSA) and paramagnetic doxyl stearic acid (5-DSA and 16-DSA) ligands is reported covering the broad pH range 0.7–12.9, mainly using electron paramagnetic resonance (EPR) methods. It is tested to which extent the pH-induced conformational isomers of HSA reveal themselves in continuous wave (CW) EPR spectra from this spin probing approach in comparison to an established spin-labeling strategy utilizing 3-maleimido proxyl (5-MSL). Most analyses are conducted on empirical levels with robust strategies that allow for the detection of dynamic changes of ligand, as well as protein. Special emphasis has been placed on the EPR spectroscopic detection of a molten globule (MG) state of HSA that is typically found by the fluorescent probe 8-Anilino- naphthalene-1-sulfonic acid (ANS). Moreover, four-pulse double electron-electron resonance (DEER) experiments are conducted and substantiated with dynamic light scattering (DLS) data to determine changes in the solution shape of HSA with pH. All results are ultimately combined in a detailed scheme that describes the pH-induced functional phase space of HSA. Full article

Here we report a series of 1-phenyl-5-substituted 2,6-di(pyrazol-3-yl)pyridine complexes with iron(II) ion found in a high spin state in solids (according to magnetochemistry) and in solution (according to NMR spectroscopy), providing experimental evidence for it being an intramolecular effect induced by the phenyl groups. According to X-ray diffraction, the high spin locking of the metal ion is a result of its highly distorted coordination environment (with a very low ‘twist’ angle atypical of 2,6-di(pyrazol-3-yl)pyridine complexes), which remains this way in complexes with different substituents and counterions, in a diamagnetic zinc(II) analogue and in their solutions. Three possible reasons behind it, including additional coordination with the phenyl group, energy penalty incurred by its rotation or intramolecular stacking interactions, are addressed experimentally. Full article

The reactions of hydrated lanthanide(III) [Ln(III)] nitrates and salicylideneaniline (salanH) have provided access to two families of mononuclear complexes depending on the reaction solvent used. In MeCN, the products are [Ln(NO₃)₃(salanH)₂(H₂O)]·MeCN, and, in MeOH, the products are [Ln(NO₃)₃(salanH)₂(MeOH)]·(salanH). The complexes within each family are proven to be isomorphous. The structures of complexes [Ln(NO₃)₃(salanH)₂(H₂O)]·MeCN (Ln = Eu, 4·MeCN_Eu, Ln = Dy, 7·MeCN_Dy; Ln = Yb, 10·MeCN_Yb) and [Ln(NO₃)₃(salanH)₂(MeOH)]·(salanH) (Ln = Tb, 17_Tb; Ln = Dy, 18_Dy) have been solved by single-crystal X-ray crystallography. In the five complexes, the Ln^III center is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms from the two monodentate zwitterionic salanH ligands, and one oxygen atom from the coordinated H₂O or MeOH group. The salanH ligands are mutually “cis” in 4·MeCN_Eu, 7·MeCN_Dy and 10·MeCN_Yb while they are “trans” in 17_Tb and 18_Dy. The lattice salanH molecule in 17_Tb and 18_Dy is also in its zwitterionic form with the acidic H atom being clearly located on the imine nitrogen atom. The coordination polyhedra defined by the nine oxygen donor atoms can be described as spherical tricapped trigonal prisms in 4·MeCN_Eu, 7·MeCN_Dy, and 10·MeCN_Yb and as spherical capped square antiprisms in 17_Tb and 18_Dy. Various intermolecular interactions build the crystal structures, which are completely different in the members of the two families. Solid-state IR data of the complexes are discussed in terms of their structural features. ¹H NMR data for the diamagnetic Y(III) complexes provide strong evidence that the compounds decompose in DMSO by releasing the coordinated salanH ligands. The solid complexes emit green light upon excitation at 360 nm (room temperature) or 405 nm (room temperature). The emission is ligand-based. The solid Pr(III), Nd(III), Sm(III), Er(III), and Yb(III) complexes of both families exhibit Ln^III-centered emission in the near-IR region of the electromagnetic spectrum, but there is probably no efficient salanH→Ln^III energy transfer responsible for this emission. Detailed magnetic studies reveal that complexes 7·MeCN_Dy, 17_Tb and 18_Dy show field-induced slow magnetic relaxation while complex [Tb(NO₃)₃(salanH)₂(H₂O)]·MeCN (6·MeCN_Tb) does not display such properties. The values of the effective energy barrier for magnetization reversal are 13.1 cm⁻¹ for 7·MeCN_Dy, 14.8 cm⁻¹ for 17_Tb, and 31.0 cm⁻¹ for 18_Dy. The enhanced/improved properties of 17_Tb and 18_Dy, compared to those of 6_Tb and 7_Dy, have been correlated with the different supramolecular structural features of the two families. The molecules [Ln(NO₃)₃(salanH)₂(MeOH)] of complexes 17_Tb and 18_Dy are by far better isolated (allowing for better slow magnetic relaxation properties) than the molecules [Ln(NO₃)₃(salanH)₂(H₂O)] in 6·MeCN_Tb and 7·MeCN_Dy. The perspectives of the present initial studies in the Ln(III)/salanH chemistry are discussed. Full article

Up to now, even if murexide-based complexometric studies are performed with all 3d or 4f ions, the crystal structures of the light-lanthanide derivatives of the lanthanide-murexide series are unknown. In this work, we report the crystal structure of the Nd^III derivative named NdMurex. Contrary to all known complexes of the 3d or 4f series, a dimeric compound was obtained. As for its already reported Dy^III and Yb^III parents, the Nd^III complex responsible for the color-change behaves as a single-molecule magnet (SMM). This behavior was observed on both the crystalline (NdMurex: U_eff = 6.20(0.80) K, 4.31 cm⁻¹; τ₀ = 2.20(0.92) × 10⁻⁵ s, H_dc = 1200 Oe) and anhydrous form (NdMurexAnhy: U_eff = 6.25(0.90) K, 4.34 cm⁻¹; τ₀ = 4.85(0.40) × 10⁻⁵ s, H_dc = 1200 Oe). The SMM behavior is reported also for the anhydrous Ce^III derivative (CeMurexAnhy: U_eff = 5.40(0.75) K, 3.75 cm⁻¹; τ₀ = 3.02(1.10) × 10⁻⁵ s, H_dc = 400 Oe). The Near-Infrared Emission NIR emission was observed for NdMurexAnhy and highlights its bifunctionality. Full article

In continuation of our work on supramolecular architectures of single-molecule magnets (SMMs) as a promising strategy in developing their magnetic performance, in this paper we report the synthesis and single crystal X-ray structure of the centered triangular tetrairon(III) SMM, [Fe₄(PhpPy)₂(dpm)₆], Fe₄ (Hdpm = dipivaloylmethane, H₃PhpPy = 2-(hydroxymethyl)-2-(4-(pyridine-4-yl)phenyl)propane-1,3-diol), and its assembly in the coordination polymer {[Fe₄(PhpPy)₂(dpm)₆Ag](ClO₄)}_n, Fe₄Ag, upon reaction with silver(I) perchlorate. Thanks to the presence of the pyridyl rings on the two tripodal ligands, Fe₄ behaves as divergent ditopic linker, and due to the Fe₄:AgClO₄ 1:1 ratio, Fe₄Ag probably possesses a linear arrangement in which silver(I) ions are linearly coordinated by two nitrogen atoms, forming 1D chains whose positive charge is balanced by the perchlorate anions. The stabilization of such a polymeric structure can be ascribed to the long distance between the two donor nitrogen atoms (23.4 Å) and their donor power. Fe₄Ag shows slow relaxation of the magnetization which follows a thermally activated process with U_eff/k_B = 11.17(18) K, τ₀ = 2.24(17) 10⁻⁷ s in zero field, and U_eff/k_B = 14.49(5) K, τ₀ = 3.88(8) 10⁻⁷ s in 1-kOe applied field, in line with what reported for tetrairon(III) SMMs acting as building blocks in polymeric structures. Full article

Understanding the correlations between magnetic nanoparticles is important for nanotechnologies, such as high-density magnetic recording and biomedical applications, where functionalized magnetic particles are used as contrast agents and for drug delivery. The ability to control the magnetic state of individual particles depends on the good knowledge of the magnetic correlations between particles when assembled. Inaccessible via standard magnetometry techniques, nanoscale magnetic ordering in self-assemblies of Fe₃O₄ nanoparticles is here unveiled via X-ray resonant magnetic scattering (XRMS). Measured throughout the magnetization process, the XRMS signal reveals size-dependent inter-particle magnetic correlations. Smaller (5 nm) particles show little magnetic correlations, even when packed close together, yielding to magnetic disorder in the absence of an external field, i.e., superparamagnetism. In contrast, larger (11 nm) particles tend to be more strongly correlated, yielding a mix of magnetic orders including ferromagnetic and anti-ferromagnetic orders. These magnetic correlations are present even when the particles are sparsely distributed. Full article