Weak Antiferromagnetic Coupling Research Articles

New heteroleptic l-argininato copper(II) complex with bromide and N, N’– heterocyclic ligands – synthesis, supramolecular, spectroscopic, magnetic and theoretical investigations

A new heteroleptic [CuBr(l−Arg)(bpy)]Br·2H2O (1) (l−Arg=l−Arginine, bpy=2,2’−bipyridine) complex has been synthesized and its molecular and supramolecular structure aspects have been discussed based on the X−ray single crystal structure and Hirshfeld analyses. In addition, the properties of complex 1 have been elucidated using different spectroscopic methods (FT−IR / FIR, Raman, NIR−Vis−UV, Q− / X− band EPR) and magnetic measurements. In this complex, the bromide anion occupies the apex of a slightly distorted square pyramid (τ = 0.12). The [CuBr(l−Arg)(bpy)]+ complex units exhibit a ribbon−like organization, with Cu(II) metal centers located at the corners of triangles where the shortest Cu∙∙∙Cu distance is 5.579 Å. The aromatic moieties of bpy are aligned nearly parallel leading to π···π interactions. Both, non−bonded and coordinated, Br− ions act as hydrogen−bond acceptors for guanidinium groups. Hirshfeld analysis indicated the importance of the Br∙∙∙H (17.9 − 18.4 %), O∙∙∙H (14.9−15.2 %), and H∙∙∙H (42.3−43.1 %) interactions in the molecular packing of the studied complex. Atoms in molecules (AIM) calculations indicated mainly the closed shell character for the Cu−N, Cu−O and Cu−Br coordination interactions. The G value calculated using the Q−band EPR spectra parameters g|| = 2.235 and g⊥= 2.05 is 4.7, what indicating negligible coupling and that local tetragonal axes are aligned parallel. A negative value of Weiss temperature θCW = -0.32 K indicates the presence of weak antiferromagnetic coupling between the neighboring Cu(II) ions mediated through hydrogen bonds and π-π stacking of aromatic moieties of bpy. The value of gav= 2.113 obtained from Q−band EPR spectra is consistent with that corresponding to the Curie constant (C = 0.4138 emu K mol−1), i.e.g = 2.101, and that obtained from the best fitting to the Brillouin function: g = 2.139.

Metal vs Ligand Oxidation: Coexistence of Both Metal-Centered and Ligand-Centered Oxidized Species.

A series of two-electron-oxidized cobalt porphyrin dimers have been synthesized upon controlled oxidations using halogens. Rather unexpectedly, X-ray structures of two of these complexes contain two structurally different low-spin molecules in the same asymmetric unit of their unit cells: one is the metal-centered oxidized diamagnetic entity of the type CoIII(por), while the other one is the ligand-centered oxidized paramagnetic entity of the type CoII(por•+). Spectroscopic, magnetic, and DFT investigations confirmed the coexistence of the two very different electronic structures both in the solid and solution phases and also revealed a ferromagnetic spin coupling between Co(II) and porphyrin π-cation radicals and a weak antiferromagnetic coupling between the π-cation radicals of two macrocycles via the bridge in the paramagnetic complex.

Coordination complexes of reduced organic dye isoviolanthrone with transition metals and lanthanides [Mn(II), Dy(III), Gd(III), Nd(III)

The reduction of a vat dye isoviolanthrone to the dianion state in the presence of metal-containing compounds yields binuclear coordination complexes. Two manganese(II) acetylacetonate units coordinate to isoviolanthrone2− (L) in {(K+)cryptand}2{[MnII(acac)2]2⋅L}2– (1) with an average Mn–O bond length of 2.022(2) Å. Dysprosium(III)- or gadolinium(III)-2,2,6,6-tetramethyl-3,5-heptanedionate (TMHD) units form {(K+)cryptand}2{[MIII(TMHD)3]2⋅L}2− (M = DyIII for 2 and GdIII for 3) with the length of the M−O bonds of 2.193(3) and 2.254(4) Å, respectively. Tris(cyclopentadienyl)neodymium(III) forms {(Cs+)cryptand}2{[Cp3NdIII]2⋅L}2–⋅C6H4Cl2 (4) with the Nd–O bonds of 2.264(2) and 2.308(2) Å length. The formation of isoviolanthrone2− is supported by their optical spectra, disappearance of the bands of the CO vibrations in the IR-spectra as well as elongation of the CO bonds in 1–4. High-spin (S = 5/2) MnII atoms in 1 have exchange interaction of J = − 0.122 cm−1 at g = 1.98. Lanthanides have weaker coupling through isoviolanthrone2− in 2 and 3 with J = − 0.029 and D = − 0.004 cm−1 for DyIII and J = 0.0019 cm−1 for GdIII. Complex 4 with two Cp3NdIII species demonstrates the χMT value of 2.33 emu⋅K/mol at 300 K, and this value is lower than the theoretically calculated value of 3.39 emu⋅K/mol. The decrease of χMT is observed in the whole 300–1.9 K range providing negative Weiss temperatures of –32 K. That is explained by magnetic behavior of Cp3NdIII similar to pristine Cp3NdIII. Complexes comprising MnII, GdIII, and NdIII show electron paramagnetic resonance (EPR) signals. Zero-field splitting (ZFS) is observed from MnII and GdIII allowing the estimation of zero-field splitting parameter D as − 0.047 and E close to 0 cm−1 for 1 and − 0.033 and − 0.020 cm−1 for 3. Complex 4 with NdIII manifests an asymmetric two-component EPR signal without ZFS. Weak antiferromagnetic couplings are explained by diamagnetism of isoviolanthrone2− and rather long distances between the metal centers of 15.47–16.25 Å. This study shows that the dianions of organic dyes are promising bidentate ligands for preparation of coordination complexes with d- and f-block metal ions.

Dysprosium‐Radical Based Single‐Molecule Magnet Containing a Seven Coordinate Dy(III) Ion: A System for ab initio Study

AbstractSingle–ion magnets (SIMs) possessing a Dy(III) ion have recently attracted huge scientific attention. Herein, we report on a Dy(III)‐radical single–molecule magnet containing a seven‐coordinate Dy(III) ion with an O3N3Cl donor set. The frequency–dependent slow relaxation of magnetization has been observed below 8 K. DC magnetic studies have shown the presence of weak antiferromagnetic coupling between the Kramers ion (Dy(III), J = 15/2) and the ligand‐centered radical electron (S = 1/2) which was confirmed by multifunctional CASSCF calculations.

Bis(ethylenedithio)tetrathiafulvalene and dibenzotetrathiafulvalene salts with dicyanoiron(III) phthalocyanine anions

Electrooxidation of bis(ethylenedithio)tetrathiafulvelene (BEDT-TTF) and dibenzotetrathiafulvelene (DB-TTF) in the presence of dicyanoiron(II) phthalocyanine (Pc) dianions in form of the {Cryptand(K+)}2{FeII(CN)2Pc}2− salt has been studied in o-dichlorobenzene and acetonitrile solutions (where Cryptand is cryptand[2.2.2]). Initially iron(II) is oxidized to iron(III) and after that tetrathiafulvelenes are oxidized forming crystalline salts (BEDT-TTF+){FeIII(CN)2Pc}−·C6H4Cl2 (1) and (DB-TTF+){FeIII(CN)2Pc}−·C6H4Cl2 (2). Both salts contain closely packed 2D layers composed of the {FeIII(CN)2Pc}− anions and cationic layers containing solvent molecules. Since cations cannot partially oxidize Pc in these salts, two S = 1/2 spins are positioned on FeIII and the cations. The χMT values are 0.90 and 0.87 emu·K/mol per formula unit at 300 K. Antiferromagnetic coupling of spins is observed in 1 and 2 due to both Fe-Cation coupling of −0.4 and −3.5 cm−1 and Fe-Fe couplings of −3.5 and −7.9 cm−1, respectively. Due to its smaller size of DB-TTF+, complex 2 has denser packing of the components and shows stronger exchange couplings. An unusual compound forms when DB-TTF and {FeII(CN)2Pc}2− are oxidized together in acetonitrile: (DB-TTF0)2{Cryptand(K+)}{FeIII(CN)2Pc}− (3) where charges on DB-TTF and Fe are closer to zero and +3, respectively. The {FeIII(CN)2Pc}− anions form layers in 3 among which chains with short C,N…C,N contacts between Pc can be outlined. Only weak intermolecular antiferromagnetic coupling is manifested in 3 with estimated zJ value of −1.6 cm−1.

Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(II) derivative anchored with a 'scorpionate' precursor as a potential electrocatalyst for heterogeneous H2 evolution.

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]2 (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, bdtbpza {bis-(3,5-di-tert-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N3O2. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian H = -J(S1·S2) for the Si = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H2 evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H2O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H2-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.

Influence of the ancillary ligand on the crystalline structures and magnetic properties of mesaconato‐bridged Mn coordination polymers

Abstract{[Mn(mes)(dmb)(H2O)] ⋅ H2O}n (1) and [Mn(mes)(bpy)]n (2); (mes=mesaconato; dmb=5,5’‐dimethyl‐2,2’‐bipyridine; bpy=2,2’‐bipyridine), were obtained by self‐assembly solution reaction at ambient conditions, and structurally characterized by single crystal X‐ray diffraction. 1 crystallizes in an orthorhombic system with a Pbca space group and forms a one‐dimensional (1D) coordination polymer; the six‐coordinated Mn(II) centers display a distorted‐trigonal prismatic configuration. 2 crystallizes in an orthorhombic system with a P21212 space group yielding a two‐dimensional (2D) coordination array, their metal centers having a distorted octahedral geometry. Magnetic susceptibility measurements reveal that 1 is paramagnetic, while 2 shows weak antiferromagnetic coupling.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion.

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Magnetic Coupling Control in Triangulene Dimers.

Metal-free magnetism remains an enigmatic field, offering prospects for unconventional magnetic and electronic devices. In the pursuit of such magnetism, triangulenes, endowed with inherent spin polarization, are promising candidates to serve as monomers to construct extended structures. However, controlling and enhancing the magnetic interactions between the monomers persist as a significant challenge in molecular spintronics, as so far only weak antiferromagnetic coupling through the linkage has been realized, hindering their room temperature utilization. Herein, we investigate 24 triangulene dimers using first-principles calculations and demonstrate their tunable magnetic coupling (J), achieving unprecedented strong J values of up to -144 meV in a non-Kekulé dimer. We further establish a positive correlation between bandgap, electronic coupling, and antiferromagnetic interaction, thereby providing molecular-level insights into enhancing magnetic interactions. By twisting the molecular fragments, we demonstrate an effective and feasible approach to control both the sign and strength of J by tuning the balance between potential and kinetic exchanges. We discover that J can be substantially boosted at planar configurations up to -198 meV. We realize ferromagnetic coupling in nitrogen-doped triangulene dimers at both planar and largely twisted configurations, representing the first example of ferromagnetic triangulene dimers that cannot be predicted by the Ovchinnikov rule. This work thus provides a practical strategy for augmenting magnetic coupling and open up new avenues for metal-free ferromagnetism.

Molecular structure, as well as optical and magnetic properties, of coordination complexes comprising the organic dye flavanthrone with transition metals (MnII, CrIII, CrIV) and lanthanides (DyIII, GdIII)

The reduction of the organic dye flavanthrone (Vat Yellow 1) to the dianion state in the presence of metal-containing compounds yields coordination complexes in which two metal fragments coordinate to flavanthrone dianions to form short M − O bonds. Flavanthrone dianion coordinates to two manganese(II) acetylacetonate units in {flavanthrone⋅[MnII(acac)2]2}2− (1) with an average Mn–O bond length of 2.048(3) Å. Two dysprosium(III) or gadolinium(III) 2,2,6,6-tetramethyl-3,5-heptanedionate (TMHD) units form longer M − O bonds in {cryptand(Cs+)}2{flavanthrone⋅[DyIII(TMHD)3]2}2− (2) and {cryptand(K+)}2{flavanthrone⋅[GdIII(TMHD)3]2}2−⋅2C6H4Cl2⋅ 2C6H14 (3), with bond lengths of 2.192(2) and 2.241(4) Å, respectively. Furthermore, the utilization of decamethylchromocene as a reductant yields the unusual neutral {(flavanthrone2−)⋅[Cp*CrIII(acac)]2} comprising acac anion (4) and {(flavanthrone2−)⋅[Cp*CrIVCl2]2} comprising two chloride anions (5). Both have zwitter-ionic nature in which negative charge of flavanthrone dianions is compensated by chromium countercations. They coordinate to two chromium-containing units with short Cr–O bonds of 1.937(2) and 1.962(5) Å, respectively. The formation of flavanthrone dianions in 1–5 is confirmed by their optical spectra, the disappearance of the bands corresponding to the C=O vibrations in the infrared spectra of the complexes, and the elongation of the carbon–oxygen bonds during the formation of 1–5. The formation of binuclear, paramagnetic species with the flavanthrone dianions induces weak antiferromagnetic couplings between the paramagnetic centers because the flavanthrone dianions are diamagnetic. Stronger couplings with exchange parameters of −0.44 and −0.55 cm−1 were observed for the chromium ions bearing the shortest M − O bonds for 5 and 4, respectively. Conversely, MnII and lanthanides (GdIII, DyIII) comprising longer M − O bonds exhibit smaller J values (close to zero in 3 or equal to −0.026 and −0.05 cm−1 in 2 and 1, respectively). The complexes comprising MnII, GdIII, and CrIII display electron paramagnetic resonance (EPR) signals. The signal from MnII is approximated considering that the exchange interaction between the two MnII ions is −0.05 cm−1. Well resolved zero-field splitting (ZFS) is displayed; it facilitates the determination of the positive ZFS parameter, D, of +0.83 cm−1 and the unusually high E value, +0.62 cm−1. The spectrum of 3 comprising two GdIII ions containing 12 lines was approximated with D and E values of −0.083 and −0.021 cm−1, respectively. The odd-spin CrIII (S = 3/2) display a broad, asymmetric EPR signal at g = 3.77 without ZFS, whereas the even-spin CrIV is EPR-silent. Taken together, this study reveals that the dianions of organic dyes represent promising bidentate ligands for preparing coordination complexes with paramagnetic d- and f-block metal ions.

Synthesis, Structures, and Magnetism of Four One-Dimensional Complexes Using [Ni(CN)4]2- and Macrocyclic Metal Complexes.

Four one-dimensional complexes, denoted as [NiL1][Ni(CN)4] (1), [CuL1][Ni(CN)4] (2), [NiL2][Ni(CN)4]·2H2O (3), and [CuL2][Ni(CN)4]·2H2O (4) (L1 = 1,8-dimethyl-1,3,6,8,10,13-hexaaza-cyclotetradecane; L2 = 1,8-dipropyl-1,3,6,8,10,13-hexaazacyclotetradecane) were synthesized by reacting nickel/copper macrocyclic complexes with K2[Ni(CN)4]. Subsequently, the synthesized complexes were characterized using elemental analysis, infrared spectroscopy analysis, thermogravimetric analysis, and X-ray powder diffraction. Single-crystal structure analysis revealed that the Ni(II)/Cu(II) atoms were coordinated by two nitrogen atoms from [Ni(CN)4]2- with four nitrogen atoms from a macrocyclic ligand, forming a six-coordinated octahedral coordination geometry. Nickel/copper macrocyclic complexes were bridged by [Ni(CN)4]2- to construct one-dimensional chain structures in 1-4. The characterization results showed that the four complexes obeyed the Curie-Weiss law with a weak antiferromagnetic exchange coupling.

An oxalato-bridged dimetallic heptanuclear [K5IFe2III] complex anion with guanidinium as counter cation: Synthesis, crystal structure, Hirshfeld surface analysis and magnetism

An oxalato-bridged heterometallic heptanuclear [K5IFe2III] hybrid salt, (CH6N3)[K5(H2O)4Fe2(C2O4)6] (1) (CH6N3+ = guanidinium cation) has been synthesized and characterized by elemental and thermal analyses, IR spectroscopy, single-crystal X-ray diffraction, EPR and SQUID magnetometry. The asymmetric unit in 1 is composed of a dimetallic heptanuclear [K5(H2O)4Fe2(C2O4)6]− anionic complex and a guanidinium CH6N3+ cation insuring the charge compensation. Each Fe3+ center in the anionic complex is six-coordinate in a distorted octahedron defined by three chelating oxalato(2-) ligands. In contrast, the coordination numbers for K+ centers range from 7 to 9. The structural feature of focal interest in 1 is formation of an extended anionic network encapsulating guanidinium cations. This structure can be considered a sheet-MOF, where metal ions and oxalates (linkers) form infinite 2D units. The 3-D supramolecular architecture is stabilized by OH∙∙∙O and NH∙∙∙O hydrogen bonds linking polymeric oxalato-bridged anionic assemblies and organic cations. The thermal studies confirmed the anhydrous character of salt 1 and showed a three-step decomposition to yield a mixture of Fe2O3 and K2O as final residue. EPR spectrum of 1 is in accordance with the oxidation state + 3 of the iron center in an octahedral environment. Temperature-dependence susceptibility measurements investigated in the temperature range 2–300 K revealed weak antiferromagnetic coupling at low temperatures in salt 1. The distributions of different types of intermolecular interactions in the crystal structure were quantified by Hirshfeld surface analysis and their associated fingerprint plots.

A Cu12 Metallacycle Assembled from Four C3-Symmetric Spin Frustrated Triangular Units

Assembling metallacycles with interesting topological arrangements is a critical task for chemists. We report here a novel dodecanuclear CuII compound, [{Cu3L(µ-N3)}4(Py)14]·2Py (Cu12) (where Py = pyridine and [H6L]Cl = tris(2-hydroxybenzylidine)triaminoguanidinium chloride, respectively), with the topology of a cycle accomplished by four two-connecting approximately flat C3-symmetric guanidine-based ligands. Each ligand affords three tridentate metal-binding cavities and the four node-to-node connections through single azido bridges are provided by pairs of metal centers. A theoretical investigation using CASSCF in addition to DFT calculations showed strong antiferromagnetic coupling within the Cu3-triangles, resulting in spin-frustrated systems. However, these calculations were not able to properly reproduce the very weak antiferromagnetic couplings between the triangle units, highlighting the challenge of describing the magnetic behavior of this compound.

Layered Co-O Cluster Applied to Photocatalytic CO2 Reduction.

In the field of recycling CO2, the photocatalytic CO2 reduction reaction (CO2RR) is a typical example, and researchers have designed a variety of photocatalysts to improve the conversion rate of CO2 over the years. In this paper, two metal-oxygen clusters are designed and formulated as [Co3Zn(OH)6(SO4)]·4H2O (1) and [Ni3Zn(OH)6(SO4)]·4H2O (2). As for compound 1, the main structure is composed of {CoO6} octahedra connected by edge-sharing to form a two-dimensional layer, on which {ZnO4} and {SO4} tetrahedra are supported. More interestingly, compound 1 has outstanding photocatalytic activity, which is mainly attributed to the open-framework structure and the cobalt ions as active sites. Upon catalysis for eight hours, its maximum CO generation rate is 9982.13 μmol g-1 h-1, with a selectivity of 81.8%. Additionally, compound 1 takes on weak antiferromagnetic coupling due to Co(II) ions.

Spin Crossover Behavior of FeII Complexes Bridged by Thiophene‐Functionalized Triazole Ligands with Different Sizes and Rigidities

AbstractFour thiophene functionalized triazole ligands (L1=4‐(thenyl)‐1,2,4‐triazole, L2=4‐(thiophene ethyl)‐1,2,4‐triazole, L3=N‐Thiophenylidene‐4H‐1,2,4‐triazole‐4‐amine, and L4=(4‐[(E)‐2‐(5‐sulfothiophene)vinyl]‐1,2,4‐triazole) were synthesized. These ligands have different lengths and rigidities, while ligand L4 has a sulfonic acid group that can form a hydrogen bond. Five 1D FeII chain complexes were synthesized: [Fe(L1)3](X)2 ⋅ nH2O [X=BF4−, n=1.5 (C1); X=ClO4−, n=1 (C2)], [Fe(L2)3](BF4)2 ⋅ 1.5H2O (C3); [Fe(L3)3](X)2 ⋅ nH2O [X=BF4−, n=2 (C4); X=ClO4−, n=2.5 (C5)]. The results of temperature‐dependent magnetic susceptibility reveal that complexes C1, C2, and C3 experienced the transition between two spin states. And C4 and C5 maintain high spin states at all temperature ranges. Binuclear complex [Fe2(L3)5(SCN)4] (C6) and mononuclear material [Fe(L4)2(H2O)4] ⋅ 2H2O (C7), these two zero‐dimensional molecules were also synthesized. They all display weak antiferromagnetic exchange coupling and a high spin state in the whole process.

Unusual stabilization of γ-Fe/α-Fe nanocrystalline junctions and antiferromagnetic transition within cm-scale buckypapers of entangled multiwall carbon nanotubes

We report the observation of an unusual weak antiferromagnetic coupling arising within μm long Fe-based nanowires encapsulated within cm-scale buckypapers of entangled multiwall carbon nanotubes (CNTs). The samples were produced in-situ by employing a custom-designed Cl-assisted chemical vapour deposition (CVD) approach, in which the chlorine-radicals have the crucial role of slowing-down the CNTs growth and enhancing the filling-ratio of the CNT-capillary. Interestingly, an enhancement of the Ar-vapour flow rate from 20 mL/min to 130 mL/min was found to have significant effects towards the stabilization of the FCC nanoscale γ-Fe phase, with a maximum relative abundance of 31% estimated by means of X-ray diffraction (XRD) and theoretical Rietveld refinement analyses. TEM, HRTEM and Fourier transform analytical methods allowed for the identification of sharp magnetic junctions established between the coexisting Fe3C, α-Fe and γ-Fe phases. Magnetometry and (field-dependent) zero-field-cooled (ZFC)/field cooled (FC) analytical methods, allowed the identification of a weak Néel temperature signal at T∼ 130 K. The origin of the antiferromagnetic component was investigated by tuning the applied field from 300 Oe to 50,000 Oe. The obtained results were further corroborated by means of magnetisation vs field acquisitions, with a significant enhancement in the coercive parameter in proximity of the Néel T-point.

Tetracoordinate Co(II) complexes with semi-coordination as stable single-ion magnets for deposition on graphene.

We present a theoretical and experimental study of two tetracoordinate Co(II)-based complexes with semi-coordination interactions, i.e., non-covalent interactions involving the central atom. We argue that such interactions enhance the thermal and structural stability of the compounds, making them appropriate for deposition on substrates, as demonstrated by their successful deposition on graphene. DC magnetometry and high-frequency electron spin resonance (HF-ESR) experiments revealed an axial magnetic anisotropy and weak intermolecular antiferromagnetic coupling in both compounds, supported by theoretical predictions from complete active space self-consistent field calculations complemented by N-electron valence state second-order perturbation theory (CASSCF-NEVPT2), and broken-symmetry density functional theory (BS-DFT). AC magnetometry demonstrated that the compounds are field-induced single-ion magnets (SIMs) at applied static magnetic fields, with slow relaxation of magnetization governed by a combination of quantum tunneling, Orbach, and direct relaxation mechanisms. The structural stability under ambient conditions and after deposition was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Theoretical modeling by DFT of different configurations of these systems on graphene revealed n-type doping of graphene originating from electron transfer from the deposited molecules, confirmed by electrical transport measurements and Raman spectroscopy.

Layered Organic Conductors Based on BEDT-TTF and Ho, Dy, Tb Chlorides

Molecular semiconductors with lanthanide ions have been synthesized based on BEDT-TTF and lanthanide chlorides: (BEDT-TTF)2[HoCl2(H2O)6]Cl2(H2O)2 (1, which contains a 4f holmium cation), and (BEDT-TTF)2LnCl4(H2O)n (Ln = Dy, Tb, Ho (2–4), which contain 4f anions of lanthanides). Conductivity and EPR measurements have been carried out along with the SQUID magnetometry, and the crystal structure has been established for 1. The structure of 1 is characterized by an alternation of organic radical cation layers composed of BEDT-TTF chains and inorganic layers consisting of chains of the [HoCl2(H2O)6]+ cations interlinked by chlorine anions and crystallization water molecules. The magnetic susceptibility of 1–3 determined mainly by lanthanide ions follows the Curie–Weiss law with the Weiss temperatures of −3, −3, −2 K for 1–3, respectively, indicating weak antiferromagnetic coupling between paramagnetic lanthanide ions. The signals attributed to the BEDT-TTF+· radical cations only are observed in the EPR spectra of 1–3, which makes it possible to study their magnetic behavior. There are two types of chains in the organic layers of 1: the chains composed of neutral molecules and those formed by BEDT-TTF+· radical cations. As a result, uniform 1D antiferromagnetic coupling of spins is observed in the BEDT-TTF+· chains with estimated exchange interaction J = −10 K. The study of dynamic magnetic properties of 1–3 shows that these compounds are not SMMs.

New HATNA(CN)6 ligand in the design of dianion magnetic assemblies with lanthanides {Cryptand(K+)}2{HATNA(CN)6·3LnIII(TMHD)3}2− (Ln = Gd, Tb and Dy)

Hexaazatrinaphthylenehexacarbonitrile ligand, HATNA(CN)6, when reduced to dianion state, forms crystalline {Cryptand[2.2.2](K+)}2{HATNA(CN)6}2−·2C6H4Cl2 (1) salt whose structure, optical and magnetic properties are studied. The HATNA(CN)6}2− dianions absorb light in the whole visible and a wide NIR range from 900 up to 2200 nm and have singlet EPR silent ground state. The dianions form trinuclear {Cryptand[2.2.2](K+)}2{HATNA(CN)6·3LnIII(TMHD)3}2−·3C6H5CH3 complexes where Ln is Gd (2), Tb (3) and Dy (4), and TMHD is 2,2,6,6-tetramethyl-3,5-heptanedionate. The {HATNA(CN)6}2− dianions are strongly distorted in coordination assemblies and have the length of the La-N{HATNA(CN)6} bonds of 2.58–2.66 Å. Dianions preserve low-energy absorption at the coordination of three metal complexes with maxima of the bands at 1440–1460 nm. DFT calculations support strong acceptor properties of HATNA(CN)6 and the non-degenerated LUMO for this ligand. As a result, dianions have singlet ground state whereas triplet state is positioned higher by 139 meV only (1620 K) and is thermally unattainable at 300 K. Coordination of the lanthanide complexes increases the singlet–triplet gap up to 210 meV (2440 K). Complexes 2–4 show weak antiferromagnetic coupling between high-spin lanthanide ions. J value estimated by PHI program for the gadolinium-complex is close to zero. Among 2–4 only gadolinium-containing complex 2 manifests an EPR signal with a complicated structure which can be approximated with D = 0.19 and E = 0.02 cm−1.

NaGdSi2O6 – A novel antiferromagnetically coupled silicate with Vierer chain structure

Ceramics based on pyroxenes and pyroxenoids present new pathways towards diverse functional materials. In this study, a novel sodium gadolinium pyroxene of the composition NaGdSi2O6 was successfully synthesised via a chelation-polymerization based sol-gel method and its crystal structure was determined by Rietveld refined synchrotron X-ray powder diffraction. NaGdSi2O6 crystallises in monoclinic symmetry with space group P21/c and unit cell parameters of a ​= ​5.44069(8) Å, b ​= ​13.83410(17) Å, c ​= ​7.66897(9) Å and β ​= ​109.9929(6)° (Z ​= ​4). The structure has been identified to be an inosilicate with a strongly undulated single tetrahedral chain along c with a chain periodicity of 4 (Vierer chain). Vierer chain-type NaGdSi2O6 shows a weak antiferromagnetic coupling, however there is no evidence for long-range magnetic order above 2 ​K.