Magnetic properties of nitronylnitroxide and iminonitroxide triradicals as model compounds for generalized ferrimagnets

A novel iminonitroxide triradical, p-triIN (1), has been synthesized, in which a π-conjugated biradical with a singlet ground state and a doublet monoradical are united by σ-bonds. The intramolecular exchange interaction within the para-phenylene-based biradical moiety has been found to be antiferromagnetic (2J(π)/k_B = −30.8 K) from magnetic susceptibility measurements on a parent biradical, p-bisIN (2). The magnetic properties of 1 have been examined by magnetic susceptibility χ_m. Upon cooling, the χ_mT value for 1 has decreased and passed across 0.38 emu mol⁻¹ K, which is expected for one mol of S = 1/2 spin with g = 2.0. The magnetic behavior of 1 indicates that the ground state of the molecular assemblage of 1 in the crystal is diamagnetic, which is attributable to one of the exotic spin states as predicted in a theoretical model of generalized ferrimagnetism. The possible occurrence of the generalized ferrimagnetic spin alignment is concluded in view of the magnetic properties of 1.     

Mesophase and magnetic behavior in cobalt(II) and iron(II) compounds

The metal complexes with long alkyl chains [Co(C16-terpy)₃](BF₄)₂ (1) and [Fe(C16-terpy)₂](BF₄)₂ (2) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mössbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mössbauer studies revealed that the complex 1 exhibited unique spin transition (T_1/2↓ = 217 K and T_1/2↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition (T_1/2↓ = 266 K and T_1/2↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.     

Magnetic interactions in p-phenylene-bis(nitronyl nitroxide) biradicals with large torsion angles

We have designed and synthesized new biradicals of p-phenylene-bis(nitronyl nitroxide) substituted with two methoxy groups at 2,3- (2) and 2,5-positions (3). A parent biradical p-phenylene-bis(nitronyl nitroxide) (1) has intramolecular antiferromagnetic exchange interaction of 2J/k_B = −104 K ∼ −106 K with a torsion angle of 28.5° between the phenyl and the imidazole rings of nitronyl nitroxide. X-ray crystal structure analysis shows that the bulky substituents in 2 and 3 give large torsion angles of 65–70°. The larger torsion angles should weaken the magnitude of intramolecular exchange interactions, which is attributed to a decrease in π-conjugation over the p-phenylene and the radical groups. Magnetic susceptibility measurements indicate that the intramolecular exchange interactions in 2 and 3 are severely weakened to about 6% of that in 1, 2J/k_B = −6 K ∼ −8 K. The relation between the torsion angle and the intramolecular exchange interaction is consistent with DFT calculations. The ground-state singlet biradicals with suppressed intramolecular exchange interactions can be a building block for exotic exchange-coupled spin systems as predicted in our theoretical studies.     

Design,synthesis and physical properties of the metal complexes using π-radical with photo-excited high-spin state as a ligand

Two π-radicals, 3-pyridinyl-phenylanthracene(iminonitroxide) (3) and 3-pyridinyl-phenylanthracene-(nitronylnitroxide) (4) were designed as candidates of the ligand for the metal complexes to clarify the exchange interactions between the paramagnetic centers of the metal ions and the photo-excited high-spin states of the purely organic π-radical. Compounds 3 and 4 were synthesized and their magnetic properties were examined, showing weak antiferromagnetic interactions, θ = −1.5 K for 3 and −0.7 K for 4. The photo-excited states of 3 and 4 were investigated by time-resolved ESR and clarified that both π-radicals have the quartet (S = 3/2) high-spin states as their lowest photo-excited states. Two metal complexes [Fe(III)(L)(4)] · (BPh₄) (Low spin) (LH₂ = N,N′-bis(1-hydroxy-2-benzyliden)-1,7-diamino-4-azaheptane) and [Cu(II)(hfac)₂(4)₂] using 4 were prepared. Their magnetic behaviors are well analyzed with the Bleaney–Bowers model with J/k_B = − 0.86 K and three S = 1/2 spin cluster model with J/k_B = −1.0 K, respectively, showing weak antiferromagnetic interactions between the paramagnetic centers of the metal ions and the π-radical in the ground state.     

Quantitative analysis of the magnetism of the meta-(methoxy)phenyl nitronyl nitroxide crystal: A bottom–up analysis of a crystal presenting competing ferro and antiferromagnetic interactions

Crystals of the meta isomer of the (methoxy)phenyl nitronyl nitroxide radical, m-(OMe)PhNN, present the characteristics of an antiferromagnet (paramagnetic θ = −0.97 K, |J| = 1.6 cm⁻¹, [L. Angeloni, A. Caneschi, L. David, A. Fabretti, F. Ferraro, D. Gatteschi, A. le Lirzin, R. Sessoli, J. Mater. Chem. 4 (1994) 1047]). Using our quantitative bottom–up approach, we analyze here the magnetism of this crystal by first computing its magnetic topology and, then, using this information, the macroscopic magnetic susceptibility χ(T) of the crystal. The crystal presents one ferro and two antiferromagnetic exchange interactions, J_AB, of similar strength (+0.20, −0.20 and −0.11 cm⁻¹) that create a complex three-dimensional magnetic topology of interacting planes. This complex network of competing ferro and antiferromagnetic pathways does not allow a sounding prediction of the macroscopic magnetic susceptibility using qualitative considerations. Our approach computes the χT versus T curve that fully reproduces the experimental shape.     

Magnetic property of 1:2 mixture of Co(p-tolsal)2; p-tolsal = N-p-tolylsalicylideniminato,and cyclic pentacarbene–pyridine with S = 10/2 in dilute frozen solution

A cyclic pentadiazo-pyridine ligand, cD5py, was prepared and its photoproduct, cC5py, in a frozen solution was confirmed to be a high-spin polycarbene with S = 10/2. The magnetic property of the 1:2 mixture of Co(p-tolsal)₂; p-tolsal = N-p-tolylsalicylideniminato, and cD5py in a dilute frozen solution after irradiation was investigated by SQUID magneto/susceptometry. In the ac magnetic susceptibility measurements, the in-phase and out-of-phase components (χ′ and χ″, respectively) with frequency dependence were observed, indicating that the 1:2 complex, Co(p-tolsal)₂-(cC5py)₂, had slow magnetic relaxation characteristic of the single-molecule magnet (SMM). From the χ″ versus T plots with various frequencies, the values of activation barrier, U_eff, for the reverse of the magnetism was estimated to be 72 K. In the dc magnetic susceptibility measurements, the magnetic hysteresis loops were observed below 3 K. The value of the coercive force, H_c, depends on the temperature and increases on cooling. The hysteresis loop with H_c = 7.1 kOe was observed at 1.9 K.     

A mixed-valence Mn6 cluster capped by nitronyl nitroxide units

A mixed-valence Mn₆ cluster, [Mn₆O₂(t-BuCO₂)₁₀(H₂O)(NNPy)₂] (2), has been synthesized by the assembly of [Mn₆O₂(t-BuCO₂)₁₀(THF)₄] · THF (1) with a nitronyl nitroxide radical molecule, 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NNPy). Complex 2 consists of [Mn₆O₂]¹⁰⁺ core and two NNPy radical attachments, where the [Mn₆O₂]¹⁰⁺ core is composed of two Mn^III ions (i.e., the inner Mn ions) bridged by two μ₄-O²⁻ and four Mn^II ions (i.e., the outer Mn ions) surrounding the inner Mn^III ions via the two μ₄-O²⁻, forming an edge-shared double-tetrahedral polygon (vertices: Mn ions, centers: μ₄-O²⁻). Bridges by pivalate groups in μ₂- and μ₃-modes assist the coordination spheres around these Mn ions. Two of the outer Mn^II ions in trans position are, to the sixth coordination site, occupied by NNPy molecule (pyridine-N coordination). One of the outer Mn^II ions is occupied by a water molecule to complete octahedral geometry, but the rest of them is vacancy taking a square-pyramidal geometry. The variable-temperature direct current (DC) magnetic susceptibility was collected in the temperature range of 1.81 to 300 K at a 1 kOe appled field. The molar magnetic susceptibility (χ_M) steadily increases from 0.053 emu mol⁻¹ at 300 K to 0.29 emu mol⁻¹ at 20 K, showing a kink (peak), and then increases rapidly to 0.70 emu mol⁻¹ at 1.81 K. The profile of this magnetic behavior indicates that the intracluster spin cancellation takes place bellow 20 K to derive an S_Mn6 = 0 by strong antiferromagnetic interaction between Mn ions. This is consequently leading to paramagnetic isolation of only the two nitronyl nitroxide units as if remaining as exchange-uncorrelated spin carriers. The variable-temperature ESR spectra show a sharp signal below 30 K with g = 2.00, proving the presence of free nitronyl nitroxide radical, in which double integral of such signal obeys the Curie–Weiss law that follows the susceptibility result in the low-temperature region.     

Ferromagnetic interaction in iron(II)–bis-Schiff base complexes

The preparation and magnetic properties of three Fe(II)–bis-Schiff base complexes, [Fe₂(L1)2(4,4′-bpy)] · MeOH (1), [Fe(L2)(EtOH)] (2) and [Fe(L3)(MeOH)] (3) (L1 = N,N′-bis(2-hydroxy-1-naphthaldehyde)-1,2-phenylenediimine; L2 = N,N′-bis(salicylidene)-1,2-phenylenediamine; L3 = N,N′-bis(5-Cl-salicylidene)-1,2-phenylenediamine; 4,4′-bpy = 4,4′-bipyridine) are reported. X-ray single crystal structure analyses for 1–3 reveal that 1 shows a dinuclear Fe(II)–bis-Schiff base complex bridged by 4,4′-bpy, while 2 and 3 show mononuclear structures. Molecular packing of 2 shows a uniform one-dimensional chain structure through hydrogen bonds and Fe?π interaction and that of 3 indicates significant π–π interaction to form a dimmer structure. The χT–T plots of 1–3 show all ferromagnetic interaction at low temperature. The origin of the ferromagnetic interaction observed in 2 is tentatively ascribed to the dimer formation through Fe?π interaction at low temperature.     

Crystal structures,luminescence, and thermal properties of lanthanide complexes with 2,3,4-trimethoxybenzoic acid and 1,10-phenanthroline

The complexes of [Ln(2,3,4-tmoba)₃phen]₂ (Ln = Dy (1), Eu (2), Tb (3); 2,3,4-tmoba = 2,3,4-trimethoxybenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by a series of techniques including the elemental analysis, IR and fluorescent spectra and TG/DSC-FTIR technology. The crystal structures were determined by X-ray crystallography. Each complex include two Ln³⁺ ions, six 2,3,4-tmoBA and two phen molecules forming a binuclear structure, giving the coordination number of nine. The three-dimensional IR accumulation spectra of gaseous products for the complexes 1 to 3 are analyzed and the thermal decomposition processes are further authenticated. Through means of differential scanning calorimeter (DSC), two solid-solid phase transition endothermic peaks were found in the complex 2, which was different from the complexes 1 and 3. The heat capacities of these complexes were measured and fitted to a polynomial equation with the least squares method for each complex on the basis of the reduce temperature x (x = [T − (T_max + T_min)/2]/[(T_max − T_min)/2]) over the range from (256.15 to 476.15) K. Subsequently, the smoothed molar heat capacities and thermodynamic functions (H_T−H_298.15 K), (S_T−S_298.15 K), and (G_T−G_298.15 K) of the complexes 1 to 3 were calculated based on the fitted polynomial of the heat capacities. The fluorescent intensity of the complexes 2 and 3 are markedly improved as well.     

Effect of counter cationic geometry on stacking structures and magnetic properties of [Ni(mnt)2]− complexes

The crystal structures and magnetic properties of two new ion-pair complexes, [N-NH₂Py][Ni(mnt)₂] (1) and [N-NH₂Ql][Ni(mnt)₂] (2) (N-NH₂Py⁺ = 1-aminopyridinium, N-NH₂Ql⁺ = 1-aminoquinolinium; mnt^2? = maleonitriledithiolate) have been investigated. The differences of the molecular topology and size of the counter cation result in distinct anionic and cationic stacking patterns in the crystals, namely, in the crystal of 1 the anions (A) and cations (C) alternate to stack into a mixed column in the fashion of …AACCAACC…, and the neighboring columns are connected together via intermolecular H-bonding interactions as well as van der Waals forces; while, in the crystal of 2, the anions and cations alternate to form a mixed columnar stack in the manner of …ACAC…, and the adjacent columns are held together via weak van der Waals forces. Investigations of variable-temperature magnetic susceptibility indicated that 1 is a spin gap system but its magnetic behavior does not follow the Bleaney–Bowers spin dimer model; 2 shows a magnetic feature of linear decrease of χ_mT value upon cooling, and this kind of magnetic behavior is probably related to the presence of temperature independent paramagnetism.     

The thermodynamic properties of DyBr3(s) and DyI3(s) from T=5 K to their melting temperatures

Low-temperature calorimetric measurements have been performed on DyBr₃(s) in the temperature range (5.5 to 420 K ) and on DyI₃(s) from T=4 K to T=420 K. The data reveal enhanced heat capacities below T=10 K, consisting of a magnetic and an electronic contribution. From the experimental data on DyBr₃(s) a C⁰_p,m (298.15 K) of (102.2±0.2) J·K⁻¹·mol⁻¹ and a value for {S⁰_m (298.15 K) − S⁰_m (5.5 K)} of (205.5±0.5) J·K⁻¹·mol⁻¹, have been obtained. For DyI₃(s), {S⁰_m (298.15 K) − S⁰_m (4 K)} and C⁰_p,m (298.15 K) have been determined as (226.9±0.5) J·K⁻¹·mol⁻¹ and (103.4±0.2) J·K⁻¹·mol⁻¹, respectively. The values for {S⁰_m (5.5 K) − S⁰_m (0)} for DyBr₃(s) and {S⁰_m (4 K) − S⁰_m (0)} for DyI₃(s) have been calculated, giving S⁰_m (298.15 K)=(212.3±0.9) J·K⁻¹·mol⁻¹ in case of DyBr₃(s) and S⁰_m (298.15 K) =(233.1±0.7) J·K⁻¹·mol⁻¹ for DyI₃(s). The high-temperature enthalpy increment has been measured for DyBr₃(s) in the temperature range (525 to 799 K) and for DyI₃(s) in the temperature range (525 to 627 K). From the results obtained and enthalpies of formation from the literature, thermodynamic functions for DyBr₃(s) and DyI₃(s) have been calculated from T→0 to their melting temperatures at 1151.0 K and 1251.5 K, respectively.     

Coordination enhancement of single-molecule magnet behavior of Tb(III)–Cu(II) dinuclear systems

Tb(III)–Cu(II) based single-molecule magnets (SMMs) were synthesized to investigate the relationship between magnetic anisotropy and the symmetry of the ligand field, by the reaction of [TbCu(o-vanilate)₂(NO₃)₃] with 2,2-dimethyl-1,3-propanediamine (3), followed by the reaction of one another equivalent of o-vaniline (4). Both complexes behave as SMM in the temperature range of 2.8–4.0 K (3) and 2.8–5.2 K (4), showing semi-circle shapes of Cole–Cole plots with α parameters in the ranges of 0.27–0.41 and 0.11–0.32. The energy barriers Δ/k_B for the spin flippings were estimated from the Arrhenius plots to be 29(2) K for 3 and 32.2(6) K for 4, respectively.     

Mechanism of reversible spin transition with a thermal hysteresis loop in [FeIII(qsal)2][Ni(dmise)2] · 2CH3CN: Selenium analogue of the precursor of an Fe(III) spin-crossover molecular conducting system

A novel Fe(III) spin-crossover complex, [Fe(qsal)₂][Ni(dmise)₂] · 2CH₃CN 1 [qsalH = N-(8-quinolyl)-salicylaldimine, dmise = 4,5-dithiolato-1,3-dithiole-2-selone] was prepared. The magnetic susceptibility measurements revealed 1 exhibited a cooperative spin transition with a thermal hysteresis loop of 15 K. The high and the low temperature structures of 1 indicated three-dimensional intermolecular π?π interactions play a key role in the cooperative spin transition, accompanying a reversible molecular slipping of π-dimer of Ni(dmise)₂ along the molecular long axis. The transfer integral calculation for 1 suggested the π-dimer of Ni(dmise)₂ is in the spin singlet state.     

New α,ω-dicarboxylate coordination polymers with 4,4′-bipyridine: Cu(bpy)(C5H6O4), Zn(bpy)(C5H6O4), Zn(bpy)(C6H8O4) and Mn(bpy)(C8H12O4) · H2O

Four 4,4′-bipyridine α,ω-dicarboxylate coordination polymers Cu(bpy)(C₅H₆O₄) (1), Zn(bpy)(C₅H₆O₄) (2), Zn(bpy)(C₆H₈O₄) (3) and Mn(bpy)(C₈H₁₂O₄) · H₂O (4) have been synthesized and structurally characterized by single crystal X-ray diffraction methods (bpy = 4,4-bipyridine, (C₅H₆O₄)²⁻ = glutarate anion, (C₆H₈O₄)²⁻ = adipate anion, (C₈H₁₂O₄)²⁻ = suberate anion). Their crystal structures are featured by dimeric metal units, which are co-bridged by 4,4′-bipyridine ligands and dicarboxylate anions such as glutarate, adipate and suberate anions to generate 2D layers with a (4,4) topology in 1, 2 and 4 as well as to form 3D frameworks in 3. Two 3D frameworks in 3 interpenetrate with each other to form a topology identical to the well-known Nb₆F₁₅ cluster compound. Over 5–300 K, the paramagnetic behavior of 4 follows the Curie–Weiss law χ_m(T − Θ) = 4.265(5) cm³ mol⁻¹ with the Weiss constant Θ = −6.3(2) K. Furthermore, the thermal behavior of 3 and 4 is also discussed.     

A family of dodecanuclear Mn11Ln single-molecule magnets

A series of four isostructural dodecanuclear complexes [Mn^III₉Mn^II₂Ln^III(O)₈(OH)(piv)₁₆(NO₃)(CH₃CN)]·xCH₃CN·yC₇H₁₆ (piv = pivalate; x = ½, y = ¾, Ln = Tb (1); x = 2, y = ½, Ln = Dy (2), Ho (3), and Y (4)) has been prepared for which the structural motif described as ‘a lanthanide ion nested in a large manganese shell’ is observed. All compounds show out-of-phase signals in their ac susceptibilities, and their single-molecule magnet behaviour was confirmed by single-crystal micro-SQUID studies of 1-3 which show hysteresis loops of molecular origin at T < 1.0 K. The SMM behaviour observed in compounds 1-3 is more pronounced than that for 4, which contains the diamagnetic Y^III ion. This is principally the result of ferromagnetic coupling between the paramagnetic anisotropic Ln^III ions (Tb^III, Dy^III and Ho^III) and the manganese shell, which enhances the total spin ground state of the complexes.     

Novel Fe (III) heterochelates: Synthesis,structural features and fluorescence studies

Fluorescence properties of five 4-acyl pyrazolone based hydrazides (H₂SB_n) and their Fe (III) heterochelates of the type [Fe(SB_n)(L)(H₂O)]·mH₂O [H₂SB_n = nicotinic acid [1-(3-methyl-5-oxo-1-phenyl-4,5-di hydro-1H-pyrazol-4yl)-acylidene]-hydrazide; where acyl = –CH₃, m = 4 (H₂SB₁); –C₆H₅, m = 2 (H₂SB₂); –CH₂–CH₃, m = 3 (H₂SB₃); –CH₂–CH₂–CH₃, m = 1.5 (H₂SB₄); –CH₂–C₆H₅, m = 1.5 (H₂SB₅) and HL = 1-cyclopropyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydroquinoline-3-carboxylic acid] were studied at room temperature. The fluorescence spectra of heterochelates show red shift, which may be due to the chelation by the ligands to the metal ion. It enhances ligand ability to accept electrons and decreases the electron transition energy. The kinetic parameters such as order of reaction (n), energy of activation (E_a), entropy (S*), pre-exponential factor (A), enthalpy (H*) and Gibbs free energy (G*) have been reported.     

Crystallography and magnetism of 1-(4-[N-tert-butylaminoxyl]-2,3,5,6-tetrafluorophenyl)pyrrole

1-(4-[N-tert-Butylaminoxyl]-2,3,5,6-tetrafluorophenyl)pyrrole (BNPPF4) was synthesized and characterized by X-ray crystallography, electron spin resonance (ESR), and magnetism. It is unusually stable by comparison to related systems. Its crystallography shows strong twisting of the nitroxide group, and dyad π-stacking that is probably assisted by fluoroarene/pyrrole interactions. There are crystallographic nitroxide chains of >5.5 Å distance between nitroxides, and chains of nitroxide to tert-butyl contacts. BNPPF4 magnetic behavior is consistent with a spin-pairing model having 2J/k = (−)8.7 ± 0.2 K, but not with a simple 1D AFM chain model.     

Molecular spin ladders self-assembly from [Ni(dmit)2]− building blocks: Syntheses,structures and magnetic properties

Two ion-pair compounds, consisting of 1-(4′-R-benzyl)pyridinium ([RBzPy]⁺, R = NO₂ (1) and Br (2)) and [Ni(dmit)₂]⁻ (dmit²⁻ = 2-thioxo-1,3-dithion-4,5-dithiolato), have been synthesized and structurally characterized. The anions of [Ni(dmit)₂]⁻ stack into dimers, which further construct into two-leg ladder through terminal S⋯S interactions in 1, lateral S⋯S interactions in 2. The weak H-bonding interactions of C–H⋯S were observed in 2, while only weak van de Waals interactions between anion and cations in 1. The magnetic susceptibilities measured in 2–300 K indicate AFM exchange interaction domination both two compounds. A peculiar magnetic transition at ∼100 K was observed in 1. An AFM ordering below ∼11 K was found in 2, and the best fit to magnetic susceptibility above 45 K in this compound, using a dimer model with s = 1/2, give rise to Δ/k_B = 36.1 K, zJ = −0.91 K, C = 3.2 × 10⁻³ emu K mol⁻¹ and χ₀ = −4.0 × 10⁻⁶ emu mol⁻¹ with g of 2.0 fixed.     

Synthesis and structures of N,N,O-scorpionatoruthenium(II) complexes featuring “non-innocent” o-benzoquinonediimines

A series of ruthenium(II) complexes bearing redox-active o-benzoquinonediimines (o-bqdi) was synthesized and characterized. Reactions of [RuCl(bdmpza)(η⁴-cod)] (bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetato; cod = 1,5-cyclooctadiene) and 1,2-benzenediamines such as o-phenylenediamine (o-pdaH₂), 4,5-difluoro-1,2-benzenediamine (o-pdaF₂), 4,5-dichloro-1,2-benzenediamine (o-pdaCl₂), and 4,5-dimethoxy-1,2-benzenediamine (o-pda(OMe)₂) afforded [RuCl(bdmpza)(o-bqdiX₂)] (X = H, 1; X = F, 2; X = Cl, 3; X = OMe, 4).