Magneto-structural Correlations Research Articles

Multimodeling Approach to Ferromagnetic Spin-Wave Excitations in the High-Spin Cluster Mn18Sr Observed by Inelastic Neutron Scattering.

The magnetism of the mixed-valence high-spin cluster [Mn18SrO8(N3)7Cl(MedhmpH)12(MeCN)6]Cl2 (1) exhibiting intramolecular ferromagnetic interactions was studied using inelastic neutron scattering (INS), and reliable values for the exchange coupling constants were determined based on the quality of simultaneous fits to the INS and magnetic data. The challenge of the huge size of the Hilbert space (3 375 000) and many exchange coupling constants (7 assuming a C3 symmetry) generally encountered in large spin clusters was resolved as follows: (a) The results of the restricted Hilbert space ferromagnetic cluster spin wave theory were compared to the experimental spectroscopic data. The observed INS transitions were thus assigned to spin wave excitations in a bounded ferromagnetic spin cluster and moreover could be visualized in a straightforward way based on this theory. (b) Simultaneously, Quantum Monte Carlo (QMC) calculations of the temperature-dependent magnetic susceptibility with the same parameter set were compared to the experimental data. Application of state-of-the-art QMC algorithms, as available in the open source ALPS package, in ferromagnetic clusters avoids the full Hamiltonian diagonalization without sacrificing calculation accuracy of the magnetic susceptibility down to the lowest temperatures, which was crucial for the successful analysis. The combined fits revealed two exchange-coupling models with equally good overall agreement to the data. Our preferred model was inspired by magnetostructural correlations and is consistent with them. The model involves three different exchange interactions, one describing the interaction between the core MnIII spins Ja = 14.3(1.0) K and two interactions linking the core and the peripheral MnII spins: Jb = 8.3(4) K and J6 = 3.6(4) K. The use of open-source QMC software and our systematic approach to fitting multiple sets of data obtained by different experimental techniques are described in detail and are generally applicable for understanding large ferromagnetically coupled clusters.

IR and Raman spectroscopic analysis, DFT modeling, and magnetic properties of a nickel(II) complex, [Ni(succ)(H2O)4

A nickel(II) coordination polymer of succinic acid (1,4-butanedioic acid, H2succ), [Ni(succ)(H2O)4]n (1), was prepared and characterized in depth using IR and Raman spectroscopic methods and theoretical (DFT) calculations. The magnetic properties of 1 were investigated to explain the magneto-structural correlation. The experimental vibrational spectroscopic studies combined with the DFT calculations are indispensable to elucidate the molecular structures of such complexes. A complete assignment of the IR and Raman spectra was made on the basis of the calculated potential energy distribution (PED). The structures of two analogues, monomer [Ni(succ)(H2O)4] (2) and dimer [Ni(succ)(H2O)4]2 (3), were modeled by DFT calculations in good agreement with the experiment. The atomic charges and spin densities for the model complexes were calculated with natural bond orbital analysis. Magnetic susceptibility measurements for 1 indicated that Ni(II) paramagnetic centers bridged by succinic ligand in the 1 D chain are almost insolated, and also a great impact of zero-field splitting effect of the Ni(II) ions is visible.

Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)4]Br2 (1), [Ni4(NO3)2(L)4(H2O)](NO3)2 (2), [Co2(L)3](ClO4)3 (3) and [Co(L)2](ClO4) (4), where L− is the monoanion of the ditopic ligand N′-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1·0.8H2O·MeOH is a strictly planar, rectangular [2 × 2] grid. Two 2.21011 L− ligands bridge adjacent CuII atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent CuII atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2·0.2H2O·3EtOH is a square [2 × 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide O atom of one 2.21011 L− ligand. The dinuclear cation of 3·0.8H2O·1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L− ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)2]+ of complex 4, the low-spin octahedral CoIII center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of π–π stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L−, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion···metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L−) dinulear units. The susceptibility data for 2 were fit to a single-J model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), 1H nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1–4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L−, and its usefulness in the synthesis of complexes with interesting structures and properties.

Determination of the magnetic principal axes of a dysprosium complex with slow relaxation on a single crystal

• The strong uniaxial anisotropy and magneto-structural correlations are investigated in a low-symmetry Dy(III) complex by angular-resolved magnetometry. • The ground state doublet component is analyzed by the magnetic susceptibility measurement along the magnetic easy axis. A mononuclear complex [Dy L 3 (H 2 O) 4 ]·5H 2 O ( 1 ) was synthesized by the assembly of an organic ligand of 2-(4-pyridyl)pyrimidine-4-carboxylic acid (H L ) with dysprosium nitrate. Magnetic studies indicate typical slow magnetic relaxation of a single-ion origin under an applied static magnetic field. Furthermore, the angular-resolved magnetometry measurements on a single crystal sample illustrates the obvious easy axial anisotropy with the g // value of 16.85 in this complex. The magnetic easy axis is determined along the direction of the second strongest coordination bond around dysprosium ion.

Crossover from Antiferromagnetic to Ferromagnetic Exchange Coupling in a New Family of Bis-(μ-phenoxido)dicopper(II) Complexes: A Comprehensive Magneto-Structural Correlation by Experimental and Theoretical Study.

Five neutral bis(μ-phenoxido)dicopper(II) complexes, [Cu2(LMe,Me,Me)2] (1), [Cu2(LMe,Me,Et)2]·CH2Cl2 (2), [Cu2(Li-Pr,i-Pr,i-Pr)2]·2H2O (3), [Cu2(Lt-Bu,Me,i-Pr)2] (4), and [Cu2(Lt-Bu,t-Bu,i-Pr)2]·H2O (5) have been synthesized and characterized by single-crystal X-ray diffraction analyses, magnetic studies, and density functional theory (DFT) calculations, in which the ligands [H2LMe,Me,Me = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2LMe,Me,Et = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2Li-Pr,i-Pr,i-Pr = N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, H2Lt-Bu,Me,i-Pr = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, and H2Lt-Bu,t-Bu,i-Pr = N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine] contain the same [O,N,N,O]-donor atoms combination but differ in substituents at phenol rings and at an amino nitrogen atom. The effect of these remote substituents on the nature of exchange coupling interactions (ferromagnetic vs antiferromagnetic) between the copper(II) ions has been investigated. The average Cu–O–Cu angle, Cu–O–Cu–O torsion angle, and Cu···Cu separation in 1–5 are varied systematically by these remote ligand substituents in the range 98.6–83.3°, 26.0–46.5°, and 2.982–2.633 Å, respectively. As a result, the intramolecular spin–spin coupling in these complexes are changing gradually from a strong antiferromagnetic (J = −395 cm–1, where Ĥ = −JŜ1Ŝ2) to a moderate ferromagnetic (J = +53.2 cm–1) regime. The crossover angle at which the magnetic interaction changes from antiferromagnetic to ferromagnetic (J = 0) is determined to be ca. 87° for this series of dicopper(II) complexes. DFT calculations support the experimentally determined crossover angle and disclose various magneto–structural correlations in the series 1–5.

Anisotropic compression effects on nanocrystalline crystals of nickel oxide

Abstract Nickel oxide (NiO) with bulk crystalline size is an antiferromagnetic insulator correlated strongly with crystal structure. A reduction in crystalline size causes a change in the magnetic sublattice, resulting in the appearance of ferromagnetic moments. Furthermore, in the nano crystals fabricated in mesoporous silica, there is a lattice distortion at the unit cell level that brings about the change in the magnetic property, suggesting a prominent magneto-structural correlation. We investigate anisotropic compression effects on nanocrystalline NiO with a crystalline size (D) of 11.4 nm to observe this remarkable magneto-structural correlation. Magneto-crystalline anisotropy is at its maximum when negative contraction occurs. The negative contraction appears even in a bulk crystal under non-ideal hydrostatic compression, and its anomalous structural change is suppressed with decreasing D.

Spin transition characteristics of molecular solvates of CuII complexes with nitroxides: sensitivity to the packing type

A method for the synthesis of solvates of dinuclear heterospin complexes of bis(hexafluoroacetylacetonato)copper(ii) with 2-(4-methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LMe) and 2-(4-ethylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) of the composition [Cu(hfac)2LR]2 · Solv (Solv = benzene, bromobenzene, toluene, o-xylene, p-xylene) was developed. It was found that a decrease in temperature induces structural transformations of the solid phases of the complexes followed by significant changes in the distances between the paramagnetic centers in the intramolecular exchange clusters {Cu2+-O·-} from 2.321 to 1.974 A. As a result, the temperature dependences of the effective magnetic moment μeff(T) exhibit magnetic features similar to spin transitions. An analysis of the shape of the μeff(T) curves suggested that the magnetic properties of the compounds under study depend primarily on the molecular packing. A comparison of the magneto-structural correlations typical of the [Cu(hfac)2LR]2 · Solv complexes studied in this work with the data obtained earlier for analogous solvates of heterospin polymer chain complexes showed that the spin transition characteristics of the [Cu(hfac)2LR]2 · Solv systems are much less sensitive to the change of the solvent than the corresponding characteristics of the heterospin polymers containing solvate molecules in the interchain space. The magnetic characteristics of the heterospin dimer molecules depend primarily on which cavity between the dimers is filled with solvate molecules. For solvates with monoclinic symmetry, changes in the solvent molecules occupying same-type cavities have almost no effect on the spin transition characteristics in the heterospin exchange cluster.

Effect of nuclearity and symmetry on the single-molecule magnets behavior of seven-coordinated β-diketonate Dy(III) complexes

A couple of mononuclear and dinuclear lanthanide complexes, Dy(thd)3(Py) (1), Ln2(thd)6(4,4′-Bpy) (Ln = Dy (2), Gd (3), Py = Pyridine, 4,4′-Bpy = 4,4′-Bipyridine) have been designed and isolated from the reaction of β-diketonate-based ligand 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd), LnCl3∙6H2O and neutral ligands. The single-crystal X-ray diffraction analysis confirms that the Dy(III) ions of complexes 1 and 2 are seven-coordinated with N1O6 coordination environment, forming C3v and C2v local symmetry. Magnetic studies reveal that mononuclear complex 1 displays butterfly-shaped hysteresis loops at 1.8 K and slow relaxation of magnetization with energy barrier 34.7 K in optimized external filed while dinuclear complex 2 not exhibits single-molecule magnet (SMM) behavior. The variable temperature magnetic data of dinuclear Gd(III) analogues (3) indicate the negligible intramolecular magnetic interactions between the two Gd(III) centers in structure due to long Ln⋅⋅⋅ Ln distance. The diluted sample by Y(III) ions of complexes 1′ and 2′ and electrostatic model calculation were employed to elucidate distinction of magnetic properties. Investigation of magneto-structural correlation emphasizes that the orientation of the magnetic axis affects the magnetic properties, which is further associated with local symmetry of the Dy(III) ions.

Theoretical Studies on Hexanuclear [M3(μ3-O/OH)]2 (M = Fe(III), Mn(III), and Ni(II)) Clusters: Magnetic Exchange, Magnetic Anisotropy, and Magneto-Structural Correlations.

Controlling spin Hamiltonian parameters such as magnetic exchange and magnetic anisotropy of polynuclear clusters is of great interest in the area of single molecule magnets (SMMs). Among large polynuclear clusters, hexanuclear clusters offer the best compromise in terms of size as they are often rigid, solution stable, and chemically amenable. The {M6O2} core is one of the common architectures known for many hexanuclear clusters and generally reported to possess a diamagnetic ST = 0 spin ground state, barring a few exceptions. In these clusters, there are several open questions that are poorly understood: (a) What controls the nature of magnetic exchange, which in turn dictates the ground state spin values? (b) For clusters possessing a nonzero spin ground state, what dictates the magnetic anisotropy? Here, using density functional methods, we have attempted to shed light on these two question by evaluating the exchange coupling constants in [Fe6IIIO2(OH)2{(C4N2H2SMe)2C(OH)O}2( tBuCO2)10] (1), [Fe6III(O)2(O2CH2)(O2CCH2 tBu)12(py)2] (2), [Fe6III(O2)(O)2(O2CCMe3)12(py)2] (3), [FeIII6O3(O2CMe)9(OEt)2(bpy)2]ClO4 (4), [MnIII6O2(O2CH2)(O2CPe t)11(HO2CPe t)2(O2CMe)] (5), and [NiII6(OH)4(O2C tBu)8( tBuCO2H)4] (6) complexes. We have estimated all the eight near-neighbor exchange coupling constants in these clusters. Our calculations not only agree with the experimental results but also offer insight on the origin of the spin ground state. Extensive magneto-structural correlations developed by varying M-O-M angles and M-O distances reveal that J values are extremely sensitive to small structural distortions. Correlations developed indicate that both the parameters are important for Fe(III), but for Mn(III) and Ni(II), the angles were found to play a dominant role. Quite interestingly, the computed zero-field splitting parameter D S=5 of complex 1 reveals that the exchange contribution to the anisotropy controls the sign of the ground state D value-an observation which differs from the general perception that the ground state D is controlled by the single-ion zero-field splitting parameter.

Magneto-structural correlations in Ni(ii) [2 × 2] metallogrids featuring a variable number of μ-aquo or μ-hydroxo extra bridges

Cationic [2 × 2]-grids of nickel(ii) and dppn exhibit strong intramolecular antiferromagnetic interactions due to complementary effects between pyridazine and aquo/hydroxo bridges.

Covalency and magnetic anisotropy in lanthanide single molecule magnets: the DyDOTA archetype.

Lanthanide ions when complexed by polyamino-polycarboxylate chelators form a class of compounds of paramount importance in several research and technological areas, particularly in the fields of magnetic resonance and molecular magnetism. Indeed, the gadolinium derivative is one of the most employed contrast agents for magnetic resonance imaging while the dysprosium one belongs to a new generation of contrast agents for T2-weighted MRI. In molecular magnetism, Single Molecule Magnets (SMMs) containing lanthanide ions have become readily popular in the chemistry and physics communities since record energy barriers to the reversal of magnetization were reported. The success of lanthanide complexes lies in their large anisotropy due to the contribution of the unquenched orbital angular momentum. However, only a few efforts have been made so far to understand how the f-orbitals can be influenced by the surrounding ligands. The outcomes have been rationalized using mere electrostatic perturbation models. In the archetype compound [Na{Dy(DOTA) (H2O)}]·4H2O (Na{DyDOTA}·4H2O) an unexpected easy axis of magnetization perpendicular to the pseudo-tetragonal axis of the molecule was found. Interestingly, a dependency of the orientation of the principal magnetization axis on the simple rotation of the coordinating apical water molecule (AWM) - highly relevant for MRI contrast - around the Dy-OAWM bond was predicted by ab initio calculations, too. However, such a behaviour has been contested in a subsequent paper justifying their conclusions on pure electrostatic assumptions. In this paper, we want to shed some light on the nature of the subtle effects induced by the water molecule on the magnetic properties of the DyDOTA archetype complex. Therefore, we have critically reviewed the structural models already published in the literature along with new ones, showing how the easy axis orientation can dangerously depend on the chosen model. The different computed behaviors of the orientation of the easy axis of magnetization have been rationalized as a function of the energy gap between the ground and the first excited doublet. Magneto-structural correlations together with a mapping of the electrostatic potential generated by the ligands around the Dy(iii) ion through a multipolar expansion have also been used to evidence and quantify the covalent contribution of the AWM orbitals.

Magneto-structural correlations of novel kagomé-type metal organic frameworks

Here, we report the in situ formation of two novel metal organic frameworks based on copper and cobalt using tetrazole-5-carboxylate ethyl ester as the ligand synthesized by a hydrothermal route.

A Chiral Bipyrimidine-Bridged Dy2 SMM: A Comparative Experimental and Theoretical Study of the Correlation Between the Distortion of the DyO6N2 Coordination Sphere and the Anisotropy Barrier.

Chiral bipyrimidine-bridged dinuclear LnIII complexes of general formula [(μ-bipym){((+)-tfacam)3Ln}2] and [(μ-bipym){((-)-tfacam)3Ln}2], have been prepared from the assembly of Ln(AcO)3·nH2O (LnIII = Dy, Gd), (+)/(−)-3-(trifluoroacetyl)camphor enantiopure ligands ((+)/(-)-Htfacam) and bipyrimidine (bipym). The structure and chirality of these complexes have been supported by single-crystal X-Ray diffraction and circular dichroism. The study of the magnetic properties of the GdIII complexes revealed a very weak antiferromagnetic interaction between the GdIII ions through the bipyrimidine bridging ligand. Ab initio CASSCF calculations indicated that the ground Kramers doublet (KD) of both DyIII centers is almost purely axial with the anisotropy axis located close to the two tfacam−ligands at opposite sides of each DyIIIatom, which create an axial crystal field. In keeping with this, ac dynamic measurements indicated slow relaxation of the magnetization at zero field with Ueff = 55.1 K, a pre-exponential factor of τo = 2.17·10−6 s and τQTM = 8 μs. When an optimal dc field of 0.1 T is applied, QTM is quenched and Ueff increases to 75.9 K with τo = 6.16 × 10−7 s. The DyN2O8 coordination spheres and SMM properties of [(μ-bipym){((+)-tfacam)3Ln}2] and their achiral [(Dy(β-diketonate)3)2(μ-bpym)]analogous have been compared and a magneto-structural correlation has been established, which has been supported by theoretical calculations. In contrast to the GdIII compounds, the magnetic exchange interaction between the DyIII ions has been calculated to be very weak and, generally, ferromagnetic in nature. Relaxation mechanisms for [(μ-bipym){((+)-tfacam)3Ln}2] and previously reported analogous have been proposed from ab initio calculations. As the magnetic exchange interaction found to be very weak, the observed magnetization blockade in these systems are primarily dictated by the single ion anisotropy of DyIII ions.

Crystal Structure of Metal Complexes with 2-Imidazoline Nitroxides and Dicyanamide

The structure of heterospin complexes[M(DCA)2L2x]·yH2O, where M = Mn, Co, Ni, Cu, Lx is the nitronyl nitroxide radical; DCA is the dicyanamide anion, y = 0, 1, or 2 is studied. It is found that the nature of[M(DCA)2L2x]·yH2O is characterized by the formation of multiple hydrogen bonds generating ribbons, layers, or frameworks in the crystals. The analysis of magnetostructural correlations by means of band calculations of the electronic structure confirms the occurrence of ferromagnetic exchange between the unpaired electrons of paramagnetic centers in solid [Cu(DCA)2L21]·2H2O when the distances between the copper ion and the coordinated N-O group are short.

Tetranuclear dicubane Ni(II) complexes with antiferromagnetically interacting Ni(II) ions: Solvothermal synthesis and magnetostructural study

The two red centrosymmetric Ni(II) tetrameric complexes with defective double-cubane core, [Ni2L2(H2O)]2·0.75 CH3OH (1) and [Ni2L2(CH3CN)]2 (2) (H2L = N-(2-hydroxy-5-methylphenyl)-salicylideneimine), have been isolated and magnetically characterized, emphasizing the effect of the reaction temperature on structural and magnetic properties of isolated complexes. Prevailing antiferromagnetic interactions in the complexes are observed leading to the non-magnetic ground state. Comparation with the known magneto-structural correlations in nickel cubane and dicubane complexes is discussed.

Synthesis, structure, physicochemical characterization and theoretical evaluation of non-covalent interaction energy of a polymeric copper(II)-hydrazone complex

One dimensional polymeric copper-hydrazone complex {[Cu(H0.5L)(µ1,3-SCN)]0.5ClO4·0.5MeOH}n (1) has been synthesized with Cu(ClO4)2·xH2O and N'-(1-(pyridin-2-yl)ethylidene)acetohydrazide (HL) in presence of NaSCN. The ligand and the complex have been characterized by several spectroscopic techniques (IR, UV–Vis and EPR), cyclic voltammetry and the structure of 1 has been determined by single crystal X-ray diffraction. The complex is an infinite one dimensional polymer bridged by thiocyanate. The magneto-structural correlation has been determined and the non-covalent interactions present in the molecule have been energetically evaluated by means of DFT calculations.

Magnetic Dynamics of a Neodymium(III) Single-Ion Magnet.

Slow relaxation of magnetization is observed in a neodymium(III) single-ion magnet based on phosphine oxide, which successfully extends our pentagonal bipyramidal family to light lanthanides. Comprehensive magnetic characterizations reveal that the magnetic dynamics follow the power law that corresponds to a Raman process, despite an energy splitting of 207 cm-1 evidenced by the ab initio calculation. Compared with a similar complex, the magnetic dynamics and magneto-structural correlations are clarified, providing deeper insight into the pursuit of promising light lanthanide single-molecule magnets.

Polynuclear Fe(II) complexes: Di/trinuclear molecules and coordination networks

We review here the recent progress made in the synthesis of spin crossover (SCO) Fe(II) coordination complexes with two or three metal atoms, as well as coordination polymers, with the main focus on that in the last five years. We discuss these as well as their magnetic properties to derive magnetostructural correlations. This manuscript is organized through the ligand types that serve to produce the various coordination systems presented. The ligand structure and essential SCO parameters of the complexes are gathered in comprehensive figures and tables. This review illustrates the richness and key parameters of coordination chemistry to develop multiple architectures with the desired properties involving this fascinating phenomenon.

A Crystallographic Study of a Novel Tetrazolyl-Substituted Nitronyl Nitroxide Radical

Spin-labelled compounds are widely used in chemistry, physics, biology, and material sciences, but the directed synthesis of some functionalized organic radicals is still a challenge. We succeeded in the preparation of a tetrazolyl-substituted nitronyl nitroxide radical in pure crystalline form. According to the single-crystal X-ray data, intra- (NH…O, 2.43 Å) and inter-molecular hydrogen bonds (NH…O, 1.91 Å) are formed between NH groups of the tetrazole cycles and O atoms of the paramagnetic moieties. The intermolecular H-bonds connect the molecules forming chains along the a-axis. Moreover, there are short intermolecular contacts between the O atoms (3.096 Å) and between the O and C atoms (3.096 Å) of the nitronyl nitroxide moieties within the chain. The spin-unrestricted broken-symmetry calculations performed at the BS-UB3LYP/def2-TZVP level of theory predicted a sufficient ferromagnetic interaction (J ≈ 20 cm–1) between the adjacent radicals inside the chain, but a weak antiferromagnetic interaction (−J ≤0.2 cm−1) between the nearest radicals belonging to the different chains. Thus, a rare case when stable radicals, the tetrazolyl-substituted nitronyl nitroxides, are ordered into ferromagnetic chains was revealed; an investigation of the magneto-structural correlations inherent in the nitroxide radical will demand a special experiment in the sub-Kelvin regime.

Theoretical insights into the magneto-structural correlation: Comparison between series of copper(I) and silver(I) metal complexes with nitronyl nitroxide radicals

Using nitronyl nitroxide radicals, we presented a comparative study of magneto-structural correlations between series of CuI and AgI models (Cu(a) ∼ Cu(d) and Ag(a) ∼ Ag(d), modified from [CuI(immepy)2]PF6 and [AgI(impy)2]PF6 (Oshio et al., 1997). Cu(a) ∼ Cu(d) possess three characteristics that are significantly different from Ag(a) ∼ Ag(d) for the more utilization of CuI than that of AgI in the ferromagnetic joints of radical networks. (1) Structural shorter Cu-N length in Cu(a) ∼ Cu(d) than corresponding Ag-N length in Ag(a) ∼ Ag(d) leads to the quite larger Wiberg bond index (WBI), ∇2ρ(r) values, and metal atomic spin densities between triplet (T) and broken-symmetry (BS) states (ΔρT-BS) of the former than that of the latter. (2) From each of AgI (Ag(a) ∼ Ag(d)) to the corresponding one of CuI (Cu(a) ∼ Cu(d)), the spin density increases up to one or two order of magnitudes, which indicates the more notable spin delocalization from ONCN groups (paramagnetic centers) to CuI than that to AgI. Changing the dihedral angle (θ) between two radical components, the variation of the magnetic coupling constants (Jab) is directly proportional to the spin density of CuI but inversely proportional to that of one ONCN group. (3) Due to the larger d orbital distributions of CuI than that of AgI in the SOMOs, large metal-ligand charge transfer (MLCT) occurs in Cu(a) ∼ Cu(d) but little in Ag(a) ∼ Ag(d). The broken orthogonality is not evidently associated with the significantly different magnetic coupling interactions between CuI and AgI models.