[Mn^III(Schiff Base)]₃[Re^IV(CN)₇], Highly Anisotropic 3D Coordination Framework: Synthesis, Crystal Structure, Magnetic Investigations, and Theoretical Analysis

A new highly anisotropic coordination heterobimetallic polymer [Mn^III(Schiff-base)]₃­[Re^IV(CN)₇] was synthesized and characterized structurally and magnetically. The single crystal X-ray analysis has revealed that this is the first framework among the complexes composed of homoleptic cyanometallate and Mn^III complex of the tetradentate Schiff base ligand. A formation of 3D assembly is possible due to both the pentagonal bipyrimidal geometry of the cyanometallate unit and suitable size of constituents: [Re­(CN)₇]^3– and [Mn^III(acacen)]⁺, where acacen = <i>N</i>,<i>N</i>′-ethylenebis­(acetylacetoneiminato). The powder and crystal magnetic studies show that the compound undergoes an antiferromagnetic ordering of a complicated character below Neel temperature of 13 K, and exhibits a metamagnetic behavior and strong magnetic anisotropy similar to those observed in related 3D Mn^II–[Mo­(CN)₇]^4– systems. Unusual magnetic properties of [Mn^III(acacen)]₃­[Re^IV(CN)₇] (<b>1</b>) originate from an interplay of Re–Mn anisotropic spin coupling and ZFS effect of Mn^III ions with a noncollinear orientation of the local magnetic axes in the cyano-bridged 3D network. A theoretical model of anisotropic spin coupling between orbitally degenerate [Re^IV(CN)₇]^3– complexes and Mn^III ions is developed, and specific microscopic mechanisms of highly anisotropic spin coupling in Re^IV–CN–Mn^III linkages in complex <b>1</b> are analyzed in detail

1D Manganese(II) Derivatives of an Imidazole-Substituted Nitronyl Nitroxide. An Approach toward Molecular Magnetic Materials of High Dimensionality

Extended linear complexes of manganese(II) with a bis-chelating nitronyl nitroxide ligand, 2-(2-imidazolato)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxy (NITIm), have been prepared where metallic and organic spin carriers alternate. Depending on the deprotonating agent, the solvent, and the counteranion, the species [Mn(NITIm)(H2O)2]CH3COO, 1, [Mn(NITIm)(DMSO)2]BPh4, 2, [Mn(NITIm)(H2O)(ImH)]NO3, 3, and [Mn(NITIm)(NITImH)]ClO4, 4, have been obtained, which differ by the additional ligands completing the metal coordination sphere. Complexes 1−3 are cis isomers and 4 is found as the mer modification; in all compounds, one observes a regular alternation of Λ and Δ metal environments. Their magnetic properties are similar, with Mn(II)−nitroxide interactions J ≈ −45 cm-1 (H = −2JSi·Sj), and they display weak ferromagnetic properties below 5 K. Canting of the manganese ions is responsible for these properties. Relevant crystallographic parameters are as follows: 1, space group Fdd2, a = 16.713(1), b = 40.111(3), c = 9.735(1), Z = 16; 2, space group Pca21, a = 30.328(3), b = 13.422(1), c = 9.589(1), Z = 4; 3, space group P21/c, a = 9.787(2), b = 22.973(5), c = 9.671(2), β = 117.32(3)°, Z = 4; 4, space group P21/c, a = 9.761(2), b = 28.668(5), c = 9.941(2), β = 96.07(3)°, Z = 4

Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a [{Fe^IIL}₂{Nb^IV(CN)₈}] Helical Chain Compound Designed with Heptacoordinate Fe^II

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear {FeIINbIV} compound with remarkable magnetic features when compared to other Fe(II)-based single chain magnets (SCM). Cyano-bridged heterometallic {3d-4d} and {3d-5d} chains are formed upon assembling Fe(II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, [M(CN)8]4− (M = NbIV, MoIV, WIV). X-ray diffraction (single-crystal and powder) measurements reveal that the [{(H2O)Fe(L1)}{M(CN)8}{Fe(L1)}]∞ architectures consist of isomorphous 1-D polymeric structures based on the alternation of {Fe(L1)}2+ and {M(CN)8}4− units (L1 stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged {Fe−Nb} exchange interaction to be antiferromagnetic with J = −20 cm−1 deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Δ/kB = 74 K and τ0 = 4.6 × 10−11 s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i.e., abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient {Fe−Nb} exchange interaction and significant anisotropy of the {Fe(L1)} unit. High field EPR and magnetization experiments have revealed for the parent compound [Fe(L1)(H2O)2]Cl2 a negative zero field splitting parameter of D ≈ −17 cm−1. The crystal structure, magnetic behavior, and Mössbauer data for [Fe(L1)(H2O)2]Cl2 are also reported

Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a [{Fe^IIL}₂{Nb^IV(CN)₈}] Helical Chain Compound Designed with Heptacoordinate Fe^II

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear {FeIINbIV} compound with remarkable magnetic features when compared to other Fe(II)-based single chain magnets (SCM). Cyano-bridged heterometallic {3d-4d} and {3d-5d} chains are formed upon assembling Fe(II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, [M(CN)8]4− (M = NbIV, MoIV, WIV). X-ray diffraction (single-crystal and powder) measurements reveal that the [{(H2O)Fe(L1)}{M(CN)8}{Fe(L1)}]∞ architectures consist of isomorphous 1-D polymeric structures based on the alternation of {Fe(L1)}2+ and {M(CN)8}4− units (L1 stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged {Fe−Nb} exchange interaction to be antiferromagnetic with J = −20 cm−1 deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Δ/kB = 74 K and τ0 = 4.6 × 10−11 s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i.e., abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient {Fe−Nb} exchange interaction and significant anisotropy of the {Fe(L1)} unit. High field EPR and magnetization experiments have revealed for the parent compound [Fe(L1)(H2O)2]Cl2 a negative zero field splitting parameter of D ≈ −17 cm−1. The crystal structure, magnetic behavior, and Mössbauer data for [Fe(L1)(H2O)2]Cl2 are also reported

Evidence for Transmission of Ferromagnetic Interactions through Hydrogen Bonds in Alkyne-Substituted Nitroxide Radicals: Magnetostructural Correlations and Polarized Neutron Diffraction Studies

A correlation between the magnetic properties and the crystal structure of the nitronyl nitroxide (NN) radical 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (6-HC⋮CPyNN, 1) and its imino nitroxide analogue 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (6-HC⋮CPyIN, 2) has been shown. Magnetic susceptibility measurements on these compounds are indicative of ferromagnetic interactions between molecular units. The imino nitroxide 2 is an organic ferromagnet with critical temperature TC = 0.19 K, whereas 1 behaves as an antiferromagnet ordering at the Néel temperature, TN = 0.54 K. Compounds 1 and 2 are isostructural and crystallize in the P21/n space group. Their crystal packings are best described as chains of molecules linked by C⋮CH···O hydrogen bonds running along the [1̄01] direction. A comparative analysis of the structures and magnetic properties of both compounds suggests that the coupling between the molecular units along the chain is ferromagnetic. Thus, the magnetic susceptibilities of 1 and 2 have been interpreted in terms of isotropic ferromagnetic Heisenberg linear chains of S = 1/2 spins. Weak values of the coupling constants (J = +0.90 K for 1, J = +0.67 K for 2) have been obtained. The spin density distribution of radical 1 has been determined by a polarized neutron diffraction experiment to gain more insight into the mechanism of transmission of magnetic interactions. As in other NN radicals, the spin density is concentrated in the two nitroxide groups and a small negative population is observed in the central carbon atom of the O−N−C−N−O fragment. However, two striking differences with respect to other nitroxides so far studied have been detected. First, the spin density is not equally shared between the two N−O functions. The oxygen atom O1, which participates in the formation of the hydrogen bond, is less populated than the “free” oxygen, O2. Ab initio spin density calculations clearly show that this effect is correlated to the presence of C⋮CH···O hydrogen bonds in the crystal. Second, a positive spin density has been detected in the ethynylic hydrogen atom, H16. These two features indicate a spin density transfer from one nitroxide group to the alkyne function of the neighboring molecule

Pentanuclear Octacyanotungstate(V)-Based Molecule with a High Spin Ground State <i>S</i> = ¹³/₂

Pentanuclear Octacyanotungstate(V)-Based Molecule with a High Spin Ground State S = 13/2</sub

Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a [{Fe^IIL}₂{Nb^IV(CN)₈}] Helical Chain Compound Designed with Heptacoordinate Fe^II

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear {FeIINbIV} compound with remarkable magnetic features when compared to other Fe(II)-based single chain magnets (SCM). Cyano-bridged heterometallic {3d-4d} and {3d-5d} chains are formed upon assembling Fe(II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, [M(CN)8]4− (M = NbIV, MoIV, WIV). X-ray diffraction (single-crystal and powder) measurements reveal that the [{(H2O)Fe(L1)}{M(CN)8}{Fe(L1)}]∞ architectures consist of isomorphous 1-D polymeric structures based on the alternation of {Fe(L1)}2+ and {M(CN)8}4− units (L1 stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged {Fe−Nb} exchange interaction to be antiferromagnetic with J = −20 cm−1 deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Δ/kB = 74 K and τ0 = 4.6 × 10−11 s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i.e., abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient {Fe−Nb} exchange interaction and significant anisotropy of the {Fe(L1)} unit. High field EPR and magnetization experiments have revealed for the parent compound [Fe(L1)(H2O)2]Cl2 a negative zero field splitting parameter of D ≈ −17 cm−1. The crystal structure, magnetic behavior, and Mössbauer data for [Fe(L1)(H2O)2]Cl2 are also reported

Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a [{Fe^IIL}₂{Nb^IV(CN)₈}] Helical Chain Compound Designed with Heptacoordinate Fe^II

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear {FeIINbIV} compound with remarkable magnetic features when compared to other Fe(II)-based single chain magnets (SCM). Cyano-bridged heterometallic {3d-4d} and {3d-5d} chains are formed upon assembling Fe(II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, [M(CN)8]4− (M = NbIV, MoIV, WIV). X-ray diffraction (single-crystal and powder) measurements reveal that the [{(H2O)Fe(L1)}{M(CN)8}{Fe(L1)}]∞ architectures consist of isomorphous 1-D polymeric structures based on the alternation of {Fe(L1)}2+ and {M(CN)8}4− units (L1 stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged {Fe−Nb} exchange interaction to be antiferromagnetic with J = −20 cm−1 deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Δ/kB = 74 K and τ0 = 4.6 × 10−11 s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i.e., abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient {Fe−Nb} exchange interaction and significant anisotropy of the {Fe(L1)} unit. High field EPR and magnetization experiments have revealed for the parent compound [Fe(L1)(H2O)2]Cl2 a negative zero field splitting parameter of D ≈ −17 cm−1. The crystal structure, magnetic behavior, and Mössbauer data for [Fe(L1)(H2O)2]Cl2 are also reported

Ferromagnetic and Antiferromagnetic Intermolecular Interactions in a New Family of Mn₄ Complexes with an Energy Barrier to Magnetization Reversal

A new family of tetranuclear Mn complexes [Mn4X4L4] (H2L = salicylidene-2-ethanolamine; X = Cl (1) or Br (2)) and [Mn4Cl4(L‘)4] (H2L‘ = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1−3 possess a square-shaped core with ferromagnetic exchange interactions between the four MnIII centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range −0.10 to −0.20 cm-1 for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of ∼1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of “exchange-biased single-molecule magnets”, where the exchange bias is controlled by chemical and structural modifications

Ferromagnetic and Antiferromagnetic Intermolecular Interactions in a New Family of Mn₄ Complexes with an Energy Barrier to Magnetization Reversal

A new family of tetranuclear Mn complexes [Mn4X4L4] (H2L = salicylidene-2-ethanolamine; X = Cl (1) or Br (2)) and [Mn4Cl4(L‘)4] (H2L‘ = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1−3 possess a square-shaped core with ferromagnetic exchange interactions between the four MnIII centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range −0.10 to −0.20 cm-1 for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of ∼1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of “exchange-biased single-molecule magnets”, where the exchange bias is controlled by chemical and structural modifications