Chem

Unprecedented anionic Fe^III spin crossover (SCO) complexes involving a weak-field O,N,O-tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO−1 of a ligand might contribute to the enhancement of a ligand-field splitting energy. The present anionic SCO complex also exhibited the light- induced excited-spin-state trapping effect.

Photoinduced spin transition: Unprecedented anionic spin crossover (SCO) complexes involving an weak-field Fe^IIIN₂O₄ coordination sphere were discovered. The tetramethylammonium salt with the present SCO anion exhibited the light-induced excited-spin-state trapping effect, which is quite rare in Fe^III SCO complexes (see scheme).

Polarized neutron diffraction (PND) experiments were carried out at low temperature to characterize with high precision the local magnetic anisotropy in two paramagnetic high-spin cobalt(II) complexes, namely [Co^II(dmf)₆](BPh₄)₂ (1) and [Co^II₂(sym-hmp)₂](BPh₄)₂ (2), in which dmf=N,N-dimethylformamide; sym-hmp=2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate, and BPh₄⁻=tetraphenylborate. This allowed a unique and direct determination of the local magnetic susceptibility tensor on each individual Co^II site. In compound 1, this approach reveals the correlation between the single-ion easy magnetization direction and a trigonal elongation axis of the Co^II coordination octahedron. In exchange-coupled dimer 2, the determination of the individual Co^II magnetic susceptibility tensors provides a clear outlook of how the local magnetic properties on both Co^II sites deviate from the single-ion behavior because of antiferromagnetic exchange coupling.

All mapped out: Polarized neutron diffraction on a paramagnetic single crystal permits mapping of the molecular magnetic anisotropy regardless of the number of molecular orientations in the crystal cell and the investigation of the competing roles of magnetic exchange and single-ion anisotropy in polynuclear complexes.

Solid-state incorporation of a single-ion magnet (SIM) within the pores of a magnetic 3D metal–organic framework (MOF) in a single-crystal to single-crystal process has been demonstrated. The resulting host–guest supramolecular aggregate affords the first deep study on the interplay of the internal magnetic field of the MOF and the slow magnetic relaxation of the SIM allowing the observation of a partial suppression of the quantum-tunneling effects operative in the SIM. More information can be found in the Full Paper by E. Pardo et al. (DOI: 10.1002/chem.201504176).

A series of heterometallic 3d–Gd³⁺ complexes based on a lanthanide metalloligand, [M(H₂O)₆][Gd(oda)₃]⋅3 H₂O [M=Cr³⁺ (1-Cr)] (H₂oda=2,2′-oxydiacetic acid), [M(H₂O)₆][MGd(oda)₃]₂⋅3 H₂O [M=Mn²⁺ (2-Mn), Fe²⁺ (2-Fe) and Co²⁺ (2-Co)], and [M₃Gd₂(oda)₆(H₂O)₆]⋅12 H₂O [M=Ni²⁺ (3-Ni), Cu²⁺ (3-Cu), and Zn²⁺ (3-Zn)], are reported. Magnetic and heat-capacity studies revealed a significant impact on the magnetocaloric effect depending on the anisotropy of the 3d transition metal ions, as confirmed by comparison of the observed maximum values of −ΔS_m between complexes 2-Co and 1-Cr. In these two complexes, the 3d metal ions have the same spin (S=3/2 for Co²⁺ and Cr³⁺ ions), and the theoretical calculation suggested a larger −ΔS_m value for 2-Co (47.8 J K⁻¹ kg⁻¹) than 1-Cr (37.5 J K⁻¹ kg⁻¹); however, the significant anisotropy of Co²⁺ ions in 2-Co, which can result in smaller effective spins, gives a smaller value of −ΔS_m for 2-Co (32.2 J K⁻¹ kg⁻¹) than for 1-Cr (35.4 J K⁻¹ kg⁻¹) at ΔH=9 T.

Cool magnets: A series of heterometallic 3d–Gd³⁺ complexes based on the [Gd(oda)₃]³⁻ (H₂oda=2,2′-oxydiacetic acid) metalloligand (see figure) are reported. According to the magnetic entropy changes of the complexes, obtained independently from magnetization and heat-capacity date, besides a high spin value, the use of isotropy 3d metal ions like Cr³⁺, Fe³⁺ and Mn²⁺, also plays an important role for obtaining heterometallic magnetic coolants with large magnetocaloric effect.

A series of mononuclear penta-coordinated halide Co^III complexes with intermediate S=1 spin state has been investigated. Their magnetic anisotropy is sensitive to the nature of the halide in an unexpected way: the largest value was found for the chloride compound and the smallest for the iodide one. This study evidences that contemporary computational methods are powerful tools to predict and rationalize the magnetic anisotropy in such systems. However, for each metal ion, at each oxidation and spin state, benchmark studies are required to define magneto-structural correlations. The authors gratefully thank Dr. Sylvain Bertaina for his assistance in the design of the cover illustration. More information can be found in the Full Paper by M. Orio, C. Duboc et al. (DOI: 10.1002/chem.201502997).

Two nanosized Mn₄₉ and Mn₂₅Na₄ clusters based on analogues of the high-spin (S=22) [Mn^III₆Mn^II₄(μ₄-O)₄]¹⁸⁺ supertetrahedral core are reported. Mn₄₉ and Mn₂₅Na₄ complexes consist of eight and four decametallic supertetrahedral subunits, respectively, display high virtual symmetry (O_h), and are unique examples of clusters based on a large number of tightly linked high nuclearity magnetic units. The complexes also have large spin ground-state values (Mn₄₉: S=61/2; Mn₂₅Na₄: S=51/2) with the Mn₄₉ cluster displaying single-molecule magnet (SMM) behavior and being the second largest reported homometallic SMM.

Stacking blocks: Two nanosized Mn₂₅Na₄ and Mn₄₉ clusters consisting of four and eight decametallic supertetrahedral subunits, respectively, are reported. These clusters are unique examples of oligomeric species based on magnetic subunits and have large spin ground-state values S=51/2 (Mn₂₅Na₄) and 61/2 (Mn₄₉). The Mn₄₉ cluster displays single-molecule magnet (SMM) behavior and is the second largest reported homometallic SMM.

Understanding the factors that control the magnitude and symmetry of magnetic anisotropy should facilitate the rational design of mononuclear metal complexes in the quest for single-molecule magnets (SMMs), based on a single metal ion, with high blocking temperatures and large energy barriers. The best strategy is to define magnetostructural correlations through the investigation of a series of metal complexes. It has been demonstrated that the main contribution to the magnetic anisotropy arises from the spin-orbit coupling (SOC) effect in metal-ion-based systems, so current studies focus particularly on the use of both ligands and metal ions possessing a large SOC. In this context, we report a unique series of halide Co^III complexes, [CoL(X)], with X=Cl, Br, I (CoX) and L=2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1-diphenylethanethiolate), which possess a rare intermediate S=1 spin ground state. The S=1 Co^III complexes are attractive species because they possess a remarkably large axial zero-field splitting (defined by D from the following Hamiltonian: H=DS_z²), as well as the halide ligands inducing large SOC constants. The single-crystal X-ray structures reveal that the CoBr and CoI complexes are isostructural with the previously described CoCl complex. Their coordination sphere displays a distorted pentacoordinated square pyramidal geometry, with the halide located in the Co^III axial position. Large positive D values of 35, 26, and 18 cm⁻¹ are found for CoCl, CoBr, and CoI, respectively, through analysis of the magnetic susceptibility data as a function of temperature. To rationalize this trend, theoretical calculations based on both density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods are performed successfully. Both the sign and magnitude of D are predicted remarkably well by these theoretical approaches. The DFT calculations also show that the resulting D values originate from a balance of several contributions, and that many factors, including differences in their structural properties and in the contribution of the halide, should be taken into account to explain the trend of D in this series of complexes.

Complex magnetism: The factors that control the magnitude and symmetry of the magnetic anisotropy are investigated for a unique series of halide Co^III complexes (see figure), which possess a rare intermediate S=1 spin ground state and remarkably large axial zero-field splitting.

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni₃^IIILn^III} star-type complexes [Ln=Gd (1), Dy (2)] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute ∢NiOGd bond angles present in 1 instigate a significant interaction between the 4f_xyz orbital of the Gd^III ion and 3d orbital of the Ni^II ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next-nearest-neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI-SO calculations performed on complex 2 suggest that the octahedral environment around the Dy^III ion is neither strong enough to stabilize the m_J |±15/2〉 as the ground state nor able to achieve a large ground-state–first-excited-state gap. The ground-state Kramers doublet for the Dy^III ion is found to be the m_J |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the J_NiDy interaction as −1.45 cm⁻¹. The strong Ni⋅⋅⋅Dy and next-nearest-neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero-field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar [Dy(AlMe₄)₃] highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established.

Quantum tunneling: DFT and ab initio calculations suggest that both Ni⋅⋅⋅Dy and 1, 3 Ni⋅⋅⋅Ni (see figure) interactions help to quench the QTM behavior in {3d–4f} single-molecule magnets.