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Two seven-coordinate Co(II)-nitrate complexes, [Co(tmpdc)(NO3)2] (1) and [Co(tmpdt)(NO3)2]·CH3CN (2) (tmpdc = N,N,N′,N′-tetramethyl-2,6-pyridinedicarboxamide, tmpdt = N,N,N′,N′-tetramethyl-2,6-pyridinedithiocarboxamide) with tridentate neutral ligands tmpdc or tmpdt have been prepared. Their crystal structures by single X-ray diffraction show markedly different coordination geometries when two donor atoms of the tridentate ligand change from oxygen in 1 to sulfur in 2. Complex 1 is a capped octahedral while complex 2 is a pentagonal bipyramid. Results of direct-current (dc) magnetic measurements and high-frequency electron paramagnetic resonance (HFEPR) indicate the anisotropic S = 3/2 spin ground state of the Co(II) ions with the easy-plane anisotropy for both 1 and 2. Frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility studies reveal slow magnetic relaxation for 1 and 2 under applied fields. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Precise modulation of the magnetic anisotropy of metal ions remains highly important for the development of high–performance single‐molecule magnets (SMMs). We herein reported the synthetic, structural, spectral, magnetic, and computational studies on four mononuclear CoII complexes in a distorted octahedral environment. The change of the organosulfonate anions triggers significant modification of the distorted octahedral CoN6 coordination sphere, which enables us to tackle the influence of the structural distortion on the magnetic anisotropy in these complexes. Magnetic measurements revealed that the four complexes possess easy–plane magnetic anisotropy with large and positive zero‐field splitting parameters, which decrease with the increasing distortion of the pseudo–octahedral CoII coordination geometry. This correlation was further confirmed by high–frequency/–field EPR (HF–EPR) spectra and theoretical calculations. Dynamic magnetic susceptibility measurements indicate that these complexes both exhibit slow magnetic relaxation under an external dc field, indicating the field‐induced single‐ion magnets (SIMs) of the four complexes. These results indicate that structural distortion plays an important role in the magnetic anisotropy of CoII complexes and should be carefully considered in the design of molecular complexes with high magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Two air-stable Co(III)–Co(II) mixed-valence complexes of molecular formulas [CoIICoIII(L)(DMAP)3(CH3COO)]·H2O·CH3OH (1) and [CoIICoIII(L)(4-Pyrrol)3 (CH3COO)]·0.5CH2Cl2 (2) (H4L = 1,3-bis-(5-methyl pyrazole-3-carboxamide) propane; DMAP = 4-dimethylaminopyridine; and 4-Pyrrol = 4-pyrrolidinopyridine) were synthesized and characterized by single-crystal X-ray crystallography, high-field electron paramagnetic resonance (HFEPR) spectroscopy, and magnetic measurements. Both complexes possess one five-coordinated paramagnetic Co(II) ion and one six-coordinated Co(III) ion with octahedral geometry. Direct-current magnetic susceptibility and magnetization measurements show the easy-axis magnetic anisotropy that is also confirmed by low-temperature HFEPR measurements and theoretical calculations. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal their field-assisted slow magnetic relaxation, which is a characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co(II) ion. The effective energy barrier of complex 1 (49.2 cm−1) is significantly higher than those of the other dinuclear Co(III)–Co(II) SMMs. This work hence presents the first instance of the dinuclear Co(III)–Co(II) single-molecule magnets with a five-coordinated environment around the Co(II) ion. [ABSTRACT FROM AUTHOR]

Three mononuclear tetrakis(pseudohalido)-cobalt(II) complexes (Ph4P)2[Co(E)4] (E = N3−, 1; NCO−, 2; NCS−, 3) have been synthesized and structurally characterized. Each compound contains a distorted tetrahedral Co2+ ion coordinated by four pseudohalide ligands. The magnetic properties of 1–3 have been studied using direct-current magnetic measurements and high-frequency and -field EPR spectroscopy (HFEPR), suggesting easy-axis magnetic anisotropy for 1 and 2 and easy-plane anisotropy for 3. Analysis of the HFEPR spectra yielded D values of −5.23 and +3.63 cm−1 for 2 and 3, respectively. The absence of the EPR signal in 1 is consistent with a large, negative value of the zero-field splitting (ZFS) parameter D in 1. The nature of magnetic anisotropies of 1–3 has also been confirmed by ab initio calculations. The calculated D values are consistent with those determined using magnetometry and HFEPR studies. Alternating current (AC) magnetic susceptibilities reveal slow magnetic relaxation under an applied magnetic field, thus indicating that 1–3 are field-induced single-ion magnets (SIMs). [ABSTRACT FROM AUTHOR]

Two five-coordinate mononuclear Co(II) complexes [Co(12-TMC)X][B(C6H5)4] (L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (12-TMC), X = Cl− (1), Br− (2)) have been studied by X-ray single crystallography, magnetic measurements, high-frequency and -field EPR (HF-EPR) spectroscopy and theoretical calculations. Both complexes have a distorted square pyramidal geometry with the Co(II) ion lying above the basal plane constrained by the rigid tetradentate macrocyclic ligand. In contrast to the reported five-coordinate Co(II) complex [Co(12-TMC)(NCO)][B(C6H5)4] (3) exhibiting easy-axis anisotropy, an easy-plane magnetic anisotropy was found for 1 and 2via the analyses of the direct-current magnetic data and HF-EPR spectroscopy. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements demonstrated that complexes 1 and 2 show slow magnetic relaxation at an applied dc field. Ab initio calculations were performed to reveal the impact of the terminal ligands on the nature of the magnetic anisotropies of this series of five-coordinate Co(II) complexes. [ABSTRACT FROM AUTHOR]

A series of mononuclear seven-coordinate Co(II) complexes [CoII(BPA-TPA)](ClO4)2·H2O (2-ClO4), [CoII(BPA-TPA)](PF6)2 (3-PF6) and [CoII(BPA-TPA)](BPh4)2 (4-BPh4) have been synthesized based on the ligand 2,6-bis(bis(2-pyridylmethyl)amino)methylpyridine (BPA-TPA), and their structures have been characterized by single-crystal X-ray diffraction. All these complexes feature an intermediate coordination polyhedron between a capped trigonal prism and a capped octahedron, which is perturbed by the variation of the counter anions. Easy-plane magnetic anisotropies for all complexes were revealed via the analyses of the direct-current magnetic data and high-field electron paramagnetic resonance (HFEPR) spectra. They show slow magnetic relaxation under applied direct current field, which is similar to the previously reported complex [CoII(BPA-TPA)](BF4)2 (1-BF4) with a capped trigonal prism. This work provides a new example of modulating the properties of single-ion magnets (SIMs) by changing the counter anions. [ABSTRACT FROM AUTHOR]

The coordination geometries of the Co(ii) site in the two complexes [Co(imidazole)6][BPh4]2·0.3CH3CN (1) and [Co(imidazole)6][NO3]2 (2) were observed to display the ideal symmetries Ci and D3d, respectively. Both complexes were shown to be field-induced single-ion magnets. The effective energy barrier was found to decrease as the local symmetry changed from low-symmetry Ci to high-symmetry D3d. [ABSTRACT FROM AUTHOR]

Magnetic anisotropy is the key element in the construction of single-ion magnets, a kind of nanomagnets for high-density information storage. This works describes an unusual large easy-plane magnetic anisotropy (with a zero-field splitting parameter D of +40.2 cm−1), mainly arising from the second-order spin-orbit coupling effect in a trigonal-planar CoII complex [Li(THF)4][Co(NPh2)3], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field-induced slow magnetic relaxation in this complex was mainly attributed to the Raman process. [ABSTRACT FROM AUTHOR]

Magnetic anisotropy is the key element in the construction of single-ion magnets, a kind of nanomagnets for high-density information storage. This works describes an unusual large easy-plane magnetic anisotropy (with a zero-field splitting parameter D of +40.2 cm−1), mainly arising from the second-order spin-orbit coupling effect in a trigonal-planar CoII complex [Li(THF)4][Co(NPh2)3], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field-induced slow magnetic relaxation in this complex was mainly attributed to the Raman process. [ABSTRACT FROM AUTHOR]

Two mononuclear Co(II) complexes [Co(L) 4 (NO 3 ) 2 ] (L = 3-phenylpyrazole for 1 and 4-methylpyridine for 2 ) featuring distorted octahedral geometry were prepared and structurally characterized by X-ray crystallographic analyses. Direct-current magnetic and high-frequency/field electron paramagnetic resonance measurements reveal that both complexes have the large and positive D values with the non-negligible transverse anisotropy ( E ). Slow magnetic relaxation effects were observed under the applied direct-current field in 1 and 2 by dynamic alternative-current magnetic susceptibility measurements, which provide two interesting examples of six-coordinate Co(II)-based single ion magnets constructed by nitrate groups and nitrogen heterocyclic compounds in mono-dentate coordination modes. [ABSTRACT FROM AUTHOR]