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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]

Cyclic 3d-4f molecular magnets have received considerable attention owing to their potential applications in high-density data storage and quantum information processing. As a rare example of ferromagnetic polynuclear Cr rings, {Cr8Y8} represents a valuable test bed to directly study the magnetic interaction between Cr3+ ions in large hexadecametallic {Cr8Ln8} (Ln = 4f metal) molecules. We have proposed a "single-J" model to approximate the low-temperature spin dynamics of {Cr8Y8} in our earlier study, while a zero-field splitting (ZFS) of the quantum levels was also suggested by the heat capacity data. In order to have a deeper understanding of the magnetism of {Cr8Y8}, it is necessary to verify the ZFS by means of high-resolution spectral methods and identify its origin. In this work, we present a high-frequency electron spin resonance (HF-ESR) study on the ZFS of {Cr8Y8}. The X-band ESR spectra consists of multi-peak structure, indicative of magnetic anisotropy that breaks the degeneracy between spin states in the absence of a magnetic field. HF-ESR spectra are collected to extract the ZFS parameters. We observed a sharp resonance peak due to the transitions between the S = 11 quantum levels and a broadband corresponding to a distribution of resonance peaks due to the ZFS of the S = 12 quantum levels. By analyzing HF-ESR spectra, we confirm the expected S = 12 ground state and determine its ZFS parameter D as −0.069 K, and, furthermore, we reproduce the spectra recorded at 154 GHz. The macrospin model proves to still be valid. The ZFS is attributed to the axial magnetic anisotropy, as found in some other Cr-based molecular wheels. The detailed HF-ESR investigation presented in this paper will benefit the studies on other {Cr8Ln8} wheels with magnetic Ln3+ ions and highlights the importance of the HF-ESR method as a high-resolution probe in determining the ZFS parameters with very small magnitude. [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]

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]

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]

• A soft tripodal ligand was used to construct a high-spin Co(II) complex with weak ligand field. • HFEPR and magnetic data reveal easy-axis anisotropy and SMM's behavior for this complex. • This complex is the first instance of Co(II)-based single-ion magnet with an octahedral CoS 6 unit. The soft donor ligands are of great importance to the pursuit of 3d transition-metal complexes with large magnetic anisotropy. Herein, a soft tripodal ligand, hydrotris(mercaptobenzothiazolyl)borate (Tbz) was used to construct a high-spin mononuclear complex [CoII(Tbz) 2 ] (1), having an octahedral geometry in an S 6 donor set. The combination of direct-current (dc) magnetic susceptibility and Low-temperature high-field electron paramagnetic resonance (HFEPR) reveals the easy-axis magnetic anisotropy of 1. Complex 1 shows field-induced slow relaxation of magnetization at an applied magnetic field of 1000 Oe, indicating the behavior of single-molecule magnet. The foregoing result constitutes the first instance of Co(II)-based single-ion magnet containing a CoS 6 unit. [ABSTRACT FROM AUTHOR]