Search

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]

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]

Recently, the choice of ligand and geometric control of mononuclear complexes, which can affect the relaxation pathways and blocking temperature, have received wide attention in the field of single-ion magnets (SIMs). To find out the influence of the coordination environment on SIMs, two four-coordinate mononuclear Co(II) complexes [NEt4][Co(PPh3)X3] (X = Cl−, 1; Br−, 2) have been synthesized and studied by X-ray single crystallography, magnetic measurements, high-frequency and -field EPR (HF-EPR) spectroscopy and theoretical calculations. Both complexes are in a cubic space group Pa3¯ (No. 205), containing a slightly distorted tetrahedral moiety with crystallographically imposed C3v symmetry through the [Co(PPh3)X3]− anion. The direct-current (dc) magnetic data and HF-EPR spectroscopy indicated the anisotropic S = 3/2 spin ground states of the Co(II) ions with the easy-plane anisotropy for 1 and 2. Ab initio calculations were performed to confirm the positive magnetic anisotropies of 1 and 2. Frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility measurements revealed slow magnetic relaxation for 1 and 2 at an applied dc field. Finally, the magnetic properties of 1 and 2 were compared to those of other Co(II) complexes with a [CoAB3] moiety. [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]

Experimental and theoretical studies of magnetic anisotropy and relaxation behavior of six-coordinate tris(pivalato)-Co(ii) and -Ni(ii) complexes (NBu4)[M(piv)3] (piv = pivalate, M = Co, 1; M = Ni, 2), with a coordination configuration at the intermediate between an octahedron and a trigonal prism, are reported. Direct current magnetic data and high-frequency and -field EPR spectra (HFEPR) of 1 have been modeled by a general Hamiltonian considering the first-order orbital angular momentum, while the spin Hamiltonian was used to interpret the data of 2. Both 1 and 2 show easy-axis magnetic anisotropies, which are further supported by ab initio calculations. Alternating current (ac) magnetic susceptibilities reveal slow magnetic relaxation at an applied dc field of 0.1 T in 1, which is characteristic of a field-induced single-ion magnet (SIM), but 2 does not exhibit single-ion magnetic properties at 1.8 K. Detailed analyses of relaxation times show a dominant contribution of a Raman process for spin relaxation in 1. [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]

The magnetization behavior of a CuFeO2 single crystal grown by the floating zone technique is investigated with a pulsed high magnetic field. We observe a series of field-induced multi-step-like transitions with hysteresis, of which the critical magnetic fields are temperature-dependent and show anisotropy. By using a pulsed high magnetic field up to 75 T, the magnetization behavior shows that the critical transition magnetic fields of spin-flip/flop shift to lower field regions with an increase in temperature. According to the magnetization curves, a complete magnetic phase diagram is depicted. [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]