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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]

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 effect of screening the CoII moment of monomeric [CoIIL2(H2O)] (L = 8-hydroxyquinaldine), having a trigonal bipyramid coordination, by diamagnetic Zn in Co x Zn 1−x solid solutions on its magnetic relaxation was explored using ac-susceptibility, high-field electron–spin-resonance measurements and CASPT2 calculations. The retention of the crystal structure for all the solid solutions was demonstrated using single crystal diffraction. The dc-magnetization and theoretical fittings of the susceptibility for Co1 and Co0.1Zn0.9 gave a large zero-field-splitting (ZFS) D of 50 ± 6 cm−1, and very weak dipole interaction between the nearest neighbors, while EPR and calculations confirmed the positive sign of the axial component (D). Consistent parameters were obtained from experiments and theory. Importantly, only field-induced relaxation was observed for the samples with less than 50% Co and a gradual change in the barrier energy to moment reversal and relaxation times was observed between 11% and 20% Co, while both were enhanced for higher dilutions. The results establish a clear barrier for extending the longevity of the magnetism for this type of single-ion species by lowering the intramolecular interactions. The results suggest that the magnetic interaction persists up to the second sphere, that is, for a dilution of 1 in 9 (11% Co). Importantly, this method is applicable to all single-ion magnet systems, that is, the optimum dilution concentration to restrain the dipole field can be given only by the single crystal structure. [ABSTRACT FROM AUTHOR]

Three mononuclear six-coordinate Co(ii)-pseudohalide complexes [Co(L)X2] with two N-donor pseudohalido coligands occupying the cis-positions (X = NCS− (1), NCSe− (2) or N(CN)2− (3)), and a five-coordinate complex [Co(L)(NCO)][B(C6H5)4] (4) [L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (12-TMC)] have been prepared and structurally characterized. Easy-plane magnetic anisotropy for 1–3 and easy-axis anisotropy for 4 were revealed via the analyses of the direct-current magnetic data, high-frequency and -field EPR (HFEPR) spectra and ab initio theoretical calculations. They display slow magnetic relaxations under an external applied dc field. Typically, two slow relaxation processes were found in 1 and 2 while only one relaxation process occurs in 3 and 4. The Raman-like mechanism is found to be dominant in the studied temperature range in 1. For 2–4, the Raman process is dominant in the low temperature region, while the Orbach mechanism dominates in the high temperature range. [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]

By utilizing well-designed bifunctional ligands derived from 1H-imidazole-4,5-dicarboxylic acid, magnetic coordination polymers (CPs) that exhibit slow magnetic relaxation at the low temperature regions were constructed and further structurally characterized. In 1, 1D cobalt–carboxyl chains were stabilized in the final structure. In contrast, by adjusting the length of the substituted arms on imidazole-4,5-dicarboxylic acid, a novel 3D CP, 2 containing 2D 63 cobalt–carboxyl layer was obtained. A combination of Quantum Monte Carlo (QMC) simulations and the first-principles Density functional theory (DFT) calculations showed that compound 2 features weak ferro- and antiferro-magnetic coupling mechanisms with two different super-exchange paths of −/+/− for syn–anti carboxylate bridges and −/−/− for syn–syn carboxylate bridges. Through HF-EPR measurements performed on polycrystalline samples over the frequency range of 60–260 GHz and field range of 0–12 T, the effective g-values of 1 and 2 were all larger than 2.00, and the signs of their D values were probably positive. [ABSTRACT FROM AUTHOR]

Two mononuclear seven-coordinate cobalt(ii) complexes [Co(L)3(NO3)2] (L = 4-tert-butylpyridine, 1; L = isoquinoline, 2) were prepared and structurally analyzed by single-crystal X-ray crystallography. The coordination spheres of 1 and 2 exhibit distorted pentagonal bipyramid geometry. Analysis of their direct-current magnetic data reveals the existence of easy plane anisotropy (D ＞ 0) with a small transverse anisotropy (E), which was further confirmed by high-field electron paramagnetic resonance (HFEPR) spectroscopy. Field-induced slow magnetic relaxations were observed under the applied dc field in complexes 1 and 2 by alternating-current magnetic susceptibility measurements. Importantly, these complexes are new instances of mononuclear high-coordinate cobalt(ii)-based single-molecule magnets. [ABSTRACT FROM AUTHOR]