Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [CoIIO4] Core.

A mononuclear four-coordinate Co(II) complex with a [CoIIO4] core, namely, PPN[Li(MeOH)4][Co(L)2] (1) (PPN = bis(phosphoranediyl)iminium; H2L = perfluoropinacol), has been studied by X-ray crystallography, magnetic characterization, and theoretical calculations. This complex presents a severely distorted coordination geometry. The O-Co-O bite angle is 83.42°/83.65°, and the dihedral twist angle between the O-Co-O chelate planes is 55.6°. The structural distortion results in a large easy-axis magnetic anisotropy with D = -104(1) cm-1 and a transverse component with |E| = +4(2) cm-1. Alternating current (ac) susceptibility measurements demonstrate that 1 exhibits slow relaxation of magnetization at zero static field. However, the frequency-dependent out-of-phase (χ"M) susceptibilities of 1 at 0 Oe do not show a characteristic maximum. Upon the application of a dc field or the dilution with a diamagnetic Zn matrix, the quantum tunneling of magnetization (QTM) process can be successfully suppressed. Notably, after dilution with the Zn matrix, the obtained sample exhibits a structure different from that of the pristine complex. In this altered sample, the asymmetric unit does not contain the Li(MeOH)4+ cation, resulting in an O-Co-O bite angle of 86.05° and a dihedral twist angle of 75.84°, thereby leading to an approximate D2d symmetry. Although such differences are not desirable for magnetic studies, this study still gives some insights. Theoretical calculations reveal that the D parameter is governed by the O-Co-O bite angle, in line with our previous report for other tetrahedral Co(II) complex with a [CoIIN4] core. On the other hand, the rhombic component is found to increase as the dihedral angle deviates from 90°. These findings provide valuable guidelines for fine-tuning the magnetic properties of Co(II) complexes.

Magnetic properties of two coordination isomeric cobalt(II) single-ion magnets.

Two isomeric Co(II) complexes with the same general molecular formula of [Co(napy)2(NO3)2] (napy = 1,8-naphthyridine) have been synthesized. X-ray single crystal structural determination demonstrates that the two compounds exhibit highly irregular six- (1) and seven-coordinate (2) geometries, respectively. The magnetic measurements, X-band EPR data and theoretical calculations were thoroughly investigated. Both complexes show field-induced slow magnetic relaxation, of which the slow magnetic relaxation in 2 is attributed to an easy-plane anisotropy.

Magneto-structural maps and bridged-ligand effect for dichloro-bridged dinuclear copper(ii) complexes: a theoretical perspective.

Theoretical understanding of magneto-structural correlations in dichloro-bridged dicopper(ii) complexes can guide the design of magnetic materials having broad-scale applications. However, previous reports suggest these correlations are complicated and unclear. To clarify possible correlations, magnetic coupling constants (J calc) of variants of a representative {Cu-(µ-Cl)2-Cu} complex A were calculated through BS-DFT. The variation of the Cu-(µ-Cl)-Cu angle (α), Cu⋯Cu distance (R 0), and Cu-Cl-Cu-Cl dihedral angle (τ) followed by structural optimization and calculation of the magnetic coupling constant (J calc) revealed several trends. J calc increased linearly with R 0 and τ, and initially increased and then decreased with α. Further, bridging ligand effects on J calc for dicopper(ii) complexes were evaluated through BS-DFT; the results revealed that J calc increased with increasing ligand field strength (I- < Br- < Cl- < N3 - < F-). Furthermore, a linear relationship was found between the spin density of the bridging ligand and J calc.

Modulation of the magnetic dynamics of pentagonal-bipyramidal Co(II) complexes by fine-tuning the coordination microenvironment.

Four air-stable mononuclear Co(II) complexes, with the formulas [Co(dapbh)(H2O)(CH3OH)] (1), [Co(Hdapbh)(N3)(CH3OH)]·(CH3OH) (2), [Co(H2aapbh)(CH3OH)2]·(NO3)2 (3) and [Co(H2bapbh)(H2O)(NO3)]·(NO3) (4), have been synthesized and structurally characterized by single crystal X-ray diffraction. In all of the complexes, the Co(II) centers constrained by the rigid pentadentate ligand H2LR with two protonable hydrogens adopt a heptacoordinated pentagonal-bipyramidal geometry. The combined analyses of magnetic data and ab initio calculations unveil large easy-plane magnetic anisotropies for these complexes (D = +37.338, +37.273, +41.138 and +41.139 cm-1 for 1-4, respectively), which indicate that the chemical alterations of the equatorial ligand and the ligand field strength in the axial positions synergistically fine-tune the magnitude of the D values. Magnetic investigations demonstrate the field-induced single-ion magnetic behavior in complexes 1, 2 and 4 with diverse energy barriers (Ueff) of 12.25 for 1, 44.15 for 2 and 48.72 K for 4, corresponding to the geometrical distortion of the heptacoordinated Co(II) ion. That is, the greatest deviation from the ideal D5h symmetry in 4 is responsible for the highest barrier.

Slow magnetic relaxation in dinuclear Co(III)-Co(II) complexes containing a five-coordinated Co(II) centre with easy-axis anisotropy.

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.

Effect of 3d heterometallic ions on the magnetic properties of azido-Cu(II) with isonicotinic acid coligands: A theoretical perspective.

Based on density functional theory and the broken-symmetry approach, the magnetic properties of an azido-Cu(II) complex with isonicotinic acid coligands were studied at the B1LYP/def2-TZVP level. According to the molecular magnetic orbitals and Mulliken spin population analysis, there are strong orbital interactions between the paramagnetic CuII/NiII ions and the bridging azide ligands and isonicotinic ions. The supposedly empty 4s/4p/4d orbitals of the MII ions are found to play an important role in the mechanism of magnetic coupling and are probed using NBO analysis. As the number of unpaired electrons on the MII ions increases, the number of electrons that occupy the empty 4d orbitals with the highest energy and overlap integrals of the magnetic orbitals in the CuIIMII (M = Cu, Ni, Co, Fe, Mn) model complexes increases accordingly, and the magnetic coupling constant gradually decreases.

Solvent-induced single-crystal-to-single-crystal transformation and tunable magnetic properties of 1D azido-Cu(ii) chains with a carboxylate bridge.

By means of the solvent effect, three new azido-copper 1D coordination polymers, [Cu(4-aba)(N3)] (1), [Cu(4-aba)(N3)(CH3OH)] (2), and [Cu(4-aba)(N3)(C2H5OH)] (3) (4-aba = 4-azidobenzoic acid), were successfully prepared in the presence of Cu2+ ion, NaN3 and 4-azidobenzoic acid. Interestingly, 1 can be employed as a precursor and transformed to 2 and 3via the coordination of methanol or ethanol, respectively. Meanwhile, the identical products of 1, namely 1a and 1b, could be obtained from both 2 and 3 by a dealcoholized process. As a result, the geometric configurations of Cu(ii) ions vary from the tetracoordinated square-planar in 1 to the hexacoordinated octahedron in 2 or 3. Compound 1 displays a well-isolated 1D chain with dual-bridges of EO-azido and syn,syn-carboxylate, while isomorphic 2 and 3 are triple-bridged chain-like motifs containing EO-azido, syn,syn-carboxylate and µ2-alkanol. The structural transformations caused by the intervention of alkanol molecules modulate the intrachain Cu-Cu distances (3.570 for 1, 3.204 Å for 2 and 3.154 Å for 3) and Cu-N-Cu angles (127.3° for 1, 106.82° for 2 and 104.81° for 3). This modulation, however, further leads to different intrachain ferromagnetic interactions (J = 28.4 cm-1 for 1, 67.6 cm-1 for 2, 40.2 cm-1 for 3) that are qualitatively demonstrated by theoretical calculation. More importantly, the significant scenarios of magnetic ordering and slow magnetic relaxation, which are infrequent in most of the reported azido-copper cases, are only observed in 2 and 3, due to the distinct interchain networks among 1-3. In addition, heat-capacity measurements highlight the characteristics of long-range ferromagnetic ordering in 2 and 3.

Potential and environmental control of carbon sequestration in major ecosystems across arid and semi-arid regions in China.

With the continuous expansion of drylands in the context of global climate change, governments have implemented a series of greening policies such as afforestation, to reduce ecological degradation. However, owing to historical conditions and technical constraints, few attempts have been made to quantitatively assess the differences in carbon sequestration capacity and the associated environmental controls among major ecosystems in the arid and semi-arid areas. Based on six flux towers located in northwestern China measuring the carbon fluxes in a maize (Zea mays L.) cropland, alpine meadow, wetland, swamp meadow, Tamarix, and gobi desert, this work revealed that all ecosystems sequestered CO2 at various magnitudes. The cropland had the highest carbon uptake, followed by the alpine meadow, swamp meadow, wetland and Tamarix, respectively. Distinct seasonal dynamics in carbon sequestration were observed across these ecosystems with the peak values in summertime, whereas the gobi desert exhibited as a weak carbon sink with considerable fluctuations around the year. In this water-limited region, soil water content instead of rainfall, is expected to be the primary environmental control on the land-atmosphere carbon fluxes, and regarded as a key linkage between hydrologic and ecologic processes. Therefore, not only the appropriate vegetation types, but also the water availability controlled by the local climatic constraints and soil characteristics, should be addressed in order to identify management strategies for ecological restoration in the dry areas.

Coligand modifications fine-tuned the structure and magnetic properties of two triple-bridged azido-Cu(ii) chain compounds exhibiting ferromagnetic ordering and slow relaxation.

Employing two benzoate derivatives with different numbers of non-coordinated fluoro-substituents, 2-fluorobenzoic acid (2-Hfba) and 2,6-difluorobenzoic acid (2,6-Hdfba), two new azido-copper coordination polymers, [Cu(2-fba)(N3)(CH3OH)]n (1) and [Cu(2,6-dfba)(N3)(CH3OH)]n (2), have been successfully isolated, and then structurally and magnetically investigated. Single crystal structure analysis demonstrates that the metal cations in the two resulting compounds are connected by the alternating triple-bridge of µ-1,1-azido, syn,syn-carboxylate and µ2-methanol, contributing to analogously linear 1D Cu(ii) chain-like motifs with slightly different intrachain and interchain geometric parameters. The fine-tuned structures lead to variant magnetic properties in the two title compounds. Although a dominant ferromagnetic coupling between adjacent Cu(ii) ions within each chain due to the counter-complementarity of the multiple superexchange pathways is observed in both compounds, the interesting plots of magnetic ordering and slow magnetic relaxation, which are rare in most of the reported azido-Cu(ii) architectures, only occur in compound 1, while 2 behaves as an antiferromagnet consisting of ferromagnetic Cu(ii) chains. The heat-capacity experiments further emphasize the characteristic long-range ferromagnetic ordering in 1 and the typical behavior of antiferromagnets in 2. Moreover, density functional theory (DFT) calculations (using different methods and basis sets) have been performed on both compounds to obtain the qualitatively theoretical interpretation of the magnetic behaviors.

Metallofullerenes encaging mixed-metal clusters: synthesis and structural studies of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80.

Metallofullerenes of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 encapsulating mixed-metal nitride clusters were synthesized. Spectroscopic characterization of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 was employed to reveal their structural and vibrational properties. The structural properties of these species were analyzed by using theoretical calculations. The studies of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 laid the foundations for these species to be used as multifunctional molecules.