Two Magnetic Switching Complexes Based on the Fe^II Ion

Two neutral mononuclear iron­(II) complexes with different spin-crossover (SCO) properties, Fe­(<b>L1</b>)₂(SCN)₂ (<b>1</b>) and Fe­(<b>L2</b>)₂(SCN)₂ (<b>2</b>) (<b>L1</b> = 2-(thiophen-3-yl)-1<i>H</i>-imidazo­[4,5-<i>f</i>]­[1,10]­phenanthroline and <b>L2</b> = 2-(thiophen-2-yl)-1<i>H</i>-imidazo­[4,5-<i>f</i>]­[1,10]­phenanthroline), were solvothermally synthesized. With the different substituted position in 1,10-phenanthroline derivatives, <b>1</b> exhibits gradual SCO around room temperature with <i>T</i>_1/2 = 280 K, while <b>2</b> displays abrupt SCO with 10 K hysteresis at <i>T</i>_1/2↓ = 210 K and <i>T</i>_1/2↑ = 220 K

Perovskite-Like Polar Lanthanide Formate Frameworks of [NH₂NH₃][Ln(HCOO)₄] (Ln = Tb–Lu and Y): Synthesis, Structures, Magnetism, and Anisotropic Thermal Expansion

A series of isostructural hydrazinium lanthanide (Ln) formate framework compounds of [NH₂NH₃]­[Ln­(HCOO)₄] for Ln³⁺ ions from Tb³⁺ to Lu³⁺ and Y³⁺ have been successfully prepared by utilizing NH₂NH₃⁺. The compounds crystallize in orthorhombic polar space group <i>Pca</i>2₁, with cell parameters at 180 K of <i>a</i> = 18.2526(7)–18.1048(5) Å, <i>b</i> = 6.5815(2)–6.5261(2) Å, <i>c</i> = 7.6362(3)–7.5044(2) Å, and <i>V</i> = 917.33(6)–886.67(4) ų, showing the effect of lanthanide contraction. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the cavities of the NaCl-like framework, in which the Ln³⁺ ions in a bicapped trigonal prism are connected by anti–anti and syn–anti formate groups. The N–H···O_formate hydrogen-bonding interactions are between the hydrazinium cations and the anionic framework. One anti–anti formate group is frustrated by the competitive N–H···O_formate hydrogen-bonding interactions. It thus twists or flips upon warming, resulting in large anisotropic thermal expansion and negative thermal expansion below 180 K. A comparison with the transition metal and magnesium analogues revealed that the structural compactness, tighter binding of the hydrazinium cation by the framework, and symmetrically better match between the framework and ammonium cation for Ln compounds could inhibit the occurrence of phase transition in the series. The IR spectroscopic, thermal, and magnetic properties are investigated

Perovskite-Like Polar Lanthanide Formate Frameworks of [NH₂NH₃][Ln(HCOO)₄] (Ln = Tb–Lu and Y): Synthesis, Structures, Magnetism, and Anisotropic Thermal Expansion

A series of isostructural hydrazinium lanthanide (Ln) formate framework compounds of [NH₂NH₃]­[Ln­(HCOO)₄] for Ln³⁺ ions from Tb³⁺ to Lu³⁺ and Y³⁺ have been successfully prepared by utilizing NH₂NH₃⁺. The compounds crystallize in orthorhombic polar space group <i>Pca</i>2₁, with cell parameters at 180 K of <i>a</i> = 18.2526(7)–18.1048(5) Å, <i>b</i> = 6.5815(2)–6.5261(2) Å, <i>c</i> = 7.6362(3)–7.5044(2) Å, and <i>V</i> = 917.33(6)–886.67(4) ų, showing the effect of lanthanide contraction. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the cavities of the NaCl-like framework, in which the Ln³⁺ ions in a bicapped trigonal prism are connected by anti–anti and syn–anti formate groups. The N–H···O_formate hydrogen-bonding interactions are between the hydrazinium cations and the anionic framework. One anti–anti formate group is frustrated by the competitive N–H···O_formate hydrogen-bonding interactions. It thus twists or flips upon warming, resulting in large anisotropic thermal expansion and negative thermal expansion below 180 K. A comparison with the transition metal and magnesium analogues revealed that the structural compactness, tighter binding of the hydrazinium cation by the framework, and symmetrically better match between the framework and ammonium cation for Ln compounds could inhibit the occurrence of phase transition in the series. The IR spectroscopic, thermal, and magnetic properties are investigated

Topology-Dependent Synthesis, Structures, and Bonding Interactions of Uranium Polyarene Complexes

Metal polyarene complexes have attracted great attention in recent years because of their appealing electronic structures and readily tunable properties and reactivity. While main group and transition metal polyarene complexes have been well studied with various degrees of reduction and different coordination modes, f-block metal polyarene complexes are rare. Here we report the synthesis of a series of uranium polyarene complexes supported by ferrocene diamide ligands. X-ray crystallography shows that the structures of uranium polyarene complexes are dependent on the topology of polyarenes. While linear polyarenes form mononuclear compounds, nonlinear polyarenes prefer an inverse-sandwich structure with a μ-η6,η6-coordination mode. Combined experimental and computational studies unveil that mononuclear uranium polyarene complexes are best described as bidentate with a three-center two-electron (3c-2e) σ bond, whereas inverse-sandwich uranium polyarene complexes are bound through two δ bonds. The correlation between the topology of polyarenes and the coordination mode of uranium polyarene complexes can be rationalized by the electronic structures and bonding interactions as well as the relative energies of coordination isomers

Series of Lanthanide Organometallic Single-Ion Magnets

The synthesis, structures, and magnetic properties of a series of lanthanide organometallic mixed sandwich molecules, (Cp*)­Ln­(COT), are investigated, where Cp* is the pentamethylcyclopentadiene anion and COT is the cyclooctatetraene dianion and Ln represents Tb^III, Dy^III, Ho^III, Er^III, and Tm^III. Among the five complexes, Dy and Ho complexes are determined to be single-ion magnets in addition to the previously reported Er complex. Both Dy and Ho complexes show obvious quantum tunneling magnetization relaxation in the absence of a static field. The diluted Ho complex behaves two sets of thermally activated relaxation as we reported previously in Er due to the COT ring static disorder. A stair-shaped hysteresis for the Er compound can be observed at 1.6 K with Hc = 1 kOe at a sweeping rate over 700 Oe/s. The quantum tunneling decoherence relaxation rate increases from Er to Ho to Dy, which may be caused by the relative increase of transverse anisotropy coming from the larger tilting of the two aromatic rings within the molecule. The fine electronic structure is analyzed with ligand-field theory employing the effective Hamiltonian method. The zero-field splitting is determined to be Ising type, and the energy gap between the ground state and the first excited one is consistent with the barrier obtained by Arrhenius analysis

Topology-Dependent Synthesis, Structures, and Bonding Interactions of Uranium Polyarene Complexes

Metal polyarene complexes have attracted great attention in recent years because of their appealing electronic structures and readily tunable properties and reactivity. While main group and transition metal polyarene complexes have been well studied with various degrees of reduction and different coordination modes, f-block metal polyarene complexes are rare. Here we report the synthesis of a series of uranium polyarene complexes supported by ferrocene diamide ligands. X-ray crystallography shows that the structures of uranium polyarene complexes are dependent on the topology of polyarenes. While linear polyarenes form mononuclear compounds, nonlinear polyarenes prefer an inverse-sandwich structure with a μ-η6,η6-coordination mode. Combined experimental and computational studies unveil that mononuclear uranium polyarene complexes are best described as bidentate with a three-center two-electron (3c-2e) σ bond, whereas inverse-sandwich uranium polyarene complexes are bound through two δ bonds. The correlation between the topology of polyarenes and the coordination mode of uranium polyarene complexes can be rationalized by the electronic structures and bonding interactions as well as the relative energies of coordination isomers

A Family of Co^IICo^III₃ Single-Ion Magnets with Zero-Field Slow Magnetic Relaxation: Fine Tuning of Energy Barrier by Remote Substituent and Counter Cation

The synthesis, structures, and magnetic properties of a family of air-stable star-shaped Co^IICo^III₃ complexes were investigated. These complexes contain only one paramagnetic Co­(II) ion with the approximate <i>D</i>₃ coordination environment in the center and three diamagnetic Co­(III) ions in the peripheral. Magnetic studies show their slow magnetic relaxation in the absence of an applied dc field, which is characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co­(II) ion with approximate <i>D</i>₃ symmetry in the center. Most importantly, it was demonstrated that the anisotropy energy barrier can be finely tuned by the periphery substituent of the ligand and the countercation. The anisotropy energy barrier can be increased significantly from 38 K to 147 K

Two-Electron Oxidations at a Single Cerium Center

Two-electron oxidations are ubiquitous and play a key role in the synthesis and catalysis. For transition metals and actinides, two-electron oxidation often takes place at a single-metal site. However, redox reactions at rare-earth metals have been limited to one-electron processes due to the lack of accessible oxidation states. Despite recent advancements in nontraditional oxidation state chemistry, the low stability of low-valent compounds and large disparity among different oxidation states prevented the implementation of two-electron processes at a single rare-earth metal center. Here we report two-electron oxidations at a cerium(II) center to yield cerium(IV) terminal oxo and imido complexes. A series of cerium(II–IV) complexes supported by a tripodal tris(amido)arene ligand were synthesized and characterized. Experimental and theoretical studies revealed that the cerium(II) complex is best described as a 4f2 ion stabilized by δ-backdonation to the anchoring arene, while the cerium(IV) oxo and imido complexes exhibit multiple bonding characters. The accomplishment of two-electron oxidations at a single cerium center brings a new facet to molecular rare-earth metal chemistry

Field-induced slow magnetic relaxation in a hydrogen-bonding linked Co(II) 1D supramolecular coordination polymer

<div><p>We have investigated the dynamic behaviour of the magnetization of a hydrogen-bonding linked Co(II) 1D supramolecular coordination polymer. In the structure, two different mononuclear Co(II) species are linked by O–H···N hydrogen bonding through coordinated H₂O and . Field-induced slow magnetic relaxation effect is observed and the anisotropy energy barrier is 33 K. <i>Ab initio</i> calculations reveal that Co(II) ion in [Co(bpm)₂(N₃)₂] species is uniaxial anisotropic with a negative axial zero-field splitting parameter of <i>D</i> = − 82.4 cm^− 1. The Co(II) ion in [Co(bpm)₂(H₂O)₂]²⁺ species, however, is easy-plane anisotropic with a positive <i>D</i> and negative <i>E</i> value (<i>D</i> = 46.3 cm^− 1, <i>E</i> = − 7.8 cm^− 1). This is an interesting complex in which slow magnetic relaxation stems from the combination contribution of uniaxial anisotropy and easy plane anisotropy.</p></div

A Series of Weakley-type Polyoxomolybdates: Synthesis, Characterization, and Magnetic Properties by a Combined Experimental and Theoretical Approach

Using DCC as the dehydrating agent, a series of Weakley-type polyoxomolybdates [Bu₄N]₃{Ln­[Mo₅O₁₃(OMe)₄(NO)]₂} (Ln = Tb, Dy, Ho, Er) were synthesized in a one-pot reaction and structurally characterized by elemental, IR, UV–vis analysis, PXRD, and single-crystal X-ray diffraction. Furthermore, the static and dynamic measurements were utilized to investigate their magnetic performances. Typically, slow relaxation of magnetization was observed for Dy analogues with an energy barrier for the reversal of the magnetization of 50 K, which is the highest barrier height observed on the polyoxomolybdates-based single-molecule magnets (SMMs). For a deep understanding of the appearance of the SMM behavior on Weakley-type polyoxomolybdates series, <i>ab initio</i> calculations on {Dy­[Mo₅O₁₃(OMe)₄(NO)]₂}^3– have been conducted