Guanidinate Rare-Earth Tetraphenylborate Complexes and Their Prospects in Single-Molecule Magnetism

The pursuit of higher-nuclearity rare-earth-metal clusters necessitates adequate ancillary ligand scaffolds as well as easily dissociable counterions. To this end, guanidinate anions are advantageous owing to their highly customizable, sterically encumbering, anionic charged framework. Here, we present the two mononuclear guanidinate rare-earth complexes [{(Me3Si)2NC(NiPr)2}2RE][(μ-η6-Ph)(BPh3)], (RE = Y (1), Dy (2)), featuring inner-sphere tetraphenylborate anions. Each complex is sterically congested and comprises a metal ion that is ligated by two ancillary guanidinate ions and one tetraphenylborate ligand. The isostructural compounds, 1 and 2, were synthesized from a protonolysis reaction between guanidinate alkyl complexes and [HNEt3][BPh4]. The isolated molecules were characterized by X-ray crystallography and IR, NMR, and UV–vis spectroscopy. The crowded coordination sphere around each metal ion implemented by the ancillary guanidinate ligands causes the tetraphenylborate ion to adopt a rare η6-binding mode where the metal center interacts asymmetrically with the carbon atoms of one phenyl ring. DFT and NBO calculations carried out on 1 provide insight into a complicated bonding picture between one of the phenyl rings of the BPh4– moiety and the rare-earth ion. Furthermore, the dysprosium congener, 2, is a single-molecule magnet displaying slow magnetic relaxation under the application of a static dc field

Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*₂Ln)₂(μ-bpym^•)]⁺ (Ln = Gd, Tb, Dy), are reported. Strong Ln^III-bpym^•– exchange coupling is observed for all species, as indicated by the increases in χ_M<i>T</i> at low temperatures. For the Gd^III-containing complex, a fit to the data reveals antiferromagnetic coupling with <i>J</i> = −10 cm^–1 to give an <i>S</i> = ¹³/₂ ground state. The Tb^III and Dy^III congeners show single-molecule magnet behavior with relaxation barriers of <i>U</i>_eff = 44(2) and 87.8(3) cm^–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature

Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*₂Ln)₂(μ-bpym^•)]⁺ (Ln = Gd, Tb, Dy), are reported. Strong Ln^III-bpym^•– exchange coupling is observed for all species, as indicated by the increases in χ_M<i>T</i> at low temperatures. For the Gd^III-containing complex, a fit to the data reveals antiferromagnetic coupling with <i>J</i> = −10 cm^–1 to give an <i>S</i> = ¹³/₂ ground state. The Tb^III and Dy^III congeners show single-molecule magnet behavior with relaxation barriers of <i>U</i>_eff = 44(2) and 87.8(3) cm^–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature

Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*₂Ln)₂(μ-bpym^•)]⁺ (Ln = Gd, Tb, Dy), are reported. Strong Ln^III-bpym^•– exchange coupling is observed for all species, as indicated by the increases in χ_M<i>T</i> at low temperatures. For the Gd^III-containing complex, a fit to the data reveals antiferromagnetic coupling with <i>J</i> = −10 cm^–1 to give an <i>S</i> = ¹³/₂ ground state. The Tb^III and Dy^III congeners show single-molecule magnet behavior with relaxation barriers of <i>U</i>_eff = 44(2) and 87.8(3) cm^–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature

Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*₂Ln)₂(μ-bpym^•)]⁺ (Ln = Gd, Tb, Dy), are reported. Strong Ln^III-bpym^•– exchange coupling is observed for all species, as indicated by the increases in χ_M<i>T</i> at low temperatures. For the Gd^III-containing complex, a fit to the data reveals antiferromagnetic coupling with <i>J</i> = −10 cm^–1 to give an <i>S</i> = ¹³/₂ ground state. The Tb^III and Dy^III congeners show single-molecule magnet behavior with relaxation barriers of <i>U</i>_eff = 44(2) and 87.8(3) cm^–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature

Taming Super-Reduced Bi₂^3– Radicals with Rare Earth Cations

Here, we report the synthesis of two new sets of dibismuth-bridged rare earth molecules. The first series contains a bridging diamagnetic Bi22– anion, (Cp*2RE)2(μ-η2:η2-Bi2), 1-RE (where Cp* = pentamethylcyclopentadienyl; RE = Gd (1-Gd), Tb (1-Tb), Dy (1-Dy), Y (1-Y)), while the second series comprises the first Bi23– radical-containing complexes for any d- or f-block metal ions, [K(crypt-222)][(Cp*2RE)2(μ-η2:η2-Bi2•)]·2THF (2-RE, RE = Gd (2-Gd), Tb (2-Tb), Dy (2-Dy), Y (2-Y); crypt-222 = 2.2.2-cryptand), which were obtained from one-electron reduction of 1-RE with KC8. The Bi23– radical-bridged terbium and dysprosium congeners, 2-Tb and 2-Dy, are single-molecule magnets with magnetic hysteresis. We investigate the nature of the unprecedented lanthanide–bismuth and bismuth–bismuth bonding and their roles in magnetic communication between paramagnetic metal centers, through single-crystal X-ray diffraction, ultraviolet–visible/near-infrared (UV–vis/NIR) spectroscopy, SQUID magnetometry, DFT and multiconfigurational ab initio calculations. We find a πz* ground SOMO for Bi23–, which has isotropic spin–spin exchange coupling with neighboring metal ions of ca. −20 cm–1; however, the exchange coupling is strongly augmented by orbitally dependent terms in the anisotropic cases of 2-Tb and 2-Dy. As the first examples of p-block radicals beneath the second row bridging any metal ions, these studies have important ramifications for single-molecule magnetism, main group element, rare earth metal, and coordination chemistry at large

Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex

We report the serendipitous discovery and magnetic characterization of a dysprosium bis­(ammonia) metallocene complex, [(C₅Me₅)₂Dy­(NH₃)₂]­(BPh₄) (<b>1</b>), isolated in the course of performing a well-established synthesis of the unsolvated cationic complex [(C₅Me₅)₂Dy]­[(μ-Ph)₂BPh₂]. While side reactivity studies suggest that this bis­(ammonia) species owes its initial incidence to impurities in the DyCl₃(H₂O)_<i>x</i> starting material, we were able to independently prepare <b>1</b> and its tetrahydrofuran (THF) derivative, [(C₅Me₅)₂Dy­(NH₃)­(THF)]­(BPh₄) (<b>2</b>), from the reaction of [(C₅Me₅)₂Dy]­[(μ-Ph)₂BPh₂] with ammonia in THF. The low-symmetry complex <b>1</b> exhibits slow magnetic relaxation under zero applied direct-current (dc) field to temperatures as high as 46 K and notably exhibits an effective barrier to magnetic relaxation that is more than 150% greater than that previously reported for the [(C₅Me₅)₂Ln]­[(μ-Ph)₂BPh₂] precursor. On the basis of fitting of the temperature-dependent relaxation data, magnetic relaxation is found to occur via Orbach, Raman, and quantum-tunneling relaxation processes, and the latter process can be suppressed by the application of a 1400 Oe dc field. Field-cooled and zero-field-cooled dc magnetic susceptibility measurements reveal a divergence at 4 K indicative of magnetic blocking, and magnetic hysteresis was observed up to 5.2 K. These results illustrate the surprises and advantages that the lanthanides continue to offer for synthetic chemists and magnetochemists alike

Solubility of Nanocrystalline Cerium Dioxide: Experimental Data and Thermodynamic Modeling

Ultrafine 5 nm ceria isotropic nanoparticles were prepared using the rapid chemical precipitation approach from cerium­(III) nitrate and ammonium hydroxide aqueous solutions. The as-prepared nanoparticles were shown to contain predominantly Ce­(IV) species. The solubility of nanocrystalline CeO₂ at several pH values was determined using ICP-MS and radioactive tracer methods. Phase composition of the ceria samples remained unchanged upon partial dissolution, while the shape of the particles changed dramatically, yielding nanorods under neutral pH conditions. According to X-ray absorption spectroscopy investigation of the supernatant, Ce­(III) was the main cerium species in solution at pH < 4. Based on the results obtained, a reductive dissolution model was used for data interpretation. According to this model, the solubility product for ceria nanoparticles was determined to be log <i>K</i>_sp = −59.3 ± 0.3 in 0.01 M NaClO₄. Taken together, our results show that the pH dependence of ceria anti- and pro-oxidant activity can be related to the dissolution of CeO₂ in aqueous media

Extraction of Lanthanide and Actinide Ions from Aqueous Mixtures Using a Carboxylic Acid-Functionalized Porous Aromatic Framework

Porous aromatic frameworks (PAFs) incorporating a high concentration of acid functional groups possess characteristics that are promising for use in separating lanthanide and actinide metal ions, as required in the treatment of radioactive waste. These materials have been shown to be indefinitely stable to concentrated acids and bases, potentially allowing for multiple adsorption/stripping cycles. Additionally, the PAFs combine exceptional features from MOFs and inorganic/activated carbons giving rise to tunable pore surfaces and maximum chemical stability. Herein, we present a study of the adsorption of selected metal ions, Sr²⁺, Fe³⁺, Nd³⁺, and Am³⁺, from aqueous solutions employing a carbon-based porous aromatic framework, BPP-7 (Berkeley Porous Polymer-7). This material displays high metal loading capacities together with excellent adsorption selectivity for neodymium over strontium based on Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. Based in part upon X-ray absorption spectroscopy studies, the stronger adsorption of neodymium is attributed to multiple metal ion and binding site interactions resulting from the densely functionalized and highly interpenetrated structure of BPP-7. Recyclability and combustibility experiments demonstrate that multiple adsorption/stripping cycles can be completed with minimal degradation of the polymer adsorption capacity