No abstract

Increasing the operating temperatures of single-molecule magnets—molecules that can retain magnetic polarization in the absence of an applied field—has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (CpMe4H) capping ligands with strong magnetic exchange coupling provided by an N2 3− radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(CpMe4H 2Tb)2(μ−)].

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*(2)Ln)(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)T at low temperatures. For the Gd(III)-containing complex, a fit to the data reveals antiferromagnetic coupling with J = -10 cm(-1) to give an S = (13)/(2) ground state. The Tb(III) and Dy(III) congeners show single-molecule magnet behavior with relaxation barriers of U(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.

Systematic analysis of related compounds is crucial to the design of single-molecule magnets with improved properties, yet such studies on multinuclear lanthanide complexes with strong magnetic coupling remain rare. Herein, we present the synthesis and magnetic characterization of the series of radical-bridged dilanthanide complex salts [(Cp*2Ln)2(μ-5,5′-R2bpym)](BPh4) (Ln = Gd, Dy; R = NMe2 (1), OEt (2), Me (3), F (4); bpym = 2,2′-bipyrimidine). Modification of the substituent on the bridging 5,5'-R2bpym radical anion allows the magnetic exchange coupling constant, JGd-rad, for the gadolinium compounds in this series to be tuned over a range from -2.7 cm -1 (1) to -11.1 cm -1 (4), with electron-withdrawing or -donating substituents increasing or decreasing the strength of exchange coupling, respectively. Modulation of the exchange coupling interaction has a significant impact on the magnetic relaxation dynamics of the single-molecule magnets 1-Dy through 4-Dy, where stronger JGd-rad for the corresponding Gd 3+ compounds is associated with larger thermal barriers to magnetic relaxation (Ueff), open magnetic hysteresis at higher temperatures, and slower magnetic relaxation rates for through-barrier processes. Further, we derive an empirical linear correlation between the experimental Ueff values for 1-Dy through 4-Dy and the magnitude of JGd-rad for the corresponding gadolinium derivatives that provides insight into the electronic structure of these complexes. This simple model applies to other organic radical-bridged dysprosium complexes in the literature, and it establishes clear design criteria for increasing magnetic operating temperatures in radical-bridged molecules.Synthesis and Structural Characterization. The molecule 2,2′-bipyrimidine (bpym) serves as an ubiquitous bridging ligand in coordination chemistry, however, symmetric 5,5′-R2bpym variants have only yet been reported for alkyl, aryl, ether, and bromine substituents. [35][36][37][38] Bipyrimidine derivatives are typically synthesized via Cu-or Ni-mediated coupling reactions, and therefore the synthesis of 5,5′-R2bpym was first attempted via a Ni-catalyzed homocoupling of the corresponding 5-R-2-chloropyrimidine. [39][40][41] This approach furnished 5,5′-R2bpym with electrondonating substituents R = Me, OEt, and NMe2 in isolated yields between 33 and 37% but did not yield electron-deficient derivatives with R = F or CF3. These results are consistent with reports of the synthesis of 5,5′-R2bpy (bpy = 2,2′-bipyridine), which found lower yields for the Ni-catalyzed homocoupling of 5-R-2-chloropyridine substrates bearing electron-withdrawing substituents. [42][43][44] Attempts to synthesize derivatives with R = F or CF3 via Cu-mediated homocoupling reactions were also unsuccessful. Instead, a Stille reaction 45,46 was used to couple 5-fluoro-2tributylstannylpyrimidine and 5-fluoro-2-chloropyrimidine, affording 5,5′-F2bpym in 57% isolated yield. Although it was not possible in our hands to isolate 5,5′-(CF3)2bpym by an analogous route, the Pd-catalyzed ...

No abstract

Assembly of the triangular, organic radical-bridged complexes Cp* Ln (μ -HAN) (Cp*=pentamethylcyclopentadienyl; Ln=Gd, Tb, Dy; HAN=hexaazatrinaphthylene) proceeds through the reaction of Cp* Ln(BPh ) with HAN under strongly reducing conditions. Significantly, magnetic susceptibility measurements of these complexes support effective magnetic coupling of all three Ln centers through the HAN radical ligand. Thorough investigation of the Dy congener through both ac susceptibility and dc magnetic relaxation measurements reveals slow relaxation of the magnetization, with an effective thermal relaxation barrier of U =51 cm . Magnetic coupling in the Dy complex enables a large remnant magnetization at temperatures up to 3.0 K in the magnetic hysteresis measurements and hysteresis loops that are open at zero-field up to 3.5 K.

Single-molecule magnets comprising one spin center represent a fundamental size limit for spin-based information storage. Such an application hinges upon the realization of molecules possessing substantial barriers to spin inversion. Axially symmetric complexes of lanthanides hold the most promise for this due to their inherently high magnetic anisotropies and low tunneling probabilities. Herein, we demonstrate that strikingly large spin reversal barriers of 216 and 331 cm(-1) can also be realized in low-symmetry lanthanide tetraphenylborate complexes of the type [Cp*2 Ln(BPh4 )] (Cp*=pentamethylcyclopentadienyl; Ln=Tb (1) and Dy (2)). The dysprosium congener showed hysteretic magnetization data up to 5.3 K. Further studies of the magnetic relaxation processes of 1 and 2 under applied dc fields and upon dilution within a matrix of [Cp*2 Y(BPh4 )] revealed considerable suppression of the tunneling pathway, emphasizing the strong influence of dipolar interactions on the low-temperature magnetization dynamics in these systems.

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 CeO 2 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 logK sp =-59.3 ± 0.3 in 0.01M NaClO 4. Taken together, our results show that the pH-dependence of ceria anti-and pro-oxidant activity can be related to the dissolution of CeO 2 in aqueous media.