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 ...