Substituent Effects on Exchange Coupling and Magnetic Relaxation in 2,2'-Bipyrimidine Radical-Bridged Dilanthanide Complexes

摘要

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'-R_2bpym)](BPh_4) (Ln = Gd, Dy; R = Nme_2 (1), Oet (2), Me (3), F (4); bpym = 2,2'-bipyrimidine). Modification of the substituent on the bridging 5,5'-R_2bpym radical anion allows the magnetic exchange coupling constant, J_(Gd-rad),f°r 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 J_(Gd-rad) f°r the corresponding Gd~(3+) compounds is associated with larger thermal barriers to magnetic relaxation (U_(eff)), 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 U_(eff) values for 1-Dy through 4-Dy and the magnitude of J_(Gd-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.