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

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 Gd3+ 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.