Ferromagnetic Exchange and Slow Magnetic Relaxation in Cobalt Bis(1,2-dithiolene)-Bridged Dilanthanide Complexes.

The construction of multinuclear lanthanide-based molecules with significant magnetic exchange interactions represents a key challenge in the realization of single-molecule magnets with high operating temperatures. Here, we report the synthesis and magnetic characterization of two series of heterobimetallic compounds, (CpLn)(μ-Co(pdt)) (Ln = Y, Gd, Dy; pdt = 1,2-diphenylethylenedithiolate) and [K(18-crown-6)][(CpLn)(μ-Co(pdt))] (Ln = Y, Gd), featuring two lanthanide centers bridged by a cobalt bis(1,2-dithiolene) complex. Dc magnetic susceptibility data collected for the Gd congeners indicate significant Gd-Co ferromagnetic exchange interactions with fits affording = +11.5 and +7.33 cm, respectively. Magnetization decay and ac magnetic susceptibility measurements carried out on the single-molecule magnet (CpDy)(μ-Co(pdt)) reveal full suppression of quantum tunneling and open-loop hysteresis persisting up to 3.5 K. These results, along with those of high-field EPR spectroscopy, suggest that transition metalloligands can enforce strong exchange interactions with adjacent lanthanide centers while maintaining a geometry that preserves molecular anisotropy. Furthermore, the magnetic properties of [K(18-crown-6)][(CpGd)(μ-Co(pdt))] show that increasing the spin of the ground state of the bridging complex may be a viable alternative to increasing in obtaining well-isolated, strongly coupled magnetic ground states.