Strong Ferromagnetic Exchange Coupling and Single-Molecule Magnetism in MoS-Bridged Dilanthanide Complexes.

We report the synthesis and characterization of the trinuclear 4d-4f compounds [Co(CMe)][(CMe)Ln(μ-S)Mo(μ-S)Ln(CMe)], (Ln = Y, Gd, Tb, Dy), containing the highly polarizable MoS bridging unit. UV-Vis-NIR diffuse reflectance spectra and DFT calculations of reveal a low-energy metal-to-metal charge transfer transition assigned to charge transfer from the singly occupied 4d orbital of Mo to the empty 5d orbitals of the lanthanides (4d in the case of ), mediated by sulfur-based 3p orbitals. Electron paramagnetic resonance spectra collected for in a tetrahydrofuran solution show large Y hyperfine coupling constants of = 23 MHz and = 26 MHz, indicating the presence of significant yttrium-localized unpaired electron density. Magnetic susceptibility data support similar electron delocalization and ferromagnetic Ln-Mo exchange for , , and . This ferromagnetic exchange gives rise to an = 15/2 ground state for and one of the largest magnetic exchange constants involving Gd observed to date, with = +16.1(2) cm. Additional characterization of and by ac magnetic susceptibility measurements reveals that both compounds exhibit slow magnetic relaxation. Although a Raman magnetic relaxation process is dominant for both and , an extracted thermal relaxation barrier of = 68 cm for is the largest yet reported for a complex containing a paramagnetic 4d metal center. Together, these results provide a potentially generalizable route to enhanced d-4f magnetic exchange, revealing opportunities for the design of new d-4f single-molecule magnets and bulk magnetic materials.