Large Hyperfine Coupling Arising from Pseudo-²S Ground States in a Series of Lutetium(II) Metallocene Complexes

The synthesis of molecules with strong coupling between electronic and nuclear spins represents an important challenge in molecular quantum information science. Here, we report the synthesis and characterization of the divalent lutetium metallocene complexes Lu(Cp^Me)(Cp) (Cp^Me = pentamethylcyclopentadienyl; Cp = pentaisopropylcyclopentadienyl), Lu(Cp^Et)₂ (Cp^Et = ethyltetraisopropylcyclopentadienyl), and Lu(Cp)₂ (Cp = tetraisopropylcyclopentadienyl). The molecular structures of these complexes, as determined through single-crystal X-ray diffraction, feature a common bent sandwich geometry, with average Cp-Lu-Cp angles ranging from 159.9° to 152.6°. Analysis of continuous-wave electron paramagnetic resonance (EPR) spectra for the complexes reveals nearly isotropic g tensors with only a slight deviation from that of a free electron. Moreover, an extremely large splitting of the eight-line spectra indicates the presence of strong hyperfine coupling, and simulations provide isotropic hyperfine coupling constants of A_iso = 4.38, 4.30, and 4.17 GHz across the series, where the value of A_iso is found to decrease as the Cp-Lu-Cp angle becomes more acute. Notably, these values are the largest yet observed for any lanthanide complex. Moreover, EPR and computational analysis show that the large values of A_iso stem from large s-orbital character─up to 41.2%─in the corresponding singly occupied molecular orbitals. To our knowledge, this degree of s-character in a molecular orbital is the largest yet reported for an open-shell isolable complex. These results outline a general strategy toward the isolation of paramagnetic molecules with strong hyperfine coupling and highly isotropic doublet electronic ground states.