Eu(II) Complexes as Model Systems for Spin‐Only Metal Polarizing Agents for Solid Effect DNP

Lanthanoid ions are valuable magnetic resonance probes due to f ‐electron–based paramagnetism, which enables pseudocontact shifts (PCS) and paramagnetic relaxation enhancement (PRE) in NMR, but often causes fast relaxation and broad EPR spectra. Gd(III) is unique among stable lanthanoid ions since its half‐filled 4 f 7 configuration yields spin‐only character ( S = 7/2, L = 0), isotropic g value, slow relaxation, and narrow EPR central transition (CT), making it ideal for EPR and high‐field solid‐state dynamic nuclear polarization (DNP). Isoelectronic Eu(II) and Tb(IV) share the same electronic ground state but differ in nuclear spin, charge, and ligand‐field effects, affecting hyperfine interactions (HFI) and zero‐field splitting (ZFS). Efficient solid effect (SE) DNP requires small ZFS, minimal g anisotropy, and weak HFI, explaining Gd(III)'s superior performance over ions like Mn(II). This work explores Eu(II) complexes as model polarizing agents to test SE DNP theory, focusing on how nuclear spin ( I = 5/2) and ZFS influence efficiency. Using a Eu(II)[2.2.2]cryptand complex, a DNP‐active agent was identified, and its magnetic properties were characterized to reproduce EPR and DNP data in sufficient detail across fields and techniques. While the DNP enhancement was modest, experimental results aligned with theory, providing mechanistic insight for SE DNP with metal‐ion polarizing agents.