Molecular Qubits for Anion Sensing by Tuning Electron Spin Relaxation via Axial Ligand Field

Previously, we reported a new macrocyclic ligand, ^MesN₆, which enables the room-temperature coherence of Cu(^MesN₆)(OTf)₂. In this study, we characterize a new series of complexes based on the square planar [M(^MesN₆)]²⁺ motif with different anions, X, (M = Co, Cu; X = SiF, OTf, and Cl) to tune the electron spin relaxation properties of Cu(II) and Co(II) through changes to the axial ligand. For Cu(II), a weakly-coordinating SiF counterion minimizes orbital angular momentum (OAM) and prolongs longitudinal relaxation. An inverted trend is observed for Co(II) where a more donating Cl axial ligand decreases OAM and prolongs spin relaxation. The most pronounced effect occurs when the singly-occupied molecular orbital (SOMO) is $d_{z^2}$ and its energy is most significantly impacted by anion binding. Using changes in coordination of Co(II) we detect chloride anions at 125 µM (0.25 equivalents) through relaxometry as measurable differences in longitudinal spin relaxation (T₁). Additionally, chloride coordination enables T_m-based detection by Hahn echo of a Co(II)-chloride species. Overall, we demonstrate predictable changes to electron spin relaxation through systematic variation of the axial ligand field, providing a strategy for molecular quantum sensing of anionic ligands.