Measurement of Line Width and Anisotropy in C₃/C₄-Symmetric Gd(III) Complexes

Gadolinium(III) complexes are frequently used as spin labels in high-field electron paramagnetic resonance spectroscopy and dynamic nuclear polarization nuclear magnetic resonance spectroscopy due to their high spin (S = 7/2) ground state and chemical robustness. A primary goal in their design is to minimize the magnetic resonance line width by engineering the zero-field splitting (ZFS). Here, we present a comprehensive experimental study of a series of C₃-symmetric TPATCN and C₄-symmetric DOTA derivatives. Using multifrequency EPR spectroscopy (34 to 390 GHz) here we try to understand trends in their ZFS parameters (D, E) and their distributions (D_strain, E_strain). We find that functionalization of the TPATCN scaffold with an electron-withdrawing nitro group (NO₂) minimizes the axial ZFS to D = 0.0110 cm^-1∥0.330 GHz, but this is coupled with a significant increase in conformational flexibility, resulting in a large D_strain. Conversely, amide-functionalized DOTA compounds demonstrate superior structural rigidity, evidenced by near-zero rhombic ZFS and small E_strain. Many complexes studied herein show considerably smaller resonance line widths than existing tags, suggesting there is scope for the design of improved Gd(III) spin labels.