Ligand-Nuclei Effects on Spin Relaxation in V(IV) Complexes

Understanding how magnetic nuclei affect spin relaxation is vital for designing robust spin coherence in magnetic materials and molecules. A key question is the extent that magnetic nuclei close to a spin (e.g., in the ligand shell of a metal complex) influence relaxation and how it varies over different classes of nuclei. Herein, we apply high-field EPR, X-band EPR, and ac magnetic susceptibility techniques to a family of five V(IV) complexes of the type [V(C₆X₄O₂)₃]^2-, featuring five different sets of 12 nuclear spins on the ligand shell: X = ¹H (1), ²H (2), ¹⁹F (3), ^35/37Cl (4), and ^79/81Br (5). We found several unanticipated results in these studies. For example, at high-field/-frequency, we found that compound 1, with the highest-magnetic-moment ligand nuclear spins, exhibits the longest phase memory relaxation times of the series. Furthermore, at lower fields, we found that the spin-lattice relaxation time and its field dependence were ligand-dependent, despite no obvious change in electronic structure across the five species. Based on this data, structural comparisons, and Raman spectroscopic data, we tentatively conclude that the spin-lattice relaxation properties of 1-5 stem from fine-tuning of the local magnetic environment with changing identity of the X atoms.