Two New Polymorphs of a Co(II) Tetra-Azide Complex with Different Magnetic Anisotropies: Structure Distortions, Slow Magnetic Relaxations, and Theoretical Studies

Two new polymorphs of (Ph4P)2[Co(N3)4] (1 and 2) were successfully synthesized via controlled temperature-mediated crystal growth. Polymorphs 1 and 2 crystallize in the monoclinic space group C2/c and triclinic space group P1̅, respectively, which are different from the two other reported polymorphs that are both in the monoclinic space group P21/n (3 and 4 with different lattice parameters). Analogous to polymorphs 3 and 4, polymorphs 1 and 2 exhibit a distorted tetrahedral coordination geometry around the Co center. Differences in the N–Co–N bond angles in the four polymorphs show a strong correlation with the deviation parameters from ideal Td geometry by SHAPE analyses, indicating that the N–Co–N bond angles (rather than Co–N bond lengths) are the main contributors to the distortions. Static magnetic and high-field EPR studies, supported by ab initio quantum chemical calculations, show that the sign of the axial zero-field splitting (ZFS) parameter (D) is positive for 1 (with rhombic magnetic anisotropy) but negative for 2 (with axial magnetic anisotropy). Furthermore, dynamic magnetic investigations confirm that both new polymorphs (1 and 2) exhibit field-induced slow magnetization relaxation, a behavior consistent with the previously reported analogues 3 and 4.