Structures and magnetic anisotropies of two seven-coordinate Co(II)-nitrate complexes showing slow magnetic relaxation

Two seven-coordinate Co(II)-nitrate complexes, [Co(tmpdc)(NO3)(2)] (1) and [Co(tmpdt)(NO3)(2)]center dot CH3CN (2) (tmpdc = N,N,N',N'-tetramethyl-2,6-pyridinedicarboxamide, tmpdt = N,N,N',N'-tetramethyl-2,6-pyridine-dithiocarboxamide) with tridentate neutral ligands tmpdc or tmpdt have been prepared. Their crystal structures by single X-ray diffraction show markedly different coordination geometries when two donor atoms of the tridentate ligand change from oxygen in 1 to sulfur in 2. Complex 1 is a capped octahedral while complex 2 is a pentagonal bipyramid. Results of direct-current (dc) magnetic measurements and high-frequency electron paramagnetic resonance (HFEPR) indicate the anisotropic S = 3/2 spin ground state of the Co(II) ions with the easy-plane anisotropy for both 1 and 2. Frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility studies reveal slow magnetic relaxation for 1 and 2 under applied fields.