Fine-tuning the effects of auxiliary ligands on two trigonal-bipyramid cobalt(ii) complexes exhibiting field-induced slow magnetic relaxation

Abstract

Two penta-coordinate Co²⁺ complexes, namely [Co(PDA)(H₂O)₂] (1) and [Co(PDA)(1,2-BIYB)_0.5(H₂O)]·(H₂O) (2), (H₂PDA = pyridine-3,5-dicarboxylate, 1,2-BIYB = 1,2-bis(imidazol-1-ylmethyl) benzene), which have similar ligand fields except for their local symmetries due to the different coordination atoms of Co²⁺, were synthesized, structurally analyzed, and magnetically investigated. Complexes 1 and 2 possess 3D supramolecular structures in which the 2D structures are further cross-linked by hydrogen bonds. Complex 1 can be observed to transform into 2 in the presence of 1,2-BIYB coligand, and 1 and 2 have distortion trigonal bipyramid geometries when the space group of Co²⁺ ion changes from C2/c in 1 to P2(1)/c in 2. Meanwhile, the average equatorial Co–O/N bond distances of CP 1 (2.1655 Å) are clearly larger than its axial Co–O bond distances (2.0704 Å); for CP 2, the average equatorial Co–O/N bond distances (2.0814 Å) are slightly shorter than the axial Co–O/N bond distances (2.0896 Å), which leads to different axial compression and distortion of the trigonal bipyramid metal environment. Fine-tuning of the structures arising from the auxiliary ligands has a definite impact on the magnetic properties of compounds 1 and 2. Both compounds share easy-plane magnetic anisotropies of their central Co²⁺ ions (D = 16 cm⁻¹ for 1 and D = 59 cm⁻¹ for 2) and different slow magnetic relaxations of magnetization with applied magnetic fields of 1500 Oe and 3000 Oe, respectively.