P–N Bond Hydrolysis Assisted Formation of a <i>pseudo</i>‐Trigonal Antiprismatic Co(II) Complex with Strong Easy‐Axis Anisotropy and Field‐Induced Slow Magnetic Relaxation: A Magneto‐Structural and Theoretical Study

The reaction of hydrated cobalt(II) perchlorate salt with the rigid tripodal ligand, tris(3,5-dimethylpyrazolyl) phosphine oxide, (O)P(3,5-DMPz)₃, results in selective in situ hydrolysis of a P-N bond affording a neutral mononuclear Co(II) complex, [Co{(O)P(O)(3,5-DMP)₂}₂] (1^Co(II)). The X-ray crystal structure of 1^Co(II) shows that it is formed by the coordination of two monoanionic N₂O-tripodal (O)P(O)(3,5-DMPz)₂ ligands, leading to a Co^IIN₄O₂ coordination sphere with an elongated pseudo-trigonal antiprismatic geometry. The analysis of the dc magnetic data revealed that this compound shows a strong easy-axis magnetic anisotropy with D=-45.3 cm^-1 and |E|= 10.2 cm^-1. Ab initio theoretical studies further supported these values and therefore confirmed the axiality in the ground state, with the magnetic anisotropy axis lying along the P-Co(II)-P direction. Dynamic magnetic measurements confirmed slow relaxation of magnetization under an applied dc field of 0.15 T. The magnetic relaxation does not take place through an Orbach process but through a combination of direct and Raman processes and it is much faster than that observed for other trigonal antiprismatic Co(II) complexes. This is probably due to the comparatively smaller magnetic anisotropy of 1^Co(II), as a result of the larger distortion of its geometry generated by the non-symmetrical (O)P(O)(3,5-DMP)₂ ligand.