Synthesis of a Neutral Mononuclear Four-Coordinate Co(II) Complex Having Two Halved Phthalocyanine Ligands That Shows Slow Magnetic Relaxations under Zero Static Magnetic Field

Abstract

Syntheses of a novel pseudotetrahedral four-coordinate mononuclear Co(II) complex that has two halved phthalocyanine moieties as the ligands, [Co(half-Pc)₂] (1), and its magnetic properties as a single molecule magnet (SMM) are reported. A one pot reaction of phthalonitrile and lithium methoxide followed by the coordination to a Co(II) ion gave 1 as an orange solid in a moderate yield. X-ray crystallography on 1 reveals tetragonally distorted coordination geometry around the Co(II) ion. The M- HT^-1 plots suggest that 1 has large axial magnetic anisotropy. The ac magnetic susceptibility data of the magnetically diluted 1 (dil.1) clearly show that the complex acts as an SMM even in the absence of the external static magnetic field ( H_dc). The influence of intermolecular and intramolecular interactions for the magnetic relaxation behaviors has been discussed by comparison of the magnetic data of 1 and dil.1. The Orbach process is suggested as the predominant mechanism of magnetic relaxations in the high-temperature range, and the Arrhenius plots provide the effective relaxation energy barrier and pre-exponential factor of U_eff = 54.0 cm^-1 and τ₀ = 3.17 × 10^-10 s, respectively. The direct estimation of the axial anisotropic parameter of 1 was successfully performed by high-field, multifrequency ESR measurements up to 55 T and 2.5 THz. The evaluated axial zero-field splitting (ZFS) energy of 57.0 cm^-1 is comparable to the U_eff energy, confirming that the magnetic relaxations are initiated by the thermal excitation from the ground | M _S⟩ = |±3/2⟩ states to the |±1/2⟩ states in the high-temperature range. The results of the ab initio calculations based on the CAS(7,5) SCF wave functions indicate that the ground states of 1 consist mainly of | M _S⟩ = |±3/2⟩ states, while the first excited states are the mixture of | M _S⟩ = |+1/2⟩ and |-1/2⟩.