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⟩.

Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [CoIIO4] Core

“…As depicted in Figure , at 300 K, the χ M T value of 1 (3.13 cm 3 ·K mol –1 ) exceeds the spin-only value of 1.875 cm 3 ·K mol –1 expected for a high-spin Co­(II) ion with S = 3/2 and g = 2, suggesting a considerable spin–orbit coupling contribution. This χ M T value is comparable to those observed in reported examples involving tetrahedral high-spin Co­(II) ion. − Upon cooling, the χ M T value gradually decreases until 140 K, below which it decreases rapidly to 2.37 cm 3 ·K mol –1 at 2 K. The final sharp decline at low temperatures is likely attributable to the strong zero-field splitting in 1 . The low-temperature magnetization measurements were performed at different applied magnetic fields ranging from 0 to 7 T at 1.9, 3.0, and 5.0 K (Figure , inset).…”

“…It should be noted that they usually feature four identical donors. Those zero-field Co­(II)-based SIMs with the CoX 4 moiety (X = O, S, Se, Te, − N − ) are summarized in Table S1. In the case of four-coordinate Co­(II)-SIMs, great efforts have been dedicated to investigating the underlying factors that govern the magnetic anisotropy and relaxations.…”

Janus Pyrrolopyrrole Aza‐dipyrrin: Hydrogen‐Bonded Assemblies and Slow Magnetic Relaxation of the Cobalt(II) Complex in the Solid State

“…Note that τ 4 ′ values of 1 and 0 represent regular tetrahedral and planar coordination geometry, respectively. Although the τ 4 ′=0.81 for Janus‐PPAD‐Co indicates that the coordination geometry is largely distorted from the regular tetrahedron, the deformation is less significant than [Co(half‐Pc) 2 ] (0.76), [17c] (HNEt 3 ) 2 [Co II (L 2− ) 2 ] (0.74), [17d] and [Co{(N t Bu) 3 SMe} 2 ] (0.66) [17e] …”

Zero-field slow relaxation of magnetization in cobalt(ii) single-ion magnets: suppression of quantum tunneling of magnetization by tailoring the intermolecular magnetic coupling

“…2.5 cm 3 mol À1 K, which coincides to previously reported values for tetrahedral Co II complexes. [44][45][46][47] The c M T product remained approximately constant upon lowering the temperature down to ca. 70 K, and it decreased abruptly below that, which is suggestive of non-zero ZFS.…”