Effect of the Distant Substituent to Slow Magnetic Relaxation of Pentacoordinate Fe(III) Complexes

Pentacoordinate Fe(III) complexes [Fe(L^MeO)₂X] and [Fe(L^EtO)₂X], X = Cl and Br, show the slow magnetic relaxation that is enhanced by the applied static magnetic field. A substitution of the distant ethoxy group to the methoxy group residing at the phenyl ring of a Schiff base N,O-donor ligand (L^MeO vs L^EtO) considerably influences the relaxation characteristics. In the chlorido complex [Fe(L^MeO)₂Cl], the following three slow relaxation channels are recognized as possessing different relaxation times: τ_LF = 0.47 s, τ_IF = 13 ms, and τ_HF = 26 μs at the static field B_DC = 0.2 T and T = 1.9 K. In the bromido complex [Fe(L^MeO)₂Br], only the following two relaxation channels are seen: τ_LF = 0.30 ms and τ_HF = 139 μs at B_DC = 0.15 T and T = 1.9 K. Due to D > 0, the Orbach relaxation mechanism does not apply, and the temperature dependence of the high-frequency relaxation time can be described by two Raman-like terms.