Spin-transition in [Fe^II(L⁵)₂][ClO₄]₂ [L⁵ = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)]: a further example of coexistence of features typical for disorder and cooperativity

Abstract

An iron(II) complex [Fe(L⁵)₂][ClO₄]₂ of a new nonplanar tridentate ligand [L⁵ = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L⁵)₂][ClO₄]₂·CH₃CN at 100 K and at 298 K revealed a mononuclear Fe^IIN₆ pseudooctahedron with the tridentate ligand in the meridional coordination mode. Magnetic susceptibility measurements revealed LS (¹A₁) ↔ HS (⁵T₂) transition starting above room temperature and remained incomplete even up to 350 K, however providing evidence for the hysteretic behaviour of the transition. Photo-magnetic measurements using a 532 nm laser revealed incomplete light-induced excited spin-state trapping (LIESST). Relaxation curves in the dark exhibited a stretched exponential shape, typical for broad distributions of relaxation times. The complete photo-magnetic data were modelled using a non-cooperative master equation including a tunnelling process with distributed parameters and accounting for the bulk absorption of light. We also measured the diffuse reflectance properties, with detailed investigation of the absorption spectra and discussed the obtained features with respect to structural properties, including the suggested presence of disorder.