A charge neutral iron(ii) complex with an above room temperature spin crossover (SCO) and hysteresis loop

Abstract

We report on the unusually abrupt spin crossover (SCO) behaviour of a tridentate-nitrogen pyrazole–pyridine–tetrazole (L¹H) based charge-neutral [Fe(L¹)₂] complex. Different reaction conditions were utilized to prepare the complex in crystalline and powder forms. X-ray crystallographic analysis of the complex at 180 K revealed distorted tetragonal bipyramidal geometry around the Fe(II) coordination center with Fe–N bond lengths and angles indicative of the low spin state of the complex. Investigation of the magnetic behaviour of the powder and crystalline forms of the complex yielded an abrupt and above room temperature first order SCO (T_1/2↓ = 346.3 K and T_1/2↑ = 349 K) with an ∼2.6 K hysteresis loop for the powder sample, whereas the crystalline form remained in the low spin state throughout the measurement temperature range. Upon irradiation with red or green light (λ = 637 nm or 532 nm, 10 mW cm⁻²) the powder form of the complex showed a light-induced excited spin state trapping (LIESST) effect with a T_(LIESST) value of 63 K, and no LIESST effect was observed for the crystalline complex. Reversible phase transition and large enthalpy (ΔH) and entropy (ΔS) changes associated with SCO of [Fe(L¹)₂] were inferred from differential scanning calorimetry (DSC) experiments. This was corroborated by variable temperature small angle X-ray scattering (SAXS) measurements wherein different crystalline phases associated with LS and HS [Fe(L¹)₂] complexes and their reversible inter-conversion upon SCO were unambiguously observed.