Solvent Coligand-Induced Spin State Variation in Iron(II) Complexes Bearing Pentadentate N5 Ligand Toward the Construction of an N5O Spin Crossover Center

Abstract

Three iron(II) complexes having a 1,2,3-triazole-containing linear pentadentate N5 ligand L3-Me-3Me and different monodentate solvent coligand X, [Fe(L3-Me-3Me)X](BPh4)2 [L3-Me-3Me = bis(N,N′-1-methyl-1H-1,2,3-triazol-4-yl-methylideneaminopropyl)methylamine; X = MeCN (1), dimethylformamide (DMF) (2), and dimethyl sulfoxide (DMSO) (3)] have been selectively prepared by the choice of reaction solvent. Along the well-known spectrochemical series, the strong-field solvent ligand MeCN generates a low-spin (LS) complex 1, the weak-field ligand DMSO generates a high-spin (HS) complex 3, and the complex 2 coordinated by the medium-field ligand DMF exhibits gradual and reversible one-step HS↔LS spin crossover (SCO) with T1/2 = 173 K in the temperature range of 400–5 K. This spin state variation among LS 1, HS 3, and SCO 2 has been investigated by the combination of temperature-dependent magnetic susceptibilities measurements, single-crystal X-ray diffraction (SCXRD) studies, Hirshfeld surface analyses, and density functional theory calculations. In addition, the details of structural change between HS and LS states of an N5O iron(II) center upon SCO are revealed for the first time by the variable-temperature SCXRD studies for 2 at nine different temperatures in which the difference of Fe–O bond length between the two spin states is shorter than that of any other Fe–N bond lengths.