A mononuclear iron(II) complex bearing the linear pentadentate N5 Schiff-base ligand containing two 1,2,3-triazole moieties and the MeCN monodentate ligand, [FeIIMeCN(L3-Me-3Ph)](BPh4)2·MeCN·H2O (1), have been prepared (L3-Me-3Ph = bis(N,N′-1-Phenyl-1H-1,2,3-triazol-4-yl-methylideneaminopropyl)methylamine). Variable-temperature magnetic susceptibility measurements revealed an incomplete one-step spin crossover (SCO) from the room-temperature low-spin (LS, S = 0) state to a mixture of the LS and high-spin (HS, S = 2) species at the higher temperature of around 400 K upon first heating, which is irreversible on the consecutive cooling mode. The magnetic modulation at around 400 K was induced by the crystal-to-amorphous transformation accompanied by the loss of lattice MeCN solvent, which was evident from powder X-ray diffraction (PXRD) studies and themogravimetry. The single-crystal X-ray diffraction studies showed that the complex is in the LS state (S = 0) between 296 and 387 K. In the crystal lattice, the complex-cations and B(1)Ph4− ions are alternately connected by intermolecular CH···π interactions between the methyl group of the MeCN ligand and phenyl groups of B(1)Ph4− ions, forming a 1D chain structure. The 1D chains are further connected by P4AE (parallel fourfold aryl embrace) interactions between two neighboring complex-cations, constructing a 2D extended structure. B(2)Ph4− ions and MeCN lattice solvents exist in the spaces of the 2D layer. DFT calculations verified that the 1,2,3-triazole-containing ligand L3-Me-3Ph gives a stronger ligand field around the octahedral coordination environment of the iron(II) ion than the analogous imidazole-containing ligand H2L2Me (= bis(N,N′-2-methylimidazol-4-yl-methylideneaminopropyl)methylamine) of the known compound [FeIIMeCN(H2L2Me)](BPh4)1.5·Cl0.5·0.5MeCN (2) reported by Matsumoto et al. (Nishi, K.; Fujinami, T.; Kitabayashi, A.; Matsumoto, N. Tetrameric spin crossover iron(II) complex constructed by imidazole⋯chloride hydrogen bonds. Inorg. Chem. Commun. 2011, 14, 1073–1076), resulting in the much higher spin transition temperature of 1 than that of 2.

Three iron(II) complexes having a 1,2,3-triazole-containing linear pentadentate N 5 ligand L 3-Me-3 Me and different monodentate solvent coligand X, [Fe(L 3-Me-3 Me )X](BPh 4 ) 2 [L 3-Me-3 Me = bis(N,N′-1-methyl-1H-1,2,3-triazol-4-ylmethylideneaminopropyl)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 T 1/2 = 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 N 5 O 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.

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