title: Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group.
abstract: Two iron(ii) molecular isomers, namely face-Λ-[Fe(L1)3](ClO4)2(1a) (L1= R-1-phenyl-N-((1-hexyl-5-methyl-1H-imidazol-2-ylmethylene)ethanamine)) and face-Λ-[Fe(L2)3](ClO4)2(1b) (L2= R-1-phenyl-N-(1-hexyl-4-methyl-1H-imidazol-2-ylmethylene)ethanamine), are obtained via a well-designed strategy based on the tautomerization of the 4(5)-methylimidazole group. Structural investigations reveal that the two isomers are extremely similar with only differences in the methyl group position of the ligands. The iron(ii) center is surrounded by three bidentate imidazole Schiff-base ligands in the face-Λ conformation, which affords a distorted [FeN6] octahedral coordination sphere. The average Fe-N bond length of 1a (1.971 Å) is shorter than that of 1b (2.185 Å). Spectroscopy analyses, X-ray crystal structures and magnetic investigations show that 1a is in the low-spin state at room temperature due to the strong ligand field imparted by the electron-donating methyl groups at the 5-position on the imidazole moieties and undergoes a partial spin transition with an estimated T1/2= 390 K. In contrast, 1b is stabilized in the high-spin state because of the strong steric hindrance when the three methyl "legs" are changed from the 5-position to the 4-position on the imidazole moieties. In addition, a new complex, 2, without methyl groups attached to the imidazole rings is introduced and characterized to further corroborate the steric influence on the spin state. Complex 2 exhibits a gradual spin-crossover behaviour with T1/2= 258 K. Moreover, the diverse spin states of these complexes are computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that the competition of the electronic effect and steric crowding influence the spin states.
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Wang-Kang
authors.initials: WK
authors.lastname: Han
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Zhi-Hua
authors.initials: ZH
authors.lastname: Li
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Wei
authors.initials: W
authors.lastname: Zhu
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Tao
authors.initials: T
authors.lastname: Li
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Zaijun
authors.identifier: http://orcid.org/0000-0003-4524-5581
authors.initials: Z
authors.lastname: Li
authors.affiliation: The Key Laboratory of Eco-textiles of Ministry of Education, College of Textiles and Clothing, Jiangnan University, Wuxi 214122, P.R. China.
authors.forename: Xuehong
authors.initials: X
authors.lastname: Ren
authors.affiliation: The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China. zhiguogu@jiangnan.edu.cn.
authors.forename: Zhi-Guo
authors.identifier: http://orcid.org/0000-0003-2227-8882
authors.initials: ZG
authors.lastname: Gu