Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands

Xue, Shufang and Guo, Yunnan and Rotaru, Aurelian and Müller-Bunz, Helge and Morgan, Grace G. and Trzop, Elzbieta and Collet, Eric and Oláh, Julianna and Garcia, Yann (2018) Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands. INORGANIC CHEMISTRY, 57 (16). pp. 9880-9891. ISSN 0020-1669

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Abstract

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5′-dimethyl-2,2′-bipyridine (bipy-CH3, 1), L = dimethyl-2,2′-bipyridyl-5,5′-dicarboxylate (MeObpydc, 2), L = diethyl-2,2′-bipyridyl-5,5′-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2′-bipyridine-5,5′-dicarboxylate (i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C–H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

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