Syntheses, three types of hydrogen-bonded assembly structures, and magnetic properties of [FeIII(Him)2(hapen)]Y·solvent (Him = imidazole, hapen =N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine, Y = BPh4−, CF3SO3−, PF6−, ClO4−,

Five iron(III) complexes of [FeIII(Him)2(hapen)]Y·solvent with five different counter anions were synthesized, where Him = imidazole, H2hapen = N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine, Y = BPh4− (1), CF3SO3− (2), PF6− (3), ClO4− (4), and BF4− (5), and solvent = methanol or H2O. Each FeIII ion has octahedral coordination geometry with N2O2 donor atoms of the equatorial tetradentate ligand (hapen) and two nitrogen atoms of two imidazoles at the axial positions. The imidazole group is hydrogen-bonded to the phenoxo oxygen atom of hapen of the adjacent unit and/or to the counter anion or solvent molecule, to give three types of assembly structures involving linear dimer, cyclic dimer, and one-dimensional zigzag chain. Compound 1, with a cyclic dimer, and 2 and 3, with a one-dimensional chain, showed spin-equilibrium behavior between high-spin (S = 5/2) and low-spin (S = 1/2) states, whereas 4 and 5, with linear dimers, are high-spin complexes. Compound 3 shows abrupt spin transition with small thermal hysteresis.