| Summary: | New Fe(III) compounds from qsal ligand, [Fe(qsal)<sub>2</sub>](CH<sub>3</sub>OSO<sub>3</sub>) (<b>1</b>) and [Fe(qsal)<sub>2</sub>](CH<sub>3</sub>SO<sub>3</sub>)·CH<sub>3</sub>OH (<b>3</b>), along with known compound, [Fe(qsal)<sub>2</sub>](CF<sub>3</sub>SO<sub>3</sub>) (<b>2</b>), were obtained as large well-shaped crystals (Hqsal = <i>N</i>-(8-quinolyl)salicylaldimine). The compounds <b>1</b> and <b>2</b> were in the low-spin (LS) state at 300 K and exhibited a cooperative spin crossover (SCO) transition with a thermal hysteresis loop at higher temperatures, whereas <b>3</b> was in the high-spin (HS) state below 300 K. The optical conductivity spectra for <b>1</b> and <b>3</b> were calculated from the single-crystal reflection spectra, which were, to the best of our knowledge, the first optical conductivity spectra of SCO compounds. The absorption bands for the LS and HS [Fe(qsal)<sub>2</sub>] cations were assigned by time-dependent density functional theory calculations. The crystal structures of <b>1</b> and <b>2</b> consisted of a common one-dimensional (1D) array of the [Fe(qsal)<sub>2</sub>] cation, whereas that of <b>3</b> had an unusual 1D arrangement by π-stacking interactions which has never been reported. The crystal structures in the high-temperature phases for <b>1</b> and <b>2</b> indicate that large structural changes were triggered by the motion of counter anions. The comparison of the crystal structures of the known [Fe(qsal)<sub>2</sub>] compounds suggests the significant role of a large non-spherical counter-anion or solvate molecule for the total lattice energy gain in the crystal of a charged complex.
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