Spin Crossover Iron(II) Coordination Polymer Chains: Syntheses, Structures, and Magnetic Characterizations of [Fe(aqin)₂(μ₂‑M(CN)₄)] (M = Ni(II), Pt(II), aqin = Quinolin-8-amine)

New Fe­(II) coordination polymeric neutral chains of formula [Fe­(aqin)₂(μ₂-M­(CN)₄)] (M = Ni^II (<b>1</b>) and Pt^II (<b>2</b>)) (aqin = Quinolin-8-amine) have been synthesized and characterized by infrared spectroscopy, X-ray diffraction, and magnetic measurements. The crystal structure determinations of <b>1</b>–<b>2</b> reveal in both cases a one-dimensional structure in which the planar [M­(CN)₄]^2– (M = Ni^II (<b>1</b>) and Pt^II (<b>2</b>)) anion acts as a μ₂-bridging ligand, and the two aqin molecules as chelating coligands. Examination of the intermolecular contacts in the two compounds reveals that the main contacts are ascribed to hydrogen bonding interactions involving the amine groups of the aqin chelating ligands and the nitrogen atoms of the two non bridging CN groups of the [M­(CN)₄]^2– (M = Ni^II (<b>1</b>) and Pt^II (<b>2</b>)) anion. The average values of the six Fe–N distances observed respectively at room temperature (293 K) and low temperature (120 K), that is, 2.142(3) and 2.035(2) Å for <b>1</b>, and 2.178(3) and 1.990(2) Å for <b>2</b>, and the thermal variation of the cell parameters (performed on <b>2</b>) are indicative of the presence of an abrupt <i>HS-LS</i> spin crossover (<i>SCO</i>) transition in both compounds. The thermal dependence of the product of the molar magnetic susceptibility times the temperature (χ_m<i>T</i>), in cooling and warming modes, confirms the <i>SCO</i> behavior at about 145 and 133 K in <b>1</b> and <b>2</b>, respectively, and reveals the presence of a small thermal hysteresis of about 2 K for each compound