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Novel pseudopolymorphic Hdabco compounds with an Fe(III) complex anion, (Hdabco)[Fe(aznp)2]·CH2Cl2 1 and (Hdabco)[Fe(aznp)2]·0.5H2O 2 [dabco = 1,4-diazabicyclo[2.2.2]octane, H2aznp = (2′-hydroxyphenylazo)-2-hydroxynaphthalene], were prepared and characterized. The magnetic susceptibility for 1 and 2 revealed that both complexes were in a high-spin (HS) state in the whole temperature range and exhibited weak ferromagnetic interactions below 40 K. The crystal structural analyses suggested that strong N–H⋯O hydrogen bonding interactions between the Hdabco cation and [Fe(aznp)2] anion may induce the distortion of a coordination structure resulting in the HS complexes, whereas π-stacking interactions between the π-ligands in the [Fe(aznp)2] anion and additional C–H⋯N hydrogen bonding interactions between the Hdabco cation and [Fe(aznp)2] anion constructed a intermolecular interaction framework structure with one-dimensional channels. The thermogravimetry analysis for compound 1 indicated the adsorption of a water molecule took place after the desorption of a dichloromethane molecule. This transformation of 1 into 2 proved to proceed in a single-crystal-to-single-crystal way by powder X-ray diffractions and single crystal X-ray structural analysis.

A novel neutral heteroleptic FeIII complex from two kinds of π-extended tridentate ligands was designed and prepared. The π-ligands formed a three-dimensional purely pi-stacking interaction network. The present complex proved to be the first neutral spin-crossover (SCO) FeIII complex with a N3O3 coordination sphere exhibiting an abrupt SCO transition with a thermal hysteresis of 10 K and the light-induced excited spin-state trapping effect.

To investigate the π-extension effect on an unusual negative-charged spin crossover (SCO) FeIII complex with a weak N2O4 first coordination sphere, we designed and synthesized a series of anionic FeIII complexes from a π-extended naphthalene derivative ligand. Acetonitrile-solvate tetramethylammonium (TMA) salt 1 exhibited an SCO conversion, while acetone-solvate TMA salt 2 was in a high-spin state. The crystal structural analysis for 2 revealed that two-leg ladder-like cation-anion arrays derived from π-stacking interactions between π-ligands of the FeIII complex anion and Coulomb interactions were found and the solvated acetone molecules were in one-dimensional channels between the cation-anion arrays. A desolvation-induced single-crystal-to-single-crystal transformation to desolvate compound 2’ may be driven by Coulomb energy gain. Furthermore, the structural comparison between quasi-polymorphic compounds 1 and 2 revealed that the synergy between Coulomb and π-stacking interactions induces a significant distortion of coordination structure of 2.