The key role of the intermolecular pi-pi interactions in the presence of spin crossover in neutral [Fe(abpt)2A2] complexes (A = terminal monoanion N ligand).

New iron(II) complexes of formulas [Fe(abpt) 2(tcm) 2] ( 1), [Fe(abpt) 2(tcnome) 2] ( 2), and [Fe(abpt) 2(tcnoet) 2] ( 3) (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, tcm (-) = [C(CN) 3] (-) = tricyanomethanide anion; tcnome (-) = [(NC) 2CC(OCH 3)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-methoxypropenide anion; tcnoet (-) = [(NC) 2CC(OC 2H 5)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-ethoxypropenide anion) have been synthesized and characterized by infrared spectroscopy, magnetic properties and by variable-temperature single-crystal X-ray diffraction. The crystal structure determinations of 1 and 2 reveal in both cases centrosymmetric discrete iron(II) monomeric structures in which two abpt chelating ligands stand in the equatorial plane and two terminal polynitrile ligands complete the distorted octahedral environment in trans positions. For 3, the crystallographic studies revealed two polymorphs, 3- A and 3- B, exhibiting similar discrete molecular structures to those found for 1 and 2 but with different molecular arrangements. In agreement with the variable-temperature single-crystal X-ray diffraction, the magnetic susceptibility measurements, performed in the temperature range 2-400 K, showed a spin-crossover phenomenon above room temperature for complexes 1, 3- A, and 3- B with a T 1/2 of 336, 377, and 383 K, respectively, while complex 2 remains in the high-spin ground state ( S = 2) in the whole temperature range. To understand further the magnetic behaviors of 1, 3-A, and 3-B, single-crystal X-ray diffraction measurements were performed at high temperatures. The crystal structures of both polymorphs could not be obtained above 400 K because the crystals decomposed. However, single-crystal X-ray data have been collected for compound 1, which reaches the full high-spin state at lower temperatures. Its crystal structure, solved at 400 K, showed a strong modification of the iron coordination sphere (average Fe-N = 2.157(3) A vs 1.986(3) A at 293 K). In agreement with the magnetic properties. Such structural behavior is a signature of the spin-state transition from low-spin (LS) to high-spin (HS). On the basis of the intermolecular pi stacking observed for the series described in this paper and for related complexes involving similar discrete structures, we have shown that complexes displaying frontal pi stacking present spin transition such as 1, 3-A, and 3-B and those involving sideways pi stacking such as complex 2 remain in the HS state.