A mononuclear iron(ii) complex, [Fe(L1)](PF) (1), and a related dinuclear one, [Fe(L2)](PF)·5HO·MeCN (2), were prepared, where L1 is a tridentate N coordinating Schiff-base ligand, 1-methyl-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and L2 is a dimeric form ligand of L1, 1,1'-(1,2-ethanediyl)bis-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and their magnetostructural relationships were investigated. Mononuclear 1 shows a gradual one-step spin crossover (SCO) from the low-spin (LS) state (S = 0) at room temperature (RT) to the mostly high-spin (HS) state (S = 2) at 470 K spanning a temperature range of more than 170 K, while dinuclear 2 shows an anomalous two-step SCO with a gradual transition in a first step from the LS-LS state at RT to around 432 K and then an abrupt spin transition centred at 432 K with 11 K wide hysteresis in a second step (T = 437 K and T = 426 K) reaching the mostly HS-HS state. Single crystal X-ray structure analyses revealed that the iron(ii) centre of mononuclear 1 has an N6 octahedral coordination environment with two tridentate L1 ligands, while the complex-cation of 2 has a dinuclear double helicate architecture as though a dimeric form of two complex-cations of 1 replaced their methyl group with a bridging ethylene chain. Both complexes have a cationic two-dimensional (2D) structure in which the 2D layer of mononuclear 1 is constructed of intermolecular CHN hydrogen bonds between adjacent complex-cations, while that of dinuclear 2 is constructed of both intermolecular π-π and CH/π interactions between neighboring helicates.

Abstract:Two new dinuclear iron(II) complexes (1·PF 6 and 1·AsF 6 ) of the general formula [Fe II 2 (L2 C3 ) 2 ](X) 4 ·nH 2 O·mMeCN (X = PF 6 , n = m = 1.5 for 1·PF 6 and X = AsF 6 , n = 3, m = 1 for 1·AsF 6 ) have been prepared and structurally characterized, where L2 C3 is a bis-1,2,3-triazolimine type Schiff-baseSingle crystal X-ray structure analyses revealed that 1·PF 6 and 1·AsF 6 are isostructural. The complex-cation [Fe II 2 (L2 C3 ) 2 ] 4+ of both has the same dinuclear double helicate architecture, in which each iron(II) center has an N6 octahedral coordination environment. Neighboring helicates are connected by intermolecular π-π interactions to give a chiral one-dimensional (1D) structure, and cationic 1D chains with the opposite chirality exist in the crystal lattice to give a heterochiral crystal. Magnetic and differential scanning calorimetry (DSC) studies were performed only for 1·AsF 6 , since the thermal stability in a high-temperature spin crossover (SCO) region of 1·PF 6 is poorer than that of 1·AsF 6 . 1·AsF 6 shows an unsymmetrical hysteretic SCO between the low-spin-low-spin (LS-LS) and high-spin-high-spin (HS-HS) states at above room temperature. The critical temperatures of warming (T c ↑) and cooling (T c ↓) modes in the abrupt spin transition area are 485 and 401 K, respectively, indicating the occurrence of 84 K-wide thermal hysteresis in the first thermal cycle.