A spin crossover (SC) Fe III complex, [Fe III (4-NH 2 py)L]BPh 4 , was synthesized and characterized, where 4-NH 2 py = 4-aminopyridine, H 2 L = pentadentate N 3 O 2 Schiff-base ligand of the 2:1 condensation product of 3-methoxysalicylaldehyde and bis(3-aminopropyl)methylamine, and BPh 4 À = tetraphenylborate. Cryomagnetic susceptibility measurements revealed a steep spin transition between Fe III low-spin (S ¼ 1=2) and high-spin (S ¼ 5=2) states at T 1=2 ¼ 120 K, but showed no hysteresis. Crystal-structural analyses were performed at 293 and 90 K in order to investigate the origin of the steep spin crossover, and to determine the structures of the high-spin and low-spin states. The crystal structure consisted of a one-dimensional chain constructed by hydrogen bonds between the amino group of 4-NH 2 py of a molecule and the methoxy group of the adjacent molecule.The spin crossover (SC) between the low-spin (LS) and high-spin (HS) states is observed in some octahedral 3d n (4 n 7) metal complexes, and is induced by an external perturbation, such as temperature, pressure, or light irradiation. 1 While the SC behavior is essentially a phenomenon of a single molecule, the interaction between SC sites is an important factor to govern the SC properties, such as the steepness of the spin transition, hysteresis, and LIESST (Light-Induced Excited Spin State Trapping) effect, which are important properties for such applications as information storage, molecular switches, and visual displays. 2 From this viewpoint, the synthesis of SC compounds exhibiting interactions between spin-transition sites is of current interest. Along this line, polymeric SC compounds with the bridging ligand and mononuclear SC compounds exhibiting intermolecular interactions, such as hydrogen bonding and -stacking, have been extensively investigated, and have shown interesting SC behaviors. 3 Previously, we reported a family of spin-equilibrium Fe III complexes with the general chemical formula of [Fe III bL]-BPh 4 , in which H 2 L denotes the N 3 O 2 pentadentate ligand derived from the 2:1 condensation reaction of the salicylaldehyde derivative and bis(3-aminopropyl)amine and b denotes monodentate ligands, such as pyridine and imidazole derivatives. 4 These Fe III complexes assume a ligand field close to the SC point; complexes with pyridine and imidazole derivatives as the axial ligand show the SC behavior. However, the SC behavior is in general gradual, and can be described as a continuous type, because there is no intermolecular interaction. In this study, we designed a Fe III complex, [Fe III (4-NH 2 py)L]-BPh 4 , in which 4-aminopyridine and bis(3-methoxysalicylideneiminopropyl)methylamine were used as the axial and pentadentate ligands, respectively. This complex can function as a self-complementary molecule to construct a 1D chain structure through a hydrogen bond between the amino group of a molecule and the 3-methoxy group of the adjacent molecule. We report here the steep SC behavior and the crystal structures at the HS and LS state...

A binuclear iron(III) complex bridged by trans-1,2-bis(4-pyridyl)ethylene, [LFeIII(vibpy)FeIIIL](BPh4)2, showed a gradual two-step spin crossover behavior, where H2L is bis(3-methoxysalicylideneaminopropyl)methylamine and BPh4− is tetraphenylborate. The single-crystal X-ray analysis at the intermediate state revealed that one of the two FeIII ions of the binuclear complex is in the high-spin (HS) state and the other is in the low-spin (LS) state. The HS and LS sites independently form layers and these HS and LS layers alternately stack.

A one-dimensional (1D) polynuclear spin-crossover FeIII complex, [FeIII(acacen)(bpyp)]n(BPh4)n, showed a gradual and complete SC over a wide temperature range of ca. 250 K between S = 5/2 and S = 1/2, where acacen = N,N′-bis(acetylacetonato)ethylenediamine, bpyp = 1,3-bis(4-pyridyl)propane, and BPh4− = tetraphenylborate. The single-crystal X-ray structures in the HS, LS, and midpoint between the HS and LS states were determined, and demonstrated that the SC may occur through the alternately arrayed HS- and LS-chains.

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Isomorphous bi- and mononuclear iron(III) complexes, [FeIII2L2(bimb)](BPh4)2 (1) and [FeIIIL(meim)]BPh4 (2), where H2L = bis(3-methoxysalicylideneaminopropyl)amine, bimb = 1,4-bisimidazolylbutane, meim = N-methylimidazole, and BPh4− = tetraphenylborate, respectively, were synthsized. Binuclear complex 1 showed a two-step spin-crossover (SCO) behavior at T1⁄2=60 and 90 K, whereas mononuclear complex 2 showed a one-step SCO behavior at T1⁄2=215 K. The different SCO behavior can be ascribed to the quasi-one-dimensional structure constructed by π–π stacking and the binuclear structure.