Spin transition and symmetry-breaking in new mononuclear Fe^II tren-complexes with up to 38 K hysteresis around room temperature

Abstract

The structurally simple complex {FeII[tren(6F-py)3]}(BF4)2 [tren(6F-py)3 = tris(3-aza-4-(6-fluoro-2-pyridyl)-3-butenyl]amine] undergoes an abrupt spin transition (ST) with the critical temperature T1/2↓ = 243 K on cooling and T1/2↑ = 281 K on...

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

“…As suggested by the literature structural data of ST complexes in both spin states, realization of narrow hysteresis loops is usually associated with moderate structural differences between the LS and HS phases, − whereas loops 20–60 K wide imply an accompanying lattice reorganization, including but not limited to order–disorder structural transitions of anions/solvent molecules/ligand substituents or lattice symmetry breaking. ,,− It can be assumed that similar mechanisms can be at the origin of hysteresis in numerous other ST compounds lacking structural characterization. ,,− In the case of complexes with tridentate ligands, these significant but hardly predictable lattice-level rearrangements can particularly lead to a trigonal distortion of the [FeN 6 ] polyhedron . This kind of geometrical deformation is known to effectively decrease the ligand field strength by removing degeneracy and decreasing the splitting energy of the 3 d -orbitals.…”

“…With this idea in mind, and also having a positive experience with discrete systems exhibiting regulated ST due to significantly varying π–π interactions, 31 , 42 , 43 we have focused our efforts on asymmetrically substituted neutral complexes with large planar ligands. The absence of nonactive “shock absorbers” of interactions such as counterions and solvent molecules and, instead, the numerous direct π–π and hydrogen intermolecular interactions between transforming ST molecules are considered important for the efficient spin-state transmission across the lattice.…”

“… 23 , 44 , 45 Furthermore, the asymmetric shape of the molecule is expected to segregate the packing into alternating regions with different interactions, which, besides favoring direct communication pathways between transforming Fe II ions, can promote the cooperative transformation of the molecular periphery and thus increase the energy cost of the ST process. 31 , 42 , 43 , 46 …”

“…With this idea in mind, and also having a positive experience with discrete systems exhibiting regulated ST due to significantly varying π–π interactions, ,, we have focused our efforts on asymmetrically substituted neutral complexes with large planar ligands. The absence of nonactive “shock absorbers” of interactions such as counterions and solvent molecules and, instead, the numerous direct π–π and hydrogen intermolecular interactions between transforming ST molecules are considered important for the efficient spin-state transmission across the lattice. ,, Furthermore, the asymmetric shape of the molecule is expected to segregate the packing into alternating regions with different interactions, which, besides favoring direct communication pathways between transforming Fe II ions, can promote the cooperative transformation of the molecular periphery and thus increase the energy cost of the ST process. ,,, …”

“…23,44,45 Furthermore, the asymmetric shape of the molecule is expected to segregate the packing into alternating regions with different interactions, which, besides favoring direct communication pathways between transforming Fe II ions, can promote the cooperative transformation of the molecular periphery and thus increase the energy cost of the ST process. 31,42,43,46 After several attempts, partially reflected in refs, 47−49 ■ RESULTS AND DISCUSSION Synthesis. The ligand was conveniently synthesized by the Suzuki cross-coupling reaction of commercially available organic precursors (see the Supporting Information).…”