Fe[4-(3-Phenylpropyl)Pyridine]₂[Fe(CN)₅NO]: A 2D Coordination Polymer with Thermally-Induced Spin Transition and Nature of Its Asymmetric Hysteresis Loop

The titled material crystallizes with an orthorhombic unit cell, in the P2₁2₁2 space group (Nr. 18). Its crystal structure was solved and refined from powder XRD patterns. The solid framework is formed by stacked undulated sheets of inorganic nature, Fe[Fe(CN)₅NO], separated by bimolecular organic pillars, [4-(3-Phenylpropyl)pyridine]₂, which remain coordinated to the axial coordination sites for the iron atom. The molecules forming these pillars remain coupled through C–H⋯π and dispersive interactions between neighboring molecules. When this solid is cooled and then warmed, a reversible spin transition, from high to low spin (HS → LS), and vice versa, is observed. This transition occurs in the temperature interval of 135–165 K, with a hysteresis between them of about 30 K. That hysteresis loop appears with a pronounced asymmetry when the slopes for the HS → LS and LS → HS transitions are compared. This effect is discussed in terms of the related structural changes in the solid structure during the spin transitions. The transition was also monitored from IR and Raman spectra recorded at 80 and 300 K. Relevant information on the electronic structure for both, the LS and HS phases of this material, was derived from the corresponding XPS spectra recorded at 114 and 270 K. This contribution emphasizes the role of the nitrosyl group (NO) as an electron buffer for tuning the bonding properties of the inorganic layer at the CN 5σ orbital to make possible the observed thermally-induced SCO behavior. Graphical Abstract: The thermally-induced spin transition in this solid shows an asymmetric hysteresis loop, which was ascribed to the nature of the pillar molecule. [Figure not available: see fulltext.]