Spin Crossover Phenomenon Accompanying Order−Disorder Phase Transition in the Ligand of [Fe^II(DAPP)(abpt)](ClO₄)₂Compound (DAPP = Bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-Amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) and Its Successive Self-Grinding Effect

Abstract

The spin crossover phenomenon of the recently described spin crossover complex [FeII(DAPP)(abpt)](ClO4)2 [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at T(trs) = 185.61 K. The transition enthalpy and entropy amounted to Delta(trs)H = 15.44 kJ mol-1 and Delta(trs)S = 83.74 J K-1 mol-1, respectively. The transition entropy is larger than the expected value 60.66 J K-1 mol-1, which is contributed from the spin multiplicity (R ln 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two perchlorate anions (2R ln 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K-1 mol-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO4)2 crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.

Molecular Spin Crossover Materials: Review of the Lattice Dynamical Properties

“…In most cases, the configurational and rotational contributions can be also neglected, but care must be taken because in some compounds the SCO is coupled with a change of a static/dynamic orientational disorder in the material and/or with a change of rotational motions of ligands, counter‐ions, or guest molecules. For example, from crystallographic studies on the SCO compound [Fe(DAPP)(abpt)](ClO 4 ) 2 , a contribution of = 11.5 J K −1 mol −1 to the total entropy change of 84 J K −1 mol −1 could be ascribed to the disordering of the DAPP ligand and one of the ClO 4 − ions . In another example, neutron‐scattering experiments performed on the compound {Fe(pyrazine)[Pt(CN) 4 ]} revealed that ≈10% of the total entropy change (85 J K −1 mol −1 ) arises from the rotational entropy contribution of the pyrazine ligands .…”

Re-investigation of the spin crossover phenomenon in the ferrous complex [Fe(HB(pz)3)2]

“…In this context, we have found only one well-documented literature report on self-grinding phenomena in a spin crossover system. Miyazaki et al 9 revealed that single-crystals of Fig. 1 The schematic structure of [Fe(HB(pz) 3…”

The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations

“…Such cooperative effects have been and continue to be of interest, as they constitute an essential ingredient for the macroscopic bistability and memory effects needed for the above-mentioned potential applications. Of particular interest, in the context of the work presented in this paper, are systems in which the spin transition triggers a crystallographic phase transitions as for instance in the model compound [Fe(ptz) 6 ](BF 4 ) 2 (ptz = 1-propyltetrazole) [9][10][11][12], or induces a super-structure such as in [Fe(pic) 3 ]Cl 2 Á EtOH [13,14], and may even lead to a self-grinding effect reducing the crystallite size to a comparatively homogeneous distribution of the order of micrometres [15].…”