Cooperative Spin Crossover and Order−Disorder Phenomena in a Mononuclear Compound [Fe(DAPP)(abpt)](ClO4)2 [DAPP = [Bis(3-aminopropyl)(2-pyridylmethyl)amine], abpt = 4-Amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole]

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Abstract
The synthesis and detailed characterization of the new spin crossover mononuclear complex [FeII(DAPP)(abpt)](ClO4)2, where DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine] and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, are reported. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed the occurrence of an abrupt spin transition with a hysteresis loop. The hysteresis width derived from magnetic susceptibility measurements is 10 K, the transition being centered at Tc↓ = 171 K for decreasing and Tc↑ = 181 K for increasing temperatures. The crystal structure was resolved in the high-spin (293 and 183 K) and low-spin (123 K) states. Both spin-state structures belong to the monoclinic space group P21/n (Z = 4). The thermal spin transition is accompanied by the shortening of the mean Fe−N distances by 0.177 Å. The two main structural characteristics of [Fe(DAPP)(abpt)](ClO4)2 are a branched network of intermolecular links in the crystal lattice and the occurrence of two types of order−disorder transitions (in the DAPP ligand and in the perchlorate anions) accompanying the thermal spin change. These features are discussed relative to the magnetic properties of the complex. The electronic structure calculations show that the structural disorder in the DAPP ligand modulates the energy gap between the HS and LS states. In line with previous studies, the order−disorder phenomena and the spin transition in [Fe(DAPP)(abpt)](ClO4)2 are found to be interrelated.