Cooperative Spin Transitions Triggered by Phonons in Metal Complexes Coupled to Molecular Vibrations

Abstract

The present article is a short overview of the theoretical modeling of spin transitions in polymetallic compounds. As distinguished from many insightful reviews on this topic, the present work is focused on the nature of cooperative interaction of the metal clusters in molecular crystals with emphasis at the physical role of molecular vibrations and phonons. The underlying model assumes that the cooperativity is triggered by phonons while the metal centers are coupled to molecular vibrations. It is demonstrated that the suggested model gives a satisfactory description of the observed spin transitions in mono-, bi- and tetranuclear compounds. In the framework of the described approach, we discuss the experimental data on spin crossover in the mononuclear [Fe(ptz)6](BF4)2, binuclear [{Fe(bt)(NCS)2}2bpym] and tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4·(H2O)2 compounds containing iron ions. The approach is also applied to the description of the charge-transfer-induced spin transition in the [{(Tp)Fe(CN)3}{Co-(PY5Me2)}](CF3SO3) complex.

The effect of fluorine substituents on the crystal structure and spin crossover behavior of the cation [MnIII(3,5-diHal-sal2323)]+ complex family with BPh4 anions

“…The theoretical modeling of spin transitions in metallocomplexes shows that the cooperativity of the transitions is triggered by phonons while the metal centers are coupled to molecular vibrations. 46 The supramolecular packing forces have a decisive influence on the cooperativity of the SCO process, which leads to abrupt transitions with large hysteresis. SCO systems with wide hysteresis loops attract considerable attention, because they have great potential for practical applications.…”

Modeling of Spin Crossover in Linear Trinuclear Complexes: Effects of External Pressure

“…The examination of spin crossover in trinuclear systems is based on the general approach suggested in our previous papers. − , The quintessence of this approach consists in the following. The spin-crossover systems represent molecular crystals.…”

“…For the first time, such a representation of the resulting vibration of the ligand surrounding of an impurity center in a crystal was introduced by Hizhnyakov et al in their papers. − Later on, this idea was explored, as it was already mentioned above, for examination of spin crossover in mono-, bi-, and tetranuclear cluster compounds − as well as of charge-transfer-induced spin transitions in Fe–Co pairs . For molecular crystals, the presentation of the resulting vibration of the ligand surrounding in such a form as in eq (1) leads to the appearance in the crystal Hamiltonian of a term that is referred to as the interaction of molecular modes with phonons. − , This term gives rise to cooperative interaction between the trinuclear clusters in the crystal. Applying the procedure of derivation of the analytical expression for this interaction, carefully described in our previous papers, − , one obtains the whole interaction Hamiltonian for a crystal containing linear trinuclear clusters as a structural unit H normali normaln normalt = prefix− 1 2 ∑ lefttrue n , n ′ n ≠ n ′ ∑ α , β = a , b , c J h s , h s false( boldn α − boldn ′ β false) t boldn α normalh normals t n ′ ...…”

“…For molecular crystals, the presentation of the resulting vibration of the ligand surrounding in such a form as in eq (1) leads to the appearance in the crystal Hamiltonian of a term that is referred to as the interaction of molecular modes with phonons. − , This term gives rise to cooperative interaction between the trinuclear clusters in the crystal. Applying the procedure of derivation of the analytical expression for this interaction, carefully described in our previous papers, − , one obtains the whole interaction Hamiltonian for a crystal containing linear trinuclear clusters as a structural unit H normali normaln normalt = prefix− 1 2 ∑ lefttrue n , n ′ n ≠ n ′ ∑ α , β = a , b , c J h s , h s false( boldn α − boldn ′ β false) t boldn α normalh normals t n ′ β normalh normals − 1 2 ∑ lefttrue n , n ′ n…”

“…Applying the procedure of derivation of the analytical expression for this interaction, carefully described in our previous papers, − , one obtains the whole interaction Hamiltonian for a crystal containing linear trinuclear clusters as a structural unit H normali normaln normalt = prefix− 1 2 ∑ lefttrue n , n ′ n ≠ n ′ ∑ α , β = a , b , c J h s , h s false( boldn α − boldn ′ β false) t boldn α normalh normals t n ′ β normalh normals − 1 2 ∑ lefttrue n , n ′ n ≠ n ′ ∑ α , β = a , b , c J …”