Guest-dependent spin transitions in Hofmann-like FeII SCO metal–organic frameworks: hints from quantum chemistry calculations

Abstract

The family of 3D Hofmann-like Fe^II coordination polymers with the formula [Fe(bpd)[M^II(CN)₄]]·2G, M = Pt, Pd, and Ni is known for having a large capacity to host guest molecules (G) such as nitrobenzene or naphthalene. The presence of the guest molecules modulates the spin-crossover properties. In fact, these clathrates present a large hysteresis loop, in one single step or two well separated steps depending on the guest. In this work, we analyse by means of state-of-the-art DFT-based calculations the main effects governing the occurrence of the multistep spin transition, and the differences due to the nature of the divalent M metal and the encapsulated guest molecules. The study determines the relative stability of the mixed-spin state, the amplitude and nature of the host–guest and guest–guest intermolecular interactions, and the key role played by the CN stretching modes of the M-cyanide units. Our results provide hints on the distinct roles of each of these factors and how they promote or prevent a direct transition from the high-spin state to the low-spin state.