Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

Norma Ortega-Villar,M Muñoz,José Real

doi:10.3390/magnetochemistry2010016

Abstract

This review provides an up to date survey of a singular class of iron(II) spin crossover (SCO) molecular materials that undergo high-spin (HS) ↔ low-spin (LS) phase transitions accompanied by crystallographic symmetry breaking (CSB). Particular interest has been focused on a variety of complexes that exhibit one-step or stepwise SCO behavior and CSB. Most of them afford excellent examples of well-ordered 1HS-1LS, 2HS-1LS or 1HS-2LS intermediate phases (IP) and represent an important platform to disclose microscopic mechanisms responsible for cooperativity and ordering in such multistable materials.

Highlights

Spin crossover (SCO) complexes represent an important class of switchable molecular materials [1,2,3,4]
These structural changes are cooperatively transmitted through elastic interactions and, in favorable cases, first-order spin transition occasionally accompanied by hysteretic behavior confers a binary function to the material
This spontaneous change may occur without significant modification of the unit cell, other than the typical structural changes associated with the LS Ø HS conversion

Summary

Introduction

Spin crossover (SCO) complexes represent an important class of switchable molecular materials [1,2,3,4] In these complexes, the energy gap between the high-spin (HS) and the low-spin (LS) states is of the order of magnitude of the thermal energy. These structural changes are cooperatively transmitted through elastic interactions and, in favorable cases, first-order spin transition occasionally accompanied by hysteretic behavior confers a binary function to the material. The other arises from subtle balances between different elastic interactions in the crystal lattice, which spontaneously generate two or more distinct SCO sites This spontaneous change may occur without significant modification of the unit cell, other than the typical structural changes associated with the LS Ø HS conversion.

Stepwise SCO Behavior

Two-Dimensional Coordination Polymers

Conclusions