Control of metal ion spin states by ligand design is important for several areas of coordination chemistry. On one hand, spin-crossover compounds showing transitions between spin states are important molecular switches for nanoscience, device and sensor applications. On the other, high- and low-spin states of base metal catalysts can also show different reactivity patterns in biological and synthetic catalysis systems. I will describe two recent studies that are relevant to this question. First, we have used the well-known chiral PyBox ligand system to observe the first discrimination of spin state in different enantiomers of the same metal complex. Thus, homochiral (R,R)-[Fe(L1)2]2+ undergoes spin-crossover at 34 K lower temperature than its heterochiral isomer (R,S)-[Fe(L1)2]2+ in CD3CN solution (Figure). The difference reflects a more twisted ligand conformation, caused by a steric clash between phenyl substituents in (R,R)-[Fe(L1)2]2+ which is not present in (R,S)-[Fe(L1)2]2+. Secondly, [Fe(L2R)2]2+ complexes are always high-spin and inactive to spin-crossover. This is surprising, because many complexes of related tridentate ligands exhibit spin-crossover properties. DFT calculations show the high-spin nature of [Fe(L2R)2]2+ reflects a narrow bite angle of the L2R ligand, reflecting the compact nature of the triazine ring.

Influence of ligand conformation on the spin-state properties of iron(ii) complexes

2019 Spring National Meeting

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