Effective S=2 antiferromagnetic spin chain in the salt (o-MePy-V)FeCl4

We present a model compound for the S=2 antiferromagnetic (AF) spin chain composed of the salt (o-MePy-V)FeCl4. Ab initio molecular-orbital calculations indicate the formation of a partially stacked two-dimensional (2D) spin model comprising five types of exchange interactions between S=1/2 and S=5/2 spins, which locate on verdazyl radical and Fe ion, respectively. The magnetic properties of the synthesized crystals indicate that the dominant interaction between the S=1/2 and S=5/2 spins stabilizes an S=2 spin in the low-temperature region, and an effective S=2 AF chain is formed for T≪10 K and H