Coordination-Induced Spin State Switching <i>via</i> a Crystal-to-Crystal Transformation of Dynamic Coordination Polymers

In this study, coordination-induced spin state switching (CISSS) functionality was achieved through a dynamic coordination polymerization process. A discrete grid [Ni₄(<b>hnih</b>)₄(py)₈]·6MeOH (<b>1·py</b>) based on a nitro-decorated isonicotinoylhydrazide ligand underwent ring-opening coordination polymerization into three one-dimensional chains [Ni(<b>hnih</b>)(py)₂·py]_<i>n</i> (<b>2·py</b>), [Ni(<b>hnih</b>)(DMSO)₂·DMSO]_<i>n</i> (<b>2·DMSO</b>), and [Ni(<b>hnih</b>)]_<i>n</i> (<b>2</b>), of which the transformation involved coordination sphere rearrangement and topologic change. Optical and magnetic studies demonstrated that the reversible high spin (HS) ↔ low spin (LS) transitions can be achieved in the solid state between the octahedral species (<b>1·py</b>, <b>2·py</b>, and <b>2·DMSO</b>) and the square planar complex <b>2</b> <i>via</i> a CISSS process. This work introduces a crystal-transformation-driven strategy to achieve and regulate CISSS functionality in dynamic coordination polymers, paving the way for solid-state stimuli-responsive materials.