Abstract
Confinement of discrete coordination cages within nanoporous lattices is an intriguing strategy to gain unusual properties and functions. We demonstrate here that the confinement of coordination cages within metal-organic frameworks (MOFs) allows the spin state of the cages to be regulated through multilevel host-guest interactions. In particular, the confined in situ self-assembly of an anionic Fe(II)(4)L(6) nanocage within the mesoporous cationic framework of MIL-101 leads to the ionic MOF with an unusual hierarchical host-guest structure. While the nanocage in solution and in the solid state has been known to be invariantly diamagnetic with low-spin Fe(II), Fe(II)(4)L(6)@MIL-101 exhibits spin-crossover (SCO) behavior in response to temperature and release/uptake of water guest within the MOF. The distinct color change concomitant with water-induced SCO enables the use of the material for highly selective colorimetric sensing of humidity. Moreover, the spin state and the SCO behavior can be modulated also by inclusion of a guest into the hydrophobic cavity of the confined cage. This is an essential demonstration of the phenomenon that the confinement within porous solids enables an SCO-inactive cage to show modulable SCO behaviors, opening perspectives for developing functional supramolecular materials through hierarchical host-guest structures.