Spin Crossover Induced by Changing the Identity of the Secondary Metal Ion from PdII to NiII in a Face-Centered FeII8MII6 Cubic Cage

The engineering of spin crossover (SCO) coordination cages is a complex endeavor with great potential in next generation multifunctional materials. Discrete metallosupramolecular cages exhibiting SCO are an exciting, though rare, class of porous polyhedral material. Incorporating the SCO property into these architectures is complicated, as there are many inter- and intramolecular factors which must be appropriately balanced. Previous investigations into the magnetic properties of a large cubic metallosupramolecular cage, [Fe8Pd6L8]28+, constructed using semi-rigid metalloligands, found that the Fe(II) centers that occupied the corners of the cubic structure did not undergo a spin transition. In this work, substitution of the linker metal on the face of the cage resulted in spin crossover behavior, as evidenced by magnetic susceptibility, Mӧssbauer and single crystal X-ray diffraction. Structural comparisons of these two cages were undertaken to shed light on the possible mechanism responsible for switching of the [Fe8MII6L8]28+ architecture from SCO inactive to active by simply changing the identity of M(II). This led to the suggestion that a possible interplay of intra- and intermolecular interactions may permit SCO in the Ni(II) analogue, 1. The distorted octahedral coordination environment of the secondary Ni(II) centers occupying the cage faces provided conformational flexibility for the eight metalloligands of the cubic architecture relative to the square planar Pd(II) environment. Meanwhile the occupation of axial coordination sites of the Ni(II) cations by CH3CN prevented the close packing of cages observed for the Pd(II) analogue, leading to a more offset, distant packing arrangement of cages in the lattice, whereby important areas of the cage that were shown to change most dramatically with SCO experienced a lesser degree of steric hindrance. Design via the effect of secondary metal centers on the flexibility of metalloligand structures and the effect of the axial donors on the packing arrangements may serve as new routes for engineering cage systems with desired magnetic properties.