Spin Crossover Induced By Changing the Identity Of the Secondary Metal Species In A Face-Centred FeII8NiII6 Cubic Cage

The assembly of multifunctional polynuclear coordination cages exhibiting spin-crossover (SCO) proves to be a challenge to researchers. Previous investigations into the magnetic properties of a large cubic metallosupramolecular cage, [Fe8Pd6L8]28+, constructed using semi-rigid metalloligands and encompassing an internal void of 41 Å3, found that the Fe(II) centres 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 the onset of spin crossover, 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 [Fe8M(II)6L8]28+ architecture from SCO inactive to active by simply changing in 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) centres 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 to conformational changes upon SCO. Design through selectivity of secondary metal centres on the flexibility of metalloligand structures and the effect of axial donors packing arrangements may serve as new routes in the engineering of SCO or non-SCO cage systems.