Spin Crossover Induced by Changing the Identity of the Secondary Metal Ion from Pd II to Ni II in a Face-Centered Fe II 8 M II 6 Cubic Cage.

Discrete spin crossover (SCO) heteronuclear cages are a rare class of materials which have potential use in next-generation molecular transport and catalysis. Previous investigations of cubic cage [Fe 8 Pd 6 L 8 ] 28+ constructed using semi-rigid metalloligands, found that Fe II centers of the cage did not undergo spin transition. In this work, substitution of the secondary metal center at the face of the cage resulted in SCO behavior, evidenced by magnetic susceptibility, Mössbauer spectroscopy and single crystal X-ray diffraction. Structural comparisons of these two cages shed light on the possible interplay of inter- and intramolecular interactions associated with SCO in the Ni II analogue, 1 ([Fe 8 Ni 6 L 8 (CH 3 CN) 12 ] 28+ ). The distorted octahedral coordination environment, as well as the occupation of the CH 3 CN in the Ni II axial positions of 1, prevented close packing of cages observed in the Pd II analogue. This led to offset, distant packing arrangements whereby important areas within the cage underwent dramatic structural changes with the exhibition of SCO.