Discrete spin crossover (SCO) heteronuclear cages are a rare class of materials which have potential use in nextgeneration molecular transport and catalysis. Previous investigations of cubic cage [Fe 8 Pd 6 L 8 ] 28 + constructed using semirigid 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.

The achievement of targeted properties in spin crossover (SCO) materials is complicated by often unpredictable cooperative interactions in the solid state. Herein, we report a dinuclear Fe(II) triple helicate 1, which is a rare example of a SCO material possessing two distinct magnetic behaviors that depend upon the thermal scan rate. Desolvated 1 was seen to undergo spin transition (ST) which was complete following slow cooling (1 K min-1), but incomplete ST (corresponding to 50% conversion) on fast cooling (10 K min-1). The incomplete ST observed in the latter case was accompanied by a higher temperature onset of ST, differing from TIESST (Temperature-Induced Excited Spin-State Trapping) materials. The two SCO pathways have been shown to arise from the interconversion between two structural phases (a and b), with both phases having associated high spin (HS) and low spin (LS) states. SCXRD (Single Crystal X-ray Diffraction) experiments using controlled cooling rates and a synchrotron light source enabled short collection times (2-3 minutes per dataset) which has enabled the identification of a mechanism by which the slow-cooled material may fully relax. In contrast, fast-cooled materials exhibit disordered arrangements of multiple structural phases, which has in turn revealed that the [HS-LS] ↔ [LS-HS] equilibria are controllable in the solid by varying the scan rate. Such behavior has been previously observed in solution studies, but its control in solids has not been reported up to now. This study demonstrates how intermolecular cooperativity can allow multiple distinct magnetic behaviors, and provides some insight into how [HS-LS] ↔ [LS-HS] equilibria can be controlled in the solid state, which may assist in the design of next-generation logic and signaling devices.

Of the multitude of [FeIII(R-sal2323)]X complexes reported in the literature, only four have demonstrated spin crossover (SCO). Herein, we report four additional examples of thermal spin crossover in [FeIII(R-sal2323)]X complexes (where R = Br, NEt2, and X = ClO4 –, BF4 –). Magnetic susceptibility measurements reveal gradual, high-temperature spin transitions in all four compounds with onsets near room temperature. To investigate the emergence of SCO behaviors being observed in these compounds, a range of intramolecular and intermolecular structural parameters were examined. The effect that ligand substituents may have on the electronic environment, as well as the effect of counterions and various intermolecular interactions on the crystal packing, were investigated and compared to the literature of [FeIII(R-sal2323)]X compounds for which magnetic measurements are reported. This comparison found that neither intramolecular subtleties nor intermolecular interactions have a large impact on whether or not these compounds are SCO active. Instead, it is shown and proposed that many compounds in the [FeIII(R-sal2323)]X family may demonstrate SCO activity if measured to higher temperatures (above 300 K). This would provide a wide range of FeIII compounds that are SCO active near or above room temperature to be explored in future work.

A mixed valence tetranuclear iron complex [(Hpmide)FeII(NCSe)2FeIII(pmide)]2•5CH3OH (1) that undergoes oxidation and ligand exchange in the solid state (H2pmide = N-(2-pyridylmethyl)iminodiethanol). Upon air oxidation, 1 was converted into [(pmide)FeIII(NCSe)FeIII(pmide)]2(NCSe)2•2H2O (2),...

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.