Polynuclear heterobimetallic coordination cages in which different metal cations are connected within a ligand scaffold are known to adopt a variety of polyhedral architectures, many of which display interesting functions. Within the extensive array of coordination cages incorporating Fe(II) centres reported so far, the majority contain low-spin (LS) Fe(II), with high-spin (HS) Fe(II) being less common. Herein, we present the synthesis and characterisation of a new tetradecanuclear heterobimetallic [Fe8Pd6L8](BF4]28 (1) cubic cage utilising the metalloligand approach. Use of the tripodal tris-imidazolimine derivative (2) permitted the formation of the tripodal HS Fe(II) metalloligand [FeL](BF4)2·CH3OH (3) that was subsequently used to form the coordination cage 1. Magnetic and structural analyses gave insight into the manner in which the HS environment of the metalloligand was transferred into the cage architecture along with the structural changes that accompanied its occupancy of the eight corners of the discrete cubic structure.

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.

We present five new dinuclear triple helicate compounds of types [Mn2L3](ClO4)4, [Co2L3](BF4)4, [Ni2L3](BF4)4, [Cu2L3](BF4)4, and [Zn2L3](BF4)4, where L is a previously reported semi-rigid ligand incorporating two α-diimine primary donor groups and two secondary 4-pyridyl donor groups. All complexes have been characterized in both solution and the solid state. Single crystal X-ray diffraction studies were used to probe the variation in the respective helical structures as the coordinated metal ion was altered, including the effect on the orientations of the secondary binding domains. The influence of the metal ion size, the spin state in the case of Fe(II), and the presence of Jahn-Teller distortions on the overall helical structure has been investigated. These results form a basis for the design and construction of new large metallosupramolecular architectures which manifest properties associated with the constituent helical metalloligand units.

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.