Spin Crossover Induced by Changing the Identity of the Secondary Metal Ion from Pd^II to Ni^II in a Face‐Centered Fe^II₈M^II₆ Cubic Cage**

Abstract

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.

Investigating the Conformations of a Family of [M2L3]4+ Helicates Using Single Crystal X-ray Diffraction

“…As such, the pitch and yaw associated with each chelate ring give rise to a complementary orientation of the secondary bonding axis. Parameters describing the orientation of secondary bonding axes have been employed to investigate the effect of the size and electronic configuration of the primary metal ion, as occurred in our previous work [ 20 , 21 , 28 ]. The “secondary bond axis” (2BA) parameter describes the angular deviation of the secondary coordination axis (as defined by the axis between N and opposite C in the pyridyl group) from the intermetallic axis [ 28 ].…”

“…The “secondary bond axis” (2BA) parameter describes the angular deviation of the secondary coordination axis (as defined by the axis between N and opposite C in the pyridyl group) from the intermetallic axis [ 28 ]. On the other hand, the “mutual bond axis angle” (MBA) describes the orientation of secondary coordination axes (proximal to one metal site) to each other [ 20 , 21 ]. These parameters provide complementary information and may be used to predict the derivative supramolecular topologies that might be targeted by these metalloligands.…”

“…The approach is to design ligands that form a complex with a primary metal while keeping additional (secondary) coordination donor sites free. The primary metal acts to direct the coordination vectors of the secondary binding sites so that they are aligned for coordination with secondary metal centers in forming the final structure [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. Using the metalloligand approach, it is thus usually possible to transfer, at least in part, the physical properties of the metalloligand units, such as luminescence and various magnetic behaviors, to the larger final architecture.…”

“…To date, the majority of the metalloligands that have been reported are mononuclear, with higher nuclearity metalloligands being comparatively rare [ 22 , 23 , 24 , 25 , 26 ]. Currently, there are a few examples of dinuclear metalloligands [ 20 , 21 , 22 ], as well as helical metalloligands [ 26 ]. However, to the best of our knowledge, no dinuclear triple helicate metalloligands have so far been incorporated in the synthesis of larger supramolecular architectures, though potential candidates have been identified [ 27 ].…”

“…We have recently shown how the magnetic properties of a tripodal mononuclear Fe(II) metalloligand, originally in the high-spin state, undergoes a spin transition when a cubic cage is formed using Ni(II) as the secondary metal ion [ 20 ]. In contrast, the substitution of Pd(II) for Ni(II) resulted in the cage maintaining its Fe(II) centers high-spin [ 21 ].…”