Abstract
We have added the {ReIVX5}− (X=Br, Cl) synthon to a pocket‐based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an FeIII system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen‐based interaction allow us to modify the temperature and nature of the SCO event.
Highlights
Supramolecular chemistry is a rapidly expanding area of research,[1] having seen the award of two Nobel prizes within the last 30 years.[2]
In practice we found that the transition metal preferentially occupies pocket 2, giving a mer-coordinated system with the general formula [ReIVX5(m-Rpch)Fe(R’-Im)3] where H2Rpch has the formula (E)-N’-(2-hydroxy-3-R-benzylidene)pyrazine-2-carbohydrazide, and R’-Im has the formula 1R’-imidazole (Figure 1, Table 1)
In the original solvated sample, the longer Br···Br distances and Re-Br···Br angle blocks the formation of a bond (Br2···Br2’ = 4.663 , ffRe-Br···Br = 1608, RXB = 1.217), the loss of the solvent leads to reorientation of molecules in the crystal resulting in a significant shortening of the Br···Br distances, “switching on” a type I halogen bond (Br2···Br2’ = 3.733 , ffRe-Br···Br = 1768, RXB = 1.009).[22]
Summary
The shortest intermolecular Br···Br distance was 4.663 and is too long to be considered a halogen bond, and is unlikely to contribute to information exchange.[19] The N4O2 coordination sphere of the FeIII ion is often observed in SCO active compounds,[20] so this complex warranted further investigation and a structure was collected at 280 K, (1)w_280. Magnetic analysis of complex (1) using a SQUID magnetometer revealed an SCO active system with an incomplete transition occurring between 220 K and 90 K, from fully HS to only 22 % of molecules in the HS electronic configuration with T1/2 = 155 K (Figure 2) This initial result was surprising given that single crystal X-ray diffraction data had indicated a HS state at 100 K. Crystallographic data were collected at 140, 180, 230 and 280 K and by plotting the adjusted
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