The First Use of a ReX5 Synthon to Modulate FeIII Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions

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.


Experimental Details General Remarks
Unless otherwise stated, all reagents were obtained from commercial sources and were used as received without further purification. All reactions were carried out under aerobic conditions. Elemental analyses (CHN) were performed using an Elementar Vario EL Analyzer. FTIR spectra were measured as KBr pellets over 4000-400 cm -1 on a Perkin Elmer Spectrum One spectrometer. Magnetic susceptibility data (2-300K) were collected on powdered polycrystalline samples on a Quantum Design MPMS-XL SQUID magnetometer under an applied magnetic field of 0.1 T (unless otherwise stated). All data were corrected from the sample holder contribution and the diamagnetism of the samples estimated from Pascal's constants.

Crystallography Collection and Refinement
Data was either measured at the SCD beamline of the ANKA Synchrotron light source, Karlsruhe, on a Bruker SMART Apex diffractometer using Si-monochromated radiation with  = 0.80000 Å, or on a Rigaku Oxford Diffraction SuperNova E diffractometer equipped with Cu-Ka and Mo-Ka microfocus sources. All data were corrected semi-empirically for absorption. Structures were solved using SHELXT [G.M. Sheldrick, Acta Cryst. A71, 3-8 (2015)] and full-matrix least-squares refinement with anisotropic thermal parameters was carried out using SHELXL [G.M. Sheldrick, Acta Cryst. C71, 3-8 (2015)]. For further details of the refinements of the structures, see _refine_special_details in the individual CIFs. Crystallographic data for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1978010-1978029. Copies of the data can be obtained, free of charge, from https://www.ccdc.cam.ac.uk/structures/

Details of adjusted unit cell volume (figure 2 of main text)
When looking at the unit cell volume of crystalline SCO materials across a range of temperatures transitions can clearly be followed, however, by normalising the volume to account for thermal expansion transitions becomes more defined. With complex (1) we investigated the analogous and isostructural Co III compound (5), which remains LS across all measured ranges, to gain a baseline for thermal expansion. At 100 K (5d_100) the unit cell volume is 7153 Å 3 , upon heating by 130 K to a temperature of 230 K (5d_230) we see expansion to 7280 Å 3 , which is a difference of 127 Å 3 or ~1 Å 3 K -1 . By subtracting this from the changes in unit cell volume observed for (1) we see a much cleaner fit, particularly for the values after 220K where the volume continues to change due to thermal expansion, but the SCO event has finished (see below).  (1) showing the significantly improved fit when a correction of 1 Å 3 K -1 is applied to the raw change in unit cell volume.

Magnetic Data
Magnetic Data for the Ligands (9), (10), and (11) The χT product of the mononuclear compound (9) at 300 K and an applied field of 0.1 T is 1.45 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.7 and 1.9. The χT product remains near constant down to 50 K where the value drops to 0.99 cm 3 K mol -1 at 4 K. Interestingly the increase to 1.05 cm 3 K mol -1 as the temperature was further decreased to 1.8 may indicate weak ferromagnetism produced by spin canting phenomena. Collecting the data again with an applied field of 1 T (up from 0.1 t) supresses a possible canted structure. The χT product of the mononuclear compound (10) at 300 K and an applied field of 0.1 T is 1.60 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.7 and 1.9. The χT product remains near constant down to 50 K where the value drops to 0.75 cm 3 K mol -1 at 1.8 K. The drastic decrease in the xT value is attributed to zero field splitting of the Re IV complex and potential weak antiferromagnetic interactions between metal centers. The χT product of the mononuclear compound (9) at 300 K and an applied field of 0.1 T is 1.77 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.5 and 1.9. The χT product remains near constant down to 50 K where the value drops to 0.64 cm 3 K mol -1 at 1.8 K. The plot χ against T shows no maximum which is infers only weak antiferromagnetic interactions are occurring. The drastic decrease in the xT value is attributed to zero field splitting of the Re IV complex and potential weak antiferromagnetic interactions between metal centers.
Magnetic Data for the Fe III containing complexes (1) -(4) For further details of the magnetic properties of these compounds please see the main text. The % low spin state is approximated using the following equation: Where XS is the maximum χT value, XRe is the χT value for the Re complexes, and XF is the χT value at the end of the spin transition. The 0.375 cm 3 K mol -1 value is that expected for a magnetically isolated Fe III in the low spin state.
This equation corrects for the contribution from the Re IV spin carrier and assumes no interaction between the Fe III and Re IV centers.
Magnetic Data for the Co III containing complexes (5) -(8) The χT product of the Co III -Re IV compound (5) at 300 K and an applied field of 0.1 T is 1.64 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.8 and 1.9, with a low spin (S = 0) Co III ion. The χT product remains near constant down to 50 K where the value drops to 0.40 cm 3 K mol -1 at 1.8 K. The drop below 1.00 cm 3 K mol -1 is due to the zero-field splitting of the Re IV ion and the existence of these interactions.
The χT product of the Co III -Re IV compound (6) at 300 K and an applied field of 0.1 T is 1.53 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.8 and 1.9, with a low spin (S = 0) Co III ion. The χT product remains near constant down to 50 K where the value drops to 0.38 cm 3 K mol -1 at 1.8 K. The drastic decrease in the xT value is attributed to zero field splitting of the Re IV complex and potential weak antiferromagnetic interactions between metal centers. The drop below 1.00 cm3 K mol-1 indicates the existence of these interactions. The χT product of the Co III -Re IV compound (7) at 300 K and an applied field of 0.1 T is 1.59 cm 3 K mol -1 . This is the expected value for a magnetically isolated Re IV complex with a spin ground state of 3/2 and a g-factor between 1.8 and 1.9, with a low spin (S = 0) Co III ion. The χT product remains near constant down to 50 K where the value drops to 0.38 cm 3 K mol -1 at 1.8 K. The drastic decrease in the xT value is attributed to zero field splitting of the Re IV complex and potential weak antiferromagnetic interactions between metal centers. The drop below 1.00 cm3 K mol-1 indicates the existence of these interactions.