Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds

Abstract

This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 1–3 of the general form [FeII2L3](BF4)4(CH3CN)n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively, and 1 is the complex when L = L1, 2 when L = L2 and 3 when L = L3 (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics. 2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T1/2 value of the two-steps in the solvated materials was equivalent to the single T1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 1–3 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J. 2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem. 2006, 3333–3339).

Supramolecular Modulation of Spin Crossover in an Fe(II) Dinuclear Triple Helicate

“…Similarly, the χ m T product plateaus until 25 K, where it drops sharply again due to zero-field splitting. Previous studies performed on analogous helicate compounds demonstrated a similar trend both before and after extended heating at high temperatures, exhibiting a one-step profile before the high-temperature isotherm and a two-step transition afterward. , Furthermore, in comparison to the PF 6 – , BF4 4 – , and ClO 4 – salts of 1 previously reported by Tuna et al, the spin transition of 1 (an I – salt) was similar in nature to that of the ClO 4 – salt, while that of the PF 6 – salt was similar to that of 3 .…”

Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

“…Here, we will attempt to correlate the observed solid-state VT behavior of dinuclear cobalt bis­(dioxolene) complexes with the solution-state electrochemistry. This comparison neglects the effects of packing in the solid-state, which are important for dinuclear SCO complexes, − as well as the effects of solvent and electrolyte on electrochemistry, so we compare values recorded in MeCN with Bu 4 NPF 6 where possible . The evaluation is supported by the excellent agreement of gas phase DFT predictions and solid-state magnetic data as well as the observation of similar two-step VT interconversions for solid 1a and 1b and for 1b in solution …”

“…Multistep transitions of molecular origin may be accessed with molecules on surfaces, in solution or as noncrystalline materials, and are therefore more versatile than traditional multistep switchable materials that depend on crystallographically unique complexes or interactions in extended solids. ,, A simple example of a two-step interconversion in a molecular species is a dinuclear metal complex that exhibits spin crossover (SCO) at each of the metal centers . The origin of the two-step SCO transitions observed for some dinuclear Fe­(II) complexes has been investigated extensively and has been linked to the use of constrained ligands to increase electronic communication, intermolecular interactions, − and the stability of the mixed low spin-high spin state . The current understanding is that small changes in one metal site, upon SCO at the other site, are insufficient to give rise to the two-step interconversion, , and instead the transition profile is directed by local electronic effects, which are difficult to control …”

Iron(II) Spin Crossover Complex with the 1,2,3-Triazole-Containing Linear Pentadentate Schiff-Base Ligand and the MeCN Monodentate Ligand

“…To sum it all up, these results demonstrate that the irreversible spin conversion at around 400 K in the first heating process is related to the crystal-to-amorphous transformation associated with the loss of lattice MeCN solvent. Desolvation effects are reported in a variety of SCO systems in both positive (occurrence of abrupt and/or hysteretic spin transition) and negative (disappearance of SCO) ways [78][79][80][81][82][83][84] but a concomitant crystal-to-amorphous transformation is rarely observed [85]. Temperature dependence of the χMT product for 1 at a sweep rate of 2 K min −1 .…”