Improved Cooperativity of Spin-Labile Iron(III) Centers by Self-Assembly in Solution

Abstract

We report on the markedly increased spin crossover activity of materials when self-assembled in solution. Spin crossover is directly related to long-range elastic interactions, typically observed in the solid state, and the transition may be gradual over a large temperature range. 1 Effects such as solvent cocrystallization often modify the transition in an unpredictable way. This is predominantly because spin transitions are crucially affected by intermolecular interactions beyond crystal packing effects, such as elastic strain. 2 Such intermolecular interactions are the classical domain to be engineered by supramolecular chemistry. 3 Specifically, the promotion of close metal-metal interactions may stimulate cooperative behavior, which is expected to result in more abrupt spin transitions. This concept has been implemented for engineering of spin crossover active materials in the solid state 4 and recently also in soluble nanoparticles. 5 Here, we have used a hydrophilic/lipophilic bias 6 for inducing a favorable arrangement of potentially spin-labile iron(III) centers. Supramolecular organization in solution leads, most remarkably, to a new type of magnetically active material for liquidphase processing.The Fe(sal 2 trien) complex 2a is known to undergo a gradual spin transition in the solid state. 7 Ligand functionalization and introduction of a lipophilic side chain was accomplished by selective alkylation of 4-hydroxysalicylaldehyde. Condensation of the aldehyde 1 with triethylenetetramine and subsequent metalation with iron(III) nitrate, either in situ or after isolation of the hexadentate ligand precursor, afforded the alkyl-tailed complexes 2b-d (Scheme 1). Metathesis of the noncoordinating anion was straightforward because of the different solubility properties of KPF 6 and the alkyltailed iron(III) complexes and gave, after extraction, complexes 3. A single crystal structure determination of complex 3c unambiguously confirmed the connectivity pattern ( Figure 1a). The bond lengths around the two crystallographically independent iron centers are diagnostic for high spin Fe III at 173 K. 1c The mutual cis coordination of the phenolate moieties arranges the alkyl chains in the same direction and maximizes intermolecular van der Waals interactions. As a consequence, the crystal packing displays apolar and polar layers consisting of the lipophilic side chains, and the charged metal coordination site and the PF 6 -anions, respectively ( Figure 1b).Despite the layered crystal packing, complex 3c does not exhibit thermochromic behavior in the solid state. Magnetic measurements indicate the absence of thermally induced spin crossover in the 30-300 K range and antiferromagnetism or zero-field splitting below 30 K. 1c Similar results have been obtained for complexes 2b-d. This suggests that alkyl functionalization in 2 generates spinstable Fe III centers. The spin stability of 2d in the solid state may also originate from the absence of favorable metal-metal interactions, as powder diffraction showed this compl...

A magnetic iron(iii) switch with controlled and adjustable thermal response for solution processing

“…While micrometre-sized superstructures reminiscent of tertiary protein structures were identified for the complexes with abrupt spin transition, 1e-1g, no such motif was found with 1c. The spherical shape of these microsize motifs is most evident in solutions of compound 1e, 18 and slightly less pronounced with 1f (average diameter 2.3 ± 0.9 μm). A lesser degree of aggregation into spherical particles is observed when the solutions are diluted, as illustrated for 1f and 1g (inset of Fig.…”

“…These molecular recognition sites require a minimum distance to the polar spin-labile site to be efficient for self-assembly. In our case, C 18 and longer units are appropriate, while C 12 units are too short. Tailoring of the spin crossover temperature is possible by adjusting the number of recognition sites per molecule or by variation of the molar concentration.…”

Inducing Spin Crossover in Amphiphilic Iron(III) Complexes

“…[13] Although alkylation of the phenolate rings was observed to quench SCO in the solid state, there was a marked enhancement in the sharpness of the transition in solution compared to the nonalkylated complex. Analysis of the crystalline packing in the C 8 amphiphile revealed a cis orientation for the lipophilic groups and a layering of the potentially spin-labile cations.…”

“…Analysis of the crystalline packing in the C 8 amphiphile revealed a cis orientation for the lipophilic groups and a layering of the potentially spin-labile cations. [13] We have therefore expanded our studies to change the morphology of the amphiphile in the solid state by moving the position of the alkylation to the secondary amino groups (Scheme 1). We now report the advantageous effect of this repositioning of the aliphatic chains on the magnetic switching properties.…”

Spin-state switches in molecular materials chemistry

“…This magnetic transition may be hysteretic, and it can also be "reverse" in character ¶. Some micellar assemblies 35 and gels 36 of spincrossover molecules have also been prepared, which undergo unexpectedly cooperative spin-state switching reflecting interaction of the switchable molecules with the semi-rigid supramolecular architectures. The multi-functional properties of spin-state switches have proven particularly useful for the production of prototype devices.…”