Dimerized Spin‐Crossover Iron(II) Complexes as Supramolecular Anion Capsules

Abstract

Two dimerized iron(II) complexes, [{Fe(L)(HL)2}{Fe(HL)3}](PF6)(BF4)2·5CH3CN·4H2O (1) and [{Fe(L)(HL)2}{Fe(HL)3}](PF6)(FeCl4)2 (2) {HL = 3‐(2‐pyridyl)‐5‐[4‐(diphenylamino)phenyl]‐1H‐pyrazole}, supported by triphenylamine‐derivatized bidentate ligands and encapsulating hexafluorophosphate ions were synthesized. The complex iron ions have octahedral coordination geometries in which they interact with six nitrogen atoms from three bidentate ligands. Both compounds have capsule‐like dimerized supramolecular structures, in which the arrangements of the bulky ligands of two discrete mononuclear complexes create a central cavity in which a single hexafluorophosphate ion is captured. The bidentate ligands interact with the anionic guest molecules through the hydrogen atoms of the ligand pyrazole moieties. Cryomagnetic studies reveal that both compounds show spin‐crossover behavior. Complex 1 showed quasi‐reversible spin‐crossover around 200–300 K, hindered by desolvation effects, whereas 2 showed reversible gradual spin‐crossover behavior. The electronic states of the iron ions resulting from the spin‐crossover were confirmed by X‐ray structure analyses and Mössbauer spectra.

A Simple Approach for Predicting the Spin State of Homoleptic Fe(II) Tris-diimine Complexes

“…The value of 2.78 Å appears to be critical, as all ligands in which the N–N separation exceeds this value furnish SCO Fe­(II) complexes (Figure ). The only exception is given by ligand 12 , which forms SCO complexes even though d (N–N) = 2.747 Å. An illustrative comparison can be made between ligands 16 and 20 , which differ by the presence of the NH group or S atom, respectively, in the five-member ring.…”

Heteroleptic iron(ii) complexes with naphthoquinone-type ligands

“…Recently, we reported a range of functional mononuclear and polynuclear supramolecular spin crossover compounds towards the preparation of responsive systems for switchable molecular materials. 10 Here we present our recent findings on the synthesis and switchable properties of heteroleptic iron(II) complexes with naphthoquinone-type ligands, HL (2-(pyridin-2-yl)-1Hnaphtho [2,3-d]imidazole -4,9-dione), and a range of capping ligands (Scheme 1). The naphthoquinone-type bidentate ligand HL was selected due to its monoanionic bidentate nature and was expected to provide an adequate ligand field for the Fe(II) ion.…”

A ferric guest inside a spin crossover ferrous helicate

“…We have recently designed the synthesis of ferrous [Fe 2 L1 3 ] 4+ supramolecular helicates where L1 comprises two pyrazolylpyridine (pzpy) fragments linked by a phenylene group. 18,19 The ability of these fragments to engender SCO on Fe(II) 20,21 provides an entry for the resulting helicates to this switchable property. In addition, the assembly exhibits a cavity able to host either a Cl À or a Br À guest as a means of tuning the SCO properties of the host in the solid state and in solution.…”