A Case Study of Mechanical Strain in Supramolecular Complexes to Manipulate the Spin State of Iron(II) Centres

Abstract

Two novel supramolecular meso-helicates have been synthesized and characterized by single-crystal X-ray diffraction, variable-temperature magnetic susceptibility measurements, NMR and UV/Vis spectroscopy, mass spectrometry and density functional theory (DFT) simulations. Both compounds show a considerably high stabilization of the high-spin state of the metal centres compared with other compounds with similar types of ligands. Whereas the pyridyl complex [Fe 2 L 2 3 ] [L 2 = N,NЈ-bis(pyridin-2-ylmethylene)benzene-1,3diamine] exhibits the beginning of a spin transition at around 350 K, the imidazolyl complex [Fe 2 L 1 3 ] [L 1 = N,NЈ-bis(1H-[a] Kekulé-

Subcomponent Exchange Transforms an FeII4L4 Cage from High- to Low-Spin, Switching Guest Release in a Two-Cage System

“…In this study, we envisaged a new approach whereby a chemical stimulus transforms a high-spin Fe II 4 L 4 cage into a low-spin analog through exchange of a more bulky aldehyde subcomponent for a less bulky one. Others 10 and our group 11 have reported Fe II cages and helicates that undergo spin-crossover 12 induced by heat and light. Mononuclear Fe II complexes incorporating 2-formyl-6-methylpyridine undergo spin-crossover, 13 attributed to a steric clash between methyl groups and adjacent pyridine rings destabilizing the low-spin state relative to the high-spin state.…”

Small Structural Variations Have Large Effects on the Assembly Properties and Spin State of Room Temperature High Spin Fe(II) Iminopyridine Cages

“…These reversibly formed systems display multiple different mechanisms of ligand exchange, including transimination as well as M–L bond cleavage, which suggests they have potential as stimuli-responsive magnets. However, there are far fewer examples of room temperature, high spin Fe­(II) helicates, − and especially the larger cage complexes − than their low spin counterparts. Fe-iminopyridyl cages are commonly (if not always) diamagnetic, − in contrast to Fe­(II) complexes with other N and O bidentate ligands. − , The strong N–Fe–N coordination that allows the formation of highly complex yet stable cage assemblies also favors low spin Fe­(II).…”

Guest‐, Light‐ and Thermally‐Modulated Spin Crossover in [FeII2] Supramolecular Helicates

“…These species are amenable to rational design; the right choice of metals and polydentate ligands allows the prediction and formation of helicates with different numbers of metals and strands . In addition, with the right choice of ligand donors, it is possible to chemically tune the crystal field around the metals (e.g., Fe II ) of the supramolecular assembly in order to facilitate the occurrence of SCO . Some of the reported examples show evidence that the spin‐active centers can be brought from the LS to a metastable HS state in response to light irradiation by the LIESST (light‐induced excited spin‐state trapping) effect .…”

“…[16,17] In addition, with the right choice of ligand donors, it is possible to chemically tune the crystal field around the metals (e.g.,F e II )o ft he supramolecular assembly in ordert of acilitate the occurrence of SCO. [18][19][20][21][22] Some of the reported examples showe vidence that the spin-active centers can be brought from the LS to am etastable HS state in response to light irradiation by the LIESST (light-induced exciteds pin-statet rapping)e ffect. [23] With am ore sophisticated design of ligands, metallohelicates are also amenable to selectively capture guest species inside them, [24][25][26][27] which offers av aluable opportunity for modulating the functional properties of their components,s uch as the switchingb ehavior of potential SCO metals.…”