A Brønsted‐Ligand‐Based Iron Complex as a Molecular Switch with Five Accessible States

Abstract

Am ononuclear Fe II complex, prepared with aB rønsted diacid ligand, H 2 L( H 2 L = 2-[5-phenyl-1H-pyrazole-3-yl] 6-benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states.T he spin crossover (SCO) complex, [Fe II (H 2 L) 2 ](BF 4 ) 2 (1 A ), exhibits abrupt spin transition at T 1/2 = 258 K, and treatment with base yields ad eprotonated analogue [Fe II -(HL) 2 ](1 B ), which shows gradual SCO above 350 K. Arange of Fe III analogues were also characterized.[ Fe III (HL)(H 2 L)]-(BF 4 )Cl (1 C )h as an S = 5/2 spin state,w hile the deprotonated complexes [Fe III (L)(HL)],( 1 D ), and (TEA)[Fe III (L) 2 ], (1 E ) exist in the low-spin S = 1/2 state.T he electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid-state.T he versatility of this simple mononuclear system illustrates howproton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.

Thermal and Light‐Activated Spin Crossover in Iron(III) qnal Complexes

“…In cases where this cooperativity is particularly high, the spin‐transition may be accompanied by a thermal hysteresis. It follows that design of SCO complexes with strong cooperativity requires the careful use of supramolecular contacts such as hydrogen and halogen bonding, and π–π stacking , , , …”

Molecular Sensors Operating by a Spin‐State Change in Solution: Application to Magnetic Resonance Imaging

“…In 2019, Shiga, Newton, Oshio and their co‐workers described a similar example of a ternary Fe(II) complex bearing two identical H 2 L ligands (H 2 L=2‐[5‐phenyl‐1H‐pyrazole‐3‐yl]‐6‐benzimidazole pyridine) (Scheme 11). [65] The complex shows a spin‐transition behavior upon cooling, but is resolutely high‐spin in acetonitrile at room temperature. Addition of 4 equivalents of the organic base DBU causes a partial switch to the low‐spin state thanks to the deprotonation of the two benzimidazole units.…”

One molecule, two states: Single molecular switch on metallic electrodes

“…For example, a mononuclear FeII complex shows five distinct magnetic states varying with the ligand-field strength after protonation, which enables higher data storage density. 176 However, when the magnetic organometallic complexes with bistable magnetic states are coupled to the metallic surfaces, the strong molecule-substrate coupling usually destroys the reversibility or even results in molecular dissociation, obstructing efficient and reliable control of the spintronic properties at the interface. 173,177,178 An alternative strategy has been implemented to protect the reversibility by integrating a switchable functional group (pyridine) into robust metalloporphyrin complexes on Ag (111).…”