Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N′-Disubstituted Bis(pyrazol-3-yl)pyridines

Abstract

Here, we report a combined study of the effects of two chemical modifications to an N,N′-disubstituted bis­(pyrazol-3-yl)­pyridine (3-bpp) and of different solvents on the spin-crossover (SCO) behavior in otherwise high-spin iron­(II) complexes by solution NMR spectroscopy. The observed stabilization of the low-spin state by electron-withdrawing substituents in the two positions of the ligand that induce opposite electronic effects in SCO–active iron­(II) complexes of isomeric bis­(pyrazol-1-yl)­pyridines (1-bpp) was previously hidden by NH functionalities in 3-bpp precluding the molecular design of SCO compounds with this family of ligands. With the recent SCO-assisting substituent design, the uncovered trends converged toward the first iron­(II) complex of N,N′-disubstituted 3-bpp to undergo an almost complete SCO centered at room temperature in a less polar solvent of a high hydrogen-bond acceptor ability.

Mononuclear Fe(iii) complexes with 2,4-dichloro-6-((quinoline-8-ylimino)methyl)phenolate: synthesis, structure, and magnetic behavior

“…This means that the SCO behavior is related to crystal packing and solvent effects. 27 At 220 K, the μ eff (T ) value of 2 was ca. 2.77μ B , which is higher than that of the LS Fe(III) ions (S = 1/2) and close to the S = 1 spin state.…”

Optical Phenomena in Molecule-Based Magnetic Materials

“…Among successful strategies, one may list the synthesis of SCO nanoparticles, core-shell heterostructures, thin film nanocrystalline phases, or even molecule-based monolayers. − Moreover, when deposited on the surface, the SCO-active systems can modify the properties of the substrate upon the occurrence of the spin transition stimulated by the change of temperature, irradiation, and other factors, while the output is registered in the form of electrical or optical response. − When combining two magnetic phases into heterostructures, the impact of SCO occurring in the first component upon the properties of the second was noticed . Generalizing to some extent, from the point of the application horizon, the materials showing the near-room-temperature spin crossover effect are the most suitable for sensing, including temperature sensing in solution, sensing of solvent vapors and gases, or mechanical strain sensing in the solid phase. − Then the related spin transition may or even should occur within a low-cooperativity process, to observe rather gradual changes within the optical absorption or the emission in the case of luminescent SCO materials. On the other hand, for the design of electronic devices, mostly relying on the modified conductivity between the HS and LS states, the presence of thermal SCO hysteresis, in the best case wide and repeatable within several thermal cycles, is required, which can be ensured by strong elastic interactions within the framework .…”

Fine-tuning of the spin-crossover properties of Fe(iii) complexes via ligand design

“…Many of the studies on ligand substituent effects on Fe(II) complexes that have appeared in the literature over the last few years report solution measurements and/or Density Functional Theory (DFT) calculations of isolated complexes. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] These techniques allow for an evaluation of the SCO properties of a given complex in the absence of crystal-packing effects and they thus allow for an unambiguous assessment of how substituents affect T 1/2 through the modulation of the electron density of the ligands and/or intramolecular steric interactions. The ability to predict in a quick and accurate way the effect of a ligand substituent on T 1/2 is key for the design of new SCO complexes with tailored switching temperatures.…”

“…Many of the studies on ligand substituent effects on Fe( ii ) complexes that have appeared in the literature over the last few years report solution measurements and/or Density Functional Theory (DFT) calculations of isolated complexes. 29–43…”