Intramolecular Electron Transfers in a Series of [Co₂Fe₂] Tetranuclear Complexes

Abstract

Discrete cyanide-bridged Co−Fe multinuclear complexes can be considered as functional units of bulk Co− Fe Prussian blue analogues, and they have been recognized as a new class of switching molecules in the last decade. The switching property of the cyanide-bridged Co−Fe complexes is based on intramolecular electron transfers between Co and Fe ions, and we herein refer to this phenomenon as an electron transfer-coupled spin transition (ETCST). Although there have been numerous reports on the complexes exhibiting ETCST behavior, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here the syntheses and thermal ETCST behavior both in the solid state and solution for a series of tetranuclear [Co 2 Fe 2 ] complexes, [Co 2 Fe 2 (CN) 6 (L1) 2 (L2) 4 ]X 2 (L1 and L2: tri-and bidentate capping ligands for Fe and Co ions, X: counteranions). All complexes showed thermal ETCST equilibrium between high-spin ([(hs-Co II ) 2 (ls-Fe III ) 2 ]) and low-spin ([(ls-Co III ) 2 (ls-Fe II ) 2 ]) states in butyronitrile, and the equilibrium temperatures (T 1/2 ) showed systematic shifts by chemical modifications and chemical stimuli. The T 1/2 values were correlated with the redox potential differences (ΔE) of the Fe and Co ions in the constituent units, and the larger ΔE values led to the lower T 1/2 . The present result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules.

Molecular Prussian Blue Analogues: From Bulk to Molecules and Low-dimensional Aggregates

“…6 shows hysteretic thermal ETCST due to strong cooperative interactions in the crystal lattice, while no ETCST behavior is observed in 7, which was caused by the different intermolecular interactions in the crystal packing ( Figure 5). 19 6 and 7 in BuCN show similar thermal ETCST behavior, but at different equilibrium temperature (T 1/2 = 175 and 161 K for 6 and 7, respectively). Because of the absence of intermolecular interactions in solution, the different ETCST behavior in BuCN is caused by substituent effects of L2.…”

Effect of Intermolecular Interactions on Metal‐to‐Metal Charge Transfer: A Combined Experimental and Theoretical Investigation

“…[21][22][23][24][25] Moreover,f ormation and breakage of hydrogen bonds between solvent molecules and the metallocyanate moiety can also tune the redox potential of the metal ions, leading to solvent-switchable MMCT behavior. [21,[26][27][28][29][30][31][32] It should be also noted that some cyanide-bridged {FeCo} systems show different MMCT behavior in the solid and solution states, [24,33,34] indicating that the MMCT behavior may depend on the state of aggregation, in which intermolecular interactions play ac rucial role.T herefore,u nderstanding the effect of intermolecular interactions on MMCT is of pivotal importance for applications in molecular switches, sensors,a nd storage devices via fabrication of molecular arrays on various substrates.H owever, the construction of solvent-free systems with the same molecular skeleton showing MMCT and tunable intermolecular interactions still remains ac hallenge.T oa chieve this goal, we utilized extended p-conjugation ligands to introduce flexible p···p interactions,w hose distances could vary in the continuous range of 3.3-3.8 , [35] thus providing ar ational strategy to tune intermolecular interactions.M oreover,t he variation in p···p interactions is expected to induce changes in coordination bond lengths and ligand field strength, which can alter the redox potential of metal centers,r esulting in different MMCT behavior.…”

Pressure‐Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis

“…In practice, this energy difference is correlated to the differences in redox potential between the Fe III/II and Co III/II couples. [17] The diamagnetic/paramagnetic transition can thus be described using an Ising-like model, whose Hamiltonian [Eq. (2)] is…”