Spin crossover phenomenon in a three-dimensional cyanido-bridged FeII–MoIV assembly

Abstract

We report a spin crossover material based on a cyanido-bridged FeII–MoIV assembly, FeII2[MoIV(CN)8](1-(3-pyridyl)ethanol)8⋅4H2O. This compound has a cubic crystal structure in the Ia3¯d space group and is composed of a three-dimensional cyanido-bridged FeII–MoIV coordination network with one crystallographic FeII site. It exhibits incomplete spin crossover, because 21% of the high-spin FeII sites (S = 2) changes to low-spin FeII sites (S = 0) in the temperature range between 200 and 50 K. Thermal hysteresis is not observed. Such an incomplete and gradual spin crossover is attributed to the elastic frustration between the high-spin and the low-spin FeII sites (e.g., alternating arrangement such as –HS–LS–HS–LS–).

Lowering the Water Oxidation Overpotential by Spin-Crossover in Cobalt Hexacyanoferrate

“…Prussian blue-type materials are known to easily undergo charge transfer and SCO. ,, In particular, the cobalt–iron Prussian blue analogue cobalt hexacyanoferrate (CoFe–PB, Figure ) has long been known to be a switchable molecular magnet. , The spin state of its Co constituents can easily be tuned by external stimuli, such as temperature, , light, − , pressure, and electric and other fields, that change its local electronic environment and surrounding crystal field. Furthermore, CoFe–PB was found to be a highly robust, easy-to-prepare, and efficient OER catalyst, which can operate in a wide range of pH (1 < pH < 13) and is made of Earth-abundant transition metals. − Given the versatility of its electronic structure, the idea that CoFe–PB may undergo SCO during OER is not far to seek.…”

“…Nonetheless, we do not only need to allow for electrochemical steps. We know that Prussian blue-type materials, and in particular CoFe–PB, , can easily undergo charge transfer and spin-crossover (SCO) under various conditions. ,,, So, we wondered if the LS *Co–OH resting state transitions to IS *Co–OH via SCO as schematized in Figure . To evaluate this possibility, we computed the minimum energy crossing points (MECP) between the potential energy surfaces (PES) of the two different magnetic states with the MECP program developed by Harvey et al , Table S4 shows the energies and properties of the MECPs for the *Co–OH intermediates of clusters (R) and (O) (as well as of the *Co–O and *Co–OOH intermediates).…”

“…In this mixed, SCO-assisted pathway, the entire OER is favorable at 1.5 V SHE . Moreover, IS *CoFe–OH is further stabilized with respect to LS *CoFe–OH by the applied electric field. , Also other stimuli such as temperature, light, or magnetic fields, could be used to increase the population of the IS configuration. − ,, …”

Guest-Induced Pore Breathing Controls Spin State in a Cyanido-Bridged Framework

“…The observed Fe-N bond lengths at 200 K are 2.080(2) Å for Fe-NC and 2.243(2) or 2.258(2) Å for Fe-Npyridine, in line with expectations for iron(II) in a high-spin (HS) state. [25][26][27][28][29][30][31][32][33][34] Octacyanidotungstate(IV) forms four cyanide bridges with iron(II), leaving four terminal cyanides engaged in hydrogen bonding with water molecules with a CN … O distance of 2.867(5) Å (Table S2). Crystallization water molecules do not leave empty spaces in the framework (no void spaces were detected for 1.3 Å probe radius, 35 while the kinetic diameter of water is estimated to be 2.65 Å).…”

“…The sudden decrease of the bond length in the 140-80 K temperature range is in line with the transition temperature determined from magnetic and Mössbauer measurements, and the variation of the bond length is consistent with observations for other SCO frameworks featuring [Fe II (py)4(μ-NC)2] moieties. [25][26][27][28][29][30][31][32][33] As the observed contraction of bond lengths falls in the typical range for similar assemblies, this raises the question about the source of unusually small ΔHHL. This could be explained by stabilization of the HS state or destabilization of the LS state.…”