Re‐Appearance of Cooperativity in Ultra‐Small Spin‐Crossover [Fe(pz){Ni(CN)₄}] Nanoparticles

Abstract

A reverse nanoemulsion technique was used for the elaboration of [Fe(pz){Ni(CN)4}] nanoparticles. Low-temperature micellar exchange made it possible to elaborate ultra-small nanoparticles with sizes down to 2 nm. When decreasing the size of the particles from 110 to 12 nm the spin transition shifts to lower temperatures, becomes gradual, and the hysteresis shrinks. On the other hand, a re-opening of the hysteresis was observed for smaller (2 nm) particles. A detailed (57)Fe Mössbauer spectroscopy analysis was used to correlate this unusual phenomenon to the modification of the stiffness of the nanoparticles thanks to the determination of their Debye temperature.

Spin-state switches in molecular materials chemistry

“…That was attributed to the influence of the relatively hard particle surface, which contains most of the atoms in a particle at such small scales. 30 Monolayer and sub-monolayer films of spin-crossover centres can also be prepared, usually by vacuum deposition methods. 29 Individual high-spin and low-spin molecules can be distinguished on gold surfaces by STM techniques, although many studies report that such molecules cannot be switched between their spin states under an STM tip below a particular surface coverage.…”

Mössbauer Spectroscopic Study on Hofmann-like Coordination Polymer Fe(4-Clpy)2[Ni(CN)4]

“…[2] The minor site 2 parameters with a large ΔEQ value as 1.31 mm s -1 at R.T. is also probably due to the FeN5(OH2) or FeN4(OH2)2 environment. The FeN5O core consists of an octahedral iron(II) [36] Taking into account of particle sizes of sample 1, the site 2 doublet may be ascribed to the iron(II) units located at the nanocrystals edge as they must have coordinative defects, that is, coordinatively unsaturated sites or coordinated terminal water molecules.…”

“…The site 1 and the site 2 are in iron(II) high spin states. Considering both the Mössbauer parameters and/or the structural data for Fe(py)2Ni(CN)4, [7,19] Fe(3-Cl-py)2Ni(CN)4, [13] Fe(3-Methyl-py)2Ni(CN)4, [14] Fe(3-Fpy)2Ni(CN)4 [17,20] and other related compounds, [2,15,16,21,27,31,32,47] the iron(II) ions in the site 1 of the sample 1 have six coordination with four nitrogen atoms of cyano groups and two nitrogen of 4-Cl-pyridine ligands. The quadrupole splitting of the major site 1 in the sample Fe(4-Clpy)2Ni(CN)4 at R.T. is smaller than that of the sample Fe(3-Cl-py)2Ni(CN)4 at R.T. because the symmetry around iron(II) in Fe(4-Cl-py)2Ni(CN)4 is higher than that in Fe(3-Clpy)2Ni(CN)4.…”

“…Recently 57 Fe Mössbauer spectroscopy has provided very powerful information on re-appearance of cooperativity in ultrasmall spin crossover Hofmann-type Fe(pz)[Ni(CN)4] nanoparticles. [2] 57 Fe Mössbauer spectroscopy and SQUID technique in 1996 revealed that the original Hofmann pyridine coordination polymer Fe(py)2Ni(CN)4 shows iron(II) SCO with one step transition containing hysteresis. [7] It should be noted that iron(II) ions in the Hofmann-type clathrate Fe(NH3)2Ni(CN)4 · 2C6H6 are in high spin states at R.T. and T = 78 K, showing the clathrate exhibits no SCO behavior.…”

Mössbauer spectroscopic study on spin crossover coordination polymer Fe(3-Clpy)2[Pd(CN)4]

“…Recently 57 Fe Mössbauer spectroscopy has provided very powerful information on re-appearance of cooperativity in ultra-small spin crossover Hofmanntype Fe(pz)[Ni (CN) 4 ] nanoparticles [2]. The original Hofmann pyridine coordination polymer Fe(py) 2 Ni(CN) 4 shows iron(II) SCO behaviour, revealed by 57 Fe Mössbauer spectroscopy and SQUID technique [3]. Pressure tuning Raman spectroscopy of the prototype compound Fe(py) 2 Ni(CN) 4 has been reported [4].…”