Large Thermal Hysteresis for Iron(II) Spin Crossover
Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide
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Abstract
A new series of iron(II)
1D coordination polymers with the general formula [FeL1(pina)]·<i>x</i>solvent with L1 being a tetradentate N<sub>2</sub>O<sub>2</sub><sup>2–</sup> coordinating Schiff-base-like ligand
[([3,3′]-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-<i>N</i>,<i>N</i>′,<i>O</i><sup>2</sup>,<i>O</i><sup>2</sup>′], and pina being a bridging
axial ligand <i>N</i>-(pyrid-4-yl)isonicotinamide, are discussed.
The X-ray crystal structure of [FeL1(pina)]·2MeOH was solved
for the low-spin state. The compound crystallizes in the monoclinic
space group <i>P</i>2<sub>1</sub><i>/c</i>, and
the analysis of the crystal packing reveals the formation of a hydrogen
bond network where additional methanol molecules are included. Different
magnetic properties are observed for the seven samples analyzed, depending
on the nature of the included solvent molecules. The widest hysteresis
loop is observed for a fine crystalline sample of composition [FeL1(pina)]·<i>x</i>H<sub>2</sub>O/MeOH. The 88 K wide thermal hysteresis loop
(<i>T</i><sub>1/2↑</sub> = 328 K and <i>T</i><sub>1/2↓</sub> = 240 K) is centered around room temperature
and can be repeated without of a loss of the spin transition properties.
For the single crystals of [FeL1(pina)]·2MeOH, a 51 K wide hysteresis
loop is observed (<i>T</i><sub>1/2↑</sub> = 296 K
and <i>T</i><sub>1/2↓</sub> = 245 K) that is also
stable for several cycles. For a powder sample of [FeL1(pina)]·0.5H<sub>2</sub>O·0.5MeOH a cooperative spin transition with a 46 K wide
hysteresis loop around room temperature is observed (<i>T</i><sub>1/2↑</sub> = 321 K and <i>T</i><sub>1/2↓</sub> = 275 K). This compound was further investigated using Mössbauer
spectroscopy and DSC. Both methods reveal that, in the cooling mode,
the spin transition is accompanied by a phase transition while in
the heating mode a loss of the included methanol is observed that
leads to a loss of the spin transition properties. These results show
that the pina ligand was used successfully in a crystal-engineering-like
approach to generate 1D coordination polymers and improve their spin
crossover properties
