Precise control of the degree and regioselectivity of functionalization in nitro- and amino-functionalized di(trispyrazolylborato)iron(II) spin crossover complexes

30 June 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Di(trispyrazolylborato)iron(II) ([Tp2Fe]) complexes represent one of the most robust class of spin crossover complexes. Their stability renders them particularly suitable for integration in nanoscale devices, e.g. as sensors or information storage units. Prior studies of the functionalization of those derivatives have been limited to the steric effect alkyl groups in position 3. The pyrazole exchange reaction between nitropyrazole and either trispyrazolylborate or its iron complex allows the regioselective installation of nitro substituents in positions 3, 4 and 5 of [Tp2Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of di- and tetra-3-NO2 substituted complexes, all derivatives display spin crossover properties in the solid state, with transition temperatures ranging from 180 to 380 K and showing different degree of abruptness, but no hysteresis. The Slichter-Drickamer model was used to extract empirical thermodynamic transition parameters, allowing a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substitution on the magnetic properties of the complexes. Steric effects dominate for substitution in position 3, but electronic effects are significant for the other positions.

Keywords

trispyrazolylborate
spin crossover
iron(II) complexes

Supplementary materials

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Title
Supplementary materials for Precise control of the degree and regioselectivity of functionalization in nitro- and amino-functionalized di(trispyrazolylborato)iron(II) spin crossover complexes
Description
Molecular structures, crystallographic parameters, crystal packing diagrams, IR spectra, UV-vis spectra, TGA curves, SQUID magnetization data and fits for all reported compounds.
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