Complete post-synthetic modification of a spin crossover complex

Article abstract of DOI:10.1039/c9dt03587g

The post-synthetic reaction between p-anisaldehyde and the spin-crossover compound [Fe(NH₂-trz)₃](NO₃)₂ was explored, obtaining different degrees of transformation from 23% to full conversion by varying the reaction time. The post-synthetic SCO complexes obtained were studied by magnetometry, powder X-ray diffraction (PXRD), elemental analysis, solid state NMR and IR and compared with the corresponding compounds obtained by direct synthetic routes, revealing new spin crossover properties.

Full text of DOI:10.1039/c9dt03587g

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Complete post-synthetic modification of a spin
crossover complex†
Cite this: Dalton Trans., 2019, 48,
16853
Alejandro Enríquez-Cabrera,
Azzedine Bousseksou*
Lucie Routaboul, Lionel Salmon * and
Received 5th September 2019,
Accepted 22nd October 2019
DOI: 10.1039/c9dt03587g
rsc.li/dalton
The post-synthetic reaction between p-anisaldehyde and the spin- increase of the spin crossover temperature and new lumine-
crossover compound [Fe(NH2-trz)3](NO3)2 was explored, obtaining scence properties.
different degrees of transformation from 23% to full conversion by
In the present paper, we wanted to demonstrate that this
varying the reaction time. The post-synthetic SCO complexes post chemical transformation can be fully achieved and can
obtained were studied by magnetometry, powder X-ray diffraction generate spin crossover compounds whose physical properties
(PXRD), elemental analysis, solid state NMR and IR and compared can be different in comparison with those of the corres-
with the corresponding compounds obtained by direct synthetic ponding complexes synthesized by direct methods, opening
routes, revealing new spin crossover properties.
the way towards new routes for SCO materials.
In order to integrate such materials into devices it is necess-
ary that they undergo a transition above room temperature, for
which we started with the complex [Fe(NH2-trz)3](NO3)2 (1 with
NH2-trz = 4-amino-1,2,4-triazole) which in our case has T1/2↑ =
308 K and T1/2↓ = 338 K. Under the experimental conditions
(see the ESI†), the post-synthetic methodology developed in
this work leads to the formation of a Schiff base by the reac-
tion of 1 with 12 eq. of p-anisaldehyde in ethanol at 35 °C
upon increasing the time: 3 h (2), 6 h (3), 12 h (4) and 72 h (5)
(Scheme 1). The final products were analysed by IR to confirm
the presence of imine by its signature peak at 1604 cm−1
(CvN), and the amount of substitution was determined by
elemental analyses resulting in 23.2% (2), 58.6% (3), 78.5% (4)
and 100% (5). It is interesting to notice that an analogous
functionalization reaction in 1,2-dichloroethane was unsuc-
cessful, leading instead to the decomposition/oxidation of the
SCO materials have attracted much attention in the past few
years due to their integration into different devices1 for
different applications in photonics,2 electronics3,4 and mech-
anics.5 In order to develop such applications, specific SCO
behaviour or transition temperatures are expected, while par-
ticular attention is paid to the nanostructuration of the SCO
materials.6,7 The post-synthetic modification (PSM) involving
the chemical transformation or exchange of pre-synthesised
materials has emerged as a powerful strategy to introduce a
functionality without affecting the original structure. In par-
ticular, such a method has proved to be very useful in the
recent development of metal–organic frameworks (MOFs)8 or
biomolecules9 and therefore it could be a good approach for
the development of new SCO materials.
There are only a few reports concerning PSM and spin
crossover materials, dealing with the functionalization of 3D
Hofmann clathrate MOFs10 or chain-like iron–triazole
complexes11,12 and more recently of a dinuclear iron(III)
complex13 and FeII[MIV(CN)8]4− (M = Mo, Nb) cyanido-bridged
frameworks.14 To the best of our knowledge, no complete reac-
tion has been reported in spite of stoichiometric conditions.
Wang et al. have recently11,12 reported a post-synthetic pro-
cedure in which they obtained up to 15% chemical transform-
ation in the final iron–triazole complex, leading to a slight
LCC, CNRS, 205 route de Narbonne, 31077 Toulouse, France.
E-mail: lionel.salmon@lcc-toulouse.fr
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
C9DT03587G
Scheme 1 The post-synthetic reaction using p-anisaldehyde.
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Fig. 2 Magnetic measurements for samples 1, 5, 6 and 7.
Fig. 1 The powder X-ray diffractograms of samples 1, 3, 5, 6 and 7.
coordination polymer. This shows that the choice of solvent is
an important factor in post-synthetic reactions.
Temperature dependent magnetic measurements carried
out for samples 1 and 5–7 are shown in Fig. 2 (see the ESI† for
We also synthesized the corresponding imino complex by other measurements). For each sample we considered the
direct synthesis (DS) starting from the imino ligand (see the second thermal cycle, the first one corresponding to the deso-
ESI†) in order to compare its properties with those of the post- lvation/‘run-in’ cycle. The SCO behaviour depends on the syn-
synthetic imino complex. It is well known that SCO behaviour thetic method used (see Scheme S0†). In comparison with the
is strongly dependent on different structural factors such as parent amino derivative the transition is less cooperative and
crystallinity15 and polymorphism.16 Interestingly, we found shifted to lower temperature with T1/2 = 278 K and 287 K for 5
that in contrast to the post synthetic reaction, the direct syn- and 7, respectively, while a very incomplete conversion is
thesis of the imino complex when carried out under normal observed for 6. In the case of the post synthetic method, the
stirring conditions resulted systematically in an amorphous completeness of the transition decreases with the transform-
powder (6); it was necessary to use a solvent diffusion tech- ation rate (see S12†). Such an effect is also observed to a lesser
nique in order to obtain a crystalline powder (7) as shown in extent for sample 7. In fact, although the general chain archi-
Fig. 1, and this is important in order to establish a fair struc- tecture of the structure is maintained in all samples as demon-
ture–property relationship.
strated by the PXRD study, the synthetic approach
The PXRD data for the post-synthetic samples (see the ESI†) implemented plays a key role in the final spin crossover pro-
reveal that the structure changes while increasing the amount perties. For sample 5, we can suggest that the difficulty in
of imine; at 23%, the diffractogram remains nearly the same incorporating a bulky moiety into the ligand by the network
as that of the amino derivative, while for higher transform- modifies the inter-chain interactions and steers the molecular
ations, we can clearly see an additional diffraction pattern. In network to form a specific packing arrangement.
fact, as considered by Wang et al.,12 the retention of the struc-
Regarding the thermal variation of susceptibility as a func-
ture is obtained for weak substitution, which could be con- tion of the degree of transformation (see Fig. S13†), it appears
sidered as grafting. It is interesting to notice that such behav- that all the intermediate transformations lead to a mixture of
iour is in contrast to that observed for the [Fe(Htrz)2(trz)](BF4) two independent complexes. It is interesting to notice that the
derivative for which a drastic modification of the structure was experiments were repeated twice and led to the same result
observed for only 3% substitution of the triazole ligand by the (the same composition and properties under the same experi-
aminotriazole ligand.17 In the latter case, the effect on the mental conditions). One explanation could be the presence of
crystal lattice was attributed to the reorganization of the inter- domains (core of the powder grains) that remain with the
actions between the Fe–triazole chains, which are directly same magnetic properties as the starting amino derivative 1
linked through N–H⋯N interactions involving Htrz and trz and other growing domains on the surface of the grains that
ligands in the pristine compound.18 Finally, in the present are transformed into the imine derivative. This hypothesis is
case, when 100% substitution is achieved the structure is com- in agreement with the fact that the starting complex 1 is in-
pletely modified and is similar to that obtained for the crystal- soluble under our experimental conditions. This trend is
line sample 7. These measurements confirm the complete totally different compared to that obtained for the direct syn-
post synthetic modification and are in agreement with the thesis in the presence of a mixture of ligands (amino and
magnetic measurements reported hereafter which clearly show imino ligands). In the latter case, the pace of the SCO curve
the presence of two species only for the partially substituted with only one singularity is consistent with the homogeneous
samples.
distribution of the imine ligand (see Fig. S14†).
16854 | Dalton Trans., 2019, 48, 16853–16856
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methods. Nevertheless, clearly the post synthetic approach
leads to a specific molecular arrangement and physical pro-
perties and opens up the way towards the preparation of
new SCO complexes. The methodology used in this work is
currently under investigation to determine the synergetic
effect between SCO and different physical or chemical pro-
perties that cannot be accessed through the direct synthetic
methodology.
Fig. 3 13C CP-MAS (left) and 13C MAS (right) NMR spectra of samples 1,
3, 5 and 6.
Conflicts of interest
In order to probe the composition of the samples in depth, There are no conflicts to declare.
we used Mössbauer spectroscopy and NMR spectroscopy, the
latter being rarely used to characterize paramagnetic SCO
materials. 13C CP-MAS and 13C MAS NMR are two different
techniques used to obtain the NMR spectra in the solid state;
Acknowledgements
while the first is adapted for diamagnetic systems, the latter A. E.-C. thanks the CONACyT (Mexico) for the postdoctoral
permits us to study also the paramagnetic entities that are grant. We thank Yannick Coppel for NMR measurements and
associated with faster nuclei relaxation.19,20 Indeed, in the discussions.
13C CP-MAS spectra at room temperature (Fig. 3), we only
observe a peak at 155 ppm corresponding to the triazole core
for samples 1–4. Although the intensities of the peaks seem to
be similar due to an increase in the number of scans (to
Notes and references
observe the peaks, see the ESI†), they decrease from sample 1
to sample 4 in accordance with the increase of susceptibility.
Indeed, from the magnetic behaviour observed in Fig. 2, it is
clear that at room temperature (heating mode) 1 is diamag-
netic, and this characteristic is lost with the degree of trans-
formation (Fig. S13†), explaining the decrease in the intensity
of the 13C CP-MAS peak. Moreover, the difficulty in obtaining a
signal for sample 5 is in agreement with the majority of HS
fractions. In this case, we cannot detect 13C spins in the direct
vicinity of the paramagnetic species, which are certainly
severely broadened to be indistinguishable from the base-
line.21 As expected sample 6 is also silent as it is fully paramag-
netic. Mössbauer data were collected for sample 5 at 80 and
325 K and confirmed the residual LS fraction at high tempera-
ture suspected by magnetic measurements (see the ESI†).
The 13C MAS NMR (Fig. 3) spectrum at room temperature
confirms the diamagnetic entities for 1, while the introduction
of the imine ligand leads to the appearance of new peaks at
59 ppm (OCH3), 115 and 137 ppm (aromatic carbons of p-ani-
saldehyde), 169 ppm (triazole core) and 200 ppm (CHvN)
attributed to the paramagnetic species that increase in inten-
sity in agreement with the degree of transformation. Clearly,
in contrast to the 13C CP-MAS experiment, 13C MAS NMR
allows for the study of the whole system in all cases.
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