TRANSMETALLATION PHENOMENA INVOLVING IRMOF-9 an XRD and EM approach
Stefano Canossa, Luca Fornasari, Paolo Pelagatti, and Alessia Bacchi Dipartimento di Chimica, Università di Parma -
[email protected]
IV MISCA
The Zn-based cluster which characterises the IRMOF-9 structure, with the coordinated DMF molecule present at r.t.
TRANSMETALLATION IRMOF-9
Transmetallation is a post-synthetic modification, which «involves the exchange of native cations in a crystalline solid with foreign ones from a surrounding solution».3 Since during this process the disposition of the coordinating species depends on the electronic characteristics of the incoming and the outgoing metal centers, the material can face a dramatic loss of cristallinity or even amorphisation. Native cation The cases in which this transformation is totally isomorphous Foreign cation and allow a single crystal to remain undamaged are indeed the Organic linker rarest ones.
IRMOF-9 is a hybrid crystalline material belonging to the class of Metal Organic Frameworks (MOFs). Its structure, first published by Yaghi et al.1, consists in a periodic arrangement of biphenyl4,4’-dicarboxylate linkers connecting together zinc atoms and forming an interpenetrated microporous network. This material has proven to have a remarkably dynamic structure, since the DMF molecules coordinating the metal centers exhibit a fluxional behaviour depending on the temperature conditions, which can also affect the overall structure of the solid2. This feature, together with the good quality of the synthesizable crystals, make it a valid candidate for fine structural investigations on its changings.
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In this work we will present different transmetallation phenomena, focusing on a particular case of successful isomorphous transformation.
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Graphical representation of the transmetallation phenomenon in a metal organic network, with the possibility of a reversible behaviour.
METHODS AND GENERAL RESULTS
A SUCCESSFUL ISOMORPHOUS TRANSFORMATION the case of the Co fully transmetalled single crystal
I - Synthesis of IRMOF-9 IRMOF-9 crystals have been synthesized according to the procedure described by Yaghi et al. 1.
Metal concentration (mg/L)
3
II - Soaking in transition metal chlorides solutions MnII,
FeII,
CoII,
The crystals were left in a DMF dilute solution of chloride in a closed vial for 10-15 days at room temperature. After this time span, remarkable changes in their aspect are observed:
Mn
Fe
Co
NiII
and
Ni
2,5 2 1,5
Zn
1
Co
Day 0
0,5
Day 16
0
CuII
0
5
10
15
20
25
IRMOF-9 soaking in CoCl2 0.2M (solvent: DMF)
Soaking time (days)
ICP-MS analyses on powdered crystals sampled at different transmetallation stages show the process is reversible.
Cu
Day 25
In fact, fully transmetalled crystals left in a DMF solution of ZnCl2 visibly start losing their blue colour within 1 day.
In all cases except for the Ni one, after the soaking treatment the crystals showed an evident transformation occurring from the external part to their centre. In the cases of Fe, Co and Cu, the coloured crystals, left in a pure DMF solution, do not lose their acquired colour.
Day 37 Day 60
PXRD patterns were recorded to evaluate any structural changing in the samples: Mn
Amorphisation
Transmetallation
Fe
Co
No reaction
Powder XRD patterns have been recorded with synchrotron radiation on policrystalline IRMOF-9 as synthesized and after complete transmetallation, which has been confirmed by XRF analyses.
Transmetallation
Ni
Cu
The isostructurality of the two phases is evident, as well as the slight distortion of the lattice before and after the transformation.
In some cases the crystals just disintegrated turning amorphous. In the case of Co and Cu, the final material is clearly crystalline and the pattern very similar to the IRMOF-9 one in terms of peaks intensities and positions.
IRMOF-9 Co-transmet. IRMOF-9
IRMOF-9 Co-transmet. IRMOF-9
EM analyses on the section of cut crystals provided an evaluation of the integrity of the solid, together with a confirmation of the cation exchange in the lattice by checking the chemical composition of the outer and inner part with EDX analysis. Finally, single crystal XRD confirmed the crystals remain single domains after the the Zn-Co exchange. The structure has been successfully solved, revealing a Co-based MOF isomorphous with IRMOF-9, never reported in the literature.
Cu
Below, the two lattices are shown along the a axis and superimposed in the middle to highlight the slight differences due to their different cations. a
17.14 Å
b
22.46 Å
S.G.: Pnnm c
25.96 Å
IRMOF-9 100 μm
a b Scan point
Normalized element abundancy (%) Region
Zn
Co
Cl
a
12.61%
86.43%
0.96%
b
93.03%
6.76%
0.21%
Electron microscopy allowed a microanalysis by EDX along the section of a cut crystal. As expected, the concentrations of Co and Zn flip progressively while going from the external part of the crystal to its core and vice versa.
IV - EM imaging and EDX analysis
Co
Normalized element abundancy (%)
III - Powder XRD analysis Amorphisation
EM/EDX analyses
Synchrotron X-ray analyses
100 μm
Co transmet. a IRMOF-9 b
17.14 Å
S.G.: Pnnm
26.27 Å
c
By repeating the synthesis of IRMOF-9 with CoCl2, a different phase is obtained, which is already reported in the literature4.
+ CoCl2 anhydrous
DMF 115°C, 3 days
21.90 Å
Single crystal XRD
In the cases of Co and Cu, the inner part is observed to be prevalently IRMOF-9, without a remarkable presence of the foreign metal cation. However, considering the morphology of the solids it is evident that the Cu transmetalled solid is powdered in smaller crystallites, whereas in the Co one the transmetalled part looks even more homogeneous than the original IRMOF-9 core. This evidence highlights the importance of transmetallation as a useful post-synthetic method to obtain a metastable phase not affordable by conventional synthesis.
V - Single Crystal XRD analysis Single crystal XRD has been performed on specimens that retained the original habit of IRMOF-9. The products of the Mn and Fe trials have been confirmed to be amorphous solids, and the Ni one unreacted IRMOF-9. In the case of Cu, the crystal integrity did not survive the transmetallation and was effectively an aggregate of crystallites. Finally, the crystals of the Co trial were observed to be the only ones to undergo transmetallation without a significant loss of crystallinity.
Bibliography: 1
Science, 2002, 295, 469–472 2 For further information please view my poster with DOI: 10.13140/RG.2.1.3224.2806 on my ResearchGate profile 3 Chem. Eur. J., 2014, 20, 6871 – 6874 4 Cryst. Growth Des., 2009, 9 (2), 1066–1071
Acknowledgements: The CNR of Trieste, and in particular Dr. Nicola Demitri, are gratefully acknowledged for the precious technical support during the XRD and XRF data collections at the ELETTRA Synchrotron facility. Dr. Monica Mattarozzi (University of Parma) is also acknowledged for the EM/EDX measurements performed with the ESEM microscope.
