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Structural studies on Cu(II) and Co(II) complexes and their free-ligands by Solid-State NMR, Solution-State NMR and X-Ray spectroscopy
Conferencia; Small Molecule NMR Conference, SMASH; 2018
Cu(II) and Co(II) ions are central atoms of complexes with heterocyclic alcohol and aldehydes that are relevant as catalysts, such as for degradation of organic pollutants present in effluents. Cu(II) complex with organic ligands are used to activate H2O2 that can produce OH? via the Cu (II)/Cu (I) cycle involving different reaction pathways [1]. This is of particular interest because allows the replacement of traditional inorganic oxidants such as K2Cr2O7 for benign ones as H2O2 and O2. Taking this into account, we decided to use heterocyclic compounds containing the gem-diol functional group obtained from the hydration of carbonyl groups [2,3], to develop new coordination complexes. Most of the time, the stability of the functional group is not studied in the free ligand before the preparation of the metal complex. Understanding the chemistry and the coordination properties of gem-diols is decisive for the development of synthetic methods to obtain new organic ligands and new coordination complexes. Usually, solution-state NMR and X-ray spectroscopy are used to study the structure and to understand the chemistry of the metal complex. Although, obtaining single-crystals is not always possible.For that reason, in this work, not only we develop new metal complexes of Cu(II) and Co(II) containing gem-diol and formyl groups in pyridine derivatives, but also, we propose a structural study combining solid-state and solution-state NMR together with single-crystal X-ray spectroscopy to characterize the Cu(II) and Co(II) complex and its free ligand. By using these NMR methodologies, we are able to approximate the functionalization present in various monomers used as the number of molecules per unit cell in cases where the samples do not yield single crystals. Here, we used 3-formylpyridine, 4-formylpyridine and di-(2-pyridyl)ketone as ligands for the coordination of copper/cobalt ions. Previously, all the isomers were studied by solution-state and solid-state NMR using different strategies to favor the gem-diol or carbonyl moiety in order to try to isolate them, as they quickly revert to the aldehyde or ketone that originated them. The 1H-NMR spectra in CD3OD of di-(2-pyridyl)ketone shows that 85% of the ligand is in the carbonyl form and 15% is in the gem-diol form. Interestingly, by X-Ray diffraction studies we could see that in the coordination complex with Cu(II) which is prepared in methanol, the ligand is in the gem-diol form. Based on these results, we studied the di-(2-pyridyl)ketone in CD3OD adding different amounts of copper ions by ls-NMR, and the relation between the gem-diol and the carbonyl group stays the same being majority product the ketone. Besides, we observed that the resolution of the 1H-NMR spectra become poor and the line widths are larger as the copper concentration increases. The most affected signals are those that belong to the gem-diol. This is due to the paramagnetic effect of the copper that induces hyperfine shifts of NMR signals and shortening of nuclear longitudinal (T1) and transverse (T2) relaxation times. The closer the copper is to the protons, the greater the paramagnetic effect. Considering the X-ray structure, we observed that it coincides that the protons that are most affected are those involved in the copper complex. In this sense, the ketone form of the di-(2-pyridyl)ketone was present in its Cu-complex as a remainder uncoordinated compound according to the 13C CP-MAS and the solution-state NMR spectra. So, ls-NMR together with ss-NMR can be a very useful tool to approximate the structure of the complex in those cases where we do not have the single-crystal. Additionally, the 1H-MAS and 2D 1H-1H SQ-DQ ss-NMR spectra (@60 kHz) can also bring structural information about the ligands surrounding the paramagnetic center.