Paramagnetic solid-state NMR assignment and novel chemical conversion of the aldehyde group to dihydrogen ortho ester and hemiacetal moieties in copper(ii)- and cobalt(ii)-pyridinecarboxaldehyde complexes
2021
Crespi, Ayelén F. | Sánchez, Verónica M. | Vega, Daniel | Pérez, Ana L. | Brondino, Carlos D. | Linck, Yamila Garro | Hodgkinson, Paul | Rodríguez-Castellón, Enrique | Lázaro-Martínez, Juan M.
The complex chemical functionalization of aldehyde moieties in Cu(ii)- and Co(ii)-pyridinecarboxaldehyde complexes was studied. X-ray studies demonstrated that the aldehyde group (RCHO) of the four pyridine molecules is converted to dihydrogen ortho ester (RC(OCH₃)(OH)₂) and hemiacetal (RCH(OH)(OCH₃)) moieties in both 4-pyridinecarboxaldehyde copper and cobalt complexes. In contrast, the aldehyde group is retained when the 3-pyridinecarboxaldehyde ligand is complexed with cobalt. In the different copper complexes, similar paramagnetic ¹H resonance lines were obtained in the solid state; however, the connectivity with the carbon structure and the ¹H vicinities were done with 2D ¹H–¹³C HETCOR, ¹H–¹H SQ/DQ and proton spin diffusion (PSD) experiments. The strong paramagnetic effect exerted by the cobalt center prevented the observation of ¹³C NMR signals and chemical information could only be obtained from X-ray experiments. 2D PSD experiments in the solid state were useful for the proton assignments in both Cu(ii) complexes. The combination of X-ray crystallography experiments with DFT calculations together with the experimental results obtained from EPR and solid-state NMR allowed the assignment of NMR signals in pyridinecarboxaldehyde ligands coordinated with copper ions. In cases where the crystallographic information was not available, as in the case of the 3-pyridinecarboxaldehyde Cu(ii) complex, the combination of these techniques allowed not only the assignment of NMR signals but also the study of the functionalization of the substituent group.
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