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

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.

. Crystal structure and numbering scheme for the copper complex for the 4pyridinecarboxaldehyde with CuCl2/CH3OH. The displacement ellipsoids for the non-H atoms in the figure were drawn at the 50% probability level. Figure S2. Crystal packing structure for the copper complex for the 4-pyridinecarboxaldehyde with CuCl2/CH3OH. The displacement ellipsoids for the non-H atoms in the figure were drawn at the 50% probability level.    (7) Cu (1)

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proportions. In a previous work, we reported that the addition of water molecules to the aldehyde group in pyridinecarboxaldehyde isomers is determined by the position of the group in the aromatic ring. 3 The most reactive compound for the addition of water is the 4pyridinecarboxaldehyde, followed by 2-and 3-pyridinecarboxaldehyde, with the fraction of gemdiol forms being 50, 40 and 10%, respectively, as determined by 1 H solution-state NMR in D2O.
This phenomenon is due to the π-deficient character of the pyridine ring, with positions 2, 4 and 6 being the most electron-deficient, which explains the low degree of hydration of 3pyridinecarboxaldehyde. Interestingly, the gem-diol content reached 95% for 4pyridinecarboxaldehyde in CD3OD. However, the NMR spectrum in CDCl3 only showed the aldehyde form ( Figure S8), indicating that the residual water molecules present in the deuterated methanol (water content ≤ 250 ppm) were added to the aldehyde group with higher reactivity than in D2O. Corresponding results were observed for the 3-pyridinecarboxaldehyde, where the gemdiol content was 75%.

*Solution-state NMR experiments with 4-pyridindecarboxaldehyde and CuCl2/CD3OD:
Addition of copper ions to the deuterated methanolic solution of both 4-and 3pyridinecarboxaldehyde produced a broadening of the NMR signals due to the relaxation effect of paramagnetic copper ions ( Figure S9). In particular, the resonance of the gem-diol (RCH(OH)2) (δ 1 H: 5.54 and 5.61 ppm for the 4-and 3-pyridinecarboxaldehyde, respectively), shifted to lowfrequency values as the copper concentration increased. These peaks then fuse with that of water to produce a peak at 4.88 ppm. However, the chemical shift of the aldehyde proton (RCHO) remained unchanged, even at the highest copper concentration (δ 1 H: 10.08 and 10.11 ppm for the 4-and 3-pyridinecarboxaldehyde, respectively). The chemical shifts of the pyridine ring protons were highly affected by the addition of copper ions at any concentration and in both systems.
Moreover, 13 C NMR spectra showed that the chemical shifts of pyridine carbon atoms were highly affected and that the peaks totally disappeared at high copper concentrations ( Figure S10). The same spectral changes were observed for cobalt complexes (Figures S21 and S22).       Figure S24. 13 C-NMR spectrum in CD3OD. Figure S25. g/A-tensor orientations of the copper(II) centers of Cu(II)-4-pyridinecarboxaldehyde (A) and Cu(II)-3-pyridinecarboxaldehyde (B) in the molecular frame A-eigenvectors in panel A are omitted for clarity, but as shown in table S13, they are nearly coincidental to those of geigenvectors.

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Solid-state NMR results. Figure S27. 2D 1 H-13 C HETCOR spectra for the single crystals obtained with 4pyridinecarboxaldehyde and CuCl2 in methanol with a contact time of 50 (left) or 500 s (right) (MAS rate: 15 kHz).