br Solution studies br NMR spectroscopy is very
3.3. Solution studies
NMR spectroscopy is very useful technique for determination of structure of Cd complexes in solution since existence of equilibrium of
free and coordinated organic ligands , two mononuclear complex species  or binuclear/mononuclear complex species  can be easily recognized by increased number of signals. 1H, 13C and 113Cd
NMR spectra (Figs. S2–S4, Supplementary material) of 1 recorded in DMSO‑d6 at ambient temperature exhibit one set of 1H, 13C and 113Cd
resonances. This refers to a single ligand environment, also found in the solid state structure of 1. Position and number of signals in 1H NMR spectrum of 1 at ambient temperature remained the same even after 48 h (Fig. S2, Supplementary material). The chemical shift of the 1H, 13C and 15N (derived from 1H-15N HSQC and HMBC spectra) resonances in 1 are very similar to those found in Cd(II) complex with related (E)‑ethyl‑2‑(2‑pyridin‑2‑yl‑methylene)hydrazylacetate , which in-dicates that the ligand in 1 is coordinated bidentately via pyridine and hydrazone nitrogen atoms. A singlet at 1.80 ppm in 1H NMR spectrum (Fig. S2, Supplementary material) and signals at 21.8 and 177.2 ppm in 13C NMR spectrum of 1 (Fig. S3, Supplementary material) correspond to an acetate ion. When acetate is coordinated to metal ions particular deshielding of δ(13C) of the carbonyl carbon is expected . The downfield shift of the signal of CH3COO in 13C NMR spectrum of 1 in comparison to the signal of carbonyl carbon ML-210 of uncoordinated acetate (171.93 ppm)  points to the presence of coordinated acetate. Further evidence for acetate ion coordination is derived from the NOESY spectrum of 1 wherein correlation signal of methyl group from acetate ion with methylene protons (CH3COO/C8H2) was observed due to mutual spatial proximity (Fig. S6, Supplementary material). Coordination of the ester carbonyl group to Cd(II) in solution can be excluded, since there is no significant change in the chemical shift of ester carbonyl carbon atom (C9) in 13C NMR spectrum of 1 in com-parison to spectra of free ligands similar to aphaOMe .
The coordination behavior of Cd(II) is typical of a soft acid and
Fig. 2. Packing in the crystal structure of 1. Hydrogen atoms, except those involved in hydrogen bonding in (A) and (B), are omitted for clarity. (A) 2D assembly parallel to (1 −1) generated by hydrogen bonding. (B) 1D supramolecular chain along  direction formed by π-π stacking interactions of pyridine rings and weak hydrogen bonds. (C) and (D): Energy frameworks for the total nearest neighbor pairwise interaction energies. The cylinders connect molecular centroids, and their thickness is proportional to the magnitude of the energy. The numbers indicate energy associated with each cylinder in kJ mol−1. Pairwise energies with magnitudes < 10 kJ mol−1 have been omitted for clarity.
S. Bjelogrlić et al.
orbital diffuseness of Cd(II) acceptor atom is well matched with diffuse sulfur donor orbitals . This is corroborated by the strong thiol-binding ability of Cd(II) . However, sulfoxides which contain O and S donor atoms, prefer coordination via O atom to Cd(II) . This is in line with the CSD survey where in all found X-ray structures of Cd
(II) complexes a DMSO molecule was coordinated via O atom ex-clusively . In presented biological experiments (vide infra) the stock solution of 1 in DMSO was used as a starting material for testing. To check if a DMSO molecule is coordinated to Cd(II), 1H NMR spectrum of freshly prepared solution of 1 in DMSO‑d6 (0.006 M), with addition of 5% (vol.) DMSO, was recorded (Fig. S11, Supplementary material). Generally, S-bound DMSO ligands exhibit 1H NMR chemical shifts ap-proximately 1 ppm downfield relative to free DMSO (~2.54 ppm), whereas O-bound DMSO exhibits a smaller downfield shift (0.05–0.5 ppm) . Since no change in 1H NMR chemical shift of free DMSO was noticed, even after 24 h (Fig. S11, Supplementary ma-terial), the coordination of DMSO to Cd(II) ion in solution of 1 can be excluded.