# Ruthenium-Thymine Acetate Binding Modes: Experimental and Theoretical Studies
## 3 Results
### Intra or Inter-Molecular Bonding Network?
In the case of the species \(\kappa^{1}\)(O)THAc-, \(\kappa^{2}\)(O,O)THAc-[Ru(CO)(PPh)\({}_{2}\)], **2** no evidence of tautomerization has been observed in the \({}^{1}\)H NMR spectra (Figures S5a,b and S7a,b), since the NH-(O)C signals at 8 7.8 and 7.9 ppm indicate exclusively the formation of amino-carbonyl intermolecular bonds. To carry out this investigation we selected CDCl\({}_{3}\), which concomitantly showed low polarity (\(\chi\) = 4.8) and scarce coordination ability [52]. Considering the low solubility of the observed mixture and the faint response of the X\(\cdots\)H-O interactions, by comparison with the intensities of the other signal, the \({}^{1}\)H NMR spectra were recorded from freshly prepared samples (within 15 min) to exclude facile isotopic exchange processes with deuterated solvents. We are convinced that Ru-carboxy coordination played a pivotal role to induce proton relocation, boosting the tautomerization from -[N(H)C(O)]-amido to -[N=C(OH)]-iminol form, which finally resulted in being energetically stabilized by the inter-molecular H-binding network. The prevalence _keto_ distribution (3.2:1) is correlated to inter-connections shown by the dimeric species, proposed as case-study model, and reported in Scheme 7. The NMR signal attribution to the X-H-O (X=N, O) intermolecular interactions was based on the electron withdrawing ability, the shielding effects of O-H-O (c-g, m and intramolecularly G) \(<\) N-H-O (h-i, n) \(<\) N-H-N (concomitant c-f) chemical shifts scale, the DFT-calculated energy scale and correlated to the resonance intensities. The carboxy-_enol_ signal interaction (8 or 11.2) and the minor hydroxy signals around \(\delta\) 8, which all belong to the minor _enol_-forms, were compared to the intense resonance at \(\delta\) 8.3 which represented the prevalent _keto_ isomers. The presence of iminol forms was also responsible for the concomitant growing of the NH-N signals (8 7.8-8.0). As suggested by DFT-space filling evaluations, the H-bond interactions have merely been considered in the case of thymine rings _anti_-disposed to minimize the steric interference with the crowded _trans_-phosphines, therefore allowing the remarkable H-bond network. (Figure 5). A variety of \({}^{1}\)H NMR spectra of intermediate mixtures, because they were analyzed at different reaction times, and although composed by different proportion of chelate-**4** and monohapto-**3** structures, showed a similar low-shifted signal pattern (two distinct examples are reported in Figure S11b,c). Considering the uncertainty of 10-15% due to integral determination and the different proportion of \(\kappa^{1}/\kappa^{2}\) coordination of the non-conjugated metal fragments, the analyzed spectra exhibited an _enol/keto_ ratio of 0.31 (Figure 5), compared to the end/keto ratio as 0.13, reported in the literature [53]. The assignments were accomplished by the help of DFT-calculated energies for THAc anionic dimers selected as models, supporting the belief that the intermolecular interac  
Figure 5: \({}^{1}\)H spectra relative to (**3 + 4**) mixture showing the intermediate trapping during the reaction course. The integrals evaluation for the tautomeric ratio results to be 1:3.2 with the prevalence of the stable amide conformation. The attributions of the H-bond functions of the type X–H–O (X=N, O) have been assigned based on the shielding effect (8) O–H–O (11) \(<\) N–H–O (10-8) \(<\) N–H–N (8), related to the calculated DFT energy scale of the dimers sketched in Scheme 8.  
tions, promoted by H-binding network, played a key role in the solution also. The larger line widths, which were observed for the intermolecular O\(\cdots\)H\(\cdots\)O bonds (violet entries c-f) may have been due to the higher degree of freedom shown by OH\(\cdots\)O(C) functions, which presented two O-lone pair able to generate in turn more than one H-bonded conformation. On the contrary, NH\(\cdots\)O bond type (green entries n-l and n) or the enol-carboxy C(O)O\(\cdots\)HO interactions at \(\delta\) 11.