# Ruthenium-Thymine Acetate Binding Modes: Experimental and Theoretical Studies
## 4 Conclusions  
The report deals with the reaction of **1** with THAcH excess, forming double coordinated mononuclear \(\kappa^{1}\)(O)THAc-, \(\kappa^{2}\)(O,O)THAc-[Ru(CO)(PPh\({}_{3}\))\({}_{2}\)] species **2**, which indicates the versatility of the coordination modes exhibited by thymine acetate. However, the X-ray characterized _cis\({}_{PP}\)-flac\({}_{THAc}\)_ isomeric form of **2** did not correspond to the _trans\({}_{PP}\)-mer\({}_{THAc}\)_ structure observed in solution. A single \({}^{1}\)H NMR signal for the THAc methyl moieties indicated a low-energy monohapto-dihapto interconversion at room temperature. DFT energetic studies confirmed the stabilization in solution of the _cis_ configuration, governed by the reduced steric hindrance and by maximizing either intra- and inter-molecular H-bonding or \(\pi\)-interactions. Metal hydride upfield-shifted \({}^{1}\)H NMR signals indicated the nature of the isolated intermediates **3** and **4**, the rotamer energies of which were studied also by DFT-calculations. To elucidate the reaction course, similar reactions were run between **1** and acetic acid, confirming the proposed mechanism of _cis-trans_ interconversion by the help of DFT calculations, despite the remarkable reduced steric requirements and the limited H-binding features. Upon ruthenium complexation, due to the further stabilization imparted by X-H-O (X=O, N) supramolecular H-binding network, the rare -(O)C-N=C(OH)- THAc-lactim tautomer results increased remarkably (0.31) compared to the predominant (O)C-NH-C(O) lactam form. [PERSON]'s pioneering work, by dealing with the mutagenic properties of metallodrugs in anticancer activity, suggested proton relocation was responsible for the stabilization of rare tautomeric forms, being promoted by direct (N,O) metal coordination of the thymine nucleobase. Herein, we further support the evidence that even non-conjugated thymine derivatives, once coordinated to ruthenium, may notably influence the stability of the minor iminol (0.31 ratio) compared to the tautomeric thymine equilibrium (pka = 10), where the formation of iminol species is promoted by acidic treatment. The occurrence of the latter tautomer, further stabilized by the preponderant H-binding network, is recognized to be active in DNA mismatching, limiting the replication of cancer cells [54; 55; 56; 57]. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) 3390/app11073113/s1, Table S1: Crystal data and structure refinement for compound _cis-_**2**, Table S2: Most relevant hydrogen bonds for _cis-_**2** [A and \({}^{\circ}\)]; Figure S1: Molecular structure of _cis-_**2** with the atom labelling, Figure S2: Molecular structure of _cis-_**2** with \(\pi\)-\(\pi\) stacking and Watson-Creek intermolecular interaction, Figure S3: View down. Fragment of the crystal packing of complex **2** illustrating the intermolecular H bonding pattern. For the sake of clarity only the H atoms engaged in H bonds are shown. Ball and stick representation is used for the dimer arising by the strong N-H\(\cdots\)O hydrogen bonding and for the atoms connected to it. H bonds are shown with blue dashed lines. The \(a\) axis of the crystal packing of _cis-_**2**.