# Ruthenium-Thymine Acetate Binding Modes: Experimental and Theoretical Studies
## 3 Results
### Reactions of [Ru(CO)H\({}_{2}\)(Pph\({}_{3}\))\({}_{3}\)] I with THAcOH
On the contrary, the THAc-methylene moiety and the phosphine \({}^{31}\)P NMR signals resulted both in being non-equivalent, since they were affected by punctual different molecular environments. No-planar disposition of the thymine rings altered the global symmetry, therefore was responsible for doublets related to \({}^{1}\)H NMR mutual coupling even in _trans_-configuration. As expected, the IR frequency pattern of the _cis_ form was totally analogous to the _trans_-, but slightly lower-shifted. Due to the scarce solubility in less polar solvent, the \({}^{13}\)C NMR spectrum, which was recorded within 24 h for resolution requirements, exhibited a broad triplet signal at 8 205.6 for the Ru-CO of the former _transp_-_merTHAc_ isomer next to a multiplet at 8 203.9 attributable to the growing _cispP-facTHAc_ species (Scheme 1). The \({}^{13}\)C signals were attributed in a spectrum of _transcis_ = 2:1 mixture, where the growing signals, related to the _cis_ form, were adjacent to the _trans_ one. Both the isomers were unstable in solution, and exhibited complete decomposition within 36 h at room temperature in polar coordinating solvents as DMSO (dimethylsulphoxide). The _cispP-facTHAc_ configuration in CDCl3 is calculated to be almost isoenergetic and stabilized by a multi-intermolecular H-binding network, which is clear in the X-ray crystal packing. In the crystal environment, the \(\kappa^{1}\)-THAc ligand is able to intermolecularly couple through Watson-Crick N-H-O interactions with the chelate \(\kappa^{2}\) (O,O)-THAc counterpart of the adjacent molecule, forming a supramolecular metallacycle, reinforced by antiparallel thymine ring \(\pi\)-\(\pi\) stacking together with a phosphine phenyl C-H-O interaction [12]. In the crystal packing (Figure S2), symmetry related molecules of **2** are connected via two pairs of N-H-O hydrogen bonds between \(\kappa^{1}\)(O)THAc -and \(\kappa^{2}\) (O,O)THAc ligands of neighboring molecules thus forming dimers centered about inversion centres. Further  
Figure 1: X-ray structure of \(\kappa^{1}\)(O)-THAc, \(\kappa^{2}\)(O,O)-THAc [Ru(CO)(PPh3)2] _cis_**-2**. Hydrogen atoms have been omitted for clarity. stabilization of the dimer is achieved by \(\pi\)-\(\pi\) stacking of the thymine rings (centroid-centroid distance 3.65 A) (Figure 2). In this way the thymine-1-acetic ligand preserves the N-H-O interactions observed in the crystal packing of thymine-1-acetic acid. A similar self-assembly showing intermolecular N-H-O hydrogen bonds and the \(\pi\)-\(\pi\) stacking of the thymine residue has been previously observed, for example, in the paddle-wheel dicopper complex [Cu2(O(O)CCH2-T)4(DMSO)2]. The supramolecular architecture is completed by an intricate intermolecular C-H-O network involving the hydrogen atoms of two phenyl rings belonging to two PPh\({}_{3}\) groups of two different molecules and the uncoordinated O atom (O6) of the monodentate THAc ligand. An analogous self-assembly interaction was observed in the THAcOH crystal structure [46; 47; 48; 49]. Therefore, the _cis_-geometry of **2** could also be ascribed to H-bonding interactions between the THAc-rings and the low solubility exhibited by the _cis_-form attributed to the inter-molecular H-bonding network in the solid packing as inferred by the small energy gap (3.7 kJ mol\({}^{-1}\)) between the _trans_ and _cis_ configurations, calculated by DFT. It is worth noting that the Ru-CO function lies on the same side (\(syn\)) of the C(O)O carboxy moiety, in accord with the DFT calculations, which designate this stereogeometry as more stable in the observed _cis_-fac and in the unstable _trans-mer_ form (See Scheme 4 for calculations). The NMR spectra relative to _trans_-2 and _cis_-2 isomers both show the presence of many solvents, which have been used during the synthesis (toluene), in chromatographic processes (CH\({}_{2}\)Cl\({}_{2}\) and Et\({}_{2}\)O), or in multi-solvent stratification procedures for crystallization (CH\({}_{2}\)Cl\({}_{2}\), Et\({}_{2}\)O and Etp).