# Ruthenium-Thymine Acetate Binding Modes: Experimental and Theoretical Studies
## 3 Results
### Reactions of [Ru(CO)H\({}_{2}\)(Pph\({}_{3}\))\({}_{3}\)] I with THAcOH
Fluxional behaviour, that plausibly promotes interconversion from _trans_ to _cis_ forms, implies generation of pentacoordinated intermediates, which in turn are stabilized by coordinating solvents, as diethyllic or methyl-cyclopentyl ether. As shown by \({}^{1}\)H NMR spectra, species **2** strongly attract solvent molecules up to the second coordination sphere, exhibiting a solvatochromic effect in solution, and solvent inclusion in the crystals, selected for X-ray diffraction studies. Studies on the Reaction Path: Isolation of the Intermediates mer-\(\kappa^{1}\)(O)-THAc [Ru(CO)H(PPh\({}_{3}\))3] **3a**, **3b** and trans(PPj)-\(\kappa^{2}\)(O,O)-THAc-[Ru(CO)H(PPh\({}_{3}\))2] **4**  
With the purpose to investigate the reaction course of **2**, a stepwise addition of two subsequent equivalents of THAcH intercepted two intermediates, which were spectroscopically characterized and analyzed by DFT calculations. The reaction (1:1 molar ratio) were stopped after 30 min. and the dried residue was eluted by Et\({}_{2}\)O on a celite pad giving two distinct fractions. The nature of the more abundant pale green band (60%) has been assigned to a monohapto-coordinated _mer_-\(\kappa^{1}\)(O)THAc-[Ru(CO)H(PPh\({}_{3}\))3] **3a**,  
Figure 2: Distinct H-bond interactions have been evidenced by different color notifications. -- Expected intermolecular Watson-Crick H-interactions Å NH-OC between two adjacent THAc rings. -- OC(O)-H (C5H5-P) between the \(\kappa^{1}\)-THAc C(O)O and the proton belonging to a phenyl of PPh\({}_{3}\) of the adjacent molecule O-H45 (2.51) O-H31 (2.44) N2H-O7 (2.12) N4H-O4 (2.05). -- \(\pi\)-\(\pi\) stacking between two aromatic rings (3.6 Å) of THAc ring belonging to opposite molecules. **3b** (Scheme 1) on the basis of a single Ru-CO IR absorption at \(\
u=1918\) cm\({}^{-1}\) and the characteristic acetate band pattern. The \({}^{31}\)P[\({}^{1}\)H] NMR broad signals at \(\delta\) 41.2 and 36.5 in reciprocal 2:1 ratio is attributed to the phosphine ligands by the hindered rotation about the CO-Ru rotation. The broad growing doublet centered at \(\delta\) 38.7, with residual Ru-H coupling (\(\sum\)J\({}_{\rm{PH}}\) = 15 Hz), supports the formation of the \(trans_{\rm{PP}}\)- \(\kappa^{2}\)-(O,O)-THAc, **4** as the more stable species (Scheme 1). The \({}^{1}\)H NMR Ru-H triplets centered at \(-6.1\) and \(-7.2\) ppm (Figure 3), (\({}^{2}\))H\({}_{\rm{\rm{\cdot}Ptans}}\) = 111 Hz and \({}^{2}\)J\({}_{\rm{\rm{H}\cdot PGs}}\) = 24 Hz) indicate that hydride substitution mainly occurred _trans_ to carbonyl ligand, which is responsible for stabilizing the donor THAc ligand. The second eluted reddish fraction is the result of acetate chelation by concomitant release of PPh\({}_{3}\). THAc-chelation has confirmed by a strong Ru-carbonyl IR band at \(\
u\) 1960 cm\({}^{-1}\) and by a narrower separation of dihapto-acetate IR bands at 1682 s, 1656 s cm\({}^{-1}\) (\(\Delta\
u\) = 26 cm\({}^{-1}\)). The highfield-shifted \({}^{1}\)H NMR hydride triplet at -17.0 ppm (t, \({}^{2}\)J\({}_{\rm{HP}}\) = 20.0 Hz) (Figure 3), due to the notable electron-donor dihapto ligand, is compatible with \(trans_{\rm{PP}}\)-phosphine location, analogously to the \(\kappa^{2}\)-O(O)-[Ru(CO)H(COH)(PCy\({}_{3}\))\({}_{2}\)] complex (\(-18.5\) ppm) [50]. Although the first reaction step, leading to \(\kappa^{1}\) intermediates, was not energetically favored, the energies required were still accessible at the experimental conditions. The distinct _cis_ conformers **3a**, **3b** exhibited comparable energy, but were separated by a quite high barrier, as predicted by DFT calculations (Scheme 2). A third isomer, having \(\kappa^{1}\)-(O) _trans_-located with respect the PPh\({}_{3}\), was predicted at too high energy to be observed. The further evolution by PPh\({}_{3}\) release, forming \(\kappa^{2}\)-intermediates **4**, exhibited energies lower than reagents only in the case of _trans_-PPh\({}_{3}\), whereas _cis_-isomers always fall at higher energies.