# Ruthenium-Thymine Acetate Binding Modes: Experimental and Theoretical Studies
## 3 Results  
### Reactions of [Ru(CO)H\({}_{2}\)(Pph\({}_{3}\))\({}_{3}\)] I with THAcOH  
The Ru-hydride _mer_-[Ru(CO)H\({}_{2}\)(Pph\({}_{3}\))\({}_{3}\)] [40], **1** reacted in refluxing toluene within 240 min with thymine-acetic acid (THAcH) excess, by molecular hydrogen release until no \({}^{1}\)H NMR signals for residual Ru-hydride in the crude solution were observed. Attribution to a structure, showing IR frequency at 1970 cm\({}^{-1}\) for Ru-carbonyl function and simultaneous mono- and di-hapto-acetate-Ruthenium coordination like \(\kappa^{1}\)(O)-THAc, \(\kappa^{2}\)(O,O)-THAc[Ru(CO)(Pph\({}_{3}\))\({}_{2}\)], **2** has been suggested on the basis of the IR frequency pattern. Namely, antisymmetric, and symmetric stretching vibrations of the COO' group of acetate at 1653 and 1363 (\(\Delta\
u\) = 290 cm\({}^{-1}\)) indicate \(\kappa^{1}\)-THAc coordination, whereas narrower frequency differences (between \(\
u\) = 1630 and 1532, \(\Delta\
u\) = 98 cm\({}^{-1}\)) are typical for \(\kappa^{2}\)-THAc chelate mode. Species **2** in CDCl\({}_{3}\) showing a single sets of \({}^{1}\)H NMR resonances has been assigned to the less sterically hindered _transp_-configuration on the basis of stability considerations reported for similar complexes [41; 42]. The \({}^{1}\)H NMR signals at 8 1.74, and 5.85 ppm, are respectively attributed to methyl and methyne thymine substituents, suggesting facile fluxional exchange at room temperature between \(\kappa^{1}\)- and \(\kappa^{2}\)-THAc rings, which may plausibly occur through a penta-coordinated transient species, in which the THAc ligands coordinatively exchange by a merry-go-round (THAc) mutual interconversion. Then, the _merTHAc_ stereogeometry designation is independent from their haptiople mode. The \({}^{1}\)H NMR signal at 8 2.35 is associated to the \(\kappa^{2}\)-NCH\({}_{2}\) group in _transp_-_merTHAc_-form, whereas a multiplet centered at 4.29 ppm showing a typical root effect (doublet CH\({}_{\alpha}\)H\({}_{\beta}\), \({}^{2}\))HH = 17; \({}^{2}\)]\({}_{\rm HH}\) = 90 Hz), is attributed to the NCH\({}_{\alpha}\)H\({}_{\beta}\) methylene moiety of the \(\kappa^{1}\)-carboxy moiety. DFT molecular modelling calculations reveal that the Ru-OC(O) carboxy rotation is hampered by the axial phosphine steric crowding. By contrast, the absence of detectable J\({}_{\rm H-H}\) coupling in the case of the dihapto-coordination may be attributed to the methylene group of chelate carboxy function, lying in planes that are mutually perpendicular (Karplus effect) [43]. This reciprocal disposition was evidenced both by the DFT _transp_- calculated isomer (94\({}^{\circ}\) in CDCl\({}_{3}\) solution) and by the X-ray crystal structure of the _cis_ isomer (87\({}^{\circ}\) A) (Figure 1). DFT calculations predicted also a low energy interconversion process from _transp_-_merTHAc_ to _cisp_-_fac_THAc_ species (\(\Delta\)E = 3.7 K) mol\({}^{-1}\) in vacuo), which decreased to 0.8 kJ mol\({}^{-1}\), if the calculations take into account of the solvent effect (CDCl\({}_{3}\)). We supposed that Ru-O cleavage of the chelate-THAc could promote PPh\({}_{3}\) migration from axial to equatorial position in a pentacoordinated intermediate species, as observed in the case of the precursor **1** solicited by light [44; 45]. Accordingly, complete interconversion from _transp_-_merTHAc_ to _cisp_-_fac_THAc_ isomer occurs at r.t. in CDCl\({}_{3}\) within 3 h. Two \({}^{31}\)P NMR doublets centered at 8 46.5, 5 43.2; (\({}_{\rm PP}\) = 25 Hz) are representative for _cis-fact_**2** form, while a broad doublet detected at 35.9 (\({}^{2}\))\({}_{\rm PP}\) = 49 Hz) is the only observed residual signal due to the _trans-mer_ form. The rapid mutual room temperature scrambling of the \(\kappa^{1}\)- and \(\kappa^{2}\)- coordinated THAc fragments made undistinguishable both the Me and the CH methyne signals in \({}^{1}\)H or \({}^{13}\)C NMR spectra.