Ru-Controlled Thymine Tautomerization Frozen by a k¹(O)-, k²(N,O)-Metallacycle: An Experimental and Theoretical Approach

The reaction of mer-(Ru(H)₂(CO)(PPh₃)₃) (1) with one equivalent of thymine acetic acid (THAcH) unexpectedly produces the macrocyclic dimer k¹(O), k²(N,O)-(Ru(CO)(PPh₃)₂THAc)₂ (4) and, concomitantly, the doubly coordinated species k¹(O), k²(O,O)-(Ru(CO)(PPh₃)₂THAc) (5). The reaction promptly forms a complicated mixture of Ru-coordinated mononuclear species. With the aim of shedding some light in this context, two plausible reaction paths were proposed by attributing the isolated or spectroscopically intercepted intermediates on the basis of DFT-calculated energetic considerations. The cleavage of the sterically demanding equatorial phosphine in the mer-species releases enough energy to enable self-aggregation, producing the stable, symmetric 14-membered binuclear macrocycle of 4. The k¹-acetate iminol (C=N-OH) unit of the mer-tautomer k¹(O)-(Ru(CO)(PPh₃)₂(THAc)) (2) likely exhibits a stronger nucleophilic aptitude than the prevalent N(H)-C(O) amido species, thus accomplishing extra stabilization through concomitant k²(N,O)-thymine heteroleptic side-chelation. Furthermore, both the ESI-Ms and IR simulation spectra validated the related dimeric arrangement in solution, in agreement with the X-ray determination of the structure. The latter showed tautomerization to the iminol form. The ¹H NMR spectra in chlorinated solvents of the kinetic mixture showed the simultaneous presence of 4 and the doubly coordinated 5, in rather similar amounts. THAcH added in excess preferentially reacts with 2 or trans-k²(O,O)-(RuH(CO)(PPh₃)₂THAc) (3) rather than attacking the starting Complex 1, promptly forming the species of 5. The proposed reaction paths were inferred by spectroscopically monitoring the intermediate species, for which the results were strongly dependent on the of conditions the reaction (stoichiometry, solvent polarity, time, and the concentration of the mixture). The selected mechanism proved to be more reliable, due to the final dimeric product stereochemistry.