A congested Ru(dps)(2) or Ru(dprs)(2) core (dps = di-2-pyridyl sulfide; dprs = di-2-pyrimidinyl sulfide) promotes sulfur inversion of N,S-chelate thioethers containing CH2R and 2- pyridyl or 2-pyrimidinyl groups

The thioethers L [L = 4-methylbenzyl 2-pyridyl sulfide (L1), 4-chlorobenzyl 2-pyridyl sulfide (L2), 3-chlorobenzyl 2-pyridyl sulfide (L3), 1,4-bis(2-pyridylthiomethyl)benzene (L4), 4- methylbenzyl 2-pyrimidinyl sulfide (L5), and 4-chlorobenzyl 2-pyrimidinyl sulfide (L6)], containing a CH2R group bonded to the sulfur atom, were prepared and characterized. Compounds L1, L2, L3, and L4 reacted with cis-Ru(N,N-dprs)2Cl2 or cis-Ru(N,N-dps)2Cl2 (dprs = di-2-pyrimidinyl sulfide, dps = di-2-pyridyl sulfide) leading to the complexes [Ru(N,N-dprs)2(N,S-L)][PF6]2 and [Ru(N,N-dps)2(N,S-L)][PF6]2. Similar products were obtained from [Ru(N,N-dps)2(NO2)(NO)][PF6]2 and L5 or L6. As a consequence of the L ligand N,S-chelation, all the complexes contain the four-membered ring RuSCN(Ru-N). Since the ruthenium and sulfur atoms are stereogenic centres, with Δ and Λ, and R and S configurations, respectively, they led to four isomers, including the enantiomers. NMR investigations show that the sulfur inversion produces an exchange between the diastereoisomers ΔR and ΔS, as well as Lambda;S and Lambda;R. The one-dimensional band-shape analysis of the exchanging methylene proton signals showed that the inversion barriers (ΔG† 298 κ) for the dprs complexes are in the 54.9-53.8 kJ mol-1 range, with the two invertomers exhibiting similar abundance. Substitution of dprs with dps affects the relative invertomer population leaving the magnitude of ΔG† 298 κ (52.0-50.6 kJ mol-1) practically unchanged. Conversely, the substitution of the pyridine thioethers (L1, L2) with pyrimidine thioethers (L5, L6) influences the inversion barriers, and ΔG† 298 κ values of 47.5 and 47.0 kJ mol-1 were found for [Ru(N,N-dps)2(N,S-L5)][PF6]2 and [Ru(N,N- dps)2(N,S-L6)][PF6]2, respectively. An intramolecular mechanism without any bond rupture is suggested on the basis of the ΔS† values (negative or close to zero) and the NMR spectra, temperature -reversible and concentration-independent. The contemporary presence of the congested Ru(N,N-dps)2 or Ru(N,N-dprs)2 core and sterically demanding N,S-coordinated thioether ligands is invoked to explain the low energy barrier of the process. This hypothesis is also corroborated by the different behaviour observed for the complex [Ru(bipy)2(N,S-L1)][PF6]2.