Dynamic Stereochemical Behaviour of Congested Ruthenium(II) Complexes Containing Asymmetric Thioether Ligands Based on Pyridine and Pyrimidine

The asymmetric thioethers L [L = 2-pyridylmethyl 2’-pyrimidyl sulfide (pps) and 2-(4-methylpyrimidyl) 2’-pyridylmethyl sulfide (mps)] reacted with cis-[RuCl2(N,N-L’)2] [L’ = di-2-pyridyl sulfide (dps); 2,2’-bis(4-methylpyridyl) sulfide (4mdps); 2,2’-bis(5-methylpyridyl) sulfide (5mdps)] to give the five-membered-ring chelate complexes [Ru(N,N-L’)2(Npyridine,S-L)]++ as the major products (92–95%). Because the sulfur and ruthenium atoms are stereogenic centres, with (R) and (S) and Δ and Λ configurations, respectively, four isomers, including the enantiomers were obtained. At low temperature and in the methylene region of the 1HNMR spectra, two AB systems due to the enantiomer couples ΔS ΛR (a) and ΔR ΛS (b) were observed with abundances of 77–89 and 6–18%, respectively. Furthermore, NMR spectroscopic investigations showed that the hybrid polydentate ligands L change their coordination mode. Thus, although a and b largely predominate, a mixture of species containing L and the Ru(N,NL’)2 unit in the ratio 1:1 are present. The four-membered ring chelate complexes [Ru(N,N-L’)2(Npyrimidine,S-L)]++ (c), as minor species (abundance 1–8%), are always observed, whereas the dinuclear species [{Ru(N,N-L’)2}2(μ-L)2]+4 (d, e) are observed when L’ = dps or 5mdps. In these cases, four AB systems are assigned to dinuclear species d and e containing two bridging L that act as Npyridine,S- or Npyridine, Npyrimidine-donor ligands. The 1H NMR spectra are temperature dependent in that at low temperature the complexes undergo inversion of the chiral centre of the coordinated sulfur atom (a b) and the dimer (d, e) and monomer (c) are in equilibrium; at higher temperatures the complexes undergo a structural dynamic rearrangement, which involves exchange between the coordinated and uncoordinated N atoms (b c). One-dimensional band-shape analysis of the exchanging methylene and methyl proton signals showed that the energy barriers for inversion of the sulfur centre are in the 50–53 kJmol–1 range, whereas those for the higher-temperatures process are in the 62–68 kJmol–1 range. The possible mechanisms of the processes are discussed. NMR spectroscopic findings suggest that inversion at the sulfur centre occurs without any bond rupture, whereas the exchange, at higher temperatures (b c), is a dissociative process involving the breaking of a Ru–Npyridine bond.