Hemilabile thioether ligands based on pyrimidine and/or pyridine derivatives that interconvert between N,S- and N-coordination in congested ruthenium(II) complexes

The thioether ligands L [L = 2-pyridylmethyl 2’-pyridyl sulfide (L1), 2-pyridylmethyl 2’-pyrimidyl sulfide (L2) and 2-pyridylmethyl 2’-(4-methylpyrimidyl) sulfide (L3)] react with cis-[Ru(N,N-diimine)2Cl2] {diimine = 2,2_-bipyridine (bipy), di- 2-pyrimidyl sulfide (dprs), 2,2’ bis(5-ethylpyrimidyl) sulfide (5edprs)} to give compounds [Ru(N,N-diimine)2L][PF6]2 {diimine = bipy, L =L1 (1), L2 (2), L3 (3); diimine = dprs, L = L1 (4); diimine = 5edprs, L = L1 (5); diimine = dprs, L = L2 (6), L = L3 (7); diimine = 5edprs, L = L2 (8), L = L3 (9)}. NMR investigations show that these potentially tridentate ligands act as N,S-bidentate species, to form a five-membered RuSCCN(Ru–N) ring, and in certain cases, as N-monodentate species coordinated to the ruthenium through the 2-pyridylmethyl group. The N,S-chelated species contain chiral sulfur and ruthenium atoms with (R) and (S), and Δ and Λ configurations, respectively. Two invertomers and two sets of NMR signals in the slow-exchange region are expected. However, the low-temperature 1H NMR spectra show that sulfur inversion is fast. The variable-temperature 1H NMR spectra allow two species to be observed (6a + 6b, 7a + 7b, 8a + 8b and 9a+ 9b), which exhibit different abundances. In the minor species (6b, 7b, 8b, and 9b), L2 or L3 exhibits an N-monodentate coordination, while in the major species, the usual N,S-coordination. At low temperatures, the population ratio is about 85:15, while when the temperature increases, the abundance of the minor species grows rapidly. The one-dimensional band-shape analysis of the exchanging methylene proton signals shows that the energy-barrier for the interchange process (ΔG#298) for 6a + 6b, 7a + 7b, 8a + 8b, and 9a + 9b is practically the same (ca. 59.5 kJ•mol–1), while the ΔS# values are negative or near to zero. The possible mechanisms for the process are discussed. The NMR spectroscopic findings strongly support the formation of an N,N-chelated labile intermediate.