Hemilabile Thioether Ligands Based on Pyrimidine and/or Pyridine Derivatives that Interconvert between N,S‐ and N‐Coordination in Congested Ruthenium(II) Complexes

Abstract

AbstractThe 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)