Restricted phenyl rotation in pyridyl thioether ligands N,S-chelated to congested diiminoruthenium cores

The thioethers 2-pyridylmethyl p-tolyl sulfide (a) and p-chlorophenyl 2-pyridylmethyl sulfide (b) react with the precursors cis-[RuCl2(N,N-diimine)2] [diimine = di-2-pyridyl sulfide (dps), 1; 2,2’-bis(4-methylpyridyl) sulfide (4mdps), 2; 2,2’-bis(5-methylpyridyl) sulfide (5mdps), 3; di-2-pyrimidyl sulfide (dprs), 4; and 2,2’-bis(5-ethylpyrimidyl) sulfide (5edprs), 5] in the presence of NH4PF6 to give the complexes [Ru(N,Ndiimine)2(N,S-a)][PF6]2 and [Ru(N,N-diimine)2(N,S-b)][PF6]2, respectively. As a consequence of the N,S chelation all the complexes contain a five-membered RuSCCN(Ru−N) ring, the sulfur and ruthenium atoms of which are stereogenic centres, with (R) and (S) and Δ and Λ configurations, respectively. Furthermore, the coordinated thioethers contain anisochronous methylene protons and phenyl protons which are sensor nuclei for pyramidal sulfur inversion and rotation of the pendant phenyl ring, respectively. In the low-temperature 1H NMR spectra of the complexes a single AB system for the methylene protons, in agreement with a fast sulfur inversion, is observed. Well-separated signals of the two ortho- as well as the meta-phenyl protons, indicating restricted phenyl rotation, are also observed. At higher temperatures the fast exchange of the two halves of the phenyl ring leads to averaged signals for the phenyl protons. The rates and activation energies of this fluxional process were measured by one-dimensional band-shape analysis. Certain trends were immediately apparent. The rotation barrier values (ΔG#298) of the dps, 4mdps and 5mdps complexes were ca.4.0 kJ•mol−1 lower than those of the dprs and 5edprs complexes. On the contrary, substitution of H4 or H5 dps protons with methyl groups and H5 dprs protons with ethyl groups, as well as substitution of a with b, leaves the rotation-barrier values practically unchanged. This effect can be correlated to the steric hindrance produced by the two ligands cis to the rotating group. The crystal structure of the enantiomeric couple ΔS/ΛR of [Ru(dps)2(2-pyridymethyl 2-pyridyl sulfide)][PF6]2 (1c), in which the 2-pyrimidylmethyl 2-pyridyl sulfide (c) ligand acts in an N,S-bidentate mode forming a five-membered RuSCCN(Ru−N) chelate ring, is also reported.