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Restricted Phenyl Rotation in Pyridyl Thioether Ligands N,S‐Chelated to Congested Diiminoruthenium Cores
Author(s) -
Baradello Laura,
Lo Schiavo Sandra,
Nicolò Francesco,
Lanza Santo,
Alibrandi Giuseppe,
Tresoldi Giuseppe
Publication year - 2004
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200400120
Subject(s) - diimine , chemistry , sulfide , methylene , sulfur , thioether , medicinal chemistry , ruthenium , chelation , ring (chemistry) , stereochemistry , crystallography , photochemistry , inorganic chemistry , organic chemistry , catalysis
The thioethers 2‐pyridylmethyl p ‐tolyl sulfide ( a ) and p ‐chlorophenyl 2‐pyridylmethyl sulfide ( b ) react with the precursors cis ‐[RuCl 2 ( 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 NH 4 PF 6 to give the complexes [Ru( N , N ‐diimine) 2 ( N , S ‐ a )][PF 6 ] 2 and [Ru( N , N ‐diimine) 2 ( N , S ‐ b )][PF 6 ] 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 1 H 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 (Δ G # 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 H 4 or H 5 dps protons with methyl groups and H 5 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 Δ S /Λ R of [Ru(dps) 2 (2‐pyridymethyl 2‐pyridyl sulfide)][PF 6 ] 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)