Syntheses, crystal structures, and electrochemical and spectroscopic properties of ruthenium complexes of the N,S-bidentate ligand 2-(2-pyridyl)benzenethiol

Conversion of the amino group of 2-(2-aminophenyl)pyridine into a thiol to give the N,S-donor chelating ligand 2-(2-pyridyl)benzenethiol (HL) afforded the oxidised disulfide L–L which was crystallographically characterised. It shows an interesting example of an intermolecular N · · · S–S interaction (N · · · S distances are 2.778 and 2.724 Å; N · · · S–S angles are both ca. 11°) in which the pyridyl lone pair interacts weakly with the σ* orbital of the S–S bond. Reaction of L–L with [Ru(bipy) 2 Cl 2 ]·2H 2 O (bipy = 2,2′-bipyridine) and RuCl 3 ·xH 2 O afforded [Ru II (bipy) 2 L][PF 6 ] 1 and [Ru III L 3 ] 2 respectively (following in situ reduction of the disulfide) which have N 5 S and mer-N 3 S 3 donor sets respectively (N of pyridyl, S of benzenethiolate). Both were crystallographically characterised and have the expected pseudo-octahedral geometries. An interesting feature of both structures is that the relatively large Ru–S distances (compared to the Ru–N) prevent the pyridyl rings from approaching the metal centre as closely as they would if they were not constrained, so the Ru–N distances are longer than usual. Electrochemical studies show that the benzenethiolate ligands are more effective electron donors to ruthenium (both +2 and +3) than are phenolates: for example, the Ru II –Ru III couple of 1 is at -0.07 V vs. ferrocene–ferrocenium, whereas the same couple of the related N 5 O-co-ordinated complex (O from phenolate) was at +0.03 V. Similarly the Ru III –Ru IV couple of 2 was at -0.21 V, compared to +0.14 V for the N 3 O 3 -co-ordinated analogue. Complex 2 also shows a reversible ligand-based oxidation which is absent for 1, arising from stabilisation of the sulfur-based radical cation by interaction with the lone pair on an adjacent sulfur atom in the co-ordination sphere of the complex, which cannot happen for 1. Electronic spectral properties show that the sulfur donor of 1 weakens the ligand field with respect to [Ru(bipy) 3 ] 2+ , and that 2 has an intense sulfur-to-Ru III ligand-to-metal charge-transfer band.