Ruthenium Bis-diimine Complexes with a Chelating Thioether Ligand: Delineating 1,10-Phenanthrolinyl and 2,2′-Bipyridyl Ligand Substituent Effects

Despite the high π-acidity
of thioether donors, ruthenium­(II)
complexes with a bidentate 1,2-bis­(phenyl­thio)­ethane
(dpte) ligand and two chelating diimine ligands (i.e., Ru­(diimine)2­(dpte)2+) exhibit room-temperature fluid
solution emission originating from a lowest MLCT excited state (diimine
= 2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine
4,4′-di-tert-butyl-2,2′-bipyridine,
1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-chloro-1,10-phenanthroline,
5-bromo-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline,
and 3,4,7,8-tetramethyl-1,10-phenanthroline). Crystal structures show
that the complexes form 2 of the 12 possible conformational/configurational
isomers, as well as nonstatistical distributions of geometric isomers;
there also are short intramolecular π–π interactions
between the diimine ligands and dpte phenyl groups. The photoinduced
solvolysis product, [Ru­(diimine)2­(CH3CN)2]­(PF6)2, for one complex
in acetonitrile also was characterized by single-crystal X-ray diffraction.
Variations in the MLCT energies and Ru­(III/II) redox couple, E°′(Ru3+/2+), can be understood in
terms of the influence of the donor properties of the ligands on the
mainly metal-based HOMO and mainly diimine ligand-based LUMO. E°′(Ru3+/2+) also is quantitatively
described using a summative Hammett parameter (σT), as well as using Lever’s electrochemical parameters (EL). Recommended parametrizations
for substituted 2,2′-bipyridyl and 1,10-phenanthrolinyl ligands
were derived from analysis of correlations of E°′(Ru3+/2+) for 99 homo- and heteroleptic ruthenium­(II) tris-diimine
complexes. This analysis reveals that variations in E°′(Ru3+/2+) due to substituents at the 4-
and 4′-positions of bipyridyl ligands and 4- and 7-positions
of phenanthrolinyl ligands are significantly more strongly correlated
with σp+ than either σm or σp. Substituents at the 5- and 6-positions of
phenanthrolinyl ligands are best described by σm and
have effects comparable to those of substituents at the 3- and 8-positions.
Correlations of EL with σT for 1,10-phenanthrolinyl and 2,2′-bipyridyl ligands show
similar results, except that σp and σp+ are almost equally effective in describing the influence
of substituents at the 4- and 4′-positions of bipyridyl ligands.
MLCT energies and d5/d6-electron redox couples
of the complexes with 5-substituted 1,10-phenanthroline exhibit correlations
with values for other d6-electron metal complexes that
can be rationalized in terms of the relative number of diimine ligands
and substituents.