Synthesis and Spectroelectrochemical Studies of Mixed Heteroleptic Chelate Complexes of Ruthenium(II) with 1,8-Bis(2-pyridyl)-3,6-dithiaoctane (pdto) and Substituted 1,10-Phenanthrolines

Reaction of dichlorotris(triphenylphosphine) ruthenium(II) [RuCl2(PPh3)3] with 1,8-bis(2-pyridyl)-3,6-dithiaoctane (pdto),
a (N2S2) tetradentate donor, yields a new compound [Ru(pdto)(PPh3)Cl]Cl (1), which has been fully characterized.
1H and 31P NMR studies of 1 in acetonitrile at several temperatures show the substitution of both coordinated
chloride and triphenylphosphine with two molecules of acetonitrile, as confirmed by the isolation of the complex
[Ru(pdto)(CH3CN)2]Cl2 (2). Cyclic voltammetric and spectroelectrochemical techniques allowed us to determine the
electrochemical behavior of compound 1. The substitution of the chloride and triphenylphosphine by acetonitrile
molecules in the Ru(II) coordination sphere of compound 1 was also established by electrochemical studies. The
easy substitution of this complex led us to use it as starting material to synthesize the substituted phenanthroline
coordination compounds with (pdto) and ruthenium(II), [Ru(pdto)(4,7-diphenyl-1,10-phenanthroline)]Cl2·4H2O (3),
[Ru(pdto)(1,10-phenanthroline)]Cl2·5H2O (4), [Ru(pdto)(5,6-dimethyl-1,10-phenanthroline)]Cl2·5H2O (5), [Ru(pdto)(4,7-dimethyl-1,10-phenanthroline)]Cl2·3H2O (6), and [Ru(pdto)(3,4,7,8-tetramethyl-1,10-phenanthroline)]Cl2·4H2O
(7). These compounds were fully characterized, and the crystal structure of 4 was obtained. Cyclic voltammetric
and spectroelectrochemical techniques allowed us to determine their electrochemical behavior. The electrochemical
oxidation processes in these compounds are related to the oxidation of ionic chlorides, and to the reversible
transformation from Ru(II) to Ru(III). On the other hand, a single reduction process is associated to the reduction
of the substituted phenanthroline in the coordination compound. The E1/2 (phen/phen-) and E1/2 (RuII/RuIII) for the
compounds (3−7) were evaluated, and, as expected, the modification of the substituted 1,10-phenanthrolines in
the complexes also modifies the redox potentials. Correlations of both electrochemical potentials with pKa of the
free 1,10-phenathrolines, λmax MLCT transition band, and chemical shifts of phenanthrolines in these complexes
were found, possibly as a consequence of the change in the electron density of the Ru(II) and the coordinated
phenanthroline.