Factors Affecting the Electrochemical and Spectroelectrochemical Properties of Diruthenium(III,II) Complexes Containing Four Identical Unsymmetrical Bridging Ligands

Factors affecting the electrochemical and spectroelectrochemical properties of diruthenium(III,II) complexes containing
four unsymmetrical bridging ligands are reported for seven related compounds which were isolated in one or two
of the four possible isomeric forms. The investigated compounds are represented as Ru2(2-CH3ap)4Cl, Ru2(2,5-F2ap)4Cl, Ru2(2,6-F2ap)4Cl, and Ru2(2,4,6-F3ap)4Cl where 2-CH3ap, 2,5-F2ap, 2,6-F2ap, and 2,4,6-F3ap are,
respectively, the 2-(2-methylanilino)pyridinate anion, the 2-(2,5-difluoroanilino)pyridinate anion, the 2-(2,6-difluoroanilino)pyridinate anion, and the 2-(2,4,6-trifluoroanilino)pyridinate anion. Ru2(2-CH3ap)4Cl and Ru2(2,5-F2ap)4Cl
exist only in a (4,0) conformation while Ru2(2,4,6-F3ap)4Cl is present in both (3,1) and (4,0) isomeric forms. Ru2(2,6-F2ap)4Cl also exists in two isomeric forms, but only the (3,1) isomer was generated in sufficient quantities to be
isolated and structurally characterized. This series of seven closely related metal−metal bonded complexes thus
provides the first possibility to systematically examine how differences in position and number of the electron-donating or electron-withdrawing groups on the anionic bridging ligands might be related to the electronic properties
and structural features of the compound as well as the type and number of geometric isomers which are formed.
Each diruthenium derivative undergoes three one-electron transfers in CH2Cl2 containing 0.1 M tetra-n-butylammonium
perchlorate (TBAP). The first reduction and first oxidation products were characterized by thin-layer UV−vis
spectroelectrochemistry, and the spectroscopic data, along with E1/2 values, were then related via linear free energy
relationships to the type of isomer and/or position of the electron-donating or electron-withdrawing substituents on
the anionic ap bridge. The electrogenerated Ru26+ and Ru24+ forms of the compounds were assigned on the basis
of electrochemical and UV−vis spectroscopic data as having the electronic configuration σ2π4δ2π*2 and σ2π4δ2π*3δ*,
respectively, and seemed to be independent of the isomer type ((3,1) or (4,0)). The spectral and electrochemical
properties of the compounds both vary substantially as a function of the isomer type, but this is not reflected in the
structural features of the compounds which are within the range of what is seen for other Ru25+ species described
in the literature. The Ru−Ru bond lengths of the four structurally characterized (4,0) isomers of the ap complexes
range from 2.275 to 2.296 Å while those of the three structurally characterized (3,1) isomers of ap derivatives fall
in the range 2.284−2.286 Å and show no significant difference among the three compounds. The Ru−Cl bond
lengths of the (3,1) isomers do not vary significantly with the bridging ligand and range from 2.458 to 2.471 Å
whereas those of the (4,0) isomers range from 2.437 to 2.487 Å and show larger variations among the compounds.
The Ru−Ru−Cl bond angle is virtually independent of the bridging ligand in the case of the (4,0) isomers but
decreases with the electron-withdrawing effect of the substituent in the case of the (3,1) isomers.