Effects of Countercations on the Structures and Redox and Spectroscopic Properties of Diruthenium Catecholate Complexes with Ligand-Unsupported Ru−Ru Bonds

The molecular structures and physicochemical properties of diruthenium complexes with ligand-unsupported
Ru−Ru bonds, generally formulated as [A2{Ru2(DTBCat)4}] (DTB = 3,5- or 3,6-di-tert-butyl; Cat2- = catecholate),
were studied in detail by changing the countercations. First, the binding structures of the cations in a family of
[{A(DME)n}2{Ru2(3,5-DTBCat)4}] (n = 2 for A+ = Li+ and Na+ and n = 1 for A+ = K+ and Rb+) were systematically
examined to reveal the effects of the cations on the molecular structures and electrochemical properties. Second,
the complex (n-Bu4N)2[Ru2(3,6-DTBCat)4] with a cation-free structure was synthesized using tetra-n-butylammonium
cations. The complex clearly demonstrates first that the ligand-unsupported Ru−Ru bonds are essentially stabilized
by the dianionic nature of the catecholate derivatives without any other bridging or supporting species. In contrast,
the redox potentials and absorption spectra of the complexes can sensitively respond to the countercations depending
upon the polarity of the solvents.