Structure and Photochemical Properties of (μ-Alkoxo)bis(μ-carboxylato)diruthenium Complexes with Naphthylacetate Ligands

Two new dinuclear Ru(III) complexes containing naphthalene moieties, K[Ru2(dhpta)(μ-O2CCH2-1-naph)2] (1) and
K[Ru2(dhpta)(μ-O2CCH2-2-naph)2] (2) (H5dhpta = 1,3-diamino-2-hydroxypropane-N,N,N‘,N‘-tetraacetic acid, naph-1-CH2CO2H = 1-naphthylacetic acid, naph-2-CH2CO2H = 2-naphthylacetic acid), were synthesized. Complex 2
crystallized as an orthorhombic system having a space group of Pbca with unit cell parameters a = 10.6200(5) Å,
b = 20.270(1) Å, c = 35.530(2) Å, and Z = 8. EXAFS analysis of 1 and 2 in the solid states and in solution
clarified that the dinuclear structures of 1 and 2 were kept in DMSO solutions. Variable-temperature magnetic
susceptibility data indicated that the two Ru(III) centers are strongly antiferromagnetically coupled as shown by the
large coupling constants, J = −581 cm-1 (1) and −378 cm-1 (2). In the cyclic voltammograms of 1 and 2, one
oxidation peak and two reduction peaks which were assigned to the redox reaction of the ruthenium moieties were
observed in DMF. The large conproportionation constants estimated from the reduction potentials of RuIIIRuIII and
RuIIIRuII indicated the great stability of the mixed-valent state. The mixed-valent species [RuIIIRuII(dhpta)(μ-O2CCH2−R)2]2- (R = 1-naph (6) and R = 2-naph (7)) were prepared by controlled potential electrolysis of 1 and 2
in DMF. The electronic absorption spectra of 6 and 7 were similar to that of [RuIIIRuII (dhpta)(μ-O2CCH3)2]2- which
is a typical Class II type mixed-valent complex. The fluorescence decay of 1 and 2 indicated that there are two
quenching processes which come from the excimer and monomer states. The short excimer lifetimes of 1 and 2
were ascribed to the energy transfer from the naphthyl moieties to the Ru centers. The different excimer ratio
between 1 and 2 suggested that the excimer formation is affected by the conformation of the naphthyl moieties in
the diruthenium(III) complexes.