Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes

Two diruthenium(II,II) naphthyridine complexes coordinated with 4-trifluoromethylbenzoate (O2CPh-4-CF3) and 3,5-bis(trifluoromethyl)benzoate (O2CPh-3,5-diCF3) ligands, formulated as [Ru2(npc)2(O2CPh-4-CF3)2] (4; npc = 1,8-naphthyridine-2-carboxylate) and [Ru2(npc)2(O2CPh-3,5-diCF3)2] (5), respectively, were synthesized and structurally characterized. Single-crystal X-ray diffraction analysis revealed that both 4 and 5 form a direct metal–metal bond between the two Ru ions (2.2893(8) and 2.2896(7) Å, respectively) and adopt a paddlewheel-type structure in which two npc and two trifluoromethyl-substituted benzoate ligands are coordinated to a Ru24+ core with a cis-2:2 arrangement. The temperature dependence of the magnetic susceptibility measurements of 4 and 5 exhibited very large zero-field splitting (D = 242 and 246 cm−1, respectively) of the triplet ground state of the Ru24+ core, similar to that of [Ru2(npc)2(O2CPh)2] (3; D = 238 cm−1). Owing to the effects of the trifluoromethyl substituents, compared with 3, 4 and 5 showed (i) a significant blue shift of the absorption bands in the visible region and (ii) a positive shift of the redox potentials, with both shifts becoming more pronounced as the number of trifluoromethyl substituents increased. These experimental results are in good agreement with the electronic structure results obtained from density functional theory calculations.