Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes with π-Conjugated Aromatic Carboxylate Ligands

Three heteroleptic diruthenium (Ru2) complexes bridged by 1,8-naphtyridine-2-carboxylate (npc) and π-conjugated aromatic carboxylate ligands, [Ru2(npc)2(O2CNap)2] (3; –O2CNap = 1-naphthalenecarboxylate), [Ru2(npc)2(O2CAnt)2] (4; –O2CAnt = 9-anthracenecarboxylate), and [Ru2(npc)2(O2CPyr)2] (5; –O2CPyr = 1-pyrenecarboxylate), were prepared by carboxylate–exchange reactions of [Ru2(npc)2(O2CMe)2] (1) with the corresponding carboxylic acids. Paramagnetic 1H NMR, ESI-MS, elemental analyses, and single crystal X-ray diffraction analyses revealed that the molecular structures of 3–5 form a “paddlewheel-type” structure, in which two npc and two aromatic carboxylate ligands were coordinated to a Ru2 core with a cis–2:2-arrangement. Variable-temperature magnetic susceptibility measurements of 3–5 showed large zero-field splitting of the triplet ground state of the Ru24+ core (D = 250, 270, and 257 cm–1, respectively). Density functional theory calculations revealed that the electronic configuration of the Ru2 core in 3–5 was π4δ2σ2δ*2π*2, which was the same configuration as that of [Ru2(npc)2(O2CPh)2] (2; –O2CPh = benzoate). The absorption spectra of 3–5 in DMF showed the intense visible absorption bands at 500–800 nm, which were due to the metal–to–ligand charge transfer (MLCT) from the Ru2 core to the npc ligands. The cyclic voltammetry diagrams of 3–5 exhibited two reversible reduction waves and one reversible oxidation wave, corresponding to the reduction of two npc ligands and the oxidation of the Ru2 core, respectively.