Paddlewheel-type and half-paddlewheel-type diruthenium(II,II) complexes with 1,8-naphthyridine-2-carboxylate

Paddlewheel-type diruthenium(II,II) complexes are paramagnetic with two unpaired electrons ( = 1) and can be utilized as versatile building blocks for higher-order structures, such as supramolecular complexes, coordination polymers, and metal-organic frameworks, although they are generally highly air-sensitive. In this study, we developed an air-stable paddlewheel-type diruthenium(II,II) complex with two electron-withdrawing 1,8-naphthyridine-2-carboxylate (npc) ligands, [Ru (μ-npc) (O CMe) ] (1). The two acetate ligands in 1 can be replaced by other carboxylate ligands; the solvothermal reactions of 1 with benzoic acid (HO CPh) yields the heteroleptic [Ru (μ-npc) (O CPh) ] (2), whereas its reaction with 1,8-naphthyridine-2-carboxylic acid (Hnpc) produces the homoleptic [Ru (μ-npc) (η -npc) ] (3). The molecular structures of 1-3 were characterized using paramagnetic H NMR, ESI-TOF-MS, elemental analyses, and single-crystal X-ray diffraction, which revealed that 1 and 2 form conventional paddlewheel-type structures, where two npc and two carboxylate ligands coordinate to the Ru core in a -2 : 2 arrangement, whereas 3 forms a half-paddlewheel-type structure, with the Ru core coordinated by two bridging μ-npc and two chelating η -npc ligands. Temperature-dependent magnetic susceptibility measurements of 1-3 showed large zero-field splittings ( = 227, 238, and 240 cm , respectively) due to the Ru center, and their effective magnetic moments at 300 K, ranging from 2.78 to 2.90 , are consistent with the spin-only value of 2.83 for an = 1 system. Electrochemical analyses revealed that 1-3 are redox-active and undergo reversible redox processes; their cyclic voltammetry (CV) diagrams showed an oxidation wave associated with the Ru /Ru process and two sequential reduction waves corresponding to the reduction of two npc ligands. Notably, 1-3 show intense broad absorption bands at approximately 500-800 nm, theoretically assigned to the metal-ligand charge transfers (MLCTs) from the d(Ru ) to π*(npc) orbitals.