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 (S = 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, [Ru₂(μ-npc)₂(O₂CMe)₂] (1). The two acetate ligands in 1 can be replaced by other carboxylate ligands; the solvothermal reactions of 1 with benzoic acid (HO₂CPh) yields the heteroleptic [Ru₂(μ-npc)₂(O₂CPh)₂] (2), whereas its reaction with 1,8-naphthyridine-2-carboxylic acid (Hnpc) produces the homoleptic [Ru₂(μ-npc)₂(η²-npc)₂] (3). The molecular structures of 1-3 were characterized using paramagnetic ¹H 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 Ru₂ core in a cis-2 : 2 arrangement, whereas 3 forms a half-paddlewheel-type structure, with the Ru₂ core coordinated by two bridging μ-npc and two chelating η²-npc ligands. Temperature-dependent magnetic susceptibility measurements of 1-3 showed large zero-field splittings (D = 227, 238, and 240 cm^-1, respectively) due to the Ru₂⁴⁺ center, and their effective magnetic moments at 300 K, ranging from 2.78 to 2.90μ_B, are consistent with the spin-only value of 2.83μ_B for an S = 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 Ru₂⁵⁺/Ru₂⁴⁺ 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(Ru₂) to π*(npc) orbitals.