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

Abstract

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 2.90 μB, are consistent with the spin-only value of 2.83 μB 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.