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 2.90μ_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.