β-Diketonate-bearing half-paddlewheel-type diruthenium(II,II) naphthyridine complexes with well-defined Ru-Ru double...

Three new diruthenium(II,II) naphthyridine complexes with β-diketonate ligands, [Ru₂(npc)₂(L)₂] (npc = 1,8-naphthyridine-2-carboxylate, L = acetylacetonate (acac) for 2, hexafluoroacetylacetonate (hfac) for 3, and 1,3-diphenyl-1,3-propanedionate (dppd) for 4), were successfully synthesized via solvothermal reactions of [Ru₂(npc)₂(O₂CMe)₂] (1) with the corresponding β-diketone ligands. Crystallographic analyses revealed that 2-4 commonly adopt half-paddlewheel-type structures, which feature (i) a direct Ru-Ru double bond (2.3671(2), 2.3742(4), and 2.3587(5) Å, respectively), (ii) two npc ligands bridging the Ru₂ core in head-to-tail fashion, and (iii) two β-diketonate ligands chelating each Ru ion. Temperature-dependent magnetic susceptibility measurements revealed that 2-4 exhibit effective magnetic moments of 2.85, 2.75, and 2.80μ_B at 300 K, respectively, which are consistent with the spin-only value of 2.83μ_B for an S = 1 system, and large zero-field splitting parameters (D = 235, 345, and 235 cm^-1, respectively), which are commonly observed for paddlewheel-type Ru₂⁴⁺ complexes. Spin-density distributions and molecular orbital analyses based on density functional theory (DFT) calculations clarified that (a) two magnetic spins in 2-4 are predominantly localized on the Ru₂⁴⁺ core, (b) the electronic configuration of the Ru₂ core is commonly π⁴δ²σ²δ*²π*², and (c) the LUMO(β)s of 2 and 4 are mainly derived from the π*(npc) orbitals, whereas that of 3 is primarily composed of the π*(Ru₂)/π*(hfac) orbital. Electrochemical properties are strongly influenced by the β-diketonate ligands; 3 exhibits a markedly positive shift of the oxidation wave attributed to the Ru₂⁵⁺/Ru₂⁴⁺ redox couple compared with 2 and 4, owing to the strong electron-withdrawing effect of the hfac ligand. Furthermore, a Ru₂⁴⁺/Ru₂³⁺ redox couple was observed for 3 at -0.37 V, whereas the corresponding reduction waves were not detected for 2 and 4. The absorption spectra of 2 and 4 exhibit intense bands in the 550-750 nm region, which are attributed to MLCT transitions from the Ru₂ moiety to the npc ligands, and are similar to that of 1. In contrast, the absorption bands of 3 observed in the 502-630 nm region, which are significantly blue-shifted by 50-120 nm relative to those of 2 and 4 due to the strong electron-withdrawing effect of the CF₃ group in the hfac ligand, were assigned to MLCT transitions from the Ru₂ core to the npc ligands as well as the hfac ligands based on time-dependent DFT calculations.