Paddlewheel-type diruthenium(iii,iii) tetrakis(2-aminopyridinate) complexes with NIR absorption features: Combined experimental and theoretical study

The reactions of [Ru₂(O₂CCH₃)₄Cl] with 2-aminopyridine (Hamp) and 2-amino-4-methylpyridine (Hammp) afforded two novel Ru₂ complexes, [Ru₂(amp)₄Cl₂] (1) and [Ru₂(ammp)₄Cl₂] (2), respectively. Single crystal X-ray diffraction analyses revealed that 1 and 2 adopted typical paddlewheel-type structures, where the Ru₂ units are coordinated with four aminopyridinate ligands with a cis-2:2 arrangement at the equatorial positions and two chloride ligands at the axial positions. The stabilities of 1 and 2 were supported by unrestricted density functional theory (uDFT) calculations. The zero-point energies of the three structural isomers (trans-2:2, 3:1, and 4:0 arrangements) of 1 and 2 were less stable than those of the respective cis-2:2 arrangements. Temperature-dependences of the magnetic susceptibility measurements and uDFT calculations showed that the oxidation and spin states of the Ru₂ units in 1 and 2 were commonly Ru₂⁶⁺ and triplet states, respectively. Cyclic voltammetry showed that 1 and 2 underwent one-electron reduction processes, i.e., 1/1^- and 2/2^-, at redox potentials (E_1/2) of -0.08 and -0.18 V vs. SCE, respectively. These results agreed well with the DFT-calculated E_1/2 values of 1 (-0.08 V vs. SCE) and 2 (-0.18 V vs. SCE) and were theoretically attributed to Ru₂-centred (δ∗(Ru₂)) redoxes. Moreover, 1 and 2 showed unique near-infrared absorption bands at approximately 1200-1500 nm, which were theoretically attributed to the ligand-to-metal charge transfer (LMCT) with π(amp or ammp) → δ∗(Ru₂) transition characteristics.