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 by 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] exhibited a one-electron reduction processes, i.e., [1]/[1]- and [2]/[2]-, at redox potentials (E1/2) of -0.08 and -0.18 V vs SCE, respectively. These results agreed well with the DFT-calculated E1/2 values of [1] (-0.08 V vs SCE) and [2] (-0.18 V vs SCE) and were theoretically assigned as Ru2-centred (δ*(Ru2)) redoxes. Moreover, [1] and [2] showed unique near-infrared absorption bands at approximately 1200–1500 nm, which were theoretically assigned as the ligand-to-metal charge transfer (LMCT) with π*(amp or ammp) → δ*(Ru2) transition characters.