Paddlewheel-Type Dirhodium Complexes with 4,6-Dimethyl-2-hydroxypyridate: A Comprehensive Study of Structures, Absorption, Electrochemical and Magnetic Properties, and DFT Calculations

The ligand-exchange reaction of [Rh2(O2CCH3)4(H2O)2] with 4,6-dimethyl-2-hydoroxypyridine (Hdmhp) yielded three structural isomers of paddlewheel-type dirhodium (Rh2) complexes: trans-2:2-[Rh2(dmhp)4] (1), cis-2:2-[Rh2(dmhp)4] (2), and 3:1-[Rh2(dmhp)4] (3). The obtained Rh2 complex isomers were characterized using 1H NMR spectroscopy, electrospray ionization-time-of-flight mass spectrometry (ESI-TOF-MS), elemental analysis, and single-crystal X-ray diffraction (SCXRD). SCXRD analysis revealed that 1 and 2 exhibit the typical discrete paddlewheel-type structure, whereas 3 forms a dimer-of-dimers structure (3-dimer), where two 3:1-[Rh2(dmhp)4] molecules are self-aggregated. Cyclic voltammetry measurements showed that 1 and 2 exhibit a reversible oxidation wave at 0.81 and 0.77 V vs SCE, respectively. In contrast, 3 shows two reversible oxidation waves at redox potentials (E1/2) of 0.69 and 0.94 V vs SCE, attributable to the dimer-of-dimers structure. When tris(4-bromophenyl)ammoniumyl hexachloroantimonate ([N(4-BrPh)3]SbCl6) was used as an oxidant, 1 was easily oxidized to 1(SbCl6) without significant structural changes. Variable-temperature magnetic susceptibility measurements revealed the following: (i) the magnetic moment of 1(SbCl6) at 300 K was 1.74 μB, indicating the presence of an unpaired electron (S = 1/2) in the paramagnetic Rh25+ core, and (ii) a weak antiferromagnetic interaction was mediated in the solid state of 1(SbCl6), as suggested by the negative Weiss temperature. Remarkably, the one-electron oxidized species of 1–3, obtained using [N(4-BrPh)3]SbCl6 as the oxidant, exhibited characteristic wide-range visible-NIR absorption, attributed to ligand-to-metal charge transfers (LMCTs). The electronic structures and absorption features of the Rh2 complexes were further investigated by using density functional theory (DFT) and time-dependent DFT (TDDFT), and the results are discussed in detail in this study.