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

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The ligand-exchange reaction of [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ] with 4,6-dimethyl-2-hydoroxypyridine (Hdmhp) yielded three structural isomers of paddlewheel-type dirhodium (Rh 2 ) complexes: trans -2:2-[Rh 2 (dmhp) 4 ] ( 1 ), cis -2:2-[Rh 2 (dmhp) 4 ] ( 2 ), and 3:1-[Rh 2 (dmhp) 4 ] ( 3 ). The obtained Rh 2 complex isomers were characterized using 1 H 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-[Rh 2 (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 ( E 1/2 ) 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 ]SbCl 6 ) was used as an oxidant, 1 was easily oxidized to 1 (SbCl 6 ) without significant structural changes. Variable-temperature magnetic susceptibility measurements revealed the following: (i) the magnetic moment of 1 (SbCl 6 ) at 300 K was 1.74 μ B, indicating the presence of an unpaired electron ( S = 1/2) in the paramagnetic Rh 2 5+ core, and (ii) a weak antiferromagnetic interaction was mediated in the solid state of 1 (SbCl 6 ), as suggested by the negative Weiss temperature. Remarkably, the one-electron oxidized species of 1 – 3, obtained using [N(4-BrPh) 3 ]SbCl 6 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 Rh 2 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.