6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties

Two new paddlewheel-type dirhodium (Rh2) complex isomers, formulated as trans-2,2- and 3,1-forms of [Rh2(bhp)4] (bhp = 6-bromo-2-hydroxypyridinate), were obtained by the reaction of 6-bromo-2-hydroxypyridine with [Rh2(O2CCH3)4(H2O)2] and characterized by NMR, ESI-MS, and elemental analyses. Single crystal X-ray diffraction analyses clarified that the crystal structure of trans-2,2-form takes a conventional paddlewheel-type dimer structure with no axial coordination ligands, i.e., trans-2,2-[Rh2(bhp)4], whereas that of the 3,1-form changed significantly depending on the kinds of solvent used for crystallization processes; dimer-of-dimers-type tetrarhodium complex, i.e., 3,1-[Rh2(bhp)4]2, and a conventional paddlewheel-type dimer complex with an axial DMF ligand, i.e., 3,1-[Rh2(bhp)4(DMF)], were observed. The 3,1-form showed unique absorption changes that were not observed in the trans-2,2-form; the trans-2,2-form showed an absorption band at approximately 780 nm both in the solid state and in solution (CH2Cl2 and DMF), whereas the 3,1-form showed a similar absorption band at 783 nm in CH2Cl2 solution, but their corresponding bands were blue-shifted in solid state (655 nm) and in DMF solution (608 nm). The molecular structures and the origin of their unique absorption properties of these Rh2 complexes were investigated using density functional theory (DFT) and time-dependent DFT (TDDFT).