Elucidation of Electronic Structures of Mixed-Valence States Induced by dσ-π Charge Delocalization in Linear-Chain and Discrete Rhodium-Dioxolene Tetrameric Complexes

We have successfully synthesized unique linear-chain and discrete mixed-valence tetrameric complexes, {[Rh(3,6-DBDiox-4,5-S2CO)(CO)2]4<middle dot>hexane}infinity (4) and [Rh(3,6-DBDiox-4,5-S2CO)(CO)2]4 (5), by carefully choosing the solvent. X-ray photoelectron spectra (XPS) confirm that 4 and 5 are in the Rh(I,II) mixed-valence state. Analyses of the metrical oxidation state (MOS) of dioxolene ligands reveal that in 4 and 5, the electron density corresponding to one electron per tetramer is transferred from Rh(I) ions to semiquinonato ligands, and the transferred charge is delocalized throughout the four dioxolene ligands. Due to their mixed-valence state, 4 and 5 are semiconductors with relatively high electrical conductivity at room temperature. Density functional theory (DFT) calculations of tetrameric complex demonstrated for the first time that the d sigma* orbitals of the Rh atoms and the pi* orbitals of the semiquinonato ligands, which are originally highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively, strongly hybridize with each other, leading to the Rh(I,II)-semiquinonato/catecholato mixed-valence state. Furthermore, time-dependent (TD)-DFT calculations have also revealed that the low energy absorption band observed centered at 5700 cm-1 is attributed to a charge transfer from [d sigma*(Rh)] (HOMO) or [pi*(SQ)-d sigma*(Rh)] (HOMO-1) to [pi*(SQ)-d sigma*(Rh)] (LUMO/LUMO+1). Although 4 and 5 are tetramers with nearly identical structures, their magnetic interactions are found to differ significantly depending on their crystal structures.