Paddlewheel Complexes with Azulenes: Electronic Interaction between Metal Centers and Equatorial Ligands.

Rhodium dinuclear complexes (1-3) with azulene moieties as equatorial ligands were obtained by reacting Rh2 (OAc)4 with guaiazulene-2-carboxylic acid, azulene-2-carboxylic acid, and azulene-1-carboxylic acid, respectively. The molecular structures in their crystalline states were determined by X-ray diffraction to be 1 ⋅ (H2 O)2 , 1 ⋅ (MeCN)2 , 2 ⋅ (MeCN)2 , and 3 ⋅ (DMF)2 , which were coordinated with the crystallization solvent at the axial positions. Among these, the crystal packing of 1 ⋅ (H2 O)2 , 1 ⋅ (MeCN)2 , and 3 ⋅ (DMF)2 revealed the formation of one-dimensional stacked chains nearly along the axial direction and of two-dimensional stacked sheets along the equatorial direction. In addition, it was found that 1 ⋅ (H2 O)2 contained cavities that could adsorb CO2 , thereby inducing structural changes. Furthermore, redox measurements revealed the stepwise one-electron redox behaviors of these complexes, indicating the intramolecular interactions between the azulene units. In addition, transient absorption measurements suggested the presence of an ultrafast intersystem crossing caused by the heavy-atom effect of rhodium, and an extended lifetime of the triplet state due to the energy migration among the azulene ligands.