Reaction of Diethyl 2-Hydroxyazulene-1,3-dicarboxylate with Metal Acetates and Alkoxides: Metal Complex Formation and Transesterification

Azulene-embedded heterometallacycles can be handled in air and are expected to have a glossy color derived from the azulenyl ligand. However, very few studies have clearly described the synthesis, structure, and effects of metal species. In this study, the reactions of diethyl 2-hydroxyazulene-1,3-dicarboxylate (DEHA, other abbreviation as H-L) with various metals were investigated to synthesize metal complexes (<b>Metals-L</b>). <b>Metals-L</b> were synthesized by the reaction of metal acetates (Pd, Cu, Ni, Co, and Zn acetate) with DEHA and Cs₂CO₃. Single-crystal X-ray structural analysis showed that <b>Pd-L</b> and <b>Cu-L</b> had mononuclear planar and binuclear structures, respectively. In the case of zinc acetate, the formation of a one-dimensional coordination polymer, [ZnCs-(OC₁₀H₅(COOC₂H₅)₂)₃] _<i>n</i> , was confirmed. For cobalt and nickel acetates, the formation of similar coordination polymers was suggested by mass spectrometry, infrared spectrometry, and energy-dispersive X-ray spectrometry. These complexes were mixed in poly-(methyl methacrylate) (PMMA), and the PMMA composite films exhibited coloration corresponding to the color of the complexes. When DEHA was mixed with titanium tetraisopropoxide and Cs₂CO₃ in an alcohol (<i>i</i>PrOH, <i>n</i>BuOH, or allyl alcohol), transesterification was observed by nuclear magnetic resonance spectroscopy. This transesterification was necessary for both titanium tetraisopropoxide and Cs₂CO₃, and a facile transesterification reaction of DEHA was observed for the first time.