Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands

The reaction between M₂(TiPB)₄ (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M₂(TiPB) ₂(6-carboethoxy-2-azulenecarboxylate)₂. Compound I (M = Mo) is blue and compound II (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate that the two azulene carboxylate π systems are coupled by their interactions with the M₂δ orbitals. Their intense colors arise from M₂δ to azulene π* electronic transitions. While compound I exhibits weak emission at ∼ 900 nm, no emission has been detected for II. Both I and II have been studied by fs and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of II in the solid state confirmed the paddlewheel nature of its W₂(O₂C) ₄ core and the trans orientation of the ligands.