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

The reaction between M-2(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 M-2(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 pi systems are coupled by their interactions with the M-2 delta orbitals. Their intense colors arise from M-2 delta to azulene pi* electronic transitions. While compound I exhibits weak emission at similar to 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 W-2(O2C)(4) core and the trans orientation of the ligands.