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Electronic Structure and Excited-State Dynamics of the Molecular Triads: trans-M2(TiPB)2[O2CC6H5-η6-Cr(CO)3]2, Where M = Mo or W, and TiPB = 2,4,6-triisopropylbenzoate

Brown-Xu, Samantha E., Chisholm, Malcolm H., Durr, Christopher B., Gustafson, Terry L., Naseri, Vesal, Spilker, Thomas F.
Journal of the American Chemical Society 2012 v.134 no.51 pp. 20820-20826
X-radiation, chromium, energy, infrared spectroscopy, methylene chloride, molybdenum, photolysis, tungsten
From the reactions between M₂(TⁱPB)₄ and HO₂CC₆H₅-η⁶-Cr(CO)₃ (2 equiv), the title compounds trans-M₂(TⁱPB)₂[O₂CC₆H₅-η⁶-Cr(CO)₃]₂, where M = Mo or W, and TⁱPB = 2,4,6-triisopropylbenzoate have been prepared and characterized. Compound I (M = Mo) was characterized by a single crystal X-ray structural determination which revealed a centrosymmetric MoMo quadruply bonded molecule. Compound I is red and the tungsten complex II is blue as a result of intense metal-to-ligand charge transfer (MLCT), which is principally M₂δ to benzoate π* with some chromium t₂g participation, according to calculations employing density functional theory. Compound I shows dual emission from S₁ and T₁ states that are assigned ¹MLCT and ³ MoMoδδ*, respectively. Both complexes have been studied by time-resolved infrared spectroscopy (TRIR) in the region of the carbonyl stretching frequency. Compound II displays a shift of ν(CO) to lower energy in both the ¹MLCT and ³MLCT states in THF, while I in CH₂Cl₂ shows ν(CO) bands shifted to both higher and lower energy. We attribute the shift to higher energy seen for I to a Cr t₂g to benzoate π* transition which mixes with the Mo₂δ to benzoate charge transfer upon excitation at 514 nm. In THF compound I undergoes a reversible photodissociation, potentially due to CO loss. Based on the TRIR of the carbonyl vibrations, it is proposed that the MLCT states are delocalized over both benzoate Cr(CO)₃ groups, as supported by calculations.