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

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.