Electronic Structure and Excited-State Dynamics of the Molecular Triads: trans-M_2(T~iPB)_2[O_2CC_6H_5-η~6-Cr(CO)_3]_2, Where M = Mo or W, and T~iPB = 2,4,6-triisopropylbenzoate

摘要

From the reactions between M_2(T_iPB)_4 and HO_2CC_6H_5-η~6-Cr(CO)_3 (2 equiv), the title compounds trans-M_2(T_iPB)_2[O_2CC_6H_5-η~6-Cr(CO)_3]_2; where M = Mo or W, and T_iPB = 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 M_2δ to benzoate π~* with some chromium t_(2g) participation, according to calculations employing density functional theory. Compound I shows dual emission from S_1 and T_1 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 CH_2C1_2 shows ν(CO) bands shifted to both higher and lower energy. We attribute the shift to higher energy seen for I to a Cr t_(2g) to benzoate π~* transition which mixes with the Mo_δ 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.