Electronic structure and excited-state dynamics of the molecular triads: trans-M2(T(i)PB)2[O2CC6H5-η6-Cr(CO)3]2, where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate.

From the reactions between M(2)(T(i)PB)(4) and HO(2)CC(6)H(5)-η(6)-Cr(CO)(3) (2 equiv), the title compounds trans-M(2)(T(i)PB)(2)[O(2)CC(6)H(5)-η(6)-Cr(CO)(3)](2), where M = Mo or W, and T(i)PB = 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(2)δ 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 (1)MLCT 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 (1)MLCT and (3)MLCT states in THF, while I in CH(2)Cl(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(2)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.