Thorium Oxo and Sulfido Metallocenes: Synthesis, Structure, Reactivity, and Computational Studies

The synthesis, structure, and reactivity of thorium oxo and sulfido metallocenes have been comprehensively studied. Heating of an equimolar mixture of the dimethyl metallocene [eta(5)-1,2,4-(Me3C)(3)C5H2](2)ThMe2 (2) and the bis-amide metallocene [eta(5)-1,2,4(Me3C)(3)C5H2](2)Th(NH-p-tolyl)(2) (3) in refluxing toluene results in the base-free imido thorium metallocene, [eta(5)-1,2,4-(Me3C)(3)C5H2](2)Th=N(p-tolyl) (4), which is a useful precursor for the preparation of oxo and sulfido thorium metallocenes [eta(5)-1,2,4-(Me3C)(3)C5H2](2)Th=E (E = O (5) and S (15)) by cycloaddition-elimination reaction with Ph2C=E (E = O, S) or CS2. The oxo metallocene 5 acts as a nucleophile toward alkylsilyl halides, while sulfido metallocene 15 does not. The oxo metallocene 5 and sulfido metallocene 15 undergo a [2 + 2] cycloaddition reaction with Ph2CO, CS2, or Ph2CS, but they show no reactivity with alkynes. Density functional theory (DFT) studies provide insights into the subtle interplay between steric and electronic effects and rationalize the experimentally observed reactivity patterns. A comparison between Th, U, and group 4 elements shows that Th4+ behaves more like an actinide than a transition metal.