Thorium Oxo and Sulfido Metallocenes: Synthesis, Structure, Reactivity, and Computational Studies
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Abstract
The synthesis, structure, and reactivity of thorium oxo and sulfido metallocenes have been comprehensively studied. Heating of an equimolar mixture of the dimethyl metallocene [η<sup>5</sup>-1,2,4-(Me<sub>3</sub>C)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>]<sub>2</sub>ThMe<sub>2</sub> (<b>2</b>) and the bis-amide metallocene [η<sup>5</sup>-1,2,4-(Me<sub>3</sub>C)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>]<sub>2</sub>Th(NH-<i>p</i>-tolyl)<sub>2</sub> (<b>3</b>) in refluxing toluene results in the base-free imido thorium metallocene, [η<sup>5</sup>-1,2,4-(Me<sub>3</sub>C)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>]<sub>2</sub>ThN(<i>p</i>-tolyl) (<b>4</b>), which is a useful precursor for the preparation of oxo and sulfido thorium metallocenes [η<sup>5</sup>-1,2,4-(Me<sub>3</sub>C)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>]<sub>2</sub>ThE (E = O (<b>5</b>) and S (<b>15</b>)) by cycloaddition–elimination reaction with Ph<sub>2</sub>CE (E = O, S) or CS<sub>2</sub>. The oxo metallocene <b>5</b> acts as a nucleophile toward alkylsilyl halides, while sulfido metallocene <b>15</b> does not. The oxo metallocene <b>5</b> and sulfido metallocene <b>15</b> undergo a [2 + 2] cycloaddition reaction with Ph<sub>2</sub>CO, CS<sub>2</sub>, or Ph<sub>2</sub>CS, 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 Th<sup>4+</sup> behaves more like an actinide than a transition metal