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

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 [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThMe₂ (<b>2</b>) and the bis-amide metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(NH-<i>p</i>-tolyl)₂ (<b>3</b>) in refluxing toluene results in the base-free imido thorium metallocene, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThN(<i>p</i>-tolyl) (<b>4</b>), which is a useful precursor for the preparation of oxo and sulfido thorium metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThE (E = O (<b>5</b>) and S (<b>15</b>)) by cycloaddition–elimination reaction with Ph₂CE (E = O, S) or CS₂. 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 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