Synthesis, Structure, and Reactivity of the Sterically Crowded Th³⁺ Complex (C₅Me₅)₃Th Including Formation of the Thorium Carbonyl, [(C₅Me₅)₃Th(CO)][BPh₄]

The Th³⁺ complex, (C₅Me₅)₃Th, has been isolated despite the fact that tris­(pentamethylcyclopentadienyl) complexes are highly reactive due to steric crowding and few crystallographically characterizable Th³⁺ complexes are known due to their highly reducing nature. Reaction of (C₅Me₅)₂ThMe₂ with [Et₃NH]­[BPh₄] produces the cationic thorium complex [(C₅Me₅)₂ThMe]­[BPh₄] that can be treated with KC₅Me₅ to generate (C₅Me₅)₃ThMe, <b>1</b>. The methyl group on (C₅Me₅)₃ThMe can be removed with [Et₃NH]­[BPh₄] to form [(C₅Me₅)₃Th]­[BPh₄], <b>2</b>, the first cationic tris­(pentamethylcyclopentadienyl) metal complex, which can be reduced with KC₈ to yield (C₅Me₅)₃Th, <b>3</b>. Complexes <b>1</b>–<b>3</b> have metrical parameters consistent with the extreme steric crowding that previously has given unusual (C₅Me₅)⁻ reactivity to (C₅Me₅)₃M complexes in reactions that form less crowded (C₅Me₅)₂M-containing products. However, neither sterically induced reduction nor (η¹-C₅Me₅)⁻ reactivity is observed for these complexes. (C₅Me₅)₃Th, which has a characteristic EPR spectrum consistent with a d¹ ground state, has the capacity for two-electron reduction via Th³⁺ and sterically induced reduction. However, it reacts with MeI to make two sterically more crowded complexes, (C₅Me₅)₃ThI, <b>4</b>, and (C₅Me₅)₃ThMe, <b>1</b>, rather than (C₅Me₅)₂Th­(Me)­I. Complex <b>3</b> also forms more crowded complexes in reactions with I₂, PhCl, and Al₂Me₆, which generate (C₅Me₅)₃ThI, (C₅Me₅)₃ThCl, and (C₅Me₅)₃ThMe, <b>1</b>, respectively. The reaction of (C₅Me₅)₃Th, <b>3</b>, with H₂ forms the known (C₅Me₅)₃ThH as the sole thorium-containing product. Surprisingly, (C₅Me₅)₃ThH is also observed when (C₅Me₅)₃Th is combined with 1,3,5,7-cyclooctatetraene. [(C₅Me₅)₃Th]­[BPh₄] reacts with tetrahydrofuran (THF) to make [(C₅Me₅)₃Th­(THF)]­[BPh₄], <b>2-THF</b>, which is the first (C₅Me₅)₃M of any kind that does not have a trigonal planar arrangement of the (C₅Me₅)⁻ rings. It is also the first (C₅Me₅)₃M complex that does not ring-open THF. [(C₅Me₅)₃Th]­[BPh₄], <b>2</b>, reacts with CO to generate a product characterized as [(C₅Me₅)₃Th­(CO)]­[BPh₄], <b>5</b>, the first example of a molecular thorium carbonyl isolable at room temperature. These results have been analyzed using density functional theory calculations