Steric and Electronic Influences of Internal Alkynes on the Formation of Thorium Metallacycles: A Combined Experimental and Computational Study

The formation of thorium metallacyclopentadiene and metallacyclopropene complexes is significantly influenced by the steric and electronic properties of the internal alkyne employed during their syntheses. The reduction of (η⁵-C₅Me₅)₂ThCl₂ (<b>2</b>) with potassium graphite (KC₈) or lithium in the presence of internal phenyl­(alkyl)­acetylenes (PhCCR) selectively yields the corresponding <i>C</i>_<i>s</i>-symmetric thorium metallacyclopentadienes (η⁵-C₅Me₅)₂Th­[η²-C­(Ph)C­(R)­C­(Ph)C­(R)] (R = Me (<b>4</b>), ^<i>i</i>Pr (<b>5</b>), C₆H₁₁ (<b>6</b>)), while the phenyl­(silyl)­acetylene PhCCSiHMe₂ gives the <i>C</i>_2<i>v</i>-symmetric metallacyclopentadiene (η⁵-C₅Me₅)₂Th­[η²-C­(SiHMe₂)C­(Ph)­C­(Ph)C­(SiHMe₂)] (<b>7</b>). However, the sterically more encumbered acetylene derivative PhCCSiMe₃ affords the chloro metallacyclopropene complex [(η⁵-C₅Me₅)₂Th­(η²-C₂Ph­(SiMe₃))­(Cl)]­[Li­(EDM)₂] (<b>8</b>), whereas Me₃SiCCSiMe₃ forms the chloro alkenyl complex [(η⁵-C₅Me₅)₂Th­[C­(SiMe₃)CHSiMe₃]­(Cl) (<b>9</b>), in which the chloro metallacyclopropene intermolecularly activates a C–H bond of the solvent (C₇H₈). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C–C bond formation