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 (2) with potassium graphite (KC8) or lithium in the presence of internal phenyl­(alkyl)­acetylenes (PhCCR) selectively yields the corresponding Cs-symmetric thorium metallacyclopentadienes (η5-C5Me5)2Th­[η2-C­(Ph)C­(R)­C­(Ph)C­(R)] (R = Me (4), iPr (5), C6H11 (6)), while the phenyl­(silyl)­acetylene PhCCSiHMe2 gives the C2v-symmetric metallacyclopentadiene (η5-C5Me5)2Th­[η2-C­(SiHMe2)C­(Ph)­C­(Ph)C­(SiHMe2)] (7). However, the sterically more encumbered acetylene derivative PhCCSiMe3 affords the chloro metallacyclopropene complex [(η5-C5Me5)2Th­(η2-C2Ph­(SiMe3))­(Cl)]­[Li­(EDM)2] (8), whereas Me3SiCCSiMe3 forms the chloro alkenyl complex [(η5-C5Me5)2Th­[C­(SiMe3)CHSiMe3]­(Cl) (9), 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.