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.