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

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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 (η 5 -C 5 Me 5 ) 2 ThCl 2 ( 2 ) with potassium graphite (KC 8 ) or lithium in the presence of internal phenyl(alkyl)acetylenes (PhC≡CR) selectively yields the corresponding C s -symmetric thorium metallacyclopentadienes (η 5 -C 5 Me 5 ) 2 Th[η 2 -C(Ph)═C(R)C(Ph)═C(R)] (R = Me ( 4 ), i Pr ( 5 ), C 6 H 11 ( 6 )), while the phenyl(silyl)acetylene PhC≡CSiHMe 2 gives the C 2 v -symmetric metallacyclopentadiene (η 5 -C 5 Me 5 ) 2 Th[η 2 -C(SiHMe 2 )═C(Ph)C(Ph)═C(SiHMe 2 )] ( 7 ). However, the sterically more encumbered acetylene derivative PhC≡CSiMe 3 affords the chloro metallacyclopropene complex [(η 5 -C 5 Me 5 ) 2 Th(η 2 -C 2 Ph(SiMe 3 ))(Cl)][Li(EDM) 2 ] ( 8 ), whereas Me 3 SiC≡CSiMe 3 forms the chloro alkenyl complex [(η 5 -C 5 Me 5 ) 2 Th[C(SiMe 3 )═CHSiMe 3 ](Cl) ( 9 ), in which the chloro metallacyclopropene intermolecularly activates a C–H bond of the solvent (C 7 H 8 ). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C–C bond formation.