Steric isomerization of alkyne–dialkyltungsten complexes with a chelating diaryloxo ligand: crystal structures of Cs- and C1-W(η2-RCCR)[2,2′-S(4-Me-6-R′C6H2O)2](CH2SiMe3)2

Dichlorotungsten complexes, W(η2-EtCCEt)(Me2tbp)Cl2 (Me2tbp=2,2′-thiobis(4,6-dimethylphenoxo)) and W(η2-EtCCEt)(tBu2tbp)Cl2 (tBu2tbp=2,2′-thiobis(4-methyl-6-tert-butylphenoxo)) were reacted with Mg(CH2SiMe3)2 to give the corresponding Cs-dialkyl complexes, Cs-W(η2-EtCCEt)(Me2tbp)(CH2SiMe3)2 (1-Cs) and Cs-W(η2-EtCCEt)(tBu2tbp)(CH2SiMe3)2 (2-Cs). These Cs symmetric dialkyltungsten complexes 1-Cs and 2-Cs were found to undergo steric isomerization to C1 symmetric isomers, 1-C1 and 2-C1, at 30 °C. The thermodynamic parameters for isomerization from 1-Cs to 1-C1 were estimated. The structures of both the Cs- and C1-isomers of 1 were determined by X-ray crystallography, indicating that the alkyne ligands in the Cs symmetric complexes could be regarded as four-electron donor ligands and that those in the C1 symmetric complexes are donating two electrons. The C1 symmetric dialkyltungsten complexes were heated to 30–60 °C to generate alkylidene species. The substituent effects of the ligands on the catalytic behavior for the ring-opening metathesis polymerization of norbornene derivatives were studied. cis-Contents of the poly(norbornene)s were controlled by the steric effect of alkyne and aryloxo ligands in these dialkyl complexes. The polymerization of 7-methylnorbornene catalyzed by 2, W(η2-PhCCPh)(tBu2tbp)(CH2Ph)2 (3-C1) and Mo(η2-PhCCPh)(tBu2tbp)Cl2 (5)–Mg(CH2SiMe3)2 gave the polymers with a cis-atactic structure.