Synthesis of cis-dichloride complexes of Group 6 transition metals bearing alkyne and chalcogen-bridged chelating bis(aryloxo) ligands as catalyst precursors for ring-opening metathesis polymerization

Dichlorotungsten complexes, W(η2-RCCR)(R′2tbp)Cl2 (1-Cs: R=Ph, R′2tbp=2,2′-thiobis(4-methyl-6-tert-butylphenoxo) (tBu2tbp); 2-Cs: R=Et, R′2tbp=2,2′-thiobis(4,6-dimethylphenoxo) (Me2tbp); 3-C1: R=Et, R′2tbp=tBu2tbp; 4-C1: R=SiMe3, R′2tbp=tBu2tbp), W(η2-PhCCPh)(tBu2Tebp)Cl2 (3-C1: tBu2Tebp=2,2′-tellurobis(4-methyl-6-tert-butylphenoxo) (5-C1) and Mo(η2-PhCCPh)(tBu2tbp)Cl2 (6-Cs) were prepared by the reaction of W(η2-RCCR)Cl4 (R=Ph, Et, SiMe3) with 2,2′-thiobis(4-methyl-6-tert-butylphenol), 2,2′-thiobis(4,6-dimethylphenol) or with 2,2′-tellurobis(4-methyl-6-tert-butylphenol). The Cs symmetric complex 1-Cs isomerized to the C1 isomer 1-C1 in solution at room temperature. The molecular structures of 1-C1, 3-C1, 5-C1 and 6-Cs were determined to reveal their six coordinated pseudo-octahedral geometry by X-ray crystallography. The MC(alkyne) bond distances in the Cs complexes 2-Cs and 6-Cs ranging from 2.014(7) to 2.035(6) Å are apparently shorter than those in the corresponding C1 complexes 1-C1, 3-C1 and 5-C1 ranging from 2.04(1) to 2.09(1) Å. In accordance with the structural data, the resonances of the alkyne carbons of the Cs complexes in 13C-NMR spectra appeared at 233.1–244.5 ppm, typical for four electron donating alkynes, and those of the C1 complexes appeared at 198.6–215.4 ppm, typical for two electron donating alkynes. The substituent effects of the ligands on the catalytic behavior of the complexes for the ring-opening metathesis polymerization of norbornene derivatives were studied in the presence of Mg(CH2SiMe3)2 as a cocatalyst. The catalytic activities of the sulfur-bridged complexes tend to increase with increasing the bulkiness of acetylene ligand. The tellurium-bridged complex 5-C1 was found to be remarkably more active than the corresponding sulfur-bridged complexes.