Control of Stereoselectivity in the Ring-Opening Metathesis Polymerization of Norbornene by the Auxiliary Ligands Butadiene and o-Xylylene in Well-Defined Pentamethylcyclopentadiene Tantalum Carbene Complexes

cis-Dialkyl complexes of tantalum, Ta(CH2Ph)2Cp*(η4-C4H6) (2a) (Cp* = η5-C5Me5), TaMe(CH2SiMe3)Cp*(η4-butadiene) (6), and TaMe(CH2CMe3)Cp*(η4-butadiene) (7), were found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene to give poly(norbornene) with a high cis-vinylene double-bond (97−99%) content, while an o-xylylene complex Ta(CH2Ph)24-o-(CH2)2C6H4)Cp* (9) was also an initiator to give poly(norbornene) with a high trans-vinylene double-bond (92−95%) content. When a Cp−butadiene complex Ta(CH2Ph)2Cp(η4-C4H6) (2b) was used as an initiator, we obtained poly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). The factors controlling these stereoselectivities have been investigated, and we obtained the following results:  (1) We isolated benzylidene complexes and found that the proportion of anti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalum center, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe3 resulted in the formation of a benzylidene complex Ta(CHPh)Cp*(η4-C4H6)(PMe3) (3a) as an anti-rotamer, which has been characterized by X-ray crystal structure analysis, while a similar treatment of 2b afforded Ta(CHPh)Cp(η4-C4H6)(PMe3) (3b), also in the anti-rotamer form, as revealed by a comparison of the chemical shift value of the benzylic proton of 3b with that of 3a. In sharp contrast to the anti-geometry, a benzylidene complex Ta(CHPh)(η4-o-(CH2)2C6H4)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, was obtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it is a syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes are considered as intermediates during the propagation step. We could not isolate any metallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane (15) was isolated in 18% yield when acenaphthylene, instead of the norbornene, was added to 12. The trans-phenyl geometry of the metallacyclobutane ring system, which indicated the retention of the TaC bond stereochemistry, was determined by the X-ray analysis of 15, whose structural features were compared with that of (14). (3) In contrast to high cis/trans stereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the 13C NMR spectra of the hydrogenated derivatives was found to be almost atactic. This suggests that the alkylidene species would be a resting state during the propagation, and thereby monomer could be added to both faces of the TaC bond, leading to the atactic polymer. Consequently, the high cis and trans stereoselectivity thus demonstrated is ascribed to the huge congestion between the Cp* ligand and auxiliary ligands 1,3-butadiene or o-xylylene.