(Arylimido)Vanadium(V)-Alkylidenes Containing Chlorinated Phenoxy Ligands: Thermally Robust, Highly Active Catalyst in Ring-Opening Metathesis Polymerization of Cyclic Olefins

Abstract

A series of (imido)vanadium(V)-alkylidene complexes containing pentachlorophenoxy ligand of type, V(CHSiMe 3 )-(N-2,6-R 2 C 6 H 3 )(OC 6 Cl 5 )(PMe 3 ) 2 [R = H, Cl, F, CH 3 ], have been prepared, and the structure of V(CHSiMe 3 )(N-2,6-Me 2 C 6 H 3 )-(OC 6 Cl 5 )(PMe 3 ) 2 was determined by X-ray crystallographic analysis. Ring-opening metathesis polymerization (ROMP) of cyclic olefins such as norbornene (NBE), cyclopentene (CPE), cycloheptene (CHPE), and cis-cyclooctene (COE) using these alkylidene catalysts have been explored, and V(CHSiMe 3 )(N-2,6-Cl 2 C 6 H 3 )(OC 6 Cl 5 )(PMe 3 ) 2 showed higher activities in the ROMP of CPE, CHPE, and COE than those of the reported V(CHSiMe 3 )(N-2,6-Cl 2 C 6 H 3 )(OC 6 F 5 )(PMe 3 ) 2 . The activity in the ROMP of COE increased at high temperature until 120 °C, and the ROMPs of CHPE and COE proceeded without chain-transfer or termination (nor catalyst decomposition); the (quasi) living nature thus maintained even at 80 °C. The activities in the ROMPs of CHPE and COE (at 25 °C) increased upon addition of 1.0 equiv of B(C 6 F 5 ) 3 , whereas the activity in the ROMP of NBE became negligible upon the addition. The order in the activity in the ROMP of cyclic olefins displayed as COE ≪ CHPE < CPE ≪ NBE, which is different from not only that in the ring strain energy but also that reported in the ROMPs using ruthenium−carbene catalysts.

Pushing the Bounds of Olefin Metathesis with Vanadium

“…Presumably, B(C 6 F 5 ) 3 accelerates the dissociation of PMe 3 from the initiator, a prerequisite for ROMP to occur, thus allowing for higher activity and conversion. [78] These results stand in contrast to previous work in which the addition of PMe 3 resulted in higher activities, [80] making the role of phosphine in vanadium-initiated ROMP less clear than in molybdenum [87] or rutheniummediated [88] reactions.…”

“…[63,73,76,77] The polymerization of monomers such as cyclopentene (CPE), cycloheptene (CHE), and cis-cyclooctene (COE) constitute the latest achievement with vanadium alkylidenes. [78][79][80] Historically, these are challenging monomers given their low ring-strains of 28.4, 28.0, and 31.0 kJ/mol, [81] respectively. However, they are also attractive in that they are commercially available, and the hydrogenation of the resulting polyalkenemers yields model, defect-free high-density polyethylene (HDPE).…”

“…Surprisingly, exchange of the fluorinated aryloxide ligand for a chlorinated version (12-14, Chart 4) resulted in initiators capable of polymerizing low strain CPE, CHE, and COE to incredibly high molar masses (M n up to 2.8 MDa) while maintaining moderately low Ð values. [78] Table 1 summarizes these results using CHE as a representative example. Catalyst 12, which bears the more electron withdrawing 2,6-dichlorophenyl group on the imido ligand, exhibited the highest activity of all initiators in this report, while 14 bearing the more electron donating 2,6-dimethylphenyl imido was the least active.…”

Ring‐Closing Olefin Metathesis Catalyzed by Well‐Defined Vanadium Alkylidene Complexes

“…The V−N and V−C bond lengths and Si‐C‐V angles in anti ‐ 1 e (1.6873(14) Å, 1.9153(14) Å, 131.64(7)°) and anti ‐ A (1.691 Å, 1.917 Å, 132.5°) are also similar. Notably, the differences in V−C bond distances and Si‐C‐V angles between anti ‐ 1 e (1.9153(14) Å, 131.64(7)°) and the reported syn ‐V( N ‐2,6‐Me 2 C 6 H 3 )(CHSiMe 3 )(OC 6 Cl 5 )(PMe 3 ) 2 [18] (1.845(3) Å, 167.53°) are more pronounced due to the agostic interaction (electron donation from C−H to V) in the syn ‐isomer, which gives the V= C bond partial triple‐bond character; this has also been observed for Mo alkylidenes [29] …”

“…In the last decade, high‐oxidation‐state vanadium alkylidene complexes of the type V(NR)(CHSiMe 3 )(X)(L), where R is an aryl [12–18] or 1‐adamantyl group; [13, 15, 16, 19] X is an amide, [12, 15] alkyl, [13] or alkoxide; [14, 16–19] and L is an NHC [13] or PMe 3 [14–19] have been extensively explored by the Nomura group for the ring‐opening metathesis polymerization (ROMP) of various cyclic alkenes [20] . The critical step of the alkylidene formation, the α‐hydrogen abstraction in the presence of PMe 3 , is shown in Scheme 1 along with examples of highly active V complexes for ROMP of norbornene (NBE) [16, 17] .…”