Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene

Abstract

A catalyst for the oligomerization of 1-hexene, generated by the activation of a benzimidazolylidene zirconium dibenzyl complex, switches to a polymerization catalyst on addition of a trialkylphosphine.Polyolefin production is practiced on an extremely large scale. 1 Over the past 20 years there have been significant advances in the development of homogeneous single-site catalysts. Welldefined early transition metal metallocene catalysts have facilitated studies on the fundamental mechanistic features of polyolefin catalysis. 2 More recently, "post-metallocene" complexes bearing multidentate oxygen-and nitrogen-based ligands have proven to be good alternatives as catalysts for ethylene polymerization; 3 however, α-olefins still present challenges to non-metallocene catalysts, which often show poor to only moderate activity and/or rapid deactivation, thus affording only traces of oligomers or polymers. 4 Furthermore, the factors that determine oligomerization vs. polymerization of α-olefins are unpredictable: subtle changes in the ligand framework, the cocatalyst, and the reaction conditions can all affect the reactivities and selectivities of these catalyst systems. As part of our studies on early transition metal complexes of tridentate NHC ligands in catalysis, 5 especially for olefin oligomerization/ polymerization, 6 we report here on a bis( phenolate)benzimidazolylidene zirconium based catalyst that can be switched between oligomerization and polymerization of 1-hexene by the simple addition of tertiary phosphines. While this work was in progress, a report appeared of a highly regioselective 1-hexene oligomerization catalyzed by a closely-related NHCzirconium complex activated by anilinium; in that case, coordination of the dimethylaniline byproduct was suggested to account for the selective behavior. 6gThe (OCO) ligand 1 is obtained by cyclization of N,N′-bis(3,5-di-tert-butyl-2-phenol)-1,2-phenylenediamide 7 with triethylformate in the presence of hydrochloric acid in 74% yield (Scheme 1). Subsequent reaction of 1 with one equivalent of KHMDS followed by addition of tetrabenzylzirconium affords (OCO)ZrBn 2 (2) in 88% yield. The 13 C NMR spectrum reveals a characteristic downfield signal (δ 199.9 ppm) assigned to the C carbene -Zr carbon.

Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

“…The N , N ′-di(2-hydroxy-3,5-di- tert -butylphenyl) imidazolium chloride ( a ) and N , N ′-di(2-hydroxy-3,5-di- tert -butylphenyl) benzoimidazolium chloride ( b ) proligands were prepared according to previously reported procedures [ 26 , 67 , 68 ]. The alcohol elimination route involving the direct and slow addition of proligands a and b to a solution of Ti(O i Pr) 4 in THF at −30 °C was found to be the most appropriate protocol for the synthesis of both ([ κ 3 - O,C,O]- I NHC)TiCl(O i Pr)(THF) 1a and ([ κ 3 - O,C,O]- Bz NHC)TiCl(O i Pr) 1b complexes, respectively, in good yields and without the formation of side compounds, such as the homoleptic and zwitterionic complexes ( Scheme 2 ).…”

“…All chemicals were obtained from Sigma-Aldrich/Merck and used as received. Proligands a , b , and c [ 26 , 67 , 68 , 82 , 83 ], compounds ([ κ 3 - O,C,O]- Is NHC)TiCl(O i Pr)(THF) 1c [ 38 ] and ([ κ 3 - O,C,O]- Is NHC)HfCl(O i Pr)(THF) 3c [ 25 , 45 ] were prepared according to the literature procedures. [PPN]Cl was recrystallized prior to use [ 84 ].…”

“…However, their dissociation from oxophilic metals was partially prevented by designing multidentate anionic carbon, nitrogen, and oxygen-functionalized NHC ligands [ 5 , 13 , 15 , 16 , 17 ]. Anchoring such functionalized NHC ligands to oxophilic metals has proved to be a successful approach for developing catalysts, notably of group 4 metals, for the (oligo-)polymerization of olefins [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ], hydroamination/cyclization of aminoalkenes [ 30 , 31 , 32 , 33 , 34 , 35 , 36 ], controlled ring-opening polymerization of rac -lactide [ 37 , 38 , 39 , 40 ], and more recently, for the copolymerization of epoxides with CO 2 [ 41 , 42 , 43 , 44 , 45 ].…”

Mechanistic Insights on the Controlled Switch from Oligomerization to Polymerization of 1-Hexene Catalyzed by an NHC-Zirconium Complex

“…Figure 5 indicates that vinyleneterminated alkenes do not reinsert, so that the ultimate ratio of vinylene to vinylidene end groups is around 85/15, as reported previously. 6 The 13 C NMR spectrum exhibits a number of signals in the range of 10−45 ppm (see Figure S30 in the Supporting Information) that would result from these isomerizations, but a complete assignment would be very difficult. The disubstituted terminal olefin 2-ethyl-1-hexene, a model vinylidene-terminated oligomer, is neither oligomerized or isomerized by 1/[Ph 3 C][B(C 6 F 5 ) 4 ], suggesting that these olefins undergo insertion with the cationic zirconium hydride intermediate generated during the course of the reaction but not with the cationic monobenzylzirconium complex 2, presumably reflecting steric hindrance with the benzyl ligand.…”

“…Young NMR tube, 1 equiv of 1-hexene was added to a solution of [Ph 3 C][B(C 6 F 5 ) 4 ] (5.7 mg, 0.0063 mmol) and complex 1 (5 mg, 0.0063 mmol) in chlorobenzene-d 5 4 ] completely shuts down activity, presumably a consequence of coordination of the resulting dimethylaniline. 6 We have now examined two other commonly used activators. When complex 1 was added to a mixture of 1000 equiv of MAO and 1000 equiv of 1-hexene and stirred overnight, followed by quenching with acidic methanol and analysis by GC, the resulting distribution of oligomers was similar to that obtained with the system 1/[Ph 3 C][B(C 6 F 5 ) 4 ] (see Figure S2 in the Supporting Information).…”