Observing a Homogeneous Ziegler Catalyst Precursor at Work: Insertion Reactions into the Zirconium – Carbon Bond of the (Butadiene)ZrCp2/B(C6F5)3 Addition Product

Planar-Chiral 1,1'-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene

The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes₂ (Mes = 2,4,6-trimethylphenyl) and ClBCy₂ as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tert-butylated boryl cyclopentadienides with MCl₂ (M = Fe, Co) selectively leads to 1,1'-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes₂ groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes₂ rings. There is only weak, if any, direct M···B interaction in 6 and 7.

Stereochemical Control Mechanisms in Propylene Polymerization Mediated by C1-Symmetric CGC Titanium Catalyst Centers

This work analyzes stereochemical aspects of olefin polymerization processes mediated by the C1-symmetric constrained geometry catalyst H2Si(ind)(tBuN)TiCH3+ (ind = indenyl), including the role of the cocatalyst/counteranion. The energetics of catalyst activation are first analyzed and shown to compare favorably with experiment. The energetics of heterolytic ion pair separation are next scrutinized, and the effects of solvation environment are assessed. Computed thermodynamic profiles for ethylene insertion at H2Si(ind)(tBuN)TiCH3+ indicate that the kinetics of insertion processes at the H2Si(ind)(tBuN)TiR+ cation can be analyzed in terms of SCF potential energies. We next compare the energetic profile for ethylene insertion at the naked H2Si(ind)(tBuN)TiCH3+ cation with that at the related H2Si(ind)(tBuN)TiCH3+H3CB(C6F5)3- ion pair to understand counterion effects. It is seen that the counterion, although affecting overall catalytic activity, does not significantly influence enchainment stereochemistry or polymer microtacticity. Next, the second ethylene insertion at H2Si(ind)(tBuN)Ti(nC3H7)+H3CB(C6F5)3- is analyzed to evaluate counteranion influence on the propagation barrier. It is found that the ethylene uptake transition state is energetically comparable to the first insertion transition state and that solvation has negligible effects on the energetic profile. These findings justify analysis of the propylene insertion process within the less computationally demanding "naked cation" model. Thus, monomer enchainment at H2Si(ind)(tBuN)TiR+ is analyzed for H2Si(ind)(tBuN)TiCH3+ + propylene (first insertion) and for H2Si(ind)(tBuN)Ti(iC4H6)+ + propylene (second insertion). Data describing the first insertion highlight the sterically dominated regioselection properties of the system with activation energies indicating that olefin insertion regiochemistry is predominantly 1,2 (primary), while the second insertion similarly reflects the catalyst stereoinduction properties, with steric effects introduced by the growing chain (mimicked by an isobutyl group) preferentially favoring insertion pathways that afford isotactic enrichment, in agreement with experiment.

Catalyst/cocatalyst nuclearity effects in single-site polymerization. Enhanced polyethylene branching and alpha-olefin comonomer enchainment in polymerizations mediated by binuclear catalysts and cocatalysts via a new enchainment pathway

The binuclear "constrained geometry catalyst" (CGC) (mu-CH2CH2-3,3'){(eta5-indenyl )[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2] and the trityl bisborate dianion (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2) have been synthesized to serve as new types of multicenter homogeneous olefin polymerization catalysts and cocatalysts, respectively. Additionally, the complex [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) was synthesized as a mononuclear control. For the bimetallic catalyst or bisborate cocatalyst, high effective local active site concentrations and catalyst center-catalyst center cooperative effects are evidenced by bringing the catalytic centers together via either covalent or electrostatic bonding. For ethylene homopolymerization at constant conversion, the branch content of the polyolefin products (primarily ethyl branches) is dramatically increased as catalyst or cocatalyst nuclearity is increased. Moreover, catalyst and cocatalyst nuclearity effects are approximately additive. Compared to the catalyst derived from monometallic Zr1 and monofunctional Ph3C+B(C6F5)4- (B1), the active catalyst derived from bimetallic Zr2 and bifunctional B2 produces approximately 11 times more ethyl branches in ethylene homopolymerization via a process which is predominantly intradimer in character. Moreover, approximately 3 times more 1-hexene incorporation in ethylene + 1-hexene copolymerization and approximately 4 times more 1-pentene incorporation in ethylene + 1-pentene copolymerization are observed for Zr2 + B2 versus Zr1 + B1.

Homogeneous single-component betaine Ziegler-Natta catalysts derived from (butadiene)zirconocene precursors

(Butadiene)zirconocene adds B(C(6)F(5))(3) at a terminal diene carbon atom to yield the zirconocene-(mu-hydrocarbyl)-borate betaine Cp(2)Zr[C(4)H(6)-B(C(6)F(5))(3)] (4). The dipolar complex 4 contains a distorted pi-allyl moiety and features an additional stabilizing Zr-F-C(arene) coordination. Under kinetic control, an isomeric betaine system is formed, characterized by an internal Zr(+).CH(2)[B](-) ion-pair interaction, that rearranges to 4 upon heating. A great variety of ansa-metallocene(butadiene) complexes and related systems cleanly form analogous metallocene-(mu-conjugated diene)-borate betaines upon treatment with B(C(6)F(5))(3) and related Lewis acids. Most of these systems represent very active homogeneous single-component Ziegler-Natta catalysts for alpha-olefin polymerization and copolymerization. In addition, these betaine catalysts are ideally suited for carrying out mechanistic studies in active Ziegler-Natta catalyst systems. They allow for an experimental observation of the first alkene insertion step at the active single-component catalyst. This feature has been used for studying the mechanism of transfer of the stereochemical information from the bent metallocene backbone and for an experimental characterization of the energy profile of the alkene addition/alkene insertion reaction sequence in active homogeneous Ziegler-Natta systems. The neutral dipolar single-component catalysts (e.g., 4) produce a polyolefin-modified R'-(CHR-CH(2))-C(4)H(6)-B(C(6)F(5))(3)(-) counterion at the end of the initiation period upon entering into the repetitive active catalytic cycle.

Solution structures and dynamic properties of chelated d(0) metal olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) (R = H, Me): Models for the [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" catalysts

To model the Ti-olefin interaction in the putative [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) have been investigated. The reaction of [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(2) (1a,b; R = H, Me) with HOCMe(2)CH(2)CH(2)CH=CH(2) yields mixtures of [eta(5)-C(5)R(4)SiMe(2)NH(t)Bu]TiMe(2)(OCMe(2)CH(2)CH(2)CH=CH(2)) (2a,b) and [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(OCMe(2)CH(2)CH(2)CH=CH(2)) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C(6)F(5))(3) yields the chelated olefin complexes [[eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][MeB(C(6)F(5))(3)] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph(3)C][B(C(6)F(5))(4)] yields [[eta(5): eta(1)-C(5)Me(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][B(C(6)F(5))(4)] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios: 4a/4a', 62/38; 4b/4b', 75/25; 5b/5b', 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d(0) metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through C(term) such that the C=C pi bond is polarized with positive charge buildup on C(int). Dynamic NMR studies show that 4b/4b' undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti ("O-shift") in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b' (i.e., 4b/4b' face exchange) are: DeltaH = 17.2(8) kcal/mol; DeltaS = 8(1) eu. 4a/4a' also undergoes olefin face exchange but with a lower barrier (DeltaH = 12.2(9) kcal/mol; DeltaS = -2(3) eu), for the conversion of 4a to 4a'.