Β-agostic Complex Research Articles

Control of Chain Walking by Weak Neighboring Group Interactions in Unsymmetrical Catalysts.

A combined theoretical and experimental study shows how weak attractive interactions of a neighboring group can strongly promote chain walking and chain transfer. This accounts for the previously observed very different microstructures obtained in ethylene polymerization by [κ2-N,O-{2,6-(3',5'-R2C6H3)2C6H3-N═C(H)-(3,5-X,Y-2-O-C6H2)}NiCH3(pyridine)], namely hyperbranched oligomers for remote substituents R = CH3 versus high-molecular-weight polyethylene for R = CF3. From a full mechanistic consideration, the alkyl olefin complex with the growing chain cis to the salicylaldiminato oxygen donor is identified as the key species. Alternative to ethylene chain growth by insertion in this species, decoordination of the monomer to form a cis β-agostic complex provides an entry into branching and chain-transfer pathways. This release of monomer is promoted and made competitive by a weak η2-coordination of the distal aryl rings to the metal center, operative only for the case of sufficiently electron-rich aryls. This concept for controlling chain walking is underlined by catalysts with other weakly coordinating furan and thiophene motifs, which afford highly branched oligomers with >120 branches per 1000 carbon atoms.

Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization

β-agostic Pd complexes are critical intermediates in catalytic reactions, such as olefin polymerization and Heck reactions. Pd β-agostic complexes, however, have eluded structural characterization, due to the fact that these highly unstable molecules are difficult to isolate. Herein, we report the single-crystal X-ray and neutron diffraction characterization of β-agostic (α-diimine)Pd–ethyl intermediates in polymerization. Short Cα–Cβ distances and acute Pd–Cα–Cβ bond angles combined serve as unambiguous evidence for the β-agostic interaction. Characterization of the agostic structure and the kinetic barrier for β-H elimination offer important insight into the fundamental understanding of agostic bonds and the mechanism of polymerization.

Structure and Isotope Effects of the β-H Agostic (α-Diimine)Nickel Cation as a Polymerization Intermediate.

Single-crystal X-ray characterization of cationic (α-diimine)Ni-ethyl and isopropyl β-agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni-Cα -Cβ angles (75.0(3)° and 74.57(18)°) and short Cα -Cβ distances (1.468(7) and 1.487(5) Å) provide unambiguous evidence for a β-agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni-H relative to the C-H bond. An Eyring plot for β-hydride elimination-olefin rotation-reinsertion is constructed from variable-temperature NMR spectra with 13 C-labeled agostic complexes. The enthalpy of activation (ΔH≠ ) for β-H elimination is 13.2 kcal mol-1 . These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β-agostic bonds and chain-migration via β-H elimination.

Hydroboration of Alkynes with Zwitterionic Ruthenium-Borate Complexes: Novel Vinylborane Complexes.

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido-[(Cp*Ru)2 B3 H9] (1) with other ligand systems was explored. In this regard, mild thermolysis of nido-1 with 2-mercaptobenzothiazole (2-mbzt), 2-mercaptobenzoxazole (2-mbzo) and 2-mercaptobenzimidazole (2-mbzi) ligands were performed which led to the isolation of bis(sigma)borate complexes [Cp*RuBH3 L] (2 a-c) and β-agostic complexes [Cp*RuBH2 L2] (3 a-c; 2 a, 3 a: L=C7 H4 NS2 ; 2 b, 3 b: L=C7 H4 NSO; 2 c, 3 c: L=C7 H5 N2 S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3 a with [PPh2 (CH2 )n PPh2 ] (n=1-3) yielded [Cp*Ru(PPh2 (CH2 )n PPh2 )-BH2 (L2)] (4 a-c; 4 a: n=1; 4 b: n=2; 4 c: n=3; L=C7 H4 NS2). Mild thermolysis of 2 a with [PPh2 (CH2)n PPh2 ] (n=1-3) led to the isolation of [Cp*Ru(PPh2 (CH2)n PPh2 )(L)] (L=C7 H4 NS2 5 a-c; 5 a: n=1; 5 b: n=2; 5 c: n=3). Treatment of 4 a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp*Ru(R-C=CH2 )BH(L2)] (6 and 7; 6: R=Ph; 7: R=COOCH3; L=C7 H4 NS2). Complexes 6 and 7 can also be viewed as η-alkene complexes of ruthenium that feature a dative bond to the ruthenium centre from the vinylinic double bond. In addition, DFT computations were performed to shed light on the bonding and electronic structures of the new compounds.

Neutral Nickel Oligo- and Polymerization Catalysts: The Importance of Alkyl Phosphine Intermediates in Chain Termination

An unconventional chain termination reaction has been explored for the SHOP (Shell higher olefin process)-type, anilinotropone, and salicylaldiminato nickel-based oligo- and polymerization catalysts by using density functional theory (DFT). Starting from the tetracoordinate alkyl phosphine complex, the termination reaction was found to involve a rearrangement of the alkyl chain to form a pentacoordinate β-agostic complex, β-hydride elimination, and olefinic chain dissociation and to compete with propagation at sufficiently high phosphine concentration and/or basicity. It provides the first complete and convincing mechanistic rationale for the decreasing chain lengths observed upon increasing phosphine concentration and basicity. The unconventional reaction was found to be a major termination pathway for the SHOP-type catalyst and is very unlikely to lead to branching and olefin isomerization, which is critical for explaining why the SHOP catalyst, in contrast to the anilinotropone and salicylaldiminato catalysts, tends to lead to the oligomerization of ethylene to form linear α-olefins. Based on our results we have proposed a new and extended catalytic cycle for the SHOP-type ethylene oligomerization catalyst. Finally, the importance of the new termination reaction for the SHOP-type catalyst suggests that this reaction may also operate with other ethylene oligomerization nickel catalysts. This prediction was confirmed for a pyrazolonatophosphine catalyst, for which the new termination route was found to be even more facile, which explains the short oligomers produced by this catalyst.

Site epimerization in ansa-zirconocene polymerization catalysts

Metallocene alkyl cations for polymerization of olefins possess two active sites involved in migratory insertion. Site epimerization, with an inversion at the metal atom, is considered to be one of the major causes for break-down of the alternating propagation model, resulting in stereoerrors whenever the two catalytic sites have substantially different enantioface selectivities. Density functional theory has been used to determine the intrinsic reaction coordinate that connects the optimized minima and transition states of inversion in the parent ansa-zirconocene [{H 2C(Cp) 2}Zr–Pr] + (Pr = n-propyl). These calculations yield a three-step reaction path for site epimerization. Starting from the pyramidal β-agostic complex, an activated rotation around the Zr–Pr bond first produces an α-agostic conformation. Continued rotation leads to an equivalent second α-agostic intermediate and then finally to the inverted β-agostic complex. The second step is rate-determining and proceeds through a planar three-coordinate transition state. In the case of [{H 2C(Cp) 2}Zr– i Bu] + ( i Bu = iso-butyl), the situation is more complicated, because there are several interconvertible α-, β- and γ-agostic intermediates, but the rate-limiting step is again an inversion process connecting two different α-agostic conformers with the alkyl group on opposite enantiosides. For both ansa-zirconocene catalysts, the computed free-energy barriers for epimerization are around 11–12 kcal/mol and almost independent of temperature, while those for insertion increase with temperature due to the entropic cost of association. According to the computational results for the isolated catalysts, insertion remains favored over epimerization for the experimentally relevant temperature range in the n-propyl case, whereas both processes are competitive in the iso-butyl case. Inclusion of bulk solvent effects by a continuum solvation model does not affect the results much, while explicit consideration of a coordinating counterion causes larger changes. The present model calculations on the role of site epimerization should thus be most relevant for propene polymerization with non-coordinating counterions.

A DFT study of ethylene polymerization by zirconocene catalysts

A DFT study of ethylene polymerization by zirconocene catalysts was carried out. Stationary points corresponding to intermediates and transition states were located on the potential energy surface of the [Cp2ZrC2H5]++C2H4 model system. Three possible reaction mechanisms involving the formation of β-agostic complexes were considered. The energy and thermodynamic characteristics for different reaction pathways were calculated. Corresponding activation energies lie in the range 3.9–6.8 kcal mol−1.

Density Functional Study of the Mechanism of the Palladium(II)-Catalyzed Ethylene Polymerization Reaction

The hybrid density functional B3LYP method has been used to study the mechanism of ethylene polymerization catalyzed by the Pd(II) diimine complex. The chain initiation starts with coordination of ethylene to the active catalyst [L2PdCH3]+, followed by ethylene migratory insertion into the Pd−alkyl bond to form a γ-agostic intermediate, which rearranges to a more stable β-agostic form. The chain propagation starting with the β-agostic complex 7 can proceed via pathway A through coordination, migratory insertion, and rearrangement, leading to linear polyethylene, or pathway B, through β-hydride elimination, olefin rotation, and reinsertion to give a branched alkyl intermediate, which can then coordinate ethylene and produce branched polyethylene. The rate-determining step is olefin insertion, and the olefin alkyl complexes are the resting states in the catalytic cycle. Associative displacement, where the coordinated polymer is associatively replaced from the same diimine−Pd−hydrido−olefin complex 7 by ethy...

Theoretical studies of inorganic and organometallic reaction mechanisms. 8. Hydrogen exchange in the β-agostic ethylene complex of cyclopentadienyl rhodium

Ab initio calculations with effective core potentials have been used to study the hydrogen exchange processes of the [CpRh(C 2H 4)(η 2-C 2H 5)] + β-agostic complex. The mirror-symmetric olefin-hydride species is found to be an intermediate (3.4 kcal mol −1 higher in energy than the β-agostic complex) rather than a transition state ( ≈ 5 kcal mol −1) in the interconversion process of the two enantiomeric forms. A higher energy process involving rotation of the methyl group in the agostic complex is determined to occur through an Rh⋯(η 2-H 2C) interaction with a calculated activation energy of ≈ 6.5 kcal mol −1. Complete loss of the Rh⋯HC agostic interaction requires an activation energy of about 14 kcal mol −1. This agostic interaction's strength depends upon the M⋯H overlap, and stronger agostic interactions result in weaker CH agostic bonds. An even higher energy process involving the exchange of all of the nine hydrogens and the four carbons is found to be due to the ethylene rotation in the agostic complex. The ethylene rotation barrier (calculated to be 9.8 kcal mol −1) depends greatly on the sizes of the transition metal atom and ligands in the complex since steric effects play an important role in the rotation.

Co-ordinatively unsaturated diphosphine platinum(II) alkyl cations: a new class of β-agostic complexes

Treatment of tris(norbornene)platinum with diphosphines and subsequent protonation affords a series of cationic complexes [Pt(C7H11)(L2)]+(L2= peralkyldiphosphine) in which the norbornyl ligand binds to platinum viaσ-alkyl and β-C–H agostic interactions; the strength of the latter bond varies and has been characterised by n.m.r. and X-ray structure analysis.