Giovanni Talarico

Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts.

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

Design of stereoselective Ziegler–Natta propene polymerization catalysts

After five decades of largely serendipitous (albeit formidable) progress, catalyst design in Ziegler–Natta olefin polymerization, i.e., the rational implementation of new active species to target predetermined polyolefin architectures, has ultimately become a realistic ambition, thanks to a much deeper fundamental understanding and major advances in the tools of computational chemistry. In this article, we discuss, as a case history, a unique class of stereorigid C2-symmetric bis(phenoxy-amine)Zr(IV) catalysts with controlled kinetic behavior. A large variety of polypropylene microstructures have been obtained with these catalysts by modulating the steric demand of one key substituent, without altering the nature and symmetry of the ancillary ligand framework, under the guidance of computer modeling. This unusual achievement is relevant per se and for the perspective implications in catalyst discovery.

Unusual hafnium-pyridylamido/ER(n) heterobimetallic adducts (ER(n) = ZnR2 or AlR3).

NMR spectroscopy and DFT studies indicate that the Symyx/Dow Hf(IV)-pyridylamido catalytic system for olefin polymerization, [{N(-),N,CNph(-)}HfMe][B(C6F5)4] (1, Nph = naphthyl), interacts with ER(n) (E = Al or Zn, R = alkyl group) to afford unusual heterobimetallic adducts [{N(-),N}HfMe(μ-CNph)(μ-R)ER(n-1)][B(C6F5)4 in which the cyclometalated Nph acts as a bridge between Hf and E. (1)H VT (variable-temperature) EXSY NMR spectroscopy provides direct evidence of reversible alkyl exchanges in heterobimetallic adducts, with ZnR2 showing a higher tendency to participate in this exchange than AlR3. 1-Hexene/ERn competitive reactions with 1 at 240 K reveal that the formation of adducts is strongly favored over 1-hexene polymerization. Nevertheless, a slight increase in the temperature (to >265 K) initiates 1-hexene polymerization.

Ethylene coordination, insertion, and chain transfer at a cationic aluminum center: A comparative study with Ab Initio correlated level and density functional methods

The performance of correlated ab initio methods and DFT methods was compared for the propagation and chain transfer steps of ethylene polymerization by a model aluminum–amidinate system, [{HC(NH)2}AlCH2CH3]+. All methods agree that the main chain transfer mechanism is β‐hydrogen transfer to the monomer (BHT), and that this is substantially easier than propagation; implications for the real Jordan system are discussed briefly. Counterpoise corrections are necessary to obtain reasonable olefin complexation energies. Activation energies are consistently lower at DFT (BP86, B3LYP) than at ab initio levels [MP2, MP3, MP4, CI, CCSD(T)]; the differences are particularly large (16 kcal/mol) for the BHT reaction. This is suggested to be related to the known problem of DFT in describing hydrogen bridged systems.

How easy is CO2 fixation by M–C bond containing complexes (M = Cu, Ni, Co, Rh, Ir)?

A comparison between different M–C bonds (M = Cu(I), Ni(II), Co(I), Rh(I) and Ir(I)) has been reported by using density functional theory (DFT) calculations to explore the role of the metal in the fixation or incorporation of CO2 into such complexes. The systems investigated are various metal based congeners of the Ir-complex 8 [(cod)(IiPr)Ir-CCPh], with a ligand scaffold based on cod and IiPr ligands (cod = 1,5-cyclooctadiene; IiPr = 1,3-bis(isopropyl)imidazol-2-ylidene). The results of this study show that the calculated CO2 insertion barriers follow the trend: Cu(I) (20.8 kcal mol−1) < Rh(I) (30.0 kcal mol−1) < Co(I) (31.3 kcal mol−1) < Ir(I) (37.5 kcal mol−1) < Ni(II) (45.4 kcal mol−1), indicating that the Cu(I) based analogue is the best CO2 fixer, while Ni(II) is the worst in the studied series.

A theoretical study of the competition between ethylene insertion and chain transfer in cationic aluminum systems

Abstract The ability of density functional models in dealing with polymerization mechanisms has been investigated by comparison with high level post-Hartree–Fock methods. Ethylene insertion and chain transfer reactions have been compared for a model of the active species suggested for the Jordan aluminum catalyst [{R′C(NR″)2}AIR]+. Conventional density functional approaches (BP86) show a strong bias in favor of chain transfer reactions via hydrogen transfer. The B1LYP model provides improved energy barriers. The aluminum model used strongly favors chain transfer over insertion, this preference being further enhanced by lengthening the growing polymer chain. These findings cast some doubts about currently accepted models of the active species in the Jordan catalyst.

Insertion and β -Hydrogen Transfer at Aluminium

Insertion and elimination of C=X bonds are two fundamental steps of organometallic chemistry. In aluminium chemistry, these reaction steps are important in olefin oligomerization and possibly polymerization, as well as in the reaction of carbonyl compounds with aluminium alkyls. The general importance of a third fundamental step, direct β-hydrogen transfer from an aluminium-bound group to a substrate, has not been recognized as widely, despite the fact that it is the key step in the reduction of ketones by alcohols (Meerwein-Pondorf-Verley reduction) and by aluminium alkyls bearing β-hydrogens. In this review we will combine experimental and theoretical results to illustrate how the delicate balance between these three reaction types determines much of the chemistry of organoaluminium compounds. Such an understanding may create new opportunities for ligand design.

Molecular modeling of the regiochemistry of olefin insertion with single-site polymerization catalysts

The regiochemistry of propene insertion promoted by metallocene and postmetallocene Column 4 metal catalysts is investigated with quantum mechanics techniques. It is shown that steric effects dominate the regiochemistry of monomer insertion with classical metallocene-based catalysts, whereas both electronic and steric effects contribute to the regiochemistry of propene insertion with postmetallocene catalysts.

LIVING PROPENE POLYMERIZATION WITH BIS(PHENOXY-IMINE) GROUP 4 METAL CATALYSTS: A THEORETICAL STUDY

Bis(phenoxy-imine)Ti catalysts with ortho-F-substituted phenyl rings on the N can promote living propene polymerization. On the basis of DFT calculations, it has been proposed that the “living” behavior originates from an unprecedented attractive interaction between the aforesaid ortho-F atoms and a β-H of the growing polymer chain, which would render the latter less prone to being transferred to the metal and/or to the monomer. In this paper, we report on a thorough full-QM and combined QM/MM investigation of representative model catalysts, demonstrating that the key factor is instead the repulsive nonbonded contact of the F-substituted rings with the growing polymer chain and an incoming propene molecule, which destabilizes the sterically demanding 6-center transition structure for chain transfer to the monomer. A conceptually similar substituent effect has been reported before for several metallocene and nonmetallocene catalysts.

“Chain-End-Controlled Isotactic” and “Stereoblock-Isotactic” Polypropylene: Where Is the Difference?

This communication describes the microstructure of “stereoblock-isotactic” polypropylene obtained with sterically hindered “oscillating” metallocene catalysts, points out in which respects it differs from that of “chain-end-controlled isotactic” polypropylene, and explains why the two definitions cannot be used as synonymous (as is commonly found in the literature).