Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

ansa-Zirconocene Catalysts for Isotactic-Selective Propene Polymerization at High Temperature: A Long Story Finds a Happy Ending.

Absolute rigidity is rare in the "soft" world of organometallics. Here we introduce two cyclopenta[a]triptycyl ansa-zirconocene catalysts for isotactic-selective propene polymerization, designed by means of an integrated high-throughput experimentation/quantitative structure-activity relationship modeling approach. An ultrarigid ligand precisely wrapped around the Zr center enforces an enzyme-like lock and key fit, effectively hampering undesired reactive events, even at high temperature. Stereodefective units are hardly detectable by 13C NMR in the polymer produced at 120 °C; this corresponds to an enantioselectivity exceeding 6-7 kcal/mol: i.e., less than 1 propene misinsertion every 4000 (and at room temperature, one every ∼40000!).

Reactivity of C1 -Symmetric Heteroarylamido Hafnium Complexes towards Unsaturated Electrophilic Molecules: Development of New Families of Olefin Polymerization Catalysts.

Heteroarylamido hafnium post-metallocenes with [C,N,N] ligands were functionalized by the insertion of small electrophilic unsaturated molecules into the CAr -Hf bond of the ligand, which gave rise to various 1,1- and 1,2-insertion complexes with the modified ligands of previously unknown [O,N,N], [N,N,N], [O,N,N,N], and [N,N,N,N] types. It was found that C1 symmetry of the starting complexes, in some cases, results in the formation of two diastereoisomers after 1,1- or 1,2-insertion. Most of the obtained novel complexes were shown to form active catalysts for olefin polymerization in the presence of MAO (methylaluminoxane). Thus, a new approach to the feasible and easy "late stage" modification of the precatalyst structure has been suggested; it can be performed immediately prior to the polymerization tests without isolation and even in a high-throughput manner.

Manganese-Catalyzed Synthesis of Polyketones Using Hydrogen-Borrowing Approach.

We report here a method of making polyketones from the coupling of diketones and diols using a manganese pincer complex. The methodology allows us to access various polyketones (polyarylalkylketone) containing aryl, alkyl, and ether functionalities, bridging the gap between the two classes of commercially available polyketones: aliphatic polyketones and polyaryletherketones. Using this methodology, 12 polyketones have been synthesized and characterized using various analytical techniques to understand their chemical, physical, morphological, and mechanical properties. Based on previous reports and our studies, we suggest that the polymerization occurs via a hydrogen-borrowing mechanism that involves the dehydrogenation of diols to dialdehyde followed by aldol condensation of dialdehyde with diketones to form chalcone derivatives and their subsequent hydrogenation to form polyarylalkylketones.

Reversible C-C bond formation in group 4 metal complexes: nitrile extrusion via ß-aryl elimination.

Pyridylamides of zirconium and hafnium with [C,N,N]-ligands reversibly insert nitriles into M-CAr bonds leading to an observable equilibrium between the starting [C,N,N]-complexes and newly formed [N,N,N]-complexes with a ketimide moiety in a 7-membered metallacycle. The discovered reversible insertion of nitriles into M-CAr bonds represents an unprecedented example of ß-aryl elimination from a ketimide ligand in early transition metal complexes. Experimental and computational studies suggest thermodynamic and electronic reasons for this reactivity. Weak orbital overlap between the ketimide nitrogen and the metal, and an unfavorable 7-membered metallacycle destabilize the product of insertion into the M-CAr bond, while the pyridylamide moiety acts as a directing group making the reverse process viable. The influence of non-chelate spectator ligands on the metal center and substituents in nitrile on the thermodynamic stability of the [N,N,N]-complexes was also studied. Exploiting ß-carbon elimination in complexes of early transition metals may extend the range of catalysts that are accessible for C-C activation processes in the future.

Triptycene as a scaffold in metallocene catalyzed olefin polymerization.

A set of metallocene olefin polymerization catalysts bearing triptycene moieties in either position 4-5 (complexes Ty1-Ty5) or in position 5-6 (complexes Ty6-Ty8) of the basic dimethylsilyl-bridged bis(indenyl) system has been tested in propene polymerization and in ethene/1-hexene copolymerization. Comparison of the results with QSPR (quantitative structure-property relationship) predictions not parametrized for these exotic ligand variations demonstrates that trends can still be identified by extrapolation. Interestingly, Ty7, upon suitable activation, provides a highly isotactic polypropylene with an exceptional amount of 2,1 regio-errors (8%). The previously developed QSPR type models successfully predicted the low regioselectivity of this catalyst, despite the fact that the catalyst structure differs significantly from the benchmark set.

Multisubstituted C2-symmetric ansa-metallocenes bearing nitrogen heterocycles: influence of substituents on catalytic properties in propylene polymerization at higher temperatures.

In this work we systematically studied the effects of modifications of substituents on the performance of the isospecific zirconocene-based catalyst family, Me2Si(2-Alk-4-(N-carbazolyl)Ind)ZrX2 (X = Cl, Me), wherein the progenitor was shown to be particularly suitable in high-temperature propylene polymerization processes. In order to obtain the required zirconocenes, we developed a novel synthetic pathway to 4-(N-carbazolyl)indenes through Pd-catalyzed cyclizations of 2,2'-dibromobiaryls with 4-aminoindenes, which were synthesized via Buchwald-Hartwig reaction or electrophilic amination of 4-indenyl Grignard reagents with trimethylsilylmethyl azide. By a number of examples, the anion-promoted rac-to-meso isomerization method was shown to work reliably well for preparation of rac-ZrMe2-complexes. Certain zirconocenes among the 21 tested in propylene polymerization at 70 and 100 °C under MAO or borate activation outperformed the parent catalyst in molecular weight capability, regio- or stereoselectivity.

Pyridylamido hafnium complexes with a silylene bridge: synthesis and olefin polymerization.

The synthesis and characterisation of six novel Cs-symmetric pyridylamido hafnium complexes with a silylene bridge of the type [ArPy(R2Si)NAr']HfAlk2 are reported. Four complexes have been structurally characterised using single crystal X-ray diffraction. Appreciable differences between the solid state structures of these complexes and the pyridylamido hafnium complexes with a CRR' bridge were noted. Reactions with B(C6F5)3, [Ph3C][B(C6F5)4] and [HMe2NPh][B(C6F5)4] yielded active catalysts for the homopolymerisations of propene and 1-hexene and ethene/1-octene copolymerization. In spite of the Cs-symmetry of the precatalysts, isotactically enriched polypropylene and poly(1-hexene) were obtained. The fact that the mechanism of the catalyst activation includes the insertion of alkene into the Hf-CAr bond was demonstrated. It was found that the structures of Ar and the R2Si bridge influence the activity, molecular weight capability and 1-octene affinity of the catalysts.

Crystal structures of di-µ-bromido-bis{di-bromido-[η5-2-(di-methyl-amino)-inden-yl]zirconium(IV)} and di-bromido-bis-[η5-2-(di-methyl-amino)-inden-yl]zirconium(IV).

In the title compounds, [Zr2Br6(C11H12N)2], (I) and [ZrBr2(C11H12N)2], (II), the positions of the η5-binding 2-di-methyl-amino-indenyl units are fixed by intra-molecular C-H⋯Br inter-actions involving aromatic or di-methyl-amino H atoms. The binuclear mol-ecule of (I) is located on a general position, while the mononuclear mol-ecule of (II) is situated on a twofold rotation axis. Both ZrIV atoms in (I) are ligated by one cyclo-penta-dienyl (CP) ring and four Br ligands (two bridging, two terminal), while in (II) the ZrIV atom is ligated by two CP rings and two terminal Br ligands. The crystal structures of both (I) and (II) comprise of strands of π-π- and N-π-bonded mol-ecules, which in turn are linked by C-H⋯Br inter-actions.