Massimiliano Delferro

Bimetallic effects for enhanced polar comonomer enchainment selectivity in catalytic ethylene polymerization.

The synthesis and characterization of the bimetallic 2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato group 10 metal polymerization catalysts {[Ni(CH(3))](2)[1,8-(O)(2)C(10)H(4)-2,7-[CH=N(2,6-(i)Pr(2)C(6)H(3))](PMe(3))(2)} and {[Ni(1-naphthyl)](2)[1,8-(O)(2)C(10)H(4)-2,7-[CH=N(2,6-(i)Pr(2)C(6)H(3))](PPh(3))(2)} [FI(2)-Ni(2)(PR(3))(2)] are presented, along with the synthesis and characterization of the mononuclear analogues {Ni(CH(3))[3-(t)Bu-2-(O)C(6)H(3)CH=N(2,6-(i)Pr(2)C(6)H(3))](PMe)(3)} and {Ni(1-naphthyl)[3-(t)Bu-2-(O)C(6)H(3)CH=N(2,6-(i)Pr(2)C(6)H(3))](PPh)(3)} [FI-Ni (PR(3))]. Monometallic Ni catalysts were also prepared by functionalizing one ligation center of the bimetallic ligand with a trimethylsilyl group (TMS), yielding {Ni(CH(3))[1,8-(O)(TMSO)C(10)H(4)-2,7-[CH=N(2,6-(i)Pr(2)C(6)H(3))](PMe(3))} [TMS-FI(2)-Ni(PMe(3))]. The FI(2)-Ni(2) catalysts exhibit significant increases in ethylene homopolymerization activity versus the monometallic analogues, as well as increased branching and methyl branch selectivity, even in the absence of a Ni(cod)(2) cocatalyst. Increasing ethylene concentrations significantly suppress branching and alter branch morphology. FI(2)-Ni(2)-mediated copolymerizations with ethylene + polar-functionalized norbornenes exhibit a 4-fold increase in comonomer incorporation versus FI-Ni, yielding copolymers with up to 10% norbornene copolymer incorporation. FI(2)-Ni(2)-catalyzed copolymerizations with ethylene + methylacrylate or methyl methacrylate incorporate up to 11% acrylate comonomer, while the corresponding mononuclear FI-Ni catalysts incorporate negligible amounts. Furthermore, the FI(2)-Ni(2)-mediated polymerizations exhibit appreciable polar solvent tolerance, turning over in the presence of ethyl ether, acetone, and even water. The mechanism by which the present cooperative effects take place is investigated, as is the nature of the copolymer microstructures produced.

Ligand Steric and Fluoroalkyl Substituent Effects on Enchainment Cooperativity and Stability in Bimetallic Nickel(II) Polymerization Catalysts

The synthesis and characterization of two neutrally charged bimetallic Ni(II) ethylene polymerization catalysts, {2,7-di-[2,6-(3,5-di-methylphenylimino)methyl]1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethylphosphine) [(CH(3) )FI(2) -Ni(2) ] and {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethyl-phosphine) [(CF(3) )FI(2) -Ni(2) )], are reported. The diffraction-derived molecular structure of (CF(3) )FI(2) -Ni(2) reveals a Ni⋅⋅⋅Ni distance of 5.8024(5) Å. In the presence of ethylene and Ni(COD)(2) or B(C(6) F(5) )(3) co-catalysts, these complexes along with their monometallic analogues [2-tert-butyl-6-((2,6-(3,5-dimethylphenyl)phenylimino)methyl)-phenolate]-Ni(II) -methyl(trimethylphosphine) [(CH(3) )FI-Ni] and [2-tert-butyl-6-((2,6-(3,5-ditrifluoromethyl-phenyl)phenylimino)methyl)phenolato]-Ni(II) -methyl-(trimethylphosphine) [(CF(3) )FI-Ni], produce polyethylenes ranging from highly branched M(w) =1400 oligomers (91 methyl branches per 1000 C) to low branch density M(w) =92 000 polyethylenes (7 methyl branches per 1000 C). In the bimetallic catalysts, Ni⋅⋅⋅Ni cooperative effects are evidenced by increased product polyethylene branching in ethylene homopolymerizations (∼3× for (CF(3) )FI(2) -Ni(2) vs. monometallic (CF(3) )FI-Ni), as well as by enhanced norbornene co-monomer incorporation selectivity, with bimetallic (CH(3) )FI(2) -Ni(2) and (CF(3) )FI(2) -Ni(2) enchaining approximately three- and six-times more norbornene, respectively, than monometallic (CH(3) )FI-Ni and (CF(3) )FI-Ni. Additionally, (CH(3) )FI(2) -Ni(2) and (CF(3) )FI(2) -Ni(2) exhibit significantly enhanced thermal stability versus the less sterically encumbered dinickel catalyst {2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethylphosphine). The pathway for bimetallic catalyst thermal deactivation is shown to involve an unexpected polymerization-active intermediate, {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1-hydroxy,8-naphthalenediolato-Ni(II) (methyl)-(trimethylphosphine).

Atom-efficient regioselective 1,2-dearomatization of functionalized pyridines by an earth-abundant organolanthanide catalyst

Developing earth-abundant, non-platinum metal catalysts for high-value chemical transformations is a critical challenge to contemporary chemical synthesis. Dearomatization of pyridine derivatives is an important transformation to access a wide range of valuable nitrogenous natural products, pharmaceuticals and materials. Here, we report an efficient 1,2-regioselective organolanthanide-catalysed pyridine dearomatization process using pinacolborane, which is compatible with a broad range of pyridines and functional groups and employs equimolar reagent stoichiometry. Regarding the mechanism, derivation of the rate law from NMR spectroscopic and kinetic measurements suggests first order in catalyst concentration, fractional order in pyridine concentration and inverse first order in pinacolborane concentration, with C=N insertion into the La–H bond as turnover-determining. An energetic span analysis affords a more detailed understanding of experimental activity trends and the unusual kinetic behaviour, and proposes the catalyst ‘resting’ state and potential deactivation pathways. Selective pyridine dearomatization processes traditionally use precious metal catalysts with reagents in stoichiometric excess, and are not well-understood mechanistically. Now, efficient 1,2-regioselective pyridine dearomatization is achieved using equimolar pinacolborane and an earth-abundant lanthanide catalyst. Mechanistic and theoretical studies elucidate the reaction mechanism and explain observed reactivity trends.

Synthesis, characterization, and heterobimetallic cooperation in a titanium-chromium catalyst for highly branched polyethylenes.

A heterobimetallic catalyst, {Ti--Cr}, consisting of a constrained-geometry titanium olefin polymerization center (CGC(Et)Ti) covalently linked to a chromium bis(thioether)amine ethylene trimerization center (SNSCr) was synthesized and fully characterized. In ethylene homopolymerizations it affords linear low-density polyethylene with molecular weights as high as 460 kg·mol(-1) and exclusively n-butyl branches in conversion-insensitive densities of ~18 branches/1000 carbon atoms, which are ~17 and ~3 times (conversion-dependent), respectively, those achieved by tandem mononuclear CGC(Et)Ti and SNSCr catalysts under identical reaction conditions.

Very large cooperative effects in heterobimetallic titanium-chromium catalysts for ethylene polymerization/copolymerization

The heterobimetallic complexes, (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-CnH2n-[N,N-bis(2-(ethylthio)ethyl)amine]CrCl3 (n = 0, Ti-C0-Cr(SNS); n = 2, Ti-C2-Cr(SNS); n = 6, Ti-C6-Cr(SNS)), (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-C2H4-[N,N-bis((o-OMe-C6H4)2P)amine]CrCl3 (Ti-C2-Cr(PNP)), and (η(5)-indenyl)[1-Me2Si((t)BuN)TiCl2]-3-C2H4-[N,N-bis((diethylamine)ethyl)-amine]CrCl3 (Ti-C2-Cr(NNN)), are synthesized, fully characterized, and employed as olefin polymerization catalysts. With ethylene as the feed and MAO as cocatalyst/activator, SNS-based complexes Ti-C0-Cr(SNS), Ti-C2-Cr(SNS), and Ti-C6-Cr(SNS) afford linear low-density polyethylenes (LLDPEs) with exclusive n-butyl branches (6.8-25.8 branches/1000 C), while under identical polymerization conditions Ti-C2-Cr(PNP) and Ti-C2-Cr(NNN) produce polyethylenes with heterogeneous branching (C2, C4, and C≥6) or negligible branching, respectively. Under identical ethylene polymerization conditions, Ti-C0-Cr(SNS) produces polyethylenes with higher activity (4.5× and 6.1×, respectively), Mn (1.3× and 1.8×, respectively), and branch density (1.4× and 3.8×, respectively), than Ti-C2-Cr(SNS) and Ti-C6-Cr(SNS). Versus a CGC(Et)Ti + SNSCr tandem catalyst, Ti-C0-Cr(SNS) yields polyethylene with somewhat lower activity, but with 22.6× higher Mn and 4.0× greater branching density under identical conditions. In ethylene +1-pentene competition experiments, Ti-C0-Cr(SNS) yields 5.5% n-propyl branches and 94.5% n-butyl branches at [1-pentene] = 0.1 M, and the estimated effective local concentration of 1-hexene is ∼8.6 M. In contrast, the tandem CGC(Et)Ti + SNSCr system yields 91.0% n-propyl branches under identical reaction conditions. The homopolymerization and 1-pentene competition results argue that close Ti···Cr spatial proximity together with weak C-H···Ti and C-H···S interactions significantly influence relative 1-hexene enchainment and chain transfer rates, supported by DFT computation, and that such effects are conversion insensitive but cocatalyst and solvent sensitive.

Temperature‐Dependent Fluorescence of Cu5 Metal Clusters: A Molecular Thermometer

The accurate measurement of temperature is of increasing importance as it is required for widespread applications (electronic devices, biology, medical diagnostics). In this context, fluorescence thermometry has already shown great potential, and a variety of molecules have been proposed as luminescent molecular thermometers. Herein, we describe Cu5 metal cluster 1 (Figure 1) that presents remarkable photophysical properties, both in solution and as the solid, characterized by temperature-dependent emission intensity and lifetime that change significantly in the range between 45 and + 80 8C. These properties allow for an unprecedented accuracy in temperature determination by fluorescence measurements, with the high sensitivity and the high temporal (sub-millisecond) and spatial (sub-micrometer) resolution typical of photoluminescence spectroscopy. Complex 1 can be seen as a metal nanoparticle composed of five copper atoms bound to three highly conjugated dianionic cationic ligands (EtNC(S)PPh2NPPh2C(S)NEt) ; Figure 1A). 14] Its absorption spectrum presents a broad and unstructured band below 450 nm (Figure 2A). The system is luminescent in all phases, both at room temperature and at 77 K (Figure 2B) and no dependence on the solvent was observed. A summary of the photophysical properties is shown in Table 1.

Single-Site Organozirconium Catalyst Embedded in a Metal–Organic Framework

A structurally well-defined mesoporous Hf-based metal-organic framework (Hf-NU-1000) is employed as a well-defined scaffold for a highly electrophilic single-site d(0) Zr-benzyl catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn is found to be a promising single-component catalyst (i.e., not requiring a catalyst/activator) for ethylene and stereoregular 1-hexene polymerization.

Supported Single-Site Organometallic Catalysts for the Synthesis of High-Performance Polyolefins

Single-site organometallic catalysts supported on solid inorganic or organic substrates are making an important contribution to heterogeneous catalysis. Early and late transition metal single-site catalysts have changed the polyolefin manufacturing industry and research with their ability to produce polymers with unique properties. Moreover, several of these catalysts have been commercialized on a large scale. Their heterogenization for slurry or gas phase olefin polymerization is important to produce polyolefin as beads and to avoid reactor fouling. The large majority of supports currently used in industry are inorganic materials (SiO2, Al2O3, MgCl2), with silica being the most important. Single-site supported catalysts are most commonly prepared by molecular-level anchoring/chemisorption, in which a molecular precursor undergoes reaction with the surface while maintaining most of the ligand sphere of the parent molecule. Chemisorption of discrete organometallic complexes on solid supports yields catalysts with well-defined active sites, greater thermal stability than the homogeneous analogues, and decreased reactor fouling versus the homogeneous analogues. This review presents a detailed account of the synthesis, characterization and polymerization properties of single-site catalysts supported on metal oxides and metal sulfated oxides, primarily carried out at Northwestern University.Graphical AbstractEarly and late transition metal single-site catalysts supported on inorganic surfaces have changed the polyolefin research and production with their unique ability to produce polymeric materials with unique architectures. This review presents a detailed account of the synthesis, characterization, and polymerization properties of single-site catalysts supported on metal oxide surfaces.

Surface structural-chemical characterization of a single-site d0 heterogeneous arene hydrogenation catalyst having 100% active sites

Structural characterization of the catalytically significant sites on solid catalyst surfaces is frequently tenuous because their fraction, among all sites, typically is quite low. Here we report the combined application of solid-state 13C-cross-polarization magic angle spinning nuclear magnetic resonance (13C-CPMAS-NMR) spectroscopy, density functional theory (DFT), and Zr X-ray absorption spectroscopy (XAS) to characterize the adsorption products and surface chemistry of the precatalysts (η5-C5H5)2ZrR2 (R = H, CH3) and [η5-C5(CH3)5]Zr(CH3)3 adsorbed on Brønsted superacidic sulfated alumina (AlS). The latter complex is exceptionally active for benzene hydrogenation, with ∼100% of the Zr sites catalytically significant as determined by kinetic poisoning experiments. The 13C-CPMAS-NMR, DFT, and XAS data indicate formation of organozirconium cations having a largely electrostatic [η5-C5(CH3)5]Zr(CH3)2+···AlS− interaction with greatly elongated Zr···OAlS distances of ∼2.35(2) Å. The catalytic benzene hydrogenation cycle is stepwise understandable by DFT, and proceeds via turnover-limiting H2 delivery to surface [η5-C5(CH3)5]ZrH2(benzene)+···AlS− species, observable by solid-state NMR and XAS.

Self-assembly of polyoxoselenitopalladate nanostars [Pd15(μ3-SeO3)10(μ3-O)10Na]9− and their supramolecular pairing in the solid state

An anionic Pd₁₅-oxo-cluster [Pd₁₅(μ₃-SeO₃)₁₀(μ₃-O)₁₀Na]⁹⁻ has been synthesized. It is the result of an unprecedented self-assembly of Pd(2+) and Na(+) cations, SeO₃²⁻ and oxo anions. It has been found for the first time, that selenito groups can give rise to supramolecular interactions in the solid state with palladium atoms, through the selenium electron lone pairs.