Qaiser Mahmood

Highly branched unsaturated polyethylenes achievable using strained imino-cyclopenta[b]pyridyl-nickel precatalysts

A new family of strained imino-cyclopenta[b]pyridines, 7-(ArN)-6-Me2C8H5N (Ar = 2,6-Me2Ph (L1), 2,6-Et2Ph (L2), 2,4,6-Me3Ph (L3), 2,6-Et2-4-MePh (L4), 2,6-i-Pr2Ph (L5)), have been synthesized in reasonable yield by a sequence of reactions from 2-chloro-cyclopenta[b]pyridin-7-one. Treatment of L1 and L3 with NiCl2·6H2O generates mononuclear bis-ligated [7-(ArN)-6-Me2C8H5N]2NiCl2 (Ar = 2,6-Me2Ph (Ni1), 2,4,6-Me3Ph (Ni3)), while with L2 and L4, the chloride-bridged binuclear complexes [7-(ArN)-6-Me2C8H5N]2Ni2(μ-Cl)2Cl2 (Ar = 2,6-Et2Ph (Ni2), 2,6-Et2-4-MePh (Ni4)), have been isolated; no apparent reaction occurred with L5. On activation with either MAO or MMAO, Ni1–Ni4 exhibited high activities towards ethylene polymerization with Ni3 the most active (5.02 × 106 g PE per mol Ni per h at 20 °C); rapid regeneration of the active species (3096–5478 h−1 at 20 °C) is a feature of their catalytic performance. A detailed microstructural analysis of the polyethylenes reveals the presence of vinyl and higher levels of internal vinylene groups indicative of high rates of chain isomerization, e.g., the ratio of (–CHCH–) to (H2CCH–) groups is 2.2 : 1 using Ni3/MAO at 60 °C. Agostic interactions involving γ-, δ- and higher-hydrogens are inferred in addition to β-hydrogen elimination to account for the vinylene groups and the longer chain alkyl branches. The molecular structures of Ni1 and Ni2·2MeOH are also reported.

Ultra-high molecular weight elastomeric polyethylene using an electronically and sterically enhanced nickel catalyst

A collection of ten related 1,2-bis(imino)acenaphthene-nickel(II) halide complexes, [1-[2,6-{(C6H5)2CH}2-4-{C(CH3)3}-C6H2N]-2-(ArN)C2C10H6]NiX2 (X = Br: Ar = 2,6-Me2C6H3Ni1, 2,6-Et2C6H3Ni2, 2,6-iPr2C6H3Ni3, 2,4,6-Me3C6H2Ni4, 2,6-Et2-4-MeC6H2Ni5) and (X = Cl: Ar = 2,6-Me2C6H3Ni6, 2,6-Et2C6H3Ni7, 2,6-iPr2C6H3Ni8, 2,4,6-Me3C6H2Ni9, 2,6-Et2-4-MeC6H2Ni10), each bearing one sterically and electronically enhanced N-2,6-dibenzhydryl-4-t-butylphenyl group, have been prepared and fully characterized. The unsymmetrical nature of the chelating bis(imino)acenaphthene is confirmed in the paramagnetic 1H NMR spectra for Ni1–Ni10, while the molecular structures of Ni1, Ni2 and Ni6 highlight the unequal steric protection of the nickel center imposed by their respective N,N-ligands. On activation with either Et2AlCl or MMAO, all the nickel complexes were highly active catalysts in ethylene polymerization [as high as 1.26 × 107 g of PE per mol of Ni per h] affording exceptionally high molecular weight (up to 3.1 × 106 g mol−1) hyper-branched polyethylene. Analysis of the mechanical properties reveals the ultra-high molecular weight polymers possess high tensile strength, excellent shape fixity and elastic recovery (up to 69%) as well as high elongation at break (eb = 843.9%); such materials offer a promising alternative to current thermoplastic elastomers (TPEs).

Bisimino-functionalized dibenzo[a,c]acridines as highly conjugated pincer frameworks for palladium(II): synthesis, characterization and catalytic performance in Heck coupling

A new pair of highly conjugated ligands, 10-[1-(arylimino)ethyl]-14-[(arylimino)methyl]dibenzo[a,c]acridine (aryl = 2,6-Me2Ph L1, 2,6-Et2Ph L2) incorporating both aldimine and ketimine units, have been prepared by a straightforward sequence of organic transformations from phenanthrene-9,10-dione. Cyclopalladation occurs readily at ambient temperature on treating L1 or L2 with PdCl2(NCCH3)2 in aprotic solvents to afford exclusively (NketimineNC)Pd(II) chloride pincer complexes Pd1 or Pd2, respectively. By contrast in methanol, Pd1 or Pd2 are isolated as the minor product with aldehyde-containing Pd3 or Pd4 as the major one, the result of hydrolysis of the pendant aldimine units in Pd1 and Pd2, respectively. All the ligands and palladium complexes have been characterized by FT-IR, 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis; the molecular structures for L1, Pd1, Pd2, Pd3 and Pd4 are also reported. Using low catalyst loadings (0.0005–0.002 mol%) and elevated temperatures (140–200 °C), Pd1–Pd4 are able to efficiently mediate the coupling of haloarenes with vinyl-containing substrates with turnover numbers as high as 174000; the effects of steric/electronic variation within the substrate and NNC-pincer complex on catalyst performance are examined.

(Co-)polymerization of methylacrylate with NBE/1-hexene by (8-arylimino-5,6,7-trihydroquinolyl)(methyl)palladium chlorides: an approaching mechanism and the polymeric microstructures

A series of methylated palladium chlorides, N-(5,6,7-trihydroquinolin-8-ylidene)arylamino PdMeCl (8-(ArN)-C9H9NPdMeCl, Ar = 2,6-Me2Ph, Pd1; 2,6-Et2Ph, Pd2; 2,6-iPr2Ph, Pd3; 2,4,6-Me3Ph, Pd4; 2,6-Et2-4-MePh, Pd5), has been prepared and fully characterized by 1H NMR, 13C NMR, FTIR spectroscopy, and elemental analysis. The solid state structures of representative complexes Pd3 and Pd4 were unambiguously confirmed by single crystal X-ray diffraction showing a slightly distorted square planar geometry around the palladium metal center. All these complexes have been found to be highly robust catalyst systems showing high catalytic activities toward the homopolymerization of methyl acrylate at elevated temperature (100 °C) and produced PMA with a molecular weight as high as 4121 kg mol−1. In addition, the Pd3 complex has good capability to incorporate norbornene (8.5%) and 1-hexene (12.7%) into the MA polymer chain. The mechanistic studies suggested that the homo(co)polymerization proceeded through a radical pathway.

N,N-chelated nickel catalysts for highly branched polyolefin elastomers: a survey

The physical properties and end applications of polyolefin materials are defined by their chain architectures and topologies. These properties can, in part, be controlled by a judicious choice of the steric and electronic properties of the catalyst and, in particular, the ligand framework. One major achievement in this field is the discovery of thermoplastic polyolefin elastomers that combine the processing and recyclable characteristics of thermoplastics with the flexibility and ductility of elastomers. These polymers are highly sought after as alternative materials to thermoset elastomers. In this perspective, works in the literature related to the development of nickel catalysts as well as their implementations for the synthesis of polyolefin elastomers are summarized in detail. Throughout the perspective, attention has been focused on developing the relationship between catalyst structure and performance, on strategies for the synthesis of polyolefin elastomer using nickel catalysts, on properties of the resultant polyolefin, such as degree of branching and crystallinity, as well as on their effects on mechanical properties. The future perspective regarding the most recent developments in single-step production of polyethylene elastomers will also be presented.