A series of new bis(phosphinite) p-XPCPIrHCl pincer complexes ([PCP = eta(3)-5-X-C(6)H(2)[OP(tBu)(2)](2)-1,3], X = MeO (4a), Me (4b), H (4c), F (4d), C(6)F(5) (4e), and Ar(F) [=3,5-bis(trifluoromethyl)phenyl] (4f)) have been synthesized. Treatment of compounds 4a-f with NatOBu in cyclooctane (COA)/tert-butylethylene (TBE) mixtures generates species with unprecedented catalytic activity for the catalyzed transfer dehydrogenation of COA with TBE as acceptor to form cyclooctene (COE) and tert-butylethane (TBA). With substrate:precatalyst ratios of 3030COA:3030TBE:1p-XPCPIrHCl (4):1.1NaOtBu, turnover numbers (TONs) between 1400 and 2200 (up to 72% conversion in TBE) and initial turnover frequencies (TOFs) between 1.6 and 2.4 s(-1) have been observed at 200 degrees C.
A series of bis(phosphinite) (p-XPCP)IrH2 pincer complexes {[PCP = η3-5-X-C6H2[OP(tBu)2]2-1,3], X = MeO (6a), Me (6b), H (6c), F (6d), C6F5 (6e), and ArF [=3,5-bis(trifluoromethyl)phenyl] (6f)} have been synthesized by dehydrochlorination of (p-XPCP)IrHCl precursor complexes 4a−f with NaOtBu in the presence of hydrogen. Dehydrochlorination of 4f in the presence of nitrogen yields {(p-ArFPCP)Ir}2{μ-N2} (11f), which was analyzed by X-ray diffraction. Complexes 6a−f exhibit identical catalytic activity in the transfer dehydrogenation of cyclooctane (COA) with tert-butylethylene (TBE) when compared to mixtures of precatalysts 4a−f and NaOtBu. The electronic properties of the fragments (p-XPCP)Ir (Aa−f) are discussed on the basis of the νCO of (p-XPCP)Ir(CO) complexes (8a−f) as well as on 1 J HD coupling constants of monodeuterated complexes (p-XPCP)IrHD (6a−f-d 1). Reaction of 4a−f with NaOtBu in arene solvents generates (p-XPCP)Ir(aryl)(H) complexes (9 and 10), which undergo rapid arene exchange on the NMR time scale. Exchange rates are zero-order in free arene, implying a dissociative exchange mechanism. More electron-deficient complexes, e.g., (p-C6F5PCP)Ir(m-xylyl)(H) (10e) or (p-ArFPCP)Ir(m-xylyl)(H) (10f), reductively eliminate m-xylene significantly faster than the more electron-rich complexes, e.g., (p-MeOPCP)Ir(m-xylyl)(H) (10a), on the basis of the line widths Δν1/ 2(0 °C) of the hydridic NMR resonances of (p-XPCP)Ir(m-xylyl)(H) complexes 10a−f. The same correlation with substituent effects applies to the catalytic activity (initial turnover frequencies) of complexes 6a−f in the transfer dehydrogenation of COA with TBE.
We present a complete analysis of the structure of polyethylene (PE) nanoparticles synthesized and stabilized in water under very mild conditions (15°C, 40 atm) by a nickel-catalyzed polymerization in aqueous solution. Combining cryogenic transmission electron microscopy (cryo-TEM) with X-ray scattering, we demonstrate that this new synthetic route leads to a stable dispersion of individual PE nanoparticles with a narrow size distribution. Most of the semicrystalline particles have a hexagonal shape (lateral size 25 nm, thickness 9 nm) and exhibit the habit of a truncated lozenge. The combination of cryo-TEM and small-angle X-ray scattering demonstrates that the particles consist of a single crystalline lamella sandwiched between two thin amorphous polymer layers ("nanohamburgers"). Hence, these nanocrystals that comprise only ca. 14 chains present the smallest single crystals of PE ever reported. The very small thickness of the crystalline lamella (6.3 nm) is related to the extreme undercooling (more than 100°C) that is due to the low temperature at which the polymerization takes place. This strong undercooling cannot be achieved by any other method so far. Dispersions of polyethylene nanocrystals may have a high potential for a further understanding of polymer crystallization as well as for materials science as, e.g., for the fabrication of extremely thin crystalline layers.Polyethylene (PE) is a commodity polymer that has become ubiquitous over the past several decades because of its low price and good mechanical properties. 1 Hence, the number of applications of the material is huge and many millions of tons are produced worldwide annually. However, PE has hardly played any role in the field of nanotechnology. This is due to the problem that PE is produced either by free radical polymerization under high pressure and temperature or with metal-organic catalysts working exclusively under strictly water-free conditions. Polymer nanoparticles and their composites with inorganic compounds, however, are very often produced in aqueous systems. 2 Recently, it was demonstrated that ethylene can be polymerized in aqueous systems in a catalytic fashion by Ni(II) complexes. [3][4][5][6] By virtue of this novel synthesis, long chains of polyethylene can be generated in a well-controlled environment and at ambient temperature. Thus, it could be shown that aqueous PE dispersions can be produced. This novel way of polymerization hence opens the way for the creation of nanostructures made from PE. Up to now, the particles synthesized in this way were semicrystalline and for the largest part consisted of stacks of several crystalline lamellae. 6
The neutral κ(2)N,O-salicylaldiminato Ni(II) complexes [κ(2)N,O-{(2,6-(3',5'-R2C6H3)2C6H3-N═C(H)-(3,5-I2-2-O-C6H2)}]NiCH3(pyridine)] (1a-pyr, R = Me; 1b-pyr, R = Et; 1c-pyr, R = iPr) convert ethylene to hyperbranched low-molecular-weight oligomers (Mn ca. 1000 g mol(-1)) with high productivities. While all three catalysts are capable of generating hyperbranched structures, branching densities decrease significantly with the nature of the remote substituent along Me > Et > iPr and oligomer molecular weights increase. Consequently, only 1a-pyr forms hyperbranched structures over a wide range of reaction conditions (ethylene pressure 5-30 atm and 20-70 °C). An in situ catalyst system achieves similar activities and identical highly branched oligomer microstructures, eliminating the bottleneck given by the preparation and isolation of Ni-Me catalyst precursor species. Selective introduction of one primary carboxylic acid ester functional group per highly branched oligoethylene molecule was achieved by isomerizing ethoxycarbonylation and alternatively cross metathesis with ethyl acrylate followed by hydrogenation. The latter approach results in complete functionalization and no essential loss of branched oligomer material and molecular weight, as the reacting double bonds are close to a chain end. Reduction yielded a monoalcohol-functionalized oligomer. Introduction of one reactive epoxide group per branched oligomer occurs completely and selectively under mild conditions. All reaction steps involved in oligomerization and monofunctionalization are efficient and readily scalable.
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