Markus Enders

Universität Heidelberg, Heidelburg, Baden-Württemberg, Germany

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Publications (73)272.75 Total impact

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    ABSTRACT: The phthalocyaninato double-decker complexes [M(obPc)2 ](0) (M= Y(III) , Tb(III) , Dy(III) ; obPc=2,3,9,10,16,17,23,24-octabutoxyphthalocyaninato), along with their reduced ([M(obPc)2 ](-) [P(Ph)4 ](+) ; M=Tb(III) , Dy(III) ) and oxidized ([M(obPc)2 ](+) [SbCl6 ](-) (M=Y(III) , Tb(III) ) counterparts were studied with (1) H, (13) C and 2D NMR. From the NMR data of the neutral (i.e., with one unpaired electron in the ligands) and anionic Tb(III) complexes, along with the use of dispersion corrected DFT methods, it was possible to separate the metal-centered and ligand-centered contributions to the hyperfine NMR shift. These contributions to the (1) H and (13) C hyperfine NMR shifts were further analyzed in terms of pseudocontact and Fermi contact shifts. Furthermore, from a combination of NMR data and DFT calculations, we have determined the spin multiplicity of the neutral complexes [M(obPc)2 ](0) (M=Tb(III) and Dy(III) ) at room temperature. From the NMR data of the cationic Tb(III) complex, for which actually no experimental structure determination is available, we have analyzed the structural changes induced by oxidation from its neutral/anionic species and shown that the interligand distance decreases upon oxidation. The fast electron exchange process between the neutral and anionic Tb(III) double-decker complexes was also studied. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry - A European Journal 08/2015; DOI:10.1002/chem.201501944 · 5.73 Impact Factor
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    ABSTRACT: The equimolar reaction of potassium (2,4,6-tri-tert-butylphenyl)amide with Cp*Ln(AlMe4)2 (Cp* = 1,2,3,4,5-pentamethyl cyclopentadienyl) yielded {Cp*Ln(AlMe4)[NH(mes*)]}x (Ln = Y, La; mes* = C6H2tBu3-2,4,6). Treatment of Cp*Ln(AlMe4)[NH(mes*)] with tetrahydrofuran led to intramolecular C−H activation of a tBu group under formation of Cp*YMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2)(thf). A similar methyl–anilide species CpQLuMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2) (CpQ = 2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl) with C−H activated ligand backbone formed from the reaction of CpQLu(AlMe4)2 and K[NH(mes*)]. The reactivity of CpQY(AlMe4)2 toward H2NAd (Ad = adamantyl) ultimately led to the methyl–amide complex CpQYMe[NH(Ad)](AlMe3), corroborating the presence of competing deprotonation and donor-induced methylaluminate cleavage reactions. The halfsandwich complexes Cp*Lu(AlMe4)2, Cp*Y(AlMe4)[NH(mes*)], Cp*YMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2)(thf), CpQLuMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2), and CpQYMe[NH(Ad)](AlMe3) as well as side-product AlMe3(H2NAd) were fully characterized by NMR/FTIR spectroscopy, elemental analysis, and X-ray crystallography.
    New Journal of Chemistry 05/2015; DOI:10.1039/C5NJ00800J · 3.09 Impact Factor
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    ABSTRACT: A series of 8-amino-2-arylquinoline ligands (16) were synthesized and reacted with CH3CrCl2(thf)(3). Under these conditions a CH bond of the 2-aryl substituent is metalated, leading to organochromium complexes with monoanionic tridentate ligands (813). The presence of a chromiumcarbon s bond in these complexes has been established by X-ray analysis. Furthermore, 8-(piperidin-1-yl)quinoline (14) was used as neutral bidentate ligand in addition to an external aryl group, leading to complex 15. Finally, the tris-aryl complex 18 was synthesized, which features a rare five-coordinate chromium(III) metal center. All chromium complexes were tested as catalysts for the selective trimerization of ethylene after activation with methylaluminoxane (MAO). Several of the new catalyst precursors show good behavior for the selective trimerization of ethylene. Although chlorido ligands in the catalyst precursor will be substituted by methyl groups during the activation with MAO, there is a clear difference in the catalytic behavior when the complex contains a methyl (or aryl) group prior to addition of MAO. The mechanism of catalyst activation has been studied in more detail with the tris-aryl complex 18.
    Organometallics 10/2014; 33(20):5758-5766. DOI:10.1021/om500459k · 4.13 Impact Factor
  • Markus Stürzel · Yi Thomann · Markus Enders · Rolf Mülhaupt
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    ABSTRACT: The catalytic ethylene polymerization on dual-site catalysts, supported on functionalized graphene, enables nanostructure formation in polyethylene reactor blends by in situ formation of uniformly dispersed ultrahigh molecular weight polyethylene (UHMWPE) nanoplatelets and in situ formed aligned UHMWPE shish-kebab nanofibers. For tailoring bimodal molar mass distributions, the quinolylsilylcyclopentadienylchromium(III) complex (Cr-1), producing UHMWPE with M-w > 3 X 10(6) g mol(-1), is blended together with bisiminopyridine complexes of either chromium(III) (CrBIP), producing polyethylene (PE) wax (2 X 10(3) g mol(-1)), or iron(II) (FeBIP), producing PE with M-w = 2.0 x 10(5) g mol(-1). Hence, the FeBIP/Cr-1 and CrBIP/Cr-1 molar ratios govern the PE/UHMWPE weight ratio without affecting the average molar mass of the individual PE fractions. In sharp contrast to conventional UHMWPE/PE reactor blends, the UHMWPE content is substantially increased up to 17 wt % without impairing melt processing. In the case of graphene-supported FeBIP/Cr-1, SEM and TEM analysis reveal that UHMWPE nanoplatelets are formed during polymerization. This is attributed to graphene-mediated mesoscopic shape replication. During injection molding, the UHMWPE nanoplatelets are transformed into aligned UHMWPE shish-kebab nanofibers, thus enabling efficient polyethylene matrix reinforcement.
    Macromolecules 08/2014; 47(15):4979-4986. DOI:10.1021/ma500769g · 5.80 Impact Factor
  • Markus Enders
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    ABSTRACT: Single-site chromium catalysts allow the polymerization of ethylene to low as well as high molecular weight polyethylenes. Chromium complexes of donor-functionalized cyclopentadienyl ligands lead to catalysts that combine both high catalytic activities and formation of high or ultra high moelcular weight polyethylene. The properties of such complexes with respect to their polymerization behavior (e.g., activation, chain termination, activity profile) are summarized.
    Molecular Catalysts, 06/2014: pages 147-160; , ISBN: 9783527335213
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    ABSTRACT: A new family of silica supported two-site Fe/Cr catalysts were prepared by means of sequential and simultaneous immobilization (“cosupporting”) of both 2,6-bis-[1-(2,6-dimethylphenyliminio)ethyl]pyridine iron(II) chloride (Fe) and η5-[3,4,5-trimethyl-1-(8-quinolyl)-2-trimethyl-silylcyclopentadienyl]chromium(III) chloride (Cr) on MAO-pretreated silica. The influence of catalyst preparation, catalyst composition, and of Fe/Cr molar ratio on catalyst activity, polyethylene (PE) molecular weight (MW), and PE molecular weight distribution (MWD), as determined by high temperature size exclusion chromatography, was examined. Catalyst activity increased with decreasing transition metal content. The average molar masses of the two PE fractions, UHMWPE (3 × 106 g/mol), produced on Cr sites, and PE (3 × 105 g/mol), produced on Fe sites, are independent of the Fe/Cr molar ratio. The sequential immobilization of the Fe complex prior to the Cr complex affords the highest catalyst activities and highest UHMWPE contents. Here, the UHMWPE content consequently increases from 0 to 32 wt% with increasing Cr content at constant Fe concentration.
    Journal of Molecular Catalysis A Chemical 03/2014; s 383–384:53–57. DOI:10.1016/j.molcata.2013.11.020 · 3.62 Impact Factor
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    ABSTRACT: The design of larger architectures from smaller molecular building blocks by element-element coupling reactions is one of the key concerns of synthetic chemistry, so a number of strategies were developed for this bottom-up approach. A general scheme is the coupling of two elements with opposing polarity or that of two radicals. Here, we show that a B-B coupling reaction is possible between two boron analogues of the ethyl cation, resulting in the formation of an unprecedented dicationic tetraborane. The bonding properties in the rhomboid B4 core of the product can be described as two B-B units connected by three-centre, two-electron bonds, sharing the short diagonal. Our discovery might lead the way to the long sought-after boron chain polymers with a structure similar to the silicon chains in β-SiB3. Moreover, the reaction is a prime textbook example of the influence of multiple-centre bonding on reactivity.
    Nature Chemistry 12/2013; 5(12):1029-1034. DOI:10.1038/nchem.1776 · 25.33 Impact Factor
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    ABSTRACT: Several small paramagnetic complexes combine large hyperfine NMR shifts with large magnetic anisotropies. The latter are a prerequisite for single molecule magnet (SMM) behavior. We choose the SMM tris(octabutoxyphthalocyaninato) diterbium (1) for a high resolution NMR study where we combined for the first time a comprehensive (1)H and (13)C chemical shift analysis of a SMM with the evaluation of large residual dipolar couplings (RDCs). The latter are a consequence of partial alignment of SMM 1 in the strong magnetic field of the NMR spectrometer. To the best of our knowledge RDCs in SMMs have never been reported before. We measured RDCs between -78 and +99 Hz for the (13)C-(1)H vectors of CH bonds and up to -109 Hz for (1)H-(1)H vectors of geminal hydrogen atoms (magnetic field of 14.09 T, temperature 295 K). Considerable negative Fermi contact shifts (up to -60 ppm) were determined for (13)C atoms at the phthalocyaninato core. Paramagnetic (13)C NMR shifts of the butoxy chains as well as all (1)H NMR chemical shifts are a result of pseudocontact shifts (pcs), and therefore it is easily possible to determine the positions of the respective nuclei in solution. Measurements of CH and HH vectors by RDC analysis are in accordance with the geometry as determined by the pseudocontact shifts, but in addition to that, RDCs give information about internal mobility. The axial component of the magnetic susceptibility tensor has been determined independently by pcs and by RDC.
    Journal of the American Chemical Society 09/2013; 135(38). DOI:10.1021/ja4069485 · 12.11 Impact Factor
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    ABSTRACT: Herein, we report on the synthesis, protonation, and coordination chemistry of chelating guanidine ligands with biphenyl, binaphthyl, and bipyridyl backbones. The ligands are shown to be proton sponges, and this protonation was studied experimentally and by using quantum-chemical calculations. Group 10 metal (Ni, Pd, and Pt) complexes with different metal/ligand ratios were synthesized. In the case of the bipyridyl systems, coordination occurs exclusively at the pyridine N atoms, as opposed to protonation. The spin-density distribution and the magnetism were evaluated for a series of paramagnetic Ni(II) complexes with the aid of paramagnetic NMR spectroscopic studies in alliance with quantum-chemical calculations and magnetic (SQUID) measurements. Through direct delocalization from the singly occupied molecular orbitals (SOMOs), a significant amount of spin density is placed on the guanidinyl groups, and spin polarization also transports spin density onto the aromatic backbone.
    Chemistry - A European Journal 07/2013; 19(27). DOI:10.1002/chem.201204294 · 5.73 Impact Factor
  • Markus Enders
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    ABSTRACT: The Aim and Scope of the ChapterBasic Theory of Paramagnetic NMRSignal AssignmentsCase StudiesReferences
    ChemInform 06/2013; 44(25). DOI:10.1002/chin.201325249
  • Chemistry 01/2013; 19(5):1513-1513. DOI:10.1002/chem.201390010 · 5.73 Impact Factor
  • Matthias Kruck · Hubert Wadepohl · Markus Enders · Lutz H Gade
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    ABSTRACT: High-spin Fe(II) -alkyl complexes with bis(pyridylimino)isoindolato ligands were synthesized and their paramagnetic (1) H and (13) C NMR spectra were analyzed comprehensively. The experimental (13) C-(1) H coupling values are temperature (T(-1) )- as well as magnetic-field (B(2) )-dependent and deviate considerably from typical scalar (1) J(CH) couplings constants. This deviation is attributed to residual dipolar couplings (RDCs), which arise from partial alignment of the complexes in the presence of a strong magnetic field. The analysis of the experimental RDCs allows an unambiguous assignment of all (13) C NMR resonances and, additionally, a structural refinement of the conformation of the complexes in solution. Moreover the RDCs can be used for the analysis of the alignment tensor and hence the tensor of the anisotropy of the magnetic susceptibility.
    Chemistry - A European Journal 01/2013; 19(5). DOI:10.1002/chem.201203450 · 5.73 Impact Factor
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    ABSTRACT: Iron(II) and cobalt(II) alkyl complexes using tridentate bis(pyridylimino)isoindolates as ancillary ligands have been synthesized from the pyridine alkyl precursor complexes [(py)2Fe(CH2SiMe3)2] and [(py)2Co(CH2SiMe3)2]. The extremely air- and moisture-sensitive compounds were structurally characterized in the solid state by X-ray diffraction as well as in solution by paramagnetic NMR spectroscopy. It is demonstrated that the paramagnetic shifts in the 13C NMR spectra are dominated by strong Fermi-contact interactions. All 13C NMR signals can be assigned by correlation with DFT-calculated spin-density distributions.
    Organometallics 01/2013; 32(3):885–892. DOI:10.1021/om301198b · 4.13 Impact Factor
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    ABSTRACT: The reaction between BF(3) ⋅OEt(2) and one of two guanidines, 1,8-bis(tetramethylguanidinyl)naphthalene (btmgn) and 1,2,4,5-tetrakis(tetramethylguanidinyl)naphthalene (ttmgn), yields the salts [(btmgn)(BF(2) )]BF(4) and [(ttmgn)(BF(2) )(2) ](BF(4) )(2) . NMR spectroscopic data show that the boron atoms in the cation and anion exchange in the case of [(ttmgn)(BF(2) )(2) ](BF(4) )(2) , but not in the case of [(btmgn)(BF(2) )]BF(4) . The rate constant for this exchange was estimated to be 4 s(-1) at 80 °C for solutions in CH(3) CN. These salts were subsequently used for the reduction of dihalides Br(2) or I(2) to give polyhalide salts. We report the synthesis and first complete characterization (including structural analysis) of salts that contain pentabromide monoanions. In these salts, the Br(5) (-) anions interact to give dimeric units or polymeric chains. Our results are compared to previous quantum chemical calculations on the gas-phase structure of the Br(5) (-) anion. The possible pathways that lead to the polyhalides are evaluated. In the case of [(ttmgn)(BF(2) )(2) ](BF(4) )(2) , reduction is accompanied by ttmgn oxidation, whereas in the case of [(btmgn)(BF(2) )]BF(4) reduction is initiated by aromatic substitution.
    Chemistry - A European Journal 10/2012; 18(44):14108-16. DOI:10.1002/chem.201202113 · 5.73 Impact Factor
  • David Sieb · Robert W. Baker · Hubert Wadepohl · Markus Enders
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    ABSTRACT: Cyclopentadienyl ligands functionalized with a 1,5-naphthyridine donor have been used for the formation of chromium(III) half-sandwich complexes. In addition to standard analytical characterization, the complexes have been investigated by paramagnetic 1H NMR spectroscopy combined with DFT calculations. After activation, highly active single-site catalysts are obtained that lead to the formation of ultrahigh molecular weight polyethylene (UHMW-PE). Evaluation of polymer formation after different polymerization times at low temperature shows that the number of active centers is low at the beginning, but increases with polymerization time.
    Organometallics 10/2012; 31(21):7368–7374. DOI:10.1021/om300582j · 4.13 Impact Factor
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    ABSTRACT: Novel families of ultrahigh-molecular-weight polyethylene (UHMWPE) nanocomposites, containing uniformly dispersed, functionalized graphene (FG) nanosheets, were prepared by means of the polymerization filling technique (PFT). Unparalleled by any other carbon and boehmite nanocomposites, FG/UHMWPE exhibited an unusual simultaneous improvement in stiffness, elongation at break, and effective nucleation of polyethylene crystallization at only 1 wt % FG content. FG nanosheets are ultrathin—with a thickness of only one carbon atom and lateral dimensions of several micrometers. Owing to the presence of surface hydroxyl groups on the FG, single FG/methylaluminoxane (MAO) nanosheets can be effectively dispersed in n-heptane, thus enabling immobilization of an MAO-activated chromium (Cr1) single-site catalyst on FG. In contrast to nanometer-scale carbon black (CB), multiwall carbon nanotubes (CNT), graphite, and nanoboehmite, which failed to form stable dispersions, FG/MAO/Cr1 afforded the highest catalyst activities and excellent morphological control. In polymerization filling, the integration of a nanoparticle dispersion into the polymerization process eliminated the need for special safety and handling precautions typically required by conventional compounding of nanoparticles with ultralow bulk densities.
    Macromolecules 09/2012; 45(17):6878-6887. DOI:10.1021/ma301376q · 5.80 Impact Factor
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    ABSTRACT: A series of functionalized diaza- and tetraazatetracenes was synthesized, either by condensation of an aromatic diamine with an ortho-quinone/diethyloxalate followed by chlorination with POCl(3) to give diazatetracenes or by palladium-catalyzed coupling of a phenylenediamine with various 2,3-dichloroquinoxalines to give tetraazatetracenes (after oxidation with MnO(2)). Representative examples included halogenated and nitrated derivatives. The optical properties of these azatetracenes were discussed with respect to their molecular structures and substitution patterns. The diazatetracenes and tetraazatetracenes formed two different groups that had significantly different electronic structures and properties. Furthermore, 1,2,3,4-tetrafluoro-6,11-bis((triisopropylsilyl)ethynyl)benzo[b]phenazine was synthesized, which is the first reported fluorinated diazatetracene. Single-crystal X-ray analysis of this compound is reported.
    Chemistry - A European Journal 04/2012; 18(15):4627-33. DOI:10.1002/chem.201103227 · 5.73 Impact Factor
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    ABSTRACT: Lithium indenide (Li-Ind) or cyclopentadienide (Li-Cp) derivatives react as nucleophiles with 8-(methylsulfinyl)-1,5-naphthyridine (Naph), leading to donor-functionalized ligands IndNaph or CpNaph, respectively. The new ligands comprise two N-donor atoms, which, for geometric reasons, cannot bind to the same metal atom. In complexes, where the metal atom is bound by the Cp or Ind moiety, the N5-donor atom is located in a distal position. The coordination behavior to Rh or Zr metal centers has been investigated. The Cp-based ligands show the expected chelating coordination mode with η5-Cp and N coordination, whereas the indenyl units act as dihapto, trihapto, or pentahapto ligands. The dinuclear Rh(I) complex 12 shows a rare coordination geometry with two η3 ligands bridging a Rh2(CO)3 fragment.
    Organometallics 01/2012; 31(1):5622-5633. DOI:10.1021/om2009638 · 4.13 Impact Factor
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    ABSTRACT: The reaction of closo-[B10H10]2− with [PtCl2(PPh3)2] in MeOH at reflux affords the B-methoxy substituted 11-vertex nido-platinaborane compound [(PPh3)2PtB10H10-8-H0.5(OCH3)0.5-10-(OCH3)] (1) and the known species [(PPh3)2PtB10H11-8-(OCH3)] (2) and 1,6-(PPh3)2B10H8 (3). The same reaction under solvothermal condition gives the partially degraded diplatinaborane [(PPh3)2(μ-PPh2)Pt2B9H7-3,9,11-(OMe)3] (4) with a novel nido-Pt2B9H10 skeleton. The new metallaborane compounds have been characterized by spectroscopic methods and single-crystal X-ray analyses. In particular, computational/theoretical chemistry supports the ultimate structural confirmation of 4. The structures of these metallaboranes exhibit interesting intra- and/or intermolecular C–H⋯O hydrogen bonding interactions.
    Polyhedron 01/2012; 31(1):607–613. DOI:10.1016/j.poly.2011.10.019 · 2.01 Impact Factor
  • Rong Xu · Günter Klatt · Markus Enders · Horst Köppel
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    ABSTRACT: We propose routes for the catalytic cycle and possible termination reactions for the polymerization of ethylene with cationic chromium complexes of the type [CpCr(L)R](+) which contain donor ligands with phosphorus or nitrogen (L = PR(3) or NR(3)). We confirm the rate-determining character of the insertion of ethylene into the chromium-alkyl bond. Contrary to the situation with late transition metals, the resulting agostic isomers will readily isomerize. The termination of the polymerization reaction by β-hydrogen elimination to the chromium center and subsequent dissociation of the resulting olefin is found to require about 25 kcal/mol and to be thermodynamically much less feasible than the alternative termination process by β-hydrogen transfer to a monomer. The latter process involves spin change; two minimum-energy crossing points as well as further transition states and intermediates have been identified. Our calculations predict that adduct formation with the polymerization additive 9-BBN should be feasible both from a Cp-quinoline-based chromium catalyst and a zirconocene catalyst. However, only the latter undergoes exergonic chain transfer, which is in accordance with the experimentally observed formation of ultrahigh M(W) polyethylene when using 9-BBN as polymerization additive in combination with Cr catalysts. For the first time, quantum dynamics simulations of such open-shell systems have been performed, which give a lifetime of the Cr-alkyl complex with regard to ethylene insertion of only 500 fs. The simulations indicate that the dissociation of ethylene from the chromium center should be relatively insignificant compared to migratory insertion.
    The Journal of Physical Chemistry A 12/2011; 116(3):1077-85. DOI:10.1021/jp209451p · 2.69 Impact Factor