Markus Enders

Universität Heidelberg, Heidelberg, Baden-Wuerttemberg, Germany

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Publications (54)174.23 Total impact

  • [show abstract] [hide abstract]
    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 01/2014; s 383–384:53–57. · 3.19 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. · 21.76 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; · 10.68 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 05/2013; · 5.93 Impact Factor
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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 01/2013; · 5.93 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 09/2012; 18(44):14108-16. · 5.93 Impact Factor
  • Macromolecules 09/2012; 45(17):6878-6887. · 5.52 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 02/2012; 18(15):4627-33. · 5.93 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. · 1.81 Impact Factor
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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. · 2.77 Impact Factor
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    ABSTRACT: Condensation of phthalodinitrile and 2-amino-5,6,7,8-tetrahydroquinoline gave the bis(2-pyridylimino)isoindole protioligand 1 (thqbpiH) in high yield. Deprotonation of thqbpiH (1) using LDA in THF at -78 °C yields the corresponding lithium complex [Li(THF)(thqbpi)] (2) in which the lithium atom enforces almost planar arrangement of the tridentate ligand, with an additional molecule of THF coordinated to Li. Reaction of cobalt(II) chloride or iron(II) chloride with one equivalent of the lithium complex 2 in THF led to formation of the metal complexes [CoCl(THF)(thqbpi)] (3a) and [FeCl(THF)(thqbpi)] (3b). The paramagnetic susceptibility of 3a,b in solution was measured by the Evans method (3a: μ(eff) = 4.17 μ(B); 3b: μ(eff) = 5.57 μ(B)). Stirring a solution of 1 and cobalt(II) acetate tetrahydrate in methanol yielded the cobalt(II) complex 4 which was also accessible by treatment of 3a with one equivalent of silver or thallium acetate in DMSO. Whereas 3a,b were found to be mononuclear in the solid state, the acetate complex 4 was found to be dinuclear, the two metal centres being linked by an almost symmetrically bridging acetate. For all transition metal complexes paramagnetic (1)H as well as (13)C NMR spectra were recorded at variable temperatures. The complete assignment of the paramagnetic NMR spectra was achieved by computation of the spin densities within the complexes using DFT. The proton NMR spectra of 3a and 3b displayed dynamic behaviour. This was attributed to the exchange of coordinating solvent molecules by an associative mechanism which was analysed using lineshape analysis (ΔS(≠)= -154 ± 25 J mol(-1) K(-1) for 3a and ΔS(≠) = -168 ± 15 J mol(-1) K(-1) for 3b).
    Dalton Transactions 06/2011; 40(40):10406-15. · 3.81 Impact Factor
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    ABSTRACT: A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.
    Inorganic Chemistry 01/2011; 50(5):1942-55. · 4.59 Impact Factor
  • Berichte der deutschen chemischen Gesellschaft 11/2010; 2011(1):83 - 90. · 2.94 Impact Factor
  • Angewandte Chemie International Edition 10/2010; 49(46):8751-4. · 13.73 Impact Factor
  • Berichte der deutschen chemischen Gesellschaft 08/2010; 2010(30):4770 - 4782. · 2.94 Impact Factor
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    ABSTRACT: A ternary blend of the bisiminopyridine chromium (III) (Cr-1) with the bisiminopyridine iron (II) (Fe-2) post-metallocenes with the quinolylsilylcyclopentadienyl chromium (III) halfsandwich complex (Cr-3) was supported on mesoporous silica to produce novel multiple single-site catalysts and polyethylene reactor blends with tailor-made molecular weight distributions (MWDs). The preferred cosupporting sequence of this ternary blend on MAO-treated silica was Fe-2 followed by Cr-1 and Cr-3. Cosupporting does not impair the single-site nature of the blend components producing polyethylene fractions with $\overline M _{\rm w}$ = 10(4) g · mol(-1) on Cr-1, $\overline M _{\rm w}$ = 3 × 10(5) g · mol(-1) on Fe-2, and $\overline M _{\rm w}$ = 3 × 10(6) g · mol(-1) on Cr-3. As a function of the Fe-2/Cr-1/Cr-2 mixing ratio it is possible to control the weight ratio of these three polyethylenes without affecting the individual average molecular weights and narrow polydispersities of the three polyethylene fractions. Tailor-made polyethylene reactor blends with ultra-broad MWD and polydispersities varying between 10 and 420 were obtained. When the molar ratio of Fe-2/Cr-1 was constant, the ultra-high molecular polyethylene (UHMWPE, $\overline M _{\rm w}$ > 10(6) g · mol(-1) ) content was varied between 8 and 16 wt.-% as a function of the Cr-3 content without impairing the blend ratio of the other two polyethylene fractions and without sacrificing melt processability. When the molar ratio Fe-2/Cr-3 was constant, it was possible to selectively increase the content of the low molecular weight fraction by additional cosupporting of Cr-1. Due to the intimate mixing of low and ultra-high molecular weight polyethylenes (UHMPEs) produced on cosupported single-site catalysts a wide range of melt processable polyethylene reactor blends was obtained.
    Macromolecular Rapid Communications 08/2010; 31(15):1359-63. · 4.93 Impact Factor
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    ABSTRACT: Quinolyl-substituted half-sandwich complexes (CpQ)Ln(AlMe4)2 (CpQ = 2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl; Ln = Y, La) were obtained in quantitative yield via protonolysis reactions utilizing HCpQ and homoleptic tetramethylaluminates Ln(AlMe4)3. X-ray structure analyses revealed that the quinolyl-substituted cyclopentadienyl ligand coordinates to the rare-earth metal centers in an η5:η1 fashion through the Cp ring carbon atoms and the N atom of the quinolyl substituent. The complexes (CpQ)Ln(AlMe4)2 show good activity and high trans-1,4-stereoselectivity (maximum 93%) in the polymerization of isoprene upon activation with the organoborates [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4]. The effects of metal size, cocatalyst, temperature, and solvent were assessed in these polymerization reactions, and the performance of such N-donor-functionalized complexes was compared to that of (CpR)Ln(AlMe4)2 containing different types of nondonor-functionalized cyclopentadienyl ligands.
    Organometallics. 05/2010; 29(11).
  • Berichte der deutschen chemischen Gesellschaft 05/2010; 2010(19):2911 - 2918. · 2.94 Impact Factor
  • European Journal of Inorganic Chemistry - EUR J INORG CHEM. 01/2010; 2010(19).
  • Berichte der deutschen chemischen Gesellschaft 10/2009; 2009(32):4795 - 4808. · 2.94 Impact Factor