-
[show abstract]
[hide abstract]
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
-
[show abstract]
[hide abstract]
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
-
[show abstract]
[hide abstract]
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
-
[show abstract]
[hide abstract]
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. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
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. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
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. · 4.60 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. · 13.45 Impact Factor
-
Berichte der deutschen chemischen Gesellschaft 08/2010; 2010(30):4770 - 4782. · 2.94 Impact Factor
-
[show abstract]
[hide abstract]
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. · 4.60 Impact Factor
-
Berichte der deutschen chemischen Gesellschaft 05/2010; 2010(19):2911 - 2918. · 2.94 Impact Factor
-
Berichte der deutschen chemischen Gesellschaft 10/2009; 2009(32):4795 - 4808. · 2.94 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The addition of aluminium alkyls to terminal olefins leads to branched organoaluminium compounds which can be converted into functionalised alkanes. This carboalumination reaction is efficiently catalysed by a donor functionalised Cp-chromium(III) complex. The active catalyst is obtained by activation of the chromium(III) dichloride precursor with MAO or with a mixture of trialkylaluminium and N,N-dimethylanilinium tetrakispentaflourphenylborate. Primary aminoalkenes deactivate the catalyst whereas secondary and tertiary aminoalkenes can also be carboaluminated.
Dalton Transactions 08/2009; · 3.84 Impact Factor
-
Angewandte Chemie International Edition 07/2009; 48(30):5538-41. · 13.45 Impact Factor
-
Angewandte Chemie 06/2009; 121(30):5646 - 5649.
-
[show abstract]
[hide abstract]
ABSTRACT: Colliding double deckers: Addition of zinc to a reaction mixture of [{Cp*RuCl}(4)]/pentamethyl-2,3-dihydro-1,3-diborole (C(3)B(2)Me(5)H) in THF leads to three known double- and triple-decker complexes of [C(3)B(2)Me(5)](-), and unexpectedly to the slipped triple-decker (see picture) with two fused diborole rings. The endo C--H bonds of two MeC--H groups donate two additional electrons to achieve the stable 34 VE configuration.
Angewandte Chemie International Edition 02/2009; 48(8):1429-31. · 13.45 Impact Factor
-
Berichte der deutschen chemischen Gesellschaft 11/2008; 2008(35):5482 - 5493. · 2.94 Impact Factor
-
Berichte der deutschen chemischen Gesellschaft 08/2008; 2008(28):4440 - 4447. · 2.94 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Reaction of the dichloro complexes [M(N(2) (TBS)N(py))Cl(2)] (M=Zr: 1, Hf: 2; TBS: tBuMe(2)Si; py: pyridine) with one molar equivalent of LiNHNPh(2) gave mixtures of the two diastereomeric chlorohydrazido(1-) complexes [M(N(2) (TBS)N(py))(NHNPh(2))Cl] (M=Zr: 3 a,b, Hf: 4 a,b) in which the diphenylhydrazido(1-) ligand adopts a bent kappa(1) coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2-) complexes [Zr(N(2) (TBS)N(py))(NNPh(2))(py)] (5) and [Hf(N(2) (TBS)N(py))(NNPh(2))(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one sigma and two pi orbitals. A high polarity of the M=N bond was found, in accordance with the description of hydrazinediide(2-) acting as a six-electron donor ligand. The pyridine ligand in [M(N(2) (TBS)N(py))(NNPh(2))(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (DeltaG(not equal)=19 kcal mol(-1)). A change in denticity of the hydrazido unit from kappa(1) to kappa(2) was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid kappa(1) to kappa(2) interconversion. This process is believed to happen early on in the N-N scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N(alpha) is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C(6)F(5))(3) to give [Zr(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (7) and [Hf(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (8). In these products, B(C(6)F(5))(3) becomes attached to the N(alpha) atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho-F atom of a C(6)F(5) ring with the metal centre.
Chemistry 08/2008; 14(27):8131 - 8146. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A unprecedented thermally induced rearrangement of 2-bromo-4-substituted oxazolines into 2-bromoisocyanates with high selectivity has been observed. Isolated yields of 85-90% were obtained with 2-bromo-4-phenyloxazoline, 2-bromo-4-isopropyloxazoline, or 2-bromo-4,4-dimethyloxazoline. In addition, chiral aziridinecarboxamides or 2-aminooxazolines could be selectively obtained from the corresponding 2-bromo isocyanate depending on reaction conditions.
Organic Letters 02/2008; 10(2):305-8. · 5.86 Impact Factor
