03 Fakultät Chemie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/4
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Item Open Access Melt-spinning of an intrinsically flame-retardant polyacrylonitrile copolymer(2020) König, Simon; Kreis, Philipp; Herbert, Christian; Wego, Andreas; Steinmann, Mark; Wang, Dongren; Frank, Erik; Buchmeiser, Michael R.Poly(acrylonitrile) (PAN) fibers have two essential drawbacks: they are usually processed by solution-spinning, which is inferior to melt spinning in terms of productivity and costs, and they are flammable in air. Here, we report on the synthesis and melt-spinning of an intrinsically flame-retardant PAN-copolymer with phosphorus-containing dimethylphosphonomethyl acrylate (DPA) as primary comonomer. Furthermore, the copolymerization parameters of the aqueous suspension polymerization of acrylonitrile (AN) and DPA were determined applying both the Fineman and Ross and Kelen and Tüdõs methods. For flame retardancy and melt-spinning tests, multiple PAN copolymers with different amounts of DPA and, in some cases, methyl acrylate (MA) have been synthesized. One of the synthesized PAN-copolymers has been melt-spun with propylene carbonate (PC) as plasticizer; the resulting PAN-fibers had a tenacity of 195 ± 40 MPa and a Young’s modulus of 5.2 ± 0.7 GPa. The flame-retardant properties have been determined by Limiting Oxygen Index (LOI) flame tests. The LOI value of the melt-spinnable PAN was 25.1; it therefore meets the flame retardancy criteria for many applications. In short, the reported method shows that the disadvantage of high comonomer content necessary for flame retardation can be turned into an advantage by enabling melt spinning.Item Open Access Reversible N‐heterocyclic carbene‐induced α‐H abstraction in Tungsten(VI) imido dialkyl dialkoxide complexes(2020) Musso, Janis V.; Benedikter, Mathis J.; Wang, Dongren; Frey, Wolfgang; Altmann, Hagen J.; Buchmeiser, Michael R.The first reversible N‐heterocyclic carbene (NHC) induced α‐H abstraction in tungsten(VI) imido‐dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH2Ph)2(OtBu)2 (Ar=2,6‐dichlorophenyl, 2,6‐dimethylphenyl, 2,6‐diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W(NAr)(CHPh)(NHC)(CH2Ph)(OtBu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert‐butoxide ligand as tBuOH in the course of the reaction was observed. The formed alkylidene complexes and tBuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1H NMR spectroscopy. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed. Kinetics of a deuterium‐labeled complex in comparison to its non‐deuterated counterpart revealed the presence of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α‐H abstraction is the rate‐determining step (RDS) of the reaction.Item Open Access Chromium(VI) bisimido dichloro, bisimido alkylidene, and chromium(V) bisimido iodo N‐heterocyclic carbene complexes(2020) Panyam, Pradeep K. R.; Stöhr, Laura; Wang, Dongren; Frey, Wolfgang; Buchmeiser, Michael R.Reaction of CrCl2(N-tBu)2 with 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3-di(2-propyl)imidazol-2-ylidene (IPr), 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-(2-Pr)2C6H3)imidazol-2-ylidene (IDipp) yields the corresponding N-heterocyclic carbene (NHC) adducts CrCl2(IMe)(N-tBu)2 (1), CrCl2(IMeCl2)(N-tBu)2 (2), CrCl2(IPr)(N-tBu)2 (3), CrCl2(IMes)(N-tBu)2 (4) and CrCl2(IDipp)(N-tBu)2 (5). Likewise, reaction of CrCl2(N-2,6-(2-Pr)2C6H3)2 and CrCl2(N-adamantyl)2 with IMes yields CrCl2(N-2,6-(2-Pr)2C6H3)2(IMes) (6) and CrCl2(N-adamantyl)2(IMes) (7), respectively. Reaction of CrCl2(N-tBu)2 with the bidentate NHCs 1-R-3-(1-(2-LiO-C6H4))imidazol-2-ylidene yields the corresponding pentacoordinated Cr(VI) complexes CrCl2(1-R-3-(1-(2-O-C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (R = 2,4,6-(CH3)3C6H2, 8), (R = tBu, 9), (R = 2-phenyl-C6H4, 10). Reaction of the chromium(VI) complex Cr(N-2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 with 1,3-dimethylimidazol-2-ylidene·AgI yields the bimetallic silver adduct of the chromium alkylidene complex (11) along with the tetrahedral chromium(V) complex CrI(N-2,6-(2-Pr)2-C6H3)2(1,3-dimethylimidazol-2-ylidene) (12). Compounds 1-4, 7, 9-12 were characterized by single-crystal X-ray analysis. Finally, the chromium(VI) bisimido-amido complexes 13-14 bearing the N-6-(2-(diethylboryl)phenyl)pyridyl-2-yl-motif are reported.Item Open Access Hydrosilylation of alkynes under continuous flow using polyurethane‐based monolithic supports with tailored mesoporosity(2022) Acikalin, Hande; Panyam, Pradeep K. R.; Shaikh, Abdul Wasif; Wang, Dongren; Kousik, Shravan R.; Atanasova, Petia; Buchmeiser, Michael R.Non‐porous polyurethane‐based monoliths are prepared under solvent‐induced phase separation conditions. They possess low specific surface areas of 0.15 m2 g-1, pore volumes of 1 µL g-1, and a non‐permanent, solvent‐induced microporosity with pore dimensions ≤1 nm. Mesoporosity can be introduced by varying the monomers and solvents. A tuning of the average solubility parameter of the solvent mixture by increasing the macroporogen content results in a decrease in the volume fraction of micropores from 70% to 40% and an increase in the volume fraction of pores in the range of 1.7-9.6 nm from 22% to 41% with only minor changes in the volume fraction of larger mesopores in the range of 9.6–50 nm. The polymeric monoliths are functionalized with quaternary ammonium groups, which allowed for the immobilization of an ionic liquid that contained the ionic Rh‐catalyst [1‐(pyrid‐2‐yl)‐3‐mesityl)‐imidazol‐2‐ylidene))(η4‐1,5‐cyclooctadiene)Rh(I) tetrafluoroborate]. The supported catalyst is used in the hydrosilylation of 1‐alkynes with dimethylphenylsilane under continuous flow using methyl‐tert‐butyl ether as second liquid transport phase. E/Z‐selectivity in hydrosilylation is compared to the one of the analogous biphasic reactions. The strong increase in Z‐selectivity is attributed to a confinement effect provided by the small mesopores.Item Open Access Olefin ring‐closing metathesis under spatial confinement and continuous flow(2021) Ziegler, Felix; Roider, Thomas; Pyschik, Markus; Haas, Christian P.; Wang, Dongren; Tallarek, Ulrich; Buchmeiser, Michael R.We report on the use of a 2nd‐generation Hoveyda-Grubbs‐type catalyst immobilized inside mesoporous silica for the application in selective macro(mono)cyclization (MMC) of an α,ω‐diene under spatially confined and continuous‐flow conditions. Reactions carried out with different flow rates allow for variations in residence time; conversion and MMC selectivity can be determined for well‐defined reaction times. Analysis of the reaction mixtures obtained for different reaction times and temperatures in a single flow experiment by NMR and MALDI‐TOF‐MS allows to address confinement effects and to determine olefin metathesis pathways. These investigations revealed that ring‐chain equilibria are quickly established but substantially affected by residence time and flow, allowing for the determination of conditions under which MMC selectivity reaches a maximum. In contrast to reactions carried out in solution, in which oligomers up to the hexamer were observed, MMC under confinement predominantly proceeds via ring‐closing metathesis of the monomer and backbiting from the dimer and trimer, but not from higher oligomers as their formation is suppressed. This leads to the observed high MMC selectivity, reaching 60 % at a 25 mM substrate concentration.Item Open Access Cationic tungsten alkylidyne N‐heterocyclic carbene complexes : synthesis and reactivity in alkyne metathesis(2020) Hauser, Philipp M.; Ende, Melita van der; Groos, Jonas; Frey, Wolfgang; Wang, Dongren; Buchmeiser, Michael R.The first cationic and neutral tungsten alkylidyne N‐heterocyclic carbene (NHC) complexes bearing one triflate ligand were synthesized and tested for their reactivity in alkyne metathesis. Both types of tungsten alkylidyne complexes display a higher productivity in alkyne metathesis than the analogous neutral tungsten alkylidyne NHC trisalkoxide complexes. Reaction of W(≡CC6H4OMe)(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethylethyl)‐imidazol‐2‐ylidene)Cl (W18) with AgB(ArF)4 (ArF = 3,5‐bis(trifluoromethyl)phenyl) resulted in the unexpected formation of, to the best of our knowledge, the first cationic ditungstatetrahedrane W2(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethyl‐ethyl)‐imidazol‐2‐ylidene)2(MeCN)(µ‐((Ar)CC(Ar)))+ (B(ArF)4)- (W19, Ar = C6H4OMe), which suggests bimolecular decomposition as a possible decomposition pathway of cationic tungsten alkylidyne NHC complexes. Reaction of the cationic tungsten alkylidyne NHC complex W(≡CC6H4OMe)(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)- (W7) with 1‐phenyl‐1‐propyne allowed for the isolation of a cationic tungstacyclobutadiene W(C3(Ph)(Me)(C6H4OMe))(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)- (W20). Its formation strongly supports a cationic active species in the alkyne metathesis with tungsten alkylidyne NHC complexes.Item Open Access Stereoselective ring‐opening metathesis polymerization with tungsten sulfido alkylidene N‐heterocyclic carbene complexes(2022) Buchmeiser, Michael R.; Probst, Patrick; Wang, Dongren; Hauser, PhilippA series of cationic tungsten sulfido alkylidene N-heterocyclic carbene (NHC) complexes (W01 - W09) of the general formula [W(S)(CHCMe3)(X)(NHC)(CMe3CN)+B(ArF)4-] (NHC = 1,3-dimesitylimidazol-2-ylidene, IMes; 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene, IMesCl2; 1,3-bis(2,6-xdiisopropyl)phenyl)imidazol-2-ylidene, IDipp; X = Cl, C6F5O, 2,6-Ph2-C6H3; B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are used as initiators in the stereoselective ring-opening metathesis polymerization (ROMP) of (+) 2,3-endo, exo-dicarbomethoxynorborn-5-ene ((+)DCMNBE, M1). Trans-isospecifity up to 84% is achieved along with varying percentages of cis-syndiospecifity. The different extent of trans-isospecifity is compared to the one of related benchmark cationic molybdenum and tungsten imido and tungsten oxo alkylidene NHC complexes. Mechanistic investigations suggest that the syn-isomer of a nitrile-free initiator reacts with M1 presumably in an eneanti fashion to yield a syn-first insertion product via turnstile rearrangement, which accounts for the predominant trans-isospecifity of the polymerization. The cis-syndiotactic sequences are proposed to stem from the competing enesyn addition of M1 to a nitrile-containing syn-isomer of the initiator.Item Open Access Synthetic and structural peculiarities of neutral and cationic molybdenum imido and tungsten oxo alkylidene complexes bearing weakly coordinating N‐heterocyclic carbenes(2024) Buchmeiser, Michael R.; Wang, Dongren; Schowner, Roman; Stöhr, Laura; Ziegler, Felix; Sen, Suman; Frey, WolfgangThe syntheses of the neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NAr)(CHCMe2Ph)(NHC)XY] (Ar=2-tBu-C6H4, 2-CF3-C6H4, 2,6-Me2-C6H3, 2,6-Cl2-C6H3, adamantyl; X, Y=OTf, OC(CF3)3, OCH(CF3)2, OC6F5, SC6F5, 2,5-bis(pentafluorophenyl)phen-1-yl) bearing electron-withdrawing NHCs (1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (TPT)) are reported. Complementary, the corresponding cationic molybdenum imido alkylidene NHC complexes of the general formula [Mo(NAr)(CHCMe2R)(NHC)X+][B(ArF)4−/Al(OC(CF3)3)4−] (R=Me, Ph; B(ArF)4-=tetrakis (3,5-bis(trifluoromethyl)phenyl)borate) have been prepared. Aiming at tungsten oxo complexes, reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)2] with [1,3-dimethyl-4,5-dichloroimidazol-2-ylidene⋅AgI] (IMeCl2⋅AgI) followed by the addition of lithium terphenoxide yields [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)2]. For comparison, [W(O)Cl(CHCMe2Ph)(IMes)(OSi(OtBu)3)] was prepared via reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)(IMes)] with KOSi(OtBu)3. [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)(Et2O)+][B(ArF)4−] (DPPO=2,6-diphenylphenoxide) became accessible via reaction of [W(O)(DPPO)2(CHCMe2Ph)(IMeCl2)] with anilinium B(ArF)4-. The structural peculiarities of selected complexes are reported. Benchmark ring-closing metathesis and homometathesis reactions revealed that the neutral complexes bearing weakly coordinating NHCs such as IMeCl2 and TPT possessed only moderate activity, which could, however, be improved by preparing the corresponding cationic metal alkylidene complexes.Item Open Access Synthesis of dihydroxy telechelic oligomers of β‐butyrolactone catalyzed by titanium(IV)‐alkoxides and their use as macrodiols in polyurethane chemistry(2021) Altmann, Hagen J.; Machat, Martin R.; Wolf, Aurel; Gürtler, Christoph; Wang, Dongren; Buchmeiser, Michael R.We report on a solvent‐free approach for the synthesis of low molecular weight, α,ω‐dihydroxy telechelic poly(β‐butyrolactone). In the presence of Ti(IV) alkoxides, mixtures of β‐butyrolactone and diols, like di‐ or triethylene glycol, were reacted in ratios between 4:1 and 10:1. The oligomerization proceeds at elevated temperatures (80-100°C). Different alkoxide substituents (R = Me, iPr, tBu) of the Ti(IV)(OR)4 catalyst were investigated. The resulting oligomers were characterized by nuclear magnetic resonance (NMR), infra‐red (IR), gel‐permeation chromatography (GPC), titration, and matrix‐assisted laser desorption‐time‐of‐flight mass spectrometry (MALDI‐ToF‐MS) analysis. Aside from low molecular weight products, special effort was devoted to achieve high O‐acyl cleavage selectivity and to circumvent the formation of unsaturated end‐groups in order to form exclusively dihydroxy‐telechelic oligomers. Optimized results in terms of selectivity and reaction rates were achieved at 100°C using catalyst loadings of 0.2 mol% with respect to the monomer. The molecular weights determined by GPC were in good accordance with the ratio of monomer to diol used, confirming successful oligomer formation. Polyurethanes prepared from crude macrodiols without any additional catalyst feature molecular weights up to 50,000 g/mol. The reported work serves as concept to utilize β‐lactones for tailored polyol synthesis; the resulting products are suitable for polyurethane chemistry.