Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes.

Molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes of the type [Mo(C-p-C6H4Y)(OC(R)(CF3)2)2 (L)(NHC)][B(ArF)4] (Y = OMe, NO2; R = CH3, CF3; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH2), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl2), 1,3-diisopropylimidazol-2-ylidene (IiPr); B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With cis-cyclooctene (cCOE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(cCOE) became available at high monomer concentrations. Also, post-REMP allowed for converting low-molecular-weight cyclic poly(cCOE) into high-molecular-weight cyclic poly(cCOE). Tailored catalysts together with suitable additives offered access to the stereoselective REMP of functional norbornenes providing functional cis-isotactic (cis-it), cis-syndiotactic (cis-st) and trans-it poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.

Melt-Spinning of an Intrinsically Flame-Retardant Polyacrylonitrile Copolymer.

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.

Reversible N-Heterocyclic Carbene-Induced α-H Abstraction in Tungsten(VI) Imido Dialkyl Dialkoxide Complexes.

The first reversible N-heterocyclic carbene (NHC) induced α-H abstraction in tungsten(VI) imido-dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH2 Ph)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)(CH2 Ph)(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 1 H 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.

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, Δ G‡303, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.

Convenient preparation of high molecular weight poly(dimethylsiloxane) using thermally latent NHC-catalysis: a structure-activity correlation.

The polymerization of octamethylcyclotetrasiloxane (D4) is investigated using several five-, six- and seven-membered N-heterocyclic carbenes (NHCs). The catalysts are delivered in situ from thermally susceptible CO2 adducts. It is demonstrated that the polymerization can be triggered from a latent state by mild heating, using the highly nucleophilic 1,3,4,5-tetramethylimidazol-2-ylidene as organocatalyst. This way, high molecular weight PDMS is prepared (up to >400 000 g/mol, 1.6 < Ð M < 2.5) in yields >95%, using low catalyst loadings (0.2-0.1 mol %). Furthermore, the results suggest that a nucleophilic, zwitterionic mechanism is in operation, in preference to purely anionic polymerization.

First acyclic diene metathesis polymerization under biphasic conditions using a dicationic ruthenium alkylidene: access to high-molecular-weight polymers with very low ruthenium contamination.

The acyclic diene metathesis (ADMET) polymerization of 6-hydroxy-1,10-undecadiene (M1) and 6-acetoxy-1,10-undecadiene (M2) by the action of two different catalysts, i.e., the second-generation Grubbs-Hoveyda system ([RuCl2(IMesH2)(CH-2-(2-PrO-C6H4)]) (1) and the dicationic ruthenium alkylidene [Ru(DMF)3(IMesH2)(CH-2-(2-PrO-C6H4)] (2, IMesH2 = 1,3-dimesitylimidazolin-2-ylidene) is reported. Biphasic conditions using 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([BDMIM(+)BF4(-)]) and 1,2,4-trichlorobenzene (TCB) are applied. Under the chosen conditions (T = 75 °C, 20 mbar), the use of catalyst 1 results only in the formation of low-molecular-weight polymers (Mn ≤ 10,000 g mol(-1)), while catalyst 2 allows for the high yield synthesis of high-molecular-weight polymers (Mn ≤ 40,000 g mol(-1), yields ≤ 99%). Irrespective of the catalyst used, all polymers display a high trans-content (>95%). Notably, Ru-contamination of the target polymers without any additional purification is as low as 1.2 ppm with catalyst 2. Together with the high yields and high molecular weights, the low Ru-contaminations clearly illustrate the advantages of the biphasic setup.

[µ-3,3'-Diisopropyl-1,1'-(propane-1,3-di-yl)bis-(1,3-diazinan-2-yl-idene)]bis-[bromido-(η(4)-cyclo-octa-1,5-diene)rhodium(I)].

The title compound, [Rh2Br2(C8H12)2(C17H32N4)], was obtained by the reaction of 3,3'-(propane-1,3-di-yl)bis-(1-isopropyl-3,4,5,6-tetra-hydro-pyrimidin-1-ium) bromide and [{Rh(cod)Cl}2] (cod is cyclo-octa-1,5-diene) in tetra-hydro-furan. The two Rh(I) atoms each have a distorted square-planar coordination environment, defined by a bidentate cod ligand, a bromide anion and one C atom of the bridging bidentate bis-N-heterocyclcic carbene (NHC) ligand. The average Rh-CNHC distance is 2.038 (7) Å, suggesting that the bond has a major σ contribution with very little back donation. The distances between the cod ligands and the Rh(I) atoms vary between 2.104 (4) and 2.210 (4) Å.

Access to Ultra-High-Molecular Weight Poly(ethylene) and Activity Boost in the Presence of Cyclopentene With Group†4 Bis-Amido Complexes.

ZrIV complexes of the type [Me2 Si{(NR)(6-{2-(diethylboryl)phenyl}pyridyl-2-yl-N)}ZrCl2 ⋅thf] (R=tBu (4), adamantyl (7 a); thf=tetrahydrofuran), [Me2 Si{(NAd)(6-{2-(diphenylboryl)phenyl}pyridyl-2-yl-N)}ZrCl2 ] (Ad=adamantyl (7 b)), the nonbridged half-titanocene complexes of the type [(N-{6-(2-diethylborylphenyl)pyrid-2-yl}-NR)Cp'TiCl2 ] (R=Me, Cp'=C5 H5 (12), Cp'=C5 Me5 (13)), and the titanium(IV)-based metallocene-type complex [bis{N-(6-{2-(diethylboryl)phenyl}pyrid-2-yl)NMe}TiCl2 ] (14) have been synthesized. The structures of complexes 7 b, 12, and 13 were determined by single-crystal X-ray diffraction analysis. In solution, complex 4 slowly rearranges to [Me2 Si{(N-tBu)(6-{2-(diethylboryl)phenyl}pyridyl-2-yl-N)}2 Zr] (4 a), the structure of which was unambiguously confirmed by single-crystal X-ray crystallography. Similarly, reaction of HfCl4 with Me2 Si({RNLi}{6-[2-(diethylboryl)phenyl]pyridyl-2-ylNLi}) yielded the corresponding HfIV complexes [Me2 Si{(NR)(6-{2-(diethylboryl)phenyl}pyridyl-2-ylN)}2 Hf] (R=tBu (8) and Ad (9)). Upon activation of these complexes with methylalumoxane (MAO), complexes 4, 7 a, 7 b, and 12-14 showed activities up to 750 kg of polyethylene (PE)/molcat. bar h in the homopolymerization of ethylene (E), producing mainly linear PE (high-density PE, HDPE) with molecular weights in the range of 1 800 000

(η(6)-p-Cymene)(1,3-dimesityl-2,3-dihydro-1H-imidazol-2-yl-idene)bis-(pentafluorobenzoato-κO)ruthenium(II) dichloro-methane disolvate.

The title compound, [Ru(C7F5O2)2(C10H14)(C21H24N2)]·2CH2Cl2, is formed as an orange crystalline powder by the reaction of RuCl2(p-cymene)(IMes) and AgOCOC6F5 in anhydrous tetra-hydro-furan (IMes = 1,3-dimesityl-2,3-di-hydro-1H-imidazol-2-yl-idene). The asymetric unit consists of two independent [Ru(C6F5COO)2(η(6)-p-cymene)(IMes)] com-plexes and four dichloro-methane solvent mol-ecules. In each complex molecule, the Ru atom presents a pseudo-octa-hedral environment with the p-cymene ligand occupying three facial coordination sites, while the remaining coordination positions are occupied by the O atoms of the penta-fluoro-benzoate ligands and by the imidazolyl-idene ligand.

Functional polyolefins: poly(ethylene)-graft-poly(tert-butyl acrylate) via atom transfer radical polymerization from a polybrominated alkane.

Poly(cis-cyclooctene) is synthesized via ring-opening metathesis polymerization in the presence of a chain-transfer agent and quantitatively hydrobrominated. Subsequent graft polymerization of tert-butyl acrylate (tBA) via Cu-catalyzed atom transfer radical polymerization (ATRP) from the non-activated secondary alkyl bromide moieties finally results in PE-g-PtBA copolymer brushes. By varying the reaction conditions, a series of well-defined graft copolymers with different graft densities and graft lengths are prepared. The maximum extent of grafting in terms of bromoalkyl groups involved is approximately 80 mol%. DSC measurements on the obtained graft copolymers reveal a decrease in T(m) with increasing grafting density.

Group†4 dimethylsilylenebisamido complexes bearing the 6-[2-(diethylboryl)phenyl]pyrid-2-yl motif: synthesis and use in tandem ring-opening metathesis/vinyl-insertion copolymerization of cyclic olefins with ethylene.

Two novel Zr(IV)- and Hf(IV)-based bisamido complexes bearing the 6-[2-(diethylboryl)phenyl]pyrid-2-yl motif, that is, [ZrCl(2){Me(2)Si(DbppN)(2)}(thf)] (9) and [HfCl(2){Me(2)Si(DbppN)(2)}(thf)(2)] (10) (DbppN=6-[2-(diethylboryl)phenyl]pyridine-2-amido) have been prepared. Their reactivities have been compared with that of a model precatalyst that does not bear the aminoborane motif. Upon activation with methylalumoxane, precatalysts 9 and 10 are active in the homopolymerization of ethylene (E) yielding high-density polyethylene (HDPE). In the copolymerization of E with cyclopentene (CPE), for example by the action of 9, the presence of CPE resulted in a dramatic increase in the polymerization activity of E, while CPE incorporation remained close to or at zero. In the vinyl-insertion copolymerization of norborn-2-ene (NBE) with E by the action of 9, statistical cyclic olefin copolymers of these two monomers were obtained. At higher NBE concentrations, however, 9 gave rise to reversible ring-opening metathesis (ROMP)/vinyl-insertion polymerization (VIP) of NBE with E, resulting in the formation of multi-block copolymers of the general formula poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E). This particular feature of precatalyst 9, that is, the ability to induce a reversible α-H elimination/α-H addition reaction, is attributed to the unique role of the 6-[2-(diethylboryl)phenyl]pyrid-2-yl ligand. Accordingly, a model precatalyst lacking this ligand does not have the ability to induce α-H elimination/α-H addition reactions. The different (11)B NMR shifts of various diethylborylphenylpyrid-2-ylamines and -amides permit a ranking of the strengths of the B-N bonds in these compounds. This strength of the B-N bond is correlated with the propensity of 9/MAO to produce poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E) at different temperatures.

Cationic versus neutral Ru(II)--N-heterocyclic carbene complexes as latent precatalysts for the UV-induced ring-opening metathesis polymerization.

A series of cationic and neutral Ru(II) complexes of the general formula [Ru(L)(X) (tBuCN)(4)](+)X(-) and [Ru(L)(X)(2)(tBuCN)(3))], that is, [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMesH(2))](+)[CF(3)SO(3)](-) (1), [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMes)](+)[CF(3)SO(3)](-) (2), [RuCl{NCC(CH(3))(3)}(4)(IMes)](+)Cl(-) (3), [RuCl{NCC(CH(3))(3)}(4)(IMesH(2))(+)Cl(-)]/[RuCl(2){NCC(CH(3))(3)}(3)(IMesH(2))] (4), and [Ru(NCO)(2){NCC(CH(3))(3)}(3)(IMesH(2))] (5) (IMes=1,3-dimesitylimidazol-2-ylidene, IMesH(2)=1,3-dimesityl-imidazolin-2-ylidene) have been synthesized and used as UV-triggered precatalysts for the ring-opening metathesis polymerization (ROMP) of different norborn-2-ene- and cis-cyclooctene-based monomers. The absorption maxima of complexes 1-5 were in the range of 245-255 nm and thus perfectly fit the emission band of the 254 nm UV source that was used for activation. Only the cationic Ru(II)-complexes based on ligands capable of forming µ(2)-complexes such as 1 and 2 were found to be truly photolatent in ROMP. In contrast, complexes 3-5 could be activated by UV light; however, they also showed a low but significant ROMP activity in the absence of UV light. As evidenced by (1)H and (13)C NMR spectroscopy, the structure of the polymers obtained with either 1 or 2 are similar to those found in the corresponding polymers prepared by the action of [Ru(CF(3)SO(3))(2)(IMesH(2))(CH-2-(2-PrO)-C(6)H(4))], which strongly suggest the formation of Ru-based Grubbs-type initiators in the course of the UV-based activation process. Precatalysts that have the IMesH(2) ligand showed significantly enhanced reactivity as compared with those based on the IMes ligand, which is in accordance with reports on the superior reactivity of IMesH(2)-based Grubbs-type catalysts compared with IMes-based systems.

Alternating ring-opening metathesis copolymerization by Grubbs-type initiators with unsymmetrical N-heterocyclic carbenes.

A series of Ru(IV)-alkylidenes based on unsymmetrical imidazolin-2-ylidenes, that is, [RuCl(2){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHPh)(pyridin)] (R = CH(2)Ph (5), Ph (6), ethyl (7), methyl (8)), have been synthesized. These and the parent initiators [RuCl(2)(PCy(3)){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHC(6)H(5))] (R = CH(2)C(6)H(5) (1), C(6)H(5) (2), ethyl (3)) were used for the alternating copolymerization of norborn-2-ene (NBE) with cis-cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE-alt-COE)(n) and poly(NBE-alt-CPE)(n) containing up to 97 and 91% alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1-3 and 5-8 were determined by using both a zero- and first-order Markov model. Finally, kinetic investigations using initiators 1-3, 6, and 7 were carried out. These revealed that in contrast to the 2nd-generation Grubbs-type initiators 1-3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE-alt-COE)(n) yielded a fully saturated, hydrocarbon-based polymer. Its backbone can formally be derived by 1-olefin polymerization of CPE (1,3-insertion) followed by five ethylene units and thus serves as an excellent model compound for 1-olefin polymerization-derived copolymers.

Formation, preservation, and cleavage of the disulfide bond by vanadium.

Reaction of the disulfide [HpicanS](2) (HpicanS is the carboxamide based on picolinate (pic) and o-mercaptoaniline (anS); the [] brackets are used to denote disulfides) with [VOCl(2)(thf)(2)] leads to reductive scission of the disulfide bond and formation of the mixed-valence (V(IV)/V(V)) complex anion [(OVpicanS)(2)mu-O](-) (1), with the dianionic ligand coordinating through the pyridine-N atom, the deprotonated amide-N atom, and thiophenolate-S atom. Reductive cleavage of the SbondS bond is also observed as [VCl(2)(tmeda)(2)] (tmeda=tetramethylethylenediamine) is treated with the disulfides [HsalanS](2) or [HvananS](2) (HsalanS and HvananS are the Schiff bases formed between o-mercaptoaniline and salicylaldehyde (Hsal) or vanillin (Hvan), respectively), yielding the V(III) complexes [VCl(tmeda)(salanS)] (2 a), or [VCl(tmeda)(vananS)] (2 b). The disulfide bond remains intact in the aerial reaction between [HsalanS](2) and [VCl(3)(thf)(3)] to yield the V(V) complex [VOCl[salanS](2)] (3), where (salanS)(2-) coordinates through the two phenolate and one of the imine functions. The S-S bond is also preserved as [VO(van)(2)] or [VO(nap)(2)] (Hnap=2-hydroxynaphthalene-1-carbaldehyde) is treated with bis(2-aminophenyl)disulfide, [anS](2), a reaction which is accompanied by condensation of the aldehyde and the diamine, and complexation of the resulting bis(Schiff bases) [HvananS](2) or [HnapanS](2) to form the complexes [VO[vananS](2)] (4 a) or [VO[napanS](2)] (4 b). In 4 a and 4 b, the phenolate and imine functions, and presumably also one of the disulfide-S atoms, coordinate to V(IV). 2-Mercaptophenyl-2'-pyridinecarboxamide (H(2)picanS) retains its identity in the presence of V(III); reaction between [VCl(3)(thf)(3)] and H(2)picanS yields [V[picanS](2)](-) (5). The dithiophenolate 2,6-bis(mercaptophenylthio)dimethylpyridine (6 a) is oxidized, mediated by VO(2+), to the bis(disulfide) octathiadiaza-cyclo-hexaeicosane 6 b. The relevance of these reactions for the speciation of vanadium under physiological conditions is addressed. [HNEt(3)]-1.0.5 NEt(3,) 3.3 CH(2)Cl(2), [HsalanS](2), [HNEt(3)]-5, and 6 b.4 THF have been characterized by X-ray diffraction analysis.

In vitro study of the insulin-mimetic behaviour of vanadium(IV, V) coordination compounds.

A representative set of vanadium(IV and V) compounds in varying coordination environments has been tested in the concentration range 1 to 10(-6) mM, using transformed mice fibroblasts (cell line SV 3T3), with respect to their short-term cell toxicity (up to 36 hours) and their ability to stimulate glucose uptake by cells. These insulin-mimetic tests have also been carried out with non-transformed human fibroblasts (cell line F26). The compounds under investigation comprise established insulin-mimetic species such as vanadate ([H(2)VO(4)](-)), [VO(acetylacetonate)(2)], [VO(2)(dipicolinate)](-) and [VO(maltolate)(2)], and new systems and coordination compounds containing OO, ON, OS, NS and ONS donor atom sets. A vitality test assay, measuring the reduction equivalents released in the mitochondrial respiratory chain by intracellular glucose degradation, is introduced and the results are counter-checked with (3)H-labelled glucose. Most compounds are toxic at the 1 mM concentration level, and most compounds are essentially non-toxic and about as effective as or more potent than insulin at concentrations of 0.01 mM and below. V(V) compounds tend to be less toxic than V(IV)compounds, and complexes containing thio functional ligands are somewhat more toxic than others. Generally, ON ligation is superior in insulin-mimetic efficacy to OO or O/ NS coordination, irrespective of the vanadium oxidation state. There is, however, no striking correlation between the nature of the ligand systems and the insulin-mimetic potency in these cell culture tests, encompassing 41 vanadium compounds, the results on 22 of which are reported in detail here. The syntheses and characteristics of various new compounds are provided together with selected speciation results. The crystal and molecular structures of [[VO(naph-tris)](2)] [where naph-tris is the Schiff base formed between o-hydroxynaphthaldehyde and tris(hydroxymethyl)amine] are reported. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0311-5.