Random propene/4‐methyl‐1‐pentene copolymers synthesized with C2 symmetric highly isospecific metallocenes

Propene (P)/4‐methyl‐1‐pentene (Y) copolymers in a wide range of composition were prepared with isospecific single center catalysts, rac‐Et(IndH₄)₂ZrCl₂ ( EBTHI ), rac‐Me₂Si(2‐Me‐BenzInd)₂ZrCl₂ ( MBI ), and rac‐CH₂(3‐^tBuInd)₂ZrCl₂ ( TBI ). ¹³C NMR analysis of copolymers and statistical elaboration of microstructural data at triad level were performed. Unprecedented and surprising results are here reported. Random P/Y copolymers were prepared with the most isospecific catalyst, TBI , that is known to prepare ethene/propene and ethene/4‐methyl‐1‐pentene copolymers with long homosequences of both comonomers, whereas longer homosequences of both comonomers were observed in copolymers from the less enantioselective metallocenes EBTHI and MBI . These findings, which are against what is acknowledged in the field, can pave the way for the preparation on a large scale of random propene‐based copolymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2575–2585     

α‐keto‐β‐diimine nickel‐catalyzed olefin polymerization: Effect of ortho‐aryl substituents and preparation of stereoblock copolymers

A series of α‐keto‐β‐diimine nickel complexes (Ar‐N = C(CH₃)‐C(O)‐C(CH₃)=N‐Ar)NiBr₂; Ar = 2,6‐R‐C₆H₃‐, R = Me, Et, iPr, and Ar = 2,4,6‐Me₃‐C₆H₃‐) was prepared. All corresponding ligands are unstable even under an inert atmosphere and in a freezer. Stable copper complex intermediates of ligand synthesis and ethyl substituted nickel complex were isolated and characterized by X‐ray. All nickel complexes were used for the polymerization of ethene, propylene, and hex‐1‐ene to investigate their livingness and the extent of chain‐walking. Low‐temperature propene polymerization with less bulky ortho‐substituents was less isospecific than the one with isopropyl derivative. Propene stereoblock copolymers were prepared by iPr derivative combining the polymerization at low temperature to prepare isotactic polypropylene (PP) block and at a higher temperature, supporting chain‐walking, to obtain amorphous regioirregular PP block. Alternatively, a copolymerization of propene with ethene was used for the preparation of amorphous block. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2440–2449     

Co- and terpolymerizations of ethene and α-olefins with metallocenes

The suitability of the (n-butCp)₂ZrCl₂/methylaluminoxane (MAO) catalyst system for the copolymerization of ethene with propene, hexene, and hexadecene was studied and Ind₂ZrCl₂/MAO was tested as a catalyst for ethene/propene and ethene/hexene copolymerizations. The synergistic effect of longer α-olefin on propene incorporation in ethene/propene/hexene and ethene/propene/hexadecene terpolymerizations was investigated with Et(Ind)₂ZrCl₂MAO and (n-butCp)₂ZrCl₂/MAO catalyst systems. The molar masses, molar mass distributions, melting points, and densities of the products were measured. The incorporation of comonomer in the chain was further studied by segregation fractionation techniques (SFT), by differential scanning calorimetry (DSC), studying the β relaxations by dynamic mechanical analysis (DMA) and by studying the microstructure of some copolymers by ¹³C-NMR. In this study (n-butCp)₂ZrCl₂ and Ind₂ZrCl₂ exhibited equal response in copolymerization of ethene and propene and both catalysts were more active towards propene than longer α-olefins. A nearly identical incorporation of propene in the chain was found for the two catalysts when a higher propene feed was used. A lower hexene feed gave a more homogeneous comonomer distribution curve than a higher hexene feed and also showed the presence of branching. In terpolymerizations catalyzed with (n-butCp)₂ZrCl₂, the hexadecene concentrations of the ethene/propene/hexadecene terpolymers were always very low, and only traces of hexene were detected in ethene/propene/hexene terpolymers. With hexene no clear synergistic effect on the propene incorporation in the terpolymer was detected and with hexadecene the effect of the longer α-olefin was even slightly negative. With an Et(Ind)₂ZrCl₂/MAO catalyst system both hexene and hexadecene were incorporated in the chain in the terpolymerizations. © 1997 John Wiley & Sons, Inc.     

Electrophilic Alkylations of Vinylsilanes: A Comparison of α‐ and β‐Silyl Effects

Kinetics of the reactions of benzhydrylium ions (Aryl₂CH⁺) with the vinylsilanes H₂C?C(CH₃)(SiR₃), H₂C?C(Ph)(SiR₃), and (E)‐PhCH?CHSiMe₃ have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second‐order kinetics, and the second‐order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium‐based nucleophilicity scale. The vinylsilane H₂C?C(CH₃)(SiMe₃), which is attacked by electrophiles at the CH₂ group, reacts one order of magnitude faster than propene, indicating that α‐silyl‐stabilization of the intermediate carbenium ion is significantly weaker than α‐methyl stabilization because H₂C?C(CH₃)₂ is 10³ times more reactive than propene. trans‐β‐(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β‐silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe₃ groups affect the nucleophilic reactivities of the corresponding C?C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes.     

New tethered ansa‐bridged zirconium catalysts: Insights into the “self‐immobilization” mechanism

New ω‐alkenyl‐substituted ansa‐bridged bisindenyl zirconium complexes are prepared and tested as self‐immobilized catalysts for ethene polymerization. But, even at very high concentration of the tethered complexes and low pressure of ethene, there is no evidence of their insertion into the polyethene chain. A “cross polymerization” test, performed by copolymerizing the tethered complexes with ethene using rac‐Me₂Si(2‐MeBenzInd)₂ZrCl₂ ( MBI ), does not lead to their incorporation into the polyethene chain. However, the corresponding ligand proves to be a suitable comonomer for ethene, and, through copolymerization promoted by MBI, innovative poly(ethene‐co‐2,2′‐bis[(1H‐inden‐3′‐yl)‐hex‐5‐ene) copolymers are prepared and characterized by ¹³C NMR. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Random ethene/propene copolymerization from a catalyst system based on a “constrained geometry” half‐sandwich complex

Ethene/propene copolymerizations were performed in solution with a single centre catalyst system composed of a “constrained geometry” half‐sandwich organometallic complex {η¹: η⁵‐[(tert‐butylamido)dimethylsilyl](2,3,4,5‐tetramethyl‐1‐cyclopentadienyl)}titanium dichloride, and methylaluminoxane. The statistical treatment of polymerization data allowed to determine the reactivity ratios for ethene and propene: r_E = 1.35 ± 0.09, r_P = 0.82 ± 0.05, r_Er_P = 1.10 ± 0.14. This catalyst system promotes an almost random distribution of ethene and propene and gives rise to values of r_P and r_E very similar to each other.     

Kinetics of Carbon Dioxide Reaction with Aqueous Mixture of Piperazine and 2‐Amino‐2‐ethyl‐1,3‐propanediol

Current researchers from environmental and industrial fields are focusing on advanced means of carbon dioxide (CO₂) capture to limit its consequences in process industries. They also intend to enhance the mitigation of environmental impart by CO₂ especially its greenhouse effect. In this study, the kinetics of CO₂ reaction with an aqueous blend of piperazine (PZ) and 2‐amino‐2‐ethyl‐1,3‐propanediol (AEPD) were investigated. It was found that blending of AEPD with a little percentage of PZ generated the observed rate constant, k_o, values that were more than twice the direct summation of the k_o values of the aqueous pure amines at the corresponding concentration and temperature. The kinetic study of the system was modeled using a termolecular mechanism. Blending 0.05 kmol/m³ of PZ with 0.5 kmol/m³ of AEPD gives an observed rate constant k_o value of 2397.9 s^?1 at 298 K. This result is comparable to rate constants of other amine mixtures. Thus, the aqueous blend of AEPD with PZ is an attractive solvent for CO₂ capture that has good advantages. The PZ that serves as the promoter in the reaction is needed in small fraction, whereas AEPD, which is a sterically hindered amine, increases CO₂ absorption capacity of the system. AEPD can be produced from renewable materials. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 161–167, 2013     

Zirconium‐chelate and mono‐η‐ cyclopentadienyl zirconium‐chelate/ methylalumoxane systems as soluble Ziegler–Natta olefin polymerization catalysts

Zirconium‐chelate and mono‐η‐cyclopentadienyl zirconium‐chelate complexes were tested as ethene and propene polymerization catalysts in combination with methylalumoxane (MAO) as a co‐catalyst: in particular (acac) _nZrCl_4−n (1a–c) (acac = acetylacetonato), (dbm) _nZrCl_4−n (2a–2c) (dbm = dibenzoylmethanato = 1,3‐diphenylpropanedionato) (n = 2–4) and (dbm)₂ZrCl₂(thf) (3) (thf = tetrahydrofuran), (η‐C₅H₅)[H₂B (C₃H₃N₂)₂]ZrCl₂ (4), (η‐C₅H₅)[HB (C₃H₃N₂)₃] ZrCl₂ (5) and (η‐C₅H₅)[(Me₃SiN)₂ CPh]ZrCl₂ (6). Polymerization productivities comparable with the (η‐C₅H₅)₂ZrCl₂ reference system were observed towards ethene for all of the above complexes. In addition, compound 6 showed some minor polymerization activity towards propene. Ethylalumoxane or isobutylalumoxane did not exhibit a co‐catalytic activity for these chelate complexes; in combination with MAO these higher alumoxanes were even found to be deactivating ⁹¹Zr NMR data are reported for 1b, 1c, 4 and 5. Copyright © 2000 John Wiley & Sons, Ltd.     

Structure and Photochemical Properties of r‐1, c‐2, t‐3, t‐4‐l, 3‐Bis [2‐ (5‐R‐benzoxazolyl) ] −2,4‐di (4‐R' ‐phenyl) cyclobutane

r‐1, c‐2, t‐3, t‐4‐1,3‐Bis[2‐(5‐R‐benzoxazolyl)]‐2,4‐di(4‐R'‐phenyl)cyclobutane (IIa: R=R' = H; IIb: R=Me, R'= H; IIc: R = Me, R' = OMe) was synthesized with high stereo‐selectivity by the photodimerization of trans‐l‐[2‐(5‐R‐benzoxazolyl)]‐2‐(4‐R'‐phenyl) ethene (Ia: R=R' = H; Ib: R = Me, R' = H; Ic: R = Me, R' = OMe) in sulfuric acid. The structures of IIa–IIc were identified by elemental analysis, IR, UV, ¹H NMR, ¹³C NMR and MS. The molecular and crystal structure of IIc has been determined by X‐ray diffraction method. The crystal of IIc (C₃₄H₃₀N₂O₄. 0.5C₂OH) is monoclinic, space group P2₁/n with cell dimensions of a = 1.5416(3), b =0.5625(1), c = 1.7875(4) nm, β = 91.56 (3)°, V= 1.550(1) nm³, Z = 2. The structure shows that the molecule of IIc is centrosymmetric, which indicates that the dimerization process is a head‐to‐tail fashion. The selectivity of the photodimerization of Ia–Ic has been enhanced by using acidic solvent and the reaction speed would be decreased when electron donating group was introduced in the 4‐position of the phenyl group. That the photodimerization is not affected by the presence of oxygen as well as its high stereo‐selectivity demonstrated that the reaction proceeded through an excited singlet state. It was also found that under irradiation of short wavelength UV, these dimers underwent photolysis completely to reproduce its trans‐monomers, and then the new formed species changed into their cis‐isomers through trans‐cis isomerization.     

High Ethene/Ethane Selectivity in 2,2′‐Bipyridine‐Based Silver(I) Complexes by Removal of Coordinated Solvent

Following removal of coordinated CH₃CN, the resulting complexes [Ag^I(2,2′‐bipyridine)][BF₄] ( 1 ) and [Ag^I(6,6′‐dimethyl‐2,2′‐bipyridine)][OTf] ( 2 ) show ethene/ethane sorption selectivities of 390 and 340, respectively, and corresponding ethene sorption capacities of 2.38 and 2.18 mmol g^?1 when tested at an applied gas pressure of 90 kPa and a temperature of (20±1) °C. These ethene/ethane selectivities are 13 times higher than those reported for known solid sorbents for ethene/ethane separation. For 2 , ethene sorption reached 90 % of equilibrium capacity within 15 minutes, and this equilibrium capacity was maintained over the three sorption/desorption cycles tested. The rates of ethene sorption were also measured. To our knowledge, these are the first complexes, designed for olefin/paraffin separations, which have open silver(I) sites. The high selectivities arise from these open silver(I) sites and the relatively low molecular surface areas of the complexes.     

Quantification of the β‐Stabilizing Effect of the Dicarbonyl(η5‐cyclopentadienyl)iron Group

Kinetic investigations of the reactions of (prop‐2‐enyl)dicarbonyl(cyclopentadienyl)iron complexes 1 with benzhydrylium ions 3 , and of dicarbonyl(cyclopentadienyl)[(1,2‐η)propene]iron(II) tetrafluoroborate ( 9 ⋅BF₄) with π‐nucleophiles have been performed to elucidate the magnitude of the β‐effect of the [(CO)₂FeCp] group (Fp group). Introduction of the Fp group into the allylic position of propene and 2‐methylpropene increases the nucleophilicity of the π‐bonds by nine and six orders of magnitude, respectively, with the result that the allyl‐Fp complexes 1a (N=6.78) and 1b (N=8.45) are among the strongest neutral π‐nucleophiles. Replacement of one β‐H‐atom in the isopropyl cation by the Fp group reduces the electrophilicity by more than 20 orders of magnitude, so that 9 ⁺ ranks among the weakest cationic C‐electrophiles (E=−11.2).     

Toward block copolymers from nonliving isospecific single‐site catalytic systems

Ethene/1‐olefin blocky copolymers were obtained through nonliving insertion copolymerizations promoted by an isospecific single site catalyst. Propene or 4‐methyl‐1‐pentene were copolymerized with ethene with metallocenes endowed with different stereospecificity in propene polymerization: (i) aspecific “constrained geometry” half‐sandwich complex, {η¹:η⁵‐([tert‐butyl‐amido)dimethylsilyl](2,3,4,5‐tetramethyl‐1‐cyclopentadienyl)}titanium dichloride [Me₂Si(Me₄Cp)(N‐^tBu)TiCl₂] ( CG ), (ii) moderately isospecific rac‐ethylenebis(indenyl)zirconium dichloride [rac‐(EBI)ZrCl₂] ( EBI ), (iii) slightly more isospecific hydrogenated homologue, rac‐ethylenebis(tetrahydroindenyl)zirconium dichloride [rac‐(EBTHI)ZrCl₂] ( EBTHI ), (iv) highly iso‐specific rac‐[methylenebis(3‐tert‐butyl‐1‐indenyl)]zirconium dichloride [rac‐H₂C‐(3‐^tBuInd)₂ZrCl₂] ( TBI ), (v) most isospecific rac‐[isopropylidene‐bis(3‐tert‐butyl‐cyclopentadienyl)]zirconium dichloride [rac‐Me₂C‐(3‐^tBuCp)₂ZrCl₂] ( TBC ). Copolymerizations were described by a 2^nd order Markovian copolymerization model and data are proposed to correlate the formation of 1‐olefin sequences with catalytic site isospecificity, made by the cooperation of organometallic complex and growing chain. Blocky copolymers were prepared over wide ranges of compositions: with any of the isospecific metallocenes when 4‐methyl‐1‐pentene was the 1‐olefin and only with the highly isospecific ones ( TBI , TBC ) when propene was the comonomer. A penultimate unit effect was observed with TBI as the metallocene, whereas a 1^st order Markov model described the ethene/propene copolymerization from TBC . A moderately isospecific metallocene, such as EBI , is shown to be able to prepare blocky ethene copolymers with 4‐methyl‐1‐pentene. These results pave the way for the synthesis of new ethene based materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2063–2075, 2010     

Three structures containing (E)‐1,2‐bis­(benzimidazol‐2‐yl)ethene groups

Two of the title compounds, namely (E)‐1,2‐bis­(1‐methyl­benzimidazol‐2‐yl)ethene, C₁₈H₁₆N₄, (Ib), and (E)‐1,2‐bis­(1‐ethyl­benzimidazol‐2‐yl)ethene, C₂₀H₂₀N₄, (Ic), consist of centrosymmetric trans‐bis­(1‐alkyl­benzimidazol‐2‐yl)ethene mol­ecules, while 3‐eth­yl‐2‐[(E)‐2‐(1‐ethyl­benzimidazol‐2‐yl)­ethen­yl]benzimidazol‐1‐ium perchlorate, C₂₀H₂₁N₄⁺·ClO₄⁻, (II), contains the monoprotonated analogue of compound (Ic). In the three structures, the benzimidazole and benzimidazolium moieties are essentially planar; the geometric parameters for the ethene linkages and their bonds to the aromatic groups are consistent with double and single bonds, respectively, implying little, if any, conjugation of the central C=C bonds with the nitro­gen‐containing rings. The C—N bond lengths in the N=C—N part of the benzimidazole groups differ and are consistent with localized imine C=N and amine C—N linkages in (Ib) and (Ic); in contrast, the corresponding distances in the benzimidazolium cation are equal in (II), consistent with electron delocalization resulting from protonation of the amine N atom. Crystals of (Ib) and (Ic) contain columns of parallel mol­ecules, which are linked by edge‐over‐edge C—H⋯π overlap. The columns are linked to one another by C—H⋯π inter­actions and, in the case of (Ib), C—H⋯N hydrogen bonds. Crystals of (II) contain layers of monocations linked by π–π inter­actions and separated by both perchlorate anions and the protruding eth­yl groups; the cations and anions are linked by N—H⋯O hydrogen bonds.     

Mechanism and Kinetics of Carbon Dioxide Capture Using Activated 2‐Amino‐2‐methyl‐1,3‐propanediol

The continued use of fossil fuels as primary sources of energy in industry and other applications stands the test of time, due to their availability and relatively lower cost than alternative sources of energy. In view of this perspective, obtaining an advanced bulk carbon dioxide (CO₂) capture medium becomes an urgent necessity so as to mitigate their effect, especially in global warming, as the use of fossil fuels produces a high rate of CO₂. In this work, the mechanism and kinetics of CO₂ capture using aqueous piperazine (PZ) as an activator to 2‐amino‐2‐methyl‐1,3‐propanediol (AMPD) were investigated. The termolecular mechanism was used to model the kinetics of the system. Reaction kinetics of the single pure amines was first obtained. The reaction rate constant, the k value of AMPD, was 77.2 m³/kmol·s, with a reaction order, n, of 1.25 at 298 K. while that of PZ was equal to 11,059 m³/kmol·s and n as 1.49 at 298 K. Blending of 0.05 kmol/m³ of PZ with 0.5 kmol/m³ of AMPD gave a rate constant, k, value of 23,319 m³/kmol·s and n equal to 1.23 at 298 K. The result obtained for the blended system is more than twice the value of the summation of the corresponding pure amines; in addition, it is comparably higher than the rate constant of monoethanolamine (MEA) in use as a commercial solvent for CO₂ capture. Therefore, an aqueous blend of PZ with AMPD deserves more comprehensive study as a solvent for commercial CO₂ capture. AMPD like other sterically hindered amines absorbs CO₂ in an equimolar ratio that is significantly higher than that of MEA. PZ serves as a promoter in the amine mixture and is required in a very small proportion.     

Construction and photocatalytic properties of two metal‐mediated coordination polymers based on benzene‐1,3,5‐tricarboxylic acid and trans‐1‐(pyridin‐3‐yl)‐2‐(pyridin‐4‐yl)ethene

With the rapid development of modern industry, water pollution has become an intractable environmental issue facing humans worldwide. In particular, the organic dyes discharged into natural water from dyestuffs, dyeing and the textile industry are the main sources of pollution in wastewater. To eliminate these types of pollutants, degradation of organic contaminants through a photocatalytic technique is an effective methodology. To exploit more crystalline photocatalysts for the degradation of organic dyes, two coordination polymers, namely catena‐poly[[(3,5‐dicarboxybenzene‐1‐carboxylato‐κO ¹)silver(I)]‐μ‐trans‐1‐(pyridin‐3‐yl)‐2‐(pyridin‐4‐yl)ethene‐κ²N :N ′], [Ag(C₉H₅O₆)(C₁₂H₁₀N₂)]_n or [Ag(H₂BTC)(3,4′‐bpe)]_n , (I), and poly[[(μ₃‐5‐carboxybenzene‐1,3‐dicarboxylato‐κ⁴O ¹,O ^1′:O ³:O ³)[μ‐trans‐1‐(pyridin‐3‐yl)‐2‐(pyridin‐4‐yl)ethene‐κ²N :N′ ]cadmium(II)] monohydrate], {[Cd(C₉H₄O₆)(C₁₂H₁₀N₂)]·H₂O}_n or {[Cd(HBTC)(3,4′‐bpe)]·H₂O}_n , (II), have been prepared by the hydrothermal reactions of benzene‐1,3,5‐tricarboxylic acid (H₃BTC) and trans‐1‐(pyridin‐3‐yl)‐2‐(pyridin‐4‐yl)ethene (3,4′‐bpe) in the presence of AgNO₃ or Cd(NO₃)₂·4H₂O, respectively. These two title compounds have been structurally characterized by IR spectroscopy, elemental analysis, single‐crystal X‐ray diffraction and powder X‐ray diffraction. In (I), the Ag^I ions and organic ligands form a one‐dimensional coordination chain, and adjacent coordination chains are connected by Ag…O interactions to give rise to a two‐dimensional supramolecular network. Each two‐dimensional network is entangled with other equivalent networks to generate an infrequent interlocked 2D→3D (2D and 3D are two‐ and three‐dimensional, respectively) supramolecular framework. In (II), the Cd^II ions are bridged by the HBTC²⁻ and 3,4′‐bpe ligands, which lie across centres of inversion, to give a two‐dimensional coordination network. The thermal stabilities and photocatalytic properties of the title compounds have also been studied.     

[Hg2(μ‐bpe)(μ‐OAc)2(μ‐SCN)2]n,a New Two‐Dimensional Coordination Polymer Involving Three Simultaneously Bridging Ligands: 1,2‐Bis(4‐pyridyle)ethene,Acetate, and Thiocyanate

A new mercury(II) complex of 1,2‐bis(4‐pyridyle)ethene (bpe) with anionic acetate and thiocyanate ligands has been synthesized and characterized by elemental analysis, IR, ¹H NMR and ¹³C NMR spectroscopy. The single crystal X‐ray analysis shows that the complex is a two‐dimensional polymer with simultaneously bridging 1,2‐bis(4‐pyridyle)ethane, acetate and thiocyanate ligands and basic repeating dimeric [Hg₂(μ‐bpe)(μ‐OAc)₂(μ‐SCN)₂] units. The two‐dimensional system forms a three‐dimensional network by packing via π‐π stacking interactions.     

Copolymerization of Propene and Buta‐1,3‐diene in the Presence of Highly Hindered C2‐Symmetric Zirconocene‐Based Catalyst

Summary: Copolymerizations of propene and buta‐1,3‐diene performed in the presence of rac‐[CH₂(3‐tert‐butyl‐1‐indenyl)₂]ZrCl₂ and methylaluminoxane (MAO) have been investigated. Buta‐1,3‐diene gives prevailingly primary coordination to the metal, producing overall 1,2 units. Cyclopropane and cyclopentane rings, although in low amounts, are also obtained. The presence of butadiene would be responsible for some regioirregular 2,1‐inserted propene units, which at high temperatures give rearrangement to 3,1 units.

     

Investigation of the Catalytic Activity of a 2‐Phenylidenepyridine Palladium(II) Complex Bearing 4,5‐Dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene in the Mizoroki‐Heck Reaction

The phenylidenepyridine (ppy) palladacycles [PdCl(ppy)(IMes)] ( 4 ) [IMes = 1,3‐bis(mesityl)imidazol‐2‐ylidene] and [PdCl(ppy){(CN)₂IMes}] ( 6 ) [(CN)₂IMes = 4,5‐dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene] were prepared by facile two step syntheses, starting with the reaction of palladium(II) chloride with 2‐phenylpyridine followed by subsequent addition of the NHC ligand to the precatalyst precursor [PdCl(ppy)]₂. Suitable crystals for the X‐ray analysis of the complexes 4 and 6 were obtained. It was shown that 6 has a shorter NHC‐palladium bond than the IMes complex 4 . The difference of the palladium carbene bond lengths based on the higher π‐acceptor strength of (CN)₂IMes in comparison to IMes. Thus, (CN)₂IMes should stabilize the catalytically active central palladium atom better than IMes. As a measure for the π‐acceptor strength of (CN)₂IMes compared to IMes, the selone (CN)₂IMes · Se ( 7 ) was prepared and characterized by ⁷⁷Se‐NMR spectroscopy. The π‐acceptor strength of 7 was illuminated by the shift of its ⁷⁷Se‐NMR signal. The ⁷⁷Se‐NMR signal of 7 was shifted to much higher frequencies than the ⁷⁷Se‐NMR signal of IMes · Se. Catalytic experiments using the Mizoroki‐Heck reaction of aryl chlorides with n‐butyl acrylate showed that 6 is the superior performer in comparison to 4 . Using complex 6 , an extensive substrate screening of 26 different aryl bromides with n‐butyl acrylate was performed. Complex 6 is a suitable precatalyst for para‐substituted aryl bromides. The catalytically active species was identified by mercury poisoning experiments to be palladium nanoparticles.     

Investigation of the C−H Activation Potential of [Hydrotris(1H‐pyrazolato‐κN1)borato(1−)]iridium (IrTpx) Fragments Featuring Aromatic Substituents x at the 3‐Position of the Pyrazole Rings,The Choice of the Precursor

A series of pyrazole‐substituted [hydrotris(1H‐pyrazolato‐κN¹)borato(1−)]iridium complexes of the general composition [Ir(Tp^x)(olefin)₂] (Tp^x=Tp^Ph and Tp^Th) and their capability to activate C−H bonds is presented. As a test reaction, the double C−H activation of cyclic‐ether substrates leading to the corresponding Fischer carbene complexes was chosen. Under the reaction conditions employed, the parent compound [Ir(Tp^Ph)(ethene)₂] was not isolable; instead, (OC‐6‐25)‐[Ir(Tp^PhκC^Ph,κ³N,N′,N″)(ethyl)(η²‐ethene)] ( 1 ) was formed diastereoselectively. Upon further heating, 1 could be converted exclusively to (OC‐6‐24)‐[Ir(Tp^Phκ²C^Ph,C^Ph,κ³N,N′,N″)(η²‐ethene)] ( 2 ). Complex 1 , but not 2 , reacted with THF to give (OC‐6‐35)‐[Ir(Tp^Phκ³N,N′,N″)H(dihydrofuran‐2(3H)‐ylidene)] ( 3 ), a cyclic Fischer carbene formed by double C−H activation of THF. Accordingly, complexes of the general formula [Ir(Tp^x)(butadiene)] (see 4 – 6 ; butadiene=buta‐1,3‐diene, 2‐methylbuta‐1,3‐diene (isoprene), 2,3‐dimethylbuta‐1,3‐diene) reacted with THF to yield 3 or the related derivative 9 . The reaction rate was strongly dependent on the steric demand of the butadiene ligand and the nature of the substituent at the 3‐position of the pyrazole rings.     

Role of Nitrite on Nitration of 2′‐Deoxyguanosine by Nitryl Chloride

Nitryl chloride and peroxynitrite are reactive nitrogen species generated by activated phagocytes against invading pathogens during infections and inflammation. In our previous report, formation of 8‐nitroxanthine and 8‐nitroguanine was observed in reaction of 2′‐deoxyguanosine or calf thymus DNA with nitryl chloride generated by mixing hypochlorous acid (HOCl) with nitrite (NC₂^?). The present study investigates factors control ling the yields of 8‐nitroxanthine and 8‐nitroguanine formation in nitration of 2′‐deoxyguanosine by nitryl chloride. We found that the yields of 8‐nitroxanthine and 8‐nitroguanine in reaction of 2′‐deoxyguanosine with nitryl chloride were highly dependent on the ratio of NO₂^? versus HOCl concentration. The yields of 8‐nitroxanthine and 8‐nitroguanine reached a plateau when the ratio of NC₂^? versus HOCl concentration was higher than 2. A possible mechanism was postulated to explain this observation. While 8‐nitroguanine is not stable in the presence of peroxynitrite, 8‐nitroxanthine is sensitive to HOCl. The stability of these two nitrated ad ducts might be a factor on their final yields in this reaction. Since HOCl is produced by neutrophils at sites of inflammation where the level of NC₂^? is elevated, it is conceivable that nitryl chloride contributes to DNA base nitration in vivo, forming 8‐nitroxanthine and 8‐nitroguanine.