UV–visible spectroscopy for zirconocene activation by MAO in olefin polymerization: activity versus wavenumber

Activation of ansa‐zirconocenes of the type Rac [Zr{1‐Me₂Si(3‐R‐(η⁵‐C₉H₅))(3‐R′‐(η⁵‐C₉H₅))}Cl₂] [R = Et, R′ = H ( 1 ); R = Pr, R′ = H ( 2 ); and R = Et, R′ = Pr ( 3 ), R, R′ = Me ( 4 ) and R, R′ = Bu ( 5 )] by MAO has been studied by UV–visible spectroscopy. Compounds 1–3 have been tested in the polymerization of ethylene at different Al:Zr ratios. UV–vis spectroscopy was used to determine a correlation between the electronic structures of ( 1–5 ) and their polymerization activity. Copyright © 2009 John Wiley & Sons, Ltd.     

Reaktionen von Lithiumhydridosilylamiden RR′(H)Si–N(Li)R″ mit Chlortrimethylsilan in Tetrahydrofuran und unpolaren Lösungsmitteln: N‐Silylierung und/oder Cyclodisilazanbildung

Reactions of Lithium Hydridosilylamides RR′(H)Si–N(Li)R″ with Chlorotrimethylsilane in Tetrahydrofuran and Nonpolar Solvents: N‐Silylation and/or Formation of Cyclodisilazanes The lithiumhydridosilylamides RR′(H)Si–N(Li)R″ ( 2 a : R = R′ = CHMe₂, R″ = SiMe₃; 2 b : R = R′ = Ph, R″ = SiMe₃; 2 c : R = R′ = CMe₃, R″ = SiMe₃; 2 d : R = R′ = R″ = CMe₃; 2 e : R = Me, R′ = Si(SiMe₃)₃, R″ = CMe₃; 2 f – 2 h : R = R′ = Me, f : R″ = 2,4,6‐Me₃C₆H₂, g : R″ = SiH(CHMe₂)₂, h : R″ = SiH(CMe₃)₂; 2 i : R = R′ = CMe₃, R″ = SiH(CMe₃)₂) were prepared by reaction of the corresponding hydridosilylamines RR′(H)Si–NHR″ 2 a – 2 i with n‐butyllithium in equimolar ratio in n‐hexane. The unknown amines 1 e – 1 i and amides 2 f – 2 i have been characterized spectroscopically. The wave numbers of the Si–H stretching vibrations and ²⁹Si–¹H coupling constants of the amides are less than of the analogous amines. This indicates a higher hydride character for the hydrogen atom of the Si–H group in the amide in comparison to the amines. The ²⁹Si‐NMR chemical shifts lie in the amides at higher field than in the amines. The amides 2 a – 2 c and 2 e – 2 g react with chlorotrimethylsilane in THF to give the corresponding N‐silylation products RR′(H)Si–N(SiMe₃)R″ ( 3 a – 3 c , 3 e – 3 g ) in good yields. In the reaction of 2 i with chlorotrimethylsilane in molar ratio 1 : 2,33 in THF hydrogen‐chlorine exchange takes place and after hydrolytic work up of the reaction mixture [(Me₃C)₂(Cl)Si]₂NH ( 5 a ) is obtained. The reaction of the amides 2 a – 2 c , 2 f and 2 g with chlorotrimethylsilane in m(p)‐xylene and/or n‐hexane affords mixtures of N‐substitution products RR′(H)Si–N(SiMe₃)R″ ( 3 a – 3 c , 3 f , 3 g ) and cyclodisilazanes [RR′Si–NR″]₂ ( 6 a – 6 c , 6 f , 6 g ) as the main products. In case of the reaction of 2 h the cyclodisilazane 6 h was obtained only. 2 c – 2 e show a very low reactivity toward chlorotrimetyhlsilane in m‐xylene and toluene resp.. In contrast to Me₃SiCl the reactivity of 2 d toward Me₃SiOSO₂CF₃ and Me₂(H)SiCl is significant higher. 2 d react with Me₃SiOSO₂CF₃ and Me₂(H)SiCl in n‐hexane under N‐silylation to give RR′(H)Si–N(SiMe₃)R″ ( 3 d ) and RR′(H)Si–N(SiHMe₂)R″ ( 3 d ′) resp. The crystal structures of [Me₂Si–NSiMe₃]₂ ( I ) ( 6 f , 6 g and 6 h ) have been determined.     

Diorganotin(IV) compounds derived from n‐methyl‐2,2′‐diphenolamine and 2,2′‐diphenolamine

The reaction of N‐methyl‐2,2′‐diphenolamine 1 and 2,2′‐diphenolamine 2 with some diorganotin(IV) oxides [R1/2SnO: R¹ = Me, n‐Bu, t‐Bu and Ph] led to the syntheses of diorgano[N‐methyl‐2,2′‐diphenolato‐O,O′,N]tin (IV) 3–6 and diorgano[2,2′‐diphenolato‐O,O′,N]tin (IV) 7–9 . All compounds (except 7 ) studied in this work were characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, infrared, and mass spectroscopy. Their ¹¹⁹Sn NMR data show that the tin atom is tetracoordinated in CDCl₃ but penta and hexacoordinated in DMSO‐d₆. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 133–139, 1999     

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.     

Substituted 1,3,2,4‐benzodithiadiazines and related compounds

Despite some limitations, the 1:1 condensation of n‐RC₆H₄‐N=S=N‐SiMe₃ (n = 2, 3, 4; R = CH₃, OCH₃, F, Cl, CF₃) with SCl₂, followed by intramolecular electrophilic ortho‐cyclization, was found to be a general synthetic approach to the corresponding 5‐R, 6‐R, and 7‐R–substituted 1,3,2,4‐benzodithiadiazines, formally antiaromatic 12π‐electron compounds. For precursors with n = 3, the high regioselectivity of the cyclization resulted in exclusive (R = OCH₃, F) or predominant (R = CH₃, Cl) formation of 6‐R isomers; the ratio of the major 6‐R isomer to the minor 8‐R one was found to be 72:28 (R = CH₃) or 78:22 (R = Cl). The preferred direction of cyclization is consistent with thermodynamics of the corresponding intermediate σ‐complexes as well as factors of kinetic control for an orbital‐controlled El‐Nu reaction. According to the X‐ray diffraction data, the molecules of 5‐CF₃ (15) and 6‐F (12) derivatives are nearly planar, while the molecules of 5‐OCH₃ (7) and 6‐CH₃ (4) derivatives are bent along the S¹ … N⁴ line by ∼11° (7) or 7° (4). An attempt to adopt CsF‐induced intramolecular nucleophilic ortho‐cyclization of Ar_F‐S‐N=S=N‐SiMe₃ into polyfluorinated 1,3,2,4‐benzodithiadiazines for polyfluoropyridine derivatives resulted in formation of polyfluorinated aminopyridines. Data obtained are consistent with a previously suggested scheme of sulfur–nitrogen chain shortening during cyclization. Mild acid hydrolysis of the title compounds was shown to be a convenient synthetic route to substituted 2,2′‐diaminodiphenyl disulfides (including polyfluorinated ones) via the corresponding 2‐aminobenzenethiols. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 113–124, 1999     

Use of Ph3P=N‐Li: Synthesis of α,β‐unsaturated nitriles from α,β‐unsaturated esters via the formation of N‐(α,β‐unsaturated acyl) phosphinimines

The reaction of Ph₃P=NLi with various α,β‐unsaturated esters gives access to new N‐(α,β‐unsaturated acyl) phosphinimines, which can undergo intramolecular aza‐Wittig reactions (at 65–110°C) to afford the corresponding nitriles. The structures of all new compounds were established by elementary analyses, IR, ¹H‐, ¹³C‐, and ³¹P‐NMR spectroscopy. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 49–54, 1999     

Kinetic study of the reaction of OH with a series of acetals at 298 ± 4 K

Rate coefficients have been measured at 298 ± 4 K and 1000 mbar total pressure for the reactions of OH with a series of symmetrical acetals (R O CH₂ O R, R = C₁ to C₄) using a relative kinetic technique. The investigations have been performed in a laboratory photoreactor and also in the large outdoor EUPHORE simulation chamber facility in Valencia, Spain. The following rate coefficients (in units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹) have been obtained: dimethoxy methane (R = CH₃), 0.49 ± 0.02; diethoxy methane (R = CH₃CH₂), 1.84 ± 0.18; di‐n‐propoxy methane (R = CH₃CH₂CH₂), 2.63 ± 0.49; di‐iso‐propoxy methane (R = (CH₃)₂CH), 3.93 ± 0.48; di‐n‐butoxy methane (R = CH₃CH₂CH₂CH₂), 3.47 ± 0.42; di‐iso‐butoxy methane (R = (CH₃)₂CHCH₂), 3.68 ± 0.57; di‐sec‐butoxy methane (R = CH₃CH₂C(CH₃)H), 4.68 ± 0.05. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 797–803, 1999     

Synthesis and characterization of functional polysilanes [RMe2Si(CH2)x(Me)Si]n (R = 2‐Fu, 5‐Me‐2‐Fu, 2‐Th, 4‐Me‐2‐Th; x = 2, 3) with appended furyl/thienyl groups on the carbosilyl side chains and their application in the generation and stabilization of palladium nanoparticles

The polysilanes [RMe₂Si(CH₂)_x(Me)Si]_n [x = 2, 3; R = 2‐Fu ( 1, 2 ), 5‐Me‐2‐Fu ( 3, 4 )] bearing furyl‐substituted carbosilyl side chains have been synthesized by dehalocondensation reaction (Wurtz coupling) of the corresponding carbosilanes using sodium dispersion in refluxing toluene. On the other hand, analogous polysilanes with appended thienyl groups [x = 2, 3; R = 2‐Th ( 5, 6 ), 4‐Me‐2‐Th ( 7, 8 )] are only accessible by the reaction of the corresponding carbosilane precursors under mild Wurtz coupling conditions (THF, RT). These polysilanes reveal monomodal molecular weight distribution with M_w/PDI = 3.3–5.4 × 10⁴/1.22–1.47 ( 1–4 ) and 9.1–14.4 × 10⁴/1.45–1.61 ( 5–8 ) and are characterized by FT‐IR, multinuclear (¹H, ¹³C{¹H}, ²⁹Si{¹H}) NMR, and UV/PL spectral studies as well as thermogravimetric analysis (TGA). Preliminary studies on the reactivity of polysilane 2 with palladium acetate (toluene, RT) reveal the formation of spherical palladium nanoparticles of size 8.2 ± 0.6 nm, which remain stable in solution for several weeks. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7816–7826, 2008     

Syntheses,characterizations and crystal structures of new triorganotin complexes with 2‐mercaptopyrimidine and 4‐amino‐2‐mercaptopyrimidine

Four triorganotin(IV) complexes with 2‐mercaptopyrimidine (HSpym) and 4‐amino‐2‐mercaptopyrimidine (HSapym) of the type, R₃SnL (L= Spym, R=Ph, 1; R=PhCH₂, 2; L=Sapym, R=Ph, 3; R=PhCH₂, 4), were synthesized. All the complexes 1–4 have been characterized by elemental, IR, ¹H NMR, and X‐ray crystallography diffraction analyses, which revealed that the structures of 1–4 are penta‐coordinated with R₃Sn‐coordinated to the thiol S and heterocyclic N atoms, and the structural distortion for each is a displacement from tetragonal toward trigonal bipyramidal geometry. The complex 1 is a one‐dimensional chain complex, while compounds 3 and 4 are dimers due to the existence of N···H hydrogen bonding. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:69–75, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20068     

Disupersilylsilane R*2, Disupersilyldisilane R*2XSi–SiX3 und Tetrasupersilyltetrasilane R*2XSi–SiX2–SiX2–SiXR*2

Compounds of Silicon. 140. Sterical Overloaded Compounds of Silicon. 24. Disupersilylsilanes R*₂SiX₂, Disupersilyldisilanes R*₂XSi–SiX₃, and Tetrasupersilyltetrasilanes R*₂XSi–SiX₂–SiX₂–SiXR*₂ Supersilylsilanes R*₂SiX₂, disupersilyldisilanes R*₂XSi–SiX₃ and tetrasupersilyltetrasilanes R*₂XSi–SiX₂–SiX₂–SiXR*₂ (R* = supersilyl = SitBu₃; X = H, Me, Ph, Hal, OH, OTf) are prepared in organic solvents (i) by reactions of supersilylhalosilanes R*X₂SiHal with supersilyl sodium NaR* (Hal/R* exchange), (ii) by reactions of halosilanes X₃SiHal with silanides NaSiXR*₂ (Hal/SiXR*₂ exchange), (iii) by dehalogenations of disupersilylhalodisilanes R*₂XSi–SiX₂Hal with Na, (iv) by insertions of supersilylsilylenes R*XSi into the NaSi‐bond of supersilylsodium NaR*, (v) by reactions of disupersilylated halosilanes and ‐disilanes R*₂XSiHal and R*₂XSi–SiX₂Hal with H^– (Hal/H exchange), (vi) by reactions of the title silanes (X = H) with halogens Hal₂ (H/Hal exchange), (vii) by reactions of the title silanes (X = Hal) first with Na (Hal/Na exchange), then with agents for protonation (Na/H exchange) or halogenation (Na/Hal exchange), (viii) by reactions of the title silanes (X = Hal) with nucleophiles like F^–, H₂O (Hal/F or Hal/OH exchange) or (ix) by reactions of the title silanes (X = H) with strong acids like HOTf (H/OTf exchange). The colorless compounds are characterized by IR, NMR and X‐ray structure analyses (structures of R*₂SiX₂ with X = H, F, Cl and R*₂HSi–SiHX–SiHX–SiHR*₂ with X = H, Br). They may thermolize under formation of silylenes (e. g. R*₂SiX₂ → R*X + R*SiX) and are normally stable for hydrolysis. For other reactions confer preparation of the title silanes (i–ix).     

Properties of the Magnesium(II) and Calcium(II) Complexes of 5‐ and 6‐Uracilmethylphosphonate (5 Umpa2– and 6 Umpa2–) in Aqueous Solution

The stability constants of the 1 : 1 complexes formed between Mg²⁺ or Ca²⁺ and 5 Umpa^2– or 6 Umpa^2– were determined by potentiometric pH titrations in aqueous solution (25 °C; I = 0.1 M, NaNO₃). Based on previously established log K^M_M(R‐PO3) versus pK^H_H(R‐PO3) straight‐line plots (M²⁺ = Mg²⁺ or Ca²⁺; R‐PO₃^2– = simple phosphate monoester or phosphonate ligands where R is a non‐interacting residue), it is shown that the Mg(5 Umpa), Ca(5 Umpa), Mg(6 Umpa) and Ca(6 Umpa) complexes have the stability expected on the basis of the basicity of the phosphonate group in 5 Umpa^2– and 6 Umpa^2–. This means, these ligands may be considered as simple analogues of nucleotides, e. g. of uridine 5′‐monophosphate. In the higher pH range deprotonation of the uracil residue in the M(5 Umpa) and M(6 Umpa) complexes occurs and this leads to the negatively charged M(5 Umpa–H)^– and M(6 Umpa–H)^– species. Based on the comparison of various acidity constants it is shown that the M(5 Umpa) complexes are especially acidic; or to say it differently, the M(5 Umpa–H)^– species are especially stable. This increased stability is attributed to the formation of a seven‐membered chelate involving next to the phosphonate group also the carbonyl oxygen atom at C4 (after deprotonation of the (N3)H site). The formation degree of this chelated isomer reaches about 45% for the Mg(5 Umpa–H)^– and Ca(5 Umpa–H)^– species. No indication for chelate formation was observed for the M(6 Umpa–H)^– complexes.     

Synthesis,Structure, and Reactivity of Hydridoiridium Complexes Bearing a Pincer‐Type PSiP Ligand

A series of iridium tetrahydride complexes [Ir(H)₄(PSiP‐R)] bearing a tridentate pincer‐type bis(phosphino)silyl ligand ([{2‐(R₂P)C₆H₄}₂MeSi]⁻, PSiP‐R, R=Cy, iPr, or tBu) were synthesized by the reduction of [IrCl(H)(PSiP‐R)] with Me₄N ⋅ BH₄ under argon. The same reaction under a nitrogen atmosphere afforded a rare example of thermally stable iridium(III)–dinitrogen complexes, [Ir(H)₂(N₂)(PSiP‐R)]. Two isomeric dinitrogen complexes were produced, in which the PSiP ligand coordinated to the iridium center in meridional and facial orientations, respectively. Attempted substitution of the dinitrogen ligand in [Ir(H)₂(N₂)(PSiP‐Cy)] with PMe₃ required heating at 150 °C to give the expected [Ir(H)₂(PMe₃)(PSiP‐Cy)] and a trigonal bipyramidal iridium(I)–dinitrogen complex, [Ir(N₂)(PMe₃)(PSiP‐Cy)]. The reaction of [Ir(H)₄(PSiP‐Cy)] with three equivalents of 2‐norbornene (nbe) in benzene afforded [Ir^I(nbe)(PSiP‐Cy)] in a high yield, while a similar reaction of [Ir(H)₄(PSiP‐R)] with an excess of 3,3‐dimethylbutene (tbe) in benzene gave the C H bond activation product, [Ir^III(H)(Ph)(PSiP‐R)], in high yield. The oxidative addition of benzene is reversible; heating [Ir^III(H)(Ph)(PSiP‐Cy)] in the presence of PPh₃ in benzene resulted in reductive elimination of benzene, coordination of PPh₃, and activation of the C H bond of one aromatic ring in PPh₃. [Ir^III(H)(Ph)(PSiP‐R)] catalyzed a direct borylation reaction of the benzene C H bond with bis(pinacolato)diboron. Molecular structures of most of the new complexes in this study were determined by a single‐crystal X‐ray analysis.     

Reduction of Disilane (tBu3Si)Br2Si‐SiBr2(SitBu3) in Presence of C2H4: A Mechanistic Approach

The reduction of R*–SiBr₂–SiBr₂–R* ( 2 ) with NaR* (R* = supersilyl = SitBu₃) in presence of C₂H₄ provides a white crystalline solid (η²‐C₂H₄)R*Si–SiR*(Br)(CH₂–CH₂–R*) ( 3 ) characterized by X‐ray diffraction analysis. Compound 3 is accompanied with an impurity of R*(Br)₂Si–Si(Br)(R*)(CH₂–CH₂–R*) ( 4 ). The formation of 3 and 4 runs complicated because of several reactive partners. However, reduction of 2 with sodium naphthalenide in presence of ethene runs straightforward with formation of a mixture of tetrahedrane R*₄Si₄ ( 1 ) and bis(silirane) R*(η²‐C₂H₄)Si–Si(η²‐C₂H₄)R* ( 5 ). The latter is formed by [1+2]‐cycloaddition reaction of intermediate disilyne R*Si≡SiR* with ethene. Compound 5 has been characterized by X‐ray structure determination. The ¹H NMR spectrum of the silacyclopropane ring protons shows AA′BB′ complex spectrum comprising of 2 sets each of 12 transitions.     

Reversible Dihydrogen Activation by Reduced Aryl Boranes as Main‐Group Ambiphiles

A new approach to main‐group H₂ activation combining concepts of transition‐metal and frustrated Lewis pair chemistry is reported. Ambiphilic, metal‐like reactivity toward H₂ can be conferred to 9,10‐dihydro‐9,10‐diboraanthracene (DBA) acceptors by the injection of two electrons. The resulting [DBA]^2? ions cleave the H?H bond with the formation of hydridoborates under moderate conditions (T=50–100 °C; p<1 atm). Depending on the boron‐bonded substituents R, the addition is either reversible (R=C≡CtBu) or irreversible (R=H). The reaction rate is strongly influenced by the nature and the coordination behavior of the countercation (Li⁺ slower than K⁺). Quantum‐chemical calculations support the experimental observations and suggest a concerted, homolytic addition of H₂ across both boron atoms. As proven by the successful conversion of Me₃SiCl into Me₃SiH, the system Li₂[DBA]/H₂ appears generally relevant for the hydrogenation of element–halide bonds.     

Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization

Synthetic routes to aluminium ethyl complexes supported by chiral tetradentate phenoxyamine (salan‐type) ligands [Al(OC₆H₂(R‐6‐R‐4)CH₂)₂{CH₃N(C₆H₁₀)NCH₃}‐C₂H₅] ( 4 , 7 : R=H; 5 , 8 : R=Cl; 6 , 9 : R=CH₃) are reported. Enantiomerically pure salan ligands 1–3 with (R,R) configurations at their cyclohexane rings afforded the complexes 4 , 5 , and 6 as mixtures of two diastereoisomers ( a and b ). Each diastereoisomer a was, as determined by X‐ray analysis, monomeric with a five‐coordinated aluminium central core in the solid state, adopting a cis‐(O,O) and cis‐(Me,Me) ligand geometry. From the results of variable‐temperature (VT) ¹H NMR in the temperature range of 220–335 K, ¹H–¹H NOESY at 220 K, and diffusion‐ordered spectroscopy (DOSY), it is concluded that each diastereoisomer b is also monomeric with a five‐coordinated aluminium central core. The geometry is intermediate between square pyramidal with a cis‐(O,O), trans‐(Me,Me) ligand disposition and trigonal bipyramidal with a trans‐(O,O) and trans‐(Me,Me) disposition. A slow exchange between these two geometries at 220 K was indicated by ¹H–¹H NOESY NMR. In the presence of propan‐2‐ol as an initiator, enantiomerically pure (R,R) complexes 4 – 6 and their racemic mixtures 7 – 9 were efficient catalysts in the ring‐opening polymerization of lactide (LA). Polylactide materials ranging from isotactically biased (P_m up to 0.66) to medium heterotactic (P_r up to 0.73) were obtained from rac‐lactide, and syndiotactically biased polylactide (P_r up to 0.70) from meso‐lactide. Kinetic studies revealed that the polymerization of (S,S)‐LA in the presence of 4 /propan‐2‐ol had a much higher polymerization rate than (R,R)‐LA polymerization (k_SS/k_RR=10.1).     

Synthesis of highly branched polyethylene using para‐phenyl‐substituted α‐diimine nickel catalysts

A series of para‐phenyl‐substituted α‐diimine nickel complexes, [(2,6‐R₂‐4‐PhC₆H₂N═C(Me))₂]NiBr₂ (R = ⁱPr ( 1 ); R = Et ( 2 ); R = Me ( 3 ); R = H ( 4 )), were synthesized and characterized. These complexes with systematically varied ligand sterics were used as precatalysts for ethylene polymerization in combination with methylaluminoxane. The results indicated the possibility of catalytic activity, molecular weight and polymer microstructure control through catalyst structures and polymerization temperature. Interestingly, it is possible to tune the catalytic activities ((0.30–2.56) × 10⁶ g (mol Ni·h)^?1), polymer molecular weights (M_n = (2.1–28.6) × 10⁴ g mol^?1) and branching densities (71–143/1000 C) over a very wide range. The polyethylene branching densities decreased with increasing bulkiness of ligand and decreasing polymerization temperature. Specifically, methyl‐substituted complex 3 showed high activities and produced highly branched amorphous polyethylene (up to 143 branches per 1000 C).     

Homo‐ and copolymerization of ethylene and norbornene with bis(β‐diketiminato) titanium complexes activated with methylaluminoxane

Homo‐ and copolymerization of ethylene and norbornene were investigated with bis(β‐diketiminato) titanium complexes [ArNC(CR₃)CHC(CR₃)NAr]₂TiCl₂ (R = F, Ar = 2,6‐diisopropylphenyl 2a; R = F, Ar = 2,6‐dimethylphenyl 2b ; R = H, Ar = 2,6‐diisopropylphenyl 2c ; R = H, Ar = 2,6‐dimethylphenyl 2d) in the presence of methylaluminoxane (MAO). The influence of steric and electric effects of complexes on catalytic activity was evaluated. With MAO as cocatalyst, complexes 2a–d are moderately active catalysts for ethylene polymerization producing high‐molecular weight polyethylenes bearing linear structures, but low active catalysts for norbornene polymerization. Moreover, 2a – d are also active ethylene–norbornene (E–N) copolymerization catalysts. The incorporation of norbornene in the E–N copolymer could be controlled by varying the charged norbornene. ¹³C NMR analyses showed the microstructures of the E–N copolymers were predominantly alternated and isolated norbornene units in copolymer, dyad, and triad sequences of norbornene were detected in the E–N copolymers with high incorporated content of norbornene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 93–101, 2008     

Vinyl polymerization of norbornene by mono‐ and trinuclear nickel complexes with indanimine ligands

A series of new indanimine ligands [ArN?CC₂H₃(CH₃)C₆H₂(R)OH] (Ar = Ph, R = Me ( 1 ), R = H ( 2 ), and R = Cl ( 3 ); Ar = 2,6‐i‐Pr₂C₆H₃, R = Me ( 4 ), R = H ( 5 ), and R = Cl ( 6 )) were synthesized and characterized. Reaction of indanimines with Ni(OAc)₂·4H₂O results in the formation of the trinuclear hexa(indaniminato)tri (nickel(II)) complexes Ni₃[ArN = CC₂H₃(CH₃)C₆H₂(R)O]₆ (Ar = Ph, R = Me ( 7 ), R = H ( 8 ), and R = Cl ( 9 )) and the mononuclear bis(indaniminato)nickel (II) complexes Ni[ArN?CC₂H₃(CH₃)C₆H₂(R)O]₂ (Ar = 2,6‐i‐Pr₂C₆H₃, R = Me ( 10 ), R = H ( 11 ), and R = Cl ( 12 )). All nickel complexes were characterized by their IR, NMR spectra, and elemental analyses. In addition, X‐ray structure analyses were performed for complexes 7 , 10 , 11 , and 12 . After being activated with methylaluminoxane (MAO), these nickel(II) complexes can polymerize norbornene to produce addition‐type polynorbornene (PNB) with high molecular weight M_v (10⁶ g mol^?1), highly catalytic activities up to 2.18 × 10⁷ gPNB mol^?1 Ni h^?1. Catalytic activities and the molecular weight of PNB have been investigated for various reaction conditions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 489–500, 2008     

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

The μ‐amino–borane complexes [Rh₂(L^R)₂(μ‐H)(μ‐H₂B=NHR′)][BAr^F₄] (L^R=R₂P(CH₂)₃PR₂; R=Ph, ⁱPr; R′=H, Me) form by addition of H₃B?NMeR′H₂ to [Rh(L^R)(η⁶‐C₆H₅F)][BAr^F₄]. DFT calculations demonstrate that the amino–borane interacts with the Rh centers through strong Rh‐H and Rh‐B interactions. Mechanistic investigations show that these dimers can form by a boronium‐mediated route, and are pre‐catalysts for amine‐borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.     

Selenoquinones Stabilized by Ruthenium(II) Arene Complexes: Synthesis,Structure, and Cytotoxicity

A new series of monoselenoquinone and diselenoquinone π complexes, [(η⁶‐p‐cymene)Ru(η⁴‐C₆R₄SeE)] (R=H, E=Se ( 6 ); R=CH₃, E=Se ( 7 ); R=H, E=O ( 8 )), as well as selenolate π complexes [(η⁶‐p‐cymene)Ru(η⁵‐C₆H₃R₂Se)][SbF₆] (R=H ( 9 ); R=CH₃ ( 10 )), stabilized by arene ruthenium moieties were prepared in good yields through nucleophilic substitution reactions from dichlorinated‐arene and hydroxymonochlorinated‐arene ruthenium complexes [(η⁶‐p‐cymene)Ru(C₆R₄XCl)][SbF₆]₂ (R=H, X=Cl ( 1 ); R=CH₃, X=Cl ( 2 ); R=H, X=OH ( 3 )) as well as the monochlorinated π complexes [(η⁶‐p‐cymene)Ru(η⁵‐C₆H₃R₂Cl)][SbF₆]₂ (R=H ( 4 ); R=CH₃ ( 5 )). The X‐ray crystallographic structures of two of the compounds, [(η⁶‐p‐cymene)Ru(η⁴‐C₆Me₄Se₂)] ( 7 ) and [(η⁶‐p‐cymene)Ru(η⁴‐C₆H₄SeO)] ( 8 ), were determined. The structures confirm the identity of the target compounds and ascertain the coordination mode of these unprecedented ruthenium π complexes of selenoquinones. Furthermore, these new compounds display relevant cytotoxic properties towards human ovarian cancer cells.