Combination of nickel and titanium complexes containing nitrogen ligands as catalyst for polyethylene reactor blending

Ethylene was polymerized using a combination of Ni(diimine)Cl₂ ( 1 ) (diimine = 1,4‐bis(2,6‐diisopropylphenyl)‐acenaphthenediimine) and {Tp^Ms*}TiCl₃ ( 2 ) (Tp^Ms* = hydridobis(3‐mesitylpyrazol‐1‐yl)(5‐mesitylpyrazol‐1‐yl)) compounds in the presence of methylaluminoxane (MAO) at 30°C. The productivity reaches a maximum at X_Ni = 0.75 (1 400 kg of PE/mol[M]· h), and the produced polyethylene (PE) showed maximal melt flow index (0.13 g/10 min) and minimal intrinsic viscosity (2.24 dL/g) compared to polyethylenes obtained with different values of nickel loading fractions (X_Ni). Productivity, intrinsic viscosity data, as well as melt flow index measurements markedly depend upon the content of the late transition metal, thus suggesting a synergic effect between nickel and titanium catalysts.     

Über die Isolierung von Lithiumhexamethylozirkonat(IV) Li2[Zr(CH3)6]

ZrCl₄ reacts with LiCH₃ at molar ratios 1 :>6 in diethylether/toluene mixtures at about ?40°C to from Li₂[Zr((CH₃)₆]. After removal of the solvents the complex compound can be separated from the ether-free residue by redissolving it in toluene. Evaporation of the toluene at ?23°C yields yellow, crystalline Li₂[Zr(CH₃)₆].     

The synthesis of poly(phenylacetylene)s with bulky chiral silyl groups and the thermal stability of their helical conformation

The polymerization of (−)‐p‐[(tert‐butylmethylphenyl)silyl]phenylacetylene (t‐BuMePhSi*PA) and (+)‐p‐[{methyl(α‐naphthyl)phenyl}silyl]phenylacetylene (MeNpPhSi*PA) with the [(nbd)RhCl]₂ Et₃N catalyst yielded polymers with very high molecular weights over 2 × 10⁶ in high yields. The optical rotations of the formed poly(t‐BuMePhSi*PA) and poly(MeNpPhSi*PA) were as high as −356 and −150° (c = 0.11 g/dL in CHCl₃), respectively. The circular dichroism (CD) spectrum of poly(t‐BuMePhSi*PA) in CHCl₃ exhibited very large molar ellipticities ([θ]) in the UV region: [θ]_max = 9.2 × 10⁴ ° · cm² · dmol⁻¹ at 330 nm and −8.0 × 10⁴ ° · cm² · dmol⁻¹ at 370 nm. The [θ]_max values of poly(MeNpPhSi*PA) were also fairly large: [θ]_max = 7.1 × 10⁴ ° · cm² · dmol⁻¹ at 330 nm and −5.3 × 10⁴ ° · cm² · dmol⁻¹ at 370 nm. The optical rotations of poly(t‐BuMePhSi*PA) and poly(MeNpPhSi*PA), measured in tetrahydrofuran, chloroform, and toluene solutions, were hardly dependent on temperature in the range 22–65 °C. The CD effects of these polymers hardly changed in the temperature range 28–80 °C, either. These results indicate that the helical structures of these polymers are thermally appreciably stable. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 71–77, 2001     

Titanium and zirconium complexes containing sterically hindered hydrotris(pyrazolyl)borate ligands: synthesis, structural characterization, and ethylene polymerization studies

The synthesis, characterization and ethylene polymerization behavior of a set of Tp^′MCl₃ complexes (4, M=Ti, Tp^′=HB(3-neopentyl-pyrazolyl)₃⁻(Tp^Np); 5, M=Ti, Tp^′=HB(3-tert-butyl-pyrazolyl)₃⁻(Tp^tBu); 6, M = Ti, Tp^′=HB(3-phenyl-pyrazolyl)₃⁻(Tp^Ph); 7, M=Zr, Tp^′=HB(3-phenyl-pyrazolyl)₃⁻(Tp^Ph); 8, M=Zr, Tp^′ = HB(3-tert-butyl-pyrazolyl)₃⁻(Tp^tBu)) is described. Treatment of these tris(pyrazolyl)borate Group IV compounds with methylalumoxane (MAO) generates active catalysts for ethylene polymerization. For the polymerization reactions performed in toluene at 60 °C and 3 atm of ethylene pressure, the activities varied between 1.3 and 5.1 × 10³ g of PE/mol[M] · h. The highest activity is reached using more sterically open catalyst precursor 4. The viscosity-average molecular weights () of the PE’s produced with these catalyst precursors varying from 3.57 to 20.23 × 10⁵ g mol⁻¹ with melting temperatures in the range of 127-134 °C. Further polymerization studies employing 7 varying Al/Zr molar ratio and temperature of polymerization showed that the activity as well as the polymer properties are dependent on these parameters. In that case, higher activity was attained at 60 °C. The viscosity-average molecular weights of the polyethylene’s decreases with increasing Al/Zr molar ratio.     

Synthesis and Chloride Affinity of Sterically Demanding Ditopic Lithium Bis(pyrazol‐1‐yl)borates

The synthesis and full characterization of the sterically demanding ditopic lithium bis(pyrazol‐1‐yl)borates Li₂[p‐C₆H₄(B(Ph)pz^R₂)₂] is reported (pz^R = 3‐phenylpyrazol‐1‐yl ( 3 ^Ph), 3‐t‐butylpyrazol‐1‐yl ( 3 ^tBu)). Compound 3 ^Ph crystallizes from THF as THF‐adduct 3 ^Ph(THF)₄ which features a straight conformation with a long Li···Li distance of 12.68(1) Å. Compound 3 ^tBu was found to function as efficient and selective scavenger of chloride ions. In the presence of LiCl it forms anionic complexes [ 3 ^tBuCl]⁻ with a central Li‐Cl‐Li core (Li···Li = 3.75(1) Å).     

Ethylene and propylene polymerization with bis(indenyl)zirconium/Mao catalytic systems modified by sterically demanding alcohols

Ethylene and propylene polymerization using Ind₂ZrCl₂ and Ind₂Zr(CH₃)₂/MAO catalytic systems modified by the sterically demanding bridged alicyclic alcohols, adamantan‐1‐ol, adamantan‐2‐ol, 2‐methyladamantan‐2‐ol, and fenchyl alcohol, was investigated. Lower alcohols like isopropanol completely deactivate the system, whereas in the case of catalysts modified by these voluminous alcohols only a slight decrease in the catalytic activity proportional to alcohol/metallocene molar ratio was observed. The addition of the modifiers gives rise to polymers with higher molecular weights than the nonmodified systems, but no structural changes in the polyethylenes were observed. The addition of the sterically demanding alcohols to the reaction medium changes the regioregularity of polypropylenes, but does not significantly influence their stereoregularity, at 30 °C. Propylene–ethylene copolymers containing up to 8.6% of ethylene units derived from 1,3‐insertion and significant amount of rr‐centered pentads were obtained by single‐monomer polymerization of propylene with Ind₂ZrCl₂/MMAO/adamantan‐1‐ol, at 70 °C. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4248–4259, 2005     

Zirconiumphthalocyanine: Darstellung und Eigenschaften chloridhaltiger Phthalocyanine des drei- und vierwertigen Zirconiums; Kristallstruktur von cis-Di(triphenylphosphin)iminium-tri(chloro)phthalocyaninato(2–)zirconat(IV)-di(dichlormethan)

Zirconiumphthalocyanines: Synthesis and Properties of Chloride Ligated Phthalocyanines of Ter- and Quadrivalent Zirconium; Crystal Structure of cis-Di(triphenylphosphine)iminium-tri(chloro)phthalocyaninato(2–)zirconate(IV)-di(dichloromethane) cis-Di(chloro)phthalocyaninato(2–)zirconium(IV) is obtained by the reaction of ZrCl₄ with phthalodinitrile in 1-chloronaphthaline at 230°C. It reacts with molten di(triphenylphosphine)iminiumchloride ((PNP)Cl) yielding cis-di(triphenylphosphine)iminium-tri(chloro)phthalocyaninato(2-)zirconate(IV), cis-(PNP)[ZrCl₃Pc^2?]. This crystallizes with two molecules of dichloromethane in the monoclinic space group P2₁/n with the lattice constants a = 15.219(4) Å, b = 20.262(10) Å, c = 20.719(4) Å, b? = 93.46(2)°, Z = 4. The seven coordinated Zr atom is situated in a “square base-trigonal cap” polyhedron. The plane of the three chlorine atoms runs parallel to the plane of the four isoindole nitrogen atoms N_iso. The Zr–Cl distances range from 2.49 to 2.55 Å, the Zr? N_iso distances from 2.26 to 2.29 Å. Due to ion packing effects the Pc^2? ligand shows an asymmetrical convex distortion. The PNP cation adopts the bent conformation. The P? N? P angle is 139°, the P? N distance 1.58 Å. As confirmed by the cyclovoltammograms cis-(PNP)[ZrCl₃Pc^2?] is oxidized (anodically or chemically by Cl₂) to yield cis-tri(chloro)phthalocyaninato(1–)zirconium(IV) and reduced (cathodically or chemically by [BH₄]^?) yielding chlorophthalocyaninato(2–)zirconium(III) and cis-di(triphenylphosphine)iminium-di(chloro)phthalocyaninato(2–)zirconate(III). The optical spectra show the typical π–π*-transitions of the Pc^2? resp. Pc^? ligand not much affected by the different states of oxidation and coordination of zirconium. The same is true for the vibrational spectra of the Pc^2? resp. Pc^? ligand. In the f.i.r. spectra between 350 and 150 cm^?1 the asym. and sym. Zr? Cl stretching and Cl? Zr? Cl deformation vibration as well as the asym. Zr? N stretching vibration of the [ZrCl_xN₄] skeleton (x = 1–3) is assigned.     

Sekundäre Hydroxyalkylphosphane: Synthese und Charakterisierung mono‐, bis‐ und trisalkoxyphosphan‐substituierter Zirconiumkomplexe und des heterobimetallischen Dreikernkomplexes [Cp2Zr{O(CH2)3PHMes(AuCl)}2]

Secondary Hydroxyalkylphosphanes: Synthesis and Characterization of Mono‐, Bis‐ and Trisalkoxyphosphane‐substituted Zirconium Complexes and the Heterobimetallic Trinuclear Complex [Cp₂Zr{O(CH₂)₃PHMes(AuCl)}₂] The secondary hydroxyalkylphosphanes RPHCH₂OH [R = 2,4,6‐Me₃C₆H₂ (Mes) ( 1 ), 2,4,6‐ⁱPr₃C₆H₂ (Tipp) ( 2 )], 1‐AdPH‐2‐OH‐cyclo‐C₆H₁₀ ( 3 ) and RPH(CH₂)₃OH [R = Ph ( 4 ), Mes ( 5 ), Tipp ( 6 ), Cy ( 7 ), ^tBu ( 8 )] were obtained from primary phosphanes RPH₂ and formaldehyde ( 1 , 2 ) or from LiPHR and cyclohexene oxide ( 3 ) or trimethylene oxide ( 4 ‐ 8 ). Starting from 5 or 7 and [Cp^R₂ZrMe₂] [Cp^R = C₅EtMe₄ (Cp°), C₅H₅ (Cp), C₅MeH₄ (Cp′)], the monoalkoxyphosphane‐substituted zirconocene complexes [Cp^R₂Zr(Me){O(CH₂)₃PHMes}] [Cp^R = Cp° ( 9 ), Cp ( 10 )] were prepared. With [Cp^R₂ZrCl₂], the bisalkoxyphosphane‐substituted complexes [Cp′₂Zr{O(CH₂)₃PHMes}₂] ( 11 ) and [Cp₂Zr{O(CH₂)₃PHCy}₂] ( 12 ) are obtained, and with [Tp^RZrCl₃], the trisalkoxyphosphane‐substituted zirconium complexes [Tp^RZr{O(CH₂)₃PHMes}₃] [Tp^R = trispyrazolylborato (Tp) ( 13 ), Tp^R = tris(3,5‐dimethyl)pyrazolylborato (Tp*) ( 14 )] are prepared. The reaction of 5 with [AuCl(tht)] (tht = tetrahydrothiophene) yielded the mononuclear complex [AuCl{PHMes(CH₂)₃OH}] ( 15 ). The trinuclear complex [Cp₂Zr{O(CH₂)₃PHMes(AuCl)}₂] ( 16 ) was obtained from [Cp₂ZrCl₂] and 15 . Compounds 1 ‐ 16 were characterized spectroscopically (¹H‐, ³¹P‐, ¹³C‐NMR; IR; MS) and compound 2 also by crystal structure determination. The bis‐ and trisalkoxyphosphane‐substituted complexes 11‐14 and 16 were obtained as mixtures of two diastereomers which could not be separated.     

Synthetic and characterization aspects of dimethylaminoethyl methacrylate reversible addition fragmentation chain transfer (RAFT) polymerization

2‐cyanoprop‐2‐yl dithiobenzoate (CPDB) mediated RAFT polymerization of dimethylaminoethyl methacrylate (DMAEMA) was carried out in dioxane at 90 °C. The influence of several parameters, such as the monomer to CPDB molar ratio (100 to 500), the monomer concentration (2 mol·L^?1 to 5.9 mol·L^?1), and CPDB to initiator molar ratio (1 to 10), was evaluated with regards to conversion and polymerization duration, as well as control of molar mass and molar mass distributions. Number average molar masses from 10,000 to 70,000 g·mol^?1 can be targeted. The determination of the molar masses has been carried out by size exclusion chromatography (SEC) with a refractometer detector with poly(methyl methacrylate) (PMMA) standards. The experimental values were lower than the expected ones. Then, SEC in aqueous medium with an online laser light scattering detector was used both to get absolute molar masses and to recalibrate the SEC column in THF. Characterization of well‐controlled PDMAEMA samples has been performed by proton NMR spectroscopy and matrix assisted laser desorption ionization time of flight mass spectrometry. Finally, a chain extension experiment was evaluated with regard to living features. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3551–3565, 2005     

Synthese und Kristallstrukturen der heteronuklearen Nitrido‐Komplexe [(Me2PhP)3(MeCN)ClRe≡N–MCl5] mit M = Sn und Zr

Synthesis and Structures of the Dinuclear Nitrido Complexes [(Me₂PhP)₃(MeCN)ClRe≡N–MCl₅] with M = Sn and Zr The water sensitive complexes [(Me₂PhP)₃(MeCN)ClRe≡N–MCl₅] (M = Sn ( 1 ) und Zr ( 2 )) are obtained in dichloromethane from [ReNCl₂(PMe₂Ph)₃] and the acetonitrile adducts of SnCl₄ or ZrCl₄. The compounds crystallize as dichloromethane solvate isotypically with [(Me₂PhP)₃(MeCN)ClRe≡N–TiCl₅] · CH₂Cl₂ in the space group P2₁/n. From toluene crystallize monoclinic crystals of 1 · MeCN · C₇H₈. In the diamagnetic complexes 1 and 2 an anion [MCl₅]^– coordinates to the nitrido ligand of the cationic complex [ReNCl(MeCN)(PMe₂Ph)₃]⁺. The resulting nitrido bridges Re≡N–M are almost linear and asymmetric with Re–N = 169.5 pm, Sn–N = 230.1 pm and Re–N–Sn = 164.5° for 1 and Re–N = 168.4 pm, Zr–N = 237.2 pm and Re–N–Zr = 165.6° for 2 . The phosphine ligands at the Re atom are in a meridional arrangement.     

Effects of ethylene polymerization conditions on the activity of SiO2-supported zirconocene and on polymer properties

The effects of polymerization conditions were evaluated on the production of polyethylene by silica-supported (n-BuCp)₂ZrCl₂ grafted under optimized conditions and cocatalyzed by methylaluminoxane (MAO). The Al : Zr molar ratio, reaction temperature, monomer pressure, and the age and concentration of the catalyst were systematically varied. Most reactions were performed in toluene. Hexane, with the addition of triisobutilaluminum (TIBA) to MAO, was also tested as a polymerization solvent for both homogeneous and heterogeneous catalyst systems. Polymerization reactions in hexane showed their highest activities with MAO : TIBA ratios of 3 : 1 and 1 : 1 for the homogeneous and supported systems, respectively. Catalyst activity increased continuously as Al : Zr molar ratios increased from 0 to 2000, and remained constant up to 5000. The highest activity was observed at 333 K. High monomer pressures (≈ 4 atm) appeared to stabilize active species during polymerization, producing polyethylenes with high molecular weight (≈ 3 × 10⁵ g mol⁻¹). Catalyst concentration had no significant effect on polymerization activity or polymer properties. Catalyst aging under inert atmosphere was evaluated over 6 months; a pronounced reduction in catalyst activity [from 20 to 13 × 10⁵ g PE (mol Zr h)⁻¹] was observed only after the first two days following preparation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1987–1996, 1999     

Zirconium-Diazabutadien-Komplexe des Typs Zr(DAD)3

Zirconium Diazabutadiene Complexes of the Type Zr(DAD)₃ 1,4-Diazabutadienes (DAD), RN?CPh? CPh?NR 1 (R = C₆H₅ a , CH₃C₆H₄ b , CH₃OC₆H₄ c ), react with ZrCl₄ · 2 THF with formation of complexes of the type ZrCl₄ · DAD 2 . At interaction with lithium diazadienides Li₂DAD deep coloured complexes of the composition Zr(DAD)₃ 3 are formed. The new compounds were characterized by elemental analysis, i.r., ¹H- and ¹³C-n.m.r. spectra.     

Polymerization of olefins with a novel dinuclear ansa-zirconocene catalyst having a biphenyl bridge

Both the rac- and meso-dinuclear ansa-zirconocene catalysts (μ-C₁₂H₈{[SiPh(Ind)₂]ZrCl₂}₂) were prepared by a coupling reaction between 2 equiv of diindenylphenylchlorosilane (rac- and meso-isomers) and 1 equiv of p-dilithiobiphenyl in diethyl ether at −80°C, followed by a successive reaction with ZrCl₄ · 2THF in THF at −78°C. Polymerizations of ethene and propene were conducted in a 1 dm³ high-pressure glass reactor equipped with a mechanical stirrer at 60, 80, 100, 120, and 150°C using methylalumoxane (MAO) as cocatalyst and toluene or decahydronaphthalene as the solvent. Copolymerization of ethene and 1-octene was also checked in brief. For ethene polymerization, the meso-catalyst was found to be more active, which displayed an extremely high activity to give linear polyethene with a high molecular weight and a narrow molar mass distribution (MMD). The apparent activity increased monotonously with rising polymerization temperature from 60°C up to 150°C, indicating that the active species are stable even at a high temperature. On the other hand, both the rac- and meso-catalysts showed very poor activities for propene polymerization. However, copolymerization of ethene and 1-octene proceeded at a high speed. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2269–2274, 1998     

Polymerization of higher linear α‐olefins with (CH3)2Si(2‐methylbenz[e]indenyl)2ZrCl2

Poly‐α‐olefins ranging from poly‐1‐pentene to poly‐1‐octadecene with narrow polydispersities were synthesized with (CH₃)₂Si(2‐methylbenz[e]indenyl)₂ZrCl₂ and methylaluminoxane at polymerization temperatures (T_p 's) ranging from −15 to 180 °C and were characterized by gel permeation chromatography, NMR spectroscopy, and differential scanning calorimetry. The molar masses of the homopolymers obtained with (CH₃)₂Si(2‐methylbenz[e]indenyl)₂ZrCl₂ were notably higher than those of poly‐α‐olefins synthesized with other zirconium‐based metallocenes under similar conditions. The temperature dependence of the molar mass distribution of the poly‐α‐olefins can be described by a common exponential decay function regardless of the investigated monomer. At T_p 's ranging from 20 to 100 °C, moderate isotacticity prevailed, but outside this temperature range, the polymers were less stereoregular. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2333–2339, 2000     

Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of the Heavier Alkaline Earth Metals: Synthesis,Characterization, and Efficient Catalytic Intramolecular Hydroamination

Heteroleptic silylamido complexes of the heavier alkaline earth elements calcium and strontium containing the highly fluorinated 3‐phenyl hydrotris(indazolyl)borate {F₁₂‐Tp^{4Bo, 3Ph}}^? ligand have been synthesized by using salt metathesis reactions. The homoleptic precursors [Ae{N(SiMe₃)₂}₂] (Ae=Ca, Sr) were treated with [Tl(F₁₂‐Tp^{4Bo, 3Ph})] in pentane to form the corresponding heteroleptic complexes [(F₁₂‐Tp^{4Bo, 3Ph})Ae{N(SiMe₃)₂}] (Ae=Ca ( 1 ); Sr ( 3 )). Compounds 1 and 3 are inert towards intermolecular redistribution. The molecular structures of 1 and 3 have been determined by using X‐ray diffraction. Compound 3 exhibits a Sr ??? MeSi agostic distortion. The synthesis of the homoleptic THF‐free compound [Ca{N(SiMe₂H)₂}₂] ( 4 ) by transamination reaction between [Ca{N(SiMe₃)₂}₂] and HN(SiMe₂H)₂ is also reported. This precursor constitutes a convenient starting material for the subsequent preparation of the THF‐free complex [(F₁₂‐Tp^{4Bo, 3Ph})Ca{N(SiMe₂H)₂}] ( 5 ). Compound 5 is stabilized in the solid state by a Ca???β‐Si?H agostic interaction. Complexes 1 and 3 have been used as precatalysts for the intramolecular hydroamination of 2,2‐dimethylpent‐4‐en‐1‐amine. Compound 1 is highly active, converting completely 200 equivalents of aminoalkene in 16 min with 0.50 mol % catalyst loading at 25 °C.     

Syntheses of Some New Group 4 non-Cp Complexes Bearing Schiff-base, Thiophene Diamide Ligands Respectively and Their Catalytic Activities for a-Olefin Polymerization

Totally sixteen new titanium and zirconium non-Cp complexes supported by Schiff-base, or thiophene diamide ligands have been synthesized. The complexes are obtained by the reaction of M(OPr-i)4(M=Ti,Zr) with the corresponding Schiff-base ligand in 1:1 molar ratio in good yield. The thiophene diamide titanium complex has been prepared from trimethylsilyl amine [N,S,N] ligand and TiCl4 in toluene at 120℃. All complexes are well charac-terized by ^1H NMR, IR, MS and elemental analysis. When activated by excess methylaluminoxane (MAO), complexes show moderate catalytic activity for ethylene polymerization, and complex If (R^1=CH3,R^2=Br) exhibits the highest activity for ethylene and styrene polymerization. When the complexes were preactivated by triethylaluminum (TEA), both polymerization activities and syndiotacticity of the polymers were greatly improved.     

Synthesis and Structure Characterization of 2‐(Dimethylaminomethyl)pyrrolate and 2,5‐Bis(dimethylaminomethyl)pyrrolate Zirconium Complexes

Tetrakis(diethylamido)zirconium reacts with 2‐(dimethylamino)methyl pyrrole (DMAMP) and 2,5‐[bis(dimethylamino)methyl]pyrrole (BDMAMP) to give Zr(NEt)₂(DMAMP)₂ 1 and Zr(NEt)₃(BDMAMP) 2 , respectively. Both 1 and 2 have been characterized by ¹H and ¹³C NMR spectroscopies and 1 has also been characterized by X‐ray crystallography. Complex 1 shows an agostic interaction between Zr and H(21A) in solid state that is not sustained in solution. Reacting 1 with 2 equivalents of trimethylsilyl chloride in toluene yields ZrCl₂(DMAMP)₂ 3 in 75% yield which was characterized by ¹H and ¹³C NMR spectroscopies.     

[{Me4C2(C5H4)2}Zr(BH4)2], a bis(tetra­hydro­borate) complex of a bridged zirconocene

Bis(tetra­hydro­borato)­[1,1,2,2‐tetra­methyl‐1,2‐ethyl­enebis­(η⁵‐cyclo­penta­dienyl)]­zirconium, (I), was synthesized by the reaction of the zirconocene dichloride with lithium tetra­hydro­borate. Crystals suitable for X‐ray structure analysis were obtained by recrystallization from toluene. The mol­ecule adopts an appproximate C_2v symmetry. Both tetra­hydro­borate ligands are η²‐coordinated and tilted by 18–19° out of the equatorial plane; the angle B1—Zr1—B2 is 104.7°. The cyclo­penta­dienyl rings show a normal η⁵‐coordinaton, with a centroid–Zr–centroid angle of 124.3°.     

A Variable-Pressure 2D 1H-NMR Study of the Mechanism of Trimethyl Phosphate Intermolecular Exchange and cis/trains-Isomerisation of Tetrachlorobis(trimethyl phosphate)zirconium(IV)

In chloroform, [ZrCl₄·2(MeO)₃PO] exists in both cis- and trans-isomeric forms. Three reactions can be envisaged in the presence of excess (MeO)₃PO = L: (1) cis-[ZrCl₄·2L] + *L?cis-[ZrCl₄·L*L]+ L; (2) trans-[ZrCl₄·2L] + *L ? trans-[ZrCl₄·L*L] + L; (3) cis-[ZrCl₄·2L]? trans-[ZrCl₄·2L]. To distinguish between these possible reaction pathways, we have used 2D ₁H-NMR spectroscopy. For the first time, variable-pressure 2D exchange spectra were used for mechanistic assignments. cis/trans-Isomerisation was found to be the fastest reaction (in CHCl₃/CDCl₃), with a small acceleration at higher pressure: it is concluded to be an intramolecular process with a slightly contracted six-coordinate transition state. The intermolecular (MeO)₃PO exchange on the cis- and trans-isomer are second-order processes and are strongly accelerated by increased pressure: I_a mechanisms are suggested without ruling out limiting A mechanisms.