Dmitrii E. Babushkin

Mechanism of dimethylzirconocene activation with methylaluminoxane: NMR monitoring of intermediates at high Al/Zr ratios

Using 1H and 13C NMR spectroscopy, the interaction between Cp2ZrMe2 and MAO in toluene solution was investigated in a wide range of Al/Zr ratios (10–4xa0000). Several intermediates (I–IV) were detected in the reaction. The structures of these intermediates were elucidated from detailed analysis of 13C-1H NMR data. Intermediate I is a weak complex of Cp2ZrMe2 with MAO. Intermediate II is the complex contact ion-pair [Cp2ZrMe(μ-Me)Cp2ZrMe]+[Me-MAO]– with symmetric binuclear cationic part. Species IIIxa0a and IIIxa0b are the heterodinuclear complex [Cp2Zr(μ-Me)2AlMe2]+[Me-MAO]– contact ion-pairs and the similar separated ion-pairs. Species IV are represented by the unsymmetrically Me-bridged complex Cp2(Me)Zr+ Me–-Al ≡ (MAO). Reaction equilibria involving intermediates I–IV were studied with varying Al/Zr ratio, MAO concentration and Al2Me6 content. With increasing Al/Zr ratio, complexes I, II, and free Cp2ZrMe2 gradually disappear and at high Al/Zr ratios at conditions, close to the usual polymerization conditions, only species III and IV were detected. At high Al/Zr ratios the molar ratio IV/III appears to be 1–4 depending on the Al2Me6 content, whereas at low Al/Zr (10–40) it is much greater and III is present only in very small amounts. These observations are rationalized assuming a gradual decrease of the average dissociation tendency of IV as the diverse Lewis acidic sites of MAO become occupied. A possible scheme of Cp2ZrMe2 interaction with MAO is suggested. Data on the interaction between Cp2ZrCl2 and MAO are given for comparison.

Polymerization of ethylene catalyzed by iron complex bearing 2,6-bis(imine)pyridyl ligand : 1H and 2H NMR monitoring of ferrous species formed via catalyst activation with AlMe3, MAO, AlMe3/B(C6F5)3 and AlMe3/CPh3(C6F5)4

1 H and 2 H NMR spectroscopic monitoring of ferrous species formed via interaction of 2,6-bis[1-2,6-dimethylphenylimino)ethyl]pyridineiron (II) chloride (1) with AlMe 3 , MAO, AlMe 3 /B(C 6 F 5 ) 3 and AlMe 3 /CPh 3 (C 6 F 5 ) 4 is reported. At interaction of 1 with MAO in toluene solution, the new stable heterodinuclear neutral complexes with proposed structures LFe(II)(Cl)(u-Me) 2 AlMe 2 and LFe(II)(Me)(u-Me) 2 AlMe 2 are formed (L is initial tridentate ligand). Complex LFe(II)(Cl)(u-Me) 2 AlMe 2 predominates at low Al/Fe ratios (less than 50), while LFe(II)(Me)(U-Me) 2 AlMe 2 at high Al/Fe ratios (more than 500). Complex assigned to LFe(II)(Me)(u-Me) 2 AlMe 2 can be prepared via interaction of 1 with AlMe 3 . Activation of LFe(II)(Me)(u-Me) 2 AlMe 2 by B(C 6 F 5 ) 3 and CPh 3 B(C 6 F 5 ) 4 gives rise to formation of new complexes with proposed structures [LFe(u-Me) 2 AlMe 2 ] + [MeB(C 6 F 5 ) 3 ] - and [LFe(u-Me) 2 AlMe 2 ] + [B(C 6 F 5 ) 4 ] - . Unexpectedly, the activity at ethylene polymerization was even higher for 1/AlMe 3 than for 1/MAO catalytic system. The co-catalytic activity of MAO towards 1 dramatically decreased with the diminishing of AlMe 3 content in the composition of MAO. Activity of the catalyst 1/AlMe 3 and the molecular structure of polyethylene produced do not change noticeably at the addition of B(C 6 F 5 ) 3 to 1/AlMe 3 . These data allow to suggest, that active species of 1/AlMe 3 and 1/MAO systems are neutral methylated ferrous complexes but not cationic intermediates. Probably, complex LFe(II)(Me) 2 AlMe 2 is the closest precursor of these active species.

Study of the ethylene polymerization over homogeneous and supported catalysts based on 2,6-bis(imino)pyridyl complexes of Fe(II) and Co(II)

Abstract The kinetic data on ethylene polymerization over homogeneous catalysts based on 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II) and cobalt(II) chlorides (LMCl2, M = Fe, Co) with different aluminium-organic activators (MAO, AlMe3, Al(i-Bu)3) are reported. LFeCl2 is very effective with AlMe3 and Al(i-Bu)3 as activator and LCoCl2 is highly active in the presence of AlMe3. For MAO as activator, the activity depends on the content of free AlMe3 in MAO. Maximal activity has been found for MAO completely purified from free AlMe3. According to 1 H and 2 H NMR study, formation of the similar neutral intermediates of the type [LFe(II)Me(μ-Me)2AlMe2] have been detected for LFeCl2/AlMe3 and LFeCl2/MAO catalysts. In the system LCoCl2/MAO, the complex of the type LCo(II)Me(X)·MAO (X = Me or Cl) with terminal Co–Me group is detected. Interaction of LCoCl2 with AlMe3 results in the formation of diamagnetic Co(I) species. Highly active supported catalysts SiO2/LFeCl2+AlR3 and SiO2/MAO/LFeCl2 have been prepared. In contrast to homogeneous systems, the supported catalysts are stable at elevated temperatures of polymerization (70xa0°C) and produce high molecular mass polyethylene with improved morphology.

1H NMR and EPR spectroscopic monitoring of the reactive intermediates of (Salen)MnIII catalyzed olefin epoxidation

Abstract Using 1H NMR and EPR spectroscopy, manganese species formed in the catalytic systems 1+iodosobenzene (PhIO) and 1+meta-chloroperoxybenzoic acid (m-CPBA), where 1 is (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) chloride ([(Salen)MnIII]) were studied. Three types of manganese complexes were characterized in the catalytic system 1+PhIO (4–6). Complex 4 is very unstable and reacts with styrene at −20°C to afford styrene oxide. It exhibits three signals of tBu groups at 1.68, 1.64 and 1.42 ppm. This pattern closely resembles that for a model complex [(Salen)MnV≡N]. Based on these data, 4 was identified as d2 low-spin oxomanganese(V) complex [(Salen)MnV=O]+. Complexes 5 and 6 are relatively stable at −20°C and poorly reactive towards styrene at this temperature. They display 1H NMR spectra characteristic for antiferromagnetically coupled μ-oxo-dinuclear MnIV species and are identified as dinuclear complexes [(Salen)LMnIV-O-MnIV(Salen)L′] with L, L′=Cl− and PhIO. It was found by EPR that the acylperoxo complex (Salen)MnIII(OOCOAr) (7) was formed at the first stage of the interaction of 1 with m-CPBA in CH2Cl2. Complex 7 is unstable and converts into manganese(IV) oxo complex [(Salen)MnIV(O)] (8). The evaluated first order rate constant of this conversion is 0.25±0.08 min−1 at −70°C. Complex 7 reacts with styrene with the rate constant 1.1±0.4 M−1 min−1 at −70°C to give epoxide and restore 1. Complex 8 is inert towards styrene at low temperature. The effect of donor ligand N-methylmorpholine-N-oxide (NMO) on the epoxidation of styrene by the system 1+m-CPBA was studied. Addition of NMO (2–5 equiv.) to the solution of 1 in CH2Cl2 before interaction with m-CPBA was found to dramatically increase the rate of undesirable transformation of the reactive acylperoxo complex 7-NMO into relatively inert oxo complex-8-NMO. However, in the presence of styrene, such undesirable conversion is entirely suppressed by very rapid reaction of 8-NMO with styrene to afford styrene oxide and restore 1-NMO.

The metallocene/methylaluminoxane catalysts formation : EPR spin probe study of Lewis acidic sites of methylaluminoxane

Abstract Using stable nitroxyl radical 2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPO) as a spin probe, Lewis acidic sites of methylaluminoxane (MAO) were identified. It was found that MAO contains two types of acidic sites. TEMPO, coordinated to the sites I and II, exhibits in the EPR spectra triplet ( g o =2.0047, a N =18.6 G) and triplet of sextets ( g o =2.0045, a N =19.6 G and a Al =1.7 G), respectively ( a N and a Al are constants of hyperfine structure from the corresponding nucleus). According to EPR measurements, concentration of sites I is close to that of sites II, and MAO contains one site of each type per 100±30 aluminium atoms. The adducts of TEMPO with sites I are less stable than those with sites II. Based on the values of a Al and relative stabilities of the adducts with TEMPO, the acidic sites I and II were attributed to coordinatively unsaturated aluminium atoms in AlOMe 2 and AlO 2 Me environment, respectively. From the EPR spectra of coordinated TEMPO, the average radius of MAO oligomers (AlOMe) n was evaluated to be 5.8 A at 20°C, which corresponds to the value of n =15–20. Thus, the major part of MAO contains not more than one Lewis acidic site per one oligomeric (AlOMe) n molecule.

Formation of Trivalent Zirconocene Complexes from ansa-Zirconocene-Based Olefin-Polymerization Precatalysts: An EPR- and NMR-Spectroscopic Study

Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. The chloro-bridged heterodinuclear ansa-zirconocenium cation [(SBI)Zr(μ-Cl)2AlMe2](+) (SBI = rac-dimethylsilylbis(1-indenyl)), present in toluene solution as its B(C6F5)4(-) salt, thus gives rise to an EPR signal assignable to the complex (SBI)Zr(III)(μ-Cl)2AlMe2, while (SBI)Zr(III)-Me and (SBI)Zr(III)(μ-H)2Al(i)Bu2 are formed by reduction of [(SBI)Zr(μ-Me)2AlMe2](+) B(C6F5)4(-) and [(SBI)Zr(μ-H)3(Al(i)Bu2)2](+) B(C6F5)4(-), respectively. These products can also be accessed, along with (SBI)Zr(III)-(i)Bu and [(SBI)Zr(III)](+) AlR4(-), when (SBI)ZrMe2 is allowed to react with HAl(i)Bu2, eliminating isobutane en route to the Zr(III) complex. Further studies concern interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation.

Stability and reactivity of low-spin ferric hydroperoxo and alkylperoxo complexes with bipyridine and phenantroline ligands

Abstract In this work the first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo complexes [Fe(bpy) 2 (OOH)Py](NO 3 ) 2 ( 2a -Py), [Fe(phen) 2 (OOH)Py](NO 3 ) 2 ( 2b -Py) and [Fe(bpy) 2 (OO t Bu)CH 3 CN](NO 3 ) 2 ( 3a -CH 3 CN) were determined in the presence of various substrates and at various temperatures. It was observed, that the alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k =1.2×10 −2 s −1 ( 3a -CH 3 CN in CH 3 CN at −10°C) and k =2×10 −4 s −1 ( 2a -Py in CH 3 CN at −10°C). The sixth ligand (Py in 2a -Py and 2b -Py; CH 3 CN in 3a -CH 3 CN) can be replaced by other donor molecules B in appropriate solvent systems. Using d 9 - t BuOOH, 2 D NMR signals of t BuOO moieties of complexes 3a -CH 3 CN, 3a -CH 3 OH and 3a -H 2 O were observed. The rate of decomposition of hydroperoxo complexes [Fe(bpy) 2 (OOH)B](NO 3 ) 2 ( 2a -B), where B are derivatives of Py (3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2 N-Py) increases with the growth of basisity of B (push effect). Such effect is markedly smaller for alkylperoxo species [Fe(bpy) 2 (OO t Bu)B](NO 3 ) 2 ( 3a -B). The addition of organic substrates (cyclohexane, cyclohexene, methyl phenyl sulfide) in concentrations up to 3 M at −10°C to +20°C does not noticeably change the rate of self-decomposition of 2a -B, [Fe(phen) 2 (OOH)B](NO 3 ) 2 ( 2b -B) and 3a -B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a -B, 2b -B and 3a -B were characteristic for free radical oxidation. OH · and HO 2 · radicals were trapped in solution containing 2a -Py, and t BuOO · free radicals were detected in solution in the presence of 3a -B. The determined rates of self-decomposition of complexes 2a -B, 2b -B and 3a -B can be used for evaluation of the upper limit for their reactivity towards organic substrates.

Multinuclear NMR spectroscopic characterization of Co(III) species: Key intermediates of cobalt catalyzed autoxidation

Abstract Using 1 H , 13 C and 59 Co NMR spectroscopies, it was shown that, the so-called `Co(III) acetate-reactive intermediate of cobalt acetate catalyzed autoxidation, is the mixture of the oxo-centered trinuclear cations [Co 3 O(OAc) 6 (AcOH) 3 ] + ( I ) and [Co 3 O(OAc) 5 (OH)(AcOH) 3 ] + ( II ) in 1:3–1:2 ratio. Cations I and II are the major species in the freshly prepared solutions of Co(III) acetate in CH 2 Cl 2 and AcOH. Immediately after dissolving in MeOH, I converts into cation [Co 3 O(OAc) 5 (OMe)(MeOH) 3 ] + and II into cation [Co 3 O(OAc) 5 (OH)(MeOH) 3 ] + . Reactivity of I and II towards p -xylene was determined. The addition of p -xylene to the sample containing I and II under anaerobic conditions in CH 2 Cl 2 (1:3 p -xylene/CH 2 Cl 2 by volume, 293 K) gave rise to gradual decrease of their concentration. The half-life was 2 h for cation II and 1.3 h for cation I .

Nature of the reactive intermediates from the titanium-induced activation of hydrogen peroxide: Studies using 1H and 17O NMR

Abstract Using 17 O and 1 H NMR, peroxotitanium species formed in the reaction of TiO(acac) 2 and Ti(OEt) 4 with H 2 O 2 in CHCl 3 were characterized. Dinuclear μ-oxo, μ-peroxo complex [Ti(acac) 2 ] 2 O(O 2 ) was prepared via reaction of TiO(acac) 2 with H 2 O 2 in CHCl 3 . This complex is inert towards alkenes oxidation. When Ti(OEt) 4 reacts with an equimolar amount of 95% H 2 O 2 in CHCl 3 at 293 K, ca. 75% of the initial titanium complex converts to the oligomeric peroxotitanium species, containing no alkoxo ligands, ca. 20% of titanium exists in solution in the form of dinuclear μ-oxo, μ-peroxo complexes of the type [Ti(OEt) 2 L 2 ] 2 O(O 2 ), where L is solvent or EtOH, and 3–5% in the form of mononuclear peroxo complexes of the type Ti(O 2 )(OEt) 2 L 2 . Oligomeric and dinuclear peroxotitanium species were inert towards alkenes and phenol; in contrast mononuclear ones oxidize cyclohexene at 293 K predominantly into cyclohexene oxide and phenol into a 1:3 catechol/hydroquinone mixture. Peroxo complex Ti(O 2 )(OEt) 2 L 2 is the first peroxotitanium complex active towards oxidation of organic substrates.

EPR and 1H NMR spectroscopic characterization of the ferric species formed in the iron picolinate/(pyridine/acetic acid)/HOOH catalytic system for the direct ketonization of methylenic carbons

Abstract Using EPR and 1H NMR spectroscopy, ferric species formed in the iron picolinate/(pyridine/acetic acid)/HOOH catalytic system for the direct ketonization of methylenic carbons were characterized. The initial complex K[FeII(PA)3] (PA-pyridine-2-carboxylato) converts into mononuclear hydroxo complex FeIII(PA)2(OH)Py via interaction with equimolar amount of HOOH in 2:1 Py/AcOH molar mixture. Addition of the great excess of HOOH to the solution of FeIII(PA)2(OH)Py in 2:1 Py/AcOH at 253 K gives rise to the formation of the unstable high-spin ferric peroxo complex with suggested structure FeIII(PA)2(OOH)Py. This complex is supposed to be reactive intermediate of oxidation.