Distinct methylalumoxane(MAO)-derived Me-MAO- anions in contact with a zirconocenium cation-a 13C-NMR studyt
Dmitrii E. Babushkin, *a Corinna N aundorF and Hans H. Brintzinger*b
Zirconocenium cations of the type [(MeC5H4)2ZrMe]+, fonned by excess methylalumoxane (MAO) from (MeC5H4)2ZrCh or (MeCsH4)2ZrMe2 with l3C-labelled ring ligands, are found to fonn ion pairs with two types of an ions, Me-MAOA - and Me-MAOB -, which differ in their coordinative strengths: More strongly coherent ion pairs [(MeC5H4)2ZrMe+ ... Me-MAOB -] are converted to more easily separable ion pairs [(MeC5H4)2ZrMe+ ... Me-MAOA -] by a sufficient excess of MAO. These react with A12Me6 to form outer-sphere ion pairs containing the cationic AIMe3 adduct [(MeC5H4)2Zr(jl-Me)2AIMe2]+;
formation of the more easily separable ion pairs might be required also for polymerisation catalysis.
Introduction
Catalytic activities of a diverse range of organometallic cations are contingent on their association with a weakly coordinating counter-anion, which allows substrate molecules to access the metal centre. 1 Zirconocene catalysts for the polymerization of u-olefins, in particular, are known to contain highly reactive methyl zirconocene cations of the type (CpX)2ZrMe+ (with Cp" = substituted, anellated and/or bridged cyclopentadienylligands) in contact with weakly coordinating MeB(C6F 5)3- or B(C6F 5)4- anions? Quite a number of ion pairs of these types have been studied with respect to their structures in the solid state and in solution,2-S their ligand exchange reactions, 3,6 and the kinetics of their reactions with u-olefins.7,8
Less well-known are structures and reactivity patterns of catalyst species formed from zirconocene pre-catalysts by the commonly used activator methylalumoxane (MAOV NMR and UV /Vis studies have shown that active catalyst systems with high [AI]MAo/[Zr]tot ratios contain (CpX)2ZrMe+ cations in direct contact with anions of the type Me-MAO-, and that heterobimetallic cations [(CpX)2Zr(jl- Me)2AIMe2]+' in outer-sphere association with Me-MAO- anions, are formed in AhMe6-dependent equilibria (eqn 1).w-14
[(Cp"),ZrMe> ... Me-MAO-] + 112 Al,Me,
"'" [(Cp'),Zr(~-Me),AlMe,]· Me-MAO- (1) Distinct Me-MAO- species appear to exist in these reaction systems, as judged from the effects of changing [AI]MAo/[Zr]tot ra- tios on equilibrium constants according to eqn 1 and on lH-NMR shifts of the cation in ion pairs of the type [(CP")2ZrMe+ ... Me- MAO-V 1,13,14 In order to characterize these Me-MAO- anions and their role in zirconocene-based catalyst systems, we have undertaken a study of these systems using zirconocene complexes with 13C-labelled ring ligands.
aBoreskov Institute of Catalysis, Russian Academy of Science, RU-630090, Novosibirsk, Russia. E-mail: dimi@catalysis.nsk.su
bPachbereich Chemie, Universitiit Konstanz, D-78457, Konstanz, Germany.
E-mail: hans.brintzinger@uni-konstanz.de
t Dedicated to Professor Heinz Berke at the occasion ontis 60th birthday.
Results and discussion
Synthesis of BC-enriched zirconocene complexes
Prochiral ring ligands allow one to distinguish C2-symmetric zirconocene complexes from those with two different ligands in the molecular mid-plane. We have thus chosen a zirconocene complex with methylcyclopentadienylligands enriched with 13C in 3-position as the main object of this study. Ofthemethods available to regioselectively incorporate 13CO into methylcyclopentadiene,ls we have used a reaction sequence described by Teuben and coworkers, 18 whereby an isoprene complex of pentamethyl- cyclopentadienyl hafnium chloride, C5Me5HfCI, reacts with CO under extrusion of (probably polymeric) C5Me5HfCIO, directly to give methylcyclopentadiene. Utilization of l3CO, conversion of the product to its Li salt and reaction of the latter with zrC~ in toluene gave acceptable yields of 13C-labelled (Me-C5H4)2ZrCI2 in the form of colorless crystals (Scheme 1).
For this complex, lH broad-band decoupled 13C NMR in C6D6 solution gave only one sharp signal at 111.9 ppm,19 as expected for regioselectively pure (1-Me-3-UCC,H,),ZrCl, (for short: {Zr lCl,).
1 H NMR spectra of {Zr }C12 show, in addition to the usual
; JCH,'H) coupling of ca. 2.5 Hz, a 'JCH,uC) coupling of165 Hz, as expected for aromatic C-H units.
Reaction of a zirconocene dichloride with excess AbMe6 is known to lead to the formation of the correspondingmonomethyl derivative.20 Upon addition of 25 equiv. of Al2Me6 to a C6D6 solution of {Zr lCl" the signal at 111.9 ppm is diminished to less than 1 0% of its original size and, at the same time, shifted to 112.5 ppm, most likely due to formation adducts of {Zr lCl, with AIMe3 or AICIMe2. The reaction product {Zr} MeCI gives rise to two signals with equal intensities at 110.5 and 109.7 ppm. The presence of two distinct mid-plane ligands in {Zr }MeCI is thus associated with a diastereotopic splitting of the signal of the 13C atom in the 3-position by 0.8 ppm.
Reaction of {Zr lCI, with slightly more than 2 equiv. ofMeMgCl in toluene/THF at -30°C, removal of solvent, extraction of the residue with toluene followed by evaporation gave the colorless dimethyl derivative {Zr}Me2 (see Experimental). As expected for a complex with identical mid-plane ligands, lH broad-band Ersch. in: Dalton Transactions ; (2006), 38. - S. 4539-4544
http://dx.doi.org/10.1039/B611028M
Konstanzer Online-Publikations-System (KOPS)
1. 13CO, benzene - [Cp*HfCIOln
..
2. n-BuLi
1. ZrCI4, 101, -30°C 2. 110°C,9h
Scheme 1
decoupled BC NMR of {Zr} Me2 gives rise to a single sharp signal with a chemical shift of 107.6 ppm in C6D6 solution. In solutions containing AbMe6 and MAO, this signal is shifted to 108.1 ppm, probably due again to the formation of Lewis acid adducts.
The gradation of chemical shifts from 112.5 ppm for {Zr}CI2 to a mean value of 110.1 ppm for {Zr}MeCI and to 108.1 ppm for {Zr} Me2, reflects an increase in electron density on the ring ligands in this series of complexes.2! From changes in iJC chemical shifts observed in charged aromatics,22 one might estimate that replacement of a Clligand at the Zr centre by a Me group increases the negative charge at each Co ring by ca. 0.05-0.1 units.
Reactions of {Zr }CI2 and of {Zr} Me2 with MAO
To study zirconocene-MAO reactions, we have used toluene solutions of MAO (nominal molar mass 800), ca. 1.3 M in AI, of which ca. 100/<, are present as "free" AbMe6.23 Solutions containing {Zr }Cb and MAO in [AI1MAo/[Zrl,o, ratios of 10 to 50 were obtained by adding 50 /-lL increments of MAO solution to a solution of 3 mg (9.3 /-lmol) of {Zr}CI2 in 500 /-lL of C6D6.
At [AI1MAo/[Zrl,o' = 10, iJC NMR spectra of these solutions show signals due to {Zr }C12 and {Zr }CIMe, which are, however, broadened and shifted to lower fields (112.9 ppm for {Zr }C12 and 110.5 and 109.5 ppm for {Zr}CIMe), due to association with some Lewis acidic MAO sites.
At increasing [AI1MAo/[Zrl,o' ratios, these signals gradually disappear to give rise, at [AI1MAo/[ZrJot = 50, to a group of broad signals between 111 and 113 ppm and to a sharp signal at 115.5 ppm. Based on its close coincidence with the signal of the ion pair [{Zr}(/-l-Me)2AIMe2J+ B(C6F')4-, observed at 115.5 ppm in a solution of {Zr}Me2 after reaction with 1 equiv. of CPh3+ B(C6Fs)4- and 50 equiv. of AbMe6 , the sharp signal at 115.5 ppm is unambiguously assigned to the cationic AIMej adduct [{ Zr }(/-l- Me )2AIMe21+
/4
which must be present in outer-sphere association with some counter-anion of type X-MAO- (X = CI or Me). The broad features at 111-113 ppm, on the other hand, are assigned to contact ion pairs of the type [{ Zr} Me+ ... X-MAO-l, which are known to be in equilibrium with the outer-sphere ion pair [{Zr}(/-l-Me)2AIMe21+ X-MAO- in these solutions.uThese contact ion pairs were studied in more detail in solutions of {Zr }C12 and MAO in higher [AI1MAo/[Zrl,o' ratios, up to ca. 1000, which were prepared by successive additions of small increments of solid {Zr }C12 to 600 /-lL of a solution of MAO in C6D6 • Due to this procedure, the total [AI1 MAo concentration (and likewise that of its AI2Me6 content) are kept constant, while decreasing [AI1MAo/[Zrltot ratios result from increasing [Zrltot concen trations.
At high [AI1MAo/[Zrl,o' ratios of ca. 400-1000, we observe, in addition to the very strong signal of [{Zr}(/-l-Me)2AIMe2J+ at
115.5 ppm, three weak but clearly separated signals in the contact ion pair region, at 111.5, 112.3 and 112.7 ppm.25 Of these, the signal at 111.5 ppm is close in size to the combined sizes of the two signals at 112.3 and 112.7 ppm (Fig. 1). The same set of signals at 111.5, 112.3 and 112.7 ppm appears in MAO solutions containing the dimethyl complex {Zr} Me2, instead of {Zr }CI2, in low concentrations ( < 10).
I i i
120
i i i 116
~ {Zr}Me+-MeMAOA -
In
{Zr} Me+-MeMAOB-i i i
112 (ppm)
i i ) 108
I I i
104
Fig. 1 i3C NMR spectrum of 3.4 mM {Zr}CL and MAO ([A1JMAo/[ZrJ"" = 388, ca. 51% Al as ALMe6) in C6D6 solution.
We propose that the three signals at 111.5, 112.3 and 112.7 ppm are due to two species, each with a diastereotopic splitting due to two different mid-plane ligands at their Zr center. One of these species would thus have a mean chemical shift of 111.9 ppm and a diastereotopic splitting of 0.8 ppm, while the other has a mean chemical shift of 112.1 ppm and a diastereotopic splitting of 1.2 ppm. The signal at 111.5 ppm then contains the superimposed high-field parts of both doublets, in accord with its intensity being close to the sum of the size of the other two signals.
Two distinct species of the type {Zr}Me+··· Me-MAO- thus appear to be present at the highest [AI1MAo/[Zrltot ratios studied in both {Zr}Me2/MAO and {Zr}CVMAO. Of these species, the one with higher chemical shift and higher diastereotopic splitting must have a more electron-deficient {Zr} center with more unequal mid-plane ligands. It is thus likely to contain a more weakly coordinating anion, denoted as Me-MAOA -, while its more strongly coordinating counterpart Me-MAO,,-, present in a contact ion pair [{ Zr} Me+ ... Me-MAO,,-l, gives rise to a BC_
NMR signal with lower chemical shift and smaller diastereotopic splitting.
While two principal classes of Me-MAO- anions can thus be distinguished, a lack of complete uniformity is indicated for both
[ AllMAoI[Zrltot
113 184 252 388 600 1005
i i i i i i I i
118 116 114 112 110 108 106 104 (ppm)
Fig.2 13C NMR spectra of C6D" solutions of MAO ([AllMAo "" 1.3 M, ca. 5% Al as free AI2Me6 ), containing {Zr }C12 in the [AllMAO/[Zrll0l concentration ratios indicated.
Me-MAOA - and Me-MAOu- by the broadness of the signals at 111.5, 112.3 and 112.7 ppm. In BC NMR spectra taken at [AllMAo/[Zrltot = 200 and temperatures between 253 and 313 K no differences are discernible with respect to the intensity ratios' or the apparent line widths of these signals. The broad appearance of these signals is thus likely to be caused by residual structural variations rather than by partial signal coalescence due to some exchange processes. This would indicate that anion ex- change between species of the type [{Zr}Me+· .. Me-MAOA -l and [{Zr} Me+ ... Me-MAOB -l is not fast on the BC-NMR time scale.
In MAO solutions containing higher concentrations of {Zr }CI2, i.e. at [AllMAO/[ZrllOl ratios decreasing from ca. 400 to 100, the sharp signal of {Zr }(Il-Me )2AIMe2 + at 115.6 ppm decreases in size relative to the broader contact ion pair signals at 111.5, 112.3 and 112.7 ppm, which become now also superimposed by an additional, somewhat broader, peak at ca. 112.3 ppm (Fig. 2).
This additional peak is absent, however, in solutions containing the dimethyl complex {Zr}Me2 in the presence of MAO, such that the set of signals at 111.5, 112.3 and 112.7 ppm appears here, free from additional absorptions, even at [AllMAO/[ZrllOl = 100.
The additional, broader feature at ca. 112.3 ppm, present at low [AllMAo/[Zrltot ratios (400-100) in {Zr}CVMAO, but not in {Zr} Me2/MAO, thus appears to be associated with the presence of Cl- in this system. One might thus tentatively assign it to some species of the type [{Zr}Me+··· CI-MAO-].
This assignment is supported by changes in the I H NMR spectra of these reaction systems (Fig. 3): While I H NMR signals due to the CH-protons of the Me-3-iJCC4H4 ligands suffer from severe overlap and complicated spin coupling to the BC-Iabel, the CH3 substituents of these ligands give singlets with useful spectral resolution. The ring-CHJ signals of the ion pairs [{Zr}Me+··· Me-MAOA -l and [{Zr}Me+··· Me-MAOu-l appear as an unresolved singlet at 1.7 ppm,25 but a clearly resolved singlet at 1.8 ppm can be assigned to the ring-CH3 group of [{ Zr} Me+ ... CI-MAO-l, since it is absent in {Zr} Me2/MAO reaction systems.26
" i ' " i ' "i" 'i' i
1.90 1.80 1.70 1.60 1.50
(ppm)
Fig. 3 1 H NMR spectra of C,D6 solutions of MAO ([AllMA 0 "" l.3 M, ca. 5% Al as free ALMe6) containing {Zr }CL in the same [AIJMAo/[Zrl""
concentration ratios as in Fig. 2.
In the system {Zr }CVMAO, the intensity of this I H NMR signal decreases in parallel to that of the 1JC NMR peak at ca. 112.3 ppm when [AllMAo/[Zrl ratios increase from 100 to 1000. At [AllMAo/[Zrl ratios of 400-1000, the ring-CH J signal at 1.8 ppm, assigned to {Zr} Me+ ... CI-MAO-, has disappeared almost completely from the I H NMR spectra (Fig. 2), while the iJC signal at 112.3 ppm still remains finite, with the total intensity of the BC NMR signals at 112.7 ppm and 112.3 ppm becoming equal to the intensity of the upper-field signal at 111.5 ppm, in accord with the assignments presented above.
Solutions in which an [AllMAo/[Zrl ratio of 100 is reached by dissolution of increasing amounts of {Zr }C12 in a 1.3 M solution of MAO give 13C NMR spectra indistinguishable from those of a solution where this [AllMAo/[Zrl ratio is reached by addition of increasing amounts of MAO to a 18.6 mM solution of {Zr}CI2 in C6D6 • This observation indicates that equilibria between the species present in these solutions are established within the time required to prepare these solutions and to measure their NMR spectra.
Experimental
Toluene-ds and benzene-d6 were dried over molecular sieves (4
A)
prior to use. All operations were carried out in a glove box under dry nitrogen (99.999°;;,). A MAO solution in toluene (1.3 M in AI, AbMe6 content ca. 301yo) was obtained as a gift from Crompton (Bergkamen) and a sample of trityl perfiuorotetraphenyl borate as a gift from BASELL GmbH. Trimethylaluminum (TMA), 2-methyl-I,3-butadiene and iJCO were purchased from Sigma- Aldrich Chemie Gmbh (Munich), HfCl4 from MCAT (Konstanz).
Syntheses of 13C-Iabelled zirconocene complexes
Following the procedure reported by Llinas et af.,27 CsMesHfCb was prepared in yields of ca. 85% by reaction of 3.8 g (18.2) of CsMesSiMe3 with 5.6 g (17.4 mmol) of HfCI4 in heptane, with the modification that the reaction mixture was stirred at 80°C for 6 h. From this material, (CsMe,)HfCl(2-Me-l,3-butadiene)
was obtained in 90% yield following the work of Teuben and coworkers." 'H NMR(250 MHz, C,D,/d,-THF 1: 1):00.27-039 (m, 2H); 0.99-1.2 (m, 2H); 2.04 (~ l5H); 2.48 (~ 3H); 5.55 (t, 7.5 Hz, lH). Traces of residual C5Me5HfCb were apparent from a 'H NMR signal at 2.28 ppm.
(1-Me-3p'C]-C,H,l,ZrCl,. A solution of 10.5 g (25.2 mmol) of (C,Me,)HfCl(2-Me-l,3-butadiene) in 160 mL of benzene was exposed to 13CO at a constant pressure of 1.5 bar (abs) and stirred overnight. The methylcyclopentadiene product was collected, together with the benzene solvent, by condensation in a double Schlenk vessel and isolated, in the form of its lithium salt, by addition of 15 mL of a 1.6 M solution of butyl lithium in pentane. The precipitate was collected by filtration, washed with pentane and dried in vacuo to yield 0.73 g (S.4 mmol, 33%) of l-methyl-3-[uC]-cyc1opentadienyllithium. uc NMR (62.5 MHz, C,D,/d,-THF 1 : 1): 0 102.7 ppm. All of this product was allowed to react in a suspension in 30 mL of toluene at -30°C with 0.92 g (4.2 mmol) of zrC~ over a period of 20 min. The reaction mixture was then allowed to reach room temperature, subsequently heated to reflux for 9 h, cooled to room temperature and filtered through kieselguhr, which was then washed with small volumes of dichloromethane. Partial evaporation of the combined filtrate and washings gave a first batch of crystalline product, which was collected by filtration, washed with ca. 5 mL of pentane and dried in vacuo. A second batch was obtained by further evaporation and cooling of the combined mother liquor and pentane washings.
Total yield 0.88 g (2.7 mmol, 65% of theory) of (l-Me-3[uC]- C,H,),ZrCl,. 'H NMR (250 MHz, C,D,): 0 2.25 (~ 6H), 5.92 (m, 165 Hz, 2.5 Hz, 2H), 6.14-6.2 (m, 7.5 Hz, 2.7 Hz, 4H), 6.25 (m, 2.4 Hz, 2H) 6.61 ppm (m, 165 Hz, 2.5 Hz, 2H). uc NMR (62.5 MHz, C,D,): 0 111.9 ppm. Found: C, 44.4; H 4.75%.
C12H I4 CbZr requires C, 45.3; H 4.3S%.
(1-Me-3p'C]-C,H,l,ZrMe,. Following a report by Samuel and Rausch," 0.4 g (1.2 mmol) of (l-Me-3[uC]-C,H,),ZrCl"
in toluene solution at -30°C, were converted to the dimethyl derivative by reaction with 2.5 mmolof methyhnagnesium chloride in THF. Stirring at room temperature for 2 h, evaporation of solvents, extraction with toluene and evaporation gave 70 mg (0.25 mmol, 21 %) of color1es~ liquid (l-Me-3[uC]-C,H,),ZrMe,.
'H NMR (250 MHz, C,D,): 0 - 0.12 (s, 6H), 2.02 (~6H), 5.11- 5.l4(q, 175 Hz, 2.75 Hz, 2H), 5.45-5.50 (m, 2.5 Hz, 2H), 5.65-5.69 (m, 6.8 Hz, 2.6 Hz, 4H), 5.80-5.83 (q, 175 Hz, 2.75 Hz, 2H). uc NMR (62.5 MHz, C,D,: 0107.6 ppm.
C6D6 solutions of MAO. A 20 mL sample of a 10% solution of MAO in toluene, obtained from Crompton Holdings GmbH (Bergkamen), was evaporated to dryness in vacuo at room temper- ature. The solid residue was kept in a dynamic vacuum at 30°C for 3 h, producing free-flowing solid MAO. Of this solid MAO, 50 mg were dissolved in 0.6 mL of C6D6 to give a MAO solution with [AI]MAo = 1.3 M, based on an assumed MAO composition close to A~Me603 and on an AIMe3 content comprising 10% of all AIMe IH NMR signals, i.e. 5% of the [AI]MAo content.
NMR spectroscopy
NMR spectra were recorded on a Varian INOVA-400 spectrometer at 22°C in standard 5 mm NMR tubes. I H NMR operating conditions: spectrometer frequency 399.76 MHz; spectral width
6.4 kHz; pulse width 2 ~s (17"); 6 s delay between pulse~ 80- 600 transients. 13C NMR parameters: spectrometer frequency 100.53 MHz; spectral width 28 kHz; pulse width 4.9 ~s (45");
5.3 s delay between pulses; broad-band decoupling, 4000-25000 transients. To determine chemical shifts, solvent peaks were taken as 7.15 ppm ('H) and 128.00 ppm (uC). For each solution, the [AI]MAo/[Zr]tot ratio was determined by comparing the combined integrated intensity of the ring-CH3 signals at 1.54, 1.7 and 1.S ppm with the combined integrals of the broad CH 3 signal of MAO centered at -0.2 ppm (assuming a MAO composition of
(A~Me,O,)") and of the CH, signal of Al,Me, at -0.33 ppm.
Conclnsions
Our NMR data indicate that the following sequence of reactions occurs when a zirconocene dichloride is activated by an increasing excess of MAO: After initial formation of the monomethyl complex {Zr }CIMe, chloride is transferred to a Lewis acidic MAO species to initially give a chloride-containing contact ion pair, denoted IV CI (eqn 2), which is characterized by 13C and I H signals at 112.3 and I.S ppm, respectively.
[{Zr} MeCl] + MAO --+ [{Zr}M" ... Cl-MAO-] (Ne» (2) Addition of larger amounts of MAO appears to give neutral MAOcI clusters and Me-MAO- anions by exchange of CI- from anionic CI-MAO- to Me- units from neutral MAO clusters, with Me-MAOA - and Me-MAOB - both in the form of their contact ion pairs IVA and IV, (eqn 3A and 3B).
[{Zr}M" ... Cl-MAO-] + MAOA --+ MAOe>
+ [{Zr}M ... Me-MAOA-] (IVA) (3A) [{Zr}M" ... Cl-MAO-] + MAO, --+ MAOe>
+ [{Zr}M ... Me-MAO,-] (IV,) (3B) Concomitantly, the presence of Al2Me6 in these solutions causes a partial displacement of Me-MAO- anions from the zirconocenium cations, forming outer-sphere ion pairs containing the heterodinuc1ear species [{Zr}(~-Me),AlMe,] (III) (eqn 4A and 4B).
[{Zr}Me· ... Me-MAOA - ] + 112 Al,Me,
"'" [{Zr}(~-Me),AlMe,]' Me-MAOA - (III) (4A) [{Zr}Me' ... Me-MAO, -] + 112 Al,Me,
","[{Zr}(~-Me),AlMe,r Me-MAO,- (III) (4B) Of these equilibria, that involving the more strongly coordinat- ing anion Me-MAOB - (eqn 4B) would be expected to stay largely on the side of the contact ion pair at the prevailing concentrations of AbMe6 , while the equilibrium involving Me-MAOA - (eqn 4A) would presumably produce the major part of the outer-sphere ion pair under these conditions.
To check this proposal, we have tried to determine the relative concentrations of the species involved in equilibria 4A and 4B, at least semiquantitatively. The I H NMR spectra in Fig. 3 give, by alternative deconvolution-integration procedures, relative concentrations for species III, NCl and for the sum of NA and IVB (Table 1). To partition the latter into individual relative concentrations of NA and IVB , rough size estimates for the 13C NMR signals in Fig. 2 are used?9 Based on the assumptions
Table 1 Relative concentrations of species III, IVa, IVA and IVg , in {Zr}Ch/MAO reaction systems, as fractions of [Zrlto\, estimated from lH (upright) and 13C (italic) NMR spectral data
[AllMAO/[Zrltot 113 184 252 388 600 1005 [HI]" 0.24 0.33 0.35 0.44 0.51 0.63 [IVa)" 0.15 0.10 0.Q7 0.04 <0.03 <0.02 [IVAj+[IV,J" 0.61 0.57 0.58 0.52 0.48 0.36 [IVA)" 0.04 0.05 0.06 0.10 0.14 0.15 [IVg]b 0.57 0.52 0.52 0.42 0.34 0.21 Estimated relative errors:" 10%. b 30%.
outlined above-that the signal at 111.5 ppm is due to both NA and Ng and that at 112.7 ppm to NA only-we arrive at concentration estimates for IVA and (by subtraction from the sum) for N, (Table 1).
From this estimate, however inaccurate, it is evident that the concentrations of the outer-sphere AlMe3 adduct III run rather closely parallel to those of contact ion pair N A, as expected for an equilibrium described by eqn 4A. At the constant A12Me6
concentrations given by our experimental conditions, we observe ratios of [III]/[NA] N 5 ± 1. The decreasing concentrations of contact ion pair N g , on the other hand, can only marginally contribute to the formation of species III through equilibrium reaction 4B; from the data in Table 1, we would roughly estimate an upper limit of[III]/[IV,] < 0.5.
These results indicate that the equilibrium constant for dis- placement of the anion Me-MAOA - by AlMe3 from a Zr+ centre is greater than that involving the anion Me-MAOg - by at least one order of magnitude. We propose that similar relations hold also for the displacement of anions Me-MAO A-and Me-MAOg -
by olefin substrates. The increase in catalytic activities associated with increasing [Al]MAo/[Zr]tot ratios thus appears to be caused by a decrease of the fraction of more coherent ion pairs NCl and IVg
from dominant (> 75%) at [Al]MAo/[Zr]", N 100 to minor « 15%) at[Al]MAo/[Zr]", N 1000.
Apparently, more strongly Lewis acidic MAO A displaces MAOg
from its methyl-containing anions according to eqn 5 to the extent that MAOA is present in the reaction medium.
Me-MAO,- + MAOA --+ Me-MAOA-+ MAO, (5) Only a small fraction of the MAO clusters present in the reaction system appears to be of the type MAO A, however, such that the concentration of MAOA becomes comparable to the total zirconocene concentration only when [Zr]tot is decreased enough for the [Al]MAo/[Zr]tct ratio to approach a value of ca. 1000.30 Apparently, the more Lewis acidic MAOA clusters comprise only a small fraction of the total Al content of the MAO activator studied,30 even if each Me-MAOA - anion contained as many as 150-200 [AlOMe] units.14 Substantial reductions of the excess of MAO required for full activation of zirconocene-based polymerisation catalysts might thus be envisioned if MAO could be prepared so as to contain higher fractions of more Lewis acidic MAO A species.
Acknowledgements
This work was supported by the European Commission, INTAS grant 00-841. We thank BAS ELL Polyolefine GmbH and Fonds
der Chemischen Industrie for financial support and Crompton Holdings GmbH (Bergkamen) for a gift of MAO.
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10 I. Tritto, R. Donetti, M. C. Sacchi, P. Locatelli and G. Zannoni, Macromolecules, 1997,30,1247.
11 D. E. Babushkin, N. V Semikolenova, V A. Zakharov and E. P. Talsi, Macromol. Chem. Phys., 2000, 201, 558.
12 D. Coevoet, H. Cramail and A. Deffieux, Macromol. Chem. Phys., 1998, 199, 1451; 1. N. Pedeutour, D. Coevoet, H. Cramail and A. Deffieux, Macromol. Chern. Phys., 1999,200,1215; A. Deffieux, H. Cramail and 1. N. Pedeutour, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chern.), 2000,41,1887.
13 U. Wieser, F. Schaper and H. H. Brintzinger, Macromol. Symp., 2006, 236,63.
14 D. E. Babushkin and H. H. Brintzinger, JAm. Chem. Soc., 2002, 124, 12869.
15 uco can be regioselectively introduced into 3-methyl-cyc1opentenone, e.g. by a reaction sequence described in ref. 16, but reduction with DIBAL and subsequent dehydration, as well as a Shapiro hydrazone reduction (ref. 17), gave the desired methyl-cyc1opentadiene only in minimal yields.
16 T. Takahashi, M. Kageyama, V Denisov, R. Hara and E. Negishi, Tetrahedron Lett., 1993, 34, 687; T. Takahashi, C. Xi, Z. Xi, M.
Kageyama, R. Fischer and E. Negishi, J Org. Chern., 1998,63,6802.
17 W G. Dauben, M. E. Lorber, N. D. Vietmeyer, R. H. Shapiro, 1. H.
Duncan and K. Tomer, J Am. Chern. Soc., 1968, 90, 4762; 1. B. Lambert, G. Wang, R. B. Finzel and D. H. Teramura, J Am. Chern. Soc., 1987, 109,7843.
18 1. Blenkers, H. 1. de Liefde Meijer and 1. H. Teuben, Organometallics, 1983,2,1483; 1. Blenkers, B. Hessen, F. van Bolhuis, A. 1. Wagner and 1. H. Teuben, Organometallics, 1987, 6, 459; B. Hessen, 1. Blenkers and 1. H. Teuben, Organometallics, 1989,8,2809.
19 Interactions between both C5-ring ligands are apparently too weak to distinguish between Crsymmetric isotopomers with re,re- or si,si- boundringligands and their Cs-symmetric re,si-counterparts, expected to occur in a 1 : 1 ratio.
20 S. Beck and H. H. Brintzinger, Inorg. Chim. Acta, 1998,270, 376.
21 Anisotropic magnetic susceptibilities of the Zr-CI and Zr-Me units can cause substantial paramagnetic contributions to 13C NMR shifts of C5ligands; here, these are likely to cancel, however, due to essentially unhindered C5-ring rotations.
22 H. GUnther, in NMR-Spektroskopie, Georg Thieme Verlag, Stuttgart, 1992, p. 442.
23 As judged by lH NMR spectra of these solutions:D. E. Babushkin, N. V Semikolenova, V N. Panchenko, A. P. Sobolev, V A.
Zakharov and E. P. Talsi, Macromol. Chem. Phys., 1997, 198, 3845.
24 M. Bochmann and 1. Lancaster, Angew. Chem., Int. Ed. Engl., 1994, 33, 1634.
25 In addition to these signals, a very broad absorption in the range 111- 114 ppm is present in solutions of {Zr }CI2/MAO. A small, sharp signal at 113.65 ppm in 13C NMR and a small narrow singlet at 1.62 ppm in lH NMR, on the other hand, can be assigned to traces of the hafnium- containing binuclear cation [(MeC5H4)2Hf().t-Me)2AlMe2]+, based on the coincidence of their chemical shifts with those of unlabelled [(MeC5H4)2Hf()l-Me)2AlMe2]+, on the close similarity of the shapes of these signals to those of [(MeC5H4)2Zr().t-Me)2AlMe2]+, and on a
chemical shift difference of 1.85 ppm to the latter, which is the same as that between corresponding pairs of 13C signals of [(n-BuC5H4)2Hf()l- Me)2AlMe2]+ and [(n-BuC5H4)2Zr().t-Me)2AlMe2]+.
26 The possibility that the signal at 112.3 ppm is due to some residual {Zr }Ch can be ruled out, since {Zr }Ch is totally converted to {Zr }CIMe even at [Al]MAO/[Zr]tot R::J 20.
27 G. H. Llinas, M. Mane, F. Palacios, P. Royo and R. Serrano, J Organomet. Chern., 1988,340,37.
28 E. Samuel and M. D. Rausch, JAm. Chem. Soc., 1973, 95, 6263.
29 By cut-and-weigh procedures.
30 An alternative explanation-that Me-MAOA - might arise from Me- MAOR - by aggregation with further MAOR units-is not compatible with our data, since the concentrations of all MAO species are constant at all [Al]MAO/[Zr]tot ratios studied.
