Effectiveness in catalyzing carboalumination can be inferred from the rate of dissociation of M/Al dimers

Article abstract of DOI:10.1021/om0495168

The rate constants for the dissociation of AlMe₃ from [ZRBt(C₆F₅)₄] and related M / Al dimers have been determined by selective population inversion (¹H NMR). The results predict - correctly - that (EBI)ZrMe⁺ will be a better carboalumination catalyst than (EBTHI)ZrMe⁺.

Full text of DOI:10.1021/om0495168

© Copyright 2004
Volume 23, Number 22, October 25, 2004
American Chemical Society
Communications
Effectiven ess in Ca ta lyzin g Ca r boa lu m in a tion Ca n Be
In fer r ed fr om th e Ra te of Dissocia tion of M/Al Dim er s
Robby A. Petros and J ack R. Norton*
Department of Chemistry, Columbia University, New York, New York 10027
Received J uly 1, 2004
Summary: The rate constants for the dissociation of
AlMe3 from [2][B(C6F5)4] and related M/ Al dimers have
been determined by selective population inversion (1H
NMR). The results predictscorrectlysthat (EBI)ZrMe+
will be a better carboalumination catalyst than (EBTHI)-
ZrMe+.
cene catalysts for olefin polymerization6,7 and from the
trialkylaluminum compounds often present8 when com-
mercially available aluminoxanes are used to activate
such catalysts.
These Zr/Al heterobimetallics appear to be involved
in chain transfer9,10 and have been assumed to be
“dormant” and incapable of inserting olefin them-
selves.11-15 We have therefore examined the kinetics of
exchange between trimethylaluminum and these ad-
ducts, first observed by Bochmann,15 of the general
formula [Cp2M(µ-Me)2AlMe2]+ (2), where M ) Ti, Zr,
Hf.
Transfer of alkyl groups from Zr to Al must occur
when Zr is used to catalyze chain growth in aluminum
alkyls1-3 and when Zr is used to catalyze carboalumi-
nation of olefins (eq 1).4,5 Little is known about the
The species that takes up olefin in both polymeriza-
tion and carboalumination reactions is believed to be a
mechanism of such transmetalation reactions, but they
seem likely to involve intermediates with alkyl bridges
between Zr and Al. Such µ-alkyl heterobimetallics are
probably also formed when trialkylaluminum com-
pounds are used as impurity scavengers with metallo-
(6) Naga, N.; Mizunuma, K. Macromol. Rapid Commun. 1997, 18,
581-589.
(7) Mohammed, M.; Nele, M.; Al-Humydi, A.; Xin, S. X.; Stapleton,
R. A.; Collins, S. J . Am. Chem. Soc. 2003, 125, 7930-7941.
(8) Pedeutour, J .-N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A.
J . Mol. Catal. A-Chem. 2002, 185, 119-125.
(9) Go¨tz, C.; Rau, A.; Luft, G. Macromol. Mater. Eng. 2002, 287,
16-22.
* To whom correspondence should be addressed. E-mail: jrn11@
columbia.edu.
(10) Lieber, S.; Brintzinger, H.-H. Macromolecules 2000, 33, 9192-
9199.
(1) Samsel, E. G. U.S. Pat. Appl. 5,210,338, 1993.
(2) Vogt, D. In Applied Homogeneous Catalysis with Organometallic
Compounds, 2nd ed.; Cornils, B.; Herrmann, W. A., Eds.; Wiley-VCH:
Weinheim, Germany, 2002; Vol. 1, pp 240-253.
(3) A similar process using Cr as a catalyst has recently been
reported: Mani, G.; Gabba¨ı, F. P. Angew. Chem., Int. Ed. 2004, 43,
2263-2266.
(11) Pedeutour, J .-N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A.
Macromol. Rapid Commun. 2001, 22, 1095-1123.
(12) Bryliakov, K. P.; Semikolenova, N. V.; Yudaev, D. V.; Zakharov,
V. A.; Brintzinger, H. H.; Ystenes, M.; Rytter, E.; Talsi, E. P. J .
Organomet. Chem. 2003, 683, 92-102.
(13) Zakharov, I. I.; Zakharov, V. A. Macromol. Theory Simul. 2002,
11, 352-358.
(4) Millward, D. B.; Cole, A. P.; Waymouth, R. M. Organometallics
2000, 19, 1870-1878.
(14) Bochmann, M.; Lancaster, S. J . J . Organomet. Chem. 1995, 497,
55-59.
(15) Bochmann, M.; Lancaster, S. J . Angew. Chem., Int. Ed. Engl.
1994, 33, 1634-1637.
(5) Huo, S.; Shi, J .-C.; Negishi, E.-I. Angew. Chem., Int. Ed. 2002,
41, 2141-2143.
10.1021/om0495168 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/18/2004

5106 Organometallics, Vol. 23, No. 22, 2004
Communications
Ta ble 1. Ra te Con sta n ts for Dissocia tion of AlMe3
fr om [2][B(C6F 5)4]
[2]
(mM)
[AlMe3]
(mM)
koff
(M-1 s-1
)
[Cp2Ti(µ-Me)2AlMe2]+ (2a )
[Cp2Zr(µ-Me)2AlMe2]+ (2b)
1.1
0.5
2.2
1.1
1.1
0.5
1.1
1.1
1.1
1.0
1.0
0.5
1.7
1.7
2.5
2.1
1.1
1.2
1.0
0.8
0.8
1.8
0.9
0.9
2.9(6)
3.3(1)
0.113(8)
0.141(9)
0.126(5)
0.118(6)
0
0.018(9)
0.010(9)
0.18(1)
0.15(1)
0.17(1)
[Cp2Hf(µ-Me)2AlMe2]+ (2c)
[(EBTHI)Zr(µ-Me)2AlMe2]+ (2d )
[(EBI)Zr(µ-Me)2AlMe2]+ (2e)
coordinatively unsaturated cation (such as the zir-
conocene methyl cation 1b in eq 2). Such cations can
F igu r e 1. 1H NMR spectrum of [2e][B(C6F5)4] + (AlMe3)2
in C6D6 at room temperature. The peak marked with the
letter a corresponds to (AlMe3)2, that with the letter b to
[(EBI)Zr(µ-Me)2Al(Me)2][B(C6F5)4], and that with the letter
c to [(EBI)Zr(µ-Me)2Al(Me)2][B(C6F5)4]. The asterisk denotes
C6D5H, and the letter x denotes MeCPh3. The remaining
peaks are those of the EBI ligand.
be trapped by aluminum alkyls (such as AlMe3 in eq 2)
to form adducts such as 2, which must dissociate and
reform the cation (e.g., 1b) before olefin insertion can
continue. The rate at which the adduct dissociates (rate
constant koff in eq 2) restricts the rate at which an alkyl
ligand can be transferred from M to Al.
Sch em e 1. Obser ved Exch a n ge of F r ee a n d
Coor d in a ted AlMe3
The equilibrium in eq 2 generally lies to the right.
No free cation 1 is observed in the proton NMR
spectrum for any of the systems 2 in Table 1 once 1
equiv of AlMe3 has been added. We have therefore
measured koff, the rate constant for regeneration of the
catalytically active cations 1.
Other exchange processes can complicate the prepa-
ration of 2. When the methyl cation 1b was generated
from Cp2ZrMe2 and B(C6F5)3, the addition of AlMe3 gave
rapid redistribution to BMe3,16 Cp2Zr(CH3)C6F5, and an
unidentified Al species (perhaps Al(C6F5)3).17-21 (The
reported synthesis of Al(C6F5)3 involves the reaction of
B(C6F5)3 and AlMe3.22) Use of [Ph3C][B(C6F5)4]23-25 with
Cp2ZrMe2 gave 1b and then with AlMe3 gave 2b cleanly;
therefore, [Ph3C][B(C6F5)4] was used to prepare 1 and
2 in all other cases.
exchange was slow on the time scale of a conventional
NMR experiment. However, a 2-D EXSY experiment
confirmed exchange between the methyl groups of 2 and
free AlMe3.26 We quantified the exchange (difficult for
a three-site problem) with selective population inversion
experiments, which can theoretically be used to find rate
constants in systems involving any number of exchang-
ing sites.27 We observed no direct exchange between the
terminal and bridging groups of 2 (with or without
excess AlMe3).
The mechanism in Scheme 1 requires the coordination
of another molecule of AlMe3 after the dissociation of
2. If the dissociation of 2 is indeed first order, the
transfer of magnetization from the methyl groups of 2
to those of free AlMe3 should be independent of [AlMe3]
and should occur (depending on which signal is inverted)
with a rate constant equal to or twice the rate constant
We began by looking for exchange between free AlMe3
and the “coordinated” AlMe3 in 2. We observed separate
1H NMR resonance signals for each of the methyl groups
of 2 and for free AlMe3 (see Figure 1), implying that
(16) BMe3 appears to be the unidentified species that Shaughnessy
and Waymouth31 generated from the reaction of Cp*2ZrMe2 or (NMInd)2-
ZrMe2 with B(C6F5)3 and then AlMe3. Al/B Me/C6F5 exchange is facile
after methyl abstraction by B(C6F5)3.
(17) Bochmann, M.; Sarsfield, M. J . Organometallics 1998, 17,
5908-5912.
(18) Wondimagegn, T.; Xu, Z.; Vanka, K.; Ziegler, T. Organometallics
2004, 23, 3847-3852.
(19) Hawrelak, E. J .; Deck, P. A. Organometallics 2003, 22, 3558-
3565.
(20) Klosin, J .; Roof, G. R.; Chen, E. Y.-X.; Abboud, K. A. Organo-
metallics 2000, 19, 4684-4686.
(21) Kim, J . S.; Wojcinski, L. M.; Liu, S.; Sworen, J . C.; Sen, A. J .
Am. Chem. Soc. 2000, 122, 5668-5669.
(22) Lee, C. H.; Lee, S. J .; Park, J . W.; Kim, K. H.; Lee, B. Y.; Oh,
J . S. J . Mol. Catal. A-Chem. 1998, 132, 231-239.
(23) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391-1434.
(24) Chien, J . C. W.; Tsai, W. M.; Rausch, M. D. J . Am. Chem. Soc.
1991, 113, 8570-8571.
(25) Song, F.; Cannon, R. D.; Lancaster, S. J .; Bochmann, M. J . Mol.
Catal. A-Chem. 2004, 218, 21-28.
(26) Bochmann reported15 “no evidence for...intermolecular exchange
between [2b and 2c]...and Al2Me6” in CD2Cl2 at 25 °C, apparently from
the absence of line broadening in ordinary 1H NMR spectra.
(27) Bain, A. D.; Cramer, J . A. J . Phys. Chem. 1993, 97, 2884-2887.

Communications
Organometallics, Vol. 23, No. 22, 2004 5107
Sch em e 2. Ca r boa lu m in a tion of Allylben zen e
4-methyl-1-pentene, and diallylsilane with rac-(EBTHI)-
ZrMe2/B(C6F5)3/AlMe3 in dichloroethane. While they did
report a 78% yield of 4 from allylbenzene with the same
catalyst and solvent (Scheme 2), we see no carboalumi-
nation activity (<5% reaction to form 3) from allylben-
zene with 1d (9 × 10-4 M) and AlMe3 (9 × 10-3 M) in
C6D6.
The relatively large koff value for 2e suggests that the
EBI cation 1e will be a much better carboalumination
catalyst. Indeed, we observe a kobs value of 3.4 × 10-4
for dissociation of 2. Selective inversion of the terminal
methyl peak of 2, and appropriate analysis of the return
of magnetization to equilibrium,28 gave the rate con-
stants in Table 1; the measured 2fAlMe3 rate con-
stants, which are independent of [AlMe3] as predicted,
must be koff. (The AlMe3 monomer/dimer equilibrium is
rapidly maintained on the time scale of methyl exchange
between Zr and Al.) Because 1 cannot be directly
observed, and [1] cannot therefore be determined, it is
impossible to determine kon.
The decrease of the dissociation rate constants koff in
the order Ti > Zr > Hf is reasonable if we think of the
dissociation of 2 as breaking a metal-ligand bond. It is
more surprising that the koff values decrease in the order
ethylenebis(indenyl) (EBI) (2e) > Cp (2b) > ethylenebis-
(tetrahydroindenyl) (EBTHI) (2d ). A variety of spectro-
scopic and thermodynamic data imply that bridged
indenyl ligands are less effective donors than cyclo-
pentadienyl ones,29 and the reduction potentials of the
dichlorides decrease in the order (EBI)ZrCl2 > Cp2ZrCl2
> (EBTHI)ZrCl2;30 therefore, the electrophilicities of the
alkylmetallocene cations 1 decrease in the order EBI >
Cp > EBTHI.
s-1 for the disappearance of allylbenzene, with [Zr] ) 7
× 10-4 M and [Al] ) 4 × 10-3 M in C6D6 at 300 K. This
result and the rigidity of the 1e/2e system make
optically active 1e/2e attractive as a catalyst for asym-
metric carboalumination.
Ackn owledgm en t. We thank SASOL North America
Inc. for their support of this work, Boulder Scientific
for donating many of the compounds used, Prof. Richard
J ordan for helpful discussions, and Prof. Alex Bain for
providing CIFIT as well as technical assistance with its
operation. This work was supported by the National
Science Foundation (Grant No. CHE-02-1131).
Note Ad d ed in P r oof. In a paper that has just
appeared (Britovsek, G. J . P.; Cohen, S. A.; Gibson, V.
C.; van Meurs, M. J . Am. Chem. Soc. 2004, 126, 10701-
10712) a “dissociative or dissociative-interchange mech-
anism” has been called “most likely” for the substitution
reactions of heterobimetallics like 2 with ethylene or a
different AlR3.
The fact that the koff value for the EBTHI cation 2d
is negligible is consistent with the fact that 2d is a poor
carboalumination catalyst. Shaughnessy and Way-
mouth31 have reported “poor selectivity for carbo-
metallated products” from 1,5-hexadiene, 1-hexene,
Su p p or tin g In for m a tion Ava ila ble: Text and figures
giving general experimental procedures and details of selective
population inversion experiments, including data analysis
using CIFIT. This material is available free of charge via the
Internet at http://pubs.acs.org.
(28) Bain, A. D. CIFIT; Chemistry Department, McMaster Univer-
sity, 2003.
(29) (a) Beck, S.; Brintzinger, H. H. Inorg. Chim. Acta 1998, 270,
376-381. (b) Wieser, U.; Babushkin, D.; Brintzinger, H.-H. Organo-
metallics 2002, 21, 920-923.
(30) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin,
G.; Brandow, C. G.; Bercaw, J . E.; J ardine, C. N.; Lyall, M.; Green, J .
C.; Keister, J . B. J . Am. Chem. Soc. 2002, 124, 9525-9546.
OM0495168
(31) Shaughnessy, K. H.; Waymouth, R. M. Organometallics 1998,
17, 5728-5745.