CH bond activation in cations of the type {[(2,4,6-Me3 C6H2NCH2CH2)2NMe] ZrR}+ and a simple solution that yields a catalyst for the living polymerization of 1-hexene

Article abstract of DOI:10.1021/om000972f

{[(MesNCH₂CH₂)₂NMe]ZrMe} [B(C₆F₅)₄] and intermediates in the polymerization reaction of 1-hexene that are formed from it decompose as a consequence of CH activation in an ortho methyl group

Full text of DOI:10.1021/om000972f

1056
Organometallics 2001, 20, 1056-1058
CH Bon d Activa tion in Ca tion s of th e Typ e
{[(2,4,6-Me3C6H2NCH2CH2)2NMe]Zr R}+ a n d a Sim p le
Solu tion th a t Yield s a Ca ta lyst for th e Livin g
P olym er iza tion of 1-Hexen e
Richard R. Schrock,* Peter J . Bonitatebus, J r., and Yann Schrodi
Department of Chemistry and the Center for Materials Science and Engineering,
Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139
Received November 15, 2000
Summary: {[(MesNCH2CH2)2NMe]ZrMe}[B(C6F5)4] and
intermediates in the polymerization reaction of 1-hexene
that are formed from it decompose as a consequence of
CH activation in an ortho methyl group in the mesityl
substituent. The intermediates in the polymerization
reaction decompose significantly more readily than does
{[(MesNCH2CH2)2NMe]ZrMe}[B(C6F5)4]. On the other
hand, analogous cationic complexes that contain the
[(2,6-Cl2C6H3NCH2CH2)2NMe]2- ligand are relatively
stable and will consume 1-hexene in a strictly first-order
and apparently living manner at 0 °C in chlorobenzene.
the polymerization of ordinary olefins, in particular
living polymerizations.20-21 Some of the most readily
accessible ligands in this category are of the type
[(ArNCH2CH2)2D]2-, where D ) O,22 S,22 NH,23,24 or
NR23,24 and Ar is a sterically protected aryl such as 2,6-
i-Pr2C6H3 or 2,4,6-Me3C6H2 (Mes). However, all poly-
merization systems based on such ligands that we have
examined so far appear to suffer from some termination
step in polymerization of 1-hexene that does not involve
formation of olefinic end groups. In this communication
we show that deactivation consists of CH bond activa-
tion in a mesityl ortho methyl group in complexes of
the type {[(MesNCH2CH2)2NMe]ZrR}+ and that, in
contrast, {[(2,6-Cl2C6H3NCH2CH2)2NMe]ZrR}+ is a highly
active living catalyst for the polymerization of 1-hexene.
We have reported that {[(MesNCH2CH2)2NMe]ZrMe}-
[B(C6F5)4] (1) will initiate the polymerization of 1-hexene
in chlorobenzene, but the molecular weight is limited
and the polydispersity is not what one would expect
from a living polymerization.24 Since no olefinic end
groups are observed, â-hydride elimination within the
growing polymer chain does not appear to be respon-
sible, at least not to a significant and detectable
degree.25 We have found that 1 decomposes in bro-
mobenzene in a first-order manner (k20°C ) 6.0 × 10-5
s-1 at [Zr]0 ) 19.0 and 38.0 mM; k60°C ) 2.1 × 10-3 s-1
at [Zr]0 ) 38.0 mM) to give methane and an insoluble
species which immediately oils out of solution and
ultimately crystallizes as a dark orange material in 85%
yield. X-ray diffraction showed that this species is the
dimeric dication shown in Figure 1. We propose that
this compound forms by dimerization of some solvated
version of the monomeric monocation (2) shown in eq
1. Apparently the configuration of the ligand in 2 leaves
the Zr exposed to binding of the arene ring (whose
methyl group has been attacked) in the “apical” position
We are interested in exploring and developing dia-
mido/donor ligands for early transition metal chemistry.
A diamido/donor ligand is one variation within a large
class of diamido ligands that have been employed for
early metal chemistry.1-19 One of the important ap-
plications of diamido/donor ligands in our laboratory is
the synthesis of Zr and Hf cations that are active for
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A. J .; McPartlin, M. Chem. Ber. Rec. 1997, 130, 1751.
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(19) Fryzuk, M. D.; Love, J . B.; Rettig, S. J . Organometallics 1998,
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(20) Baumann, R.; Schrock, R. R. J . Organomet. Chem. 1998, 557,
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(21) Schrock, R. R.; Baumann, R.; Reid, S. M.; Goodman, J . T.;
Stumpf, R.; Davis, W. M. Organometallics 1999, 18, 3649.
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(25) In contrast, zirconium complexes that contain [(RN-o-C6H4)2O]2-
ligands (R ) isopropyl, cyclohexyl, or mesityl) appear to be catalysts
only for the oligomerization of 1-hexene as a consequence of â hydride
elimination.
10.1021/om000972f CCC: $20.00 © 2001 American Chemical Society
Publication on Web 02/10/2001

Communications
Organometallics, Vol. 20, No. 6, 2001 1057
merization intermediates when 1‚PhNMe2 is the initia-
tor is due to CH bond activation similar to that shown
in eq 1. Interestingly, it is clear that these intermediates
decompose considerably faster than does 1 (k0°C ) 5.0
× 10-4 s-1 compared to k20°C ) 6.0 × 10-5 s-1 for 1). We
suggest that the larger alkyl chain keeps the base
(dimethylaniline, bromobenzene, or [B(C6F5)4]-) further
away from the zirconium cationic center, thereby al-
lowing a faster intramolecular CH bond activation by
the lower coordinate metal center. The dimethyaniline
adduct of 1 was found to decompose in a first-order
manner at 60 °C (k60°C ) 1.3 × 10-4 s-1 at [Zr]0 ) 21.0
mM) to give a dimethylaniline adduct of 2, although
identification of that adduct was based solely on NMR
studies.23
trans to the amine donor. This structure is related to
that proposed (on the basis of NMR studies) for the
product of CH activation in an amido TMS methyl group
in {[(TMSNCH2CH2)2NTMS]ZrCH2Ph}[PhCH2B(C6F5)3],
in which the anion forms a π arene complex through
the benzyl’s phenyl ring.10
To eliminate CH activation, but provide approxi-
mately the same steric protection of the amido nitro-
gens, we considered replacing the mesityl group with a
2,6-Cl2C6H3 group. The reaction between commercially
available 2,6-dichlorobromobenzene and (H2NCH2CH2)2-
NH under conditions employed for the synthesis of
(MesNHCH2CH2)2NH proceeded smoothly in 5 days at
100 °C to give (ArClNHCH2CH2)2NH (ArCl ) 2,6-
Cl2C6H3) in 80% yield. The proton on the central
nitrogen can be replaced by a methyl group in 60% yield
under the conditions shown in eq 2. The (ArClNHCH2-
Polymerization reactions of 1-hexene initiated by 1
are too rapid to be monitored readily by NMR methods.
(Approximately 250 equiv of 1-hexene are polymerized
within ∼5 min by 1 (0.25 mM) at 0 °C in bromobenzene.)
Addition of 20 equiv of 1-hexene to a solution of 1 in
bromobenzene at -30 °C followed by warming to 20 °C
gave crystals of the dimer of 2 in high yield, according
to X-ray crystallography (unit cell match). Therefore the
intermediates in the polymerization reaction also de-
compose by the same CH activation process.
1-Hexene is polymerized by {[(MesNCH2CH2)2NMe]-
ZrMe(PhNMe2)}[B(C6F5)4] (1‚PhNMe2), which is formed
upon addition of [PhNHMe2][B(C6F5)4] to [(MesNCH2-
CH2)2NMe]ZrMe2 at 0 °C in bromobenzene, but at a rate
that is slow enough to be followed readily by NMR
methods. A plot of ln[1-hexene] versus time in this
reaction shows considerable curvature. The rate of
consumption can be modeled assuming that a concomi-
tant first-order decomposition of intermediates in the
polymerization reaction is responsible for the curvature.
The rate constant for the first-order and irreversible
decomposition of the intermediates in the polymeriza-
tion reaction obtained in this manner is k0°C ) 5.0 ×
10-4 s-1. We propose that the decomposition of poly-
CH3I, K2CO3
(ArClNHCH2CH2)2NH
8
CH3CN
(ArClNHCH2CH2)2NMe
60%
(2)
CH2)2NMe ligand then can be added to Zr(NMe2)4 to
produce [(ArClNCH2CH2)2NMe]Zr(NMe2)2 in 90% yield,
which upon treatment with Me3SiCl gives [(ArClNCH2-
CH2)2NMe]ZrCl2 in 90% yield. Alkylation of [(ArClNCH2-
CH2)2NMe]ZrCl2 with MeMgBr in diethyl ether then
affords [(ArClNCH2CH2)2NMe]ZrMe2 (3) in 70% yield.
(See Supporting Information for all experimental de-
tails.) All reactions are identical to those employed to
prepare (MesNHCH2CH2)2NMe and analogous Zr com-
plexes that contain the [(MesNHCH2CH2)2NMe]2-
ligand.23 An X-ray structure26 of [(ArClNCH2CH2)2NMe]-
Zr*Me2 (3*; *Me ) 13CH3) showed it to be isostructural
with [(MesCH2CH2)2NMe]ZrMe2.23,24 (See Supporting
Information for crystallographic details.)
The reaction between [(ArClNCH2CH2)2NMe]Zr*Me2
and [PhNMe2H][[B(C6F5)4] in bromobenzene at -20 °C
yielded a bright yellow solution, proton and 13C NMR
spectra of which are consistent with formation of a
cationic species that has mirror symmetry that we
propose is {[(ArClNCH2CH2)2NMe]Zr*Me(PhNMe2)]-
[B(C6F5)4] (4). The methyl resonance is found at -0.03
ppm in the proton NMR spectrum (J CH ) 117 Hz) and
37.80 ppm in the carbon NMR spectrum at -14 °C.
Compound 4 is stable for hours at 0 °C. Addition of
F igu r e 1. ORTEP diagram of the dimer of 2. Selected bond
distances (Å): Zr(1)-N(1) ) 2.068(7); Zr(1)-N(3) ) 2.190-
(6); Zr(1)-C(1) ) 2.251(7); Zr(1)-N(2) ) 2.355(7); Zr(1)-
C(3) ) 2.608(8); Zr(1)-C(2) ) 2.617(7); Zr(1)-C(7) )
2.671(8); Zr(1)-C(4) ) 2.722(7); Zr(1)-C(6) ) 2.854(8).
Selected bond angles (deg): N(1)-Zr(1)-N(3) ) 132.8(3);
N(1)-Zr(1)-C(1) ) 98.6(3); N(3)-Zr(1)-C(1) ) 73.9(2);
N(1)-Zr(1)-N(2) ) 74.2(3); N(3)-Zr(1)-N(2) ) 70.8(2);
C(1)-Zr(1)-N(2) ) 120.4(3).
(26) Crystal data and structure refinement details for [ArClN2NMe]-
Zr(13Me)2‚THF (3*): a ) 8.186(4) Å, b ) 12.142(7) Å, c ) 13.747(6) Å,
R ) γ ) 90°, â ) 95.17(4)°, V ) 1360.9(12) Å3, monoclinic P21/m, T )
183(2) K, λ ) 0.71073 Å, Z ) 2, Fcalcd ) 1.449 g/cm3, µ ) 0.816 mm-1
,
F(000) ) 603, 2θ range ) 5.00-44.96°, limiting indices -4ehe8,
-10eke8, -13ele14, no. of reflections collected/unique ) 2380/1568,
Rint ) 0.0162, FMLS on F2 refinement method, no. of data/restraints/
parameters ) 1568/0/170, GoF on F2 ) 1.051, final R indices [I>2σ(I)]
are R1 ) 0.0399, wR2 ) 0.1061 and R indices (all data) are R1 ) 0.0417,
wR2 ) 0.1080 with largest difference peak and hole ) 0.508 and -0.519
e Å-3
.

1058 Organometallics, Vol. 20, No. 6, 2001
Communications
Ta ble 1. P oly(1-h exen e) P r ep a r ed Usin g
{[Ar ClN2NMe]Zr *Me(P h NMe2)}[B(C6F 5)4]a
equiv
1-hexene
103 Mn
(calcd)
103 Mn
(found)
Mn
(found/calcd)
103 Mw
PDI
100
150
200
600
150b
150c
8.4
12.6
16.8
50.5
12.6
12.6
24.2
31.5
35.6
78.5
48.8
62.1
2.9
2.5
2.1
1.6
3.9
4.9
24.6
32.1
36.3
79.6
50.5
64.9
1.02
1.02
1.02
1.01
1.03
1.04
a
b
[4] ) 3 mM in chlorobenzene, 0 °C, 1 h. 4 equiv of PhNMe2
added. c 8 equiv of PhNMe2 added.
1-hexene (50 equiv) at 0 °C leads to its consumption in
strictly a first-order manner with an observed first-order
rate constant of k0°C ) 2.5 × 10-3 s-1. According to NMR
spectroscopy the initiator ([Zr]0 ) 3.0 mM) is completely
consumed only when the amount of 1-hexene exceeds
200 equiv. The PDI values are uniformly low for all
polymers in bulk polymerizations, and the measured
molecular weights (by light scattering) are closest to
those expected for a living polymerization (in which kp
is of the same order as ki) for the highest number of
equivalents of 1-hexene (see Table 1, Mn(found/calcd)
) 1.6 for 600 equiv of 1-hexene).
F igu r e 2. Competitive inhibition of 1-hexene polymeri-
zation (50 equiv) by PhNMe2 using {[ArClN2NMe]Zr*Me-
(PhNMe2)}[B(C6F5)4] (3 mM) in C6D5Br at 0 °C (followed
by 1H NMR). For 16 equiv of PhNMe2 kobsd ) 2.78(3) × 10-4
s-1; for 8 equiv of PhNMe2 kobsd ) 5.37(6) × 10-4 s-1; for 4
equiv of PhNMe2 kobsd ) 7.78(6) × 10-4 s-1
.
Addition of free dimethylaniline (4-16 equiv) to
{[(ArClNCH2CH2)2NMe]Zr*Me(PhNMe2)}[B(C6F5)4] be-
fore 1-hexene is added leads to increasingly slower rates
of consumption of 1-hexene (Figure 2), consistent with
competitive inhibition of 1-hexene polymerization by
dimethylaniline. A simple treatment of the data shown
in Figure 2 reveals that Keq for formation of the
dimethylaniline adduct of the cationic complex formed
during 1-hexene polymerization is ∼160 M-1, while the
rate constant for the reaction between the “base-free”
cationic propagating species and 1-hexene is 0.80 M-1
s-1 at 0 °C.27 The more dimethylaniline that is added,
the greater amount of {[(ArClNCH2CH2)2NMe]Zr*Me-
(PhNMe2)][B(C6F5)4] that remains after polymerization
is complete and the greater the disparity between the
observed Mn and that calculated for a perfect living
polymerization in which initiation and propagation are
roughly comparable in rate. For example Mn(found/
calcd) values increase from 2.5 in a run in which no
dimethylaniline has been added to 3.9 in a run in which
4 equiv of PhNMe2 has been added, to 4.9 in a run in
which 8 equiv of PhNMe2 has been added (Table 1). We
propose that dimethylaniline binds more strongly to the
methyl initiator than to the metal in the complex that
contains the growing polymer chain (for steric reasons)
and that the dimethylaniline that is “liberated” upon
initiation represses dissociation of dimethylaniline from
the remaining initiator, thereby effectively preventing
further initiation. For this reason, the polydispersities
of the resulting polymers are uniformly low and the
molecular weights approach those expected for a well-
behaved living process only at high initial monomer
concentrations.
The findings reported here suggest that CH activation
in a mesityl ortho methyl group in an intermediate of
the type {[(MesNCH2CH2)2NMe]ZrR}[B(C6F5)4] is a
termination step in a 1-hexene polymerization reaction
and that comparable polymerizations by a catalyst that
contains a 2,6-dichlorophenyl group are well-behaved.
Use of 2,6-dichlorophenyl groups might be useful in
other circumstances in which 2,6-disubstituted aryl
groups bound to amido nitrogen ligands have been
employed in the last several years, at least those in
which CH activation might be taking place. At this stage
we do not know the extent to which the 2,6-dichlorophen-
yl group in the cations discussed here might actually
stabilize cations via “light” (or transiently dative)
coordination of the chloride to the metal or the degree
to which electronic differences between the 2,6-dichlo-
rophenyl group and the mesityl group might contribute
to the high activity of the 2,6-dichlorophenyl catalysts.
Ack n ow led gm en t. R.R.S. thanks the Department
of Energy (DE-FG02-86ER13564) and the MRSEC
program of the National Science Foundation (DMR 98-
08941) for supporting this research.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and fully labeled ORTEP drawings, atomic coordinates,
bond lengths and angles, and anisotropic displacement pa-
rameters for the dimer of 2 and 3* are available free of charge
via the Internet at http://pubs.acs.org.
(27) If [cat] + [cat(B)] ) [Zr]T (where cat ) {[(ArClNCH2CH2)2NMe]-
ZrR}[B(C6F5)4], cat(B) ) {[(ArClNCH2CH2)2NMe]ZrR(PhNMe2)}[B(C6F5)4],
R is the growing polymer chain, and B is dimethylaniline), and the
equilibrium between cat and B to give cat(B) is fast in both directions,
then [Zr]T/k(obs) ) 1/k + K[B]/k where k is the rate constant for the
reaction between cat and 1-hexene and K ) [cat(B)]/[cat][B]. A plot of
[Zr]T/k(obs) vs [B] gives 1/k and K/k.
OM000972F