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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(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