Synthesis of a new olefin polymerization catalyst supported by an sp 3-C donor via insertion of a ligand-appended alkene into the Hf-C bond of a neutral pyridylamidohafnium trimethyl complex

Article abstract of DOI:10.1039/b811384j

A new living and isoselective propylene polymerization precatalyst was generated via the intramolecular insertion of a ligand-appended vinyl group into the Hf-C bond of a neutral pyridylamidohafnium trimethyl complex. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811384j

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Synthesis of a new olefin polymerization catalyst supported by an sp3-C
donor via insertion of a ligand-appended alkene into the Hf–C bond
of a neutral pyridylamidohafnium trimethyl complexw
Gregory J. Domski, Joseph B. Edson, Ivan Keresztes, Emil B. Lobkovsky
and Geoffrey W. Coates*
Received (in Cambridge, UK) 4th July 2008, Accepted 17th September 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b811384j
A new living and isoselective propylene polymerization precatalyst
was generated via the intramolecular insertion of a ligand-
appended vinyl group into the Hf–C bond of a neutral pyridyl-
amidohafnium trimethyl complex.
set out to prepare a pyridylamidohafnium trimethyl complex
bearing a vinyl group in the ligand framework. We initially
targeted the pyridylamidohafnium trimethyl complex
(1, Fig. 1) envisioning that methide abstraction with B(C6F5)3
would generate the pyridylamidohafnium dimethyl cation,
which would subsequently undergo intramolecular migratory
insertion of the vinyl group. Geometry optimization calcula-
tions indicate that the pathway leading to 1,2-insertion of this
compound is energetically less favorable than 2,1-insertion. A
2,1-insertion would be interesting in its own right since it could
lead to the generation of a polymerization catalyst bearing a
stable Z2-CR2Ar moiety bound to the active metal center.
Treating the pyridylamidohafnium trichloride complex
(Fig. 1) with 3.7 equivalents of MeMgBr furnished the crude
product as a yellow-orange foam-like solid. Characterization
of the crude product by 1H NMR spectroscopy revealed a
15 : 85 mixture of products formulated as the expected pyridyl-
amidohafnium trimethyl complex (1) and a diastereomeric
mixture of pyridylamidohafnium dimethyl complexes (rac-
2a,b). Heating a C6D6 solution of this mixture at 80 1C
overnight led to the disappearance of the vinyl proton reso-
nances and complete conversion to rac-2a,b. The 1H NMR
spectrum revealed two sets of two singlets corresponding to
the Hf–CH3 groups, which are consistent with the formulation
of the product as a mixture of diastereomers. Characterization
of the product via 13C{1H} NMR reveals 70 resonances, 35
corresponding to each diastereomer. Single crystals suitable
for structural determination via X-ray crystallography were
grown from a saturated, pentane-layered toluene solution over
several days at À30 1C. The X-ray structure (Fig. 2) revealed
Recent advances in non-metallocene olefin polymerization
catalysts1 allow unprecedented access to new and useful
polyolefin architectures. In particular, advances in living
olefin polymerization,2 and more recently, chain shuttling
polymerization,3 have allowed researchers to prepare olefin-
based block copolymers that exhibit promising material
properties.3a,4 One of the most exciting classes of catalysts to
emerge in the past decade is derived from the C1-symmetric
arylpyridylamidohafnium complexes ((APA)HfMe2, R =
2-iPrC6H4, Fig. 1) developed by Dow and Symyx.5 For
propylene polymerization several catalysts in this family are
highly isoselective, thermally robust, and are capable of
producing high molecular weight (Mw 4 105 g molÀ1) poly-
propylene (PP). Detailed mechanistic studies of the C1-sym-
metric (APA)HfMe2 complex by Froese et al. have shown that
1,2-insertion of an olefin into the Hf–CAryl bond generates an
sp3-hybridized carbon donor atom that supports the active
metal center rather than participating in further olefin inser-
tion.5b We have shown that the catalyst derived from a
Cs-symmetric (APA)HfMe2 complex (R
= H, Fig. 1)
furnished moderately isotactic PP (iPP).6 Statistical analysis
of the stereoerrors in the iPP sample via 13C{1H} NMR
spectroscopy revealed that, quite unexpectedly, an enantio-
morphic site control mechanism of isotactic monomer
enchainment was operative for this catalyst.7 These observa-
tions suggest that the isoselectivity exhibited by catalysts
derived from the Cs-symmetric (APA)HfMe2 complex might
result from the 1,2-insertion of an a-olefin into the Hf–CAryl
bond generating a C1-symmetric catalyst supported by an
sp3-hybridized carbon donor atom.
With the goal of incorporating an ancillary sp3-C donor into
the ligand framework of an olefin polymerization catalyst, we
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell University, Ithaca, NY 14853-1301, USA.
E-mail: gc39@cornell.edu; Fax: +1 607 255-4137;
Tel: +1 607-255-5447
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis and characterization of rac-2a,b and precur-
sors, polymerization procedures and X-ray data. CCDC 693767. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b811384j
Fig. 1 Synthesis of complexes 2a and 2b.
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Fig. 2 ORTEP diagram of rac-2a. Thermal ellipsoids are drawn at
the 40% probability level. The unlabelled atoms are carbon.
Fig. 3 Plot of Mn (K) and Mw/Mn (’) versus polymer yield for
propylene polymerization at 20 1C catalyzed by rac-2a,b/B(C6F5)3.
that the coordination geometry about hafnium is best de-
scribed as distorted trigonal bipyramidal with C(34) and
N(1) occupying the axial positions (C(34)–Hf(1)–N(1) =
167.1(2)1). From the short Hf(1)–C(4) distance (2.684(7) A),
it is apparent that the 1-(naphthalen-2-yl)propyl moiety is
bound in an Z2-fashion.8 Inspection of the extended unit cell
revealed the presence of only one diastereomer. However, the
1H NMR spectrum of the redissolved single crystals revealed
the presence of both diastereomers in the same ratio (61 : 39)
as that observed in the crude product. This result suggests that
an equilibrium exists for the mixture of diastereomers in
solution; it is possible to envision interconversion of the
diastereomers via a ring-flip of the six-membered metalla-
cycle.9 These results represent a rare example of olefin inser-
tion into the metal–alkyl bond of a neutral group IV metal
complex,10–13 and to the best of our knowledge represents the
only report of intramolecular insertion of an alkene into a
Hf–CAlkyl bond for a neutral hafnium alkyl compound.
Although we cannot rule out Grignard attack of the alkene
followed by chloride displacement on the hafnium, it is
unlikely, since treating the 15 : 85 mixture of 1 : rac-2a,b with
a further 3.7 equivalents of MeMgBr led to no decrease in
the intensity of the 1H NMR resonances corresponding to the
vinyl protons of 1. Additionally, the formation of 1 in the
crude reaction mixture and subsequent conversion to rac-2a,b
upon heating as monitored by 1H NMR spectroscopy suggests
a migratory insertion of the alkene.
ligand derived from rac-2a,b was the exclusive small molecule
product thereby indicating that the Hf–C(sp3) bond remains
intact throughout the course of polymerization. Therefore,
rac-2a,b/B(C6F5)3 represents a new, highly isoselective catalyst
for 1-hexene polymerization.
We proceeded to investigate the propylene polymerization
behavior of rac-2a,b/B(C6F5)3. Several polymerizations of
varying duration were conducted at 20 1C under 2 atm of
propylene. The Mn of the PP increased linearly with increasing
polymer yield over the course of 45 min while the molecular
weight distribution remained narrow (Mw/Mn r 1.05) (Fig. 3).
The average turnover frequency (TOF) was 2800 hÀ1
,
which is significantly lower than that observed for the analo-
gous ortho-metalated precatalyst under identical conditions
(25 000 hÀ1).14 Analysis of the PP via 13C{1H} NMR spectro-
scopy revealed that the polymer was isotactic ([m4] = 80%).
The degree of isotacticity was also somewhat lower than that
of the iPP produced by the analogous ortho-metalated pre-
catalyst ([m4] = 91%).14 Statistical analysis of the methyl
region revealed a 2 : 2 : 1 ratio of peaks corresponding to
[mmmr] : [mmrr] : [mrrm] stereoerrors which indicated that an
enantiomorphic site-controlled mechanism of isotactic mono-
mer enchainment was operative.7 It is unclear at this time
whether the rate of interconversion between diastereomers is
similar to the rate of propagation, however, if it is, it does not
result in a change in the enantiofacial selectivity of the catalyst
since no resonance attributable to the mmrm stereoerror was
observed. The 13C{1H} NMR spectrum (Fig. 4) also revealed
resonances due to regioerrors (ca. 9 mol%) arising from head-
to-head or tail-to-tail misinsertions5a which may explain the
relatively low melting temperature (Tm = 120 1C relative to
123 1C for a regioregular iPP with a similar degree of
isotacticity).15a The catalyst rac-2a,b/B(C6F5)3 is a rare exam-
ple of one that is both living and isoselective for propylene
polymerization.4,6,15
Addition of B(C6F5)3 to a solution of rac/2a,b and 1-hexene
(1 : 1 : 580) in toluene at 25 1C led to the formation of 0.48 g of
poly(1-hexene) (62% conversion) in 30 minutes. The polymer
possessed an Mn = 49 000 g molÀ1 and a relatively narrow
molecular weight distribution (Mw/Mn = 1.85). Analysis of
the poly(1-hexene) microstructure via 13C{1H} NMR spectro-
scopy revealed that the polymer was regioregular and highly
isotactic; the spectrum revealed no resonances corresponding
to stereoerrors (see ESIw). It is likely that the high degree of
stereoselectivity observed for this catalyst is due to the chiral
structure of rac-2a,b. Analysis of the organic residues isolated
after a 1-hexene polymerization revealed that the protonated
Since rac-2a,b contains three hafnium-bound alkyl groups
(two methyl groups, and one 2-naphthyl-1-propyl group), it
was necessary to determine which of these were being
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Fig. 4 13C{1H} NMR spectrum (125 MHz, 1,1,2,2-C2D2Cl4, 135 1C)
of iPP produced by rac-2a,b/B(C6F5)3 at 20 1C.
abstracted by B(C6F5)3. The reaction between rac-2a,b and
B(C6F5)3 was monitored using 1H NMR spectroscopy. The
spectrum revealed that two of the four resonances correspond-
ing to Hf–CH3 groups had disappeared. The two remaining
Hf–CH3 resonances appeared at À0.25 and À0.44 ppm in a
70 : 30 ratio. Two broad, overlapping resonances appeared at
1.75 ppm which were assigned to [(C6F5)3BCH3]À. Importantly,
no resonance attributable to (C6F5)3B–CH(CNaphthyl)(Et) was
present. These observations suggest that rac-2a,b reacts with
B(C6F5)3 via methide abstraction to produce a diastereomeric
mixture of pyridylamidohafnium methyl cations.
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We set out to synthesize 1 and subsequently generate the
corresponding pyridylamidohafnium dimethyl cation via
methide abstraction. Unexpectedly, intramolecular 2,1-inser-
tion of the appended vinyl group into the Hf–CH3 bond
of 1 occurred, thereby furnishing rac-2a,b in a rare example
of olefin insertion into the Hf–C bond of
a neutral
hafnium–methyl complex. Upon activation with B(C6F5)3,
rac-2a,b formed an active polymerization catalyst that isose-
lectively polymerized propylene in a living fashion. The origin
of isoselectivity in these systems remains unclear; current
studies focus on modeling the chiral framework of the putative
active species. The catalyst derived from methide abstraction
of rac-2a,b represents a rare example of an olefin polymeriza-
tion catalyst supported by an sp3-C donor.
12 For an example of intramolecular olefin insertion into a Ti–vinyl
bond see: R. A. Himes, P. E. Fanwick and I. P. Rothwell, Chem.
Commun., 2003, 18–19.
13 For an example of intramolecular olefin insertion into a Zr–
benzyne see: J. H. Tidwell and S. L. Buchwald, J. Am. Chem.
Soc., 1994, 116, 11797–11810.
The authors acknowledge Mitsubishi Chemical Corpora-
tion for support.
14 G. J. Domski, PhD Thesis, Cornell University, 2008.
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