Intramolecular σ-bond metathesis/protonolysis on zirconium(IV) and hafnium(IV) pyridylamido olefin polymerization catalyst precursors: Exploring unexpected reactivity paths

Article abstract of DOI:10.1021/ic100947q

A temperature-controlled metathesis/protonolysis takes place on group 4 amidopyridinate polymerization catalyst precursors. Unraveling this unprecedented reactivity path allowed us to highlight the importance of the metal precursor choice while preparing improved catalytic structures or studying new catalytic processes.

Full text of DOI:10.1021/ic100947q

Inorg. Chem. 2010, 49, 6811–6813 6811
DOI: 10.1021/ic100947q
Intramolecular σ-Bond Metathesis/Protonolysis on Zirconium(IV) and Hafnium(IV)
Pyridylamido Olefin Polymerization Catalyst Precursors: Exploring Unexpected
Reactivity Paths
†
,†
†
†
‡
ꢀ
´
Lapo Luconi, Giuliano Giambastiani,* Andrea Rossin, Claudio Bianchini, and Agustı Lledos
†Institute of Chemistry of OrganoMetallic Compounds, ICCOM-CNR, Via Madonna del Piano 10,
‡
´
ꢁ
50019 Sesto F.no, Florence, Italy, and Departamento de Quımica, Universitat Autonoma de Barcelona,
08193 Bellaterra, Barcelona, Spain
Received May 12, 2010
A temperature-controlled metathesis/protonolysis takes place on
group 4 amidopyridinate polymerization catalyst precursors. Un-
raveling this unprecedented reactivity path allowed us to highlight
the importance of the metal precursor choice while preparing
improved catalytic structures or studying new catalytic processes.
Hf-CAr bond.4 In addition to classical transamination reac-
tions, early amido group 4 metal complexes can undergo
intramolecular σ-bond metathesis, providing unconventional
cyclometalated frameworks.1,2,4 In particular, one or more
NHMe2 groups are generated that can remain in the metal
coordination sphere5 or be definitively removed.
Herein we describe the first examples of zirconium(IV) and
hafnium(IV) amidopyridinate tautomers that reversibly inter-
convert via a temperature-controlled CAr-H/Zr-NMe2
σ-bond metathesis/Me2N-H/Zr-CAr protonolysis. We feel
that the results presented in this Communication are of broad
interest because they contribute to a better understanding of
the organometallic chemistry of MIV(NMe2)4-based com-
plexes, especially toward a rational design of metal catalysts
for polymerization reactions.
As part of our continuing interest in developing early-
transition-metal-based systems for efficient and selective
olefin upgrade,6 we have studied the reactivity of ZrIV(NMe2)4
and HfIV(NMe2)4 with a series of aminopyridinate ligands bear-
ing different aryl or heteroaryl substituents on the pyridine 6
position. We have observed that the reaction of 1with Zr(NMe2)4
proceeds smoothly in benzene at room temperature with com-
plete substrate conversion within 3 h and the loss of 1 equiv of
dimethylamine. Solvent evaporation provides a pale-yellow solid
highly soluble in aromatic and aliphatic hydrocarbons, whose
1D and 2D NMR spectroscopy (toluene-d8, 298 K) reveals the
coexistence of two distinct isomeric forms (tautomers 2 and 3) in
an about 70:30 ratio, with the minority being unambiguously
attributed to the hexacoordinated cyclometalated N2ThZr-
(NMe2)2(η1-HNMe2) species (3; Scheme 1) with a dimethyl-
amine ligand as part of the metal coordination sphere.
In conjunction with a variety of nitrogen ligands, hafnium(IV)
and zirconium(IV) tetrakis(dimethylamidates), M(NMe2)4,
are largely used as precursors to organometallic species and,
in particular, to generate olefin polymerization catalysts via
transamination reactions. Within the latter field, cyclometa-
lated hafnium(IV) complexes, stabilized by dianionic amido-
pyridinate ligands, have recently attracted much interest in
view of their outstanding catalytic performance,1-3 as well as
the wealth of their activation chemistry centered on the unusual
*To whom correspondence should be addressed. E-mail: giuliano.
giambastiani@iccom.cnr.it.
(1) (a) Boussie, R. T.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe,
A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen,
R. K.; tevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Angew.
Chem., Int. Ed. 2006, 45, 3278–3283. (b) Domski, G. J.; Lobkovsky, E. B.;
Coates, G. W. Macromolecules 2007, 40, 3510–3513. (c) Busico, V.; Cipullo, R.;
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2009, 28, 5445–5458. (d) Frazier, K. A.; Boone, H. W.; Vosejpka, P. C.; Stevens,
J. C. (Dow Global Techologies Inc.). U.S. Patent Appl. 2004/0220050 A1.
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r
2010 American Chemical Society
Published on Web 06/28/2010
pubs.acs.org/IC

6812 Inorganic Chemistry, Vol. 49, No. 15, 2010
Luconi et al.
Figure 1. Selected variable-temperature 1H NMR spectra [400 MHz,
C7D8, 4 g δ (ppm) g 1] of 2 and 3. Full spectroscopic data are provided in
the Supporting Information.
Figure 2. Selected variable-temperature 1H NMR spectra [400 MHz,
C7D8, 4 g δ (ppm) g 1] of 5 and 6. Full spectroscopic data are provided in
the Supporting Information.
Scheme 1. Synthesis of ZrIV (2-3 and 4) and HfIV (5-6 and 7) complexes
alcohols or amines.7 Therefore, the present case constitutes a
useful reference to gain further insight into C-H bond func-
tionalization reactions of early-transition-metal complexes.
Finally, a prolonged heating (24 h) of the 2-3 mixture in
refluxing toluene, combined with static vacuum cycles, yields
the amine-free species N2ThZr(NMe2)2 (4; Scheme 1) as an
analytically pure yellow-brown solid.5d
The very fast (2-3) tautomer equilibration at different
temperatures does not allow the assignment of a precise
kinetic order to the process, while a standard reaction
enthalpy (ΔH°) of 5.17 ( 0.2 kcal mol-1 has been calculated
from the van’t Hoff plot. Finally, NMR studies conducted on
the hafnium(IV) congeners 5 and 6 (Scheme 1) have shown
analogous temperature-induced tautomeric interconversion.
Indeed, temperature variation between 298 and 373 K led to
isomeric mixtures of 5 and 6 from 83:17 to 11:89 ratios,
respectively, with complete retention of reversibility after
several thermal cycles (Figure 2).
This result suggests that the initial N,N-chelation faci-
litates cyclometalation at the β position of the thienyl moiety,
by locking the heteroaryl group close to the orientation for
σ-bond metathesis of [Zr-NMe2].
Similarly to 4, complex 7 was isolated as a dark-brown
microcrystalline solid after prolonged heating in refluxing
toluene (18 h) and degassing cycles to remove volatiles.5d
A theoretical analysis of the energy profile for the 2-3
equilibrium has contributed to a better process understanding
(Figure 3). An independent optimization of 2 starting with an
initial S-coordination to zirconium led to a final geometry
where the thiophene ring is not coordinated [optimized
Varying the temperature from 238 to 373 K results in
interconversion of the isomeric mixture of 2 and 3 from 94:6
1
to 25:75 ratios, respectively (from H NMR signal integra-
tion, Figure 1). It is worth noting that a complete reversibility
of the process was maintained throughout several thermal
cycles with no apparent mixture decomposition. From a
careful process analysis, it appears that Me2N-H/CβTh-Zr
protonolysis competes with CβTh-H/Zr-NMe2 σ-bond meta-
thesis, with the latter being predominant at high temperature.
The dimethylamine generated upon cyclometalation reaction
(2 f 3) strongly coordinates the metal center5 as a noninno-
cent ligand, ultimately acting as a temperature-controlled
proton shuttle for the M-CAr bond protonolysis.7
To the best of our knowledge, no example of temperature-
controlled metathesis/protonolysis has been reported in the
literature so far. Moreover, it is generally accepted that
C-H bond activations on early-transition-metal species are
irreversibly inhibited in the presence of protic groups like
˚
d(Zr-S)=3.76 A], with the heterocycle tilted away from the
metal center [optimized θ(NPy-CPy-CTh-S) = 47°; Figure
S16 in the Supporting Information]. This suggests that the
presence of three amido groups on zirconium is enough to
satisfy the electronic requirements of the electron-poor early-
transition-metal ion.
Thus, the overall process starts from a rotational isomer of
2 (20), in which the CβTh-H bond is pointing toward one
Py
Py
˚
NMe2 group [C-H N = 2.24 A, optimized θ(N -C -
3 3 3
CTh-S) = 149°; Figure S17 in the Supporting Information].
20 lies 1.2 kcal mol-1 below 2, providing evidence for a facile
thiophene rotation around the CPy-CTh bond [an additional
experimental proof is provided by the (1H-1H) NOESY
(7) (a) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959.
(b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507.

Communication
Inorganic Chemistry, Vol. 49, No. 15, 2010 6813
Figure 3. Energy profile in toluene for the C-H bond activation of the thiophene ring of 2 and optimized structures for 2, 20, TS, 3, and 4.
spectrum; see Figure S7 in the Supporting Information].
Density functional theory calculations carried out on the
real system at the M06/6-31G* level (toluene, PCM) reveal
that the σ-bond metathesis reaction has a rather low barrier
(19.2 kcal mol-1; Figure 3), and it is slightly endothermic
(6.1 kcal mol-1, in excellent agreement with the experimental
ΔH0 value reported above). An extra energy of 15.6 kcal
mol-1 is finally required in order to obtain the amine-free
complex 4, in line with the experimental results.
This result is of major importance in the field of polymer-
ization catalysis by zirconium(IV) and hafnium(IV) pyridyl-
amido catalysts, in light of their well-established efficiency
(in high-temperature solution processes in particular) for the
production of specialty polyolefins.1-4 Indeed, the dramatic
change at the metal coordination sphere of the zirconium(IV)
and hafnium(IV) pyridylamido systems [from five-coordi-
nated species (2 and 5) stabilized by bidentate monoanionic
(N-) ligands to six-coordinated complexes (3 and 6) with
tridentate dianionic (N-, C-) ones] as a consequence of a
simple temperature variation, makes the true catalyst identity
in solution elusive. In spite of this, it is intriguing to note that
related zirconium(IV) complexes containing either a phenyl
or a 2-furanyl group instead of the 2-thienyl fragment do not
show any detectable aryl metalation at room temperature,
with the cyclometalated isomers appearing on the NMR
spectra only above 310 K (Figures S11 and S12 in the Sup-
porting Information).
Ethylene polymerization tests have been carried out with
precatalysts (2 and 3) and 4 using methylaluminoxane as the
activator (Al/Zr > 1000) in the 40-80 °C temperature range.
Turnover frequencies (TOFs) and gel permeation chromato-
graphy (GPC) profiles for the high-density polyethylenes
(HDPEs) produced suggest that only the ortho-metalated
species are responsible for generation of the catalytically
active forms.1-3 The TOFs observed with 2-3 are, in fact,
comparable to those obtained with 4 when only the molar
percentage of 3, at the corresponding temperature, is taken
into account (see the Supporting Information). Finally, per-
fectly matching GPC traces for the HDPEs produced by the
two catalyst precursors imply identical active species at work
(Figures S21 and S22 in the Supporting Information).
In conclusion, we have reported an original example of
temperature-controlled σ-bond metathesis/protonolysis oc-
curring at N,N group 4 amido complexes, a discovery that
highlights how the chemistry of well-known metal precursors
such as MIV(NMe2)4 (M = Zr, Hf) may deserve new and
unexpected reactivity paths. We also feel that the results
presented here may lead to a better understanding of the
structure-activity relationships of group 4 amido-based
olefin polymerization catalysts.
Acknowledgment. Thanks are due to Firenze HYDRO-
LAB for financial support of this work. G.G. thanks
Dr. A. Meli and Prof. A. Vacca for fruitful discussions.
A.L. thanks the Spanish MICINN (Project CTQ2008-
06866-CO2-02). CESCA is also acknowledged for pro-
viding computational resources.
These findings ultimately highlight the importance of
the metal precursor choice8 for the obtainment of tailored
catalysts of unambiguous identity. A reliable identification of
the catalyst precursor, along with knowledge of the true
active speciesinvolved in the process,4 is indeed of mandatory
importance while designing improved catalytic structures
and studying catalytic processes.
Supporting Information Available: Text giving full experi-
mental data and details on VT NMR experiments, 1D and 2D
NMR spectra of 2-4, van’t Hoff plots, computational details,
optimized Cartesian coordinates, internal energies, and pictures
of 2, 20, TS, 3, and 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
(8) The reaction of 1 with Zr(CH2Ph)4 results in the formation of the
unique cyclometallated N2ThZr(CH2Ph)(η2-CH2Ph) complex. These results
will be reported elsewhere.