Mechanistic insights on acrylate insertion polymerization

Article abstract of DOI:10.1021/ja910760n

Complexes [{(PΛO)PdMe]_n] (1_n; PΛO = K²-P, O-Ar₂PC₆H₄SO₂O with Ar = 2-MeOC₆H₄) are a single-component precursor of the (PΛO)PdMe fragment devoid of additional coordinating ligands, which also promotes the catalytic oligomerization of acrylates. Exposure of 1_n to methyl acrylate afforded the two diastereomeric chelate complexes [(PΛO)Pd{k²-C,O-CH(C(O)OMe)CH₂CH(C(O)OMe)CH ₂CH₃}] (3-meso and 3-rac) resulting from two consecutive 2,1-insertions of methyl acrylate into the Pd-Me bond with the same or opposite stereochemistry, respectively, in a 3:2 ratio as demonstrated by comprehensive NMR spectroscopic studies and single crystal X-ray diffraction. These six-membered chelate complexes are direct key models for intermediates of acrylate insertion polymerization, and also ethylene-acrylate copolymerization to high acrylate content copolymers. Studies of the binding of various substrates (pyridine, dmso, ethylene and methyl acrylate) to 3-meso and 3-rac show that hindered displacement of the chelating carbonyl moiety by π-coordination of incoming monomer significantly retards, but does not prohibit, polymerization. For 3-meso,3-rac + C₂H₄ ? 3-meso-C₂H₄ 3-rac-C₂H₄ an equilibrium constant K(353 K) ≈ 2 × 10^-3 L mol_-1 was estimated. Reaction of 3-meso, 3-rac with methyl acrylate afforded higher insertion products [(PΛO)Pd(C₄H₆O₂).,Me] (n = 3, 4) as observed by electrospray ionization mass spectrometry (ESI-MS). Theoretical studies by DFT methods of consecutive acrylate insertion provide relative energies of intermediates and transition states, which are consistent with the aforementioned experimental observations, and give detailed insights to the pathways of multiple consecutive acrylate insertions. Acrylate insertion into 3-meso,3-rac is associated with an overall energy barrier of ca. 100 kJ mol^-1

Full text of DOI:10.1021/ja910760n

Published on Web 03/05/2010
Mechanistic Insights on Acrylate Insertion Polymerization
Damien Guironnet,† Lucia Caporaso,*,‡ Boris Neuwald,† Inigo Go¨ttker-Schnetmann,†
Luigi Cavallo,‡ and Stefan Mecking*,†
Chair of Chemical Materials Science, Department of Chemistry, UniVersity of Konstanz,
78464 Konstanz, Germany, and Department of Chemistry, UniVersity of Salerno, Via Ponte Don
Melillo, 84084-Fisciano (SA), Italy
Received December 21, 2009; E-mail: stefan.mecking@uni-konstanz.de; lcaporaso@unisa.it
Abstract: Complexes [{(P∧O)PdMe}n] (1n; P∧O ) κ2-P,O-Ar2PC6H4SO2O with Ar ) 2-MeOC6H4) are a
single-component precursor of the (P∧O)PdMe fragment devoid of additional coordinating ligands, which
also promotes the catalytic oligomerization of acrylates. Exposure of 1n to methyl acrylate afforded the two
diastereomeric chelate complexes [(P∧O)Pd{κ2-C,O-CH(C(O)OMe)CH2CH(C(O)OMe)CH2CH3}] (3-meso
and 3-rac) resulting from two consecutive 2,1-insertions of methyl acrylate into the Pd-Me bond with the
same or opposite stereochemistry, respectively, in a 3:2 ratio as demonstrated by comprehensive NMR
spectroscopic studies and single crystal X-ray diffraction. These six-membered chelate complexes are direct
key models for intermediates of acrylate insertion polymerization, and also ethylene-acrylate copolymerization
to high acrylate content copolymers. Studies of the binding of various substrates (pyridine, dmso, ethylene
and methyl acrylate) to 3-meso and 3-rac show that hindered displacement of the chelating carbonyl moiety
by π-coordination of incoming monomer significantly retards, but does not prohibit, polymerization. For
3-meso,3-rac + C2H4 a 3-meso-C2H4, 3-rac-C2H4 an equilibrium constant K(353 K) ≈ 2 × 10-3 L mol-1
was estimated. Reaction of 3-meso, 3-rac with methyl acrylate afforded higher insertion products
[(P∧O)Pd(C4H6O2)nMe] (n ) 3, 4) as observed by electrospray ionization mass spectrometry (ESI-MS).
Theoretical studies by DFT methods of consecutive acrylate insertion provide relative energies of
intermediates and transition states, which are consistent with the aforementioned experimental observations,
and give detailed insights to the pathways of multiple consecutive acrylate insertions. Acrylate insertion
into 3-meso,3-rac is associated with an overall energy barrier of ca. 100 kJ mol-1
.
Introduction
unique microstructure, and the relevant underlying organome-
tallic species. κ2-C,O-coordinated Pd(II)-alkyl species [(di-
imine)Pd(κ2-CHRCH2CH2C(O)OCH3)]+ (R ) growing polymer
chain) with a carbonyl group coordinating in a chelating fashion
are formed by insertion of methyl acrylate (MA), followed by
a series of ꢀ-hydride elimination and reinsertion. Opening of
these chelates by coordination of incoming monomer to form
the corresponding olefin complexes is the turnover limiting step
during copolymer chain-growth.2,5
Catalytic insertion polymerization of ethylene and propylene
is one of the most well-studied chemical reactions. In terms of
applications, it is employed for the production of more than 70
million tons of polyolefins annually.1 By contrast, an insertion
polymerization of electron deficient polar-substituted vinyl
monomers like acrylates has remained elusive. In fact, until most
recently not even consecutive incorporation of such monomers
in polymerization reactions had been observed.
It was not until the mid 1990s that cationic Pd(II) diimine
complexes were reported to catalyze the insertion copolymer-
ization of ethylene and 1-olefins with acrylates.2 These d8-metal
(late transition metal) complexes are less oxophilic by com-
parison to early transition metal polymerization catalysts, and
therefore more tolerant toward polar moieties.3 Due to “chain
walking” of the catalyst, the highly branched copolymers, which
consist of ethylene as the major component (g75 mol %),
contain acrylate units at the end of branches preferentially.2,4
In-depth NMR spectroscopic studies, for which these systems
are exceptionally well suited, provide an understanding of this
While the copolymers obtained with cationic Pd(II) diimine
catalysts are highly branched due to extensive chain walking,
with analogous Ni(II) complexes, and with neutral Pd(II)
(3) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169–1203. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003,
103, 283–316. (c) Mecking, S. Coord. Chem. ReV. 2000, 203, 325–
351. (d) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534–540. (e)
Guan, Z. Chem. Eur. J. 2002, 8, 3086–3092. (f) Domski, G. J.; Rose,
J. M.; Coates, G. M.; Bolig, A. D.; Brookhart, M. Prog. Polym. Sci.
2007, 32, 30–92. (g) Chen, E. Y.-X. Chem. ReV. 2009, 109, 5157–
5214. (h) Nakamura, A.; Ito, S.; Nozaki, K. Chem. ReV. 2009, 109,
5215–5244.
(4) For a mechanistic study of the reactivity towards vinyl ether as the
sole (electron-rich) substituted vinyl monomer, for which three
consecutive insertions were observed, cf.: Chen, C.; Luo, S.; Jordan,
R. F. J. Am. Chem. Soc. 2008, 130, 12892–12893.
† University of Konstanz.
‡ University of Salerno.
(1) Mu¨lhaupt, R. Macromol. Chem. Phys. 2003, 204, 289–327.
(2) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 267–268. (b) Mecking, S.; Johnson, L. K.; Wang, L.;
Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888–899.
(5) Related stoichiometric studies of acrylate insertion in Pd(II) complexes:
(a) Braunstein, P.; Frison, C.; Morise, X. Angew. Chem., Int. Ed. 2000,
39, 2867–2870. (b) Agostinho, M.; Braunstein, P. Chem. Commun.
2007, 58–60.
9
4418 J. AM. CHEM. SOC. 2010, 132, 4418–4426
10.1021/ja910760n  2010 American Chemical Society

Acrylate Insertion Polymerization
A R T I C L E S
phosphinesulfonato complexes, linear ethylene-MA copolymers
are formed.6-8 Note that the latter are also compatible with a
broad scope of functional vinyl monomers.3h,9,10 By comparison
with the cationic Pd(II) diimine system the formally neutral
Pd(II) phosphinesulfonato complexes appear to be less reactive,
which can be compensated, however, by higher polymerization
temperatures by virtue of their remarkable temperature stability.
These neutral Pd(II) catalysts have been studied as in situ
mixtures of metal sources and ligands,7,11 and employing
complexes [(P∧O)PdMe(L)] (L ) pyridine, lutidine, PPh3, 1/2
Me2NCH2CH2NMe2, dimethylsulfoxide) as well-defined single-
component catalyst precursors8,12-15 and as reagents for mecha-
nistic studies.16-18 Polyethylene chain growth,16 and also the
incorporation of single isolated acrylate monomer units in
polyethylenes have also been studied theoretically.19 By com-
parison to the aforementioned N- and P-based ligands L,
dimethylsulfoxide (dmso) binds less strongly to the metal center
and is more readily displaced by olefin substrate. This enabled
insertion homooligomerizations of methyl acrylate to products
with degrees of oligomerization of up to ca. DPn ) 20. Also,
ethylene copolymers with up to 50 mol % acrylate incorporation
containing consecutive acrylate-derived repeat units were
prepared. By contrast to the well-studied catalytic dimerization
of acrylates,20 these homooligomerization reactions possess all
mechanistic features of a polymerization reaction, namely
multiple insertions occur prior to chain transfer. In view of this
fundamental relevance, and also the accessibility of linear
ethylene-acrylate copolymers with unprecedented compositions,
an insight into the underlying reaction steps is desirable.
We now report on the observation and isolation of organo-
metallic polymerization intermediates from multiple consecutive
acrylate insertions, and a combined experimental and theoretical
study of their reactivity.
Results and Discussion
Reactive Precursors. A prerequisite for such studies are
precursors providing reactive (P∧O)PdR fragments (1), the
reactivity of which is not overly diminished by the presence of
any other strongly coordinating ligands or bases. As outlined,
the aforementioned dmso complexes are relatively labile, and
reaction with acrylate allows for observation of the single
insertion product of MA into the Pd-Me bond, [(P∧O)Pd-
{CH(COOMe)CH2CH3}(dmso)] (2-dmso).15
However, the preparation and isolation of multiple acrylate
insertion products was complicated by competing ꢀ-hydride
elimination. An alternative concept for providing weakly
coordinated precursors to the (P∧O)PdR fragment devoid of
additional ligands and reagents is replacement of the ligand L
in complexes [(P∧O)MR(L)] by the O-donor of another
(P∧O)MR fragment. This has been demonstrated for dimeric
Ni(II) complexes [{(P∧O)NiR}2].21 Recently, isolated Pd(II)
complexes [{(P∧O)PdR}n] (R ) -CH2SiMe3, -CH2CMe3) with
a chelating phosphinesulfonato ligand were obtained by abstrac-
tion of pyridine from [(P∧O)PdR(pyridine)] with B(C6F5)3. In
the solid state, a dimeric structure (n ) 2) was determined.12
The reaction of [{(P∧O)PdCH2CMe3}2] with 6-chlorohex-1-
ene afforded [(P∧O)PdCl] (trapped as the pyridine adduct) and
oligomerization products of the 1-olefin. For [{(P∧O)PdMe}n]
the formation of a π-complex with vinyl ethers as well as slow
1,2 insertion was observed by NMR.9b Note, that reactive
[(P∧O)PdMe] species have also been generated in situ without
isolation.9a,11,14,22,25f
(6) (a) Johnson, L.; Bennett, A.; Dobbs, K.; Hauptman, E.; Ionkin, A.;
Ittel, S.; McCord, E.; McLain, S.; Radzewich, C.; Yin, Z.; Wang, L.;
Wang, Y.; Brookhart, M. Polym. Mater. Sci. Eng. 2002, 86, 319. (b)
Wang, L.; Hauptman, E.; Johnson, L. K.; Marshall, W. J.; McCord,
E. F.; Wang, Y.; Ittel, S. D.; Radzewich, C. E.; Kunitsky, K.; Ionkin,
A. S. Polym. Mater. Sci. Eng. 2002, 86, 322. (c) Johnson, L.; et al.
ACS Symp. Ser. 2003, 857, 131–142.
(7) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744–745.
(8) Berkefeld, A.; Mecking, S. Angew. Chem., Int. Ed. 2008, 47, 2538–
2542.
(9) (a) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem.
Soc. 2007, 129, 8948–8949. (b) Luo, S.; Vela, J.; Lief, G. R.; Jordan,
R. F. J. Am. Chem. Soc. 2007, 129, 8946–8947. (c) Weng, W.; Shen,
Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 15450–15451. (d) Ito,
S.; Munakata, K.; Nakamura, A.; Nozaki, K. J. Am. Chem. Soc. 2009,
131, 14606–14607. (e) Skupov, K. M.; Piche, L.; Claverie, J. P.
Macromolecules 2008, 41, 2309–2310.
(10) Ethylene-CO copolymerizations: (a) Drent, E.; van Dijk, R.; van
Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964–
965. (b) Hearley, A. K.; Nowack, R. J.; Rieger, B. Organometallics
2005, 24, 2755–2763.
(11) Kochi, T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25–27.
(12) Vela, J.; Lief, G. R.; Shen, Z.; Jordan, R. F. Organometallics 2007,
26, 6624–6635.
(13) Skupov, K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.;
Conner, D.; Goodall, B. L.; Claverie, J. P. Macromol. Rapid Commun.
2007, 28, 2033–2038.
The weakly coordinated Pd(II)-Me complex 1n was obtained
by pyridine abstraction from 1-pyr in CH2Cl2 (eq 1). Layering
with pentane afforded crystals of 1n. While crystallization allows
for the clean isolation of 1n, unfortunately the crystals were not
suited for single crystal structure analysis. 1n dissolves only in
the presence of coordinating solvents (e.g., mixtures of dichlo-
romethane with methanol or acetone) in concentrations sufficient
for NMR analysis (cf. experimental section for complete
(14) Borkar, S.; Newsham, D. K.; Sen, A. Organometallics 2008, 27, 3331–
3334.
(15) Guironnet, D.; Roesle, P.; Ru¨nzi, T.; Go¨ttker-Schnetmann, I.; Mecking,
S. J. Am. Chem. Soc. 2009, 131, 422–423.
(16) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.;
Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088–14100.
(17) Skupov, K. M.; Hobbs, J.; Marella, P.; Conner, D.; Golisz, S.; Goodall,
B. L.; Claverie, J. P. Macromolecules 2009, 42, 6953–6963.
(18) Luo, R.; Newsham, D. K.; Sen, A. Organometallics 2009, 28, 6994–
7000.
(19) Haras, A.; Anderson, G. D. W.; Michalak, A.; Rieger, B.; Ziegler, T.
Organometallics 2006, 25, 4491–4497.
(21) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123–134.
(22) A related in situ activation of η-allyl-Pd complexes incorporating the
(P∧O) ligand motif with B(C6F5)3 has been reported: Liu, S.; Borkar,
S.; Newsham, D.; Yennawar, H.; Sen, A. Organometallics 2007, 26,
210–216.
(20) (a) Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.;
Garner, I. M.; Fagan, P. J.; Calabrese, J. C. J. Am. Chem. Soc. 1994,
116, 8038–8060, and references cited. Also cf.: (b) Braunstein, P.;
Chetcuti, M. J.; Welter, R. Chem. Commun. 2001, 2508–2509.
9
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A R T I C L E S
Guironnet et al.
Figure 1. Molecular structures of 3-meso (left) and 3-rac at 50% probability ellipsoids. All hydrogen atoms and cocrystallized solvent molecules are
omitted for clarity.
analytical data). This low solubility is a key advantage of 1n,
as it allows for the separation of unreacted starting material from
the reaction products in insertion reactions (vide infra). Exposure
of 1n to one equivalent of 2,6-lutidine or dmso resulted in the
clean formation of soluble 1-lut9a or 1-dmso,15 respectively.
Noteworthy, the formation of 1-lut occurred much faster than
the formation of 1-dmso, which required prolonged ultrasoni-
cation. This is inline with the stronger relative binding of lutidine
vs dmso.15 1n is a single component precursor to very active
catalysts for ethylene polymerization and ethylene-acrylate
copolymerization at ethylene pressures of only 5 bar, and for
the homopolymerization of acrylates (for comprehensive po-
lymerization data cf. Supporting Information).
Isolation of Multiple Insertion Products. Ultrasonication of
1n in a 4 mol L-1 MA solution in dichloromethane (with added
2,6-di-t-4-methylphenol (BHT) stabilizer) at 60 °C for 3 h results
in partial consumption of 1n and generation of a ca. 3:2 mixture
of two palladium complexes 3-meso and 3-rac (eq 2, identifica-
tion vide infra). Preparative workup allows for the isolation of
3-meso and 3-rac, containing traces of BHT radical inhibitor
and poly(methyl acrylate) from free-radical polymerization as
observed by 1H NMR analysis. An alternative approach,
affording pure 3-meso and 3-rac in high yield, was found to
be halide abstraction from the recently reported dimeric cation-
bridged anion [{(1-Cl)-µ-Na}2] which is easily available by the
reaction of the sodium salt of the ligand (P∧ONa) with
[(cod)Pd(Me)Cl].23 Reaction of [{(1-Cl)-µ-Na}2] with AgBF4
in the presence of MA (20 equivalents) in dichloromethane at
60 °C in a sealed tube for 90 min, followed by evaporation of
volatiles and extraction with toluene afforded pure 3-meso,3-
rac (3:2) in good yields (>70%, eq 2).
1H- 13C-, 1H,1H-gCOSY, 1H{13C}-gHSQC- and gHMBC-experi-
ments (Supporting Information).24 Key 1H NMR spectroscopic
features comprise palladium-R-methine resonances at δ 2.07 and
1.94 ppm as well as palladium-γ-methine resonances at 3.16
(3-meso) and 2.98 (3-rac) with corresponding 13C NMR
resonances at δ 28.08 and 27.19 (R-methine) and remarkably
discriminated γ-methine resonances at δ 52.06 and 44.71 ppm.
The existence of a six-membered chelate ring in 3-meso and
3-rac is evidenced by carbonyl resonances of a Pd(II)-
coordinated C(O)OMe-group (∆) at δ 185.67 (3-meso) and
185.40 (3-rac) as well as a noncoordinated carbonyl-group (Β)
at δ 179.62 (3-meso) and 177.95 (3-rac) ppm.25 Thermolysis
of 3-meso,3-rac in methylene chloride-d2 at 90 °C for 48 h in
the absence of additional MA yields methyl 4-carbomethoxy-
hex-2-enoate by ꢀ-hydride elimination as the sole product.
The stereochemistry of 3-meso,3-rac could be assigned by
single-crystal X-ray diffraction analysis. Suitable crystals were
obtained from a saturated solution of 3-meso,3-rac in a benzene-
pentane mixture at 4 °C. The solid state structure is disordered
with both diastereomers 3-meso and 3-rac occupying identical
positions. The refinement converged to an occupancy of 64:36
for both stereoisomers which is in agreement with the ratio
determined by NMR spectroscopy (vide supra). In the major
stereoisomer 3-meso,26 which results from two consecutive 2,1
acrylate insertions with the same stereochemistry the ethyl group
(C8, C9) is positioned anti to the carbomethoxy group (O3-C6-
O4-C7), while a syn-disposition is found in 3-rac,26 which
results from two insertions with opposite stereochemistry (Figure
1).
(23) Ru¨nzi, T., Master Thesis, UniVersity of Konstanz, 2009.
(24) In this context note, that organometallic intermediates resulting from
tail-to-tail dimerization of MA featuring either a η3-allyl bonding of
the dimer to the metal centre or σ-bonding of the dimer resulting in
the formation of a bis-chelate have been isolated (ref. 20).
(25) For other examples of Pd(II) alkyl/carbonyl chelate complexes cf.:
(a) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746–4764. (b) ref. 2. (c) ref. 5. (d) Fujita, T.; Nakano, K.;
Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2006, 128, 1968–1975.
(e) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc.
2007, 129, 7770–7771. (f) Nakamura, A.; Munakata, K.; Kochi, T.;
Nozaki, K. J. Am. Chem. Soc. 2008, 130, 8128–8129.
Complexes 3-meso and 3-rac were identified as diastereo-
meric six-membered chelates 3-meso (major isomer) and 3-rac
resulting from two consecutive 2,1-insertions of MA into the
palladium-methyl bond on the basis of a detailed analysis of
(26) This designation is based on consideration of the stereocenters as part
of a growing chain that is in this respect of identical (3-meso) or
opposite (3-rac) stereoconfiguration.
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Acrylate Insertion Polymerization
A R T I C L E S
Table 1. Selected Bond Distances (Å) and Angles (°) for
white precipitate upon addition of pentane to the ether extract.
Electrospray ionization (ESI) as a particularly mild technique
allowed for the recording of mass spectra from MeOH
solutions. Triethylammonium salts were found to be suited
as a cationizing agent.27 HNEt3+ adducts of the [(P∧O)PdR]
complexes (M + 102.13) represent the major species
observed by ESI-MS (Figure 2). The observed very specific
isotope pattern is in agreement with the expected peak
structure (Figure 2, insets).28 Apart from the starting material
(m/z ) 796), the products of two further consecutive methyl
acrylate insertion into 3-meso,3-rac are clearly detected (m/z
) 882; 968). Further minor signals (m/z ) 782; 868) result
from the consecutive insertion of methyl acrylate into the
Pd-H bond of the hydride [(P∧O)PdH], which is formed
by ꢀ-hydride elimination. In a similar manner products from
multiple insertion of butyl acrylate into 3-meso,3-rac and
[(P∧O)PdH] were generated and characterized (cf. Supporting
Information). The direct observation of these higher insertion
products further confirms that this polymerization of acrylate
follows an insertion pathway.29
3-meso,3-rac
Pd1-P1
Pd1-C1
Pd1-O1
Pd1-O5
C1-C2
C1-C6
2.217(1)
2.062(4)
2.129(3)
2.113(3)
1.509(7)
1.502(6)
C6-O3
1.207(6)
1.337(7)
1.217(6)
1.295(6)
95.11(9)
86.43(17)
C6-O4
C4-O1
C4-O2
P1-Pd1-O5
C1-Pd1-O1
Table 2. Ethylene Polymerization with Complexes 3-meso,3-raca
TOF [mol (C2H4)
mol (Pd)-1 h-1
polymer
yield [g]
b
entry
p [bar]
]
Mn [103 g mol-1
]
Mw/Mnb
2-1
2-2
2-3
2-4
2-5
2-6
1
2
5
10
15
20
0.1 × 105
0.5 × 105
1.1 × 105
1.4 × 105
1.5 × 105
1.6 × 105
0.33
1.72
3.90
5.03
5.36
5.44
6.7
12.9
19.0
22.0
20.1
22.3
2.1
2.0
1.9
1.9
2.2
2.1
a Reaction conditions: 2.5 µmol Pd, 100 mL of toluene; 80 °C, 30
min polymerization time. b Determined by GPC at 160 °C vs linear PE.
Coordination Equilibria. The reactivity of 3-meso,3-rac
towards the olefinic substrates is of interest, as π-coordination of
substrate (ethylene, MA) is a prerequisite for further chain growth.
Exposure of CD2Cl2-solutions of 3-meso,3-rac (0.04 mol L-1) to
excess ethylene (30 equivalents) or methyl acrylate (27 equivalents)
at temperatures down to -80 °C resulted in no reaction as observed
by 1H and 13C NMR spectroscopy, that is the chelating carbonyl
group is not displaced by the substrates under these conditions.
This corresponds to equilibrium constants of Keq < 10-2 (eq 3).30
By comparison, the more strongly coordinating ligand pyridine
appears to react virtually completely with 3-meso,3-rac to 3-meso-
pyr, 3-rac-pyr (Figure 3).
The bond lengths around the Pd-atom (Table 1) are compa-
rable to those of the 5-membered chelate complex
[(P∧O)Pd((MeOOC)CHCH2C(O)Me)]25f and other related CO-
methyl acrylate insertion products differing from the latter in
the nature of the (P∧X) ligand.5 The enlarged chelate size in
3-meso,3-rac accounts for an increase of the binding angle
O1-Pd-C1 (86.43(17) vs 82.61(21) in the aforementioned
5-membered chelate complex).
Reactivity of Multiple Insertion Products. Polymerization
Studies. A first indication of the reactivity of 3-meso,3-rac is
given by studies of polymerization with the compounds as
catalyst precursors. Exposure of 3-meso,3-rac to ethylene results
in polymerization with high rates also at low pressures. Activities
in 30 min polymerization experiments are independent of
monomer concentration at p(C2H4) g 5-10 bar for 3-meso,3-
rac (Table 2). Activation of 3-meso,3-rac for ethylene polym-
erization requires an initial opening of the chelate by coordi-
nation of ethylene (vide infra), which apparently occurs
efficiently under these conditions. 3-meso,3-rac exhibits a
slightly higher activity than 1-dmso (1-dmso: 1.0 × 105 TO
h-1, 3-meso,3-rac: 1.5 × 105 TO h-1 under identical conditions).
This is related to the irreversibility of the opening of the chelate.
Once ethylene is inserted, the oxygen atom of the carbonyl group
does not compete anymore with monomer coordination due to
increasingly disfavored chelate sizes with their entropic con-
tribution, while the entropic penalty of dmso coordinating to
the metal center remains essentially unchanged over the whole
polymerization reaction.
With dmso as a ligand, a directly observable equilibrium
exists at room temperature. The equilibrium constants K1(T) )
[3-meso-dmso,3-rac-dmso]/([3-meso,3-rac] × [dmso]) were
determined by 13C NMR spectroscopy in the temperature range
between 278 to 308 K, based on the chemical shift of the
∆-carbonyl resonances.31 The values of the reaction enthalpy
∆H° ) -3.0(6) kJ mol-1 and entropy ∆S° ) -3(2) J mol-1
K-1 were obtained by a Van’t Hoff plot (Figure S19,
(27) The addition of a cationizing agent is necessary to obtain reasonable
signal intensities.
In acrylate insertion oligomerization, productivities of
3-meso,3-rac are similar to the activity of 1-dmso (DPn ∼
7, TON ∼ 100). After every consecutive insertion, 6-mem-
bered chelated complexes similar to 3-meso,3-rac are invari-
ably formed. NMR characterization of these higher insertion
products is hampered by the extreme complexity of the
spectra resulting from the growing number of stereocenters
in the metal-bound growing chain, and simultaneous ꢀ-hy-
dride elimination reactions. However, mass spectrometry
enabled the direct observation of the organometallic products
of further insertion of acrylates into 3-meso,3-rac (Scheme
1, Figure 2). A solution of 3-meso,3-rac and 30 equivalents
methyl acrylate in benzene was kept for 4 h at 60 °C.
Evaporation of solvent and extraction with Et2O yielded a
(28) The isotope patterns of the masses for MA insertion into 3-meso,3-
rac (m/z ) 796, 882, 968) show a tailing. This can be explained by
appearance of minor peaks from the Na adducts (M+23) of these
complexes, which becomes evident from the isolated peak for the
starting material [3-meso,3-rac+Na]+ (m/z ) 717).
(29) In this context note that for other types of organometallic Pd(II)
complexes free-radical polymerization of acrylates to high-molecular-
weight polymer triggered by acrylate insertion has been found: (a)
Albeníz, A. C.; Espinet, P.; López-Fernández, R. Organometallics
2003, 22, 4206–4212. (b) López-Fernández, R.; Carrera, N.; Albeníz,
A. C.; Espinet, P. Organometallics 2009, 28, 4996–5001.
(30) Conservatively assuming a detection limit of 5% for the chelate opening.
(31) The temperature range was limited by the cryo-coil probehead
employed to obtain sufficient spectral quality. A fast site exchange
between free and chelated carbonyl resulted in the observation of one
averaged resonance (Figure 3 and supporting information).
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A R T I C L E S
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Scheme 1. Insertion of Methyl Acrylate into Complexes 3-meso,3-raca
a M represents major mass of the isotope pattern, detected by ESI-MS as the HNEt3-adducts (M+102.13).
Figure 2. Positive-ion ESI-mass spectrum of multiple insertion products (methanol solution with added [HNEt3][B(C6F5)4]). Inset A: found isotope pattern;
inset B: calculated isotope pattern for C39H55NO11PPdS.
Figure 3. 13C NMR spectra (carbonyl region, CD2Cl2, 298 K, [Pd] ∼ 0.04 moL-1) of pure 3-meso,3-rac (a) and in the presence of 10 equiv MA (b); 8 equiv
of dmso (c); 1 equiv of pyridine (d).
Supporting Information).32 The observed entropy loss is
surprisingly low for a chelate opening process A + B a C.
Indeed, for cationic (N∧N)Pd(II) complexes much more
negative entropies have been observed.2b,33 However, it has
also been noted that the nature of the incoming ligand (L;
eq 3) in chelate opening can have an unexpectedly strong
impact on the reaction entropy.33
(K2) is of relevance in the context of copolymerizations affording
linear ethylene-acrylate copolymers with high MA contents
(> ca. 20%), in which consecutive MA insertions occur to a
considerable extent.15 As outlined, an opening of the chelate
by olefinic substrate could not be observed even at low
temperatures; thus, a direct determination especially at the
elevated temperatures typically employed for polymerization
reactions (80 °C) was not possible. K2(353 K) could be estimated
by combining the equilibrium constant for chelate opening with
The equilibrium constant for the opening of the double
acrylate insertion product 3-meso,3-rac with ethylene substrate
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Acrylate Insertion Polymerization
A R T I C L E S
Figure 4. Methyl acrylate coordination and insertion in 1. Relative energies of the various species are given in parentheses (in kJ mol-1; black: intermediates;
red: transition states).
Figure 5. Methyl acrylate and ethylene insertion from 2. Relative energies of the various species are given in parentheses (in kJ mol-1; black: intermediates;
red: transition states).
dmso (K1) with the relative binding strength (K3) of dmso vs
ethylene ([(P∧O)PdR(dmso)] + C2H4 a [(P∧O)PdR(C2H4)]
+ dmso). The relative binding strength was determined from
the observed inhibition of polymerization rates by variable
amounts of added dmso in ethylene polymerization experiments
at 80 °C and 10 bar ethylene pressure (Table S5, Supporting
Information). A plot of TOFmax/TOF against the ratio of
concentrations [dmso]/[ethylene] gives K3(353 K) ) 1.1(2) ×
10-3 (Figure S22, Supporting Information). Recently, Claverie
Computational Studies. In order to gain a deeper understanding
of the catalytic methyl acrylate insertion polymerization, reaction
pathways of chain growth were studied theoretically employing
DFT methods. Regarding consecutive insertions of acrylate into a
metal-alkyl species, the first three insertions are expected to exhibit
distinctly different energy profiles, as the starting species are
different in energy due to different modes of chelating coordination
of already incorporated acrylate repeat units via the carbonyl group,
and also the substitution of the migrating alkyl group differs
particularly between the first and the second insertion. Consecutive
insertions were studied starting from the palladium methyl species
1. Exclusively the 2,1-insertion modus of methyl acrylate was
considered, since it is the predominant insertion mode observed
experimentally. Note that the first insertion is similar to previous
studies by Ziegler et al. of methyl acrylate insertion into a Pd-n-
propyl bond in the context of acrylate/ethylene copolymerization
with this catalyst system.19
et al. estimated the relative binding strength of ethylene vs
17
pyridine for 1-pyr to K4 ∼ 10-7
.
The higher value for K3
compared to K4 is consistent with dmso being the more labile
ligand. Combining K3 with K1(353 K) ≈ 1.9 L mol-1 extrapo-
lated from the thermodynamic data (Figure S19, Supporting
Information), K2 ) K1 × K3, yielded an estimated value of
K2(353 K) ≈ 2 × 10-3 L mol-1 for the chelate opening with
ethylene at 80 °C. This illustrates the stability of the six-
membered chelate, which hinders but does not suppress po-
lymerization.34
(33) For the (5-membered) chelate [(N∧N)Pd(RC(H)OC(O)CH3)], it was
found that opening with acetonitrile resulted in a much lower entropy
(∆S ) -38 J mol-1 K-1) in comparison to the opening with ethylene
(∆S ) -96 J mol-1 K-1). This was tentatively related to acetonitrile
losing fewer degrees of freedom by its linear coordination by
comparison to η2-coordination of ethylene, also a possible stronger
organization of the solvent by free acetonitrile vs. ethylene; and motion
of the OC(O)CH3-moiety being more extensively restricted by ethylene
coordination: Williams, B. S.; Leatherman, M. D.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2005, 127, 5132–5146.
(32) Observations of the 3-meso,3-rac + dmso a 3-meso-dmso,3-rac-dmso
equilibrium at 193 K are in qualitative agreement with the low temperature
dependence of the equilibrium derived from the NMR data acquired at
278 to 308 K. At low temperature, exchange is slow on the NMR time
scale, which actually facilitates the experiment, but additional non-
identified species observed (possibly as a result of slow exchange between
conformers) hamper a full analysis (cf. supporting information).
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A R T I C L E S
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As expected, methyl acrylate coordination to 1 is strongly
favorable and the most stable coordination intermediate 1MA-
cis, with the Pd-C σ-bond cis to the P atom, is nearly 60 kJ
mol-1 lower in energy than 1 (Figure 4). This π-complex is
slightly more stable, by about 15 kJ mol-1, than the alternative
isomer in which the acrylate molecule is κ-O bound to the metal
by the carbonyl oxygen atom. In agreement with previous
calculations,16,19 the preferred pathway for methyl acrylate
insertion into the Pd-C bond proceeds through isomerization
of 1MA-cis, with a barrier of 33 kJ mol-1, to the less stable
species 1MA-trans which is roughly 20 kJ mol-1 higher in
energy than 1MA-cis. 2,1- insertion into the Pd-C bond of
1MA-trans, associated with an energy barrier of ca. 40 kJ
mol-1, affords the kinetic product 2-KP, stabilized by a γ-agostic
interaction, and finally the monoinsertion product 2. 2 is
stabilized by coordination of the CdO group of the inserted
methyl acrylate unit, forming a four-membered chelate.
The second methyl acrylate insertion requires dissociation
of the O-coordinated carbonyl moiety from the Pd center in 2,
with simultaneous methyl acrylate coordination (Figure 5). Due
to the stabilizing effect of the carbonyl coordination, this step
is less favorable by ca. 35 kJ mol-1 by comparison to methyl
acrylate coordination to the alkyl species 1. By comparison to
methyl acrylate, π-coordination of ethylene to 2 is remarkably
favored with an energy gain of 41 kJ mol-1, in line with previous
calculations on both cationic and neutral Pd catalysts.19,35,36
Again, migratory insertion requires first cis/trans isomerization
to the less stable geometry with the monomer coordinated cis
to the P donor (2MA-trans both meso and rac, and 2E-trans
in Figure 5).37 This isomerization proceeds through a concerted
mechanism without dissociation of the coordinated monomer,
as already indicated by Ziegler et al.,19 and Morokuma and
Nozaki et al.16 for polyethylene chain growth or an isolated
insertion of acrylate, respectively. It requires ca. 30 to 40 kJ
mol-1 for the methyl acrylate complex, while the barrier is
roughly 20 kJ mol-1 lower for the ethylene complex. This
slightly lower barrier for isomerization of the ethylene complex
is consistent with the reduced destabilization of 2E-trans relative
to 2E-cis (ca. 15 kJ mol-1) by comparison to a destabilization
Figure 6. Transition states for methyl acrylate insertion affording 3-meso-
KP and 3-rac-KP, respectively.
of ca. 25 kJ mol-1 for both 2MA-trans species relative to the
corresponding 2MA-cis species. Both methyl acrylate- and the
ethylene-insertion transition states collapse into γ-agostic
complexes as the first kinetic product, which convert to the six-
membered carbonyl-coordinated chelates (3-meso, 3-rac and
3E) as the final stable products. Overall, the insertion liberates
ca. 90 to 100 kJ mol-1 (energy difference 2 vs 3-meso, 3-rac
or 3E). At this point, the main difference between coordination-
insertion of methyl acrylate and ethylene in 2 is the more
favorably π-coordination of ethylene.
Considering that experimental studies2b,25a (the results of
which have also been employed previously for theoretical
considerations)19 indicate an unfavorable entropic contribution
of about 40 kJ mol-1 for coordination of either monomer,
formation of the olefin complexes 2MA-cis from 2 should be
endoergonic by ca. 20 kJ mol-1, whereas formation of 2E-cis
is probably exoergonic. Further, still on a free energy profile,
and considering a minor entropic change between the coordina-
tion intermediates and the corresponding transition states, for
insertion of both methyl acrylate and ethylene the transition
states are roughly 70 kJ mol-1 higher than 2 + free monomer.
Concerning the stereochemistry of acrylate insertion from 2,
all species along the reaction pathway leading to 3-meso are
slightly lower in energy than the corresponding species leading
to 3-rac. The preference for the transition state leading to
3-meso is rather small, 2 kJ mol-1 only, which agrees well
qualitatively with the experimentally observed formation of a
ca. 3:2 meso:rac mixture of the two diastereomers.26 The
preferred transition state (Figure 6) is likely favored due to the
relative anti disposition of the two -COOMe groups, which
minimizes steric and electrostatic repulsion.
Concerning the third insertion of methyl acrylate, in view of
the rather similar energetics revealed for the pathways corre-
sponding to insertion of the two enantiofaces of methyl acrylate
in the second insertion, only coordination-insertion from the
slightly more stable complex 3-meso leading to another meso
diad, thus overall affording an isotactic triad, is considered
(Figure 7).
The third methyl acrylate insertion, which is representative
for acrylate insertion polymerization chain growth, requires
dissociation of the carbonyl moiety from the Pd center in
3-meso, with simultaneous methyl acrylate coordination, as-
sociated with an energy gain of about 15 kJ mol-1. By
comparison, ethylene coordination to 3-meso releases ca. 35
kJ mol-1. The equilibrium constant for the chelate opening with
ethylene K2(353 K) ≈ 2 × 10-3 L mol-1 estimated from the
experimental data corresponds to a ∆H of -32 kJ mol-1 at 353
(34) As a commentary concerning the significance and utility of this
equilibrium data, note that while it is subject to experimental error
and includes the assumption of similar relative binding strength of
ligands in different Pd-alkyls, otherwise from the viewpoint of
thermodynamics the argument is fully valid and only includes the
assumption that ∆H1° and ∆S1° of the chelate opening with dmso are
temperature-independent. A K2(353 K) ) 2 × 10-3 L mol-1 corre-
sponds to ∆G2°(353 K) ca. 18 kJ mol-1. Working-hypothesis-level
considerations at other temperatures utilizing this data require an
assumption on the contributions of ∆H2° and ∆S2° of chelate opening
by monomer to this free enthalpy value. It is common practice to
assume that for equilibria ML1 + L2
a ML2 + L1 the entropy
contribution is negligible. For the particular case discussed (L1,2
)
dmso, monomer), this would translate to the assumption that the
entropy of chelate opening by monomer (∆S2°) is the same as for
dmso (∆S1°), the latter being amenable to experimental observation.
However, the aforementioned (ref 33) experimental findings of very
different ∆S° for a cationic diimine systems for (L1,2 ) acetonitrile,
ethylene) question the validity of this argument. An, arguable,
alternative is an educated guess of ∆S2° assuming that it is similar to
the entropies of comparable carbonyl-chelate opening process in
cationic Pd(II) diimine complexes, which have been determined
experimentally (for example ∆S ) -9633 and -15125a J mol-1 K-1
(five-membered chelates), -10025a and -121 to -1422b J mol-1 K-1
(six-membered chelates)).
(35) Michalak, A.; Ziegler, T. Organometallics 2001, 20, 1521–1532.
(36) Michalak, A.; Ziegler, T. J. Am. Chem. Soc. 2001, 123, 12266–12278.
(37) Calculations indicated that for both monomers the insertion transition
state from 2-monomer-cis is at least 25 kJ mol-1 higher in energy
than from 2-monomer-trans.
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Acrylate Insertion Polymerization
A R T I C L E S
Figure 7. Methyl acrylate and ethylene insertion from 3-meso. Relative energies of the various species are given in parentheses (in kJ mol-1; black:
intermediates; red: transition states).
ethylene are roughly 100-110 kJ mol-1 above 3-meso + free
monomer. This indicates that the high stability of the six-
membered resting state formed by chelating coordination of the
penultimate inserted acrylate unit is a key limitation in acrylate
homopolymerization.
At this point, a comparison with the behavior of cationic
Pd(II) catalyst systems is instructive. From calculations by
Ziegler et al.,19,36 the main difference between cationic and
neutral Pd-systems is the oxophilicity of the metal center, which
is higher in the cationic Pd-systems. Consequently, the rupture
of the six-membered chelate resting state in the cationic Pd(II)-
Figure 8. Six- and four-membered chelate structures of 3-meso, energies
in kJ mol-1 in parentheses.
system by monomer π-coordination is much more disfavored
relative to the same process in the neutral Pd-systems. This can
rationalize the better ability of neutral Pd(II) catalysts to enchain
consecutive methyl acrylate units. The different oxophilicity of
cationic and neutral Pd systems is also exemplified by the
unability of the neutral Pd-system to assume square pyramidal
geometries in the olefin complexes (Figure 9). Both for methyl
acrylate and ethylene coordination to 3-meso, the isomer with
K. This is in good agreement with the calculated energy gain.38
The coordination energies are about 10 kJ mol-1 lower by
comparison to monomer coordination to 2, due the higher
stability of the six-membered chelate ring of 3-meso relative
to the four-membered chelate 2. Indeed, calculations show that
also the four-membered chelate geometry of 3-meso is less
stable than the six-membered geometry by about 25 kJ mol-1
(Figure 8).
a four-membered chelation of the carbonyl group of the last
Considering the strong prevalence found for the second
inserted methyl acrylate unit in the axial position of the square
insertion for this pathway, it can be assumed that also the third
pyramid is not stable, and all geometry optimizations resulted
migratory insertion requires isomerization of the more stable
geometries 3MA-meso-cis and 3E-cis to the less stable 3MA-
meso-trans and 3E-trans, respectively. Also in this case
isomerization barriers were calculated to amount to roughly
in the breaking of chelation. Differently, the isomer with a six-
membered chelation of the carbonyl group of the penultimate
inserted methyl acrylate unit in the axial position of the square
pyramid is stable, but quite high in energy, roughly 20 to 40 kJ
30-40 kJ mol-1. The insertion transition states are thus reached
from the 3MA-meso-trans and 3E-trans intermediates, with
barriers of 73 and 95 kJ mol-1 relative to 3MA-meso-cis and
3E-cis, respectively. Both methyl acrylate and ethylene insertion
transition states collapse first into the kinetic γ-agostic product,
which rearranges to the six-membered O-coordinated chelates
as the stable final products. Overall, the third insertion liberates
ca. 80 kJ mol-1 (energy difference 3-meso vs 4-meso or 4E).
On a free energy profile, assuming an entropic contribution
of roughly 40 kJ mol-1 for coordination of either monomer (vide
infra), formation of 3MA-meso-cis and 3E-cis from 3 are
endoergonic by ca. 30 and 10 kJ mol-1 respectively. Further,
the transition states for insertion of both methyl acrylate and
mol-1 relative to the simple square planar complex 3-meso. This
differs strikingly from ethylene insertion after a methyl acrylate
unit promoted by cationic Pd(II) complexes.36 In this case the
square pyramidal isomer with a six-membered chelating car-
bonyl moiety in the axial position was found to be the most
stable isomer in the case of ethylene coordination, the square
pyramidal four-membered chelate isomer was found to be stable
and ca. 5 kJ mol-1 higher in energy, and the least stable isomer
is the simple four-coordinate square planar isomer at least 10
kJ mol-1 higher in energy relative to six-membered chelate
square pyramidal geometry.36
Summary and Conclusions
The exposure of reactive species [{(P∧O)PdMe}n] (1) to
methyl acrylate (MA) yields the isolable product of two
consecutive acrylate insertions into the Pd-Me bond (3-meso
and 3-rac). Single crystal X-ray diffraction analysis as well as
comprehensive NMR studies reveal 3-meso,3-rac to be two
(38) The calculation of ∆H includes an unfavorable entropic contribution
of 50 kJ mol-1 at 353 K, estimated from the experimentally observed
typical entropy for olefin complexation to related cationic Pd
complexes, which varies between -113 and -151 J mol-1 K-1 (refs
2a and 25a).
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A R T I C L E S
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Figure 9. Relative energies (in kJ mol-1) of different isomers formed by coordination of methyl acrylate or ethylene to 3-meso.
diastereomers formed by 2,1-insertions of MA, in which the
carbonyl moiety of the first inserted monomer unit coordinates
to the metal center in a chelating fashion. This finding is
consistent with theoretical studies, which demonstrate that
significant stabilization of the primary multiple acrylate insertion
product results from such chelating coordination. The experi-
mentally observed low preference for the major diastereomer
(3:2) is in agreement with theoretical studies of the insertion
steps, and it is in line with the nonstereoregular nature of the
methyl acrylate oligomers and low-molecular-weight polymers
generated with catalysts based on the same phosphinesulfonato
ligand.15
experiments showing that 3-meso,3-rac are precursors for very
active ethylene polymerization catalysts at 5 bar and 80 °C.
Theoretical studies give insight also to key steps which were
not amenable to direct experimental observation. π-coordination
of acrylate to the products of multiple acrylate insertion is associated
with a relatively low energy gain and, on a free energy level, likely
endergonic, as a consequence of the aforementioned relatively
favorable chelating coordination. The overall insertion barrier
amounts to ca. 100 kJ mol-1, which does not differ much from
the overall insertion barrier for ethylene, in-line with an insertion
mechanism of acrylate polymerization with these catalysts. Indeed,
products of multiple insertion of acrylate into a Pd-alkyl bond
[(P∧O)Pd(C4H6O2)nMe] up to n ) 4 formed by the reaction of
3-meso,3-rac with acrylates could be directly observed by mass
spectrometry.
3-meso,3-rac are a direct analogue of the key intermediates
in catalytic acrylate insertion polymerization as well as ethylene-
acrylate copolymerization to high acrylate content copolymers.
Displacement of the chelating carbonyl group by incoming
olefinic substrate is a prerequisite for further chain growth. The
relative stability of the chelate 3 is reflected by the fact that an
opening with neither ethylene nor methyl acrylate could be
observed by NMR spectroscopy even at low temperature. A
fast equilibrium is observed at room temperature with dmso
and the thermodynamic parameters ∆H1° ) -3.0(6) kJ mol-1
and ∆S1° ) -3(2) J mol-1 K-1 could be obtained via a 13C
NMR study from a Van’t Hoff plot. Combined with polymer-
ization inhibition studies affording the relative binding strength
of ethylene vs dmso, an approximation for the chelate opening
with ethylene (3-meso,3-rac + C2H4 a 3-meso,3-rac-C2H4)
under typical polymerization conditions (80 °C) yielded K2(353
K) ≈ 2 × 10-3 L mol-1. Energy values from DFT, which does
not consider entropy contributions, are in good qualitative
agreement with this conclusion that chelating coordination of a
second-last inserted acrylate unit retards chain growth signifi-
cantly, but not prohibitively. This is in line with polymerization
Acknowledgment. This article is dedicated to Professor Wil-
helm Keim on the occasion of his 75th birthday. B.N. acknowledges
support by the state of Baden-Wu¨rttemberg by a Landesgraduierten-
fo¨rderung-Stipend. This work is financially supported by the DFG
(Me 1388/10). S.M. is indebted to the Fonds der Chemischen
Industrie. The authors thank Anke Friemel for recording of VT-
13C NMR spectra as well as the HPC team of Enea (www.enea.it)
for use of the ENEA-GRID and the HPC facilities CRESCO
(www.cresco.enea.it) in Portici, Italy. Access to the ESI-MS
instrument was kindly provided by Andreas Marx and co-workers.
S.M. thanks the Italian Chemical Society for kind hosting as a Karl
Ziegler-Giulio Natta-Lecturer.
Supporting Information Available: Full list of authors for
ref 6c, experimental procedures, and analytical data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA910760N
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4426 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010