Reactivity of a cationic alkyl amino-functionalized cyclopentadienyl aluminum compound with olefins: NMR observation and computational investigation of the single propene insertion product into an Al-C bond

Article abstract of DOI:10.1021/om800993e

In this study the reactivity of the compound dimethyl [2-(N,N- dmiemylethylen)cyclopentadienyl)]Al(III) toward ionizing species and the subsequent reactivity toward ethylene and propene have been explored. Reactions were studied via NMR tube experiments. Upon methyl abstraction by the Lewis acid B(C6F5)3, the amine donor on the ligand side arm coordinates to aluminum, stabilizing the resulting cationic species versus secondary reactions. The obtained cationic species was able to polymerize ethylene, albeit with low activity. Reaction with propene resulted in the selective 1,2-insertion of one propene molecule into an Al-C bond of the Al-Cp moiety. Density functional and ab initio calculations were used to characterize the energy landscape of the insertion into the Al-Cp bond for both ethylene and propene. The computational results suggest this reaction to be more facile than the insertion into the Al-Me bond.

Full text of DOI:10.1021/om800993e

2554
Organometallics 2009, 28, 2554–2562
Reactivity of a Cationic Alkyl Amino-Functionalized
Cyclopentadienyl Aluminum Compound with Olefins: NMR
Observation and Computational Investigation of the Single Propene
Insertion Product into an Al-C Bond
Daniela Pappalardo,*,† Claudio Pellecchia,‡ Giuseppe Milano,‡ and Massimo Mella*,§
Dipartimento di Studi Geologici ed Ambientali, UniVersita` del Sannio, Via dei Mulini 59/A,
I-82100, BeneVento, Italy, Dipartimento di Chimica, UniVersita` di Salerno, Via Ponte don Melillo,
I-84084, Fisciano (SA), Italy, and School of Chemistry, Cardiff UniVersity, Main Building, Park Place,
Cardiff CF10 3AT, U.K.
ReceiVed October 15, 2008
In this study the reactivity of the compound dimethyl [2-(N,N-dimethylethylen)cyclopentadienyl)]Al(III)
toward ionizing species and the subsequent reactivity toward ethylene and propene have been explored.
Reactions were studied via NMR tube experiments. Upon methyl abstraction by the Lewis acid B(C6F5)3,
the amine donor on the ligand side arm coordinates to aluminum, stabilizing the resulting cationic species
versus secondary reactions. The obtained cationic species was able to polymerize ethylene, albeit with
low activity. Reaction with propene resulted in the selective 1,2-insertion of one propene molecule into
an Al-C bond of the Al-Cp moiety. Density functional and ab initio calculations were used to characterize
the energy landscape of the insertion into the Al-Cp bond for both ethylene and propene. The
computational results suggest this reaction to be more facile than the insertion into the Al-Me bond.
salicylaldimine,7 2-anilinotropone,8 and, more recently, a bis-
(iminophosphorano)methandiide aluminum complex, based on
Introduction
a spirocyclic carbon center subtended by two AlMe2 units,9 have
also been shown to polymerize ethylene. Activation of these
compounds was usually achieved by the ionizing agents
traditionally used in homogeneous Ziegler-Natta catalysis (i.e.,
B(C6F5)3 or [(C6H5)3C][B(C6F5)4]). Very interestingly, Sen et
al. reported that simple aluminum alkyls, after reaction with
ionizing agents, catalyze the polymerization not only of ethylene
but also of propene,10 while recently simple aluminum alkyls
in the presence of chloro activators were shown to promote
ethylene polymerization in mild conditions with higher activ-
ity.11
The nature of the active species involved in the ethylene
polymerization with aluminum-based systems has been inves-
tigated; although cationic alkyl complexes have been isolated
and characterized, it is still unclear if they are directly
responsible for the catalytic activity. Monitoring the reaction
of perdeuterio-ethylene with an aminotroponiminate aluminum
complex, Jordan excluded that intact cationic species are the
active ethylene polymerization catalysts, while the main reaction
is a ꢀ-H transfer to generate the corresponding cationic
Oligomerization and polymerization of ethylene and olefins
are among the most important homogeneous catalytic processes.
Since the discovery by Ziegler of the Aufbau reaction, aluminum
alkyls have long been known as ethylene oligomerization
catalysts1 and widely used as “cocatalysts” or “scavengers” in
Ziegler-Natta olefin polymerization catalysis.2 On the contrary,
olefin polymerization by means of transition metal-free aluminum-
based catalytic species is a quite recent achievement. In 1992
Martin demonstrated that the simple bis(dichloroaluminum)-
ethane and trialkylaluminum are able to produce polyethylene
of high molecular weight in considerably mild conditions.3 In
the past decade several examples have been described in the
literature concerning olefin polymerization at aluminum, al-
though with low activity. Following the reports of Jordan on
mono- and bis(amidinate) complexes of aluminum,4 a few
related dialkyl aluminum derivatives carrying chelating monoan-
ionic ligands, such as aminotroponimine,5 bis(imino)pyridine,6
* Corresponding authors. E-mail: pappalardo@unisannio.it; MellaM@
cardiff.ac.uk.
† Universita` del Sannio.
‡ Universita` di Salerno.
(6) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, A. J. P.;
Williams, D. J. Chem. Commun. 1998, 2523–2524.
§ Cardiff University.
(1) Ziegler, K.; Gellert, H.-G.; Zosel, K.; Holzkamp, E.; Schneider, J.;
So¨ll, M.; Kroll, W.-R. Justus Liebigs Ann. Chem. 1960, 629, 121–166.
(2) For leading references see: (a) Britzinger, H. H.; Fischer, D.;
Mullhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl.
1995, 34, 1143. (b) Kamisky, W.; Arndt, M. AdV. Polym. Sci. 1997, 127,
143.
(3) Martin, H.; Bretinger, H. Makromol. Chem. 1992, 193, 1283–1288.
(4) (a) Coles, M. P.; Swenson, D. C.; Jordan, R. F. Organometallics
1997, 16, 5183–5194. (b) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1997, 119, 8125–8126. (c) Duchateau, R.; Meetsma, A.; Teuben, J. H. Chem.
Commun. 1996, 22, 3–224.
(7) (a) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce,
M. D.; White, A. J. P.; Williams, D. J. Chem. Commun. 1999, 1883–1884.
(b) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; White,
A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 415–422. (c)
Pappalardo, D.,; Tedesco, C.; Pellecchia, C. Eur. J. Inorg. Chem. 2002,
621–628.
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C. Eur. J. Inorg. Chem. 2004, 1292.
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Catal. A: Chem. 2002, 189, 137–143.
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2007, 26, 2252–2257.
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1998, 120, 8277–8278. (b) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young,
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10.1021/om800993e CCC: $40.75
2009 American Chemical Society
Publication on Web 03/31/2009

Propene Insertion Product into an Al-C Bond
Organometallics, Vol. 28, No. 8, 2009 2555
aluminum hydride.5a According to a theoretical study on
ethylene polymerization at aluminum centers, mononuclear
aluminum species are unlikely to produce polymers, because
chain transfer is much faster than propagation.12 Therefore, the
observed polymerization activity could be due to minor uni-
dentified species produced in situ, and probably more complex
structures have to be considered.
literature,19 was allowed to react with 1 equiv of B(C6F5)320 to
give the cationic species [C5H5(CH2CH2NMe2)AlMe]+-
[MeB(C6F5)3]- (2). The reaction was studied via NMR in
C6D5Cl solution at 25 °C (Figure 1a). Diagnostic resonances in
the 1H NMR spectrum of 2 are a singlet at -1.07 ppm
attributable to the AlMe (3 H) and a singlet at +1.07 ppm (3
H) attributable to the “free” anion MeB(C6F5)3 .20 Consistently,
-
in the 13C NMR spectrum, a signal at -16.3 ppm attributable
We noted that there has been little investigation into the
reactivity with olefins of aluminum(III) compounds bearing
cyclopentadienyl ligands,13 which instead were widely used for
olefin polymerization catalysts.2 Moreover, although cationic
species represent potential candidates for catalysis due to the
increased Lewis acidity,14 cationic aluminum species are quite
unstable, and very often they undergo ligand scrambling
reactions with the anion.15 The stability of the cationic species
could be strongly improved by the presence of a Lewis base in
the solvent medium16 or, even better, in the ligand itself.7a,b
Cyclopentadienyl ligands with tethered donor units have been
largely used in the preparation of several compounds of s-, p-,
d-, and f-block elements.17 In particular, cyclopentadienyl
compounds with a dialkylaminoalkyl side chain have also been
employed in transition metal olefin polymerization catalytic
systems.17a,18 In this study we have explored the reactivity of
dimethyl [2-(N,N-dimethylethylene)cyclopentadienyl]Al(III)19
toward ionizing species and the subsequent reactivity toward
ethylene and propene. Monitoring the reaction of the obtained
cationic species with propene allowed the observation of an
unprecedented propene insertion product into an Al-C bond.
The energy landscape of this reaction channel was also
investigated by means of electronic structure calculations in the
gas phase and shown to proceed through a low-lying transition
state.
-
to AlMe and a signal at 10.1 ppm attributable to MeB(C6F5)3
appear. In addition, the ligand resonances are shifted to higher
field in comparison with those of the neutral starting compound.
Concerning the cyclopentadienyl ligand, the presence of two
singlets (6.15 and 5.79 ppm, 2 H each) in the 1H NMR spectrum
and of three signals (129.2, 121.3, and 96.5 ppm) in the 13C
NMR spectrum is compatible with a “windscreen-wiper” fast
haptotropic rearrangement process, already observed in analo-
gous neutral compounds (Scheme 1).17c,19 Lower temperature
1H NMR experiments performed in CD2Cl2 resulted in more
complex spectra, with new signals in the Cp regions and two
broad singlet at -1.09 and -1.78 ppm attributable to AlMe
hydrogen atoms; the 13C NMR spectrum, registered at -60 °C,
displayed five signals (130.1, 128.0, 124.7, 116.1, 111.6 ppm).
These data indicate that at lower temperature the “windscreen-
wiper” fluxional behavior is frozen out. It is worth noting that
such a cationic species is stable for days in C6D5Cl solution at
room temperature. On the contrary, the analogous reaction of
CpAlMe2 and B(C6F5)3 did not result in stable or observable
cationic species. Probably in our case the extra donor -NMe2
unit could stabilize the cation against side reactions such as
ligand redistribution.15
The reactivity of the cationic species 2 toward olefins was
explored: compound 1, when activated with 1 equiv of B(C6F5)3,
polymerized ethylene (1 atm) to solid polyethylene albeit with
low activity (80 g (PE) mol-1 h-1 atm-1), while, in the same
conditions, it was inactive in the propene polymerization.
Results and Discussion
Interesting results were derived from the study of the
reactivity of the cationic species 2 toward olefins by NMR tube
reactions. To a solution of compound 2 in C6D5Cl was added
propylene, and the reaction was monitored via 1H NMR (Figure
1b,c). New resonances appeared just after a few minutes from
the injection of propylene. After 2 h their intensities were in
1:2 ratio with respect to the original cationic aluminum species
2. These resonances were attributed to a new organometallic
cationic aluminum species (3), arising from the insertion of
propylene into an Al-C bond of 2 (Scheme 2). Unexpectedly,
the propylene does not insert into the Al-Me bond, but into
the Al-Cp moiety of 2. Density functional and ab initio
calculations (Vide ultra) were used to characterize the energy
landscape of the reaction for both ethylene and propene and
predicted a barrier of at least 23.4 kcal/mol for the insertion of
the ethylene into the Al-Me bond, while the insertion into the
Al-Cp bond presented very low barriers (4.3-4.8 kcal/mol).
Experimental Results. Dimethyl [2-(N,N-dimethylethyl-
ene)cyclopentadienyl)]Al(III) (1), synthesized according to the
(12) (a) Talarico, G.; Budzelaar, P. H. M. Organometallics 2000, 19,
5691–5695. (b) Talarico, G.; Busico, V.; Budzelaar, P. H. M. Organome-
tallics 2001, 20, 4721–4726. (c) Talarico, G.; Budzelaar, P. H. M.
Organometallics 2002, 21, 34–38.
(13) (a) Shapiro, P. J. Coord. Chem. ReV. 1999, 189, 1–17. (b)
Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. 1996, 35, 2226.
(c) Burns, C. T.; Shapiro, P. J.; Budzelaar, P. H. M.; Willett, R.; Vij, A.
Organometallics 2000, 19, 3361. (d) Lee, S.-J.; Shapiro, P. J.; Twamley,
B. Organometallics 2006, 25, 5582–5588.
(14) For reviews on group 13 cationic compounds, see: (a) Atwood,
D. A. Coord. Chem. ReV. 1998, 176, 407–430. (b) Atwood, D. A.; Dagorne,
S. Chem. ReV. 2008, 108 (10), 4037–4071.
(15) (a) The reaction between AlR3 and B(C6F5)3 was described in a
patent application as a convenient approach to the synthesis of Al(C6F5)3:
Biagini, P.; Lugli, G.; Abis, L.; Andreussi, P. (Enichem Elastomeri S.r.l.)
Eur. Patent Appl. EP 0 694 548 A1, 1996, pp 1-9. (b) Qian, B.; Ward,
D. L.; Milton, R. S., III Organometallics 1998, 17, 3070–3076. (c)
Bochmann, M.; Sarfield, M. J. Organometallics 1998, 17, 5908–5912.
(16) (a) Jegier, J. A.; Atwood, D. A. Inorg. Chem. 1997, 36, 2034–
2039. (b) Klosin, G.; Roof, R. G.; Chen, E. Y.-X.; Abboud, K. A.
Organometallics 2000, 19, 4684–4686.
1
The structure of 3 was completely elucidated by the use of H
and 13C, mono- and bidimensional NMR experiments, including
1
DEPT, COSY, and direct and long-range H-13C correlation
(Tables 1, 2, and 3), disclosing interesting features. First, the
anion MeB(C6F5)3- is not involved in the reaction, neither does
it interact with the new cationic species. In the 1H NMR
spectrum the intensity of the singlet at +1.07 ppm (3 H,
(17) (a) Jutzi, P.; Redeker, T. Eur. J. Inorg. Chem. 1998, 663–674. (b)
Mu¨ller, C.; Vos, D.; Jutzi, P. J. Organomet. Chem. 2000, 600, 127–143.
(c) Bensiek, S.; Bangel, M.; Neumann, B.; Stammer, H.-G.; Jutzi, P.
Organometallics 2000, 19, 1292–1298.
(18) (a) Flores, J. C.; Chien, J. C. W.; Rausch, M. D. Organometallics
1994, 13, 4140–4142. (b) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kru¨ger,
C.; Verhovnik, G. P. J. Organometallics 1997, 16, 1511–1513. (c) Bradley,
S.; Camm, K. D.; Furtado, S. J.; Gott, A. L.; McGowan, P. C.; Podesta,
T. J.; Thornton-Pett, M. Organometallics 2002, 21, 3443–3453.
(19) Jutzi, P.; Dahlhaus, J.; Bangel, M. J. Organomet. Chem. 1993, 460,
C13-C15.
-
MeB(C6F5)3 ) remains constant. Consistently, the 19F NMR data
(20) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245–
250. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
3623–3625. (c) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc.
1994, 116, 10015–10031.

2556 Organometallics, Vol. 28, No. 8, 2009
Pappalardo et al.
Figure 1. (a) 1H NMR spectrum (C6D5Cl, 25 °C) of [C5H5(CH2CH2NMe2)AlMe]+[MeB(C6F5)3 ]- (2). (b, c) 1H NMR spectra (C6D5Cl, 25
°C) monitoring the reaction of 2 5 min and 2 h, respectively, after the addition of propene.
Table 1. 1H NMR (C6D5Cl, 25 °C) Data for the Species 3a
Scheme 1
δ (1H)
assgnt
-0.81 (3 H, s)
AlMe
Al-CHaHb-
Al-CHaHb-
2
3
4
-0.55 (1 H, dd, J ) 15.0, J ) 6.8, J ) 1.5)
2
3
-0.25 (1 H, dd, J ) 15.0, J ) 1)
3
1.01 (3 H, d, J ) 6.6)
CHaHb-CHcMe
[MeB(C6F5)3]-
CH2CH2NMeMe′
CH2CH2NMeMe′
CH2CH2NMeMe′
CH2CH2NMeMe′
CHaHb-CHcMe
Hd (C5 ring)
1.07 (3 H, s)
1.92 (3 H, s)
1.97 (3 H, s)
2.18 (2 H, m)
2.30 (2 H, m)
2.42 (1 H, m)
2.90 (1 H, m)
Scheme 2
3
6.05 (1 H, d, J ) 5.6)
H (C5 ring)
H (C5 ring)
H (C5 ring)
6.47 (1 H, s, br)
3
6.50 (1 H, d, J ) 5.6)
a All chemical shifts are in ppm and J values in Hz.
Hb) bound to the carbon in R-position with respect to Al. H-H
COSY analysis confirms that both hydrogen atoms Ha and Hb
correlate with the same CH hydrogen (Hc, 2.18 ppm); likewise,
this Hc hydrogen is coupled with a methyl group, appearing as
a doublet at 1.01 ppm (Figure 2). A HMBC (heteronuclear
multiple bond correlation) 1H-13C 2D NMR experiment
established unambiguously the structure, through observation
of the C-H correlation between the ꢀ-carbon atom of the
propene inserted unit (Al-CH2-CHMe, 35.6 ppm) and the
hydrogen bound to the sp3-hybridized C5-ring carbon atom (Hd,
are as expected for symmetric methyl-borate anions, showing
no evidence of methyl or fluorine interactions with the cationic
center.
1
1
In the H NMR spectrum a characteristic resonance of the
2.90 ppm). In addition, in the H NMR spectrum a long-range
new species 3 is the singlet at -0.81 ppm (3 H), accounting
for the Al-Me protons. In the 13C NMR spectrum the corre-
sponding carbon displays a resonance at -11.4 ppm. In the 1H
NMR spectrum diagnostic resonances for the propene insertion
are the two doublets of doublets at -0.25 and -0.55 ppm,
relative to the diastereotopic geminal hydrogen atoms (Ha and
coupling constant (4J ) 1.5 Hz) between Ha and the hydrogen
atom bound to the sp3 carbon atom of the C5 ring (Hd) was
observed, consistent with a stereorigid “W” conformation of
the four bonds between these two hydrogen atoms.
The insertion of propylene therefore was highly regiospecific,
affording exclusively the 1,2-insertion product. The structure

Propene Insertion Product into an Al-C Bond
Organometallics, Vol. 28, No. 8, 2009 2557
Table 2. 13C NMR (C6D5Cl, 25 °C) Data for the Species 3a
new cationic aluminum species, and two doublets of doublets
at -0.26 and -0.46 ppm, attributable to the diastereotopic
geminal hydrogen atoms (Ha and Hb) bound to the carbon in
R-position with respect to Al.
δ (13C)
assgnt
-11.4
7.9
AlMe
Al-CH2-CHMe
11.2
25.0
25.5
35.6
44.4
44.7
58.9
68.0
129.6
138.0
149.5
157.0
BMe
Al-CH2-CHMe
Consistently with the above structure, compound 3 does not
further react with propene; formation of polypropene was not
detected even at a prolonged reaction time (24 h). Following a
reviewer’s suggestion, in another experiment, 3 was generated
in situ, and then some ethylene was further added. The reaction
(C5H4)CH2CH2NMeMe′
Al-CH2-CHMe
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
CHd (C5 ring)
CH (C5 ring)
CH (C5 ring)
CH (C5 ring)
C (quaternary; C5 ring)
1
was monitored by H NMR. Interestingly, after a 4 h period,
the concentration of 3 remained unchanged. On the contrary,
the concentration of 2 decreased, as well as the concentration
of ethylene, and as already observed above formation of 4 was
detected. In conclusion while at the moment the identification
of the real catalytic species responsible for the ethylene
polymerization is still elusive, the performed NMR experiments
clearly indicate a preference for olefin insertion into the Al-Cp
bond.
a All chemical shifts are in ppm.
of the product was identified by careful consideration of the
coupling constants to Ha in the 1H NMR spectrum. In fact, the
signal at -0.55 ppm appears as a doublet of doublets, with
further fine structure due to the long-range coupling. The
geminal (2J(Ha-Hb) ) 15.0 Hz) and the vicinal (3J(Ha-Hc) )
6.8 Hz) coupling constants reveal significantly different values
depending on the small Ha-C-C-Hc dihedral angle. The
molecule seems to be locked in a pseudo-metallacycle, where
the methyl group originating from propene insertion occupies
preferentially an equatorial position. The nitrogen atom should
be strongly coordinated to the Al, as evidenced by the
appearance of two singlets for the NMe2 groups both in the 1H
(1.92 and 1.97 ppm) and 13C (44.4 and 44.7 ppm) NMR spectra.
Concerning the C5 ring, in the 13C NMR spectrum five distinct
resonances are observed (68.0, 129.6, 138.0, 149.0, 157.0),
clearly indicating a locked position of the two C-C double
Computational Results. The final structure of the stationary
points on the B3LYP/6-311+G(d,p) potential energy surface
for the ethylene insertions (i.e., the cation 2, its complex with
ethylene (CE), the transition state (TS) describing the insertion
of ethylene into the Al-Cp bond (TS-E1), the TS for the
insertion of ethylene into the Al-Me bond (TS-E2), and the
end product (PE1) of the insertion into the Al-Cp bond) is
shown in Figure 3. Figure 4 shows the equivalent structures
for the propene insertion into the Al-Cp bond. Table 4 provides
the relative energetics of TS-E1, TS-E2, and PE1 with respect
to the ethylene-cation (CE) complex and of the transition states
(TS-P1-4) and products (PP1-4) with respect to the related
complexes (CP1-4) for the propene insertion.
From Figure 3, we notice that the geometries of CE, TS-E1,
and PE1 present a relatively short Al-Cp distance, suggesting
the presence of an interaction between Al and the Cp ring during
all reaction stages. The structures of CE and TS-E1 seem to
suggest Al features a η1-coordination with the Cp ring, whereas
the proximity of Al to the ring in PE1 is likely due to the
polarization of the Cp ring by the positively charged metal ion.
One also notices that the gas phase equilibrium structure of 2
has Al located over the electron-rich Cp ring, thus suggesting
a η5-coordination with the latter. The low-temperature 13C NMR
spectrum for this species indicates an asymmetric chemical
environment for the ring carbons. Theoretical 13C NMR chemi-
cal shifts for 2 (129.7 ppm for C1, 101.4 ppm for C2, 125.4
ppm for C3, 123.2 ppm for C4, and 108.1 ppm for C5, as
computed at the B3LYP/6-311+G(2d,2p) level) show, accord-
ingly, a nonsymmetric pattern in agreement with the experi-
mental data. A similar lack of symmetry is found also in the
distances between Al and the carbon atoms in the Cp ring (2.165,
2.180, 2.314, 2.322, and 2.192 Å) and in the Al-C bond orders
(0.2779, 0.3175, 0.2308, 0.2259, and 0.3095) computed using
the natural bond order (NBO) approach. The difference in Al-C
distances appears to be due to a weak strain in the structure
induced by the specific conformation of the CH2CH2NMe2
ligand obtained during geometry optimization, which should
disappear upon increasing the system temperature thanks to a
more fluxional behavior of the CH2CH2NMe2 group. As for the
Al-C bond orders, we notice that our data indicate the presence
of a fractional bonding of Al with all the ring carbons, although
a weak preference is present toward the substituted carbon in
the ring and its two neighbors. In our view, this suggests Al to
have either a η3- or η5-coordination with the Cp ring in 2.
1
bonds. The low-field H NMR spectrum pattern, in fact, is the
one expected for a 1,3-disubstituted cyclopentadiene ring.
Consistently, density functional and ab initio calculations
(Vide ultra) also indicated a better stability and a lower transition
state energy barrier for the regioisomers deriving from the 1,2-
insertion, which could also be explained in terms of Mulliken
population of the olefin sp2 carbon atoms. Moreover, as a further
support to the whole structure, the calculated proton chemical
shifts (Vide ultra) are in good agreement with the experimental
results.
In order to force the reaction to completion, the solution was
warmed at higher temperature (i.e., 50 °C), but as a result only
decomposition products were obtained. Removal of propene did
not cause any deinsertion reaction. Consistently with this result,
the computed barrier to the deinsertion reaction was high (see
Table 4). On the contrary, when more propene was added to
replace the consumed one, the reaction was forced to comple-
tion; at room temperature in a 30 h reaction period, 3 became
the prevalent species (in 1.5:1 molar ratio with respect to 2).
Attempts to isolate the species 3 as analytically pure material,
performed on larger scale reaction, were unsuccessful: the
reactions gave no clean products, probably due to the extreme
air sensitivity of the cationic aluminum species.
The reactivity of compound 2 toward ethylene was studied
in analogous experiments. Monitoring the reaction of compound
2 with ethylene (3 equiv) in C6D5Cl solution at 23 °C showed
that the amount of 2 and ethylene gradually decreased in 1:1
molar ratio. After 6 h, the concentrations of 2 and of ethylene
were respectively 55% and 80% of the initial ones. Conversely
new resonances, attributable to the Al-Cp ethylene insertion
product 4, analogous to 3, appeared. Characteristic resonances
were the singlet at -0.85 ppm, attributable to the AlMe+ of the
Despite the aforementioned similarity between experimental
and theoretical 13C NMR shift patterns and the fact that ring-

2558 Organometallics, Vol. 28, No. 8, 2009
Pappalardo et al.
Table 3. Summary of the Long-Range Correlations Observed in the HMBC Spectrum (C6D5Cl, 25 °C) for the Species 3 (∆ )100 µs)a
δ (1H)
assgnt
δ (13C)
assgnt
2
3
4
-0.55 (1 H, dd, J ) 15.0, J ) 6.8, J ) 1.5)
Al-CHaHb-ChcMe
35.6
25.0
35.6
68.0
35.6
58.9
44.4
58.9
44.7
129.6
58.9
44.7
157.0
58.9
157.0
138.0
68.0
Al-CH2-CHcMe
Al-CH2-CHcMe
Al-CH2-CHcMe
CHd C5ring
Al-CH2-CHcMe
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
C5 ring
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
C5 ring
(C5H4)CH2CH2NMeMe′
C5 ring
2
3
-0.25 (1 H, dd, J ) 15.0, J ) 11)
Al-CHaHb-CHcMe
Al-CHaHb-CHcMe
3
1.01 (3 H, d, J ) 6.6)
1.92 (3 H, s)
1.97 (3 H, s)
2.18 (2 H, m)
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
(C5H4)CH2CH2NMeMe′
2.30 (2 H, m)
2.42 (1 H, m)
(C5H4)CH2CH2NMeMe′
Al-CHaHb-CHcMe
C5 ring
CHd C5 ring
2.90 (1 H, m)
Hd C5 ring
157.0
138.0(w)
129.6
35.6
138.0
157.0
68.0
C5 ring
C5 ring
C5 ring
Al-CH2-CHcMe
C5 ring
C5 ring
CHd C5 ring
C5 ring
3
6.05 (1 H, d, J ) 5.6)
H C5 ring
H C5 ring
6.47 (1 H, s)
3
6.50 (1 H, d, J ) 5.6)
H C5 ring
129.6
a All chemical shifts are in ppm and J values in Hz. Weaker correlations, corresponding to smaller coupling constants, are identified with “w”.
Table 4. Energetics of Ethylene and Propene Insertion Processes
into the Al-Cp and Al-Me Bonds
relative energya
B3LYP/6-311
MP2/6-211
+G(d,p)
+G(d,p)
Ethylene
TS-E1 Al-Cp bond insertion
TS-E2 Al-Me bond insertion
product (PE1) Al-Cp bond insertion
TS-E1 Al-Cp bond deinsertion
4.3
23.4
-14.1
18.3
4.8
24.2
-18.3
23.1
Propene
TS-PP1 Al-Cp bond insertion
TS-PP2 Al-Cp bond insertion
TS-PP3 Al-Cp bond insertion
TS-PP4 Al-Cp bond insertion
product (PP1) Al-Cp bond insertion
product (PP2) Al-Cp bond insertion
product (PP3) Al-Cp bond insertion
product (PP4) Al-Cp bond insertion
TS-PP1 Al-Cp bond deinsertion
TS-PP2 Al-Cp bond deinsertion
TS-PP3 Al-Cp bond deinsertion
TS-PP4 Al-Cp bond deinsertion
7.6
7.4
4.6
4.7
9.4
16.5
13.7
-6.4
-8.0
-3.3
-4.1
13.9
15.4
19.8
17.8
9.4
-14.1
-15.9
-14.4
-11.6
18.6
20.6
23.8
21.1
Figure 2. Aliphatic region H-H COSY NMR spectrum (C6D5Cl,
25 °C) of the reaction of [C5H5(CH2CH2NMe2)AlMe]+[MeB-
(C6F5)3]- (2) with propene.
coordinated cationic complexes between Al and dicarbolly-
lamine ligands similar to 2 were previously characterized by
X-ray crystallography, NMR spectroscopy, and DFT calcula-
tions, we feel that it would be a hazard to draw a definitive
conclusion about the possible changes induced by temperature
in the structure of 2.21 This uncertainty is mostly due to the
absence of explicit solvent molecules in our calculations, which
may induce further distortions in the structure of 2 following
coordination with the Al cation as found in the case of CE.
As for the energetics of the ethylene insertions, Table 4
indicates already at first glance a good agreement between the
energy profile for the insertion into both the Al-Cp and Al-Me
bonds provided by the two levels of theory. Overall, our
theoretical results indicate the insertion into the Al-Cp bond
to be more facile than the one into the Al-Me bond by roughly
19 kcal/mol, in good agreement with the experimental results
that indicate the preferential formation of 3.
a Energies (kcal/mol) computed using the complex between the
cationic species and each olefin as reference.
Vice Versa) for the latter reaction, Mulliken atomic charges were
computed for CE. Unfortunately, this method failed to provide
a clear-cut suggestion, with the C atoms involved in the reaction
in both ethylene and the Cp ring bearing negative partial charges.
In fact, if one excludes the C atoms in positions 1 (+0.08) and
2 (-0.36) with respect to the ring substituent and in close
proximity to the positive Al atom in CE, the remaining carbons
in the Cp ring bear smaller negative charges (-0.20 on C3,
-0.11 on C4, and -0.16 on C5) than the olefin ones (-0.36
on the carbon closer to the ring and -0.30 on the other). An
inspection of the DFT molecular orbitals on TS-E1 highlighted,
however, the presence of an overlap between the unoccupied
ethylene π* and an occupied orbital of the Cp ring (TS/HOMO,
Figure 5a). The latter interaction appears to be stabilizing the
HOMO orbital (-10.4 kcal/mol at the B3LYP/6-311+G(d,p)
level) at the TS geometry with respect to the HOMO in the
complex, which is highly localized on the ring. Roughly
In an attempt to interpret the driving force and mechanism
(electrophilic attack of the activated olefin on the Cp ring or
(21) Lee, J.-D.; Kim, S.-K.; Kim, T.-J.; Han, W.-S.; Lee, Y.-J.; Yoo,
D.-H.; Cheong, M.; Ko, J.; Kang, S. O. J. Am. Chem. Soc. 2008, 130, 9904–
9917.

Propene Insertion Product into an Al-C Bond
Organometallics, Vol. 28, No. 8, 2009 2559
(CH2dCHMe, +0.02) geometrically available for the nucleo-
philic attack by the Cp ring thanks to a preferential η1-
coordination between Al and the unsubstituted CH2 carbon atom.
Conversely, the formation of PP3 and PP4 from CP3 and CP4
requires the olefin to shift closer to the Cp ring in order to
facilitate the nucleophilic attack onto the negatively charged
carbon (-0.51) in CH2, a displacement that should be expected
to somewhat contribute to the increase in the energy barrier for
the process. The larger negative charge and the structural
requirement to form PP3-4 also help in explaining the higher
barrier that needs to be surmounted in forming PP3 and PP4
than in the case of PE1. As a final comment, it is also useful to
mention that the reaction energy, computed as the difference
between complex and product energies (Table 4), indicates a
somewhat better stability for PP1 and PP2 than for PP3 and
PP4 with respect to their parent complexes, thus suggesting that
PP1 and PP2 should be preferentially found even in the case
of an equilibrium process.
An additional support for the preferential formation of 3, in
either the PP1 or PP2 form, is provided by the computed
chemical shifts22 of Ha, Hb, and Hc (Scheme 2) reported in Table
5 for the four possible regioisomers. As discussed in the
Experimental Section, the signals of these three protons play a
key role in elucidating the structure of 3. Similarly, the
theoretical results indicate a distinctively different pattern for
the PP1-2 and PP3-4 pairs: whereas the former presents a
single proton resonating around 3 ppm (lower field) and two
protons absorbing around 0 ppm (higher field), the latter has
two protons shifted to lower field and one at slightly higher
field. Comparing with the chemical shifts reported in Table 1,
it appears evident that only the chemical shifts of the PP1-2
pair match the experimentally observed pattern of signals. As
for the absolute accuracy of the computed NMR shifts, we point
out that an offset of roughly 0.5 ppm between the experimental
and theoretical data is a quite acceptable result23 considering
that possible effects due to solvation have not been taken into
account in our model calculations. Due to this, a definitive
discrimination between PP1 and PP2 based on a finer matching
of chemical shifts would be inappropriate.
Figure 3. Optimized structures of the cationic species 2, its complex
with ethylene (CE), the transition state for the ethylene insertion
into the Al-Cp (TS-E1) and Al-Me (TS-E2) bonds, and the
product of the ethylene insertion into the Al-Cp bond (PE1).
speaking, this finding may be interpreted as indicating that the
electron donation from the occupied Cp HOMO to the unoc-
cupied ethylene π* may be identified as largely responsible for
lowering the barrier. In fact, the other high lying valence orbitals
(HOMO-1-HOMO-3) describing the π-systems on both Cp and
ethylene increase in energy at the TS geometry. In other words,
the mechanism of the transformation appears to be mostly driven
by the nucleophilic attack of the Cp ring onto ethylene despite
the smaller Mulliken charges borne by the ring carbon atoms
(also, Vide infra for propene).
A similar conclusion is reached studying the regioselectivity
of the propene insertion into the Cp-Al bond, on which we
decided to focus given the substantially lower energy barrier
found in the study with ethylene. Similarly to the latter case,
the barriers separating the complexes from the insertion products
are predicted to be low by both theoretical methods, with MP2
suggesting slightly lower barriers than DFT. From these results,
however, it also appears that the formation of products having
the methyl group of the propene closer to the ring (PP1 and
PP2) is more facile (roughly 4-6 kcal/mol depending on the
theoretical method) than for the other two regioisomers (PP3
and PP4). This finding is clearly in optimal agreement with the
NMR characterization of the product, suggesting the formation
of species with only a hydrogen close to the Cp ring. This result
can be easily understood on the basis of our mechanistic
suggestion surveying the coordination and Mulliken population
of the olefin sp2 carbon atoms in the complexes as shown for
two representative complexes (CP1 and CP4) in Figure 5b. Due
to their geometry, both CP1 and CP2 present the almost neutral
secondary carbon atom in the coordinated propene
Conclusions
In this study, we have explored the reactivity of dimethyl
[2-(N,N-dimethylethylene)cyclopentadienyl]Al(III) toward ion-
izing species and the subsequent reactivity toward ethylene and
propene. After abstraction of the methyl group by the ionizing
agent B(C6F5)3, the amine function on the side arm coordinates
to aluminum, stabilizing the resulting cationic species versus
secondary reactions. The cyclopentadienyl ligand having a
tethered amine unit offers a good balance of stability and
reactivity. The obtained cationic species was able to polymerize
ethylene, albeit with low activity. Quite surprisingly, conversely,
reaction with propene resulted in the selective insertion of one
propene molecule into an Al-C bond of the Al-Cp moiety.
The energy landscape of both processes was explored by means
of electronic structure methods, with results for the transition
state barrier heights and proton chemical shifts strongly sup-
porting the interpretation of the experimental results.
The carbalumination, i.e., the insertion of olefins into an
Al-C bond, is a well-known reaction;24 it is enough to mention
(22) McWeeny, R. Phys. ReV. 1962, 126, 1028.
(23) Helgaker, T.; Jaszunski, M.; Ruud, K. Chem. ReV. 1999, 99, 293.
(24) Eish J. J. In ComprehensiVe Organometallic Chemistry II; Abel,
Stone, Wilkinson, Eds.; Pergamon: Exeter, 1995; Chapter 10: Aluminum,
pp 464-467.

2560 Organometallics, Vol. 28, No. 8, 2009
Pappalardo et al.
Figure 4. Optimized structure for the complexes between the cationic species 2 and propene (CP1-4), the transition states (TS-P1-4),
and products (PP1-4) for the propene insertion into the Al-Cp bond.
that the insertion of ethene into Al-ethyl bonds is the basis of
the original Aufbau reaction for the preparation of a mixture of
R-alkenes or alcohols (C4-C30).1 Nevertheless, to our knowl-
edge, direct observation of the reaction product deriving from
the single insertion of propene into an Al-C bond has never
been reported. Single olefin insertion products have been instead
observed with some early transition metals and constituted
models of intermediates for the insertion mechanism in
Ziegler-Natta polymerization.25 The observation that propene
undergoes an insertion reaction into the Al-Cp moiety of the
species 2 showed that this bond is more reactive than the Al-Me
bond, which acts instead as a spectator ligand. It is worth nothing
that this behavior is the opposite of the one observed with the
classical homogeneous transition metal-based catalytic systems
for olefin polymerization, for which the metal-alkyl bond is
the reactive one, while the cyclopentadiene acts as an “ancillary”
ligand.2 Actually, the present case has much in common with
an earlier report by Jordan on the reversible cycloaddition
reactions of ethylene and alkynes to cationic ꢀ-diketiminate
aluminum complexes.26 Density functional theory27 was used
to study the factors that influence these reactions and predicted
barriers for the cycloaddition reactions significantly lower than
those previous calculated for ethylene insertion in the Al-alkyl
bonds of cationic aluminum alkyl complexes.12
Such results nicely agree with our present conclusions and
further confirm the experimental observation that the cationic
species 2 selectively inserts one propene molecule into an Al-C
bond of the Al-Cp moiety rather than into the Al-methyl bond.
(25) (a) Pellecchia, C.; Grassi, A.; Zambelli, A. J. Chem. Soc., Chem.
Commun. 1993, 947–949. (b) Pellecchia, C.; Immirzi, A.; Grassi, A.;
Zambelli, A. J. Organomet. Chem. 1994, 479, C9-C11. (c) Pellecchia, C.;
Grassi, A.; Zambelli, A. Organometallics 1994, 13, 298–302. (d) Horton,
A. D.; de With, J. Organometallics 1997, 16, 5424–5436. (e) Gielens,
E. C. G.; Dijkstra, T. W.; Berno, P.; Meetsma, A.; Hessen, B.; Teuben,
J. H. J. Organomet. Chem. 1999, 591, 88–95. (f) Corradi, M.; Jimenez
Pindado, G.; Sarsfield, M. J.; Thornton-Pett, M.; Bochmann, M. Organo-
metallics 2000, 19, 1150–1159. (g) Shafir, A.; Arnold, J. Organometallics
2003, 22, 567–575. (h) Anderson, L. L.; Schmidt, J. A. R.; Arnold, J.;
Bergman, R. G. Organometallics 2006, 25, 3394–3406.
(26) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1998, 120, 9384–9385.
(27) Ariafard, A.; Lin, Z.; Jordan, R. F. Organometallics 2005, 24, 5140–
5146.

Propene Insertion Product into an Al-C Bond
Organometallics, Vol. 28, No. 8, 2009 2561
B(C6F5)3 (33 mg, 0.065 mmol) in 0.5 mL of C6D5Cl at 20 °C. The
solution was analyzed by NMR spectroscopy at room temperature.
1H NMR (ClC6D5, 293 K): δ -1.07 (s, 3H, Al-CH3), 1.07 (s, 3H,
B-CH3), 1.69 (s, 6H, N-(CH3)2), 2.11 (t, 2H, Cp-CH2), 2.35 (t, 2H,
NCH2), 5.79 (s, 2H, Cp-H), 6.15 (s, 2H, Cp-H). 13C NMR (ClC6D5,
293 K): δ -16.35 (Al-CH3), 10.12 (B-CH3), 25.25 (Cp-CH2), 44.17
(N-(CH3)2), 59.30 (NCH2), 96.55 (Cp-C2,5), 116.94 (Cp-C3,4), 129.22
3
(Cp-C1). 19F NMR (ClC6D5, 293 K): δ -132.2 (d, J ) 22 Hz),
3
3
-164.9 (t, J ) 21 Hz), -167.3 (t, J ) 21 Hz).
Reaction of [C5H5(CH2CH2NMe2)AlMe]+[MeB(C6F5)3]- (2)
with Ethylene. In a J-Young NMR tube, dimethyl [2-(N,N-
dimethylethylene)cyclopentadienyl]Al(III) (0.001 mmol) and
B(C6F5)3 (0.001 mmol) were dissolved in 0.7 mL of C6D5Cl
containing C6H6 (0.04 M) as internal standard and previously
saturated with ethylene (3 equiv, measured by 1H NMR) at 23 °C.
The reaction was monitored via NMR spectroscopy at 23 °C. The
NMR spectra showed that the amount of 2 and ethylene gradually
decreased in a 1:1 molar ratio. After 3 h, the concentrations of 2
and of ethylene were respectively 70% and 89% of the initial ones,
and after 6 h 55% and 80% of the initial ones. Conversely new
resonances, attributable to the Al-Cp ethylene insertion product
1
4, appeared. After 6 h the molar ratio of 4:2 was almost 1:1. H
NMR for compound 4, selected resonances (ClC6D5, 293 K): δ
-0.85 (s, 3H, Al-CH3), 1.07 (s, 3H, B-CH3), -0.46 (dd, 1H, Al-
CHaHb-), -0.26 Al-CHaHb- (dd, 1 H, Al-CHaHb-), 1.84 (s, 3H,
NMeMe′), 1.89 (s, 3H, N-MeMe′), 2.95 (m, 1H, Hd), 5.94 (d, 1H,
H Cp), 6.26 (d, 1H, H Cp), 6.47 (s br, 1H, H Cp). 19F NMR
Figure 5. (a) Transition state HOMO molecular orbital at the
B3LYP/6-311+(d,p) level (energies in hartree). (b) Mulliken charge
population for complexes CP1 and CP4 at the B3LYP/6-21(d) level.
3
3
(ClC6D5, 293 K): δ -132.2 (d, J ) 22 Hz), -164.9 (t, J ) 21
3
Hz), -167.3 (t, J ) 21 Hz).
Reaction of [C5H5(CH2CH2NMe2)AlMe]+[MeB(C6F5)3]- (2)
with Propene. In a glovebox dimethyl [2-(N,N-dimethylethyl-
ene)cyclopentadienyl]Al(III) (13 mg, 0.065 mmol) was allowed to
react with B(C6F5)3 (33 mg, 0.065 mmol) in 0.5 mL of C6D5Cl
containing C6H6 as internal standard (0.08 M) at 23 °C. Propylene
Table 5. 1H Chemical Shifts Characterizing the Four Possible
Products of the Propene Insertion into the Al-Cp Bond Computed
at the B3LYP/6-311+G(2d,2p) Level
species
1H δ (ppm) Ha, Hb, Hc
PP1
PP2
PP3
PP4
3.1098, 0.2855, -0.1894
2.9345, 0.3317, -0.4666
2.1623, 2.896, 0.4032
3.2951, 1.9425, 0.6886
1
was injected by syringe (3 equiv, measured by H NMR). The
reaction was monitored via NMR spectroscopy at room temperature,
giving the reaction product 3. Two hours after the addition of
propene the amount of species 3 resulted in a 1:2 ratio with respect
to species 2. Species 3 was characterized by 1H, 13C, DEPT,
homonuclear and heteronuclear COSY, and direct and long-range
1H-13C correlation NMR experiments. The NMR data are reported
Experimental Section
General Procedures. Manipulations of sensitive materials were
carried out under a dry nitrogen atmosphere using Schlenk or
glovebox techniques. ClC6H5 and ClC6D5 were dried over CaH2
and distilled prior to use. Polymerization grade ethylene and propene
(SON) were used without purification. The compounds dimethyl
[2-(N,N-dimethylethylene)cyclopentadienyl]Al(III)19 and B(C6F5)320
were synthesized according to literature procedures. NMR spectra
were recorded on a Bruker Advance 400 MHz spectrometer (1H,
400 MHz; 13C, 100 MHz; 19F 376 MHz); chemical shifts were
referenced to the residual protio impurity of the deuterated solvent.
Ethylene Polymerization Tests. A typical polymerization test
was carried out in a 100 mL glass flask charged under nitrogen
with 12 mL of dry ClC6H5 and thermostatted at 23 °C. The inert
gas was replaced by ethylene at 1 atm, then 0.05 mmol of dimethyl
[2-(N,N-dimethylethylene)cyclopentadienyl]Al(III) and 0.05 mmol
of B(C6F5)3 (each dissolved in 1.0 mL of ClC6H5) were injected
into the flask. The flask was fed with constant monomer pressure,
and after 1 h the reaction was stopped by injecting methanol. The
mixture was poured into acidified methanol, and the polymer was
recovered by filtration, washed with fresh methanol, dried under
vacuum, and analyzed by DSC and NMR (mp: 137 °C; 13C NMR
in C2D2Cl4; 120 °C, δ in ppm from TMS: 30.00).
3
in Tables 1, 2, and 3. 19F NMR (ClC6D5, 293 K): δ -132.2 (d, J
3
3
) 22 Hz), -164.9 (t, J ) 21 Hz), -167.3 (t, J ) 21 Hz).
A second experiment was performed as above, but after a 2 h
period, some of the propene was removed by three freeze-pump-
thaw cycles, becoming one-third of the species 2. The tube was
maintained at 23 °C, and the reaction was monitored by 1H NMR.
The NMR spectra, registered every 15 min in a 12 h period, showed
that the molar ratio between the species 2, 3, and propene was
unchanged.
In another experiment, performed as above, after the 2 h reaction
time, more propene was added by syringe, replacing that consumed.
The tube was kept at 23 °C, and the reaction was monitored by 1H
NMR. The NMR spectra showed that species 2 was gradually
converted in species 3; in a 30 h reaction period species 3 became
the prevalent one (in 1.5:1 molar ratio with respect to species 2).
In a further experiment, performed as above, 2 h after the addition
of propene, ethylene was added by syringe and the reaction was
1
monitored by H NMR. After 4 h the concentration of species 3
Generation of [C5H5(CH2CH2NMe2)AlMe]+[MeB(C6F5)3]-
(2). In a glovebox dimethyl [2-(N,N-dimethylethylene)cyclopenta-
dienyl]Al(III) (13 mg, 0.065 mmol) was allowed to react with
remained unchanged. On the contrary, the concentration of species
2 decreased, as well as the concentration of ethylene, and resonances
attributable to species 4 were detected.

2562 Organometallics, Vol. 28, No. 8, 2009
Pappalardo et al.
suggested in Scheme 1 were carried out using two different DFT
functionals starting from sensible initial geometries with the
aluminum coordinated to the amine group and the Cp ring.
Structures for the complex between the cation and ethylene (CE)
were subsequently optimized adding the olefins in positions favoring
the interaction between the vacant coordinative site of Al and the
olefin π-bond to the lowest energy structure of the cationic species.
The search for a transition state (TS) describing the insertion of
ethylene into the Al-Cp bond (TS-E1) was conducted by means
of constrained scans along the distance between an ethylene carbon
atom and one of carbon atoms in position 3 from the dimethylami-
noalkyl group on the Cp ring; putative TS structures were
successively refined with a normal TS optimization carried out with
the Berny algorithm in conjunction with analytical second deriva-
tives. Geometries for the end product (PE1) were obtained relaxing
structures having surmounted the energy barrier for the reaction.
Similarly, the TS search for the insertion of ethylene into the
Al-Me bond (TS-E2) was performed by means of a two-step
procedure. First, an energy minimization was carried out with the
system being constrained to have the distance between the C atom
of the Me-Al group and a sp2 carbon of the olefin at 2.1 Å. The TS
geometry was successively refined as indicate above for the insertion
into the Al-Cp bond. A similar strategy was used for locating the
stationary points on the potential energy surface of the reaction
between propene and 2.
Computational Details
Gas phase electronic structure calculations were carried out using
the ADF28 and Gaussian0329 suites of codes, employing both
density functional theory (DFT) and post Hartree-Fock ab initio
methods, the latter at the second-order Møller-Plesset (MP2) level
of theory. The DFT calculations employing the ADF program were
carried out using the local exchange-correlation potential by Vosko
et al.,30 augmented in a self-consistent manner with Becke’s
exchange-gradient correction31 and Perdew’s correlation-gradient
correction,32 and mostly used to obtain initial geometries for further
refinement. Double-ꢁ Slater-type orbitals (STO) were used for
carbon (2s, 2p) and hydrogen (1s), augmented with single 3d and
2p functions (respectively). Aluminum was described using a
triple-ꢁ STO quality basis set augmented with a 4d polarization
function. Calculations carried out with the Gaussian suite employed
the 6-311+G(d,p) basis set in conjunction with DFT-B3LYP and
MP2 models. Mulliken population analysis was carried out using
the 6-31G(d) basis set. Geometries for all species (i.e., cation, olefin
complexes, transition states (TSs), and final products) were fully
optimized using B3LYP/6-311+G(d,p), and the stationary points
found were characterized by means of frequency calculations.
Structural optimizations of the putative catalytic cationic species 2
(28) Amsterdam Density Functional Package, Release 2000.1; Vrije
Universiteit: Amsterdam, 2000.
Single-point MP2 calculations using the 6-311+G(d,p) basis set
were carried out on the B3LYP/6-311+G(d,p)-optimized structures.
NMR chemical shifts for carbon and hydrogen were computed at
the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) level of theory
in the gas phase with the gauge independent atomic orbitals
(GIAO)22 using analytical derivatives.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A. ; Vreven, T., Jr.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003.
Acknowledgment. The authors are grateful to Dr. Patrizia
Oliva for performing NMR experiments. This work was
supported by the Italian Ministry of University and Research
(FAR 2007-Universita` del Sannio).
Supporting Information Available: Additional NMR spectra
and Cartesian coordinates for the optimized species are available
free of charge via the Internet at http://pubs.acs.org.
(30) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
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