Synthesis and characterization of cationic and zwitterionic allyl zirconium complexes derived from trimethylenemethane (TMM) cyclopentadienylzirconium acetamidinates

Article abstract of DOI:10.1016/S0022-328X(03)00315-2

Protonation of the trimethylenemethane derivatives, Cp*Zr(σ ²,π-C₄H₆) [N(R¹)C(Me)N (R²)] (1a: R¹=R²=i-Pr and 1b: R¹=Et, R²=t-Bu) (Cp&=η⁵-C ₅

Full text of DOI:10.1016/S0022-328X(03)00315-2

Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
www.elsevier.com/locate/jorganchem
Synthesis and characterization of cationic and zwitterionic allyl
zirconium complexes derived from trimethylenemethane (TMM)
cyclopentadienylzirconium acetamidinates
Denis A. Kissounko, James C. Fettinger, Lawrence R. Sita *
Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
Received 6 March 2003; received in revised form 1 April 2003; accepted 17 April 2003
Abstract
Protonation of the trimethylenemethane derivatives, Cp*Zr(s2,p-C4H6)[N(R1)C(Me)N(R2)] (1a: R1ꢀ
R2ꢀ h5-C5Me5), by [PhNMe2H][B(C6F5)4] in chlorobenzene at ꢁ
t-Bu) (Cp*ꢀ 10 8C provides the cationic methallyl complexes,
Cp*Zr(h3-C4H7)[N(R1)C(Me)N(R2)] (2a: R1ꢀR2ꢀi-Pr and 2b: R1ꢀEt, R2ꢀ
t-Bu), which are thermally robust in solution at
elevated temperatures as determined by H NMR spectroscopy. Addition of B(C6F5)3 to 1a and 1b provides the zwitterionic allyl
complexes, Cp*Zr{h3-CH2C[CH2B(C6F5)3]CH2}[N(R1)C(Me)N(R2)] (3a: R1ꢀR2ꢀi-Pr and 3b: R1ꢀEt, R2ꢀ
t-Bu). The crystal
/
R2ꢀ
/
i-Pr and 1b: R1ꢀ
Et,
/
/
/
/
/
/
/
/
1
/
/
/
/
structures of 2b and 3a have been determined. Neither the cationic complexes 2 or the zwitterionic complexes 3 are active initiators
for the Ziegler-Natta polymerization of ethylene and a-olefins.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Ziegler-Natta polymerization; Cationic allyl complexes; Zwitterionic allyl complexes
1. Introduction
B(C6F5)3], derived from reaction of a zirconocene
butadiene complex with B(C6F5)3, can serve as a highly
active initiator for the Ziegler-Natta polymerization of
propene. Given the opposing behavior presented by
these two cases, there is a clear need to further establish
the structural and electronic factors that govern the
barriers to olefin insertion into allyl ligands of cationic
and zwitterionic zirconium complexes, and in particular,
of non-metallocene-based Ziegler-Natta initiators and
propagating species. With respect to this, we recently
reported that cationic cyclopentadienylzirconium acet-
amidinates of the general structure, [(h5-C5R5)-
ZrX{N(R1)C(Me)N(R2)}]ꢂ[B(C6F5)4]ꢁ, are highly ac-
tive initiators for the living Ziegler-Natta polymeriza-
tion of a-olefins and a,v-nonconjugated dienes, and
Cationic zirconium allyl complexes have been impli-
cated as playing critical roles in the deactivation of
zirconocene-based propagating species involved in the
Ziegler-Natta polymerization of a-olefins [1], and pos-
sibly, in the formation of stereoerrors that occur during
such polymerizations carried out at reduced monomer
concentrations [2,3]. In a recent study, Brintzinger and
co-workers [4] investigated the rate of propene insertion
into the methallyl ligand of the cationic zirconocene
complex,
[(h5-C5H5)2Zr(h3-C4H7)]ꢂ[MeB(C6F5)3]ꢁ,
which serves as a model for an allyl complex carrying
a polymer alkyl chain, and found it to be significantly
lower than rates of propene insertion into the
where Rꢀ
/
Xꢀ
/
Me, R1ꢀ
/
Et and R2ꢀ
t-Bu, these poly-
/
zirconiumꢃcarbon bond of cationic zirconium alkyl
/
species. On the other hand, Erker and co-workers [5]
demonstrated that the zwitterionic zirconocene allyl-
merizations proceed in an isospecific fashion [6].
Further, in the pursuit of structurally related cationic
zirconium alkyl derivatives that might prove useful as
either initiators or models of the propagating species
borate
species,
(h5-C5H5)2Zr[h3-CH2CHCHCH2-
(e.g., Xꢀ
diisobutyl cyclopentadienylzirconium acetamidinates,
Cp*Zr(i-Bu)2[N(R1)C(Me)N(R2)] h5-C5Me5)
(Cp*ꢀ
/
i-Bu), it was discovered that the neutral
* Corresponding author. Tel.: ꢂ
9121.
E-mail address: ls214@umail.umd.edu (L.R. Sita).
/
1-301-405-5753; fax: ꢂ1-301-314-
/
/
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00315-2

30
D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
(R1ꢀ
/
R2ꢀ
/
i-Pr and R1ꢀ
/
Et, R2ꢀ
/t-Bu), decomposed
method that involved the use of a two-fold excess of
n-butyllithium [9]. B(C6F5)3 and [PhNMe2H][B(C6F5)4]
were obtained from Aldrich and Boulder Scientific,
at 50 8C in solution to provide the novel s2,p-trimethy-
lenemethane (TMM) cyclopentadienylzirconium aceta-
midinates, Cp*Zr(s2,p-C4H6)[N(R1)C(Me)N(R2)] (1a
and 1b, respectively), albeit not in highly reproducible
amounts given the apparent complexity of the decom-
position mechanism [7,8]. With these compounds in
hand, however, we were immediately interested in
determining whether they could be viable precursors to
Ziegler-Natta cationic or zwitterionic allyl zirconium
initiators or model complexes through direct ‘activation’
with either borate or borane reagents, respectively,
according to Eq. (1). In the present work, we now
present a reliable direct synthesis of the TMM com-
pounds 1a and 1b, the results of more extensive solution
and solid-state structural characterizations of these
species, and a documentation of their utility as pre-
cursors to the cationic and zwitterionic allyl zirconium
complexes, 2 and 3, respectively, shown in Eq. (1).
1
respectively, and used without further purification. H
and 13C{1H} NMR spectra were recorded at 400 (or
500) and 100 MHz, respectively, using either benzene-d6,
toluene-d6 (VT NMR experiments) or chlorobenzene-d5
as the solvent. Elemental analyses were performed by
Midwest Microlab.
2.2. Synthesis of Cp*Zr(s2,p-C4H6)[N(i-
Pr)C(Me)N(i-Pr)] (1a)
To a suspension of 0.63 g (2.30 mmol) of K2(TMM) in
70 ml of pentane, cooled to ꢁ78 8C, a solution of 1.03 g
/
(2.30 mmol) of 4a in 5 ml of THF was added dropwise.
The resulting mixture was allowed to warm up to
ambient temperature during 2 h, whereupon, the
volatiles were removed in vacuo. The residue was then
(1)
2. Experimental
extracted with pentane, the extracts filtered through a
small pad of Celite, and the solvent removed in vacuo to
provide a crude material that was recrystallized at
2.1. General procedures
ꢁ30 8C from a 5:1 pentane/toluene mixture (ca. 4 ml)
/
to give 0.51 g (51% yield) of 1a as a red crystalline
1
Manipulations were performed under an inert atmo-
sphere of dinitrogen using standard Schlenk techniques
or a Vacuum Atmospheres glovebox. Dry, oxygen-free
solvents were employed throughout. Diethyl ether
(Et2O), tetrahydrofuran (THF) and pentane were dis-
tilled from sodium/benzophenone (with a few milliliters
of triglyme being added to the pot in the case of
pentane), and chlorobenzene (PhCl) was refluxed for
several days over calcium hydride before being used for
polymerizations. The monomer 1-hexene (99%) was
obtained from Aldrich and stirred over NaK alloy
(1:1) overnight before being vacuum-transferred. Ben-
zene-d6 was likewise vacuum transferred from NaK
prior to being used for NMR spectroscopy. Chloroben-
zene-d5 was dried over CaH prior to vacuum transfer.
Cp*ZrCl2[N(i-Pr)C(Me)N(i-Pr)] and Cp*ZrCl2[N(Et)-
C(Me)N(t-Bu)] (4a and 4b, respectively) were prepared
as previously reported [7] and K2(TMM) was prepared
according to a slight modification of the literature
material. For 1a: H NMR (benzene-d6, 25 8C): d 0.77,
0.80 (d, 6H, Jꢀ
1.91 (s, 15H, C5Me5); 2.57 (br s, 6H, C(CH2)3); 3.38
(sept, 2H, Jꢀ
6.4 Hz, CHMe2). 13C {1H} NMR
/6.4 Hz, CHMe2); 1.52 (s, 3H, CMe);
/
(benzene-d6, 25 8C): d 12.1 (C5Me5); 12.7 (CMe); 24.8,
25.5 (CHMe2); 47.6 (CHMe2); 75.5 (C(CH2)3); 119.5
(C5Me5), 163.8, 165.4 (CMe, C(CH2)3). Anal. Calc. for
C22H38N2Zr: C, 62.65; H, 9.08; N, 6.64. Found: C,
61.86; H, 8.86; N, 6.05.
2.3. Synthesis of Cp*Zr(s2,p-C4H6)[N(t-
Bu)C(Me)N(Et)] (1b)
In the glovebox, 0.68 g (2.48 mmol) of solid
K2(TMM) was added to a suspension of 1.09 g (2.49
mmol) of 4b in 30 ml of a 10:1 Et2O/pentane mixture,
cooled to ca. ꢁ
/
30 8C in an internal refrigerator. The
resulting mixture was then allowed to gradually warm

D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ
/
38
31
up to ambient temperature. After stirring overnight, the
volatiles were removed in vacuo, the residue was
extracted with pentane, the extracts filtered through a
small pad of Celite, and then the solvent removed in
vacuo to provide a crude material that was recrystallized
2.5. Synthesis of Cp*Zr{h3-
CH2C[CH2B(C6F5)3]CH2}[N(i-Pr)C(Me)N(i-Pr)]
(3a)
Within a glovebox, a solution of 182 mg (0.43 mmol)
of 1a in 1 ml of chlorobenzene was added to a solution
of 100 mg (0.43 mmol) of B(C6F5)3 in 1 ml of
chlorobenzene at ambient temperature. The solution
was then layered with pentane (ca. 10 ml) and stored
at ꢁ30 8C from a 5:1 pentane/toluene mixture (ca. 6 ml)
/
to provide 0.24 g (23% yield) of 1b as a red crystalline
1
material. For 1b: H NMR (benzene-d6, 25 8C): d 0.87
(t, 3H, Jꢀ
(s, 3H, CMe); 1.87 (s, 15H, C5Me5); 2.57 (br s, 6H,
C(CH2)3); 2.89 (dq, 1H, 2Jꢀ13.8 Hz, 3Jꢀ
7.2 Hz,
13.8 Hz, Jꢀ7.2 Hz,
/7.2 Hz, CH2CH3); 0.92 (s, 9H, CMe3); 1.56
overnight at ꢁ
redꢃorange crystals of 3a (228 mg, 82% yield). For
compound 3a: H NMR (chlorobenzene-d5, 25 8C): d
0.67 (d, 6H, Jꢀ6.0 Hz, CHMe2), 0.71 (d, 6H, Jꢀ6.0
/
30 8C in an internal refrigerator to give
/
/
/
1
2
3
CHAHBCH3), 2.91 (dq, 1H, Jꢀ
/
/
CHAHBCH3). 13C {1H} NMR (benzene-d6, 25 8C): d
11.7 (C5Me5); 14.4 (CMe); 17.1 (CH2CH3); 31.2
(CMe3); 41.5 (CHAHBCH3); 53.0 (CMe3), 74.4
(C(CH2)3); 118.3 (C5Me5), 165.0, 166.8 (CMe,
C(CH2)3). Anal. Calc. for C22H38N2Zr: C, 62.65; H,
9.08; N, 6.64. Found: C, 61.86; H, 8.86; N, 6.05.
/
/
Hz, CHMe2); 1.70 (s, 15H, C5Me5); 1.93 (s, 3H, CMe);
2.57 (s, 2H, CH2C[CH2B(C6F5)3]CH2), 2.71 (s, 2H,
CH2C[CH2B(C6F5)3]CH2);
2.57
(br
s,
2H,
CH2C[CH2B(C6F5)3]CH2); 3.38 (sept, 2H, Jꢀ
/
6.0 Hz,
CHMe2). 13C {1H} NMR (chlorobenzene-d5, 25 8C): d
10.9, 12.0, 21.0, 22.4, 40.6, 48.9, 88.1, 124.5, 173.3,
189.9, Anal. Calc. for C40H38BF15N2Zr: C, 51.45; H,
4.10; N, 3.0. Found: C, 51.21; H, 4.18; N, 3.04.
2.4. NMR investigation of protonation of Cp*Zr(s2,p-
C4H6)[N(R1)C(Me)N(R2)] (1a: R1ꢀ
/
R2ꢀ
t-Bu) using [PhNMe2H][B(C6F5)4]
/
i-Pr; 1b:
2.6. Crystal structure determinations
R1ꢀ
/
Et, R2ꢀ
/
Data were collected on a Bruker SMART CCD
A solution of 1a or 1b (0.02 mmol in ca. 0.5 ml of
chlorobenzene-d5) was added to a suspension of 16 mg
(0.02 mmol) of [PhNMe2H][B(C6F5)4] in ca. 0.3 ml of
chlorobenzene-d5 at ambient temperature. The resulting
system operating at ꢁ80 8C. All crystallographic calcu-
/
lations were performed on a personal computer with a
Pentium 1.80 GHz processor and 512 MB of extended
memory. The SHELXTL program package was imple-
mented to determine the probable space group and set
up the initial files. Table 1 provides information on the
data collection and refinement parameters for com-
pounds 2b and 3a.
clear redꢃ
2b was then transferred to a NMR tube. In the case of
2b, upon cooling to ꢁ20 8C, small orangeꢃred crystals
/
orange solution of the cationic species 2a or
/
/
deposited upon the walls of the NMR after several
weeks. Although insufficient material was obtained for
chemical analysis, these crystals proved suitable for
single crystal X-ray analysis.
2.7. Crystal structure determination of compound 2b
An orange block with approximate orthogonal di-
mensions 0.23ꢄ
cally centered on the Bruker SMART CCD system at
100 8C. The initial unit cell was indexed using a least-
squares analysis of a random set of reflections collected
from three series of 0.38 wide v-scans, 10 s per frame,
and 25 frames per series that were well distributed in
/
0.21ꢄ
/
0.17 mm3 was placed and opti-
2.4.1. For compound 2a
1H NMR (chlorobenzene-d5, 25 8C): d 0.92 (d, 6H,
ꢁ
/
Jꢀ
/
6.4 Hz, CHMe2), 1.03 (d, 6H, Jꢀ6.4 Hz, CHMe2);
/
2.05 (s, 3H, CMe); 2.07 (s, 18H, C5Me5,
CH2C(Me)CH2); 3.0 (s, 2H, CH2C(Me)CH2), 3.18 (s,
reciprocal space. Data frames were collected [MoꢃKa]
/
2H, CH2C(Me)CH2); 3.51 (sept, 2H, Jꢀ6.4 Hz,
/
with 0.28 wide v-scans, 40 s per frame and 909 frames
per series. Six data series were collected, five at varying
CHMe2).
8
angles (8ꢀ
composed of 909 0.28 wide 8-scans with fixed vꢀ
308 and finally a partial repeat of the first series, 200
/
08, 728, 1448, 2168, 2888), a sixth
/
2.4.2. For compound 2b
1H NMR (chlorobenzene-d5, 25 8C): d 0.82, (t, 3H,
ꢁ
/
frames, for decay purposes. The crystal to detector
distance was 4.442 cm, thus providing a complete sphere
Jꢀ7.0 Hz, CH2CH3); 0.92 (s, 9H, CMe3); 1.85 and 1.90
/
(s, 21H, C5Me5, CH2C(Me)CH2, CMe); 2.73 (br s, 1H,
CH2C(Me)CH2), 2.97 (br s, 1H, CH2C(Me)CH2), 3.04
(br s, 1H, CH2C(Me)CH2); 2.88 (m, 2H, CHAHBCH3).
Note: the resonance for PhNMe2 is presumed to be
obscuring a methallyl resonance at 2.68 ppm.
of data to 2umax
ꢀ55.18. A total of 86 458 reflections
/
were collected and corrected for Lorentz and polariza-
tion effects and absorption using Blessing’s method as
incorporated into the program SADABS with 10 738
unique [Rint
ꢀ0.0379]. System symmetry, systematic
/

32
D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
Table 1
Crystallographic data and details of refinement for compounds 2b and 3a
2b
3b
¯
P1
C2/c
/
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C46H39BF20N2Zr
1101.82
173(2)
[C40H38BF15N2Zr][C6H5Cl]1.5
[C40H38BF15N2Zr][C6H5Cl]
1102.58
173(2)
monoclinic
C2/c
1046.30
173(2)
triclinic
monoclinic
P21/n
¯
P1
/
˚
a (A)
13.1664(6)
23.2144(11)
15.2561(7)
90
22.320(4)
10.7833(18)
39.856(7)
90
10.5876(3)
12.2659(4)
18.1468(5)
95.4760(10)
92.4220(10)
107.8310(10)
2226.85(11)
2
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
103.6160(10)
90
98.711(3)
90
3
˚
V (A )
Z
4532.0(4)
4
1.615
9482(3)
8
1.545
rcalc (g cmꢁ3
)
1.560
0.406
m(Mo) (mmꢁ1
)
0.362
0.413
Crystal size (mm)
2u range (8)
Total reflections
0.23ꢄ
/
0.21ꢄ
/
0.17
0.24ꢄ
1.85ꢃ25.00
63505
8362 [Rint
8362/6/817
R1ꢀ0.0584, wR2ꢀ
R1ꢀ0.0666, wR2ꢀ
1.317
/
0.20ꢄ
/
0.10
0.36ꢄ
/
0.19ꢄ0.13
/
1.82ꢃ25.00
/
/
2.19ꢃ30.00
/
31 283
51 284
Independent reflections
Data/restraints/parameters
7980 [Rint
ꢀ
/
0.0383]
ꢀ
/
0.0416]
12 837 [Rint
ꢀ/0.0253]
7980/3/690
R1ꢀ0.0448, wR2ꢀ
R1ꢀ0.0651, wR2ꢀ
1.086
12 837/7/742
R1ꢀ0.0378, wR2ꢀ
R1ꢀ0.0474, wR2ꢀ
1.104
Final R indices [I ꢀ
R indices (all data)
/
2s(I)]
/
/
0.1226
/
/
0.1145
0.1165
/
/
0.1076
/
/0.1335
/
/
/
/0.1139
Goodness-of-fit
absences and intensity statistics indicated the centro-
symmetric monoclinic non-standard space group P21/n
(no. 14). The structure was determined by direct
methods with the successful location of nearly the entire
molecule using the program XS4. The structure was
refined with XL5. The 86 458 data collected were merged
optically centered on the Bruker SMART CCD system
at ꢁ100 8C. The initial unit cell was indexed using a
least-squares analysis of a random set of reflections
collected from three series of 0.38 wide v-scans, 10 s per
frame, and 25 frames per series that were well distrib-
uted in reciprocal space. Data frames were collected
/
based upon identical indices yielding 41 531 data [Rint
0.0407] that were further truncated to 2umax 50.08 and
merged and during least-squares refinement to 7980
unique data [Rint 0.00383]. A series of least-squares
difference-Fourier cycles were required to locate the
remaining non-hydrogen atoms. All full-occupancy non-
hydrogen atoms were refined anisotropically. All of the
hydrogen atoms were located directly from an addi-
tional difference-Fourier map and allowed to refine
freely. Disorder was modeled for the ethyl and t-butyl
groups. A centroid was calculated for the pentamethyl-
cyclopentadienyl ligand. The final structure was refined
ꢀ
/
[Moꢃ
frames per series. Five data series were collected at
varying 8 angles [8ꢀ08, 728, 1448, 2168, 2888], includ-
ing a partial repeat of the first series, 200 frames, for
decay purposes. The crystal to detector distance was
4.371 cm, thus providing a complete sphere of data to
/
Ka] with 0.38 wide v-scans, 40 s per frame and 606
ꢀ
/
/
ꢀ
/
2umax
ꢀ50.08. A total of 63 505 reflections were col-
/
lected and corrected for Lorentz and polarization effects
and absorption using Blessing’s method as incorporated
into the program SADABS with 8362 unique [Rint
ꢀ
/
0.0416]. System symmetry, systematic absences and
intensity statistics indicated the centrosymmetric mono-
clinic non-standard space group C2/c (no. 15). The
structure was determined by direct methods with the
successful location of nearly the entire molecule of
interest using the program XS. The structure was refined
with XL. A single least-squares difference-Fourier cycle
was required to locate the remaining non-hydrogen
atoms. All non-hydrogen atoms were refined anisotro-
pically. All of the hydrogen atoms were located directly
from an additional difference-Fourier map and allowed
to refine freely. A centroid was calculated for the
pentamethylcyclopentadienyl ligand and also for the
to convergence [D/s 5
wR(F2)ꢀ
13.35%, GOFꢀ
reflections [R(F)ꢀ
4.48%, wR(F2)ꢀ
6174 data with Foꢀ4s(Fo)]. The final difference-Four-
/
0.001] with R(F)ꢀ
1.086 for all 7980 unique
12.26% for those
/
6.51%,
/
/
/
/
/
ier map was featureless indicating that the structure is
both correct and complete.
2.8. Crystal structure determination of compound 3a
A reddish orange block with approximate orthogonal
0.2ꢄ
0.10 mm3 was placed and
dimensions 0.24ꢄ
/
/

D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ
/
38
33
allylic. The final structure was refined to convergence [D/
s 50.001] with R(F)ꢀ 11.65%,
6.66%, wR(F2)ꢀ
GOFꢀ1.317 for all 8362 unique reflections [R(F)ꢀ
5.84%, wR(F2)ꢀ
11.45% for those 7364 data with
Foꢀ4s(Fo)]. The final difference-Fourier map was
of the decomposition pathway leading to their forma-
tion that also produces difficult to separate crystalline
coproducts [8]. Accordingly, a direct synthetic route to
these materials was sought and as Eq. (2) reveals, this
was secured through reaction of the corresponding
dichloro cyclopentadienylzirconium acetamidinates 4
[7] with the dipotassium salt of the trimethylenemethane
dianion, K2(TMM) [9]. While the unoptimized yields of
1a and 1b that can be obtained by this procedure are
modest for 1a and low for 1b, 51 and 24%, respectively,
the amounts that can be obtained are reproducible and
purification of the compounds can be achieved by a
simple single recrystallization.
/
/
/
/
/
/
/
featureless indicating that the structure is both correct
and complete.
A second reddish orange block with approximate
orthogonal dimensions 0.362ꢄ
placed and optically centered on the Bruker SMART
CCD system at ꢁ100 8C. The initial unit cell was
/
0.194ꢄ
0.125 mm3 was
/
/
indexed using a least-squares analysis of a random set
of reflections collected from three series of 0.38 wide v-
scans, 10 s per frame, and 30 frames per series that were
well distributed in reciprocal space. Data frames were
collected [Moꢃ
frame and 909 frames per series. Six data series were
collected, five at varying 8 angles (8ꢀ08, 728, 1448,
2168, 2888), a sixth composed of 1200 0.28 wide v-scans
08 and finally, 200 frames, a partial repeat
/
Ka] with 0.28 wide v-scans, 20 s per
/
with fixed 8ꢀ
/
of the first series for decay purposes. The crystal to
detector distance was 4.371 cm, thus providing a
ð2Þ
complete sphere of data to 2umax
ꢀ60.08. A total of
/
51 284 reflections were collected and corrected for
Lorentz and polarization effects and absorption using
Blessing’s method as incorporated into the program
SADABS with 12 837 unique [Rint
symmetry, lack of systematic absences and intensity
ꢀ
/
0.0253]. System
3.2. Solid-state and solution structural studies of
complexes 1a and 1b
statistics indicated the centrosymmetric triclinic space
¯
1 (no. 2). The structure was determined by
group P
/
Only three TMM derivatives of early transition metals
other than 1a and 1b are known [10], and of these, only
two have been the subject of crystallographic studies;
anionic [Cp*Zr(TMM)Cl2]ꢁ (5) in which the TMM
ligand is proposed to coordinate in a h4-fashion [10b],
direct methods with the successful location of nearly the
entire molecule of interest using the program XS. The
structure was refined with XL. A series of least-squares
difference-Fourier cycles were required to locate the
remaining non-hydrogen atoms. All non-hydrogen
atoms were refined anisotropically. All of the hydrogen
atoms were located directly from an additional differ-
ence-Fourier map and allowed to refine freely. A
centroid was calculated for the pentamethylcyclopenta-
dienyl ligand and also for the allylic region of the large
multidentate ligand. The final structure was refined to
ꢅ
and the neutral zirconocene, Cp2 Zr(TMM) (6), in which
the h2-TMM group is clearly interacting with the metal
center through only two s-bonds. The h2-TMM inter-
action of 6 will henceforth then be regarded as an
example of s2-TMM bonding. To probe the nature of
bonding in 1, the crystal structures of 1a and 1b were
obtained and presented in our initial report where we
stated, without extensive comment, that their structural
parameters supported a s2,p-TMM bonding mode [7].
Here, we now make a stronger case for this designation
through a comparison of the molecular structures and
selected structural parameters of 1a and 1b that are
presented in Fig. 1 and Table 2, respectively. To begin,
upon first inspection, compounds 1a and 1b would
appear to present nearly identical structures with respect
to the cyclopentadienyl, acetamidinate, and TMM
fragments. However, closer analysis reveals that, due
most likely to increased steric interactions, the TMM
moiety dissymmetrically lies closer to the N-Et side of
the acetamidinate in 1b rather than being almost
symmetrically disposed as in the structure of 1a (cf.
convergence
wR(F2)ꢀ
11.39%, GOFꢀ
reflections [R(F)ꢀ
3.78%, wR(F2)ꢀ
10 968 data with Foꢀ4s(Fo)]. The final difference-
[D/s 5
/
0.001]
1.104 for all 12 837 unique
10.76% for those
with
R(F)ꢀ4.74%,
/
/
/
/
/
/
Fourier map was featureless indicating that the structure
is both correct and complete.
3. Results and discussion
3.1. Preparation of TMM complexes 1a and 1b
Although compounds 1a and 1b have previously been
reported by us [7], reliable amounts of these TMM
complexes were difficult to obtain due to the complexity
the nonbonded distances for N(1)ꢀ
/
C(20) and N(2)ꢀ
/

34
D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
Fig. 1. Molecular structures (30% thermal ellipsoids) of (a) compound 1a and (b) compound 1b. Hydrogen atoms have been removed for the sake of
clarity.
that involving C(22) now falls outside a conventional
zirconiumꢃcarbon bonding distance [cf. 2.642 and 2.553
A for 1a and 1b, respectively]. Within the TMM
C(21)
bond distances are distinctly longer in both complexes
at 1.440(3); 1.442(2) in 1a and 1.431(3); 1.437(3) in 1b
Table 2
Selected distances (A) and angles (8) for compounds 1a and 1b
˚
/
˚
1a
1b
fragment itself, the C(19)ꢀ
/
C(20) and C(19)ꢀ
/
Distances
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
C(19)ꢀ
C(19)ꢀ
C(19)ꢀ
Zr(1)ꢀ
N(1)ꢀ
N(2)ꢀ
/
N(1)
2.2306(12)
2.2306(12)
2.3239(14)
2.3774(17)
2.3888(17)
1.442(2)
1.440(3)
1.385(2)
2.642
2.2412(15)
2.2421(14)
2.3102(17)
2.4144(19)
2.3926(19)
1.431(3)
1.437(3)
1.394(3)
2.553
/
N(2)
relative to that for the C(19)ꢀ
observed values are 1.385(2) for 1a and 1.394(3) for 1a.
This last structural comparison suggests that the C(19)ꢀ
/
C(22) bond for which the
/
C(19)
C(20)
C(21)
/
/
/
/
C(20)
C(21)
C(22)
C(22) bond has a greater degree of ‘double bond’
character which suggests that the s2,p-TMM bonding
mode that is depicted in Fig. 1 is operative in both
structures. It must be pointed out, however, that C(19)
in both 1a and 1b is pyramidalized to a large extent as
indicted by the bond angle sums for auC(19) of 348.78 in
1a and 348.08 in 1b. Thus, an argument could also be
made for a strong contribution to Zr-TMM bonding of
/
/
/
C(22)
/
C(20)
C(21)
3.462
3.763
3.149
4.103
/
Angles
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
C(20)ꢀ
C(20)ꢀ
C(21)ꢀ
/
N(1)ꢀ
N(2)ꢀ
N(2)ꢀ
/
C(13)
C(16)
C(15)
145.7(10)
142.65(10)
142.05(12)
/
/
ꢃ/
a h4-interaction where the Zr(1)ꢀ
/
C(22) bond is sub-
/
/
ꢃ/
135.29(11)
113.77(19)
118.12(19)
116.2(2)
/
C(19)ꢀ
C(19)ꢀ
C(19)ꢀ
/
C(21)
C(22)
C(22)
113.83(16)
116.86(18)
117.96(17)
stantially elongated due to a strong trans-effect of the
cyclopentadienyl group as originally proposed by Bazan
et al. [10b]. To illuminate the TMM-bonding picture in 1
further, solution studies involving variable temperature
1H NMR spectroscopy of 1a and 1b were carried out.
/
/
/
/
˚
C(21) of 3.462 and 3.763 A in 1a vs. the corresponding
˚
distances of 3.149 and 4.103 A in 1b). The increased
1
At room temperature, H NMR spectra showed that
the TMM fragments in 1a and 1b are not static on the
NMR timeframe, but rather, they are engaged in a
dynamic process that serves to exchange certain sets of
formally magnetically inequivalent protons by ‘rotation’
steric interactions in 1b are also manifested in the slight
pyramidalization observed for N(2) which bears the
tert-butyl substituent (cf. the sum of the bond angles
about N(2), auN(2)ꢀ356.28, in 1b vs. 3608 for all the
/
of the TMM group about the Zr(1)ꢀ
/
C(19) ‘bond’. Due
other nitrogen atoms in 1a and 1b). Regarding bonding
to the TMM fragment, in both structures, the shortest
to the Cs-symmetric nature of 1a, all six protons of the
TMM group are expected to become equivalent from
this process, whereas the C1-symmetry of 1b should
compartmentalize the six protons into two sets of three
site-exchanging protons, if a TMM ‘face-flipping’ pro-
cess does not additionally exist. For 1a, an apparent
zirconiumꢃ/carbon distance is found between the metal
and the central carbon atom, C(19), of the TMM group
˚
at 2.3239(14) A in 1a and 2.3102(17) A in 1b. In
˚
comparison, the zirconiumꢃcarbon distances involving
/
1
C(20) and C(21) are significantly longer [cf. 2.3774(17);
2.3888(17) in 1a and 2.4144(10); 2.3926(19) in 1b], while
slow exchange limit H NMR (400 MHz, toluene-d8)
spectrum taken at ꢁ80 8C revealed three separate
/

D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ
/
38
35
resonances at 1.56 (2H), 2.19 (2H) and 4.32 (2H) ppm
for three sets of magnetically inequivalent protons
expected for a static TMM group. The higher field
resonances at 1.56 and 2.19 ppm are assigned to being
those for the diastereotopic protons on the carbon
atoms that are s-bonded to zirconium within the s2,p-
bonding scheme (i.e. C(20) and C(21) in Fig. 1). The
remaining downfield resonance at 4.32 ppm is then
assigned to belonging to the magnetically equivalent
protons on the carbon which is postulated as having
significant sp2-character due to its involvement in the
formal double bond within this same s2,p-bonding
scheme (i.e. C(22) in Fig. 1). The absence of coupling
between the two sets of diastereomeric protons indi-
cated, however, that dynamic exchange was still occur-
obtained previously for Cp*Ta(TMM)Me2 [10a], but in
sharp contrast, a high barrier for this process was
reported for the Zr-TMM complex 5, and compound 6
apparently does not engage in TMM rotation at all
[10b,10c].
1
Fig. 2 shows selected variable temperature H NMR
spectra for 1b taken at the apparent slow exchange limit,
at coalescence, and above the coalescence temperature
of the dynamic process involving TMM group rotation.
In keeping with expectations, the apparent slow ex-
change limit 1H NMR spectrum for 1b displays the
expected six resonances for each of the six magnetically
inequivalent protons of the TMM group within the
static structure (d 1.48, 1.96, 2.07, 2.50, 3.74, and 4.21
ppm). Qualitatively, the barrier to rotation in this
complex is higher than that of 1a presumably as a result
of the increased steric interactions of the TMM and
acetamidinate fragments discussed earlier. Importantly,
the fast exchange spectrum taken at 70 8C reveals two
sets of resonances for the TMM fragment at 2.37 and
2.50 ppm (3H each) which demonstrates that neither
face-flipping of the TMM, or any change in its hapticity
that would serve to exchange any two diastereomeric
protons, occurs at these elevated temperatures.
ring at an appreciable rate even at ꢁ80 8C and this
/
1
assumption was verified by a 2D H EXSY spectrum
taken at this temperature which produced significant
crosspeaks for the exchanging protons within a wide
range of mixing times [11]. Upon warming, coalescence
of all three resonances for the TMM group was
observed to occur at ꢁ40 8C and at 25 8C, a broad
/
singlet for these six protons materialized at 2.53 ppm. A
similarly low barrier for TMM group rotation was
Fig. 2. Partial 1H NMR (400 MHz, toluene-d8) spectra of 1b recorded at (a) ꢁ
resonance for the methyl substituent of the acetamidinate group is marked with an asterisk (*) and the resonances for the TMM fragment are
indicated by pluses (ꢂ). The two resonances appearing in the range of 2.7ꢃ3.1 ppm are for the two diastereotopic protons of the N-Et substituent of
the acetamidinate group.
/
70 8C, (b) the coalescence temperature of ꢁ10 8C, and (c) 60 8C. The
/
/
/

36
D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
3.3. Generation of cationic methallyl complexes 2a and
2b and the solid-state structure of 2b
Table 3
Selected bond distances (A) and angles (8) for compounds 2b and 3a
˚
2b
3a
Attempts by Brintzinger and co-workers [4] to gen-
erate the cationic zirconocene complex, [(h5-
C5H5)2Zr(h3-C4H7)]ꢂ, possessing the less strongly
bound borate anion, [B(C6F5)4]ꢁ, were thwarted by
non selective protonation of both the methyl and
methallyl groups of (h5-C5H5)2ZrMe(h3-C4H7) by
[PhNMe2H][B(C6F5)4]. Indeed, to the best of our
knowledge, such ‘looser’ ion pairs of cationic zirconium
allyl complexes have yet to be prepared and character-
ized. Accordingly, it was with great satisfaction that it
¯
P1
C2/c
/
Distances
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
B(1)ꢀ
C(19)ꢀ
C(19)ꢀ
C(19)ꢀ
/
N(1)
2.149(3)
2.188(3)
2.500(4)
2.448(5)
2.395(4)
2.190(4)
2.162(4)
2.512(4)
2.392(4)
2.432(5)
1.684(6)
1.411(6)
1.398(6)
1.477(6)
2.2022(14)
2.1586(14)
2.4991(15)
2.3864(16)
2.4714(17)
1.687(2)
/
N(2)
/
C(19)
C(20)
C(21)
/
/
/C(22)
ꢃ/
/
C(20)
C(21)
C(22)
1.374(7)
1.378(6)
1.500(6)
1.413(2)
1.406(3)
1.493(2)
/
/
was found that addition of
1
equivalent of
Angles
Zr(1)ꢀ
Zr(1)ꢀ
Zr(1)ꢀ
C(20)ꢀ
C(20)ꢀ
C(21)ꢀ
C(19)ꢀ
[PhNMe2H][B(C6F5)4] to either 1a or 1b in chloroben-
zene at 25 8C resulted in quantitative formation of the
cationic methallyl complexes, 2a and 2b, as determined
/
N(1)ꢀ
N(2)ꢀ
N(2)ꢀ
/
C(13)
C(16)
C(17)
136.1(2)
144.6(3)
145.5(3)
144.90(14)
143.74(12)
/
/
ꢃ/
/
/
138.7(2)
119.0(5)
120.8(5)
118.4(5)
ꢃ/
ꢃ/
1
by H NMR spectroscopy (see Eq. (1)). More specifi-
/
C(19)ꢀ
C(19)ꢀ
C(19)ꢀ
C(22)ꢀ
/
C(21)
C(22)
C(22)
B(1)
117.5(4)
229.6(4)
121.2(4)
114.1(3)
117.16(16)
120.29(15)
120.50(16)
113.07(13)
/
/
cally, an apparent static NMR spectrum was obtained
for 2a at 20 8C in chlorobenzene-d5 that shows two
sharp methylene resonances for the methallyl ligand at
2.85 and 3.02 ppm and two resonances for the diaster-
eotopic methyl groups of the isopropyl substituents on
the acetamidinate fragment at 0.75 and 0.86 ppm.
Warming the NMR sample of 2a eventually led to
coalescence of the two methylene resonances at 60 8C
/
/
/
/
ꢃ/
2b recorded in chlorobenzene-d5 similarly showed an
apparent static structure. Finally, it can be mentioned
that both 2a and 2b are quite robust in solution at
elevated temperatures with no indication of decomposi-
tion after several hours according to H NMR.
Upon cooling the NMR sample of 2b to ꢁ
for which a h30
/
h1 allyl rearrangement barrier of
1
DG%c ꢀ16.2 kcal molꢁ1 could be estimated. This barrier
/
/
10 8C, a
is slightly higher than that observed for the similar
process in [(h5-C5H5)2Zr(h3-C4H7)]ꢂ[MeB(C6F5)4]ꢁ [4]
and it can most likely be attributed to the somewhat
stronger preference for h3-coordination of the methallyl
ligand in 2a due to an increased charge separation in the
small number of orangeꢃred crystals were deposited
/
upon the walls of the NMR tube after several weeks.
Single crystal X-ray analysis confirmed that these
crystals were that of 2b and Fig. 3 and Table 3
respectively present the molecular structure and selected
structural parameters for this complex. Of particular
1
ion pair. The room temperature H NMR spectrum for
note are the zirconiumꢃnitrogen bond distances of
/
˚
2.149(3) and 2.188(3) A in 2b which are significantly
shorter than those found for the neutral TMM com-
plexes 1a and 1b. The methallyl fragment comprising
C(19), C(20), C(21), and C(22) possesses geometrical
parameters that are consistent with h3-allyl ligation
where the shortest zirconiumꢃ
/
carbon contacts are found
for the Zr(1)ꢀC(20) and Zr(1)ꢀ
/
/C(21) distances. It must
also be noted that no close contacts between the
electrophilic zirconium center and any fluorine atoms
of the borate counterion (not shown for the sake the
clarity) are observed in the solid state packing scheme.
3.4. Preparation and characterization of the zwitterionic
complexes 3a and 3b
Following the example of Erker and co-workers [5],
we were interested in determining whether addition of
B(C6F5)3 to 1a and 1b would provide the corresponding
zwitterionic allyl-borate complexes 3a and 3b according
to Eq. (1). Satisfactorily, addition of 1 equivalent of this
Fig. 3. Molecular structure (30% thermal ellipsoids) of compound 2b.
Hydrogen atoms and the [B(C6F5)4] counterion have been removed for
the sake of clarity.

D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ
/
38
37
structural parameters suggest that no significant differ-
ences between the two structures exist apart from the
observation that the nitrogen atoms are slightly more
¯
pyramidalized in the P1 structure vs. those of the C2/c
/
modification. This observation is consistent with our
previous experiences where it has been noted that the
soft potential surface for the amidinate group allows it
to accommodate otherwise strong steric interactions
within a structure through distortions that incur little
or no energy penalties. Comparing the structure of 3a to
those of 1a and 1b, it is interesting to note that the
carbon atoms of the allyl moiety in the former [i.e.
C(19), C(20), and C(21)] are all positioned further away
from the zirconium center than the corresponding atoms
within the TMM derivatives. Finally, it can be noted
that C(19) in 3a is essentially trigonal coplanar with a
auC(19) value of 3588 in both crystalline modifications.
With respect to solution studies of 3a and 3b, variable
1
temperature H NMR spectra revealed a static h3-allyl
Fig. 4. Molecular structure (30% thermal ellipsoids) of compound 3a
from the monoclinic C2/c crystalline form. Hydrogen atoms have been
removed for the sake of clarity.
structure for these compounds in the temperature range
of 20ꢃ120 8C. Further, as in the case of 2a and 2b, these
/
zwitterionic species were found to be thermally robust in
solution for extended periods of time at elevated
temperatures.
borane to the TMM complexes 1 in chlorobenzene at
room temperature led to instantaneous formation of a
redꢃ
could be obtained in the case of 3a upon addition of
pentane and cooling to ꢁ10 8C. Single-crystal X-ray
analysis revealed the existence of two different crystal-
/
orange colored solution, from which, red crystals
3.5. Ziegler-Natta polymerization activity of 2 and 3 with
ethylene and 1-hexene
/
¯
line polymorphs, C2/c and P1 modifications, that
/
With both the cationic methallyl complexes 2 and the
zwitterionic allyl complexes 3 having been prepared, it
was of interest to determine whether they could serve as
well defined initiators for the Ziegler-Natta polymeriza-
tion of ethylene and a-olefins. Given the general
insolubility of 2 and 3 in any solvent other than
chlorobenzene, polymerization studies were limited to
this solvent medium. Surprisingly, a series of studies
revealed that 2 and 3 are both inactive for polymeriza-
possess different solid-state structures for 3a. As Figs.
4 and 5 and Table 3 show, the differences in these two
structures mainly involve the relative orientation of the
two isopropyl substituents on the acetamidinate frag-
¯
ments. In the P1 modification, it would appear from
/
Fig. 5 that the isopropyl groups might be engaged in
stronger steric interactions with the allyl fragment than
in the C2/c polymorph. However, an inspection of the
¯
1 crystalline
Fig. 5. Comparison of the molecular structures (30% thermal ellipsoids) of 3a obtained from the (a) monoclinic C2/c and (b) triclinic P
/
modifications. Hydrogen atoms and the aryl ring carbon and fluorine atoms of the B(C6F5)3 group have been removed for the sake of clarity.

38
D.A. Kissounko et al. / Journal of Organometallic Chemistry 683 (2003) 29ꢃ38
/
(b) G. Jeske, H. Lauke, H. Mauermann, P.N. Swepstone, H.
Schumann, T.J. Marks, J. Am. Chem. Soc. 107 (1985) 8091;
(c) J.J. Eshuis, Y.Y. Tan, A. Meetsma, J.H. Teuben, J. Renkema,
G.G. Evens, Organometallics 11 (1992) 362;
tion in this solvent over a range of pressure and
1
temperature, including a H NMR study that further
showed that even a single insertion of 1-hexene into 2
and 3 does not occur at temperatures up to 60 8C. Upon
inspection of space-filling models, the reluctancy of 2
and 3 to engage in olefin insertion maybe largely steric
in nature. Indeed, past observations made by us have
shown that subtle differences in steric interactions can
have a profound effect on the polymerization activity of
the even more open [Cp*ZrMe{N(R1)C(Me)N-
(R2)}]ꢂ[B(C6F5)4]ꢁ initiators [6]. The present study
then serves to indicate that the formation of cationic
allyl complexes from the propagating species derived
from these initiators during polymerization, if it ever
occurs, would be an irreversible terminating event.
(d) D.E. Richardson, G.N. Alameddin, M.F. Ryan, T. Hayes,
J.R. Eyler, A.R. Siedle, J. Am. Chem. Soc. 118 (1996) 11244;
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4. Supplementary material
[5] (a) B. Temme, G. Erker, J. Karl, H. Luftmann, R. Frohlich, S.
¨
Kotila, Angew. Chem. Int. Ed. 34 (1995) 1755;
Crystallographic (excluding structure factors) for the
structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication nos. CCDC 205019 (for compound 1a),
205020 (for compound 1b), 207303 (for compound 2b),
(b) J. Karl, G. Erker, R. Frohlich, J. Organometal. Chem. 535
¨
(1997) 59.
[6] (a) K.C. Jayaratne, L.R. Sita, J. Am. Chem. Soc. 122 (2000) 958;
(b) K.C. Jayaratne, R.J. Keaton, D.A. Henningsen, L.R. Sita, J.
Am. Chem. Soc. 122 (2000) 10490;
205523 (for compound 3a in the C2/2 crystalline
¯
(c) R.J. Keaton, K.C. Jayaratne, J.C. Fettinger, L.R. Sita, J. Am.
Chem. Soc. 122 (2000) 12909;
modification) and 205524 (for compound 3a in the P
/
1
(d) R.J. Keaton, K.C. Jayaratne, D.A. Henningsen, L.A. Koter-
was, L.R. Sita, J. Am. Chem. Soc. 123 (2001) 6197;
(e) K.C. Jayaratne, L.R. Sita, J. Am. Chem. Soc. 123 (2001)
10754;
crystalline modification). Copies of this data can be
obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
ꢂ44-1223-336-033; email: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
/
(f) D.A. Kissounko, J.C. Fettinger, L.R. Sita, Inorg. Chim. Acta
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[7] R.J. Keaton, L.A. Koterwas, J.C. Fettinger, L.R. Sita, J. Am.
Chem. Soc. 124 (2002) 5932.
[8] R.J. Keaton, L.R. Sita, Organometallics 21 (2002) 4315.
[9] A.M.M. Simmons, N.S. Mills, S.R. Iyer, Inorg. Chim. Acta 116
(1986) 43.
Acknowledgements
[10] (a) J.M. Mayer, C.J. Curtis, J.E. Bercaw, J. Am. Chem. Soc. 105
(1983) 2651;
This work was supported by the National Science
Foundation (CHM-0092493) for which we are grateful.
(b) G.C. Bazan, G. Rodriguez, B.P. Cleary, J. Am. Chem. Soc.
116 (1994) 2177;
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