Synthesis and structure of trialkyltantalum complexes stabilized by aminopyridinato ligands

Article abstract of DOI:10.1002/ejic.200600120

(4-Methylpyridin-2-yl)(trimethylsilyl)amine (1), (6-methylpyridin-2-yl) (trimethylsilyl)amine (2), and (2,6-diisopropylphenyl)(pyridin-2-yl)ainine (3) were deprotonated and used as ligands to synthesize trialkyltantalum complexes. The reaction of 2 equiv.

Full text of DOI:10.1002/ejic.200600120

FULL PAPER
DOI: 10.1002/ejic.200600120
Synthesis and Structure of Trialkyltantalum Complexes Stabilized by
Aminopyridinato Ligands
Awal Noor,[a] Winfried Kretschmer,[b] and Rhett Kempe*[a,c]
Keywords: Amido ligands / Aminopyridinato ligands / N ligands / Olefin polymerization / Tantalum
(4-Methylpyridin-2-yl)(trimethylsilyl)amine (1), (6-methyl-
pyridin-2-yl)(trimethylsilyl)amine (2), and (2,6-diisopropyl-
phenyl)(pyridin-2-yl)amine (3) were deprotonated and used
as ligands to synthesize trialkyltantalum complexes. The re-
action of 2 equiv. of 1 or 2 with pentabenzyltantalum afforded
compounds like methyllithium affords the corresponding tri-
alkyltantalum complexes. X-ray diffraction studies of four of
the synthesized complexes were carried out. They adopt two
different coordination environments, either slightly distorted
capped octahedrons (sterically less demanding aminopyridin-
ato ligands) or pentagonal bipyramids (bulkier aminopyridin-
ato ligands). The alkyl species were surprisingly stable at el-
evated temperatures and no formation of mixed alkyl/alk-
ylidene complexes was observed. Alkyl cation formation and
the behaviour of a selection of these compounds in olefin po-
lymerization were explored.
tribenzyltantalum(V) complexes by toluene elimination.
Analogous reaction using 3 failed. Lithiation of 3 followed by
the reaction with tribenzyltantalum dichloride gave rise to
the corresponding tribenzyl complex. Other alkyltantalum
complexes stabilized by this ligand environment can be pre-
pared by treating tantalum pentachloride with 2 equiv. of
lithiated 3 to form a bis(aminopyridinato)tantalum trichloride. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
The reaction of this trichloride with 3 equiv. of alkyllithium
Germany, 2006)
Introduction
Aminopyridinato ligands[1] are interesting amido ligands
because of the flexibility of their binding mode and the li-
gand “asymmetry”. The dominating binding mode in early
transition metal and lanthanide chemistry[2] is the strained
η2 coordination (Scheme 1, left). The bridging binding
mode (Scheme 1, right) is characteristic for late transition
metal complexes. Of course, there are exceptions like homo-
leptic strained η2-bound (aminopyridinato)palladium com-
plexes.[3] The ligand “asymmetry” caused by the two dif-
ferent donor functionalities−the pyridine and amido
function−might be considered as an additional interesting
feature especially in comparison to the closest “relatives”,
the amidinates.[4]
Scheme 1. Important binding modes of deprotonated 2-aminopyri-
dines ([M] and [MЈ] = transition metal moiety; R = aryl, silyl, or
alkyl substituent).
Scheme 2. Aminopyridinato (left) and amidinate (right) ligands
Group 5 metal complexes stabilized by aminopyridinato
ligands (Scheme 2, left) have been investigated to a lesser
extent than related amidinate[5] complexes (Scheme 2,
right).
The first Ap (Ap = aminopyridinato) group 5 metal com-
plex was published by Gambarotta 15 years ago.[6] A more
detailed investigation of some aspects of the coordination
([M] = group 5 metal complex moiety; R, RЈ = substituent).
chemistry of these ligands was started by Polamo. He ap-
plied the “direct synthesis” method to prepare a variety of
niobium and tantalum complexes stabilized by aminopyrid-
inato ligands.[7–10] Interesting examples of aminopyridinato
low oxidation state group 5 metal complexes have been de-
scribed by Gambarotta and Cotton.[11] Reactivity studies
employing organometallic species are still very rare.[12,13]
Because of this lack of information we became interested in
alkyltantalum complexes. We also expect an easy access
into this chemistry because of the recently published Kol
synthesis of [Ta(CH2C6H5)5].[14,15] We report here on the
synthesis and structure of organotantalum complexes stabi-
lized by aminopyridinato ligands. The formation of organ-
[a] Lehrstuhl Anorganische Chemie II, Universität Bayreuth,
95440 Bayreuth, Germany
E-mail: kempe@uni-bayreuth.de
[b] Stratingh Institut for Chemistry and Chemical Engineering,
Center for Catalytic Olefin Polymerization, University of
Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
[c] Leibniz-Institut für Katalyse,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
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A. Noor, W. Kretschmer, R. Kempe
FULL PAPER
ocations and the relevance to olefin polymerization of a se- groups give two doublets at δ = 2.98 and 3.18 ppm. The
lection of these species is also discussed.
product was characterized by X-ray analysis (molecular
structure is shown in Figure 2) in addition to NMR spec-
troscopy. Details of the X-ray crystal structure analysis are
summarized in Table 3. The coordination of 5 is best de-
scribed as a capped octahedron (first triangle N1, N2, and
N4; second triangle N3, C15, and C22). In both cases, the
rigid aminopyridinato ligands induce distortion from the
ideal symmetry. The Ta–C bond lengths lay in the ranges
of 2.247–2.318 Å and 2.248–2.264 Å, respectively, for 4 and
5, which clearly indicate η1 binding of all benzyl groups in
the solid state.
Results and Discussion
Synthesis of Ligands
Compounds 1 and 2 were prepared as reported.[16,17] The
aminopyridine 3 can be synthesized by palladium-catalyzed
arylamination.[18] The reaction of 2-bromopyridine with
2,6-diisopropylaniline and sodium tert-butoxide in the pres-
ence of a Pd catalyst in toluene (90 °C, 48 h) leads after
workup and purification by crystallization to compound 3
(Scheme 3) in good yield (70%).
Scheme 3. Applied aminopyridines.
Synthesis and Structure of Tantalum Complexes
Reaction of 2 equiv. of 1 with [Ta(CH2C6H5)5] in hexane
at 60 °C results in the formation of 4 by toluene elimination
(Scheme 4). No formation of benzylidene functionalities is
observed by NMR spectroscopy in the course of this reac-
tion. Complex 4 was isolated after workup in hexane in
moderate yield as a yellow crystalline material.
Figure 1. Molecular structure of 4; selected bond lengths [Å] and
angles [°]: Ta–C1 2.248(9), Ta–N4 2.230(8), Ta–N2 2.266(8), Ta–
N1 2.123(10), Ta–N3 2.132(8), Ta–C8 2.264(1), Ta–C15 2.257(10),
N2–C22 1.332(13), N1–C22 1.400(13), N3–C28 1.376(13); N1–Ta–
N2 61.00(3), N1–C22–N2 109.49(8), Ta–N2–C22 92.05(6), Ta–N1–
C22 96.31(6), N4–Ta–N3 59.10(3), N3–C28–N4 109.06(8), Ta–N3–
C28 96.53(6), Ta–N4–C28 93.21(6), C1–Ta–C8 78.28(4), C15–Ta–
C8 78.60(4), C1–Ta–C15 126.83(4).
Scheme 4. Synthesis of 4 and 5 (4: R = H, RЈ = CH3; 5: R = H,
RЈ = CH3).
The three benzyl groups show a broad doublet at δ =
2.96 ppm at room temperature indicative of a fast ligand
exchange. Crystals suitable for X-ray analysis were grown
from a hexane solution. Details of the X-ray crystal struc-
ture analysis are summarized in Table 3. The molecular
structure of 4 is shown in Figure 1. The coordination of
4 is best described as a distorted capped octahedron. The
methylene carbon atom of one of the benzyl ligands (C15)
as well as the two pyridine nitrogen atoms occupy the first
triangle and C8 as well as the two amido nitrogen atoms
the second triangle. In order to apply a slightly increased
steric bulk in close proximity to the metal center 2 equiv. of
2 were treated with [Ta(CH2C6H5)5] under the same condi-
tions as applied to 4. The resulting benzyl complex 5 was
Figure 2. Molecular structure of 5; selected bond lengths [Å] and
angles [°]: Ta–C1 2.250(11), Ta–N2 2.382(10), Ta–N4 2.248(10),
Ta–N1 2.069(11), Ta–N3 2.212(9), Ta–C15 2.247(11), Ta–C22
2.318(10), N2–C2 1.379(16), N4–C9 1.355(14), N1–C2 1.375(15),
N3–C9 1.391(13); C1–Ta–C15 81.09(5), C1–Ta–C22 76.15(4), C15–
Ta–C22 130.18(5), N1–Ta–N2 60.31(4), N1–C2–N2 109.77(12),
Ta–N1–C2 101.02(8), Ta–N2–C2 87.22(9), N4–Ta–N3 61.44(3),
obtained in moderate yield (Scheme 4). The three benzyl Ta–N3–C9 93.36(7), Ta–N4–C9 92.80(7), N4–C9–N3 112.12(11).
2684 Eur. J. Inorg. Chem. 2006, 2683–2689
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Trialkyltantalum Complexes Stabilized by Aminopyridinato Ligands
FULL PAPER
In order to increase the steric bulk of the used aminopyr-
idinato ligands drastically we introduced the 2,6-diisopro-
pylphenyl substituent at the amido nitrogen atom instead
of the SiMe3 group. Application of the toluene elimination
route starting from [Ta(CH2C6H5)5] was not successful and
salt metathesis was adopted. The reaction of 2 equiv. of the
lithium salt of 3 with [TaCl2(CH2C6H5)3][14] at room tem-
perature (Scheme 5) resulted in the formation of the triben-
zyl complex 6. The 1H NMR spectra of this compound
shows two different sets of signals for benzyl protons. One
signal corresponds to four methylene protons of the benzyl
substituents and the other to the remaining two. We pro-
pose a pentagonal-bipyramidal arrangement in solution
with the two amino and one of the benzyl moieties in the
equatorial plane. Because of the steric bulk of the deproton-
ated 3 no exchange on the NMR time scale at room tem-
perature is observed. Two benzyl resonances were found,
one at δ = 3.60 ppm and the second at δ = 2.79 ppm.
Scheme 6. Synthesis of 7 and 8.
ride ion form the pentagonal (equatorial) plane. In 8 methyl
ligands adopt the same coordination sites as the chloro li-
gands in 7. A distortion is caused by the small N–Ta–N
angles due to the strained binding mode of the aminopyrid-
inato ligands. In 7 they are 60.1(3) and 60.4(3)° and in 8
59.25(2)°. Both lead to a situation in which all other angles
in the pentagonal plane are over 72°. In 7 N–Ta–Cl(2) cis
angles are 77.9(2) and 77.3(2)° and in 8 the N–Ta–C(1) cis
angles are 77.43(10)°. N–Ta–N angles involving the amido
bonds of the aminopyridinato ligands are the widest,
84.4(3)° in 7 and 86.65(18)° in 8. The Clax-Ta–Cleq angles
for 7 are 99.87(9)° and 99.64(9)° and the Cax-Ta–Ceq angles
for 8 are 94.02(15)° and 93.23(16)°. Details of the X-ray
crystal structure analyses of 7 and 8 are summarized in
Table 3. The 1H NMR spectrum shows two different sets of
signals for methyl protons. The two methyl groups present
above and below the pentagonal plane (axial positions)
show a resonance at δ = 1.26 ppm and the in-plane methyl
ligand at δ = 0.63 ppm. Similar observations have been
made for 6. The trimethyl complex 8 was found to be stable
Scheme 5. Synthesis of 6.
The first Schrock-type alkylidene complex, [Ta(CH2tBu)3-
(CHtBu)], was prepared in 1974.[19] Various mono- and po-
lydentate ligand systems that support this functionality
have been introduced in recent years and several synthetic
methodologies are known that lead to the alkylidene com-
plexes like, for instance, an α-elimination reaction sequence
from dialkyl precursors.[20] However, the complexes 4, 5,
and 6 do not form mixed benzyl/benzylidene complexes
neither during the synthesis nor by treatment of the tri-
alkyltantalum complexes at elevated temperatures (80 °C).
In order to prepare other trialkyltantalum complexes sup-
ported by deprotonated 3 −− the sterically most demanding
of the three ligands described in this publication −− we syn-
thesized the corresponding tantalum trichloride species 7
(Scheme 6).
Compound 7 can be synthesized by the reaction of 3 with
TaCl5 (Polamo method “direct synthesis”[7]). Because of a
relatively low yield using this “direct synthesis” salt metath-
esis (Scheme 6) was applied and gave rise to 7 in better
yield. The success of salt metathesis using lithiated 3 and
TaCl5 is in contrast to Polamo’s observations. He developed
his method due to problems with the salt metathesis ap-
proach in similar reactions. The trimethyltantalum complex
8 results when 3 equiv. of LiMe were treated with 7. The X-
ray crystal structures of 7 (Figure 3) and 8 (Figure 4) re-
vealed a distorted pentagonal-bipyramidal coordination. In
7 the two chloro ligands occupy the axial positions of the
Figure 3. Molecular structure of 7; selected bond lengths [Å] and
angles [°]: C1–N2 1.339(11), C1–N1 1.376(11), C5–N2 1.337(11),
C6–N1 1.438(11), C15–N3 1.391(10), C15–N4 1.352(12), C19–N4
1.337(11), C20–N3 1.422(11), N1–Ta1 2.072(8), N2–Ta1 2.264(8),
N3–Ta1 2.098(7), N4–Ta1 2.263(7), Cl1–Ta1 2.352(2), Cl2–Ta1
2.349(2), Cl3–Ta1 2.376(2); C1–N1–Ta 100.45(6), C1–N2–Ta
92.92(5), C1–N2–C5 118.55(8), C15–N4–C19 118.13(8), C15–N3–
Ta 99.66(6), C15–N4–Ta 93.42(5), N1–Ta–N2 60.05(3), N3–Ta–N4
60.38(3), N2–C1–N1 106.52(8), N4–C15–N3 106.53(8), Cl1–Ta–
polyhedra and the two pairs of nitrogen and the third chlo- Cl3 160.49(8), Cl1–Ta–Cl2 99.87(9), Cl2–Ta–Cl3 99.63(9).
Eur. J. Inorg. Chem. 2006, 2683–2689
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A. Noor, W. Kretschmer, R. Kempe
FULL PAPER
towards elevated temperatures in terms of the formation of Table 2. Ethylene polymerization results, activation of 8 using tri-
alkylammonium tetrakis(pentafluorophenyl)borate.[a]
mixed methyl/methylidene species. Heating of 8 in toluene
–
at 80 °C for 40 min did not affect the structure of the com-
pound. When harsher conditions were employed, i.e., heat-
ing to 120 °C for 30 min, 8 started to decompose slowly
without any signs of methylidene formation.
Run
no.
Pre-cat. B(C6F5)4
TIBAO
[mmol]
PE
[g]
Activity
[kg(PE)mol(Ta)–1h–1bar–1
]
[µmol]
[µmol]
1
2
3
–
10
10
11
11
22
0.1
0.1
0.1
0.05
0.06
0.16
–
5
13
[a] 260 mL of toluene as solvent, 5 bar of ethene, 80 °C, 15 min
run time, Ta/Al = 1:20 (m/m); TIBAO = tetraisobutylaluminoxane
([iBu2Al]2O).
plex 8 and activating with trialkylammonium tetrakis-
(pentafluorophenyl)borate. No significant improvement in
terms of activity was found by changing the activation pro-
cedure. Because of the low activity no molecular weight
analysis of the polymers was accomplished. The generation
of cationic complexes was studied in an NMR scale reac-
tion. Reaction of 8 with 1 equiv. of B(C6F5)3 in CD2Cl2
results in the abstraction of one of the axial methyl groups.
A new resonance at δ = 1.86 ppm was observed for this
methyl group in the 1H NMR spectra. The resulting organo-
cation is not stable in solution and decomposes rapidly into
unidentified species.
Figure 4. Molecular structure of 8; selected bond lengths [Å] and
angles [°]: C1–Ta1 2.190(6), C2–Ta1 2.203(4), N1–Ta1 2.105(3),
N2–Ta1 2.332(4), N1–Ta1–N1A 154.86(19), C1–Ta1–C2
106.89(12), N1–Ta1–C2 93.23(16), C2A–Ta(1)–C2 146.2(2), N1–
Ta1–N2 145.88(13), N1–Ta1–N2 59.25(13), C1–Ta1–N2 136.67(9),
C2–Ta1–N2 77.25(16), N2–Ta1–N2A 86.65(18).
Conclusions
Formation of Organocations and Their Application in
Ethylene Polymerization
Trialkyltantalum complexes stabilized by aminopyridin-
ato ligands can be synthesized by salt metathesis or toluene
elimination. They adopt the coordination of either a capped
octahedron or a pentagonal bipyramid depending on the
steric demand of the aminopyridinato ligand. These trialkyl-
tantalum complexes are thermally unusually stable towards
α-H elimination. They form rather unstable organocations
and the instability of these cations might be the reason for
the low activity in ethylene polymerization.
Because of the high ethylene polymerization activity ob-
served for the tantalum complexes 9 and 10[12] (Scheme 7)
we became interested in exploring the polymerization be-
havior of some of the complexes discussed in this study.
Experimental Section
General Procedures
Synthesis and Structure Analysis: All manipulations were per-
formed with rigorous exclusion of oxygen and moisture in Schlenk-
type glassware on a dual manifold Schlenk line or in an argon-
filled glove box (mBraun 120-G) with a high-capacity recirculator
(Ͻ0.1 ppm O2). Non-halogenated solvents were dried by distil-
lation from sodium wire/benzophenone. Commercial TaCl5 (Lanc-
aster) was used as received. Deuterated solvents were obtained
from Cambridge Isotope Laboratories and were degassed, dried,
and distilled prior to use. NMR spectra were recorded with a
Bruker ARX at 250 MHz and chemical shifts are reported in ppm
relative to the deuterated solvent. Elemental analyses (CHN) were
carried out using a Vario EL III instrument. X-ray crystal structure
analyses were performed by using a STOE-IPDS II diffractometer
equipped with an Oxford Cryostream low-temperature unit. Crys-
tal structure data are presented in Table 3. Structure solution and
refinement was accomplished using SIR97,[21] SHELXL97,[22] and
WinGX.[23] CCDC-297697 (4), -297698 (5), -297699 (7), and
Scheme 7. Structures of 9 and 10.
Ethylene polymerization studies were performed using 7
(Table 1) and 8 (Table 2) as pre-catalysts.
Table 1. Ethylene polymerization results, activation of 7 using
PMAO [PMAO = polymethylaluminoxane (AlOMe)x].[a]
Pre-cat.
[µmol]
PMAO
[mmol]
PE
[g]
Activity
[kg(PE)mol(Ta)–1h–1bar–1]
10
5
0.2
16
[a] 260 mL of toluene as solvent, 5 bar of ethene, 50 °C, 15 min run
time, Ta/Al = 1:500 (m/m).
Ethylene polymerization activity of 7 after activation
with PMAO is low. This is in contrast to what was observed
for 9 and 10. Thus, we expected to see a significantly better
catalyst performance when starting from the methyl com- -297700 (8) contain the supplementary crystallographic data for
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Trialkyltantalum Complexes Stabilized by Aminopyridinato Ligands
Table 3. Details of the X-ray crystal structure analyses.
FULL PAPER
Compound
4
5
7
8
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
monoclinic
C2/c
23.279(3)
18.761(2)
20.204(3)
90
117.557(8)
90
7823(3)
0.12×0.1×0.08
1.381
monoclinic
P21/n
13.955(3)
14.704(3)
18.617(3)
90
91.680(2)
90
3818(2)
0.12×0.11×0.06
1.414
triclinic
P1
monoclinic
C2/c
14.1240(10)
12.5620(10)
22.694(2)
90
93.375(5)
90
4019.5(6)
0.99×0.34×0.44
1.333
¯
10.3627(9)
19.7101(18)
21.343(2)
96.120(7)
103.540(7)
90.410(7)
4211.6(7)
0.16×0.1×0.05
1.434
γ [°]
V [Å3]
Crystal size [mm]
ρcalcd. [g cm–3]
µ [mm–1] (Mo-Kα)
T [K]
0.7107
193(2)
0.7107
193(2)
0.7107
193(2)
0.7107
193(2)
θ range [°]
1.47–26.19
7732
4508
415
0.1282
1.77–26.21
7506
2470
409
0.1033
1.35–25.83
15909
9228
880
0.1018
1.80–25.68
3604
3508
214
0.0924
No. of unique refl.
No. of obsd. refl. [I Ͼ 2σ(I)]
No. of parameters
wR2 (all data)
R value [I Ͼ 2σ(I)]
0.0755
0.0542
0.0632
0.0312
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(0.636 g, 1 mmol) in hexane (15 mL) at –33 °C. The mixture was
warmed to room temperature and was further heated at 60 °C for
2 h as a color change was observed from brown to deep red. The
mixture was filtered and the volume of the solvent was reduced in
vacuo. The solution was cooled at –25 °C overnight to afford a
yellow crystalline material. Yield 0.365 g (43%). C39H51N4Si2Ta
(812.97): calcd. C 57.62, H 6.32, N 6.89; found C 57.36, H 6.62, N
6.29. 1H NMR (C6D6, 298 K): δ = 0.40 [s, 18 H, Si(CH3)3], 1.61 (s,
6 H, CH3), 2.96 (br. d, 6 H, CH2), 5.76 (d, 2 H, Py-5-H), 5.88 (s,
2 H, Py-3-H), 6.74 (t, 3 H, Ar-4-H), 6.89–7.10 (m, 14 H, Py-6-H,
Ar-2, 3, 5, 6 H) ppm. 13C NMR (C6D6, 298 K): δ = 2.45 (6 C, 6
CH3), 21.6 (2 C, 2 CH3), 99.40 (br. and flat s, 3 CH2), 111.87 (2
CH, Py-C-3/5), 114.05 (2 CH, Py-C-3/5), 122.49 (3 CH, Benz-C-4),
126.83 (6 CH, Benz-C-3,5), 128.20 (6 CH, Benz-C-2,6), 141.51 (3
C, Benz-C-1), 151.53 (2 C, Py-C-4), 153.26 (2 CH, Py-C-6), 164.91
(2 C, Py-C-2) ppm.
Polymerization: Toluene (Aldrich, anhydrous, 99.8%) was passed
through columns of Al2O3 (Fluka), BASF R3-11 supported Cu
oxygen scavenger, and molecular sieves (4 Å, Aldrich). Ethylene
(AGA polymer grade) was passed over BASF R3-11 supported Cu
oxygen scavenger and molecular sieves (4 Å, Aldrich). PMAO
(4.9 wt.-% Al in toluene, Akzo Nobel), N,N,N-trialkylammonium
–
tetrakis(pentafluorophenyl)borate [6.2 wt.-% B(C6F5)4 in Isopar,
DOW Chemicals], and TIBA (Witco) were used as received. TI-
BAO was prepared according to published procedures.[24]
Synthesis of 3:[25] Toluene (25 mL) was added to a Schlenk vessel
charged with tris(dibenzylideneacetone)dipalladium (0.082 g,
0.09 mmol), NaOtBu (0.750 g, 7.8 mmol), and 1,3-bis(diphenyl-
phosphanyl)propane (0.074 g, 0.18 mmol). To the resulting suspen-
sion 2-bromopyridine (0.57 mL, 6 mmol) and 2,6-diisopropylani-
line (1.47 mL, 7.8 mmol) was added. The solution was stirred and
heated to 90 °C for 48 h. On cooling to room temperature, water
and diethyl ether were added to the resulting red solution. The
organic phase was extracted and the remaining inorganic phase
was further washed with diethyl ether (3×20 mL). The combined
organic phases were washed with a saturated sodium chloride solu-
tion and dried with sodium sulfate. The solvent was removed under
reduced pressure and the resulting reddish solid was purified using
silica gel chromatography (eluent: dichloromethane). The solvent
was removed under reduced pressure and the product was recrys-
tallized from pentane as a white crystalline material. Yield 1.07 g
(70%). C17H22N2 (254.37): calcd. C 80.27, H 8.72, N 11.01; found
Synthesis of 5: Compound 2 (0.463 g, 2.57 mmol, 546 µL) in hexane
(5 mL) was added slowly to a stirred solution of [Ta(CH2C6H5)5]
(0.817 g, 1.28 mmol) in hexane (15 mL) at –33 °C. The mixture was
warmed to room temperature and further heated at 60 °C for 2 h.
The mixture was filtered and the volume of the solvent was reduced
in vacuo. The solution was cooled at –25 °C overnight to afford
a yellow crystalline material. Yield 0.460 g (44%). C39H51N4Si2Ta
(812.97): calcd. C 57.62, H 6.32, N 6.89; found C 56.73, H 6.24, N
6.54. 1H NMR (C6D6, 298 K): δ = 0.27 [s, 18 H, Si(CH3)3], 1.92 (s,
6 H, CH3), 3.15–2.96 (2 d, 6 H, CH2), 5.65 (d, 2 H, Py-5-H), 5.83
(d, 2 H, Py-3-H), 6.69 (dd, 6 H, Benz-2,6-H), 7.00 (t, 6 H, Benz-
3,5-H), 7.44 (t, 2 H, Py-4-H) ppm. 13C NMR (C6D6, 298 K): δ =
2.57 (6 C, 6 CH3), 21.56 (2 C, 2 CH3), 100.34 (3 CH2), 108.34 (2
CH, Py-C-3/5), 113.95 (2 CH, Py-C-3/5), 122.66 (3 CH, Benz-C-4),
126.67 (6 CH, Benz-C-3,5), 129.42 (6 CH, Benz-C-2,6), 138.32 (3
C, Benz-C-1), 151.97 (2 CH, Py-C-4), 153.17 (2 C, Py-C-6), 161.49
(2 C, Py-C-2) ppm.
1
C 80.30, H 8.75, N 10.82. H NMR (C6D6, 298 K): δ = 1.10 [d, 12
H, CH(CH3)2], 3.39 [sept, 2 H, CH(CH3)2], 5.88 (d, 1 H, Py-3-H),
6.20 (dd, 1 H, Py-5-H), 6.89 (t, 1 H, Py-4-H), 7.17–7.29 (m, 3 H,
Ar-CH), 7.99 (dd, 1 H, Py-6-H), 8.34 (br. s, 1 H, NH) ppm. 13C
NMR (C6D6, 298 K): δ = 23.98 [4 CH3, CH(CH3)2], 28.67 [2 CH,
CH(CH3)2], 105.62 (CH, Py-C-5), 112.94 (CH, Py-C-3), 124.22 (2
CH, Ar-C-9,11), 134.69 (CH, Py-C-4), 137.68 (2 CH, Ar-C-8,12),
148.45 (1 C, Ar-C-7), 148.57 (1 CH, Py-C-6), 160.51 (1 CH, Py-C-
2) ppm.
Synthesis of 6: To a stirred solution of 3 (0.557 g, 2.14 mmol) in
diethyl ether (10 mL), nBuLi (1.34 mL, 1.6 , 2.14 mmol) was
added at 0 °C. This “ligand solution” was warmed slowly to room
temperature and stirred for 2 h. This solution was slowly added to
[TaCl2(CH2C6H5)3] (0.562 g, 1.07 mmol) in hexane (10 mL) at
room temperature. The mixture was stirred overnight and a color
change from red to dark brown was observed. LiCl was filtered off
Synthesis of 4: Compound 1 (0.380 g, 2 mmol, 425 µL) in hexane
(5 mL) was added slowly to a stirred solution of [Ta(CH2C6H5)5]
Eur. J. Inorg. Chem. 2006, 2683–2689
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A. Noor, W. Kretschmer, R. Kempe
FULL PAPER
and the volume of the solvent was reduced in vacuo. The solution
was cooled at –25 °C overnight to afford an orange crystalline ma-
steel 1-L autoclave (Medimex) in semi-batch mode (ethylene was
added by replenishing flow to keep the pressure constant). The re-
terial. Yield 0.564 g (55%). C55H63N4Ta (961.06): calcd. C 68.74, actor was temperature- and pressure-controlled and equipped with
1
H 6.61, N 5.83; found C 67.44, H 6.64, N 5.51. H NMR (C6D6,
298 K): δ = 1.15–1.27 [dd, 24 H, 2 CH(CH3)2], 2.79 (s, 2 H, CH2),
3.60 (s, 4 H, 2 CH2), 3.85 [sept, 4 H, 4 CH(CH3)2], 5.52 (d, 2 H,
separated toluene, catalyst, and cocatalyst injection systems and a
sample outlet for continuous reaction monitoring. At 5 bar of eth-
ylene pressure multiple injection of the catalyst with a pneumati-
Py-3-H), 5.81 (t, 2 H, Py-5-H), 6.22 (d, 4 H, Ar-H), 6.27 (d, 2 H, cally operated catalyst-injection system was used. During a polyme-
Py-4-H), 6.62 (dd, 2 H, Py-6-H), 6.78–6.67 (m, 5 H, Ar-H), 6.94– rization run the pressure, ethylene flow, inner and outer reactor
6.86 (m, 6 H, Ar-H), 7.19–7.16 (m, 6 H, Ar-CH) ppm. 13C NMR
(C6D6, 298 K): δ = 23.22 [8 CH3, CH(CH3)2], 26.28 [2 CH,
CH(CH3)2], 28.80 [2 CH, CH(CH3)2], 83.50 (3 C, CH2), 106.53 (2
CH, Py-C-3/5), 112.39 (2 CH, Py-C-3/5), 123.18 (4 CH, Ar-C-9,11),
125.31 (6 CH, Benz-C-3,5), 126.57 (6 CH, Benz-C-2,6), 127.09 (3
CH, Benz-C-4), 129.96 (4 CH, Ar-C-8,12), 139.55 (2 CH, Ar-C-
10), 141.87 (2 CH, Py-C-4), 143.08 (2 C, Ar-C-7), 146.36 (3 C,
Benz-C-1), 148.46 (2 CH, Py-C-6), 169.21 (2 CH, Py-C-2) ppm.
temperature, and the stirrer speed were monitored continuously. In
a typical semi-batch experiment, the autoclave was evacuated and
heated at 125 °C for 1 h prior to use. The reactor was then brought
to the desired temperature with stirring at 600 rpm and charged
with 230 mL of toluene together with either the required amount
of PMAO [2.76 g, Ta/Al = 1:500 (m/m)] or TIBAO scavenger [1 mL
of a 0.1  stock solution in toluene, Ta/Al = 1:20 (m/m)]. After
pressurizing with ethylene to reach 5 bar total pressure the auto-
clave was equilibrated for 5 min. Subsequently 1 mL of a 0.01 
stock solution of the tantalum complex in toluene was injected to-
gether with 30 mL of toluene, to start the reaction. In the case
where trialkylammonium tetrakis(pentafluorophenyl)borate was
the activator, 1 mL of the tantalum complex stock solution and the
appropriate amount of borate [0.12 g, Ta/B = 1:1.1 (m/m)] were
premixed before injection. During the run the ethylene pressure was
kept constant to within 0.2 bar of the initial pressure by replen-
ishing flow. After the desired reaction time, the reactor was vented
and the residual PMAO/TIBAO was destroyed by addition of
20 mL of ethanol. Polymeric product was collected, stirred in acidi-
fied ethanol for 30 min, and rinsed with ethanol and acetone on a
glass frit. The polymer was initially dried in air and subsequently
in vacuo at 80 °C.
Synthesis of 7. Method I: TaCl5 (1.432 g, 4 mmol) and 3 (2.035 g,
8 mmol) were heated at 110 °C. The melt started to turn dark
brown immediately. During the reaction gas formation was ob-
served. After 1 h, toluene (30 mL) was added and the dark-red re-
action mixture was further refluxed for 2 h. The solution was fil-
tered while hot and the volume was reduced until red crystals began
to appear. The solution was cooled and a red crystalline material
was obtained overnight. Yield 1.103 g (35%). C34H42Cl3N4Ta + 0.5
C7H8 (840.10): calcd. C 53.61, H 5.51, N 6.67; found C 53.67,
1
H5.54, N 6.08. H NMR (C6D6, 298 K): δ = 1.13–1.48 [dd, 24 H,
CH(CH3)2], 2.10 (s, 3 H, toluene) 3.86 [sept, 4 H, CH(CH3)2], 5.74
(d, 2 H, Py-3-H), 6.05 (t, 2 H, Py-5-H), 6.83 (t, 2 H, Py-4-H), 7.28–
7.16 (m, 6 H, Ar-H), 8.06 (d, 2 H, Py-6-H) ppm. 13C NMR (C6D6,
298 K): δ = 24.73 [4 CH3, CH(CH3)2], 24.95 [4 CH3, CH(CH3)2],
28.22 [4 CH, CH(CH3)2], 107.56 (2 CH, Py-C-3/5), 113.50 (2 CH,
Py-C-3/5), 124.46 (4 CH, Ar-C-9,11), 140.19 (2 CH, Ar-C-10),
140.66 (CH, Py-C-4), 141.15 (2 C, Ar-C-7), 147.37 (4 C, 2 CH, Ar-
C-8,12, Py-C-6), 169.86 (2 C, Py-C-2) ppm. Method II: Lithiated 3
(1.041 g, 4 mmol) in diethyl ether (20 mL) was added to TaCl5
(0.716 g, 2 mmol) in toluene (5 mL) at room temperature and the
reaction mixture was stirred at room temperature overnight. The
solution was filtered and the solvent was removed in vacuo. The
red residue was washed with hexane (5 mL) and dried under vac-
uum. Yield 0.92 g (58%).
Acknowledgments
Financial support from the Fonds der Chemischen Industrie and
Deutsche Forschungsgemeinschaft is gratefully acknowledged.
[1] Review article on aminopyridinato ligands: R. Kempe, Eur. J.
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stirred suspension of 7 (0.794 g, 1 mmol) in hexane (20 mL) at –
40 °C. The red suspension was warmed to room temperature as the
color started changing from red to yellow. The suspension was fur-
ther stirred for 2 h. The brown solution was filtered and the residue
washed with diethyl ether (5 mL). The volume was reduced in
vacuo and the solution was cooled at –25 °C overnight to afford a
yellow crystalline material. Yield 0.432 g (59%). C37H51N4Ta
(732.78): calcd. C 60.65, H 7.02, N 7.65; found C 60.37, H 7.04, N
1
7.50. H NMR (C6D6, 298 K): δ = 0.63 (s, 3 H, CH3), 1.13 [d, 12
H, CH(CH3)2], 1.26 (s, 6 H, 2 CH3),1.34 [d, 12 H, CH(CH3)2], 3.61
[sept, 4 H, CH(CH3)2], 5.54 (d, 2 H, Py-3-H), 6.18 (dd, 2 H, Py-5-
H), 6.86 (t, 2 H, Py-4-H), 7.24–7.17 (m, 6 H, Ar-8,9,10-H) ppm.13C
NMR (C6D6, 298 K): δ = 24.10 [4 CH3, CH(CH3)2], 24.18 [4 CH3,
CH(CH3)2], 25.10 (3 CH3), 27.75 [2 CH, CH(CH3)2], 27.93 [2 CH,
CH(CH3)2], 105.67 (2 CH, Py-C-3/5), 113.36 (1 CH, Py-C-3/5),
113.56 (1 CH, Py-C-5), 124.45 (4 CH, Ar-C-9,11), 127.09 (1 CH,
Ar-C-10), 127.14 (1 CH, Ar-C-10), 138.91 (1 CH, Py-C-4), 139.11
(1 CH, Py-C-4), 141.32 (4 CH, Ar-C-8,12), 146.09 (4 CH, Py-C-6,
2 C, Ar-C-7), 168.32 (1 C, Py-C-2) ppm.
General Description of Polymerization Experiments: The catalytic
ethylene polymerization reactions were performed in a stainless
2688
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Eur. J. Inorg. Chem. 2006, 2683–2689

Trialkyltantalum Complexes Stabilized by Aminopyridinato Ligands
FULL PAPER
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Received: February 9, 2006
Published Online: April 27, 2006
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