Hafnium trialkyls stabilized by bulky, electron-rich aminopyridinates

Article abstract of DOI:10.1002/zaac.201100475

A series of hafnium aminopyridinates were synthesized and spectroscopically analyzed. In addition, selected examples of these hafnium complexes were characterized by X-ray single crystal structure analysis. The aminopyridinato ligands used here carry an additional dialkylamine substituent to enhance the electron donating ability of the ligands. Structural data and low temperature NMR spectroscopic investigations are indicative of the increased donating ability of the ligands. Ethylene polymerization studies revealed a rather low catalytic activity most likely due to catalyst instability. Details of the deactivation reaction were investigated by NMR spectroscopy. Copyright

Full text of DOI:10.1002/zaac.201100475

ARTICLE
DOI: 10.1002/zaac.201100475
Hafnium Trialkyls Stabilized by Bulky, Electron-Rich Aminopyridinates
Muhammad Hafeez,[a] Winfried P. Kretschmer,[a] and Rhett Kempe*[a]
Keywords: Hafnium; N ligands; Aminopyridinates; NMR spectroscopy; X-ray diffraction
Abstract. A series of hafnium aminopyridinates were synthesized and ity of the ligands. Structural data and low temperature NMR spectro-
spectroscopically analyzed. In addition, selected examples of these scopic investigations are indicative of the increased donating ability
hafnium complexes were characterized by X-ray single crystal struc-
ture analysis. The aminopyridinato ligands used here carry an ad-
ditional dialkylamine substituent to enhance the electron donating abil-
of the ligands. Ethylene polymerization studies revealed a rather low
catalytic activity most likely due to catalyst instability. Details of the
deactivation reaction were investigated by NMR spectroscopy.
Introduction
Group 4 metal alkyls stabilized by aminopyridinato (= Ap)
ligands (Figure 1) have been frequently used as alternatives to
metallocenes or cyclopentadienyl ligand stabilized alkyl com-
plexes of these metals, also in terms of developing catalysts
for olefin polymerization.[1–4] Ap ligands are non-symmetric
versions of bidentate mono-anionic N-ligands suited to stabi-
lize early (and late) transition metals and can be considered as
related to amidinate[5] or diketiminate[6] ligands.
Figure 2. Schematic representation of the increased electron donor
ability of Ap ligands by introducing a dialkylamine moiety (R, RЈ, RЈЈ
= alkyl substituents).
Results and Discussion
Figure 1. Aminopyridinato ligands (R, RЈ = alkyl, aryl or silyl substitu-
ents).
Synthesis of the Complexes
The ligands used here are listed in Figure 3. They were syn-
thesized following a published procedure.[7t]
Most of the (coordination) chemistry involving Ap ligands
has been dedicated to titanium[7] and zirconium.[7a,f,i,r,8] Haf-
nium complexes are nearly unexplored. Pioneering work in
this regard was done by Polamo et al. employing his direct
synthesis route – the reaction of the metal chloride in the
melted ligand.[9] The Ap ligands used by him have a rather low
steric bulk and highly nitrogen coordinated complexes were
observed. The application of bulky aminopyridinates can ad-
dress ligand redistribution problems.[10] We recently reported
hafnium complexes stabilized by bulky aminopyridinates[8l]
and report here the synthesis and structure of hafnium tribenzyl
complexes stabilized by bulky Ap ligands possessing a strong
electron donation ability due to the presence of an addition
dialkylamine functionality (Figure 2).
* Prof. Dr. R. Kempe
Fax: +49-921-55-2157
E-Mail: kempe@uni-bayreuth.de
[a] Lehrstuhl für Anorganische Chemie II
Universität Bayreuth
Universitätsstr. 30
95440 Bayreuth, Germany
Figure 3. Applied aminopyridines.
324
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Z. Anorg. Allg. Chem. 2012, 638, (2), 324–330

Hafnium Trialkyls Stabilized by Bulky, Electron-Rich Aminopyridinates
These ligands react smoothly (1:1 ratio) with tetrabenzylhaf- idyl proton resonances at δ = 2.8 and 1.1 ppm but did not result
nium (HfBn4) to give mono-Ap complexes (ApHfBn3) in a chemical inequivalence of the same protons. However, for
(Scheme 1). NMR scale reactions are indicative of a quantita- the benzyl ligand resonances at 2.4 ppm, a split into two sets
tive conversion and isolated yields of around 80% could be of multiplet signals below –50 °C with a ratio of 1:2 was ob-
obtained. Zr/Hf trialkyls stabilized by anionic N-ligands are served. This is most likely due to a reversible η3 coordination
documented for a few cases and known for amidinates,[11] gu- of one of the benzyl ligands to reduce the electron deficit of
anidinates[12] diketiminate,[13] macrocyclic amides,[14] tropo- the metal atom.
coronands[15] and tris(pyrazolyl)borates.[16]
Structural Studies
Crystals of 2a suitable for X-ray analysis were grown from
a concentrated n-hexane solution at room temperature. For the
molecular structure of 2a please see Figure 5. Crystallographic
Scheme 1. Synthesis of 2, for the assignment of 1a–1h and 2a–2h
please refer to Figure 3 (Bn = benzyl).
NMR investigation of the compounds 2 revealed (as ex-
pected) one set of proton resonances for the Ap ligand and
another set for all three benzyl ligands indicating a dynamic
1
coordination. In the H NMR spectra of 2c–h the proton reso-
nances for the 3 and 5 position of the pyridine ring appear
between 4.9 and 5.6 ppm. These signals are quite up field for
pyridine or aromatic protons and more typical for olefinic
protons. The dialkylamine substituent increases the electron
donating ability but weakens the ring current of the pyridine
ring. In the case of the earlier reported ApTiCl3 complexes7t
Figure 5. Molecular structure of complex 2a: Selected bond
lengths /Å and bond angles /°: Hf1–C1 2.259(7), Hf1–C8 2.213(8),
Hf1–C9 2.645(9), Hf1–C15 2.230(8), Hf1–N1 2.389(7), Hf1–N2
2.146(6), C1–C2 1.477(11), C8–C9 1.466(11), C15–C16 1.494(10),
C26–Cl1 1.736(8), N1–C22 1.370(9), N1–C26 1.329(10), N2–C22
1.348(9), N2–C27 1.447(9), C2–C1–Hf1 109.5(1), C9–C8–Hf1
we could confirm the increased bond order for the
pyridine–Ndialkylamine bond by observing a high rotation barrier
C
in low temperature NMR experiments. A similar experiment
for hafnium compound 2c (Figure 4) between –70 to –10 °C
showed a very strong temperature depending shift of the piper- 89.5(5), C16–C15–Hf1 122.2 (5), N2–Hf1–N1 58.3(2).
1
Figure 4. Variable H low temperature NMR spectra of 2c ([D8]toluene, –70 to –10 °C, 0–8 ppm).
Z. Anorg. Allg. Chem. 2012, 324–330
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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325

M. Hafeez, W. P. Kretschmer, R. Kempe
ARTICLE
data are given in Table 1. Two of the three benzyl ligands are
η1 coordinated [C2–C1–Hf1 109.6 (4)°, C16–C15–Hf1 122.3
(5)°] whereas the third one is η2-coordinated [C9–C8–Hf1
90.5 (5)°]. The Hf–CH2 bond lengths of the η2 coordinated
benzyl ligand is slightly shorter than that of the two other
Hf–CH2 distances. The Hf–Npyridine bond length [2.385(6) Å]
is significantly longer than the Hf–Namido bond length
[2.143(6) Å], indicating that the anionic function of the ligand
is localized on the amido nitrogen atom (N2).
Table 1. Selected experimental details of the X-ray crystal structure
analyses.
Compound
2a
2e
Formula
Formula weight
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
C38H41ClHfN2
739.67
C42H49HfN3O
790.33
monoclinic
P21/n
9.878 (1)
34.175 (2)
11.904 (1)
triclinic
¯
P1
Figure 6. Molecular structure of complex 2e: Selected bond
lengths /Å and bond angles /°: Hf1–C1 2.218(7), Hf1–C8 2.238(7),
Hf1–C15 2.210(8), Hf1–N2 2.364(6), Hf1–N3 2.092(5), N2–C22
1.378(8), N2–C26 1.362(8), N1–C26 1.379(9), N3–C22 1.345(8), C2–
C1–Hf1 122.0(5), C9–C8–Hf1 113.8(5), C16–C15–Hf1 117.2(6), N3–
C22–N2 109.7(6), N2–C26–N1 116.4(7), N3–Hf1–N2 59.43(19).
10.836 (1)
11.069 (1)
14.320 (1)
74.569 (6)
87.029 (7)
78.008 (7)
1619.5 (2)
2
112.831 (3)
γ /°
Cell volume
Z
3704(3)
4
ylaluminum component was removed). Borate activation in the
presence of a aluminum scavenger did not improve the activity
significantly. We ascribe this observation to transfer of Ap li-
gand from hafnium to aluminum. The Ap ligand might become
transferred from hafnium to aluminum and the Ap ligand free
hafnium compound is inactive.[8i] To support this hypothesis
we studied the stability of 2c against trimethylaluminum
(TMA, Figure 7). A very fast coordination of TMA (Scheme 2,
spectrum after 5 min) with a subsequent ligand transfer reac-
tion was observed (spectrum after 16 h). To confirm the ligand
transfer we independently synthesized the corresponding Ap
containing dimethylaluminum complex 3c by the reaction of
the aminopyridine 1c with one equivalent of TMA in toluene
according to a published procedure (Figure 7, above).[7i]
Crystal size /mm
Habit
0.41ϫ 0.37ϫ 0.31 0.34ϫ 0.23ϫ 0.11
prism
yellow
1.52
plate
yellow
1.42
Color
Density /g·cm–3
T /K
133
133
Theta range
Unique reflections
Observed reflections 4973
No of parameters
wR2 (all data)
R value
1.48–25.72
6090
1.2–25.64
6982
3339
424
379
0.117
0.049
0.038
0.079
Compound 2e was crystallized from n-hexane solution at
–24 °C. The molecular structure of 2e is given in Figure 6 and
crystallographic details are listed in Table 1. In contrast to 2a,
all the three benzyl ligands are η1-coordinated. Hf–CH2–Cipso
angles between 117.2(6) and 109.7(6)° were observed. The
averaged Hf–CH2 bond is 2.222 Å, which is slightly shorter
than the average value of such a bond (2.2696 Å)[17]. The N1–
C26 bond length is 1.379(9) Å, which is in between a Csp2–N
double (approx. 1.29 Å) and a Csp2–N single bond (approx.
1.48 Å) indicating a bond order higher than one.[18] The calcu-
lated sum of all angles around N1 is 351o indicating an almost
planar nitrogen atom, which is more typical for a sp2-hy-
bridized nitrogen atom. The sp2-hybridization of N1 together
with the short N1–C26 distances is indicative that the lone pair
of N1 participats in the ligands π-system increasing the elec-
tron donating ability of the amine functionalized Ap ligand.
Conclusions
Several conclusions can be drawn from this study. First,
mono-Ap hafnium complexes are selectively formed by re-
acting tetrabenzylhafnium with the corresponding bulky ami-
nopyridines. Second, the additional dialkylamine substituent
increases the electron donor ability of the Ap ligand. Third, a
low ethylene polymerization activity was observed due to fast
aluminum coordination at the Ap ligand and a subsequent but
slower transfer of the Ap ligand towards aluminum during the
polymerization reaction.
Experimental Section
Reactions of the Hafnium Complexes with Trialkyl
Aluminum Compounds
All manipulations were performed with rigorous exclusion of oxygen
and moisture in Schlenk-type glassware on a dual manifold Schlenk
line or in an nitrogen filled glove box (mBraun 120-G) with a high-
capacity recirculator (Ͻ 0.1 ppm O2). Deuterated solvents were ob-
tained from Eurisotop. Benzene-d6 was dried with sodium/potassium
Ethylene polymerization studies with selected examples of
the tribenzyl complexes revealed no activity for the
activation with MAO and very low activities of around 5–10
kgPE/molcat·h·barethylene in the presence of dMAO (trimeth- alloy and bromobenzene-d5 over molecular sieves, degassed and dis-
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Hafnium Trialkyls Stabilized by Bulky, Electron-Rich Aminopyridinates
1
Figure 7. H NMR spectra (C6D6, 26 °C, 0–8 ppm) of ApAlMe2 (3c) top, ApHfBn3 (2c) bottom, 2c + 30 equiv. TMA after 5 min and after 16
h (middle).
Synthesis of Complexes:
Synthesis of 2a: Aminopyridine 1a (0.288 g, 1.0 mmol in 15.0 mL
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
20.0 mL toluene). The color changed from yellow to red. The resultant
solution was stirred for 12h at room temperature. The toluene was
removed under vacuum and the yellow product was extracted with n-
hexane. The volume of the n-hexane extract was reduced and the prod-
uct was crystallized at –24 °C. Yield 0.50 g (67%). C38H41ClHfN2
(739.27): calcd. C 61.68, H 5.59, N 3.79 found; C 61.45, H 5.50, N
3.38 %.
Scheme 2. Ligand transfer reaction.
tilled prior to use. Commercial HfCl4 (Across Organics) was used as
received. Tetrabenzylhafnium was prepared according to the literature
procedures.[19] NMR spectra were recorded on a Varian Inova
(400 MHz) or Varian Inova (300 MHz) spectrometer. The chemical
1H NMR (400 MHz, C6D6, 298 K): δ = 0.96 (d, 6 H, H14,15,17,18), 1.16
(d, 6 H, H14,15,17,18), 2.40 (s, 6 H, HCH2benzyl), 3.05 (sept, 2 H, H13,16),
5.20 (d, 1 H, H3), 5.91 (d, 1 H, H5), 6.35 (t, 1 H, H4), 6.86 (d, 6
H, HCHbenzyl), 6.87–7.20 (m, 12 H, H9,10,11,CHbenzyl) ppm. 13C NMR
(100 MHz, C6D6, 298 K): δ = 23.98 (C14,15,17,18), 24.13 (C14,15,17,18),
25.20 (C13,16), 80.02 (CCH2benzyl), 96.25 (C3), 97.90 (C5), 122.90
(Cbenzyl), 128.20 (C9,11), 129.20 (Cbenzyl), 129.62 (C10), 133.50 (C8,12),
137.80 (C4), 142.87 (Cbenzyl), 143.09 (C7), 144.14 (Cbenzyl), 144.67
(C2), 159.38 (C6) ppm.
1
shifts are reported in ppm referenced to internal TMS for H and 13C.
Elemental analyses (CHN) were determined using Vario ELIII instru-
ment. X-ray crystal structure analyses were performed by using a
STOE-IPDS II equipped with an Oxford Cryostream low-temperature
unit. Structure solution and refinement was accomplished using
SIR97,[20] SHELXL97,[21] and WinGX.[22] Crystallographic data (ex-
cluding structure factors) for the structures have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC-849066 (compound 2a) and CCDC-849067 Synthesis of 2b: Aminopyridine 1b (0.246 g, 1.0 mmol in 20.0 mL
(compound 2e). Copies of the data can be obtained free of charge on toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
25.0 mL toluene). The color changed from yellow to red. The resultant
[Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk; web: solution was stirred for 12h at room temperature. The toluene was
www.ccdc.cam.ac.uk/conts/retrieving.html.
removed under vacuum and the yellow product was extracted and crys-
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Z. Anorg. Allg. Chem. 2012, 324–330

M. Hafeez, W. P. Kretschmer, R. Kempe
ARTICLE
tallized with n-hexane at –24 °C. Yield 0.485 g (69%). C35H35ClHfN2 (d, 1 H, H3), 5.93 (d, 1 H, H5), 6.79 (s, 2 H, H 9,11), 6.84 (t, 1 H, H4),
(697.22): calcd. C 60.24, H 5.06, N 4.02; found C 59.78, H 5.50, N
6.95–7.20 (m, 9 H, HCHbenzyl) ppm. 13C NMR (100 MHz, C6D6, 298
4.22 %. H NMR (400 MHz, C6D6, 298 K): δ = 2.18 (s, 6 H, H13,14), K): δ = 18.39 (C13,14), 20.85 (C15), 25.62 (C19), 28.60 (C18,20), 49.50
2.24 (s, 3 H, H15), 2.38 (s. 6 H, HCH2benzyl), 5.18 (d, 1 H, H3), 6.42 (C17,21), 89.40 (CCH2benzyl), 95.40 (C3), 96.58 (C5), 122.79 (C9,11),
1
(d, 1 H, H5), 6.65 (t, 1 H, H4), 6.84 (d, 6 H, HCH2benzyl), 6.90–7.20 127.60 (Cbenzyl), 128.90 (C10), 129.22 (Cbenzyl), 129.40 (Cbenzyl),
(m, 11 H, H9,11,CHbenzyl) ppm. 13C NMR (100 MHz, C6D6, 298 K): δ 134.20 (C8,12), 141.86 (C10), 143.78 (C4), 143.96 (Cbenzyl), 156.30
= 18.60 (C13,14), 20.68 (C15), 95.51 (C3), 97.12 (C5), 122.80 (C9,11), (C7), 158.78 (C2) 164.70 (C6) ppm.
127.60 (Cbenzyl), 128.76 (C10), 129.20 (Cbenzyl), 129.40 (Cbenzyl),
134.69 (C8,12), 143.78 (C4), 143.94 (Cbenzyl), 156.30 (C7), 158.78 (C2),
164.70 (C6) ppm.
Synthesis of 2e: Aminopyridine 1e (0.339 g, 1.0 mmol in 15.0 mL
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
20.0 mL toluene). The color changed from yellow to red. The resultant
solution was stirred for 12h at room temperature. The toluene was
removed under vacuum and the yellow product was extracted and crys-
tallized with n-hexane at –24 °C. Yield 0.68 g (86%). C42H49HfN3O
(790.35): calcd. C 63.81, H 6.25, N 5.32; found, C 64.35, H 5.80, N
5.36 %.
Synthesis of 2c: Aminopyridine 1c (0.337 g, 1.0 mmol in 15.0 mL
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
25.0 mL toluene). The color changed from yellow to red. The resultant
solution was stirred for twelve hours at room temperature. The toluene
was removed and the yellow product was extracted and crystallized
with n-hexane at –24 °C. Yield 0.630 g (80%). C43H51HfN3 (787.90):
calcd. C 65.49, H 6.52, N 5.33; found C 64.99, H 6.15, N 5.59 %.
1H NMR (400 MHz, C6D6, 298 K): δ = 1.11 (d, 6 H, H14,15,17,18), 1.21
(d, 6 H, H14,15,17,18), 2.41 (s, 6 H, HCH2benzyl), 2.64 (t, 4 H, H19,23),
2.71 (t, 4 H, H20,22), 3.19 (sept, 2 H, H13,16), 5.10 (d, 1 H, H3), 5.49
(d, 1 H, H5), 6.77 (t, 1 H, H4), 6.88 (d, 6 H, Hbenzyl), 6.94–7.20 (m,
12 H, H9,10,11,CHbenzyl) ppm. 13C NMR (75 MHz, C6D6, 298 K): δ =
23.98 (C14,15,17,18), 24.13 (C14,15,17,18), 25.18 (C13,16), 48.25 (C19,23),
66.20 (C20,22), 79.23 (CCH2benzyl), 96.25 (C3), 97.90 (C5), 122.90
(Cbenzyl), 128.20 (C9,11), 129.20 (Cbenzyl), 129.62 (C10), 133.50 (C8,12),
137.80 (C4), 142.87 (Cbenzyl), 143.09 (C7), 144.14 (Cbenzyl), 144.67
(C6), 159.38 (C2) ppm.
1H NMR (400 MHz, C6D6, 298 K):
δ = 1.11 (m, 12 H,
H14,15,17,18,21,22,23), 1.21 (d, 6 H, H14,15,17,18), 2.41 (s, 6 H, HCH2benzyl),
2.77 (t, 4 H, H20,24), 3.19 (sept, 2 H, H13,16), 5.03 (d, 1 H, H3), 5.49
(d, 1 H, H5), 6.77 (t, 1 H, H4), 6.94 (d, 6 H, HCHbenzyl), 6.98–7.22 (m,
12 H, H9,10,11,CHbenzyl) ppm.13C NMR (100 MHz, C6D6, 298 K): δ =
23.98 (C14,15,17,18), 24.13 (C14,15,17,18), 25.18 (C13,16), 25.66 (C22),
28.78 (C21,23), 45.20 (C20,24), 79.23 (CCH2benzyl), 96.25 (C3), 97.90
(C5), 122.90 (Cbenzyl), 128.20 (C9,11), 129.20 (Cbenzyl), 129.62 (C10),
133.50 (C8,12), 137.80 (C4), 142.87 (Cbenzyl), 143.09 (C7), 144.14
(Cbenzyl), 144.67 (C6), 159.38 (C2) ppm.
Synthesis of 2f: Aminopyridine 1f (0.296 g, 1.0 mmol in 20.0 mL tol-
uene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in 20.0 mL
toluene). The resultant solution was stirred for 12h at room tempera-
ture. The toluene was removed and the yellow product was extracted
and crystallized with n-hexane at –24 °C. Yield 0.63 g (84%).
C39H43HfN3O (747.83): calcd. C 62.58, H 5.79, N 5.62; found C
63.15, H 5.24, N 5.33 %.
Synthesis of 2d: Aminopyridine 1d (0.295 g, 1.0 mmol in 20.0 mL
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
25.0 mL toluene). The color changed from yellow to red. The solution
was stirred for 12h at room temperature. The toluene was removed
under vacuum and the yellow product was extracted and crystallized
with n-hexane at –24 °C. Yield 0.60 g (80%). C40H45HfN3 (745.85):
calcd. C 64.36, H 6.08, N 5.63; found C 64.75, H 5.65, N 5.43 %.
1H NMR (400 MHz, C6D6, 298 K): δ = 2.04 (s, 6 H, HCH2benzyl), 2.19
(s, 6 H, H13,14), 2.26 (s, 3 H, H15), 2.64 (t, 4 H, H17,21), 3.28 (t, 4 H,
H18,20), 5.16 (d, 1 H, H3), 5.40 (d, 1 H, H5), 6.80 (t, 6 H, Hbenzyl),
6.84 (t, 1 H, H4), 6.95–7.20 (m, 11 H, H9,11,CHbenzyl) ppm. 13C NMR
(100 MHz, C6D6, 298K): δ = 18.39 (C13,14), 20.85 (C15), 48.02
(C17,21), 65.86 (C18,20), 89.52 (CCH2benzyl), 95.51 (C3), 96.66 (C5),
122.79 (C9,11), 127.60 (Cbenzyl), 128.90 (C10), 129.22 (Cbenzyl), 129.40
1H NMR (400 MHz, C6D6, 298 K): δ = 1.40 (m, 6 H, H18,19,20), 2.14 (Cbenzyl), 134.20 (C8,12), 141.86 (C10), 143.78 (C4), 143.96 (Cbenzyl),
(s, 9 H, H13,14,15), 2.43 (s. 6 H, HCH2benzyl), 3.44 (t, 4 H, H17,21), 5.47 156.30 (C7), 158.78 (C2), 164.70 (C6) ppm.
328
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Hafnium Trialkyls Stabilized by Bulky, Electron-Rich Aminopyridinates
Synthesis of 2g: Aminopyridine 1g (0.323 g, 1.0 mmol in 20.0 mL
Kashmir and the HEC (Higher Education Commission) Pakistan for a
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in scholarship. We thank T. Bauer for his help in the X-ray lab.
20.0 mL toluene). The resultant solution was stirred for 12h at room
temperature. The toluene was removed and the yellow product was
References
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(77%). C42H49HfN3 (773.88): calcd. C 65.13, H 6.38, N 5.43; found
C 64.78, H 6.12, N 5.90 %.
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1H NMR (400 MHz, C6D6, 298 K):
δ = 1.17 (m, 10 H,
H14,15,17,18,20,21), 1.30 (d, 6 H, H14,15,17,18), 2.45 (t, 4 H, H19,22), 2.67
(s, 6 H, HCH2benzyl), 3.41 (sept, 2 H, H13,16), 4.96 (d, 1 H, H3), 5.27
(d, 1 H, H5), 6.75 (t, 1 H, H4), 6.86 (d, 6 H, Hbenzyl), 6.94–7.20 (m,
12 H, H9,10,11,CHbenzyl) ppm. 13C NMR (100 MHz, C6D6, 298 K): δ =
24.42 (C14,15,17,18), 24.13 (C14,15,17,18), 25.57 (C13,16), 28.95 (C20,21),
47.81 (C19,22), 89.65 (CCH2benzyl), 91.39 (C3), 96.19 (C5), 122.08
(Cbenzyl), 124.02 (C9,11), 126.47 (Cbenzyl), 128.38 (C10), 133.50 (C8,12),
134.30 (C7), 142.20 (C4), 142.87 (Cbenzyl), 143.09 (C7), 144.14
(Cbenzyl), 154.49 (C6), 165.32 (C2) ppm.
Synthesis of 2h: Aminopyridine 1h (0.281 g, 1.0 mmol in 15.0 mL
toluene) was added to tetrabenzylhafnium (0.543 g, 1.0 mmol in
20.0 mL toluene). The resultant solution was stirred for 12h at room
temperature. The toluene was removed and the yellow product was
extracted and crystallized with n-hexane at –24 °C. Yield 0.560 g
(76%). C39H43HfN3 (732.27): calcd. C 63.97, H 5.92, N 5.74; found
C 63.85, H 5.72, N 5.62 %.
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1H NMR (400 MHz, C6D6, 298 K): δ = 1.20 (t, 4 H, H17,18), 2.25 (s,
6 H, H13,14), 2.26 (s, 3 H, H15), 2.41 (s, 6 H, HCH2benzyl), 2.70 (t, 4 H,
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6.84 (t, 1 H, H4), 6.95–7.20 (m, 11 H, H9,11,CHbenzyl) ppm. 13C NMR
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(C17,18), 47.87 (C16,19), 89.39 (CCH2benzyl), 90.41 (C3), 96.25 (C5),
122.20 (C9,11), 127.03 (Cbenzyl), 128.90 (C10), 129.46 (Cbenzyl), 133.91
(C8,12), 134.60 (C7), 142.60 (C10), 143.78 (C4), 142.71 (Cbenzyl),
154.84 (C2), 163.38 (C6) ppm.
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Acknowledgments
We thank the Deutsche Forschungsgemeinschaft (DFG) SFB 840 for
supporting this work. M. H. thanks the University of Azad Jammu &
Z. Anorg. Allg. Chem. 2012, 324–330
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Z. Anorg. Allg. Chem. 2012, 324–330