Synthesis of bis(amino)pyridines by the stepwise alkylation of bis(imino)pyridines: An unexpected and selective alkylation of the aminoiminopyridine by AlMe3

Article abstract of DOI:10.1021/om200899f

Bis(imino)pyridines 2-[2,6-(R¹)₂C₆H ₃N=C(CH₃)]-6-[2,6-(R²)₂C ₆H₃N=C(CH₃)](C₅H₃N)(1a, R¹ = R² = i-Pr; 1b, R¹ = i-Pr, R² = CH₃; 1c, R¹ = R² = CH₃) were first alkylated by AlMe₃ and then hydrolyzed to aminoiminopyridines 2-[2,6-(R¹)₂C₆H₃N=C(CH ₃)]-6-[2,6-(R²)₂C₆H ₃NHC(CH₃)₂](C₅H₃N)(3a, R¹ = R² = i-Pr; 3b, R¹ = i-Pr, R² = CH₃; 3c, R¹ = R² = CH₃). When the aminoiminopyridines were treated with AlMe₃, the expected methane elimination was not found; alkylation at the imine functional group led to the aluminum complexes {2-[2,6-(R¹)₂C₆H ₃NC(CH₃)₂]-6-[2,6-(R²) ₂C₆H₃NHC(CH₃)₂]-(C ₅H₃N)}AlMe₂ (4a, R¹ = R² = i-Pr; 4b, R¹ = i-Pr, R² = CH₃; 4c, R ¹ = R² = CH₃), which were further hydrolyzed to the bis(amino)pyridines 2-[2,6-(R¹)₂C₆H ₃NHC(CH₃)₂]-6-[2,6-(R²) ₂C₆H₃NHC(CH₃)₂](C ₅H₃N)(5a, R¹ = R² = i-Pr; 5b, R ¹ = i-Pr, R² = CH₃; 5c, R¹ = R ² = CH₃). The selective alkylation over elimination may come from the noninnocent property of the conjugated iminopyridine group. New magnesium, yttrium, and zirconium complexes supported by 5a ({2,6-[2,6-(i-Pr) ₂C₆H₃NC(CH₃)₂] ₂(C₅H₃N)}Mg(THF)(6), {2,6-[2,6-(i-Pr) ₂C₆H₃NC(CH₃)₂] ₂(C₅H₃N)}Y(CH2SiMe3)(THF)(7), and {2,6-[2,6-(i-Pr)₂C₆H₃NC(CH₃) ₂]₂(C₅H₃N)}Zr(CH ₂SiMe₃)₂ (8)) were formed via the alkane elimination method.

Full text of DOI:10.1021/om200899f

Article
pubs.acs.org/Organometallics
Synthesis of Bis(amino)pyridines by the Stepwise Alkylation of
Bis(imino)pyridines: An Unexpected and Selective Alkylation of the
Aminoiminopyridine by AlMe3
Boon-Ying Tay, Cun Wang,* Sze-Chen Chia, Ludger P. Stubbs, Pui-Kwan Wong, and Martin van Meurs
Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), Singapore, 1 Pesek Road,
Jurong Island, Singapore 627833
S
* Supporting Information
ABSTRACT: Bis(imino)pyridines 2-[2,6-(R1)2C6H3NC(CH3)]-6-[2,6-(R2)2C6H3NC(CH3)](C5H3N) (1a, R1 = R2 = i-Pr;
1b, R1 = i-Pr, R2 = CH3; 1c, R1 = R2 = CH3) were first alkylated by AlMe3 and then hydrolyzed to aminoiminopyridines 2-[2,6-
(R1)2C6H3NC(CH3)]-6-[2,6-(R2)2C6H3NHC(CH3)2](C5H3N) (3a, R1 = R2 = i-Pr; 3b, R1 = i-Pr, R2 = CH3; 3c, R1 = R2 =
CH3). When the aminoiminopyridines were treated with AlMe3, the expected methane elimination was not found; alkylation at
the imine functional group led to the aluminum complexes {2-[2,6-(R1)2C6H3NC(CH3)2]-6-[2,6-(R2)2C6H3NHC(CH3)2]-
(C5H3N)}AlMe2 (4a, R1 = R2 = i-Pr; 4b, R1 = i-Pr, R2 = CH3; 4c, R1 = R2 = CH3), which were further hydrolyzed to the
bis(amino)pyridines 2-[2,6-(R1)2C6H3NHC(CH3)2]-6-[2,6-(R2)2C6H3NHC(CH3)2](C5H3N) (5a, R1 = R2 = i-Pr; 5b, R1 = i-Pr,
R2 = CH3; 5c, R1 = R2 = CH3). The selective alkylation over elimination may come from the noninnocent property of the
conjugated iminopyridine group. New magnesium, yttrium, and zirconium complexes supported by 5a ({2,6-[2,6-(i-
Pr)2C6H3NC(CH3)2]2(C5H3N)}Mg(THF) (6), {2,6-[2,6-(i-Pr)2C6H3NC(CH3)2]2(C5H3N)}Y(CH2SiMe3)(THF) (7), and
{2,6-[2,6-(i-Pr)2C6H3NC(CH3)2]2(C5H3N)}Zr(CH2SiMe3)2 (8)) were formed via the alkane elimination method.
ligand frame with bulky steric demands. The starting material
was fully recovered when 1a was treated in the same way.
Direct alkylation of 1a to bis(amino)pyridine 5a also does not
work.10 A general synthesis of bis(amino)pyridines was
achieved by the substitution reaction of 2,6-bis(bromomethyl)-
pyridine by amines.11
We disclose here a convenient way to the bis(amino)pyridine
5 through selective alkylation of 3 by AlMe3 followed by
hydrolysis (Scheme 2). The expected formation of aluminum
complex 2 from 3 via methane elimination was not found.
Since the first independent reports by Gibson, Bennett, and
Brookhart that bis(imino)pyridine-supported iron and cobalt
complexes were very active for olefin polymerization,1
tremendous research has been undertaken to understand this
efficient system.2 Studies have shown that the conjugated ligand
itself is noninnocent and is involved in a series of trans-
formations such as deprotonation reactions, alkylation, and
participation in redox reactions.3,4 One of the transformations is
the monoalkylation of the bis(imino)pyridine 1a to the
aminoiminopyridine 3a by AlMe3 via the aluminum complex
2a as the intermediate (Scheme 1).3d,5 The aminoiminopyr-
idine 3a has offered a platform to numerous complexes with
unique properties and applications.6 A further straightforward
modification to the ligand framework is bis(amino)pyridine, in
which both of the imine groups are replaced by amine donors.
Bis(amino)pyridines exert quite different electronic and steric
environments and have been applied as useful supporting
ligands for transition metals which mainly have applications to
polymer formation.4,7,8 While the direct reduction of bis-
(imino)pyridine to bis(amino)pyridine by sodium borohydride
was previously reported,9 this method cannot be applied to this
RESULTS AND DISCUSSION
■
We found this reaction by serendipity. When we prepared the
ligand 3a according to the literature (Scheme 1),3d,5 the
bis(imino)pyridine 1a was not completely transformed into 3a
(>90% conversion). In order to obtain a higher conversion rate,
which is necessary for ease of purification of the product, the
Received: September 27, 2011
Published: October 12, 2011
© 2011 American Chemical Society
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Scheme 1. Monoalkylation of Bis(imino)pyridine 1a by AlMe3
Scheme 2. Alkylation of 3 by AlMe3
crude mixture of 1a and 3a was treated with a second excess
portion of AlMe3. To our surprise, the main product was not 3a
but the bis(amino)pyridine 5a after workup. This spurred us to
investigate the reaction further. We tried the reaction of pure 3a
with 1 equiv of AlMe3 in toluene at room temperature for 12 h.
Instead of the product 2a, formed via methane elimination like
other group III and IV metal complexes depicted in the
literature,6 only the selective alkylation product 4a was obtained
in quantitative yield (Scheme 2). 4a has been isolated as a
byproduct and characterized by X-ray diffraction in the reaction
of ligand 3a with [Ln(AlMe4)3] in the literature.10 Although the
author claims the alkylation proceeds via the [Ln−Me] moiety
rather than [Al−Me], we think the high reactivity of the Al−Me
bond and the noninnocent conjugated iminopyridine group
may dominate the alkylation reaction selectively. 4a was further
hydrolyzed to the bis(amino)pyridine 5a with an isolated yield
of 93% after recrystallization from methanol. 4a and 5a have
been characterized by NMR spectroscopy, elemental analysis,
etc. Single crystals for X-ray structure analysis were also
obtained from 5a by the slow evaporation of its solution in
methanol/ether (Figure 1).
Figure 1. ORTEP drawing of the molecular structure of 5a
(anisotropic displacement parameters set at the 30% probability
level). Selected bond lengths (Å) and angles (deg): C(1)−N(1) =
1.336(5), C(1)−C(21) = 1.548(5), C(21)−N(2) = 1.485(5), C(5)−
N(1) = 1.355(5), C(5)−C(6) = 1.531(6), C(6)−N(3) = 1.500(5);
C(1)−C(21)−N(2) = 109.6(3), C(5)−C(6)−N(3) = 107.2(3).
With the large number of bis(imino)pyridine ligands that
have been reported, it is interesting to see if this route is more
general for sterically less bulky ligands. We have prepared a
series of bis(imino)pyridine compounds 1 with different
substituents at the imine nitrogen atoms according to literature
methods.1d,9,12 The results show that the substituents with
steric demands are necessary for the successful alkylation of the
imine functional groups. While 1b,c were smoothly trans-
formed to the corresponding 3b,c and 5b,c, symmetric
bis(imino)pyridine ligands with less bulky subsituents such as
n-butyl and p-methylphenyl groups underwent a complicated
reaction in the presence of trimethylaluminum, and no clean
products could be obtained for the first step of alkylation. Just
as expected, in the case of 1b, the alkylation took place on the
imine with the less bulky 2,6-dimethylphenyl substituent. The
structures of the new compounds 3b,c have been also
confirmed by NMR spectroscopy, elemental analysis, and X-
ray diffraction studies (see the Experimental Section and
Supporting Information).
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1
Figure 2. H NMR spectrum of 4b (400 MHz, d6-benzene, 298 K).
Scheme 3. Synthesis of the Bis(amido)pyridinato Ligand 5a Supported Complexes of Magnesium, Yttrium, and Zirconium
It is worthy of note that, when 3b was reacted with
trimethylaluminum, only the selective alkylation of the more
bulky imine group over the methane elimination at the other
side of amine was found, and the aluminum complex 4b was
confirmed as the sole product (Figure 2). The noninnocent
conjugated iminopyridine group may again be the reason. 3c
can be alkylated to 4c in the same way. Upon hydrolysis, 4b,c
released the free ligands 5b,c. Although the second step of
alkylation proceeds smoothly at room temperature, it is
beneficial to do the reaction with an excess of trimethylalumi-
num (1.2 equiv) at higher temperature (50 °C). The complete
conversion of 3 makes it easier for the separation of product 5,
which can be recrystallized from methanol.
magnesium, yttrium, and zirconium complexes supported by 5a
(Scheme 3). The bis(amino)pyridine 5a reacted smoothly with
metal alkyls by alkane elimination and gave the desired
complexes cleanly with or without donor solvent (THF)
coordinated for electronic and steric requirements. In solution,
complexes 6−8 exhibit 1H NMR spectra typical for rigid
coordination. There is a high rotational barrier for the phenyl
rings, making the two methyl groups of each isopropyl group
diastereotopic at room temperature (Experimental Section). In
the solid-state structure of 6 (Figure 3), the four-coordinate
magnesium metal center adopts a planar geometry and a
symmetry plane along O(1)−Mg(1)−N(1) (180°) bisects the
whole molecule. The doubly alkylated ligand 5a exhibits a
spatially congested environment around the metal center. Only
one molecule of THF is coordinated to the magnesium atom,
To test the feasibility of using bis(amino)pyridines 5 as facile
ligands for transition metals, we have successfully synthesized
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Figure 4. ORTEP drawing of the molecular structure of 8 (anisotropic
displacement parameters set at 30% probability, hydrogen atoms and
the toluene molecule omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zr(1)−N(4) = 2.288(3), Zr(1)−N(5) = 2.133(2),
N(5)−C(8) = 1.502(4), C(8)−C(9) = 1.517(4), N(4)−C(9) =
1.362(3), Zr(1)−C(14) = 2.245(4), Zr(1)−C(23) = 2.230(5); N(5)−
Zr(1)−N(4) = 70.48(7), C(14)−Zr(1)−N(4) = 102.7(1), C(23)−
Zr(1)−N(4) = 151.1(1), C(23)−Zr(1)−C(14) = 106.2(2), Zr(1)−
C(23)−Si(3) = 145.1(2), Zr(1)−C(14)−Si(2) = 133.5(2), N(5)i−
Zr(1)−N(5) = 136.11(9).
Figure 3. ORTEP drawing of the molecular structure of 6 (anisotropic
displacement parameters set at 30% probability, hydrogen atoms
omitted for clarity). Selected bond lengths (Å) and angles (deg):
Mg(1)−N(1) = 2.0616(18), Mg(1)−O(1) = 2.0271(16), Mg(1)−
N(2) = 2.0169(12), C(3)−N(1) = 1.3407(15), C(3)−C(4) =
1.5453(18), C(4)−N(2) = 1.4730(17), C(7)−N(2) = 1.4143(16);
O(1)−Mg(1)−N(1) = 180.0, N(1)−C(3)−C(4) = 114.38(12),
N(2)−C(4)−C(3) = 108.02(10), C(4)−N(2)−Mg(1) = 116.40(8),
N(2)−Mg(1)−N(1) = 78.65(4), C(7)−N(2)−Mg(1) = 118.87(9).
over elimination reactions may come from the noninnocence of
the conjugated iminopyridine group.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under a dry argon atmosphere using standard Schlenk techniques or in
a glovebox. Solvents were dried and purified with MBRAUN solvent
purification system. NMR spectra were recorded on a Bruker 400
MHz spectrometer. Chemical shifts are reported in δ, referenced to 1H
(of residual protons) and 13C signals of deuterated solvents as internal
standards. Elemental analysis was performed on aThermo Flash 2000
CHNS analyzer. 1a,1d 1b,12a 1c,1d 2a,3d,5 2c,14 3a,5 (TMEDA)-
while two molecules of THF and/or a five-coordinate geometry
prevail in other cases.3b,13 This is also true for the yttrium
complex 7, in which the ligand frame’s spatial congestion
prevents the coordination of a second molecule of THF to the
metal center. On the other hand, two coordinated THF
molecules were found in a similar yttrium compound supported
by a less bulky ligand.7f For the zirconium complex 8, although
MgMe2,15 Y(CH2SiMe3)3(THF)2,16 and Zr(CH2SiMe3)4 were
17
prepared according to literature methods.
1
X-ray Crystal Structure Analyses. Data sets were collected with
a Rigaku Saturn 70 CCD diffractometer at 110 K with graphite-
monochromated Mo Kα radiation (λ = 0.710 70 Å). The data was
corrected for Lorentz and polarization effects and absorption
(REQAB). The structure was solved by direct methods (SHELXS-
97) and refined on F2 against all reflections (SHELXL-97).18 Crystal
data for 5a: C35H5N3, Mr = 513.79, orthorhombic, space group Pccn, a
= 15.873(3) Å, b = 32.135(6) Å, c = 12.193(2) Å, α = 90°, β = 90°, γ =
90°, V = 6219(2) Å3, Z = 8, ρ(calcd) = 1.097 g/cm−3, R1 (I > 2σ(I))
= 0.1219, wR2 (all data) = 0.2615, GOF = 1.234. Crystal data for 6:
C39H57MgN3O, Mr = 608.19, monoclinic, space group C2/c, a =
9.4750(19) Å, b = 16.603(3) Å, c = 22.150(4) Å, α = 90°, β = 95.26(3)
°, γ = 90°, V = 3469.8(12) Å3, Z = 4, ρ(calcd) = 1.164 g/cm−3, R1 (I
> 2σ(I)) = 0.0593, wR2 (all data) = 0.1704, GOF = 1.211. Crystal data
for 8: C50H79N3Si2Zr, Mr = 869.59, orthorhombic, space group Pnma,
a = 24.951(10) Å, b = 13.912(5) Å, c = 14.168(6) Å, α = 90°, β = 90°,
γ = 90°, V = 4917.7(33) Å3, Z = 4, ρ(calcd) = 1.174 g/cm−3, R1 (I >
3σ(I)) = 0.082, wR2 (I > 3σ(I)) = 0.080, GOF = 3.807.
2-[2,6-(i-Pr)2C6H3NC(CH3)]-6-[2,6-(CH3)2C6H3NHC(CH3)2]-
(C5H3N) (3b). AlMe3 (1.4 mL, 2.0 M in toluene, 2.8 mmol) was
added slowly to a solution of 1b (1.13 g, 2.65 mmol) in 20 mL of
toluene. The resultant dark brown solution was stirred at 110 °C for
12 h. After it was cooled to room temperature, the dark red solution
was treated with water (0.4 mL) slowly under argon until a yellow
solution was obtained. The toluene solution was filtered and
evaporated under reduced pressure. The residue was purified by
recrystallization from methanol to give the product as a yellow powder.
Yield: 0.92 g, 2.07 mmol, 78%. Anal. Calcd for C30H39N3: C, 81.59; H,
8.90; N, 9.51. Found: C, 80.88; H, 8.62; N, 9.16. MS: m/z (%) 442.3
its H NMR spectrum shows a C2v symmetry like that of 6 at
room temperature, there must be a fast fluxion between the two
(trimethylsilyl)methyl groups as all six methyl groups show
only one signal at −0.05 ppm. This can be easily seen from the
X-ray structure of 8 in its solid state (Figure 4). The geometry
around the zirconium center is best described as distorted
square pyramidal. C23 forms an equatorial plane with N4, N5,
and N5i, while C14 occupies the apical position. This makes an
evident contrast to the reported example with a similar but less
bulky ligand, for which a perfect C2v symmetry is adopted.11
The bulk of the isopropyl substituents in compound 8 prevents
the adjacent residence of trimethylsilyl groups and makes them
point outward. This is also found in another literature report.7g
Since this bis(amino)pyridine ligand frame induces a
different electronic and steric effect on the metal center, new
properties and catalytic behaviors would be expected from the
complexes supported by the dianionic bis(amido)pyridinato
ligands. Work to explore the applications of this kind of
reaction in metal catalysis is still under way in our laboratory.
In summary, bulky bis(imino)pyridines can be alkylated in a
stepwise fashion selectively by AlMe3. The clean and easily
handled reaction should open the door to a series of
bis(amino)pyridine derivatives with different electronic and
steric requirements, due to the ease of modification of the
bis(imino)pyridine framework. The selectivity of alkylation
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[M+ + H]. 1H NMR (400 MHz, d-chloroform, 298 K): δ 8.21 (dd,3JHH
= 7.6 Hz, 4JHH = 0.8 Hz, 1H, Py-H), 7.80 (t, 3JHH = 7.6 Hz, 1H, Py-H),
7.69 (dd, 3JHH = 7.6 Hz, 4JHH = 0.8 Hz, 1H, Py-H), 7.18−6.86 (m, 6H,
Ar-H), 4.39 (s, 1H, NH), 2.77 (sept, 3JHH = 6.8 Hz, 2H, CHMe2), 2.21
(s, 3H, NC(CH3)), 2.14 (s, 6H, Ar-(CH3)), 1.56 (s, 6H,
was added slowly to a solution of 3b (315 mg, 0.71 mmol) in 20 mL of
toluene cooled with a water bath. The resultant dark brown solution
was stirred at room temperature for 4 h. A small portion of the
solution was taken out, and toluene was removed. The red solid
obtained was analyzed by NMR spectroscopy. NMR studies showed
the quantitative formation of 4b. 1H NMR (400 MHz, d6-benzene, 298
3
NC(CH3)2), 1.17, 1.15 (each d, JHH = 6.8 Hz, 12H, CH(CH3)2).
13C{1H} NMR (100 MHz, d-chloroform, 298 K): δ 167.4, 166.9,
K): δ 7.23 (m, 3H, Ar-H or/and Py-H), 7.00 (t, 3JHH = 7.8 Hz, 1H, Ar-
3
H or/and Py-H), 6.84 (m, 4H, Py-H or/and Py-H), 6.48 (dd, JHH
=
=
154.7, 146.6, 144.1, 136.9, 135.8, 134.7, 128.4, 123.5, 123.2, 123.0,
120.7, 118.7, 60.0, 29.3, 28.2, 23.2, 22.9, 20.3, 17.4. Crystals suitable
for X-ray structure analysis were also obtained for 3b by the slow
evaporation of its solution in methanol/dichloromethane at room
temperature.
7.8 Hz, 4JHH = 1.2 Hz, 1H, Py-H), 4.43 (s, 1H, NH), 3.93 (sept, 3JHH
6.4 Hz, 2H, CHMe2), 1.93 (s, 6H, Ar-(CH3)), 1.50 (s, 6H,
3
3
NC(CH3)2), 1.39 (d, JHH = 6.8 Hz, 6H, CH(CH3)2), 1.36 (d, JHH
= 6.8 Hz, 6H, CH(CH3)2), 1.22 (s, 6H, NC(CH3)2), −0.49 (s, 6H,
Al(CH3)2). 13C{1H} NMR (100 MHz, d6-benzene, 298 K): δ 173.2,
164.2, 150.7, 146.4, 143.3, 139.7, 131.3, 129.5, 124.1, 124.0, 123.8,
119.1, 116.7, 61.7, 58.4, 31.1, 30.8, 28.1, 27.0, 24.8, 20.5, −2.3.
2-[2,6-(i-Pr)2C6H3NHC(CH3)2]-6-[2,6-(CH3)2C6H3NHC(CH3)2]-
(C5H3N) (5b). The above toluene solution was heated at 50 °C
overnight. A 0.2 mL portion of water was added dropwise to hydrolyze
the aluminum complex 4b until a yellow solution was obtained. The
resulting solution was filtered, and the residue was washed with
toluene. The combined toluene solution was evaporated under
vacuum to yield the crude product, which was recrystallized from
methanol to give yellow crystals. Yield: 247 mg, 0.54 mmol, 76%. Anal.
Calcd for C31H43N3: C, 81.35; H, 9.47; N, 9.18. Found: C, 81.27; H,
9.39; N, 9.18. MS: m/z (%) 458.4 [M+ + H]. 1H NMR (400 MHz, d-
2-[2,6-(CH3)2C6H3NC(CH3)]-6-[2,6-(CH3)2C6H3NHC(CH3)2]-
(C5H3N) (3c). 3c was obtained similarly from 1c (1.64 g, 4.44 mmol)
and AlMe3 (2.3 mL, 2.0 M in toluene, 4.6 mmol). Yield: 1.12 g, 2.91
mmol, 65%. Anal. Calcd for C26H31N3: C, 81.00; H, 8.10; N, 10.90.
Found: C, 80.53; H, 8.19; N, 10.48. MS: m/z (%) 386.3 [M+ + H]. 1H
3
NMR (400 MHz, d-chloroform, 298 K): δ 8.23 (dd, JHH = 7.6 Hz,
4JHH = 0.8 Hz, 1H, Py-H), 7.79 (t, 3JHH = 7.6 Hz, 1H, Py-H), 7.72 (dd,
4
3JHH = 7.6 Hz, JHH = 0.8 Hz, 1H, Py-H), 7.08−6.86 (m, 6H, Ar-H),
4.33 (s, 1H, NH), 2.19 (s, 3H, NC(CH3)), 2.13, 2.05 (each s, each
6H, Ar-(CH3)), 1.55 (s, 6H, NC(CH3)2). 13C{1H} NMR (100 MHz,
d-chloroform, 298 K): δ 167.6, 167.0, 154.6, 148.8, 144.1, 136.8, 134.6,
128.4, 127.9, 125.5, 123.2, 122.9, 120.8, 118.7, 60.0, 29.4, 20.3, 18.0,
16.6. Crystals suitable for X-ray structure analysis were also obtained
for 3c by the slow evaporation of its solution in methanol/ether at −30
°C.
chloroform, 298 K): 7.65 (t, 3JHH = 7.8 Hz, 1H, Py-H), 7.51 (d,3JHH
=
7.8 Hz, 1H, Py-H), 7.39 (d, 3JHH = 7.8 Hz, 1H, Py-H), 7.07 (peudo s,
3H, Ar-H), 6.98 (d, 3JHH = 7.6 Hz, 2H, Ar-H), 6.87 (t, 3JHH = 7.6 Hz,
1H, Ar-H), 4.24 (s, 1H, NH), 4.18 (s, 1H, NH), 3.28 (sept, 3JHH = 6.8
Hz, 2H, CHMe2), 2.09 (s, 6H, Ar-(CH3)), 1.55, 1.47 (each s, each 6H,
NC(CH3)2), 1.07 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(100 MHz, d-chloroform, 298 K): δ 167.1, 166.5, 146.7, 144.3, 140.4,
136.7, 134.5, 128.4, 124.4, 123.0, 122.9, 117.2, 116.7, 59.8, 59.4, 29.4,
29.1, 28.2, 24.1, 20.2.
{2-[2,6-(i-Pr)2C6H3NC(CH3)2]-6-[2,6-(i-Pr)2C6H3NHC(CH3)2]-
(C5H3N)}AlMe2 (4a). AlMe3 (2.5 mL, 2.0 M in toluene, 5.0 mmol)
was added slowly to a solution of 3a (2.5 g, 5.02 mmol) in 20 mL of
toluene cooled with a water bath. The resultant dark brown solution
was stirred at room temperature overnight. A small portion of the
solution was taken out, and toluene was removed. The brown solid
obtained was analyzed by NMR spectroscopy and elemental analysis.
NMR studies showed the quantitative formation of 4a, and the
elemental analysis results were in agreement with the expected
composition. Anal. Calcd for C37H56AlN3: C, 77.99; H, 9.91; N, 7.37.
2,6-[2,6-(CH3)2C6H3NHC(CH3)2]2(C5H3N) (5c). 5c was obtained
similarly from 3c (870 mg, 2.26 mmol) and AlMe3 (1.3 mL, 2.0 M in
toluene, 2.6 mmol) as yellow crystals. Yield: 753 mg, 1.88 mmol, 83%.
Anal. Calcd for C27H35N3: C, 80.75; H, 8.78; N, 10.46. Found: C,
1
Found: C, 78.18; H, 10.32; N, 7.10. H NMR (400 MHz, d6-benzene,
1
80.30; H, 8.74; N, 10.22. MS: m/z (%) 402.3 [M+ + H]. H NMR
298 K): δ 7.26 (m, 3H, Ar-H), 7.01 (m, 3H, Ar-H), 6.88 (t, 3JHH = 7.8
(400 MHz, d-chloroform, 298 K): δ 7.65 (t, 3JHH = 7.8 Hz, 1H, Py-H),
7.48 (d, 3JHH = 7.8 Hz, 2H, Py-H), 6.97 (d, 3JHH = 7.6 Hz, 4H, Ar-H),
6.86 (t, 6H, 3JHH = 7.6 Hz, 2H, Ar-H), 4.17 (s, 2H, NH), 2.09 (s, 12H,
Ar-(CH3)), 1.52 (s, 12H, NC(CH3)2). 13C{1H}NMR (100 MHz, d-
chloroform, 298 K): δ 166.7, 144.4, 136.7, 134.5, 128.4, 123.0, 117.1,
59.8, 29.4, 20.3.
3
4
Hz,1H, Py-H), 6.87 (dd, JHH = 7.8 Hz, JHH = 1.2 Hz, 1H, Py-H),
6.64 (dd, 3JHH = 7.8 Hz, 4JHH = 1.2 Hz, 1H, Py-H), 4.55 (s, 1H, NH),
3.82 (sept, 3JHH = 6.4 Hz, 2H, CHMe2), 2.76 (sept, 3JHH = 6.8 Hz, 2H,
CHMe2), 1.50 (s, 6H, NC(CH3)2), 1.37 (s, 6H, NC(CH3)2), 1.35 (d,
3
3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.34 (d, JHH = 6.8 Hz, 6H,
3
CH(CH3)2), 1.10 (d, JHH = 6.4 Hz, 12H, CH(CH3)2), −0.23 (s, 6H,
Al(CH3)2). 13C{1H} NMR (100 MHz, d6-benzene, 298 K): δ 175.6,
165.9, 150.7, 145.4, 144.0, 140.0, 139.4, 125.0, 124.3, 124.2, 123.7,
119,3, 117.2, 62.4, 58.4, 31.3, 30.3, 28.5, 28.1, 26.9, 25.0, 24.5, −2.5.
2,6-[2,6-(i-Pr)2C6H3NHC(CH3)2]2(C5H3N) (5a). A 1.1 mL portion
of water was added dropwise to the above toluene solution cooled with
an ice bath. Effervescence was found immediately, and a white
precipitate started to form. The dark reaction mixture gradually
lightened and was stirred until all of the aluminum complex 4 was
completely hydrolyzed to an orange solution. The resultant solution
was filtered, and the residue was washed with toluene. The combined
toluene solution was evaporated under vacuum to yield the crude
product, which was recrystallized from methanol to give yellow
crystals. Yield: 2.41 g, 4.7 mmol, 93%. Anal. Calcd for C35H51N3: C,
81.82; H, 10.00; N, 8.18. Found: C, 81.99; H, 10.19; N, 7.92. MS: m/z
{2,6-[2,6-(i-Pr)2C6H3NC(CH3)2]2(C5H3N)}Mg(THF) (6).
(TMEDA)MgMe2 (34 mg, 0.20 mmol) in 1 mL of toluene was
added to a solution of 5a (103 mg, 0.20 mmol) in 2 mL of toluene and
0.1 mL of THF at room temperature. The yellow solution turned
darker brown immediately and was stirred for another 10 min. The
solvent was evaporated, and the residue was dissolved in 0.2 mL of
toluene and 0.8 mL of pentane and stored at −20 °C overnight. The
product precipitated as crystalline yellow crystals. Yield: 79 mg, 0.13
mmol, 65%. Anal. Calcd for C39H57MgN3O: C, 77.02; H, 9.45; N,
1
6.91. Found: C, 76.86; H, 9.43; N, 6.93. H NMR (400 MHz, d6-
benzene, 298 K): δ 7.23 (t, 3JHH = 7.0 Hz, 1H, Py-H), 7.18 (m, 4H, Ar-
3
H), 7.09 (m, 2H, Ar-H), 6.94 (d, 2H, JHH = 7.0 Hz, Py-H), 3.98
3
(sept, JHH = 7.0 Hz, 4H, CHMe2), 2.88 (pseudo s, 4H, THF-α-H),
3
1.61 (s, 12H, NC(CH3)2), 1.31 (d, JHH = 7.0 Hz, 12H, CH(CH3)2),
3
1
(%) 513.7 [M+ − H]. H NMR (400 MHz, d-chloroform, 298 K): δ
1.18 (d, JHH = 7.0 Hz, 12H, CH(CH3)2), 0.96 (m, 4H, THF-β-H).
13C{1H} NMR (100 MHz, d6-benzene, 298 K): δ 171.9, 155.3, 149.6,
138.5, 123.3, 121.8, 115.8, 70.4, 62.3, 33.1, 27.7, 25.8, 25.1, 25.0. Single
crystals for X-ray structure analysis were obtained by the diffusion of
pentane vapor into a toluene solution of 6 at −20 °C.
7.65 (t, 3JHH = 8.0 Hz, 1H, Py-H), 7.39 (dd, 3JHH = 8.0 Hz, 4JHH = 0.8
Hz, 2H, Py-H), 7.08 (m, 6H, Ar-H), 4.26 (s, 2H, NH), 3.31 (sept,
3
3JHH = 6.8 Hz, 4H, CHMe2), 1.49 (s, 12H, NC(CH3)2), 1.07 (d, JHH
= 6.8 Hz, 24H, CH(CH3)2). 13C{1H} NMR (100 MHz, d-chloroform,
298 K): δ 167.0, 146.8, 140.3, 136.8, 124.5, 123.0, 116.7, 59.4, 29.0,
28.2, 24.1. Crystals suitable for X-ray structure analysis were also
obtained for 5a by the slow evaporation of its solution in methanol/
ether −30 °C.
{2,6-[2,6-(i-Pr)2C6H3NC(CH3)2]2(C5H3N)}Y(CH2SiMe3)(THF) (7).
Y(CH2SiMe3)3(THF)2 (315 mg, 0.64 mmol) in 2 mL of toluene was
added to a solution of 5a (327 mg, 0.64 mmol) in 5 mL of toluene at
room temperature. The yellow solution was stirred for 1 h. The
solvent was evaporated, and the residue was treated with 5 mL of
pentane and filtered. The filtrate was stored at −20 °C for 2 days. The
{2-[2,6-(i-Pr)2C6H3NC(CH3)2]-6-[2,6-(CH3)2C6H3NHC(CH3)2]-
(C5H3N)}AlMe2 (4b). AlMe3 (0.42 mL, 2.0 M in toluene, 0.84 mmol)
6032
dx.doi.org/10.1021/om200899f|Organometallics 2011, 30, 6028−6033

Organometallics
Article
product precipitated as a pale yellow solid containing cocrystallized
pentane, which was isolated by filtration and dried under vacuum. The
solid product changed to a powder under vacuum, while the
cocrystallized pentane was lost. Yield: 374 mg, 0.49 mmol, 77%.
Anal. Calcd for C43H68N3OiY: C, 67.95; H, 9.02; N, 5.53. Found: C,
(3) (a) Sugiyama, H.; Gambarotta, S.; Yap, G. P. A.; Wilson, D. R.;
Thiele, S. K.-H. Organometallics 2004, 23, 5054. (b) Blackmore, I. J.;
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S.; Yap, A. P.; Budzelaar, P. H. M. Angew. Chem., Int. Ed. 2002, 41,
3873. (d) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White,
A. J. P.; Williams, D. J. Chem. Commun. 1998, 2523. (e) Scott, J.;
Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am. Chem. Soc.
2005, 127, 13019. (f) Sugiyama, H.; Aharonian, G.; Gambarotta, S.;
Yap, G. P. A.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2002, 124, 12268.
(g) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G.; Wang, Q. J. Am.
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Hawrelak, E. J.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Chem.
Commun. 2005, 3402.
1
67.53; H, 9.03; N, 5.36. H NMR (400 MHz, d6-benzene, 298 K): δ
7.25 (m, 2H, Ar-H), 7.16 (m, 1H, Py-H), 7.08 (m, 4H, Ar-H), 6.83 (d,
3
3JHH = 7.6 Hz, 2H, Py-H), 4.64 (sept, JHH = 6.6 Hz, 2H, CHMe2),
3.20 (sept, 3JHH = 6.6 Hz, 2H, CHMe2), 2.88 (m, 4H, THF-α-H), 1.94
(s, 6H, NC(CH3)2), 1.50 (d, 3JHH = 6.6 Hz, 6H, CH(CH3)2), 1.48 (d,
3JHH = 6.6 Hz, 6H, CH(CH3)2), 1.24 (s, 6H, NC(CH3)2), 1.20 (d,
3
3JHH = 6.6 Hz, 6H, CH(CH3)2), 0.81 (d, JHH = 6.6 Hz, 12H,
CH(CH3)2), 0.80 (m, 4H, THF-β-H), 0.39 (s, 9H, CH2Si(CH3)3),
2
−0.35 (d, JYH = 3.6 Hz, 2H, CH2Si(CH3)3). 13C{1H} NMR (100
(4) For recent review articles on redox-active ligands: (a) Chirik, P.
J. Inorg. Chem. 2011, 50, 9737. (b) Tondreau, A. M.; Milsmann, C.;
Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2011, 50, 9888.
MHz, d6-benzene, 298 K): δ 175.1, 149.7, 149.2, 148.9, 138.7, 124.4,
1
123.4, 123.3, 115.6, 70.4, 66.9, 40.7, 29.7 (d, JYC = 47 Hz,
CH2Si(CH3)3), 28.2, 28.1, 27.4, 27.2, 27.1, 26.5, 24.9, 23.9, 4.6
(5) Britovsek, G. J. P.; Gibson, V. C.; Mastroianni, S.; Oakes, D. C.
H.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J.
Inorg. Chem. 2001, 431.
(CH2Si(CH3)3).
{2,6-[2,6-(i-Pr)2C6H3NC(CH3)2]2(C5H3N)}Zr(CH2SiMe3)2 (8). Zr-
(CH2SiMe3)4 (377 mg, 0.86 mmol) in 2 mL of toluene was added to a
solution of 5a (440 mg, 0.86 mmol) in 4 mL of toluene at room
temperature. The reaction mixture was stirred at 50 °C for 1 h. The
solvent was evaporated, and the residue was washed with 5 mL of
pentane and dried under vacuum to give a white solid. Yield: 575 mg,
0.74 mmol, 86%. Anal. Calcd for C43H71N3Si2Zr: C, 66.43; H, 9.21; N,
(6) (a) Zimmermann, M.; Tornroos, K. W.; Waymouth, R. M.;
Anwander, R. Organometallics 2008, 27, 4310. (b) Knijnenburg, Q.;
Smits, J. M. M.; Budzelaar, P. H. M. Organometallics 2006, 25, 1036.
(c) Cameron, T. M.; Gordon, J. C.; Michalczyk, R.; Scott, B. L. Chem.
Commun. 2003, 2282. (d) Baugh, L. S.; Sissano, J. A. J. Polym. Sci., Part
A: Polym. Chem. 2002, 40, 1633. (e) Gibson, V. C.; Kimberley, B. S.;
Solan, G. A. PCT Int. Appl. WO 2000020427 A1 20000413, 2000.
(7) (a) Kang, K. K.; Hong, S.-P.; Jeong, Y.-T..; Shiono, T.; Ikeda, T.
1
5.40. Found: C, 66.21; H, 9.16; N, 4.83. H NMR (400 MHz, d6-
benzene, 298 K): δ 7.22 (m, 6H, Ar-H), 7.05 (t, JHH = 7.8 Hz, 1H,
3
3
3
Py-H), 6.60 (d, JHH = 7.8 Hz, 2H, Py-H), 3.68 (sept, JHH = 6.4 Hz,
4H, CHMe2), 1.45 (d, 3JHH = 6.4 Hz, 12H, CH(CH3)2), 1.38 (s, 12H,
́
J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3756. (b) Duhovic, S.;
Khan, S.; Diaconescu, P. L. Chem. Commun. 2010, 46, 3390. (c) Jie, S.;
Diaconescu, P. L. Organometallics 2010, 29, 1222. (d) Zimmermann,
3
NC(CH3)2), 1.26 (d, JHH = 6.4 Hz, 12H, CH(CH3)2), 0.76 (s, 4H,
CH2Si(CH3)3), −0.05 (s, 18H, CH2Si(CH3)3). 13C{1H} NMR (100
MHz, d6-benzene, 298 K): δ 173.1, 147.7, 146.8, 139.9, 125.8, 124.9,
116.5, 70.9, 63.3, 33.6 (CH2Si(CH3)3), 28.4, 27.4, 25.5, 2.8
(CH2Si(CH3)3). Single crystals suitable for X-ray structure analysis
were obtained by the diffusion of pentane vapor into a toluene
solution of 8 at −20 °C.
M.; Estler, F.; Herdtweck, E.; Tornroos, K. W.; Anwander, R.
̈
Organometallics 2007, 26, 6029. (e) Cruz, C. A.; Emslie, D. J. H.;
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Organometallics 1999, 18, 3649.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for 3b,c, 5a, 6, and 8 and
figures giving ORTEP drawings for 3b,c and NMR spectra for
all of the compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
(9) Lavery, A.; Nelson, S. M. J. Chem. Soc., Dalton Trans. 1985, 1053.
(10) Zimmermann, M.; Tornroos, K. W.; Anwander, R. Angew.
Chem., Int. Ed. 2007, 46, 3126.
̈
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wang_cun@ices.a-star.edu.sg. Tel: +(65)-6796-3838.
Fax: +(65)-6316-6182.
(11) Guer
15, 5586.
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in, F.; McConville, D. H.; Vittal, J. J. Organometallics 1996,
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F.; Laschi, F.; Sommazzi, A. Eur. J. Inorg. Chem. 2003, 1620.
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ACKNOWLEDGMENTS
■
We thank the Institute of Chemical and Engineering Sciences
for its financial support (Project ICES/09-514A02).
(13) Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G. Organometallics
2010, 29, 4203.
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2002, 40, 1633.
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