Aluminium complexes containing bidentate and symmetrical tridentate pincer type pyrrolyl ligands: Synthesis, reactions and ring opening polymerization

Article abstract of DOI:10.1039/c1dt10442j

A series of aluminium derivatives containing substituted bidentate and symmetrical tridentate pyrrolyl ligands, [C₄H₃NH(2-CH ₂NH^tBu)] and [C₄H₂NH(2,5-CH ₂NH^tBu)₂], in toluene or diethyl ether were synthesized. Their reactivity and application for the ring opening polymerization of ε-caprolactone have been investigated. The reaction of AlMe₃ with one equiv. of [C₄H₃NH(2-CH ₂NH^tBu)] in toluene at room temperature affords [C ₄H₃N(2-CH₂NH^tBu)]AlMe₂ (1) in 70% yield by elimination of one equiv. of methane. Interestingly, while reacting AlMe₃ with one equiv. of [C₄H₃NH(2- CH₂NH^tBu)] in toluene at 0°C followed by refluxing at 100°C, [{C₄H₃N(2-CH₂N^tBu)}AlMe] ₂ (2) has been isolated via fractional recrystalliztion in 30% yield. Similarly, reacting AlMe₃ with two equiv. of C₄H ₃NH(2-CH₂NH^tBu) generates [C₄H ₃N(2-CH₂NH^tBu)]₂AlMe (3) in a moderate yield. Furthermore, complex 1 can be transformed to an aluminium alkoxide derivative, [C₄H₃N(2-CH₂NH ^tBu)][OC₆H₂(-2,6-^tBu ₂-4-Me)]AlMe (4) by reacting 1 with one equiv. of HOC ₆H₂(-2,6-^tBu₂-4-Me) in toluene via the elimination of one equiv. of methane. The reaction of AlR₃ with one equiv. of [C₄H₂NH(2,5-CH₂NH ^tBu)₂] in toluene at room temperature affords [C ₄H₂N(2,5-CH₂NH^tBu) ₂]AlR₂ (5, R = Me; 6, R = Et) in moderate yield. Surprisingly, from the reaction of two equiv. of [C₄H ₂NH(2,5-CH₂NH^tBu)₂] with LiAlH ₄ in diethyl ether at 0°C, a novel complex, [C₄H ₂N(2-CH₂N^tBu)(5-CH₂NH ^tBu)]₂AlLi (7) has been isolated after repeating re-crystallization. Furthermore, reacting one equiv. of C₄H ₂NH(2,5-CH₂NH^tBu)₂ with AlH ₃·NMe₃ in diethyl ether generates an aluminium dihydride complex, [C₄H₂N(2,5-CH₂NH ^tBu)₂]AlH₂ (8), in high yield. Additionally, treating 8 with one equiv. of HOC₆H₂(-2,6- ^tBu₂-4-Me) in methylene chloride produces [C ₄H₂N(2,5-CH₂NH^tBu) ₂][OC₆H₂(-2,6-^tBu ₂-4-Me)]AlH (9) with the elimination of one equiv. of H₂. The aluminium alkoxide complex 4 shows moderate reactivity toward the ring opening polymerization of ε-caprolatone in toluene.

Full text of DOI:10.1039/c1dt10442j

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PAPER
Aluminium complexes containing bidentate and symmetrical tridentate pincer
type pyrrolyl ligands: synthesis, reactions and ring opening polymerization†
Wen-Yen Huang,a Sheng-Jie Chuang,a Nien-Tsu Chunag,a Ching-Sheng Hsiao,a Amitabha Datta,a
Shau-Jiun Chen,a Ching-Han Hu,a Jui-Hsien Huang,*a Ting-Yu Leeb and Chia-Her Linc
Received 16th March 2011, Accepted 6th May 2011
DOI: 10.1039/c1dt10442j
A series of aluminium derivatives containing substituted bidentate and symmetrical tridentate pyrrolyl
ligands, [C4H3NH(2-CH2NHtBu)] and [C4H2NH(2,5-CH2NHtBu)2], in toluene or diethyl ether were
synthesized. Their reactivity and application for the ring opening polymerization of e-caprolactone
have been investigated. The reaction of AlMe3 with one equiv. of [C4H3NH(2-CH2NHtBu)] in toluene
at room temperature affords [C4H3N(2-CH2NHtBu)]AlMe2 (1) in 70% yield by elimination of one
equiv. of methane. Interestingly, while reacting AlMe3 with one equiv. of [C4H3NH(2-CH2NHtBu)] in
toluene at 0 ◦C followed by refluxing at 100 ◦C, [{C4H3N(2-CH2NtBu)}AlMe]2 (2) has been isolated via
fractional recrystalliztion in 30% yield. Similarly, reacting AlMe3 with two equiv. of C4H3NH(2-
CH2NHtBu) generates [C4H3N(2-CH2NHtBu)]2AlMe (3) in a moderate yield. Furthermore, complex 1
can be transformed to an aluminium alkoxide derivative, [C4H3N(2-CH2NHtBu)][OC6H2(-2,6-tBu2-
4-Me)]AlMe (4) by reacting 1 with one equiv. of HOC6H2(-2,6-tBu2-4-Me) in toluene via the elimination
of one equiv. of methane. The reaction of AlR3 with one equiv. of [C4H2NH(2,5-CH2NHtBu)2] in
toluene at room temperature affords [C4H2N(2,5-CH2NHtBu)2]AlR2 (5, R = Me; 6, R = Et) in moderate
yield. Surprisingly, from the reaction of two equiv. of [C4H2NH(2,5-CH2NHtBu)2] with LiAlH4 in
diethyl ether at 0 ◦C, a novel complex, [C4H2N(2-CH2NtBu)(5-CH2NHtBu)]2AlLi (7) has been isolated
after repeating re-crystallization. Furthermore, reacting one equiv. of C4H2NH(2,5-CH2NHtBu)2 with
AlH3·NMe3 in diethyl ether generates an aluminium dihydride complex, [C4H2N(2,5-CH2NHtBu)2]-
AlH2 (8), in high yield. Additionally, treating 8 with one equiv. of HOC6H2(-2,6-tBu2-4-Me) in
methylene chloride produces [C4H2N(2,5-CH2NHtBu)2][OC6H2(-2,6-tBu2-4-Me)]AlH (9) with the
elimination of one equiv. of H2. The aluminium alkoxide complex 4 shows moderate reactivity toward
the ring opening polymerization of e-caprolatone in toluene.
(3) alkylalumoxanes5 as co-catalysts for olefin polymerization, (4)
Introduction
reducing agents6 in organic synthesis and (5) hydrogen storage
Aluminium alkyl1 and aluminium hydride2 complexes supported
by various types of ligands have been extensively investigated
and reviewed in the past decades due to their potential usages
such as synthesizing (1) aluminium cation complexes3 as catalysts
for olefin polymerization, (2) aluminium alkoxide complexes4 as
initiators for ring opening polymerization of lactones and lactides,
materials.7 The reactions of aluminium alkyl or hydride complexes
involve deprotonation, insertion, and abstraction as shown in
Scheme 1.
aDepartment of Chemistry, National Changhua University of Education,
Changhua, Taiwan, 500
bDepartment of Applied Chemistry, National Kaohsiung University, Kaohsi-
ung, Taiwan, 500
cDepartment of Chemistry, Chung-Yuan Christian University, Chung-Li 320,
Taiwan. E-mail: juihuang@cc.ncue.edu.tw
† Electronic supplementary information (ESI) available: NMR spectra of
complex 7 and Crystal data and structure refinement for complex 2 (second
set of crystal data). CCDC reference numbers 792978–792982 and 808790–
808793. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10442j
Scheme 1
Concerning different ligand systems, the pyrrolyl entity has
the capability to bring metal atoms into close proximity and
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provides an intramolecular or intermolecular pathway for bonding
interactions. Substituted pyrrole heterocycles also show important
biological activity. For example, various substituted pyrroles are
known to act as analgesics, anti-inflammatory agents, or to have
antibacterial properties.8 However, relatively few metal–pyrrole
complexes have been found to be synthetically useful in the
preparation of substituted pyrrole rings, and the syntheses of
substituted pyrroles have been largely limited to electrophilic
additions to the free ligand and to ring-closing reactions of ap-
propriate precursors.9 To summarize the chemistry of the pyrrolyl
based system, the reactions of aluminium hydride complexes
with ketones generate aluminium alkoxide complexes10 and we
also showed the reactivity of monomeric aluminium hydride
complexes with pyrrolyl ligands11 reflecting insertion and the C–
C coupling mechanistic pathway. The role of organoaluminium
hydrides in facilitating the reactions of donor-substituted pyrrole
ligands with PhNCO, CO2, H2O, and tertiary alcohols have
been recognized12 but considerable work remains to establish a
thorough understanding of metal coordination effects on pyrrole
reactivity as nitrogen based polydentate ancillary ligands.
We have been investigating the development of non-toxic metal
complexes as initiators for the lactone ring opening polymerization
in the past years using substituted pyrrole as supporting ligands.13
As an extension of our continuous interest in the pyrrolyl
ligand system, the present contribution focuses on the synthesis,
characterization and reactivity study of aluminium complexes
using bidentate and symmetric tridentate pincer type pyrrolyl
ligands. In addition, complex 4 can participate in the ring-opening
polymerization of e-caprolactone, acting as a catalyst.
Scheme 2
Results and discussion
Syntheses and characterization of complexes 1–9
The reactions of AlMe3 with C4H3NH(2-CH2NHtBu) are shown in
Scheme 2. The reaction of AlMe3 with [C4H3NH(2-CH2NHtBu)]
in toluene at room temperature affords a four-coordinate alu-
minium dimethyl complex [C4H3N(2-CH2NHtBu)]AlMe2 (1) in
70% yield along with the elimination of one equiv. of methane.
1
The H and 13C NMR spectra of the methyl groups of AlMe2
show a singlet at d -0.28 and a broad signal at d -7.8, respectively.
The broad 13C NMR resonance is resulting from the quadruple
interaction of Al atoms (I = 5/2).14 The methylene protons of
CH2NHtBu show two doublets at d 3.40. Alternatively, while
reacting AlMe3 with [C4H3NH(2-CH2NHtBu)] in toluene at 0 ◦C
followed by heating at 93 ◦C, complex 1 and the dimeric aluminium
complex [{C4H3N(2-CH2NtBu)}AlMe]2 (2) were both obtained
and 2 can be isolated via fractional recrystalliztion in 30% yield.
Scheme 3
amine simultaneously at low temperature to generate the dimeric
complex 2.
1
The H NMR spectrum of 2 shows a singlet at d -0.17 for the
methyl protons of the AlMe and two doublets at d 3.83 and 4.21 for
the methylene protons of CH2NtBu. No amino proton resonance
of NHtBu was observed for 2, although it appeared at d 2.13 for
1. Complex 1 is quite thermally stable, which remains unchanged
while heating at 110 ◦C for 12 h. This phenomenon demonstrates
that the mechanistic pathway does not proceed through 1 during
the formation of 2 from a combination of AlMe3 and C4H3NH(2-
CH2NHtBu). Possible reaction mechanisms for the formation of
complexes 1 and 2 are shown in Scheme 3 where AlMe3 might have
a better chance to bind to the acidic protons of the pyrrole and
Similarly, reacting AlMe3 with two equiv. of C4H3NH(2-
CH2NHtBu) affords a four-coordinate aluminium methyl complex
1
[C4H3N(2-CH2NHtBu)]2AlMe (3). The H NMR spectra of 3 at
room temperature show a singlet at d -0.17 for the methyl protons
of the AlMe and two doublets at d 3.44 appearing as a broad band
for the methylene protons of CH2NHtBu. A small broad band
is observed at d 3.83, which is assigned as the amino proton of
NHtBu. Variable temperature 1H NMR spectra of 3 in d8-toluene
were obtained ranging from 213~343 K showing no significant
changes for the 1H NMR resonances. This phenomenon indicates
7424 | Dalton Trans., 2011, 40, 7423–7433
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a fast exchange happened between the two substituted pyrrolyl
ligands bonding to the Al atom as shown in Scheme 4.
groups. Similarly, complex 6, [C4H2N(2,5-CH2NHtBu)2]AlEt2, can
be obtained from the reaction of C4H2NH(2,5-CH2NHtBu)2 with
1
one equiv. of AlEt3 in toluene. The H and 13C NMR spectra of
6 are consistent, similarly to those of complex 5 and no further
discussion has been included here.
While LiAlH4 was reacting with one equiv. of C4H2NH(2,5-
CH2NHtBu)2 in diethyl ether at 0 ◦C, the evolution of hydrogen gas
was observed and a dinuclear complex 7, [C4H2N(2-CH2NtBu)(5-
CH2NHtBu)]2AlLi, was isolated after repeating re-crystallization.
It is interesting to note that while LiAlH4 reacted with two equiv.
of C4H2NH(2,5-CH2NHtBu)2, convoluted products were observed
from the 1H NMR spectra without differentiation of pure complex
7. The asymmetrical geometry of 7 results in complicated NMR
spectra. Detailed assignment of the 1H-13C HSQC 2D NMR
Scheme 4
Complex 1 can be converted to the aluminium alkoxide com-
plex [C4H3N(2-CH2NHtBu)][OC6H2(-2,6-tBu2-4-Me)]AlMe (4) by
reacting 1 with one equiv. of HOC6H2(-2,6-tBu2-4-Me) in toluene
along with the elimination of one equiv. of methane (Scheme 2).
On the addition of excess alcohol, no further methane elimination
was observed, an indication of the large sterically crowded alkoxide
ligand blocking the pathway of a second alcohol into penetration
with the aluminium methyl fragment. The 1H and 13C NMR
spectra of 4 show a singlet at d -0.09 and -7.0, respectively, proving
the existence of the aluminium methyl.
1
spectra and H-1H NOESY 2D NMR spectra were provided in
the ESI.† In addition, the nNH stretching frequency appearing at
3248 cm-1 indicates that one of the two side arms of the pyrrolyl
ligands retains its neutrality of amino functionality.
Reacting one equiv. of C4H2NH(2,5-CH2NHtBu)2 with alane,
AlH3·NMe3, in diethyl ether and recrystallizing from methy-
lene chloride results in an aluminium dihydride complex 8,
[C4H2N(2,5-CH2NHtBu)2]AlH2, in high yield. A deuterated com-
plex [C4H2N(2,5-CH2NHtBu)2]AlD2 (8-D) can also be obtained
from C4H2NH(2,5-CH2NHtBu)2 and AlD3·NMe3. The IR spectra
show that the nAl-H stretching frequency shifted from 1801 to
1318 cm-1 for nAl-D (calculated at 1319 cm-1). The 1H NMR
spectra of 8 show a singlet at d 2.58 to the correspond-
ing amino proton of NHtBu. Again the methylene protons
of CH2NHtBu appear as a doublet at d 3.72, further prov-
The reactions of [C4H2NH(2,5-CH2NHtBu)2] with AlR3,
LiAlH4 and alane and its corresponding derivative with
HOC6H2(2,6-tBu2-4-Me) are shown in Scheme 5. The pincer
type15 tri-dentate ligand, C4H2NH(2,5-CH2NHtBu)2 was obtained
following a similar procedure as for synthesizing the bidentate
pyrrolyl ligand C4H3NH(2-CH2NHtBu).16 Reacting C4H2NH(2,5-
CH2NHtBu)2 with one equiv. of AlMe3 in toluene affords complex
5, [C4H2N(2,5-CH2NHtBu)2]AlMe2, in 67% yield along with the
elimination of one equiv. of methane. Complex 5 was characterized
3
ing the JHH coupling of CH2 and NH. Treating 8 with one
equiv. of HOC6H2(-2,6-tBu2-4-Me) in methylene chloride yields
[C4H2N(2,5-CH2NHtBu)2]AlH[OC6H2(-2,6-tBu2-4-Me)] (9) with
by H and 13C NMR spectra. The chemical shift of the amino
1
1
proton of CH2NHtBu appears at d 1.59, which shifts slightly
depending upon the concentration of the complex in D-solvent due
to hydrogen bonding. The methylene protons of CH2NHtBu show
two doublets of doublets at d 3.62 and 3.65 while showing different
the elimination of one equiv. of H2. The H NMR spectra of
methylene protons of CH2NHtBu appeared as multiplets at d 3.77
due to the rigidity of the complex geometry arising from the bulky
aryloxide substituent. The amino proton of CH2NHtBu shows one
singlet at d 2.32, which overlaps with the signal of the methyl group
of the aryloxide fragment. The hydride signal was not observed in
the 1H NMR spectra, therefore, a deuterated complex [C4H2N(2,5-
CH2NHtBu)2]AlD[OC6H2(-2,5-tBu2-4-Me)] (9-D) was synthesized
2
diastereomericity with the JHH coupling constant of 8.4 Hz.
The H and 13C NMR spectra of the aluminium methyl groups
1
show one singlet at d -0.31 and -4.9, respectively, representing
the strong electronic shielding properties of the anionic alkyl
Scheme 5
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Table 1 Summary of data collections for complexes 1–7 and 9
1
2
3
4
5
6
7
9
Formula
FW
C11H21AlN2 C20H34Al2N4 C19H33AlN4 C25H41AlN2O C16H32AlN3
C18H36AlN3 C63H108Al2Li2N12 C29H50AlN3O
208.28
384.47
344.47
412.58
293.43
Triclinic
P -1
9.0135(8)
9.9804(9)
10.8491(8)
103.080(5)
96.371(5)
102.129(5)
321.48
Monoclinic Monoclinic
P21/c P21/c
10.4122(7) 9.2957(3)
17.7071(11) 20.3506(8)
10.7696(7) 17.9295(6)
1101.45
483.70
Monoclinic
P21/c
17.1684(3)
10.1390(2)
16.2407(3)
90
92.7340(10)
90
Crystal system
Space group
Monoclinic Monoclinic Monoclinic Monoclinic
P21/n
C2/c
P21/c
C2/c
˚
a/A
8.0962(3)
12.8293(4)
12.8195(4)
90
100.424(2)
90
10.8437(8) 15.980(3)
15.3133(11) 15.156(3)
26.1398(19) 17.705(3)
24.0326(19)
12.5009(7)
18.4002(16)
90
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
90
96.2580(10) 99.467(13)
90 90
90
90
102.188(7) 99.594(2)
90 90
90
116.566(7)
90
3
˚
V/A , Z
1309.57(8), 4 4314.7(5), 8 4229.6(14), 8 4944.3(6), 8
916.46(13), 2 1940.8(2), 4 3344.3(2), 2
2823.81(9), 4
1.138
0.097
150(2)
1064
Dc/Mg m-3
1.056
0.125
150(2)
456
1.184
0.146
150(2)
1664
1.082
0.103
150(2)
1504
1.109
0.099
150(2)
1808
1.063
0.107
150(2)
324
1.100
0.107
295(2)
712
1.094
0.089
150(2)
1204
m/mm-1
T/K
F(000)
no. of reflns collected
no. of ind reflns
Rint
data/restraints/params
goodness of fit on F2
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
wR2 (all data)
10232
3141
0.0236
16760
5116
0.0255
29976
10184
0.1005
19862
5975
0.0437
11521
4721
0.0265
8712
4358
50486
8383
48630
6832
0.0539
0.0389
0.0606
6832/0/323
1.020
3141/0/136 5116/0/243 10184/0/447 5975/0/277
4721/0/189 4358/0/199 8383/3/373
1.037
1.103
1.050
0.761
0.937
0.946
1.106
0.0351
0.0971
0.0447
0.1017
0.0391
0.1072
0.0513
0.1131
0.291 and
-0.246
0.0527
0.1206
0.2024
0.1524
0.344 and
-0.325
0.0451,
0.1023
0.0980
0.1126
0.233 and
-0.254
0.0403
0.1041
0.0547
0.1136
0.271 and
-0.253
0.0548
0.1208
0.0945
0.1298
0.547 and
-0.278
0.0528
0.1351
0.0819
0.1491
0.560 and
-0.338
0.0418
0.0943
0.0694
0.1086
0.305 and
-0.270
largest diff peak and hole/e 0.235 and
-3
˚
A
-0.164
from 8-D and HOC6H2(-2,5-tBu2-4-Me). The IR spectra show that
the nAl-H stretching frequency shifted from 1852 cm-1 to 1352 cm-1
for nAl-D
.
Molecular structures of 1–7 and 9
The crystallographic data for complexes 1–7 and 9 are summarized
in Table 1, and selected bond lengths and angles are listed
in Table 2. Orange-red crystals ◦of 1 were obtained from a
saturated toluene solution at -20 C and its molecular structure
is shown in Fig. 1. The four-coordinated aluminium center forms
a distorted tetrahedral geometry where the bond lengths of Al-
Fig. 2 The molecular structure of complex 2 with the thermal ellipsoids
drawn at 30% probability. All hydrogen atoms are omitted for clarity.
t
˚
N(pyrrole) and Al-N(NH Bu) are 1.8788(10) and 2.0275(10) A,
respectively. The geometry of 2 is relatively similar to that of the
published [C4H3N(2-CH2NMe2)]AlMe2 complex,17 indicating the
steric congestion of NHtBu is similar to that of NMe2.
Pure yellow-orange crystals of 2 were generated from a
toluene solution after repeating recrystallization. Fig. 2 shows a
perspective drawing of the molecule 2, containing two molecular
units, together with the selective atomic labelling. The geometries
of two independent molecular units in 2 both contain planar
Al2N2 cores. The pyrrolyl ligands here act as a bidentate dianionic
ligand, [C4H3N(2-CH2NtBu)]2-, showing the binding of the
pyrrolyl nitrogen terminally to the aluminium atom and the
bridging participation of the NtBu amino nitrogen among two
aluminium atoms. The corresponding three rings, AlNCCN,
Al2N2, and AlCCN are presented in both molecular units of 2,
which are arranged as a chair conformation in the geometry of
2a and as a boat conformation in the geometry 2b. Concerning
the chair conformation with regard to the Al2N2 core, this
represents trans while cis refers to boat form. A schematic drawing
showing the geometries of 2 is depicted in Scheme 6. The bond
lengths of Al–N(pyrrole) and Al–N(NtBu) for the geometries
of 2a and 2b are relatively similar and belong in the range of
Fig. 1 The molecular structure of complex 1 with the thermal ellipsoids
drawn at 30% probability. All hydrogen atoms excepting the amino proton
of NHtBu are omitted for clarity.
˚
˚
1.8365–1.8459 A and 1.9655–1.9665 A, respectively. The results are
7426 | Dalton Trans., 2011, 40, 7423–7433
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◦
of [{C4H3N(2-CH2NMe)}AlMe]2.17 Comparing the geometries
of 1 and [C4H3N(2-CH2NMe2)]AlMe2, we have concluded that
both the NHtBu and NMe2 fragments face similar sterical
hindrance. Therefore, the geometries of 2a and 2b in the unit cell
of 2 may arise accidentally from the crystal packing. Another
batch of crystals of complex 2 has been selected and subjected
for structural determination. Only the chair form of 2 can be
observed in that structure (see ESI†). Obviously, the formation
energies of the two conformations of complex 2 (chair and boat
form) are relatively similar and this will be discussed vide infra.
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 1–7 and
9
1
Al(1)–N(1)
Al(1)–C(11)
Al(1)–C(10)
Al(1)–N(2)
N(1)–Al(1)–C(10)
2
Al(1)–N(1)
Al(1)–C(10)
Al(1)–N(2A)
Al(1)–N(2)
Al(1)–Al(1A)
Al(2)–N(3)
Al(2)–C(20)
Al(2)–N(4A)
Al(2)–N(4)
Al(2)–Al(2A)
N(1)–Al(1)–C(10)
3
1.8788(10)
1.9554(15)
1.9608(14)
2.0275(10)
111.78(5)
C(11)–Al(1)–C(10)
N(1)–Al(1)–N(2)
C(11)–Al(1)–N(2)
C(10)–Al(1)–N(2)
115.68(7)
85.58(4)
111.26(6)
114.63(5)
1.8459(14)
1.9550(15)
1.9665(12)
1.9818(12)
2.7832(8)
1.8365(13)
1.943(2)
1.9655(14)
1.9668(13)
2.8073(10)
112.38(7)
N(1)–Al(1)–N(2A)
C(10)–Al(1)–N(2A)
N(1)–Al(1)–N(2)
C(10)–Al(1)–N(2)
N(2A)–Al(1)–N(2)
N(3)–Al(2)–C(20)
N(3)–Al(2)–N(4A)
C(20)–Al(2)–N(4A)
N(3)–Al(2)–N(4)
C(20)–Al(2)–N(4)
N(4A)–Al(2)–N(4)
109.33(6)
121.54(6)
90.17(5)
127.70(6)
90.35(5)
112.42(15)
114.08(6)
117.85(8)
90.01(5)
129.40(8)
88.89(5)
Al(1)–N(1)
Al(1)–N(3)
Al(1)–C(19)
Al(1)–N(2)
Al(2)–N(5)
Al(2)–C(38)
Al(2)–N(6)
Al(2)–N(7)
N(5)–Al(2)–N(6)
N(7)–Al(2)–N(6)
4
Al(1)–O(1)
Al(1)–N(1)
Al(1)–C(10)
Al(1)–N(2)
C(11)–O(1)
N(1)–Al(1)–N(2)
5
1.842(2)
1.855(2)
1.935(2)
1.987(2)
1.850(2)
1.937(2)
1.974(2)
1.854(2)
86.75(10)
104.04(9)
N(3)–Al(1)–C(19)
N(1)–Al(1)–N(2)
N(3)–Al(1)–N(2)
C(19)–Al(1)–N(2)
N(5)–Al(2)–N(7)
N(1)–Al(1)–N(3)
N(1)–Al(1)–C(19)
N(7)–Al(2)–C(38)
N(5)–Al(2)–C(38)
C(38)–Al(2)–N(6)
113.04(11)
86.87(10)
105.67(9)
118.47(11)
113.72(10)
113.61(10)
116.35(10)
112.43(11)
117.48(10)
119.52(12)
Scheme 6
Dark red crystals of 3 were obtained from a saturated toluene
solution at -20 ◦C and its molecular structure is shown in Fig. 3.
There are two independent molecules in a unit cell; however, these
two molecules are very similar. Two equiv. of pyrrolyl ligands
[C4H3N(2-CH2NHtBu)] coordinate to one aluminium atom. One
acts as a bidentate ligand while the other one acts as a monodentate
ligand. The aluminium atom is surrounded by three nitrogen atoms
and one methyl group forming a distorted tetrahedral geometry.
The biting angle of the bidentate pyrrolyl ligand is 86.75(10)◦. The
two Al–N(pyrrole) bond lengths are relatively close, (1.842(2) A
and 1.855(2) A) whereas one of the Al–N(NH Bu) bond lengths
belongs in the coordinating range (1.987(2) A) while the other
differentiates far from the bonding range (~3.788 A).
1.7178(11)
1.8618(14)
1.9470(18)
2.0303(15)
1.3700(17)
85.72(6)
C(11)–O(1)–Al(1)
O(1)–Al(1)–N(1)
O(1)–Al(1)–N(2)
O(1)–Al(1)–C(10)
N(1)–Al(1)–C(10)
C(10)–Al(1)–N(2)
161.82(10)
114.17(6)
98.92(6)
118.26(7)
113.81(7)
121.09(7)
˚
t
˚
18
Al(1)–N(1)
Al(1)–C(2)
Al(1)–C(1)
Al(1)–N(2)
Al(1)–N(5)
6
1.8709(11)
1.9662(13)
1.9687(12)
2.2855(11)
2.5301(13)
N(1)–Al(1)–C(2)
N(1)–Al(1)–C(1)
C(2)–Al(1)–C(1)
N(1)–Al(1)–N(2)
C(2)–Al(1)–N(2)
117.88(6)
120.16(5)
121.19(6)
77.94(4)
˚
˚
103.07(5)
Al(1)–N(3)
Al(1)–C(15)
Al(1)–C(18)
Al(1)–N(1)
C(18)–Al(1)–N(1)
7
Li(1)–N(2)
Al(1)–N(5)
Li(1)–N(6)
Al(1)–N(3)
Li(1)–N(1)
Al(1)–N(4)
Li(1)–N(4)
N(2)–Li(1)–N(4)
N(6)–Li(1)–N(4)
Al(1)–N(1)–Li(1)
Al(1)–N(4)–Li(1)
9
1.881(2)
1.970(2)
1.971(2)
2.112(2)
109.48(8)
N(3)–Al(1)–C(15)
N(3)–Al(1)–C(18)
C(15)–Al(1)–C(18)
N(3)–Al(1)–N(1)
C(15)–Al(1)–N(1)
117.53(9)
114.01(9)
120.14(10)
81.96(7)
105.86(8)
2.023(3)
1.7993(14)
2.030(3)
1.8083(14)
2.238(4)
1.8963(15)
2.308(3)
124.93(16)
90.49(12)
84.02(9)
N(2)–Li(1)–N(6)
N(5)–Al(1)–N(3)
N(2)–Li(1)–N(1)
N(5)–Al(1)–N(4)
N(6)–Li(1)–N(1)
N(3)–Al(1)–N(4)
Al(1)–N(1)
N(5)–Al(1)–N(1)
N(3)–Al(1)–N(1)
N(4)–Al(1)–N(1)
131.29(17)
133.72(7)
92.25(14)
90.50(6)
126.28(16)
117.35(7)
1.8973(14)
116.73(7)
89.99(6)
Fig. 3 The molecular structure of complex 3 with the thermal ellipsoids
drawn at 30% probability. All hydrogen atoms excepting the amino proton
of NHtBu are omitted for clarity.
108.54(6)
82.13(9)
Dark red crystals of 4 were obtained from a saturated toluene
solution at -20 ◦C and its molecular structure is shown in Fig. 4.
The geometry of 4 is similar to that of 3. The aluminium center
is surrounded by two nitrogen atoms of the bidentate pyrrolyl
ligand, methyl group, and aryloxide fragment forming a distorted
tetrahedral geometry where the biting angle of the bidentate
pyrrolyl ligand is 85.72(6)◦. The bond angle of Al(1)–O(1)–C(11)
is 161.82(10)◦ and the bond lengths of Al(1)–O(1) and O(1)–C(11)
Al(1)–O(1)
Al(1)–N(1)
Al(1)–N(3)
Al(1)–N(2)
C(21)–O(1)
1.7612(11)
2.2374(13)
2.2552(13)
1.8299(13)
1.3646(17)
C(21)–O(1)–Al(1)
O(1)–Al(1)–N(2)
N(1)–Al(1)–N(3)
N(2)–Al(1)–N(3)
N(1)–Al(1)–N(2)
134.20(9)
116.11(6)
156.97(5)
78.13(5)
79.03(5)
comparable with the previously published article.17 It is
remarkable to note that 2 consists of two independent geometries
(chair form, 2a; boat form, 2b), which is not observed in the case
˚
˚
are 1.7178(11) A and 1.3700(17) A, respectively. The short Al–O
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Fig. 6 The molecular structure of complex 6 with the thermal ellipsoids
drawn at 50% probability. All hydrogen atoms excepting the amino protons
are omitted for clarity.
-60 ◦C, representing the fast exchange of the bonding of the two
NHtBu fragments to the Al atom.
Pale ◦yellow crystals of 7 were obtained from a toluene solution
at -20 C. The structure of 7 is shown in Fig. 7. Complex 7 is a
heterobimetallic complex consisting of two pyrrolyl ligands, one
aluminium, and one lithium atom,20 with one disordered toluene
molecule remaining in the lattice. The two pyrrolyl nitrogen
atoms bridge to the aluminium and lithium atoms forming an
AlN2Li four-membered ring. The aluminium atom also individ-
ually generates a distorted tetrahedron with four nitrogen donor
sites, from a pair of pyrrolyl ligands manifesting with pyrrolyl-
N and NtBu fragments of corresponding ligands. The relatively
Fig. 4 The molecular structure of complex 4 with the thermal ellipsoids
drawn at 30% probability. All hydrogen atoms excepting the amino proton
of NHtBu are omitted for clarity.
bond length and the large Al–O–C bond angle are consistent with
the results of Barron’s study showing the p-bonding between the
aryloxide oxygen and the vacant aluminium pz orbital.19
The molecular structures of 5 and 6 are relatively similar
and possess highly distorted geometries, shown in Fig. 5 and 6,
respectively, into which the pincer type [C4H2(2,5-CH2NHtBu)2]
acts most likely as a bidentate ligand. The nitrogen atom of the
CH2NHtBu fragment on the pyrrolyl ring binds the Al atom in an
asymmetrical manner. For complex 5, the Al(1)–N(2) and Al(1)–
˚
short bond lengths of Al(1)–N(3) and Al(1)–N(5) (ca. 1.80 A)
indicating the aluminium with terminal t-butylamido bonding
modes is comparable to previous literature.21 The lithium atom
also possesses a distorted tetrahedral environment initiating from
four nitrogen donor sites (pyrrolyl-N and NHtBu fragments) of
two corresponding pyrrolyl ligands and the related Li–N(NHtBu)
˚
˚
N(5) bond lengths are 2.2855(11) A and 2.5301(13) A, respectively,
which correspond to the coordination of one of the two side
arms of the pyrrolyl ligand, CH2NHtBu, to the Al atom while
the other belongs on the borderline of bonding or nonbonding
range. Similarly in 6, the corresponding Al(1)–N(1) and Al(1)–
˚
bond lengths, Li(1)–N(2) and Li(1)–N(6) (av. 2.026 A) belong in
the normal range, like in other published articles.22
˚
˚
N(2) bond lengths are 2.112(2) A and 3.018(2) A, respectively,
representing the coordination of one fragment of the pyrrolyl
ligand, CH2NHtBu, to the Al atom while the other exists as
nonbonding to the central tetrahedron unit. However, the solution
1H NMR spectra of 5 and 6 both show only one tBu group even at
Fig. 7 The molecular structure of complex 7 with the thermal ellipsoids
drawn at 50% probability. All hydrogen atoms excepting the amino protons
are omitted for clarity.
Orange crystals of 9 were obtained from a diethyl ether
solution at -20 ◦C and its molecular structure was shown in
Fig. 8. The geometry of 9 can be emphasized as a distorted
trigonal bipyramidal. The tridentate pyrrolyl ligand is connected
to aluminium atom in a symmetrical mer-type where the pyrrolyl
ring nitrogen occupies the equatorial position and two NHtBu
Fig. 5 The molecular structure of complex 5 with the thermal ellipsoids
drawn at 50% probability. All hydrogen atoms excepting the amino protons
are omitted for clarity.
7428 | Dalton Trans., 2011, 40, 7423–7433
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Dynamic properties of complex 2
The geometries of complexes containing one Al2N2 core and
two rings formed by the coordination of bidentate dianionic
ligands to the aluminium atom exist in two possible confor-
mations, chair-form and boat-form. Most of the complexes in
the systems A-E in Scheme 7 have chair-form geometries due
to the steric congestion.13a,23 However, we have found that for
system E in Scheme 6 there exists one complex with a boat-
form conformation.24 In the case of complex 2 of this paper,
two geometries (boat- and chair-form) are observed. Theoretical
computations using the B3LYP density functional and the 6-
31G(d) basis set revealed that the enthalpy of the chair-form
(2a) at 298 K is 2.63 kcal mol-1 lower than that of the boat-
form (2b). The results suggest that in the presence of the bulky t-
butyl group plays an important role on its conformation and that
thermodynamically the chair- and boat-forms can be converted
(Scheme 7). The variable temperature 1H NMR spectra of 2
(Fig. 9, from 353 K to 188 K) also proves this phenomenon.
At high temperature (353 K), only one set of methyl, t-butyl,
methylene, and pyrrolyl protons were observed. While the solution
temperature was lowered to 188 K, the t-butyl and pyrrolyl groups
both split into two resonances and the others remain unchanged.
A reasonable explanation for the dynamic properties of 2 is that
Fig. 8 The molecular structure of complex 9 with the thermal ellipsoids
drawn at 50% probability. All hydrogen atoms excepting the hydride and
amino protons are omitted for clarity.
nitrogen atoms feature the axial sites with the bond angle of N(3)–
Al(1)–N(1) at 156.97(5)◦. The trigonal plane has been formed
by the pyrrolyl ring nitrogen, hydride and the oxygen atom of
aryloixde fragment. In contrast to complex 4, the relative small
bond angle of Al(1)–O(1)–C(21) (134.20(9)◦) representing less
interactions of the p-bonding between the aryloxide oxygen and
the vacant aluminium pz orbital.
Fig. 9 The variable temperature 1H NMR spectra of complex 2 in d8-toluene using 300 MHZ NMR spectrometer. The solvent peak was designated as
a start mark.
Scheme 7
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Table 3 Polymerization of e–caprolactone (CPL) using complex 4 as an initiator
b
c
entry
[CPL]0/[4]0
conversion (%)
activitya
Mn,theor
Mn,cor
PDI
f (%)d
1
2
3
4
50
99
99
99
98
5500
5650
39700
51900
126000
166700
1.68
1.42
1.21
1.09
14
21
13
20
100
150
300
11140
16340
33280
11300
16950
33557
a gpolmolcat ·h-1; b Mn,theor = ([CPL]0/[4]0) ¥ 114.14 ¥ conversion (%) for CPL; c Mn,cor = 0.259 Mn,GPC
,
for CPLe ; d initiator efficiency = Mn,theor/Mn,cor
;
-1
1.073
St
e P. Dubois, I. Barakat, R. Jerome, P. Teyssie Macromolecules, 1993, 26, 4407. rxn temp/time(m) = 75 ◦C
the low differentiate of formation enthalpy of chair-form 2 (2a)
and boat-form 2 (2b) results in the fast exchange between the two
conformations. In addition, the spectrum at 188 K only slightly
slows the exchange rate and most of the dynamic properties of 2
remain unchanged.
group metal chemistry and inserting lanthanides, as well as differ-
ent organic molecules. It will be our priority to differentiate metal–
ligand based architecture over hydroalumination and insertion
reactions.
Experimental
Ring opening polymerization of e-caprolactone catalyzed by 4
Physical measurements and materials
Aluminium phenoxide 4 can be treated as an initiator for the ring
opening polymerization of e-caprolactone, and the corresponding
results are presented in Table 3. There was an increase in molecular
weight (Mn) and decrease in molecular weight distribution (PDI)
on increasing the feeding ratio of the monomer to the catalyst
([CPL]0/[4]0). However, poor linear correlation between Mn and
[CPL]0/[4]0 was observed, which could result from slow initiation
rate. The observed Mn’s significant deviations from the theoretical
molecular weights also indicates that the initiation would be slow
in comparison with propagation, and the initiation might have not
been quantitative. The polymerization activity of 4 is comparable
to that of previously reported neutral and cationic aluminium
complexes25 supported by bidentate O,P-phosphinophenolate
ligands, neutral aluminium heteroscorpionate,26 diamides27 or
ketiminate13c complexes. Comparing the increase in molecular
weight (Mn) while applying different entries, there is a decrease
in the PDI values for aluminium phenoxide with the treatment of
e-caprolactone whereas the numerical molecular weights obtained
for the polymers increase with the [M]/[Al] ratio in the feed of
polymerization runs. These are subsequently higher than theoret-
ical ones (Mn,theor) for heteroscorpionate aluminium complexes26
and subject to the Mn,theor value remaining constant, the PDI
values varied with the Mn,obs values for O,P-phosphinophenolate
mediated aluminium derivatives.25
All reactions were performed under a dry nitrogen atmosphere
using standard Schlenk techniques or in a glove box. Toluene and
diethyl ether were dried by refluxing over sodium benzophenone
ketyl. CH2Cl2 was dried over P2O5. All solvents were distilled
and stored in solvent reservoirs, which contained 4 A molecular
sieves and were purged with nitrogen. H and 13C NMR spectra
were recorded on a Bruker Avance 300 spectrometer. Chemical
shifts for 1H and 13C spectra were recorded in ppm relative to the
residual protons of CDCl3 (d 7.24, 77.0) and C6D6 (d 7.15, 128.0).
Elemental analyses were performed on a Heraeus CHN-OS Rapid
Elemental Analyzer at the Instrument Center of the NCHU. Due
to higher air sensitivity, we could not collect elemental analysis
data for compounds 2, 3, 5 and 7. [C4H3NH(2-CH2NHtBu)] and
[C4H2NH(2,5-CH2NHtBu)2] were prepared in a similar procedure
as reported in the literature.13a All the chemicals (Aldrich, Acros)
were used as received.
˚
1
Preparation of the complexes
Synthesis of [C4H3N(2-CH2NHtBu)]AlMe2 (1). To a solution
of AlMe3 (3.16 mL, 3.28 mmol) in 10 mL of toluene was added
dropwise a C4H3NH(2-CH2NHtBu) (0.50 g, 3.28 mmol) toluene
solution (20 mL) at 25 ◦C. The mixture was stirred at room
temperature for 12 h and the color changed from colorless to
pale orange yellow. Volatiles were removed under vacuum and
the solid was re-crystallized from a toluene solution at -20 ◦C to
yield 0.51 g (74%) of orange-red solid. 1H NMR (C6D6): -0.28 (s,
6H, AlMe2), 0.65 (s, 9H, NHCMe3), 2.13 (s, 1H, CH2NHCMe3),
3.40 (dd, 2H, CH2NH), 6.23 (d, 1H, pyrrolyl CH), 6.63 (t, 1H,
pyrrolyl CH), 7.01 (s, 1H, pyrrolyl CH). 13C NMR (C6D6): -7.77
(AlMe2), 27.2 (q, JCH = 126 Hz, NHCMe3), 42.4 (t, JCH = 141 Hz,
CH2NHCMe3), 54.1 (s, NCMe3), 103.5 (d, JCH = 164 Hz, pyrrolyl
CH), 111.1 (d, JCH = 165 Hz, pyrrolyl CH), 122.1 (d, JCH = 178 Hz,
pyrrolyl CH), 132.2 (s, pyrrolyl Cipso). Anal. calcd for C11H21AlN2
(208.28): C 63.43, H 10.16, N 13.45. Found: C 62.44, H 9.95, N
13.68%.
Conclusions
In summary, the reactivities of [C4H3NH(2-CH2NHtBu)] and
[C4H2NH(2,5-CH2NHtBu)2] with AlR3 and LiAlH4 under dif-
ferent conditions were investigated and characterized mainly by
NMR spectroscopic data analyses. Using the aluminium alkoxide
derivative [C4H3N(2-CH2NHtBu)][OC6H2(-2,6-tBu2-4-Me)]AlMe
(4) as an initiator, moderate reactivity towards ring opening
polymerization of e-caprolactone was observed . We have isolated
one novel complex, [C4H2N(2-CH2NtBu)(5-CH2NHtBu)]2AlLi
(7), after repeating re-crystallization for the reaction between
LiAlH4 and one equiv. of C4H2NH(2,5-CH2NHtBu)2 in diethyl
ether at 0 ◦C, where we subsequently observed the evolution of
hydrogen gas. Future work will explore the design of synthesizing
asymmetric pyrrolyl ligands and study the reactivity for main
Synthesis of [{C4H3N(2-CH2NtBu)}AlMe]2 (2). Similar proce-
dures as for synthesizing complex 1 were used except the addition
was performed at 0 ◦C and then heated at 93 ◦C for 24 h. Repeating
7430 | Dalton Trans., 2011, 40, 7423–7433
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re-crystallizations were carried out due to the mixture of complexes
1 and 2. 0.39 g of yellow-orange crystals were obtained (31% yield).
1H NMR (C6D6): -0.17 (s, 6H, AlMe), 0.75 (s, 18H, NHCMe3),
Synthesis of [C4H2N(2,5-CH2NHtBu)2]AlEt2 (6). To a solution
of AlEt3 (1.05 mL, 2.10 mmol) in 10 mL of toluene was added
dropwise a C4H2NH(2,5-CH2NHtBu)2 (0.50 g, 2.10 mmol) toluene
solution (20 mL) at 0 ◦C. The mixture was stirred at room
temperature for 12 h and then volatiles were removed under
vacuum. The solid was re-crystallized from a toluene solution to
yield 0.42 g (62%) of pale yellow-orange solid. 1H NMR (C6D6):
0.31 (q, 4H, AlCH2CH3), 0.93 (s, 18H, NHCMe3), 1.26 (t, 6H,
2
3.83 4.21 (dd, JHH = 16.2 Hz, 4H, CH2NHCMe3), 6.20 (d, 2H,
pyrrolyl CH), 6.67 (t, 2H, pyrrolyl CH), 6.85 (d, 2H, pyrrolyl CH).
13C NMR (C6D6): 28.8 (q, JCH = 126 Hz, NCMe3), 45.3 (t, JCH
=
138 Hz, CH2NCMe3), 56.4 (s, NCMe3), 104.2 (d, JCH = 165 Hz,
pyrrolyl CH), 114.6 (d, JCH = 161 Hz, pyrrolyl CH), 120.9 (d, JCH
180 Hz, pyrrolyl CH), 137.4 (s, pyrrolyl Cipso).
=
2
AlCH2CH3), 1.63 (s, 2H, CH2NHCMe3), 3.67 (dd, JHH = 9 Hz,
2
1H, CHaHbNH), 3.70 (dd, JHH = 9 Hz, 1H, CHaHbNH), 6.34
Synthesis of [C4H3N(2-CH2NHtBu)]2AlMe (3). Similar proce-
dures as for synthesizing complex 1 were adopted. C4H3NH(2-
CH2NHtBu) (1.00 g, 6.56 mmol) and AlMe3 (3.16 mL, 3.28 mmol)
were used and the reaction was stirred at room temperature for
12 h. Dark red crystals were obtained from a toluene solution
(s, 1H, pyrrolyl CH). 13C NMR (C6D6): 2.6 (t, JCH = 112 Hz,
AlCH2CH3), 9.8 (q, JCH = 118 Hz, AlCH2CH3), 28.5 (q, JCH
=
125 Hz, NHCMe3), 42.5 (t, JCH = 136 Hz, CH2NHCMe3), 52.4 (s,
NCMe3), 105.2 (d, JCH = 165 Hz, pyrrolyl CH), 133.4 (s, pyrrolyl
C
ipso). Anal. calcd for C18H36AlN3 (321.48): C 67.25, H 11.29, N
1
to afford 0.42 g of 3 in 37% yield. H NMR (C6D6): -0.17 (s,
13.07. Found: C 67.29, H 10.73, N 13.63%.
3H, AlMe), 0.82 (s, 18H, NHCMe3), 3.44 (dd, 4H, CH2NH), 3.83
(s, 2H, CH2NHCMe3), 6.18 (d, 2H, pyrrolyl CH), 6.46 (t, 2H,
pyrrolyl CH), 6.89 (s, 2H, pyrrolyl CH). 13C NMR (C6D6): 28.1
(q, JCH = 126 Hz, NHCMe3), 41.4 (t, JCH = 137 Hz, CH2NHCMe3),
52.4 (s, NCMe3), 108 (d, JCH = 165 Hz, pyrrolyl CH), 111.4 (d,
JCH = 165 Hz, pyrrolyl CH), 124.4 (d, JCH = 179 Hz, pyrrolyl CH),
134.8 (s, pyrrolyl Cipso).
Synthesis of [C4H2N(2-CH2NtBu)(5-CH2NHtBu)]2AlLi (7). To
a solution of LiAlH4 (0.080 g, 2.10 mmol) in 10 mL of toluene
was added dropwise a C4H2NH(2,5-CH2NHtBu)2 (0.50 g, 2.10
mmol) toluene solution (20 mL) at 0 ◦C. The mixture was stirred at
room temperature for 12 h and then volatiles were removed under
vacuum. The solid was re-crystallized from a toluene solution to
yield 0.223 g (21%) of pale yellow solid. 1H NMR (C6D6): 0.59 (s,
18H, NHCMe3), 1.31 (s, 18H, NCMe3), 3.51 (dd, 2H, CHaHbNH),
3.90 (dd, 2H, CHaHbNH), 4.36 (q, 4H, CH2NCMe3), 6.29 (s, 2H,
pyrrolyl CH), 6.41 (s, 2H, pyrrolyl CH). 13C NMR (C6D6): 8.3
(q, JCH = 128 Hz, NCMe3), 30.4 (q, JCH = 124 Hz, NHCMe3),
41.1 (t, JCH = 136 Hz, CH2NHCMe3), 44.0 (t, JCH = 133 Hz,
Synthesis
of
[C4H3N(2-CH2NHtBu)][OC6H2(-2,6-tBu2-4-
Me)]AlMe (4). To a solution of 1 (0.20 g, 0.96 mmol) in 10 mL
of toluene was added dropwise a HOC6H2(-2,6-tBu2-4-Me) (0.21
g, 0.96 mmol) toluene solution (20 mL) at 0 ◦C. The mixture was
stirred at room temperature for 4 h and the color changed from
colorless to dark red. Volatiles were removed under vacuum and
the solid was re-crystallized from a toluene solution to yield 0.18
g (45%) of dark red solid. 1H NMR (C6D6): -0.09 (s, 3H, AlMe),
0.8 (s, 9H, NHCMe3), 1.46 (s, 18H, CMe3), 2.33 (s, 3H, Me), 3.27
CH2NCMe3), 50.5 (s, NHCMe3), 50.8 (s, NCMe3), 104.3 (d, JCH
=
167 Hz, pyrrolyl CH), 112.4 (d, JCH = 163 Hz, pyrrolyl CH), 129.3
(s, pyrrolyl Cipso), 130.3 (s, pyrrolyl Cipso), 145.9 (s, toluene Cipso).
Synthesis of [C4H2N(2,5-CH2NHtBu)2]AlH2 (8). To a solution
of Me3N·HCl (2.00 g, 2.090 mmol) in 20 mL of diethyl ether was
added dropwise a LiAlH4 (0.95 g, 25.1 mmol) diethyl ether solution
(20 mL) at 0 ◦C. The mixture was stirred at room temperature for
2 h and the solution was filtered through Celite. To the filtrate, a
diethyl ether (20 mL) solution of C4H2NH(2,5-CH2NHtBu)2 (4.96
2
(s, 1H, CH2NHCMe3), 3.46 (d, JHH = 15 Hz, 1H, CHaHbNH),
4.08 (d, 2JHH = 9 Hz, 1H, CHaHbNH), 6.2 (d, 1H, pyrrolyl CH),
6.62 (t, 1H, pyrrolyl CH), 7.11 (s, 1H, pyrrolyl CH), 7.21 (s, 2H,
phenyl CH). 13C NMR (C6D6): -7.0 (br, AlMe), 21.4 (q, JCH = 125
Hz, Me), 27.8 (q, JCH = 127 Hz, NHCMe3), 31.2 (q, JCH = 128 Hz,
CMe3), 34.7(s, CMe3), 42.9 (t, JCH = 140 Hz, CH2NHCMe3), 56.4
◦
g, 20.9 mmol) was added slowly at 0 C. The solution was then
(s, NCMe3), 103.9 (d, JCH = 160 Hz, pyrrolyl CH), 111.7 (d, JCH
=
stirred at room temperature for 12 h. Volatiles were removed to
yield a white solid. The solid was re-crystallized from a methylene
chloride solution to yield 5.546 g (70%) of colorless solid. 1H NMR
(C6D6): 1.31 (s, 18H, NHCMe3), 2.58 (s, 2H, CH2NHCMe3), 3.72
(d, 4H, CH2NH), 5.83 (s, 2H, pyrrolyl CH). 13C NMR (C6D6): 28.4
(q, JCH = 125 Hz, NCMe3), 41.9 (t, JCH = 134 Hz, CH2NHCMe3),
54.7 (s, NCMe3), 104.2 (d, JCH = 168 Hz, pyrrolyl CH), 132.3 (s,
pyrrolyl Cipso). Anal. calcd for C14H28AlN3 (265.379): C 63.36, H
10.63, N 15.83. Found: C 63.29, H 10.76, N 15.81%. IR (KBr):
166 Hz, pyrrolyl CH), 123.4 (d, JCH = 180 Hz, pyrrolyl CH),
125.8 (s, phenyl CH), 126.5 (s, phenyl Cipso), 133.4 (s, pyrrolyl
C
ipso), 138.3 (s, phenyl Cipso), 154.2 (s, phenyl Cipso). Anal. calcd for
C25H41AlN2O (412.59): C 72.78, H 10.02, N 6.79. Found: C 71.83,
H 10.92, N 6.55%.
Synthesis of [C4H2N(2,5-CH2NHtBu)2]AlMe2 (5). To a solu-
tion of AlMe3 (1.05 mL, 2.10 mmol) in 10 mL of toluene was
added dropwise a C4H2NH(2,5-CH2NHtBu)2 (0.50 g, 2.10 mmol)
toluene solution (20 mL) at 0 ◦C. The mixture was stirred at
room temperature for 12 h and then volatiles were removed
under vacuum. The solid was re-crystallized from a diethyl ether
1801 cm-1 (nAl-H), 3201 cm-1 (nNH), 1318 cm-1 (nAl-D
)
Synthesis of [C4H2N(2,5-CH2NHtBu)2]AlH[OC6H2(-2,6-tBu2-4-
Me)] (9). To a solution of 8 (0.20 g, 0.75 mmol) in 10 mL of
methylene chloride was added dropwise a HOC6H2(-2,6-tBu2-4-
Me) (0.17 g, 0.75 mmol) methylene chloride solution (20 mL) at 0
◦C. The mixture was stirred at room temperature for 2 h and the
color changed from colorless to orange. Volatiles were removed
under vacuum and the solid was re-crystallized from a diethyl ether
1
solution to yield 0.41 g (67%) of yellow-orange solid. H NMR
(C6D6): -0.31 (s, 6H, AlMe2), 0.94 (s, 18H, NHCMe3), 1.59 (s, 2H,
2
CH2NHCMe3), 3.62 (dd, JHH = 8.4 Hz, 1H, CHaHbNH), 3.65
(dd, 2JHH = 8.4 Hz, 1H, CHaHbNH), 6.38 (s, 2H, pyrrolyl CH). 13
C
NMR (C6D6): -4.9 (s, AlMe2), 25.6 (q, JCH = 125 Hz, NHCMe3),
42 (t, JCH = 128 Hz, CH2NHCMe3), 52.5 (s, NCMe3), 104.7 (d,
JCH = 164 Hz, pyrrolyl CH), 132.6 (s, pyrrolyl Cipso).
1
solution to yield 0.24 g (67%) of orange solid. H NMR (C6D6):
0.97 (s, 18H, NHCMe3), 1.48 (s, 18H, CMe3), 2.28 (s, 3H, phenyl
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Me), 2.32 (s, 2H, CH2NHCMe3), 3.66–3.80 (m, 4H, CH2NH), 6.28
(s, 2H, pyrrolyl CH), 7.10 (s, 2H, phenyl CH). 13C NMR (C6D6):
21.1 (q, JCH = 125 Hz, -Me), 28.1 (q, JCH = 126 Hz, NHCMe3), 32.3
Acknowledgements
We acknowledge the financial assistance to the National Science
Council of Taiwan. We would wish to thank the National
Changhua University of Education for supporting the X-ray
diffractometer and NMR spectrometer.
(q, JCH = 125 Hz, o-PhCMe3), 35.7 (s, o-PhCMe3), 42.2 (t, JCH
=
137 Hz, CH2NHCMe3), 54.2 (s, NCMe3), 104.8 (d, JCH = 164 Hz,
pyrrolyl CH), 125.3 (s, phenyl CH), 126.6 (s, phenyl Cipso), 131.8 (s,
pyrrolyl Cipso), 139.2 (s, phenyl Cipso), 156.5 (s, phenyl Cipso). Anal.
calcd for C29H50AlN3O (483.71): C 72.01, H 10.42, N 8.69. Found:
C 71.56, H 9.65, N 8.78%. IR (KBr): 1852 cm-1 (nAl-H), 3214 cm-1
(nNH), 1342 cm-1 (nAl-D).
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©