Aluminum complexes containing cyclohexane-1,2-diyl linked bis(ketiminato) ligands and proton-promoted demethylation

Article abstract of DOI:10.1002/ejic.200800057

A series of cyclohexane-1,2-diyl linked bis(ketiminato)aluminum complexes were synthesized. Reaction of a mixture of cis- and trans-diaminocyclohexane and 2,4-pentanedione gave a mixture of cis- and trans-cyclohexane-1,2-diyl linked bis(ketimine) ligands (cis-1 and trans-1), which can be separated by fractional recrystallization. Reactions of trans-1 with 1 and 2 equiv. AlMe3 gave monoaluminum complex trans-C₆H₁₀[(NCMeCHCMeO)AlMe ₂](NHCMeCHCMeO) (2) and dialuminum complex trans-C₆H ₁₀[(NCMeCHCMeO)AlMe₂]₂ (3), respectively. Contrarily, when 1 equiv. or excess cis-1 reacted with AlMe₃, only five-coordinate, linked bis(ketiminato) aluminum complex cis-C₆H ₁₀[(NCMeCHCMeO)₂AlMe] (4) could be isolated in moderate yield. Heating complex 2 at reflux in a toluene solution resulted in the elimination of 1 equiv. methane to generate a five-coordinate, linked bis-(ketiminato)aluminum complex, trans-C₆H₁₀[(NCMeCHCMeO) ₂AlMe] (5). Complex 2 reacts with allyl alcohol and 2,6-dimethylphenol to form dimeric aluminum complexes [trans-C₆H ₁₀(NCMeCHCMeO)₂Al]₂(μ-O-CH ₂CH=CH₂)₂ (6) and trans-C₆H ₁₀(NCMeCHCMeO)₂Al(O-C₆H₃-2,6-Me ₂) (7), respectively. All the complexes have been characterized by ¹H and ¹³C NMR spectroscopy and X-ray diffractometry. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800057

FULL PAPER
DOI: 10.1002/ejic.200800057
Aluminum Complexes Containing Cyclohexane-1,2-diyl Linked Bis(ketiminato)
Ligands and Proton-Promoted Demethylation
Chun-Hsueh Huang,[a] Li-Feng Hsueh,[a] Pei-Cheng Kuo,[a] Hon Man Lee,[a]
Chia-Liang Uno,[a] Jui-Hsien Huang,*[a] Cheng-Yi Tu,[a] Ching-Han Hu,*[a]
Gene-Hsiang Lee,[b] and Cheng-Hsiung Hung[c]
Keywords: Aluminum / Bis(ketiminato) ligands / Diaminocyclohexane / Demethylation
A series of cyclohexane-1,2-diyl linked bis(ketiminato)alumi-
num complexes were synthesized. Reaction of a mixture of
cis- and trans-diaminocyclohexane and 2,4-pentanedione
gave a mixture of cis- and trans-cyclohexane-1,2-diyl linked
(4) could be isolated in moderate yield. Heating complex 2
at reflux in a toluene solution resulted in the elimination of
1 equiv. methane to generate a five-coordinate, linked bis-
(ketiminato)aluminum complex, trans-C6H10[(NCMeCHC-
bis(ketimine) ligands (cis-1 and trans-1), which can be sepa- MeO)2AlMe] (5). Complex 2 reacts with allyl alcohol and 2,6-
rated by fractional recrystallization. Reactions of trans-1 with
1 and 2 equiv. AlMe3 gave monoaluminum complex trans-
C6H10[(NCMeCHCMeO)AlMe2](NHCMeCHCMeO) (2) and
dialuminum complex trans-C6H10[(NCMeCHCMeO)AlMe2]2
dimethylphenol to form dimeric aluminum complexes [trans-
C6H10(NCMeCHCMeO)2Al]2(µ-O–CH2CH=CH2)2 (6) and
trans-C6H10(NCMeCHCMeO)2Al(O–C6H3–2,6-Me2)
(7),
respectively. All the complexes have been characterized by
(3), respectively. Contrarily, when 1 equiv. or excess cis-1 re- 1H and 13C NMR spectroscopy and X-ray diffractometry.
acted with AlMe3, only five-coordinate, linked bis(ketimin-
ato)aluminum complex cis-C6H10[(NCMeCHCMeO)2AlMe]
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
tuned through substituted fragments; (ii) the ketiminato li-
gands leave an open end on the O site, which allows mono-
meric substrates to approach the metal center to increase
the probability of successive reactions; (iii) the β-hydrogen
of the ketiminato backbone presents higher acidity, and it
may further react with other metal–alkyl or metal–amide
complexes. In previous publications, we used ketiminato li-
gands as ancillary ligands to form a series of hydrido-
aluminum complexes.[31–33] The versatile (hydrido)bis(ket-
iminato)aluminum complexes react with small organic
molecules through insertion or abstraction reactions. Here
we extend our ketiminato–aluminum chemistry to include
linked diketiminato–aluminum systems. We report the syn-
thesis and characterization of aluminum complexes con-
taining cyclohexane-1,2-diyl linked diketimines as ancillary
ligands.
Introduction
Common strategies for producing new catalysts with en-
hanced activities toward small molecules include changes in
the types of ancillary ligand and variations in their elec-
tronic and steric effects. Numerous ligands have been used
for synthesizing organometallic complexes; cyclopen-
tadienyl (Cp) ligands dominate the field.[1–4] However, many
studies have been aimed at non-Cp ligand systems, resulting
in the development of many new types of ancillary li-
gands.[5–7] Among the non-Cp-type ancillary ligands, li-
gands containing heteroatoms such as O, N, S, and P have
been broadly studied. Bidentate anionic ligands containing
N and/or O atoms such as Schiff bases[8–15] and diketimin-
ato ligands[16–24] have been widely used as ancillary ligands
with many types of metals. However, monoanionic ketimin-
ato or linked dianionic ketiminato ligands[25–30] have re-
ceived less attention. The reasons for using ketiminato li-
gands instead of Schiff bases and diketiminates are: (i) ket-
iminato ligands contain one substituted imine, with which
the electronic and steric effect of the ligands can be fine-
Results and Discussion
Synthesis and Characterization of Compounds 1–4
The reaction of a mixture of cis- and trans-diaminocyclo-
hexane and 2,4-pentanedione in methanol under mild con-
[a] Department of Chemistry, National Changhua University of
Education,
Changhua 50058, Taiwan
ditions gave
a
mixture of cis- and trans-C6H10-
E-mail: juihuang@cc.ncue.edu.tw
(NHCMeCHCMeO)2 (1) (Scheme 1), which were then sep-
arated by fractional recrystallization. Pure trans-1 and cis-
1 were obtained in 39.7% and 6.0% yield, respectively. The
advantage of this method is that we can use inexpensive
[b] Department of Chemistry and Instrumentation Center,
National Taiwan University,
Taipei 10764, Taiwan
[c] Institute of Chemistry, Academia Sinica,
128 Academia Rd. Sec. 2, Nankang Taipei 115, Taiwan
3000
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3000–3008

Aluminum Complexes with Cyclohexane-1,2-diyl Linked Bis(ketiminato) Ligands
mixtures of cis- and trans-diaminocyclohexane as starting stoichiometric ratio resulted in the isolation of only com-
material to obtain the corresponding cis- and trans-cyclo- plex 4, and no other ketiminatoaluminum complexes were
1
hexane-1,2-diyl linked diketimine ligands. Both cis- and found. The H NMR spectrum of 4 shows that the reso-
1
trans-1 have been characterized by H and 13C NMR spec- nances for the methyl groups attached to aluminum appear
troscopy (see Experimental Section).
at δ = –0.88 ppm, while those for the methine protons of
the ketiminato backbones appear at δ = 4.98 ppm.
Scheme 1.
Scheme 3.
Reactions of trans-1 with 1 or 2 equiv. AlMe3 in dichlo-
romethane at room temperature gave high yields of trans-
C6H10[(NCMeCHCMeO)AlMe2](NHCMeCHCMeO) (2)
or trans-C6H10[(NCMeCHCMeO)AlMe2]2 (3), respectively
(Scheme 2) along with the elimination of 1 or 2 equiv. meth-
ane, respectively. Complex 3 can also be obtained from the
Proton-Promoted Demethylation of the Dimethylaluminum
Complex
1
Monoketiminatodimethylaluminum complex 2 is ther-
mally quite stable. It can be converted into a linked bis-
(ketiminato)methylaluminum complex, 5, trans-C6H10-
[(NCMeCHCMeO)2AlMe], only under harsh conditions,
i.e. by heating at reflux in toluene for 96 h (Scheme 4); this
reaction is accompanied by the elimination of 1 equiv.
methane. Pure complex 5 was obtained after recrystalli-
zation from a THF solution in high yield (80%). Unlike
complex 4, the asymmetrical complex 5 shows two methine
reaction of 1 equiv. AlMe3 with 2. The H NMR spectrum
of 2 shows two methine proton resonances at δ = 4.83 and
5.00 ppm, a clear differentiation between the ketiminato
and ketimine backbones. The 13C NMR spectrum of 2
shows that ketimine that is not coordinated to aluminum
exhibits a more downfield-shifted resonance for C=O at δ
= 194.8 ppm, whereas the resonance for the C=O of the
ketiminato ligand coordinated with aluminum appears at δ
= 178.9 ppm. The AlMe2 fragment of 2 shows two 13C
NMR resonances at δ = –9.1 and –7.2 ppm, indicating the
1
signals for the two ketiminate fragments in the H NMR
1
spectrum. Similarly, the methine protons of the cyclohex-
ane-1,2-diyl unit exhibited two multiplets at δ = 3.04 and
3.90 ppm.
diastereotopic properties of the two methyl groups. The H
NMR spectrum of 3, however, exhibited a more symmetri-
cal pattern. Complex 3 consists of two ketiminato–AlMe2
fragments linked by cyclohexane-1,2-diyl. Only one methine
proton resonance of the ketiminato backbones was ob-
served at δ = 3.88 ppm, indicating that the coordinating en-
vironments of the two ketiminato–AlMe2 fragments are the
same.
Complex 2 reacted with alcohols to produce linked bis-
(ketiminato)aluminum complexes along with the elimi-
nation of methane. The reactions of 2 with 1 equiv. allyl
alcohol and 2,6-dimethylphenol (Scheme 4) generated
alkoxidoaluminum complexes [trans-C6H10(NCMeCHC-
MeO)2Al]2(µ-O–CH2CH=CH2)2 (6) and trans-C6H10-
(NCMeCHCMeO)2Al(O–C6H3–2,6-Me2) (7), respectively.
Elemental analysis of complex 6 did not match with the
calculated values, as the isolation of pure complex 6 by re-
peated recrystallization from a dichloromethane solution
was hindered by a small amount of ketimine ligands and
unknown impurities in the product. Complex 6 has been
characterized by 1D and 2D 1H and 13C NMR spec-
troscopy. The 1H NMR spectra of 6 show two methine sig-
nals for the ketiminato backbones at δ = 4.83 and 5.06 ppm
due to the steric environment and two resonances for the
cyclohexane-1,2-diyl CH protons at δ = 3.19 and 2.51 ppm.
Scheme 2.
Treatment of cis-1 with 2 equiv. AlMe3 afforded a five- The methylene protons of O–CH2 (δ = 5.09 and 4.87 ppm)
coordinate, linked bis(ketiminato)aluminum complex, cis- and =CH2 (δ = 4.28 and 4.25 ppm) for the allyl fragments
C6H10[(NCMeCHCMeO)2AlMe] (4), in moderate yield all show distinguishing chemical shifts. Similarly, the 1H
(Scheme 3). Although the cis-1 and AlMe3 were employed NMR spectrum of 7 shows two methine signals for the
in a 1:2 stoichiometric ratio, complex 4 was the only prod- linked bis(ketiminato) backbones at δ = 4.95 and 5.08 ppm
uct isolated. Attempts to prepare an asymmetrical ketimin- and two resonances for the cyclohexane-1,2-diyl CH pro-
atoaluminum complex similar to 2 with a 1:1 cis-1/AlMe3 tons at δ = 3.13 and 4.25 ppm.
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J. Huang, C.-H. Hu et al.
FULL PAPER
Scheme 4.
Scheme 5.
with available X-ray data. It is seen that geometries pre-
dicted by DFT are in good agreement with the experimental
data. The largest deviations occur in the Al–N and Al–O
distances.
The calculation shows that adding 1 equiv. AlMe3 to
trans-1 and cis-1 to form 2 and cis-2 are very exothermic;
∆E are –53.4 and –44.4 kcal/mol, respectively. For trans-1
we have located two minima, an equatorial and an axial
isomer. The equatorial isomer is 4.3 kcal/mol lower in en-
ergy. The complex of the equatorial trans-1 with AlMe3 is
lower in energy than its axial counterpart by 15.5 kcal/mol.
The results are consistent with the resolved X-ray structure
of trans-1 and -2.
Comparing the conversions of monoketiminatoalumi-
num complex 2 to linked bis(ketiminato)aluminum com-
plexes 5, 6, and 7, we see that the presence of protic organic
molecules has enhanced the elimination of the methyl group
from the methylaluminum fragments. A possible reaction
mechanism is shown in Scheme 5. The acidic proton of H–
OR protonates the methylaluminum group and eliminates
1 equiv. methane to form AlMe(OR) complexes. The
greater steric crowding of aluminum alkoxides enhances the
further elimination of a second equivalent of methane to
form the final products.
The reactions of 2 and cis-2 with the second equivalent
of AlMe3 are both very exothermic. In our investigation, 3
is the product of 2 and AlMe3, while 4 is the only product
Theoretical Computations
Theoretical methods have been applied in order to of treating cis-1 with either one or two equivalents of
understand the probable driving force that contributes to AlMe3. A possible cause of this difference is the relative
the differences in the reaction products of cis-1 and trans- stability of 2 and cis-2 with respect to the consecutive coor-
1. Structures and important geometrical parameters of the dination (into 5 and 4). The conversion of 2 to 5 may in-
studied species are illustrated in Figure 1 and are compared volve a larger reaction barrier than that of cis-2 to 4.
3002
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Eur. J. Inorg. Chem. 2008, 3000–3008

Aluminum Complexes with Cyclohexane-1,2-diyl Linked Bis(ketiminato) Ligands
Figure 1. Structures and geometrical parameters obtained at the B3LYP/6-31G* level. The parameters obtained by X-ray diffraction data
are shown in the second entries.
Molecular Structures for Complexes 1–7
fore, only one molecule is described here. Complex 2 con-
tains one ketiminatodimethylaluminum fragment and one
The crystals of trans-1 and cis-1 were obtained, and their free ketimine (Figure 3). The ketiminate and ketimine rings
molecular structures were solved as well (Figure 2). The are both perpendicular to the cyclohexane-1,2-diyl ring and
structure of trans-1 is the same as that reported in the litera- are almost parallel to each other. The ketiminatodimethylal-
ture.[28] Single crystals of aluminum complexes 2–7 suitable uminum fragment exhibits a distorted tetrahedral geometry
for X-ray structure determination were crystallized from with a biting angle of O(1)–Al(1)–N(1) equal to 106.6(3)°,
dichloromethane solutions at –20 °C. The collection and re- which is much smaller than standard tetrahedral angles;
finement data of the analyses are summarized in Table 1. however, it is much larger than that in normal coordinated
Selected bond lengths and angles for complexes 2–7 are ketiminatoaluminum complexes[22,28,29]. The free ketimine
shown in Table 2. Although there are two independent that is not coordinated to aluminum shows a mixed keto–
molecules in a unit cell for complex 2, the bond lengths and enol form with bond lengths for C(15)–C(16) and C(17)–
angles of these two molecules are relatively similar. There- O(2) of 1.392(10) Å and 1.242(9) Å, respectively.
Figure 2. The comparison of the molecular structures of cis- and trans-1, in which thermal ellipsoids were drawn at 30% probability and
all hydrogen atoms were omitted for clarity.
Eur. J. Inorg. Chem. 2008, 3000–3008
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J. Huang, C.-H. Hu et al.
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Table 1. Summary of crystallographic data for complexes 2–7.
2
3
4·CH2Cl2
5
6·CH2Cl2
7
Formula
Formula weight
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [deg]
β [deg]
C18H31AlN2O2 C20H36AlN2O2 C18H29AlCl2N2O2 C17H27AlN2O2 C39H60Al2Cl2N4O6 C24H33AlN2O3
334.43
150(2)
triclinic
390.47
150(1)
orthorhombic triclinic
403.31
150(1)
318.39
150(2)
monoclinic
P21/n
805.77
150(2)
triclinic
424.50
150(2)
monoclinic
P21/n
¯
¯
P1
¯
P1
Pccn
P1
12.117(4)
12.417(4)
13.965(4)
80.003(5)
75.081(5)
73.492(5)
1935.2(10)
4
11.1513(4)
11.5438(4)
17.6683(7)
90
90
90
2274.41(14)
4
1.140
8.0184(1)
10.9668(2)
12.5820(2)
84.8258(10)
76.1238(8)
70.4327(8)
2530.39(6)
2
8.0963(5)
9.8450(7)
21.4805(14)
90
96.762(2)
90
1700.26(19)
4
1.244
11.7913(9)
13.4011(10)
13.6351(11)
77.041(2)
83.100(2)
83.473(2)
2076.1(3)
2
10.8736(6)
19.3299(10)
11.5527(7)
90
111.075(3)
90
2265.8(2)
4
1.244
γ [deg]
V [Å3]
Z
dc [Mg/m3]
1.148
1.324
1.289
µ [mm–1]
0.116
0.143
0.378
0.128
0.248
0.117
F(000)
λ [Å]
No. of rflns. collected
Ind. rflns.
Rint
728
0.71073
13636
8416
0.0371
8416/0/415
0.871
848
428
0.71073
17890
4643
0.0397
4643/0/227
1.054
688
0.71073
16630
860
0.71073
33818
11060
0.0428
11060 /0/ 486
1.075
912
0.71073
24463
6002
0.0421
6002/0/ 277
1.077
0.0423, 0.1027
0.0725, 0.1118
0.262, –0.311
0.71073
28118
2619
0.0380
2619/0/122
1.061
4539
0.0204
4539/0/204
1.037
0.0330, 0.0935 0.0544, 0.1419
0.0401, 0.0973 0.0849, 0.1572
Data/restraints/params.
Goodness-of-fit on F2
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
0.0609, 0.1721 0.0417, 0.1087 0.0490, 0.1405
0.1589, 0.2064 0.0486, 0.1137 0.0584, 0.1500
Largest diff peak, hole [e/Å3] 0.508, –0.464
0.356, –0.189
0.651, –0.499
0.338, –0.276
1.444, –1.098
Figure 3. The molecular structure of complex 2, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
were omitted for clarity.
Figure 4. The molecular structure of complex 3, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
were omitted for clarity.
The molecular structure of complex 3 (Figure 4) is very two ketiminate fragments is ca. 0.53 Å. The bond lengths
similar to that of 2 except that the two ketiminate rings are of aluminum to the ketiminate N and O atoms, 1.9974–
now both bound to aluminum atoms and form two ketimin- 2.0067 Å and 1.8501–1.8428 Å, respectively, are apparently
atodimethylaluminum fragments. Again, the biting angle of longer than those in complexes 2 and 3 (1.954–1.965 Å and
the ketiminatoaluminum fragment, O(1)–Al(1)–N(1), is 1.794–1.804 Å, respectively). Presumably, the more elec-
96.16(5)°, which is smaller than the bond angle for regular tron-rich ketiminato ligands lessen the electrophilicity of
tetrahedral geometry.
the high oxidation state of the aluminum atom.
In the molecular structure of complex 4 (Figure 5), the
The linked bis(ketiminato) fragments of complex 5 are
five-coordinate aluminum atom adopts a square-pyramidal bound to an aluminum atom along with a methyl group,
geometry in which the two ketiminato ligands occupy the forming a five-coordinate trigonal bipyramidal geometry
square base and the methyl group takes the apical position. (Figure 6). The nitrogen atom of one ketiminate group and
The cyclohexane-1,2-diyl fragment exhibits a boat confor- the oxygen atom of the other ketiminate group occupy the
mation. The deviation of the aluminum atom from the axial positions with an angle of 167.53(4)°. The cyclohexane-
square base formed by the nitrogen and oxygen atoms of 1,2-diyl fragment is arranged in the chair conformation.
3004
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Eur. J. Inorg. Chem. 2008, 3000–3008

Aluminum Complexes with Cyclohexane-1,2-diyl Linked Bis(ketiminato) Ligands
Table 2. The selected bond distances [Å] and angles [°] for com-
plexes 2–7.
2
Al(1)–O(1)
Al(1)–C(1)
O(1)–Al(1)–C(2)
C(1)–Al(1)–C(2)
C(2)–Al(1)–N(1)
1.804(5)
1.951(9)
106.0(3)
119.0(3)
117.1(3)
Al(1)–C(2)
Al(1)–N(1)
O(1)–Al(1)–C(1)
O(1)–Al(1)–N(1)
C(1)–Al(1)–N(1)
1.946(7)
1.954(6)
109.7(3)
106.6(3)
106.6(3)
3
Al(1)–O(1)
Al(1)–N(1)
O(1)–Al(1)–C(2)
C(1)–Al(1)–C(2)
C(2)–Al(1)–N(1)
1.7935(12) Al(1)–C(1)
1.9649(12) Al(1)–C(2)
111.13(7) O(1)–Al(1)–C(1)
116.44(7) O(1)–Al(1)–N(1)
109.03(6) C(1)–Al(1)–N(1)
1.9640(16)
1.9657(17)
106.77(7)
96.16(5)
Figure 5. The molecular structure of complex 4, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
and one dichloromethane molecule were omitted for clarity.
115.39(6)
4
Al(1)–O(1)
Al(1)–N(1)
Al(1)–N(2)
O(2)–Al(1)–C(1)
O(2)–Al(1)–N(1)
C(1)–Al(1)–N(1)
O(1)–Al(1)–N(2)
N(1)–Al(1)–N(2)
1.8501(13) Al(1)–O(2)
1.9974(16) Al(1)–C(1)
2.0067(15) O(2)–Al(1)–O(1)
104.99(7) O(1)–Al(1)–C(1)
143.17(6) O(1)–Al(1)–N(1)
111.74(7) O(2)–Al(1)–N(2)
152.24(7) C(1)–Al(1)–N(2)
78.98(6)
1.8428(13)
1.9807(19)
85.59(6)
102.11(7)
89.33(6)
88.88(6)
105.60(7)
5
Al(1)–O(1)
Al(1)–N(2)
Al(1)–C(17)
O(1)–Al(1)–O(2)
O(2)–Al(1)–N(2)
O(2)–Al(1)–C(17)
O(1)–Al(1)–N(1)
N(2)–Al(1)–N(1)
1.8162(8) Al(1)–O(2)
1.9732(9) Al(1)–N(2)
1.9857(11) Al(1)–N(1)
1.8581(8)
1.9732(9)
2.0425(9)
117.84(4)
114.93(4)
126.89(4)
167.53(4)
95.59(4)
90.82(4)
89.57(4)
95.35(4)
89.99(4)
79.06(4)
O(1)–Al(1)–N(2)
O(1)–Al(1)–C(17)
N(2)–Al(1)–C(17)
O(2)–Al(1)–N(1)
C(17)–Al(1)–N(1)
Figure 6. The molecular structure of complex 5, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
(excepting those on C5 and C6) and one dichloromethane molecule
were omitted for clarity.
6
Al(1)–O(2)
Al(1)–O(6)
Al(1)–N(2)
Al(2)–O(3)
Al(2)–O(5)
Al(2)–N(3)
O(5)–C(33)
C(34)–C(35)
1.8582(15) Al(1)–O(1)
1.8994(15) Al(1)–O(5)
2.0205(18) Al(1)–N(1)
1.8490(15) Al(2)–O(4)
1.9035(15) Al(2)–O(6)
2.0402(18) Al(2)–N(4)
1.8682(15)
1.9107(14)
2.0535(17)
1.8742(15)
1.9077(15)
2.0453(17)
1.431(2)
1.494(3)
91.01(6)
75.94(6)
168.04(7)
165.40(7)
92.40(6)
75.92(6)
167.83(7)
87.99(7)
The crystal of 6 contains one molecule of 6 and one
molecule of dichloromethane, which is omitted in Figure 7
for clarity. The molecular structure of 6 can be described as
an edge-shared bi-octahedron in which the two oxygen
atoms of the allyloxide groups occupy the sharing edge and
the two allyl fragments extend in the same direction. All of
the Al–O(bridge) bond lengths in complex 6 are very sim-
ilar and lie in the range 1.8994(15)–1.9107(14) Å. They are
much longer than the Al–O(ketiminate) bond lengths
reported here, which range from 1.8490(15) Å to
1.8742(15) Å. Here, the Al–O(ketiminate) bond lengths are
longer than the Al–O bond length in Schiff base aluminum
complexes.[34,35] The Al2O2 core in complex 6 forms a per-
fect square plane with a dihedral angle of 7.8°, which is
similar to the core structure of alkoxidodiketiminatoalumi-
num complexes reported by Wengrovius.[36]
Unlike the dimeric molecular structure of 6, aryloxido-
aluminum complex 7 exhibits a five-coordinate trigonal bi-
pyramidal geometry in which the bulky 2,6-dimethylphe-
noxido ligand occupies the axial position (Figure 8). The
short aryloxido–Al [1.7417(11) Å] bond length along with
the large Al(1)–O(3)–C(17) [139.66(9)°] bond angle indicate
that the delocalized O(pπ) electrons move towards the
1.431(2)
1.321(3)
1.246(4)
92.42(6)
88.88(7)
95.51(7)
88.18(7)
O(6)–C(36)
C(36)–C(37)
C(37)–C(38)
O(1)–Al(1)–O(5)
O(6)–Al(1)–O(5)
O(6)–Al(1)–N(2)
O(2)–Al(1)–N(1)
O(4)–Al(2)–O(5)
O(1)–Al(1)–O(6)
O(2)–Al(1)–N(2)
O(5)–Al(1)–N(2)
O(1)–Al(1)–N(1)
O(4)–Al(2)–O(6)
O(3)–Al(2)–N(3)
O(3)–Al(2)–N(4)
C(35)–C(34)–C(33)
168.17(7) O(5)–Al(2)–O(6)
89.17(7) O(5)–Al(2)–N(3)
166.08(7) O(4)–Al(2)–N(4)
124.1(2) C(38)–C(37)–C(36)
128.1(3)
7
Al(1)–O(3)
Al(1)–O(2)
Al(1)–N(1)
O(3)–Al(1)–O(2)
O(3)–Al(1)–N(2)
O(2)–Al(1)–N(2)
O(1)–Al(1)–N(1)
N(2)–Al(1)–N(1)
1.7417(11) Al(1)–O(1)
1.8474(10) Al(1)–N(2)
2.0221(12) O(3)–Al(1)–O(1)
1.7947(10)
1.9453(12)
111.91(5)
90.88(5)
125.15(5)
90.21(5)
98.58(5)
O(1)–Al(1)–O(2)
122.16(5) O(1)–Al(1)–N(2)
89.82(5)
91.17(5)
80.57(5)
O(3)–Al(1)–N(1)
O(2)–Al(1)–N(1)
169.48(5)
Eur. J. Inorg. Chem. 2008, 3000–3008
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Huang, C.-H. Hu et al.
FULL PAPER
complexes can be useful in olefin or lactide polymerization
chemistry, and we are currently developing their applica-
tions.
Experimental Section
General Procedure: All reactions were performed under a dry nitro-
gen atmosphere by using standard Schlenk techniques or in a glove
box. Toluene, heptane, diethyl ether, and tetrahydrofuran were
dried by heating at reflux over sodium benzophenone ketyl. CH2Cl2
was dried over P2O5. 1,2-diaminocyclohexane (99%, mixture of cis
and trans) and 2,4-pentanedione were obtained from Aldrich and
used as received. All solvents were distilled and stored in solvent
reservoirs that contained 4-Å molecular sieves and were purged
with nitrogen. 1H and 13C NMR spectra were recorded with a
Bruker Avance 300 spectrometer. Chemical shifts for 1H and 13C
NMR spectra were recorded in ppm relative to the residual protons
and the 13C isotope of CDCl3 (δ = 7.24 and 77.0 ppm, respectively)
and C6D6 (δ = 7.16 and 128.0 ppm, respectively). Elemental analy-
ses were performed with a Heraeus CHN–OS Rapid Elemental An-
alyzer at the Instrument Center, NCHU.
Figure 7. The molecular structure of complex 6, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
(excepting those on C6, C11, C22, and C27) and one dichlorometh-
ane molecule were omitted for clarity.
cis- and trans-C6H10[(NHCMeCHCMeO)]2 (cis- and trans-1): A
mixture of cis- and trans-diaminocyclohexane (87.7 g, mmol) was
dissolved in ethanol (30 mL), and a solution of 2,4-pentanedione
(50.0 g, mmol) in methanol (30 mL) was added slowly. A few drops
of formic acid were added to the mixed solution as catalyst. The
solution was stirred at room temperature for another 12 h. Pure
trans-1 was first obtained from recrystallization (48.4 g, 39.7%
yield), and cis-1 was obtained from the last crop of recrystallization
(7.2 g, 6.0% yield).
high-oxidation-state aluminum atom. A comparison of the
structures of 6 and 7 shows that the bulkier aryloxide
group of 7 is bound to the aluminum atom and occupies a
large space in the Al coordination sphere, preventing com-
plex 7 from forming a dimeric structure. In contrast, in
complex 6, the smaller allyloxide fragments are bridged to
two aluminum atoms, forming a more stable dimeric struc-
ture.
trans-1: 1H NMR (CDCl3): 1.31 (m, 4 H, CH2CH2CHN), 1.79–
2.03 (m, 4 H, CH2CH2CHN), 1.93 (s, 6 H, CCH3 ), 1.98 (s, 6 H,
CCH3 ), 3.18 (m, 2 H, CH2CH2CHN), 4.84 (s, 2 H, CCHC), 10.95
(s, 2 H, NH ) ppm. 13C NMR (CDCl3): δ = 18.7 (q, JCH = 128 Hz,
CCH3), 24.4 (t, JCH = 129 Hz, CH2CH2CHN), 28.7 (q, JCH
=
126 Hz, CCH3), 32.9 (t, JCH = 130 Hz, CH2CH2CHN), 57.7 (d,
JCH = 137 Hz, CH2CH2CHN), 95.3 (d, JCH = 161 Hz, CCHC),
162.8 (s, C–N), 194.9 (s, C = O) ppm. C16H26N2O2 (278.39): calcd.
C 69.03, H 9.41, N 10.06; found C 69.16, H 10.17, N 10.46.
cis-1: 1H NMR (CDCl3): δ = 1.44 (m, 2 H, CHHCH2CH), 1.67
(m, 2 H, CHHCH2CHN), 1.67 (m, 4 H, CH2CH2CHN), 1.88 (s, 6
H, CCH3), 1.95 (s, 6 H, CCH3), 3.62 (m, 2 H, CH2CH2CHN), 4.98
(s, 2 H, CCHC), 11.12 (s, 2 H, OH) ppm. 13C NMR (CDCl3): δ =
18.5 (q, JCH
= 128 Hz, CCH3), 21.6 (t, JCH = 128 Hz,
CH2CH2CHN), 28.5 (q, JCH = 126 Hz, CCH3), 29.5 (t, JCH
=
126 Hz, CH2CH2CHN), 52.9 (d, JCH = 137 Hz, CH2CH2CHN), 96
(d, JCH = 160 Hz, CCHC), 161.5 (s, C–N), 194.7 (s, C = O) ppm.
C16H26N2O2 (278.39): calcd. C 69.03, H 9.41, N 10.06; found C
69.02, H 10.15, N 10.18.
trans-C6H10[(NCMeCHCMeO)AlMe2](NHCMeCHCMeO) (2): To
a 30-mL Schlenk flask charged with dichloromethane (15 mL) and
trans-1 (3.0 g, 10.8 mmol), was added AlMe3 toluene solution
(5.4 mL, 2 , 10.8 mmol) at –78 °C. The solution was stirred for
10 h, and the volatiles were removed under vacuum. The solid was
recrystallized from a saturated dichloromethane solution at –20 °C
to yield a white solid of 2 (2.67 g) in 74% yield. 1H NMR (CDCl3):
δ = –0.99 (s, 3 H, AlMe), –0.61 (s, 3 H, AlMe), 1.33 (br.m, 4 H,
cyclohexane-1,2-diyl-CH2), 1.76–2.04 (br.m, 4 H, cyclohexane-1,2-
diyl-CH2), 1.80 (s, 3 H, CMe), 1.91 (s, 3 H, CMe), 1.94 (s, 3 H,
CMe), 1.97 (s, 3 H, CMe), 3.54 (br.m, 2 H, cyclohexane-1,2-diyl-
Figure 8. The molecular structure of complex 7, in which thermal
ellipsoids were drawn at 30% probability and all hydrogen atoms
(excepting those on C6 and C11) and one dichloromethane mole-
cule were omitted for clarity.
Conclusions
A series of aluminum complexes containing cis- and
trans-cyclohexane-1,2-diyl linked bis(ketiminato) ligands
has been synthesized. These ligands exhibit versatile coordi-
nation modes and the ability of forming mono- or dialumi-
CH-N), 4.83 (s,
1 H, NCMeCHCMeO), 5.00 (s, 1 H,
num complexes. The allyloxido and aryloxidoaluminum NCMeCHCMeO), 10.91 (br. s, 1 H, NH) ppm. 13C NMR (CDCl3):
3006
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3000–3008

Aluminum Complexes with Cyclohexane-1,2-diyl Linked Bis(ketiminato) Ligands
δ = –9.1 (q, JCH = 119 Hz, AlMe), –7.2 (q, JCH = 118 Hz, AlMe),
19.1 (q, JCH = 128 Hz, CMe), 22.8 (q, JCH = 128 Hz, CMe), 24.5
(t, JCH = 128 Hz, cyclohexane-1,2-diyl-CH2), 24.7 (t, JCH = 127 Hz,
cyclohexane-1,2-diyl-CH2), 25.3 (q, JCH = 127 Hz, CMe), 28.8 (q,
JCH = 126 Hz, CMe), 31.7 (t, JCH = 128 Hz, cyclohexane-1,2-diyl-
CH2), 33.9 (t, JCH = 133 Hz, cyclohexane-1,2-diyl-CH2), 55.9 (d,
JCH = 137 Hz, cyclohexane-1,2-diyl-CH-N), 63.7 (d, JCH = 135 Hz,
clohexane-1,2-diyl-CH-N), 99.3 (d, JCH = 161 Hz, OCMeCHC-
MeN), 101.1 (d, JCH = 156 Hz, OCMeCHCMeN), 165.2 (s, OC-
MeCHCMeN), 173.8 (s, OCMeCHCMeN), 176.1 (s, OCMeCHC-
MeN) 180.8 (s, OCMeCHCMeN) ppm. C17H27AlN2O2 (318.39):
calcd. C 64.13, H 8.55, N 8.80; found C 63.95, H 8.08, N 8.60.
[trans-C6H10(NCMeCHCMeO)2Al]2(µ-O–CH2CH=CH2)2 (6): To a
100-mL Schlenk flask charged with 2 (1.0 g, 2.99 mmol) and tolu-
ene (50 mL) was added allyl alcohol (0.174 g, 2.99 mmol), and the
solution was heated at reflux for additional 4.5 h. Volatiles were
removed, and the residue was recrystallized from a CH2Cl2/heptane
solvent system to yield 1.53 g (71%) pale yellow crystals. A small
amount of the original cyclohexane-1,2-diyldiketimine ligand was
observed in the final product even when repetitive recrystallization
cyclohexane-1,2-diyl-CH-N), 95.2 (d, JCH
= 161 Hz, OC-
MeCHCMeNH), 101.2 (d, JCH = 162 Hz, OCMeCHCMeN), 162.9
(s, OCMeCHCMeNH), 175.2 (s, OCMeCHCMeN), 178.9 (s, OC-
MeCHCMeN), 194.8 (s, OCMeCHCMeNH) ppm. C18H31AlN2O2
(334.43): calcd. C 64.65, H 9.34, N 8.38; found C 64.58, H 8.85, N
8.41.
1
trans-C6H10[(NCMeCHCMeO)AlMe2]2 (3): A similar reaction pro-
cedure to that used for synthesizing 2 was applied here. trans-1
(1.0 g, 3.6 mmol) and AlMe3 (2 , 3.6 mL, 7.2 mmol) were used,
was performed. H NMR (C6D6): δ = 1.32 (br., 16 H, CH2 cyclo-
hexane-1,2-diyl), 1.84 (s, 6 H, CH3), 1.96 (s, 6 H, CH3), 1.97 (s, 6
H, CH3), 2.02 (s, 6 H, CH3), 2.48 (d, 2 H, CH cyclohexane-1,2-
diyl), 3.06 (d, 2 H, CH cyclohexane-1,2-diyl), 4.18 (br., 4 H,
OCH2), 4.86 (d, 2 H, CH allyl), 5.09 (d, 2 H, CH allyl), 5.03 (s, 2
H, CCHC), 5.25 (s, 2 H, CCHC), 6.01 (m, 2 H, CH allyl) ppm.
13C NMR (C6D6): δ = 21.3 (q, JCH = 128 Hz, CCH3), 23.6 (q, JCH
= 128 Hz, CCH3), 25.6 (q, JCH = 128 Hz, CCH3), 24.7 (t, CH2
cyclohexane-1,2-diyl), 26.5 (t, CH2 cyclohexane-1,2-diyl), 32.8 (t,
JCH = 122 Hz, CH2 cyclohexane-1,2-diyl), 33.4 (t, JCH = 129 Hz,
CH2 cyclohexane-1,2-diyl), 63.0 (d, JCH = 138 Hz, CH cyclohex-
ane-1,2-diyl), 64.0 (d, JCH = 137 Hz, CH cyclohexane-1,2-diyl),
64.3 (t, JCH = 138 Hz, OCH2), 99.4 (d, JCH = 161 Hz, CCHC),
1
and 1.21 g product was isolated (86% yield). H NMR (CDCl3): δ
= –1.00 (s, 6 H, AlMe), –0.55 (s, 6 H, AlMe), 1.30 (br.m, 2 H,
cyclohexane-1,2-diyl-CH2), 1.83–2.00 (br.m, 6 H, cyclohexane-1,2-
diyl-CH2), 1.92 (s, 6 H, CMe), 1.93 (s, 6 H, CMe), 3.88 (br.m, 2 H,
cyclohexane-1,2-diyl-CH-N), 4.99 (s, 2 H, NCMeCHCMeO) ppm.
13C NMR (CDCl3): δ = –9.3 (q, JCH = 112 Hz, AlMe), –7.0 (q,
JCH = 112 Hz, AlMe), 23.4 (q, JCH = 128 Hz, CMe), 24.3 (t, JCH
= 128 Hz, cyclohexane-1,2-diyl-CH2), 25.3 (q, JCH = 127 Hz,
CMe), 31.8 (t, JCH = 126 Hz, cyclohexane-1,2-diyl-CH2), 61.9 (d,
JCH = 133 Hz, cyclohexane-1,2-diyl-CH-N), 101.0 (d, JCH
=
161 Hz, OCMeCHCMeN), 175.4 (s, OCMeCHCMeN), 178.8 (s, 101.8 (d, JCH = 159 Hz, CCHC), 111.5 (t, JCH = 155 Hz, CH2
OCMeCHCMeN) ppm. C20H36Al2N2O2 (390.48): calcd. C 61.52,
H 9.29, N 7.17; found C 60.79, H 9.33, N 7.20.
allyl), 142.6 (d, JCH = 155 Hz, CH allyl), 167.0 (s, N-C), 174.4 (s,
N-C), 176.3 (s, O =C), 181.7 (s, O =C) ppm.
cis-C6H10[(NCMeCHCMeO)2AlMe] (4): A similar reaction pro-
cedure to that used for synthesizing 2 is applied here. cis-1 (1.0 g,
3.6 mmol) and excess AlMe3 (2 , 3.6 mL, 7.2 mmol) were used,
trans-C6H10(NCMeCHCMeO)2Al(O–C6H3–2,6-Me2) (7): To a 30-
mL Schlenk flask charged with toluene (15 mL) and trans-1 (0.50 g,
1.50 mmol) was added a toluene solution of 2,6-dimethylphenol
(0.183 g, 1.50 mmol in 10 mL toluene) at 0 °C. The solution was
stirred for 20 min and heated at reflux for additional 14 h. The
volatiles were removed under vacuum, and the solid was recrys-
tallized from a saturated dichloromethane solution at –20 °C to
1
and 0.21 g product was isolated (86% yield). H NMR (CDCl3): δ
= –0.88 (br. s, 3 H, AlMe), 1.40 (br.m, 2 H, cyclohexane-1,2-diyl-
CH2), 1.54–1.69 (br.m, 4 H, cyclohexane-1,2-diyl-CH2), 1.86–1.96
(m, 2 H, cyclohexane-1,2-diyl-CH2), 1.951 (s, 6 H, CMe), 1.954 (s,
6 H, CMe), 1.96 (s, 3 H, CMe), 3.74 (br.m, 2 H, cyclohexane-
1,2-diyl-CH-N), 4.98 (s, 2 H, NCMeCHCMeO) ppm. 13C NMR
(CDCl3): δ = 19.3 (t, JCH = 127 Hz, cyclohexane-1,2-diyl-CH2),
20.9 (q, JCH = 128 Hz, CMe), 25.6 (q, JCH = 127 Hz, CMe), 27.1 (t,
JCH = 128 Hz, cyclohexane-1,2-diyl-CH2), 55.0 (d, JCH = 140 Hz,
cyclohexane-1,2-diyl-CH-N), 99.6 (d, JCH = 160 Hz, OCMeCHC-
MeN), 170.6 (s, OCMeCHCMeN), 179.1 (s, OCMeCHCMeN)
ppm. C17H27AlN2O2 (318.39): calcd. C 64.13, H 8.55, N 8.80;
found C 64.32, H 8.59, N 8.86.
1
afford a white solid of 7 (0.47 g) in 79% yield. H NMR (C6D6): δ
= 1.37 (br.m, 4 H, cyclohexane-1,2-diyl-CH2), 1.67–2.53 (m, Me
and cyclohexane-1,2-diyl-CH2), 3.13 (br.m, 1 H, cyclohexane-1,2-
diyl-CH-N), 4.25 (br.m, 1 H, cyclohexane-1,2-diyl-CH-N), 4.95 (s,
1 H, NCMeCHCMeO), 5.08 (s, 1 H, NCMeCHCMeO), 6.48-6.87
(m, 3 H, Ph) ppm. 13C NMR (C6D6): δ = 17.52 (q, JCH = 128 Hz,
Ph-Me), 21.54 (q, JCH = 128 Hz, Me), 23.73 (q, JCH = 128 Hz,
Me), 24.56 (t, JCH = 128 Hz, cyclohexane-1,2-diyl-CH2), 25.50 (q,
JCH = 127 Hz, Me), 26.36 (t, JCH = 127 Hz, cyclohexane-1,2-diyl-
CH2), 32.77 (t, JCH = 132 Hz, cyclohexane-1,2-diyl-CH2), 33.34 (t,
JCH = 133 Hz, cyclohexane-1,2-diyl-CH2), 63.03 (d, JCH = 137 Hz,
cyclohexane-1,2-diyl-CH-N), 64.41 (d, JCH = 137 Hz, cyclohexane-
1,2-diyl-CH-N), 99.36 (d, JCH = 161 Hz, OCMeCHCMeNH),
102.3 (d, JCH = 162 Hz, OCMeCHCMeN), 116.64, 127.25, 127.46,
157.2 (Ph), 166.9 (s, OCMeCHCMeNH), 174.62 (s, OCMeCHC-
MeN), 175.9 (s, OCMeCHCMeN), 182.4 (s, OCMeCHCMeNH)
ppm. C22H33AlN2O3 (400.49): calcd. C 67.90, H 7.84, N 6.60;
found C 67.78, H 7.28, N 5.90.
trans-C6H10[(NCMeCHCMeO)2AlMe] (5):
A toluene (20 mL)
solution of 2 (0.30 g,) was heated at reflux for 4 d, and the volatiles
were removed under vacuum. The resulting solid was recrystallized
from a THF solution to yield 0.23 g (80.4%) final product. 1H
NMR (CDCl3): δ = –0.88 (s, 3 H, AlMe), 1.36 (br.m, 3 H, cyclohex-
ane-1,2-diyl-CH2), 1.75–2.05 (br.m, 4 H, cyclohexane-1,2-diyl-
CH2), 1.85 (s, 3 H, CMe), 1.92 (s, 3 H, CMe), 1.94 (s, 3 H, CMe),
1.97 (s, 3 H, CMe), 2.48 (m, 1 H, cyclohexane-1,2-diyl-CH2), 3.04
(br.m, 1 H, cyclohexane-1,2-diyl-CH-N), 3.90 (br.m, 1 H, cyclohex-
ane-1,2-diyl-CH-N), 4.77 (s, 1 H, NCMeCHCMeO), 5.03 (s, 1 H,
X-ray Structure Determination of Complexes 1–7: The crystals were
sealed in glass fibers under nitrogen and transferred to a goniostat.
NCMeCHCMeO) ppm. 13C NMR (CDCl3): δ = 21.3 (q, JCH
=
128 Hz, CMe), 23.3 (q, JCH = 128 Hz, CMe), 24.6 (t, JCH = 124 Hz, Data were collected with a Bruker SMART CCD diffractometer
cyclohexane-1,2-diyl-CH2), 25.66 (q, JCH = 127 Hz, CMe), 25.74 with graphite-monochromated Mo-Kα radiation with a radiation
(q, JCH = 127 Hz, CMe), 26.5 (t, JCH = 127 Hz, cyclohexane-1,2- wavelength of 0.71073 Å. Structural determinations were made by
diyl-CH2), 32.8 (t, JCH = 132 Hz, cyclohexane-1,2-diyl-CH2), 33.6 using the SHELXTL package of programs.[37] A SADABS absorp-
(t, JCH = 130 Hz, cyclohexane-1,2-diyl-CH2), 63.28 (d, JCH
=
tion correction was made.[38] All refinements were carried out by
135 Hz, cyclohexane-1,2-diyl-CH-N), 63.30 (d, JCH = 135 Hz, cy- full-matrix least-squares and by using anisotropic displacement pa-
Eur. J. Inorg. Chem. 2008, 3000–3008
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3007

J. Huang, C.-H. Hu et al.
FULL PAPER
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rameters for all non-hydrogen atoms. All the hydrogen atoms were
placed with the use of a riding model. The crystal data are summa-
rized in Table 1.
CCDC-665882 (cis-1), -665881 (trans-1), -665879 (2), -665875 (3),
-665880 (4), -665876 (5), -665877 (6), and -665878 (7) contain the
supplementary crystallographic data for this paper. These data can
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Theoretical Computations: The gradient-corrected hybrid density
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Table 3. Reaction energies computed by using the B3LYP/6-
31G*level of theory.
Reactions
∆E [kcal/mol]
trans-1 + AlMe3
2 + AlMe3
2
cis-1 + AlMe3
cis-2 + AlMe3
cis-2
Ǟ
Ǟ
Ǟ
Ǟ
Ǟ
Ǟ
2 + CH4
–53.4
–51.7
–28.3
–44.4
–48.7
–42.1
3 + CH4
M. McCann, S. Townsend, M. Devereux, V. McKee, B. Walker,
Polyhedron 2001, 20, 2799–2806.
5 + CH4
cis-2 + CH4
cis-3 + CH4
4 + CH4
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Received: January 17, 2008
Published Online: May 30, 2008
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