Electronic and steric factors affecting the formation of four or five-coordinated aluminum complexes: Syntheses and crystal structures of some aluminum alkoxides

Article abstract of DOI:10.1016/S0022-328X(98)00968-1

The reaction of 2-phenoxyethanol with AlX₃ affords a five-coordinated dimeric product [(μ-OCH₂CH₂OPh)AlX₂]₂ (1, X=Cl; 2, X=Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Buⁱ)₃ yields a four-coordinated dimeric product [(μ-O(CH₂)₂OPh)Al(Buⁱ)₂] ₂ (3). A mixture, [(μ-O(CH₂)₂OPh)Al(Et)_0.75(Cl) _1.25]₂ (4) is obtained from the reaction of 2-phenoxyethanol with one equivalent of Et₂AlCl. The reaction of 2-methoxybenzyl alcohol with AlR₃ gives a semi-pentacoordinated dimeric product [(μ-OCH₂C₆H₄OMe)AlR₂]₂ (5, R=Me; 6, X=Buⁱ) in high yields. Crystal structure studies of these compounds reveal that the distance between Al and the phenoxy oxygen or the methoxy oxygen is 3>6>5?4>1~2. Although both the electronic and steric effect influence the nature of the O,O′-bifunctional ligand chelating on the aluminum center to generate a five- or four-coordinated aluminum, the electronic effect is considered to play a major role. The interaction of aluminum and ethereal oxygen can be monitored by ¹³C-NMR spectroscopic studies.

Full text of DOI:10.1016/S0022-328X(98)00968-1

Journal of Organometallic Chemistry 575 (1999) 67–75
Electronic and steric factors affecting the formation of four or
five-coordinated aluminum complexes: syntheses and crystal structures
of some aluminum alkoxides
Chia-Her Lin a, Bao-Tsan Ko a, Fa-Cherng Wang a, Chu-Chieh Lin a,*, Chen-Yuan Kuo b
a Department of Chemistry, National Chung-Hsing Uni6ersity, Taichung, Taiwan 402, ROC
b Department of Chemical Engineering, Yung-Ta Industrial Commercial College, Pingtung, Taiwan 909, ROC
Received 6 August 1998; received in revised form 8 September 1998
Abstract
The reaction of 2-phenoxyethanol with AlX3 affords a five-coordinated dimeric product [(v-OCH2CH2OPh)AlX2]2 (1, X=Cl;
2, X=Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Bui)3 yields a four-coordinated dimeric product
[(v-O(CH2)2OPh)Al(Bui)2]2 (3). A mixture, [(v-O(CH2)2OPh)Al(Et)0.75(Cl)1.25 2
] (4) is obtained from the reaction of 2-phe-
noxyethanol with one equivalent of Et2AlCl. The reaction of 2-methoxybenzyl alcohol with AlR3 gives a semi-pentacoordinated
dimeric product [(v-OCH2C6H4OMe)AlR2]2 (5, R=Me; 6, X=Bui) in high yields. Crystal structure studies of these compounds
reveal that the distance between Al and the phenoxy oxygen or the methoxy oxygen is 3\6\5ꢀ4\1ꢁ2. Although both the
electronic and steric effect influence the nature of the O,O%-bifunctional ligand chelating on the aluminum center to generate a five-
or four-coordinated aluminum, the electronic effect is considered to play a major role. The interaction of aluminum and ethereal
oxygen can be monitored by 13C-NMR spectroscopic studies. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Aluminum; Ether alcohol; Bifunctional; 2-Phenoxyethanol; 2-Methoxybenzyl alcohol
1. Introduction
plexes with O,O%-chelate ligands have been reported by
the Benn [4], Oliver [5], Schumann [6], Lewinski [7] and
Barron [8] groups, respectively. However, structurally
characterized dimeric four-coordinated aluminum com-
plexes with O,O%-bifunctional ligands are rare. Al-
though [Et2Al{O(CH2)2O(CH)2OCH2CH3}]2 has been
characterized as a four-coordinated complex using 27Al-
NMR spectroscopic studies, however, no crystal struc-
ture has been reported [9]. Most recently, we found that
the reaction of 2-phenoxyethanol with AlMe3 affords a
four-coordinated dimeric compound [10]. In the mean-
time, Lewinski pointed out that the rearrangement of
aluminum complexes in solution is determined by the
electronic effect of the ligand [7]. Due to the potential
utility of aluminum chelate complexes, the formation of
four-coordinate complex led us to further study this
system in order to determine the factors influencing the
coordination numbers on the Al center. In this paper,
Aluminum alkoxides are interesting because of their
regioselective and stereoselective catalytic activities in
organic synthesis [1] and their practical uses as precur-
sors for ceramic materials [2]. Organoaluminum com-
plexes coordinated with O,O%-bifunctional ligands are
potentially important due to the fluxionalities of the
oxygen–aluminum dative bond. Typically, when
organoaluminum derivatives react with O,O%-bifunc-
tional ligands, five-coordinated dimeric aluminum com-
plexes are obtained. The first structurally characterized
example is [(v-OC(CH3)N(Ph)CO(Ph))AlMe2]2 in
which the carbonyl group coordinated to the Al center
through an oxygen atom [3]. Recently, many more
pentacoordinated or tetracoordinated aluminum com-
* Corresponding author. Tel.: +886-4-2840412, ext. 718.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00968-1

68
C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
we report the preparation and crystal structures of
several four- and five-coordinated aluminum com-
plexes. Factors that influence the coordination sphere
of aluminum complexes will also be discussed.
yield a white powder. The white residue was extracted
with CH2Cl2 (40 ml). The extract was dried under
vacuum to give a white powder. Yield: 1.24 g (96%).
Found: C, 29.48, H, 2.65%. Anal. Calc. for
C16H18O4Al2Br4: C, 29.66; H, 2.80%. 1H-NMR
(CDCl3): l 7.32–7.53 (m, 5H, Ph), 4.56 (t, 2H, JH–H
=
2. Experimental section
6.0 Hz, CH2), 4.28 (t, 2H, JH–H=6.0 Hz, CH2). 13C-
NMR (CDCl3): l 154.39, 129.89, 126.73, 121.02 (Ph),
74.12 ( CH2OPh), 59.03 (–CH2OAl). M.p.: 130–132°C
(dec.).
2.1. Reagents and general techniques
All manipulations were carried out under dry nitro-
gen atmosphere. Solvents were dried by refluxing at
least 24 h over sodium/benzophenone (toluene, hexane,
ether) or over P2O5 (CH2Cl2) and freshly distilled be-
fore use. Deuterated solvents (Aldrich) were dried over
molecular sieves. The compounds AlMe3 (2.0 M in
hexane), AlBr3 and AlCl3 were purchased from Aldrich
and used without further purification. The compounds
Et2AlCl (15% in hexane) and Al(Bui)3 (0.58 M in
hexane) were purchased from TCI and used as ob-
tained. 2-Phenoxyethanol and 2-methoxybenzyl alcohol
(Aldrich) were purchased and stored over molecular
sieves prior to use. Melting points were determined with
2.4. Preparation of [(v-OCH2CH2OPh)Al(Bui)2]2 (3)
To an ice cold (0°C) solution of 2-phenoxyethanol
(0.46 ml, 4.0 mmol) in ether (20 ml), (i-Bu)3Al (0.58 M
in hexane, 8.0 ml, 4.0 mmol) was added slowly. After
addition of all the (i-Bu)3Al, the mixture was stirred for
2 h and then was dried under vacuum to give a white
powder. The white residue was extracted with 30 ml of
toluene. The extract was concentrated to ca. 5 ml at
30°C. Colorless crystals were obtained after 24 h after
cooling to 4°C. Yield: 0.99 g (89%). Found: C, 67.83;
H, 9.39%. Anal. Calc. for C32H54Al2O4: C, 69.04; H,
1
1
a Buchi 535 digital melting point apparatus. H- and
9.78%. H-NMR (CDCl3, ppm): l 6.9–7.3 (m, 5H, Ph),
13C-NMR spectra were recorded on a Varian VXR-300
or Varian 400 spectrometer with chemical shifts given
in ppm from the internal TMS. Microanalyses were
performed using a Heraeus CHN-O-RAPID instru-
ment. IR spectra were obtained from a Bruker Equnox
55 spectrometer.
4.05–4.11 (m, 4H, CH2), 1.77–1.81 (m, 2H, CH), 0.85–
0.90 (m, 12H, (CH3)2CH), −0.07 (d, 2H, CH2Al, JH–
H=6.0 Hz). 13C-NMR (CDCl3, ppm): l 157.6, 129.3,
121.3, 114.3 (Ph), 67.7 (–CH2OPh), 61.6 (–CH2OAl),
28.4, 25.7, 21.8 (Bui). IR (KBr, cm−1): 2937.9 (s, br),
2872.1 (s, br), 1600.5 (s), 1498.5 (s), 1458.5 (s), 1239.6
(s), 1083.7 (s), 928.9 (s), 754.1 (s), 669.6 (s, br). M.p.:
95–100°C (dec.).
2.2. Preparation of [(v-OCH2CH2OPh)AlCl2]2 (1)
To a solution of 2-phenoxyethanol (0.50 ml, 5.0
mmol) in toluene (20 ml), AlCl3 (0.533 g, 4.0 mmol) in
toluene (50 ml) was added slowly at 25°C. The mixture
was stirred for 4 h and was then dried in vacuo to give
a white powder. The white residue was extracted with
CH2Cl2 (50 ml) and filtered. Volatile materials were
removed under vacuum to give a white powder. Yield:
0.79 g (84%). Found: C, 40.55, H, 3.91%. Anal. Calc.
for C16H18O4Al2Cl4: C, 40.88; H, 3.86%. 1H-NMR
2.5. The reaction of 2-phenoxyethanol with Et2AlCl
To an ice cold (0°C) solution of 2-phenoxyethanol
(0.50 ml, 5.0 mmol) in hexane (20 ml), Et2AlCl (5.8 ml,
15% in hexane, 5.0 mmol) was added slowly. The
mixture was stirred for 3 h and was then dried in vacuo
to give a white solid. The white residue was extracted
with toluene (20 ml) and then was concentrated to ca.
10 ml. Colorless crystals, 4, were obtained after 2 days
(CDCl3): l 7.26–7.44 (m, 5H, Ph), 4.53 (t, 2H, JH–H
=
at 27°C. Yield: 0.80
g (70%). Anal. Calc. for
5.6 Hz, CH2), 4.26 (t, 2H, JH–H=5.6 Hz, CH2). 13C-
NMR (CDCl3): l 154.30, 129.94, 126.89, 120.92 (Ph),
74.22 (–CH2OPh), 58.66 (–CH2OAl). IR (KBr, cm−1):
2965.2 (s, br), 2904.6 (s), 1598.8 (s), 1489.1 (s), 1455.0
(s), 1359.7 (s), 1238.0 (s), 1184.2 (s), 1084.9 (s, br).
M.p.: 141.0–142.0°C (dec.).
C9.5H12.75AlCl1.25O2: C, 49.56; H, 5.58%. Found: C,
1
49.45, H, 5.62%. H-NMR (CDCl3): 7.12–7.42 (m, 5H,
Ph), 4.16–4.40 (m, 4H, CH2), 1.02 (t, 3H, JH–H=8.4
Hz, CH3(AlEt)), 0.99 (t, 3H, JH–H=8.4, CH3(AlEt)),
−0.131 (q, 2H, JH–H=8.0 Hz, CH2(AlEt)), −0.134
(q, 2H, JH–H=8.0 Hz, CH2(AlEt)). 13C-NMR (CDCl3):
l 156.20, 155.11, 129.78, 129.64, 125.68, 124.01, 123.92,
119.50, 117.37, 117.20 (Ph), 72.15, 69.58, 69.36 (–
CH2OPh), 60.39, 60.32, 59.36 (–CH2OAl), 8.68, 8.52
(–CH3CH2Al), −0.69, −0.78 (–CH2Al). IR (KBr,
cm−1): 2956.9 (m, br), 2938.7 (m, br), 2898.1 (s), 2866.0
(s), 1592.4 (s), 1497.4 (s), 1463.8 (s), 1212.6 (s), 1082.6
(s), 923.3 (s).
2.3. Preparation of [(v-OCH2CH2OPh)AlBr2]2 (2)
To a solution of 2-phenoxyethanol (0.50 ml, 5.0
mmol) in toluene (20 ml), AlBr3 (1.28 g, 4 mmol) in
toluene (30.0 ml) was added slowly at 25°C. The mix-
ture was stirred for 4 h and was then dried in vacuo to

C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
69
2.6. Preparation of [v-OCH2C6H4OMe)AlMe2]2 (5)
3. Results and discussion
To an ice cold (0°C) solution of 2-methoxybenzyl
alcohol (0.41 ml, 3.0 mmol) in ether (20 ml), AlMe3 (1.8
ml, 2.0 M in toluene, 3.6 mmol) was added slowly. The
mixture was stirred for 2 h and was then dried in vacuo
to yield a white powder. The white residue was ex-
tracted with ether (15 ml) and filtered. The filtrate was
concentrated to about 10 ml at 27°C and cooled to
−20°C. White crystals were obtained after 12 h. Yield:
0.55 g (95%). Found: C: 61.90, H: 7.70; Anal. Calc. for
C10H15AlO2: C, 61.85; H, 7.79%. 1H-NMR (CDCl3,
ppm) l 6.86–7.33 (m, 4H, Ph), 4.65 (s, 2H, PhCH2O),
3.84 (s, 3H, OCH3), −0.96 (s, 6H, AlCH3). M.p.:
105–106°C.
3.1. Syntheses and spectroscopic studies
The reaction of 2-phenoxyethanol with AlX3 in
toluene at 25°C affords the pentacoordinated com-
plexes 1 and 2 (1: X=Cl; 2: X=Br) in a moderate to
high yield as shown in Scheme 1. However, when
2-phenoxyethanol reacts with Al(Bui)3 in diethyl ether,
a four-coordinated compound 3 is obtained as shown in
Scheme 2. The reaction of 2-methoxybenzyl alcohol
with AlR3 in diethylether furnishes semi four-coordi-
nated compounds 5 and 6 (5: R=Me; 6: R=Bui) as
illustrated in Scheme 3. Compounds 1 and 2 are insolu-
ble in hydrocarbon solvents but are only scarcely solu-
ble in methylene chloride. The others are readily soluble
in hydrocarbon solvents. 1H-NMR spectra of com-
plexes 1–3, 5, and 6 are essentially identical as ex-
pected. There is no OH resonance indicating loss of
hydroxyl proton. These compounds are stable as solids
or in solution under an inert atmosphere. However,
compounds 3, 5 and 6 are rather less stable than 1 and
2 toward moisture. In order to distinguish two different
methylene groups on [(v-PhOCH2CH2O)AlR2, nuclear
overhauser effect was observed for complex 1. It
demonstrated a H-1 (−OCH2Al, 4.12 ppm) and H-3
(Ph, 7.21 ppm) hydrogen signal enhancement upon
irradiation of H-2 (PhOCH2−, 4.26 ppm). A 2D-Het-
cor NMR spectrum study of 1 indicates that PhOCH2–
resonance at lower field and –CH2CH2OAl resonance
at higher field for both 1H- and 13C-NMR spectra.
Et2AlCl reacts with 2-phenoxyethanol in hexane to give
a mixture of 4. 1H-NMR spectroscopic studies of 4
illustrates three sets of triplets peaks for each methylene
2.7. Preparation of [(v-OCH2C6H4OMe)Al(Bui)2]2 (6)
To an ice cold (0°C) solution of 2-methoxybenzyl
alcohol (0.27 ml, 2.0 mmol) in ether (30 ml), Al(Bui)3
(4.1 ml, 0.58 M in toluene, 2.4 mmol) was added
slowly. The mixture was stirred for 2 h and was then
dried in vacuo to yield a white powder. The white
residue was extracted with ether (15 ml) and filtered.
The filtrate was concentrated to about 5 ml at 27°C and
cooled to −20°C to give white crystals after 24 h.
Yield: 0.45 g (80%). 1H-NMR (CDCl3): l 6.86–7.33 (m,
4H. Ph), 4.65 (s, 2H, PhCH2O), 3.84 (s, 3H, OCH3),
−0.96 (s, 6H, Al–CH3). 13C-NMR (CDCl3): l 157.46,
130.73, 129.93, 126.08, 120.28, 110.16 (Ph), 61.05
(PhCH2O), 55.00 (OCH3), 28.37, 25.47, 21.70 (Bui). IR
(KBr, cm−1): 2947.6 (s, br), 2886.1 (s, br), 1606.5 (s),
1494.4 (s), 1466.6 (s), 1322.3 (m), 1241.3 (s), 1204.1 (m),
1178.9 (m), 1123.7 (s), 1069.7 (s, br), 1030.8 (s, br),
1002.8 (s, br), 818.5 (m). M.p.:71–74°C
protons indicating that
4 is a mixture of [(v-
2.8. X-ray crystallographic studies
O(CH2)2OPh)AlCl2]2, and [(v-O(CH2)2OPh)Al(Et)(Cl)]2
with a ratio of 1:3. The result has been verified by the
elemental analysis and X-ray crystal structure determi-
nation. Attempts to separate these two compounds
have been unsuccessful. The observation of [(v-
O(CH2)2OPh)AlCl2]2 indicates that a small degree of
disproportionation of Et2AlCl into Et3Al and EtAlCl2
may occur in solution. EtAlCl2 then reacts with 2-phe-
noxyethanol independently to give the product [(v-
O(CH2)2OPh)AlCl2]2.
Suitable crystals of 1–6 were sealed in thin-walled
glass capillaries under nitrogen atmosphere and
mounted on a Siemens P4 diffractometer. The crystallo-
graphic data were collected using a q-2q scan mode
with Mo–Kh radiation. Cell constants were obtained by
least-squares analysis on positions of 42 randomly se-
lected reflections for 1, 33 reflections for 2, 26 reflec-
tions for 3, 42 reflections for 4, 38 reflections for 5 and
42 reflections for 6 in the 2q range of 9–30°. The space
group determination was based on a check of the Laue
symmetry and systematic absences, and was confirmed
by the structure solution. The structure was solved by
direct methods using Siemens SHELXTL PLUS package
[11]. All non-H atoms were located from successive
Fourier maps. Anisotropic thermal parameters were
used for all non-H atoms, and fixed isotropic parame-
ters were used for H atoms that were refined using a
riding model [11,12]. Crystallographic data of 1–6 are
given in Table 1.
Both cis and trans isomers (shown in Scheme 4) of
[(v-O(CH2)2OPh)Al(Et)(Cl)]2 are detected in the 1H-
NMR spectrum. However only trans-form is observed
in the X-ray crystal structure determination.
3.2. Molecular structures of 1–6
Single crystals of 1, 2 and 4 suitable for X-ray
structure determination were recrystallized from
CH2Cl2. Selected bond lengths and bond angles are
listed in Tables 2–4, respectively. ORTEP [13] diagrams

70
C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
Table 1
Crystallographic data of complex 1–6
1
2
3
4
5
6
Empirical formula
C8H9AlCl2O2
C8H9AlBr2O2
C16H27AlO2
C9.5H12.75AlCl1.25 C10H15AlO2
O2
C16H27AlO2
Formula weight
Crystal system
Space group
235.0
Orthorhombic
Pbca
324.0
Orthorhombic
Pbca
278.4
Triclinic
P1
228.6
Monoclinic
P21/c
194.2
Monoclinic
P21/n
278.4
Triclinic
P1
Unit cell dimensions
˚
a (A)
11.120(1)
7.882(1)
23.946(3)
8.082(1)
11.224(2)
24.306(3)
9.675(2)
10.050(2)
10.242(2)
91.76(2)
116.59(2)
98.91(2)
874.1(3)
2
1.058
0.71073
0.113
03.5–46.0
q-2q
12.061(3)
12.839(3)
7.641(2)
9.609(2)
7.790(2)
15.388(2)
8.666(2)
10.423(2)
11.406(2)
101.46(2)
102.59(2)
114.42(2)
865.8 (3)
2
1.068
0.71073
0.114
4.0–45.0
q-2q
˚
b (A)
˚
c (A)
h (°)
i (°)
106.08(2)
100.23(2)
k (°)
3
˚
V (A )
2098.7(4)
8
2204.7(7)
8
1136.8(6)
4
1133.5(4)
4
Z
Dcalc. (Mg m−3
)
1.488
0.71073
0.666
3.5–46.0
q-2q
1.952
0.71073
7.399
3.5–46.
q-2q
1.345
0.71073
0.443
4.0–45.0
q-2q
1.138
0.71073
0.148
4.0–48.0
q-2q
˚
u(Mo–Kh) (A)
Absorption coefficient (mm−1
)
2q range (°)
Scan type
Reflections collected
1445
1519
2549
1593
1842
2339
1092
1026
1784
1200
1409
1854
Observed reflections
(F\4.0|(F))
118
3.29
3.67
1.02
(F\4.0|(F))
118
5.98
6.00
1.65
(F\4.0|(F))
190
6.21
6.94
1.98
(F\4.0|(F))
136
4.77
5.32
1.41
(F\4.0|(F))
118
5.70
6.55
1.77
(F\4.0|(F))
172
6.71
7.96
2.56
No. of refined parameter
Ra for significant reflections (%)
b
Rw (%)
GOFc
a R=ꢀꢁ(ꢀFo–Fcꢀ)/ꢁꢀFoꢀꢀ.
b
ꢀꢁꢂw(ꢀFo–Fcꢀ)/ꢁꢂwꢀFoꢀꢀ; w=1/[|2(F)+0.001)F2 for 1–4 and 6 and w=1/[|2(F)+0.0015)F2 for 5.
c GOF, [ꢁw(ꢀFoꢀ–ꢀFcꢀ)2/(Nrf ln s–Nparams)]1/2
.
inversion and occupies an apical site of the second
Al(a) atom with a longer Al(a)–O(1) distance (0.052(2)
of 1, 2 and 4 with atomic numbering are shown in Fig.
1. Complex 1 and 2 crystallize in the centrosymmetric
orthorhombic space group Pbca (no. 61). The structure
of 1 shows dimeric feature which contains 5.4.5-fused
rings with an Al2O2 core and the phenoxy group
bonded to Al through the oxygen atom. The Al2O2 core
is undoubtedly coplanar due to its symmetry. Although
similar fused rings have been found in many cases
[5,10,14], however, the interesting feature is that the
mean plane of the five-membered ring is almost copla-
nar with the Al2O2 core in which the dihedral angle
between the five and four-membered ring is only 3.7°.
The constraints of the fused 5.4.5 ring system result in
the rather small O(1)–Al–O(2) angle of 79.1(1)°. The
coordination geometry around Al is ca. a trigonal
bipyramidal, with two chlorides and an oxygen atom,
O(1), on equatorial positions. O(1) sits over a center of
˚
A longer). The four atoms Al, Cl(1), Cl(2), and O(1) are
˚
almost coplanar with the mean deviation of 0.0098 A.
The phenoxy oxygen atom and another bridged oxygen
atom are in axial positions with the O(2)–Al–O(1a)
angle of 154.1(1)°. The distances between bridging
alkoxides and aluminum, Al–O(1) and Al–O(1a) are
˚
1.811(2) and 1.863(2) A, respectively within the normal
range for an Al2O2 core [15]. The distance between
phenoxy oxygen and aluminum, Al(1)–O(2), with
˚
2.028(2) A is much longer than the distance between
bridging alkoxide and aluminum indicating that O(2) is
apparently much more weakly bonded to aluminum
than O(1). The distance between Al and Cl with
˚
2.131(1) and 2.137(1) A, respectively is also within a
normal range for an Al–Cl bond [16]. The structure of
Scheme 1.
Scheme 2.

C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
71
Table 2
Selected bond lengths (A) and bond angles (°) of 1
˚
˚
Bond distance (A)
Al–Cl(1)
Al–O(1)
Al–AlA
O(1)–C(1)
O(2)–C(2)
2.131(1)
1.811(2)
2.901(2)
1.425(4)
1.456(4)
Al–Cl(2)
Al–O(2)
Al–O(1A)
O(1)–AlA
O(2)–C(3)
2.137(1)
2.028(2)
1.863(2)
1.862(2)
1.424(3)
Scheme 3.
Bond angle (°)
Cl(1)–Al–Cl(2)
Cl(2)–Al–O(1)
Cl(2)–Al–O(2)
Cl(1)–Al–AlA
O(2)–Al–AlA
Cl(2)–Al–O(1A)
O(2)–Al–O(1A)
Al–O(1)–AlA
Al–O(2)–C(2)
2 is essentially the same as that of 1 with the difference
only in the Br instead of Cl. The distance between the
phenoxy oxygen and aluminum, Al(1)–O(2), with
114.5(1)
126.5(1)
93.2(1)
114.9(1)
117.3(1)
97.1(1)
154.1(1)
104.3(1)
110.3(2)
Cl(1)–Al–O(1)
Cl(1)–Al–O(2)
O(1)–Al–O(2)
Cl(2)–Al–AlA
Cl(1)–Al–O(1A)
O(1)–Al–O(1A)
Al–O(1)–C(1)
C(1)–O(1)–AlA
Al–O(2)–C(3)
118.9(1)
96.4(1)
79.1(1)
116.8(1)
100.7(1)
75.7(1)
122.3(2)
133.1(2)
129.2(2)
˚
2.001(7) A is within the same range as the chloride
analogs. The Al–O distances in the Al2O2 core are
˚
asymmetric with 1.798(7) and 1.860(7) A, respectively,
which are also compatible with the chloride congener.
Al is almost coplanar with O(1)Br(1)Br(2) and it is only
˚
0.0190 A above the O(1)Br(1)Br(2) plane.
The structure of 4 is determined in the P21/c space
group. The geometry of Al also shows a distorted
trigonal bipyramidal. The ethyl group positions are
disordered with respect to chloride/ethyl occupancy,
and therefore the least-square refinement method is
carried out so that for each chloride and ethyl atom site
a single set of coordinates and temperature factors were
used, but the site occupancy is refined as a fraction of
a chlorine atom and a fraction of a ethyl atom such
that two fractions summed to unity. The value corre-
sponds to ca. 0.25 Cl and 0.75 C for Cl%/C(1). The
Al–O(2) distance of 2.253(3) is somewhat longer than
that of 1 and 2. The average bond distance of Al–O in
˚
which is about 0.7–1.0 A longer than a normal Al–O
coordinated covalent bond in a pentacoordinated O,O%-
chelate aluminum complex [17]. It is the longest dis-
tance between an aluminum and alkoxy oxygen ever
found for an organoaluminum complex with a O,O%-bi-
functional ligand. Because it is only somewhat shorter
˚
than the combined van der Waals radius (3.45 A) of Al
and O, it is considered that no or very weak interaction
exists between Al and O(2) [18]. The Al2O2 core of 3 is
asymmetric in which the Al–O(1) distance of 1.828(3)
A is 0.036 A shorter than the Al–O(1A) distance of
1.864(3) A. One of the iso-butyl groups are disordered
with two positions for C(2) and C(4), respectively. The
site occupancies were refined as a fraction of ca. 0.70/
0.30 for C(2)/C(2%) and C(4)/C(4%).
Single crystals of 5 and 6 suitable for X-ray structure
determination is recrystallized from ether at −20°C.
The atomic geometries of 5 and 6 are listed in Table 6
and Table 7, respectively. The ORTEP diagram of 5 and
˚
˚
˚
˚
the Al2O2 core of 1.845(2) A is within a normal range
for an Al2O2 ring. Due to disorder of the chloro and
ethyl groups, the average bond distances of the Al–C,
Al–Cl and the bond angle of C–Al–Cl are not reliable.
Single crystals of 3 suitable for X-ray structure deter-
mination were recrystallized from toluene at −20°C.
Complex 3 crystallizes in the triclinic space group P1
(no. 2). The atomic geometries of 3 are listed in Table
5. The ORTEP diagram of 3 with atomic numbering is
shown in Fig. 2. The coordination geometry around Al
Table 3
˚
Selected bond lengths (A) and bond angles (°) of 2
˚
is a distorted tetrahedral in which Al is 0.3184 A above
˚
the O(1)C(1)C(5) basal plane. The result is totally dif-
ferent to that observed for five-coordinated aluminum
complexes in which Al is coplanar with the basal plane.
It is interesting to note that the distance between Al
Bond lengths (A)
Al–Br(1)
Al–O(1)
Al–AlA
O(1)–C(1)
O(2)–C(2)
2.284(4)
1.801(8)
2.890(6)
1.421(14)
1.482(14)
Al–Br(2)
Al–O(2)
Al–O(1A)
O(1)–AlA
O(2)–C(3)
2.298(3)
1.999(8)
1.860(7)
1.855(8)
1.446(12)
˚
and the phenoxy oxygen atom, O(2), is 2.999(3) A
Bond angles (°)
Br(1)–Al–Br(2)
Br(2)–Al–O(1)
Br(2)–Al–O(2)
Br(1)–Al–AlA
O(2)–Al–AlA
Br(2)–Al–O(1A)
O(2)–Al–O(1A)
Al–O(1)–AlA
Al–O(2)–C(2)
113.9(1)
126.7(3)
93.6(2)
115.0(2)
117.2(3)
96.9(3)
153.9(3)
104.5(4)
110.9(6)
Br(1)–Al–O(1)
Br(1)–Al–O(2)
O(1)–Al–O(2)
Br(2)–Al–AlA
Br(1)–Al–O(1A)
O(1)–Al–O(1A)
Al–O(1)–C(1)
C(1)–O(1)–AlA
Al–O(2)–C(3)
119.3(3)
96.9(2)
79.0(3)
116.7(2)
100.5(3)
75.5(4)
123.0(6)
132.5(7)
130.8(6)
Scheme 4.

72
C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
Table 4
Table 5
Selected bond lengths (A) and bond angles (°) of 3
˚
˚
Selected bond lengths (A) and bond angles (°) of 4
˚
˚
Bond lengths (A)
Bond lengths (A)
Al–Cl
2.152(2)
1.966(20)
1.423(5)
1.424(5)
Al–O(1)
Al–O(1A)
Al–O(2)
1.804(2)
1.871(3)
2.253(3)
1.400(4)
Al–O(1)
Al–C(5)
Al–O(1A)
O(1)–AlA
O(2)–C(11)
1.828(2)
1.951(4)
1.864(2)
1.864(2)
1.368(5)
Al–C(1)
Al–AlA
O(1)–C(9)
O(2)–C(10)
1.957(6)
2.836(2)
1.442(4)
1.425(4)
Al–C(1)
O(1)–C(3)
O(2)–C(4)
O(2)–C(5)
Bond angles (°)
Cl–Al–O(1)
O(1)–Al–C(1)
O(1)–Al–O(1A)
Al–O(1)–C(3)
C(3)–O(1)–AlA
122.1(1)
115.2(8)
76.4(1)
122.9(2)
133.0(2)
Cl–Al–C(1)
121.3(7)
98.9(1)
106.1(6)
103.6(1)
116.9(11)
Bond angles (°)
O(1)–Al–C(1)
C(1)–Al–C(6)
C(6)–Al–AlA
C(1)–Al–O(1A)
Al–O(1)–C(9)
C(9)–O(1)–AlA
Al–C(6)–C(6)
Cl–Al–O(1A)
C(1)–Al–O(1A)
Al–O(1)–AlA
Al–C(1)–C(2)
109.2(2)
121.7(2)
121.1(1)
110.2(2)
126.7(2)
127.5(2)
122.6(2)
O(1)–Al–C(5)
C(1)–Al–AlA
O(1)–Al–O(1A)
C(6)–Al–O(1A)
Al–O(1)–AlA
Al–C(1)–C(2)
120.9(2)
116.0(1)
79.6(1)
106.4(1)
100.4(1)
129.1(6)
6 with atomic numbering is shown in Fig. 3. While this
work was in progress, the preparation and crystal struc-
ture of [Me2Al(v-OCH2C6H4CH2OMe)] was reported
[6], which is almost the same as the structure of 5.
Therefore, a detailed discussion of this structure will
not be presented here. However, because their prepara-
tion and crystallization conditions are quite different,
the bond distance between Al and the methoxy oxygen
is somewhat different with 2.625(3) in 5 compared to
˚
2.572(2) A as reported. Compound 6 was determined in
the triclinic P1 space group. The coordination geome-
try around Al is a distorted tetrahedral in which Al is
˚
0.2700 A above the O(1)C(1)C(5) basal plane. The
distance between Al and the methoxy oxygen atom,
˚
O(2) is 2.800(3) A which is much longer than a normal
Al–O bond in a pentacoordinated O,O%-chelate alu-
minum complexes. Therefore, it is also considered that
no or very weak interaction exists between Al and O(2).
The Al2O2 core of 6 is also asymmetric in which the
˚
Al–O(1) distance of 1.832(2) is 0.045 A shorter than the
˚
Al–O(1A) distance of 1.877(3) A. The Al–O(2) dis-
˚
tance in 5 or 6 is ca. 0.5–0.7 A longer than that of
˚
compounds 1 and 2, however it is ca. 0.7–0.9 A shorter
than the sum of the van der Waals radii for an Al–O
bond. It is considered the methoxy oxygen is semi-coor-
dinated to Al.
Fig. 1. Molecular structures of (a) [(v-OCH2CH2OPh)AlCl2]2 (1) (b)
[(v-OCH2CH2OPH)AlBr2]2
O(CH2)2OPh)Al(Et)0.75(Cl)1.25 2
(2)
(4).
and
(c)
[(v-
]
Fig. 2. Molecular structure of (v-O(CH2)2OPh)Al(Bui)2]2 (3).

C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
73
Table 6
Selected bond lengths (A) and bond angles (°) of 5
˚
˚
Bond lengths (A)
Al–O(1)
Al–C(2)
Al–O(1A)
O(1)–AlA
O(2)–C(10)
1.821(2)
1.936(3)
1.878(2)
1.878(2)
1.422(6)
Al–C(1)
Al–AlA
O(1)–C(3)
O(2)–C(9)
1.960(3)
2.868(2)
1.421(4)
1.372(4)
Bond angles (°)
O(1)–Al–C(1)
C(1)–Al–C(2)
C(2)–Al–AlA
C(1)–Al–O(1A)
Al–O(1)–C(3)
C(3)–O(1)–AlA
117.2(1)
122.9(2)
116.6(1)
104.7(1)
133.3(2)
126.3(2)
O(1)–Al–C(2)
C(1)–Al–AlA
O(1)–Al–O(1A)
C(2)–Al–O(1A)
Al–O(1)–AlA
116.6(1)
117.2(1)
78.8(1)
106.3(1)
101.2(1)
When organoaluminum derivatives react with O,O%-
bifunctional ligands, five-coordinated aluminum com-
plexes are usually obtained. Five-coordinated Al
possessing a trigonal bipyramidal geometry has been
found in many cases in which the distance between Al
˚
and alkoxy oxygen atom ranges from 1.93 to 2.27 A
[3–7,17]. For example in [(v-O(CH2)2OMe)AlMe2]2, the
methoxy group coordinates on Al with the Al–O dis-
˚
tance of 2.269 A [5]. However, the distance between
aluminum and the phenoxy oxygen is 2.999(3) in 3, and
˚
2.761(3) A in a. The distance between the phenoxy
oxygen
and
aluminum
(Al–O(2))
in
[(v-
O(CH2)2OPh)AlR2]2 is significantly longer than that in
[(v-O(CH2)2OMe)AlMe2]2 (shown in Table 8) due to
the reduced Lewis basicity of the phenoxy oxygen
compared to the methoxy oxygen resulting from the
conjugation of the oxygen lone pair with the y system
of the phenyl ring. It is interesting to note that a
five-coordinated complex is obtained when the Me or
Bui groups attaching on Al are substituted by Cl or Br.
The Al–O(2) distance decreases from 2.999(3) for 3 to
Fig. 3. Molecular structures of (a) [(v-OC6H4OMe)AlMe2]2 (5) and
(b) [(v-OC6H4OMe)Al(Bui)2]2 (6).
derivatives are better electron withdrawing groups than
Me and Bui groups with the result that AlCl2(OR) is a
stronger Lewis acid than AlMe2(OR). The average dis-
tance between Al and the bridging oxygen of the five-
coordinated complexes is also slightly shorter than that
of four-coordinated complexes. This is also consistent
with the fact that compounds 3, 5 and 6 are rather
more unstable than 1 and 2 toward moisture due to the
coodinatively unsaturation of these compound.
˚
2.028(2) for 1 and 2.010(2) A for 2, respectively. The
shortening of the Al–O(2) distance is believed to be due
to the electronic effect because chloride and bromide
Table 7
˚
Selected bond lengths (A) and bond angles (°) of 6
˚
Bond lengths (A)
Al–O(1)
Al–C(6)
Al–O(1A)
O(1)–AlA
O(2)–C(16)
1.832(2)
1.962(6)
1.877(3)
1.877(3)
1.412(7)
Al–C(1)
Al–AlA
O(1)–C(9)
O(2)–C(16)
1.968(4)
2.860(2)
1.447(4)
1.363(4)
The distance between Al and methoxy oxygen in the
pentacoordinated complex [Me2Al(v-OC6H4-2-OCH3)]2
˚
is 2.198(2) A [7]. However, when 2-methoxy benzyl
alcohol reacts with AlR3 (R=Me or Bui), only a
four-coordinated aluminum complex is obtained with
the distance between aluminum and the methoxy oxy-
Bond angles (°)
O(1)–Al–C(1)
C(1)–Al–C(6)
C(6)–Al–AlA
C(1)–Al–O(1A)
Al–O(1)–C(9)
C(9)–O(1)–AlA
Al–C(5)–C(6)
120.6(1)
121.6(2)
116.6(2)
104.7(2)
128.4(2)
126.7(2)
119.9(4)
O(1)–Al–C(6)
C(1)–Al–AlA
O(1)–Al–O(1A)
C(6)–Al–O(1A)
Al–O(1)–AlA
Al–C(1)–C(2)
111.8(1)
119.6(1)
79.6(1)
108.4(2)
100.4(1)
121.6(3)
˚
gen being 2.625(3) for 5 and 2.800(3) A for 6. The
longer Al–O distance in [Me2Al(v-OCH2C6H4-2-
OCH3)]2 than that of [Me2Al(v-OC6H4-2-OCH3)]2 indi-
cates that the methoxy group of [–OC6H4-2-OCH3] is
more basic than that of [–OCH2C6H4-2-OCH3] result-
ing from the conjugation of the alkoxide lone pair with

74
C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
Table 8
˚
Comparison of selected bond distances (A) and bond angles (°) of 1–6 and compounds reported in literature
Compound
1
2
3
4
aa,d
5
6
bb,e
1.827(3)
1.892(3)
2.269(3)
1.951(5)
–
cc,f
1.859(3)
1.952(3)
2.249(3)
1.955(5)
–
Al–O(1) (bridged)
1.811(2)
1.863(2)
2.028(2)
–
2.134(1)
–
1.798(7)
1.860(7)
2.001(7)
–
1.828(2)
1.864(2)
2.999(3)
1.954(6)
–
1.804(2)
1.871(3)
2.253(3)
1.97(2)
2.152(2)
–
1.817(3)
1.821(2)
1.878(2)
2.625(3)
1.943(3)
–
1.832(2)
1.877(3)
2.800(3)
1.965(5)
–
1.865(3)
2.761(3)
1.945(5)
–
Al–O(2) (alkoxy group)
Al–C (ave.)
Al–Cl (ave.)
Al–Br (ave.)
Al–Al(a)
–
2.292(3)
2.890(6)
–
–
–
–
–
–
2.901(2)
2.836(2)
2.955(3)
2.855(2)
2.858(2)
2.850(2)
2.924(2)
3.018(2)
Al–O(1)–Al(a) (Al2O2)
O(1)–Al–O(1a) (Al2O2)
O(1)–Al–O(2) (ring)
Coordination number
104.3 (3)
75.7(1)
79.1(2)
5
104.4(3)
75.6(3)
79.1(6)
5
100.4(1)
79.6(1)
–
4
103.6(1)
76.4(1)
76.5
101.7(1)
78.3(1)
–
4
101.2(1)
78.8(1)
–
4
100.4(1)
79.6(1)
–
4
103.1
104.7(1)
76.3
75.9
5
73.3(1)
75.2(1)
5
5
a a, [(v-O(CH2)2OPh)AlMe2]2.
b b, [(v-O(CH2)2OMe)AlMe2]2.
c c, [(v-OC6H4-2-OMe)AlEt2]2.
d Reference [10].
e Reference [4].
f Reference [5].
Table 9
The comparison of 13C-NMR spectra (in ppm)
the phenyl ring, avoiding the conjugation of the
methoxy oxygen with the phenyl ring as shown in
Scheme 5. However, a resonance occurs between the
methoxy oxygen with the y system in [–OCH2C6H4-2-
OCH3]. Therefore, the methoxy oxygen of [–OC6H4-2-
ROCH2CH2O (D) ROCH2CH2O (D) C.N.
PhOCH2CH2OH 69.13
61.34
–
5
5
4
4
1
74.22 (5.09)
74.12 (4.99)
67.59 (−1.54)
67.67 (−1.46)
58.66 (−2.88)
59.03 (−2.31)
60.46 (−0.88)
61.61 (0.27)
OCH3] is
a
better Lewis base than that of
2
[–OCH2C6H4-2OCH3].
The distance between aluminum and the methoxy
aa
3
oxygen in the iso-butyl aluminum derivative, 6, with
CH3OC6H4 (D)
C6H4CH2O (D)
–
˚
2.800(3) A is somewhat longer than that of methyl
MeOC6H4CH2OH 55.20
61.75
˚
congener, 5, with 2.625(3) A. In addition, the distance
5
6
54.88 (−0.32)
55.00 (−0.20)
60.87 (−0.88)
61.05 (−0.70)
4
4
between aluminum and the phenoxy oxygen of 3 with
˚
2.999(3)
A
is also longer than that of [(v-
˚
O(CH2)2OPh)AlMe2] with 2.761(3) A. The lengthening
of the Al–O distance emerged by changing the Me to
the Bui group, results from both more steric hindrance
a Reference [9].
dinated aluminum) emerged in ca. 1.5 ppm upfield
compared to PhOCH2OCH2OH. A similar result was
observed for the MeOC6H4CH2OH system. The 13C-
i
and better electron donor of Bu than Me group.
Though 27Al-NMR spectroscopic studies have been
widely used to determine the coordination number on
Al in solution, it is interfered by the presence of Al
impurity in NMR tubes. An interesting result is ob-
served in 13C-NMR spectroscopic studies. The chemical
shift for C-2 in 1 or 2 (four-coordinated aluminum) was
observed in ca. 5 ppm downfield compared to that in
PhOCH2OCH2OH. However, C-2 in a or 3 (four-coor-
NMR
spectra
of
PhOCH2CH2OH
and
MeOC6H4CH2OH with 1–3, 5, 6 and a [10] are com-
pared in Table 9. Therefore, 13C-NMR of the carbon
adjacent to ethereal oxygen can be used as a model to
the interaction between ethereal oxygen and aluminum.
4. Conclusion
The coordination number of the Al center is deter-
mined by both electronic and steric effects of ligand
and Al center. Though a bulky ligand attaching on Al
would keep oxygen away, the steric effect only plays as
a minor role in these cases. The electronic effect can be
attributed to the basicity of oxygen donor and the
acidity of aluminum center. 13C-NMR spectra can be
used as a model to indicate the bonding mode between
ethereal oxygen and aluminum.
Scheme 5.

C.-H. Lin et al. / Journal of Organometallic Chemistry 575 (1999) 67–75
75
[4] R. Benn, A. Rufinska, H. Lehmkuhl, E. Janssen, C. Kruger,
Angew. Chem. Int. Ed. Engl. 22 (1983) 779.
5. Supplementary materials
[5] D.G. Hendershot, M. Barber, R. Kumar, J.P. Oliver,
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[6] H. Schumann, M. Frick, B. Heymer, F. Girgsdies, J. Organomet.
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[7] (a) J. Lewinski, J. Zachara, I. Justyniak, Organometallics 16
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kiewicz, J. Organomet. Chem. 454 (1993) 5.
For 1–6, tables give full details of the crystal data,
data collection, structure solution parameters, atomic
coordinates of all atoms, bond distances, bond angles,
anisotropic thermal parameters of nonhydrogen atoms,
and isotropic thermal parameters of hydrogen atoms.
[8] M.B. Power, A.W. Apblett, S.G. Bott, J.L. Atwood, A.R. Bar-
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Acknowledgements
[10] B.-T. Ko, M.-D. Lee, C.-C. Lin, J. Chin. Chem. Soc. 44 (1997)
163.
Financial support from The National Science Coun-
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is gratefully acknowledged.
[11] G.M. Sheldrick, SHELXTL PLUS Users Manual. Revision 4.1
Nicolet XRD Corporation, Madison, Wisconsin, USA, 1991.
[12] Hydrogen atoms were ride on carbons or oxygens in their
idealized positions and held fixed with the C–H distances of 0.96
˚
and the O–H distances of 0.85 A.
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