Synthesis and structure of aluminum 2,2′-oxobis(phenolate)s

Article abstract of DOI:10.1016/j.molstruc.2011.10.013

Two aluminum complexes with OOO-tridentate bis(phenolate) ligands have been synthesized and structurally characterized. Reactions of 2,2′- oxobis(phenol) with ^tBu₃Al and Ph₃Al produced the dimeric compounds [RAl(OC₆

Full text of DOI:10.1016/j.molstruc.2011.10.013

Journal of Molecular Structure 1006 (2011) 606–610
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and structure of aluminum 2,20-oxobis(phenolate)s
a
b
a
Wanda Ziemkowska a, , Paweł Zuk , Michał K. Cyranski , Antoni Kunicki
⇑
_
´
a Warsaw University of Technology, Faculty of Chemistry, Koszykowa 75, 00-662 Warsaw, Poland
b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2011
Received in revised form 9 October 2011
Accepted 10 October 2011
Available online 20 October 2011
Two aluminum complexes with OOO-tridentate bis(phenolate) ligands have been synthesized and struc-
turally characterized. Reactions of 2,20-oxobis(phenol) with tBu3Al and Ph3Al produced the dimeric com-
pounds [RAl(OC6H4OC6H4O)]2 [R = tBu (1), R = Ph (2)]. A single crystal X-ray analysis showed that
molecules of the compounds contained two 2,20-oxobis(phenolate) units and two five-coordinate alumi-
num atoms, each bonded to four oxygen atoms and to one carbon atom.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Aluminum
Aluminum alkoxides
Aluminum bis(phenolate)s
2,20-Oxobis(phenol)
1. Introduction
application of the bis(phenolato)bis(amine) ligand precursors,
bearing an additional nitrogen atom in the dative arm bonded to
Alkyl aluminum 2,20-methylene-bis(phenolate)s have been
extensively studied in the Ring-Opening Polymerization (ROP) of
cyclic esters and cyclohexene oxide, and organic synthesis [1].
The structure of the compounds depends on reaction conditions
and the bulkiness of the bis(phenolate) moiety. Reactions of
aluminum trialkyls with 2,20-methylene-4,6-alkylbis(phenol)s in
non-coordinative solvents lead to a formation of the dimeric com-
pounds A, whereas, the monomeric complexes B are obtained in
the presence of Lewis bases (Scheme) [2]. Recently, a bis(pheno-
late) ligand variation with an additional functionality has been
used as a platform for the preparation of new aluminum com-
plexes. Bridges of the bis(phenolate) moiety containing sulfur
and nitrogen atoms act as additional coordination sites and are
able to increase the coordination number of aluminum atoms.
The monometallic structures C with five-coordinate aluminum
atoms are favored in reactions of 2,20-tiobis(4,6-dialkylphenol)s
with trialkyl aluminum in Lewis basic solvents [3]. It was found
by Sobota et al. [4] that, in non-coordinative solvents, the alumi-
num complexes of tridentate (OSO) bis(phenolate) ligands (D) are
dimeric. Using various ratios of reagents and reaction conditions,
Okuda et al. [3b] and Munoz-Hernandez et al. [3a] obtained unu-
sual dimetallic complexes E with six- and four-coordinate alumi-
num atoms (Scheme 1).
the bridged nitrogen, resulted in the formation of a series of mono-
meric aluminum compounds G (Scheme 1). A bowl-shaped envi-
ronment around the metal center prevented complexes from
dimerization [6].
In comparison with aluminum tio- and aminobis(phenoxide)s,
gallium and indium complexes are rare [7]. Oxygen-linked bis(phe-
nolate) ligands have been not yet used in the chemistry of the
group 13 metals. The only tridentate bis(phenolato) ligands of
the (OOO)-type, used for synthesis of groups 1 and 4 metal com-
plexes, are ligands with a flexible CH2OCH2 link between the phe-
nolato functions [8]. We describe herein the synthesis and
characterization of aluminum 2,20-oxobis(phenolate)s as the com-
plexes supported by (OOO)-tridentate ligand, in which aromatic
rings are directly linked by the oxygen atom.
2. Results and discussion
Treatment of 2,20-oxobis(phenol) with one molar equivalent of
R3Al in toluene produces dimeric compounds [RAl(OOO)]2 [where
(OOO) = 2,20-oxobis(phenolate) unit, R = tBu (1), R = Ph (2)], as
shown in Scheme 2.
The compounds were characterized by NMR spectroscopy, ele-
mental analyses and melting point measurements. The molecular
structures were determined on the basis of an X-ray diffraction
study and are shown in Figs. 1 and 2. Data collection and structure
analyses are listed in Table 1. Selected interatomic distances (Å)
and angles (°) for molecules are shown in Table 2. Crystals of 1
and 2 suitable for X-ray structure determination were grown from
In contrast to the dimeric aluminum tiobis(phenoxide)s D, alu-
minum complexes with CH2N(R0)CH2-bridged bis(phenolato) li-
gands F are monomeric in non-coordinative solvents [5]. An
⇑
Corresponding author. Tel.: +48 22 2347316.
E-mail address: ziemk@ch.pw.edu.pl (W. Ziemkowska).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.10.013

W. Ziemkowska et al. / Journal of Molecular Structure 1006 (2011) 606–610
607
atom and four oxygen atoms; three of them originating from one
bis(phenotate) unit, the fourth is a Ophenoxide atom of the second
bisphenolate unit. The tBu and Ph groups are mutually cis with re-
spect to the Al2O2 core.
In a cell, crystals of 1 contain two kinds of molecules (A and B)
differing in bond lengths and angles. The angles [O(1A)–Al(1A)–
C(13A) 121.81(6)°, O(1A)–Al(A1)–O(2A) 116.93(6)°, C(13A)–
Al(1A)–O(2A) 119.26(6)° and O(3A)–Al(1A)–O(2A)0 149.77(5)°
[O(2B)–Al(1B)–O(1B) 119.79(6)°, O(2B)–Al(1B)–C(13B) 121.79(6)°,
O(1B)–Al(1B)–C(13B) 116.96(6)°, O(1B)0–Al(1B)–O(3B) 149.67(5)°]
indicate that the coordination sphere geometry of the aluminum
O
O
R
O
O
Al
R
Al
O
O
L
Al
R
A
R
B
O
O
R
Al
S
O
O
atom can be regarded as
arrangement.
a distorted trigonal bipyramidal
S
Al
In 1A molecules, the O(3A) and O(2A)0 atoms occupy the axial
positions and the O(1A), O(2A) and C(13A) atoms define the equa-
torial sites of the trigonal bipyramid (Fig. 1). In 1B molecules, the
O(1B)0 and O(3B) atoms are situated in the axial positions and
L
S
Al
O
O
R
C
D
the O(1B), O(2B) and C(13B) lie in the equatorial sites. The sums
P
O
R'
of the angles about the oxygen atoms O(3) { [O(3A)] 330.0°,
P
[O(3B)] 333.8°} are less than 360°, which indicates a steric strain
in the molecules. The Al–Ophenoxide bond lengths [Al(1A)–O(1A)
1.757(1) Å, Al(1A)–O(2A) 1.866(1) Å, Al(1B)–O(1B) 1.853(1) Å,
Al(1B)–O(2B) 1.761(1) Å] are similar to those found in other struc-
tures of five-coordinate aluminum aryloxides [3–6].
S
R
O
O
O
N
Al
R
Al
Al
S
R
O
O
In the compound 2, one molecule of toluene is present in the
formula unit as a crystalline net stabilizing factor. Similarly to 1,
each metal atom is coordinated in a distorted trigonal bipyramidal
fashion by O(3) and O(2)0 atoms in axial positions and O(1), O(2)
and C(13) atoms in equatorial positions [O(1)–Al(1)–O(2)
E
F
O
NR"2
L = Lewis base
N
Al
G
122.2(1)°,
O(1)–Al(1)–C(13)
119.0(2)°,
O(2)–Al(1)–C(13)
117.5(2)°, O(2)0–Al(1)–O(3) 151.4(1)°] (Fig. 2). The sum of the an-
R
P
R, R', R" = alkyls
gles about the oxygen atom O(3) { [O(3)] 337.6°} indicates a ste-
ric strain in the molecule. Bond lengths of Al(1)–Ophenoxide and
Al(1)–O(3) [2.157(3) Å] in 2 are similar to those of the compound 1.
The Al(1)–O(3) distances of 2.227(1) Å (1A), 2.177(1) Å (1B) and
2.157(3) Å (2) are significantly longer than those of Al–Ophenoxide
and longer than typical coordinate Al–O bonds in R3AlꢁB adducts
(where R = alkyl and phenyl groups, B = Lewis base with an oxygen
atom) [10] and in aluminum alkoxide complexes with THF, [1i,11].
Since the Al(1)–O(3) distances are much closer to the sum of their
covalent radii (1.86 Å) than to the sum of their van der Waals radii
(3.57 Å) [12], the Al(1)–O(3) distances are considered to be weak
coordinate bonds. Zevaco [1i] found, that the distances between
aluminum atoms and non-coordinative carbon atoms linking phe-
nyl rings of a ligand in the dinuclear aluminum para-chloro-bis-
phenoxide [Al–C 3.796(9) and 3.899(9) Å] (see Scheme 1,
structure A) are even longer than the sum of their van der Waals
radii (3.70 Å). This indicates, that the position of Al(1) and O(3)
atoms in 1 and 2 is not due to the arrangement of the phenyl rings
and a rigid structure of the 2,20-oxobis(phenolate) ligand, but in-
stead is forced by the bonding between two atoms. Similar coordi-
native Al–S bonds were observed in aluminum complexes with
sulfide-linked bis(phenolato) ligands [3,4].
O
O
O
O
O
N
= bis(phenolate) units
S
O
O
Scheme 1.
n-hexane and toluene solutions at ꢀ10 °C, respectively. The molec-
ular studies showed that the molecules of 1 and 2 were dimeric
and centrosymmetric. They are composed of two 2,20-oxobis(phe-
nolate) units, two five-coordinate aluminum atoms and two groups
(tBu or Ph) bonded to the aluminum atoms. The central planar
four-membered ring is composed of two aluminum atoms and
two bridging oxygen atoms of the bis(phenolate) ligands. This type
of bridging has been observed in phenolate aluminum compounds
[9]. Each of the metal atoms in 1 and 2 is bonded to one carbon
The 1H NMR spectrum of 1 in solution revealed signals of aro-
matic protons in the range of 7.69–6.71 ppm and a singlet of
(CH3)3CAl at 0.71 ppm in a 8:9 relative intensity ratio which indi-
cated one bis(phenolate) moiety per one tBu group. In the 13C
NMR spectrum, twelve signals of aromatic carbon atoms and sig-
OH
OH
O
R
O
O
O
- 4 RH
t
2
nals of Bu group carbon atoms {29.59 [(CH3)3C], 15.28 [(CH3)3C]
O
+ 2 R3Al
Al
Al
ppm} were present. The presence of 12 signals of Caromat indicated
a nonequivalence of all carbon atoms of bis(phenolate) unit. It is in
agreement with the molecular structure, however, the separated
signals of 1A and 1B solid-state structures were not observed in
the NMR spectra. The NMR spectra of 2 showed the following sig-
nals: a multiplet of the aromatic protons (7.73–681 ppm) in the 1H
NMR spectrum and sixteen signals of Caromat atoms [12 signals of
O
R
O
R = tBu (1)
R = Ph (2)
Scheme 2.

608
W. Ziemkowska et al. / Journal of Molecular Structure 1006 (2011) 606–610
Fig. 1. Molecular structure of [tBuAl(OOO)]2: (1A) (top) and (1B) (bottom). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity.
Fig. 2. Molecular structure of [PhAl(OOO)]2 (2). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and a disordered molecule of toluene are
omitted for clarity.

W. Ziemkowska et al. / Journal of Molecular Structure 1006 (2011) 606–610
609
Table 1
aluminum atoms is higher in comparison with that of typical alu-
minum bisphenoxides A and B (Scheme 1) due to the formation of
additional bonds between the oxygen atoms linking aromatic rings
in the ligands and aluminum atoms.
Crystal data and data collection parameters for 1 and 2.
1
2ꢁC7H8
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C32H34Al2O6
568.55
100(2)
0.71073
Triclinic
P ꢀ 1
C36H26Al2O6ꢁC7H8
700.66
100(2)
3. Experimental
0.71073
Monoclinic
P2(1)/n
8.962(1)
11.911(2)
17.386(2)
90
93.83(1)
90
1851.7(4)
2
3.1. Materials and instrumentation
10.0974(6)
11.0859(6)
15.0020(9)
76.813(5)
76.772(5)
64.121(6)
1455.0(2)
2
b (Å)
c (Å)
All manipulations were carried out using standard Schlenk
techniques under an inert gas atmosphere. Solvents were distilled
over a blue benzophenone–K complex. tBu3AlꢁOEt2 and Ph3AlꢁTHF
a
(°)
b (°)
[
13C NMR (CDCl3) d 138.24, 128.32, 127.66, 127.22 (Caromat), 72.25,
c
(°)
V (Å3)
25.36 (CTHF) ppm] were synthesized as described in the literature
[13,14], as well as 2,20-oxobis(phenol) [15]. 1H NMR spectra were
obtained on a Mercury-400BB spectrometer. Chemical shifts were
referenced to the residual proton signals of CDCl3 (7.26 ppm). Ele-
mental analyses of 1 and 2 were obtained on a Perkin–Elmer 2400
analyzer.
Z
Dcalc (g cmꢀ3
)
1.298
1.257
0.126
Absorption coefficient 0.143
(mmꢀ1
F (000)
Crystal size (mm)
)
600
732
0.2 ꢂ 0.2 ꢂ 0.2
0.25 ꢂ 0.10 ꢂ 0.10
H
range for data
2.66–28.74
2.85–25.00
collection (°)
Index ranges
3.2. Synthesis of [tBuAl(OOO)]2 (1)
ꢀ13 6 h 6 13,
ꢀ14 6 k 6 14,
ꢀ20 6 l 6 19
27,433
ꢀ10 6 h 6 8,
ꢀ14 6 k 6 14,
ꢀ20 6 l 6 20
13,586
t
A solution of Bu3AlꢁOEt2 (0.544 g, 2 mmol) in 10 cm3 of Et2O
Reflections collected
Independent
reflections
was injected to a stirred solution of 2,20-oxobis(phenol) (0.404 g,
2 mmol) in Et2O (15 cm3) at ꢀ76 °C. The reaction mixture was al-
lowed to warm to room temperature. After 24 h a white solid of
the pure compound 1 precipitated from the post-reaction mixture
(yield 0.505 g, 89%). At 234 °C the compound 1 undergoes the
decomposition becoming a gray solid.
7031 [Rint = 0.0395]
3255 [Rint = 0.1216]
Refinement method
Full-matrix least-squares Full-matrix least-
on F2
squares on F2
3255/0/211
Data/restraints/
parameters
7031/0/367
Goodness-of-fit on F2
Final R indices
0.646
0.939
1H NMR (CDCl3) d 7.69 (dd, JH–H = 9,9 Hz, 4JH–H = 1,8 Hz, 2H,
3
R1 = 0.0342,
wR2 = 0.0937
R1 = 0.0626,
wR2 = 0.1072
0.297 and ꢀ0.272
R1 = 0.0589,
wR2 = 0.1396
R1 = 0.1475,
wR2 = 0.1674
0.693 and ꢀ0.253
3
[I > 2r(I)]
R indices (all data)
H
aromat), 7.54 (dd, 2H, JH–H = 9,4 Hz, 4JH–H = 1,2 Hz, Haromat), 7.43
3
(dd, 2H, JH–H = 9,9 Hz, 4JH–H = 1,8 Hz, Haromat), 7.34–7.28 (m, 2H,
H
aromat), 7.16–7.00 (m, 6H, Haromat), 6.85–6.71 (m, 2H, Haromat),
Max/min of residual
electron density
0.71 (s, 18H, (CH3)3Al). 13C NMR (CDCl3) d 151.60, 147.28, 145.86,
145.40, 127.37, 126.75, 122.56, 120.73, 118.67, 118.20, 117.47,
115.78 (Caromat), 29.59 [(CH3)3C], 15.28 [(CH3)3C] ppm.
Elemental anal.: Calcd. for C32H34Al2O6: C, 67.54; H, 5.98.
Found: C, 67.20; H, 6.30 wt.%.
Table 2
Selected bond distances (Å) and angles (°) for 1 and 2.
1A 1B
2
3.3. Synthesis of [PhAl(OOO)]2 (2)
Bond length
Al–Ophenoxide
1.757(1)
1.761(1)
1.757(3)
1.839(3)
1.883(3)
2.157(3)
1.952(4)
A solution of Ph3AlꢁTHF (0.660 g, 2 mmol) in 20 cm3 of Et2O was
injected to a stirred solution of 2,20-oxobis(phenol) (0.404 g,
2 mmol) in Et2O (15 cm3) at ꢀ76 °C. The reaction mixture was al-
lowed to warm to room temperature. The solvent was removed
in vacuo from the post-reaction mixture. A white solid of the com-
pound 2 was quantitatively obtained. Mp.: >300 °C.
1H NMR (CDCl3) d 7.73–6.81 (Haromat, m) ppm. 13C NMR (CDCl3)
d 117.21, 117.62, 118.02, 118.95, 119.81, 122.82, 127.34, 127.62,
128.33, 128.68, 128.87, 137.43, 145.91, 146.12, 147.93, 151.87
(Caromat) ppm.
1.866(1)
1.884(1)
2.227(1)
1.967(2)
1.853(1)
1.890(1)
2.177(1)
1.970(2)
Al(1)–O(3)
Al(1)–C(13)
Bond angles
O(1)–Al(1)–O(2)
O(2)–Al(1)–C(13)
O(1)–Al(1)–C(13)
O(2)0–Al(1)–O(3)
C(12)–O(3)–C(6)
C(12)–O(3)–Al(1)
C(6)–O(3)–Al(1)
116.93(6)
119.26(6)
121.81(6)
149.77(5)
117.5(1)
119.79(6)
121.79(6)
116.96(6)
149.67(5)
119.0(1)
122.2(1)
117.5(2)
119.0(2)
151.4(1)
121.0(3)
110.7(2)
105.9(2)
107.96(8)
104.55(8)
106.26(9)
108.52(8)
Elemental anal.: Calcd. for C36H26Al2O6: C, 71.05; H, 4.28.
Found: C, 70.71; H, 4.49 wt.%.
Colorless crystals of 2ꢁC7H8 for X-ray diffraction studies were
obtained from a toluene solution at ꢀ10 °C.
carbon atoms of the oxobis(phenolate) unit and four signals of car-
bon atoms of the PhAl groups], 29.59 [(CH3)3C], 15.28 [(CH3)3C]
ppm in the 13C NMR spectrum, which is in accordance with the so-
lid-state structure.
3.4. X-ray crystal structure determinations
Determination of the crystal structures was performed on a
Summarizing, tri-tert-butyl- and triphenyl aluminums react
with 2,20-oxobis(phenyl) to yield novel dimeric compounds sup-
ported by a tridentate (OOO) ligand. The structural analysis
showed a strain in the molecules of 1 and 2 probably due to rigid
structure of the ligand in which aromatic rings are directly linked
by an oxygen atom. The coordination number of five-coordinate
KUMA CCD
j-axis diffractometer with graphite-monochromated
Mo K radiation. A crystal of 1 was positioned at 62.25 mm from
a
the KM4CCD camera and 2000 frames were measured at 0.6° inter-
vals with a counting time of 15 s. A crystal of 2 was positioned at
62.25 mm from the KM4CCD camera and 600 frames were mea-
sured at 1.0° intervals with a counting time of 35 s. Data reduction

610
W. Ziemkowska et al. / Journal of Molecular Structure 1006 (2011) 606–610
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Acknowledgments
We thank Warsaw University of Technology for financial sup-
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ka for her kind assistance.
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Appendix A. Supplementary material
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Trans. (1995) 177.
CCDC 805949 and 805950 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
[10] V.S.J. De Mel, J.P. Oliver, Organometallics 8 (1989) 827.
[11] W. Ziemkowska, S. Kucharski, A. Kołodziej, R. Anulewicz-Ostrowska, J.
Organomet. Chem. 689 (2004) 2930.
[12] Values are based on an analysis from Cambridge Structural Database. <http://
www.ccdc.cam.ac.uk/products/csd/radii/table.php4>.
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