A structural study of dimethylaluminum carboxylato and hydrazonato complexes

Article abstract of DOI:10.1016/j.jorganchem.2004.03.043

The reaction of trimethylaluminum with 2,4,6-triphenylbenzoic acid and the isoelectronic fluoren-9-one phenylhydrazone results in the formation of the dialkylaluminum complexes [(CH₃) ₂Al{μ-O₂CPh(Ph₃)}]₂ (4) and [Me₂AlN(C₆H₅) N=C₁₃H₈]₂ (5). The synthesis of these complexes, as well as their spectroscopic and structural characterization are reported. X-ray crystallographic studies show that the carboxylato complex 4 adopts a dimeric structure containing an 8-membered O-Al-O-C core whereas the hydrazonato complex 5 likewise is dimeric in the solid state, but exhibits a 6-membered N-Al-N heterocyclic ring.

Full text of DOI:10.1016/j.jorganchem.2004.03.043

Journal of Organometallic Chemistry 689 (2004) 2186–2191
www.elsevier.com/locate/jorganchem
A structural study of dimethylaluminum carboxylato and
hydrazonato complexes
a
a
c,2
Diane A. Dickie , Dharinee D. Choytun , Michael C. Jennings b,1, Hilary A. Jenkins
,
a,
*
Jason A.C. Clyburne
a
Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby BC, Canada V5A 1S6
b
Department of Chemistry, University of Western Ontario, 1151 Richmond Street, London ON., Canada N6A 5B7
c
Department of Chemistry, Saint Mary’s University, 5950 Inglis Street, Halifax NS, Canada B3H 3C3
Received 24 February 2004; accepted 29 March 2004
Dedicated to Professor Alan H. Cowley, University of Texas at Austin, for his many important contributions to chemistry,
chemical education, and service to the scientific community.
Abstract
The reaction of trimethylaluminum with 2,4,6-triphenylbenzoic acid and the isoelectronic fluoren-9-one phenylhydrazone results
in the formation of the dialkylaluminum complexes [(CH3)2Al{l-O2CPh(Ph3)}]2 (4) and [Me2AlN(C6H5)N@C13H8]2 (5). The
synthesis of these complexes, as well as their spectroscopic and structural characterization are reported. X-ray crystallographic
studies show that the carboxylato complex 4 adopts a dimeric structure containing an 8-membered O–Al–O–C core whereas the
hydrazonato complex 5 likewise is dimeric in the solid state, but exhibits a 6-membered N–Al–N heterocyclic ring.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Aluminum; Carboxylate ligands; Hydrazones; Group 13 elements; N ligands; O ligands
1. Introduction
chosen to study the coordination chemistry of two iso-
electronic species 2,4,6-triphenylbenzoic acid 2 and flu-
oren-9-one phenylhydrazone 3, which can also be
considered a structural isomer of an amidine in which
the positions of the imino carbon and nitrogen atoms
have been reversed (Fig. 2). To the best of our knowl-
edge, there have been no reports to date of monomeric
dialkylaluminum carboxylato complexes, however base-
stabilized monomeric dialkylaluminum hydrazonato
complexes are known [8].
Herein we report our attempts to enforce small ring
sizes or monomeric structures using in one case, steric
factors, and in the other, electronic factors. By main-
taining the steric bulk of 1 in the carboxylato ligand 2,
the coordination chemistry with aluminum should also
be maintained. In the case of the hydrazonato ligand 3,
the electron withdrawing ability of the fluorenylidene
portion will alter the electronic environment around the
nitrogen atoms and therefore also affect the coordina-
tion chemistry to aluminum.
The coordination chemistry and reactivity of alumi-
num carboxylato complexes (A) and related ligands such
as amido (B), amidinato (C), hydrazonato (D) and
triazenido (E) (Fig. 1) have been of importance recently,
particularly in the areas of olefin polymerization [1,2]
and materials science [3,4]. One of the reasons these li-
gands are of such interest is due to the variety of
bonding modes possible [5], which can affect not only
the structure but also the reactivity of the complexes.
We recently reported the syntheses of monomeric
m-terphenyl amidine 1 [6] and bis-amidine [7] aluminum
complexes. As a continuation of this work, we have
* Corresponding author. Tel.: +604-2914885; fax: +604-2913765.
E-mail addresses: mjenning@uwo.ca (M.C. Jennings), Hilary.
Jenkins@smu.ca (H.A. Jenkins), clyburne@sfu.ca (J.A.C. Clyburne).
1
Tel.: 1-519-661-2167.
Tel.: 1-902-420-5091.
2
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.03.043

D.A. Dickie et al. / Journal of Organometallic Chemistry 689 (2004) 2186–2191
2187
O
N
N
O
O
N
R
CR2
NR
C
R
C
C
R
N
N
R
R
N
N
Ar
Ar
Ar
E
D
C
A
B
Fig. 1. Isoelectronic carboxylato (A), amido (B), amidinato (C), hydrazonato (D) and triazenido (E) ligands.
R
N
N
R
O
O
N
N
R = Isopropyl or cyclohexyl
1
2
3
Fig. 2. Amidinato 1, carboxylato 2 and hydrazonato 3 ligands.
2. Experimental
evaporated slowly at room temperature to give colour-
less crystals. Additional crystals formed upon cooling
the solution to )30 °C. The solvent was removed via
cannula and the crystals were characterized as 4. Yield:
2.1. General
1
An MBraun UL-99-245 dry box and standard
Schlenk techniques on a double manifold vacuum line
were used in the manipulation of air and moisture sen-
sitive compounds. NMR spectra were recorded on a
Varian AS 500 MHz spectrometer or a Bruker AMX
600 MHz spectrometer in five millimeter quartz tubes.
1H and 13C{1H} chemical shifts are reported in parts per
million (ppm) downfield from tetramethylsilane (TMS)
and are calibrated to the residual signal of the solvent.
Infrared spectra were obtained using a Bomem MB
spectrometer with the percent transmittance values re-
ported in cmꢀ1. Melting points were measured using a
Mel-Temp apparatus and are uncorrected. Elemental
analysis were obtained by M.K. Yang on a Carlo Erba
Model 1106 CHN analyzer. UV–Vis spectra were
recorded on a Shimadzu UV-3150 spectrometer. 2,4,6-
Triphenylbenzoic acid [9] and fluoren-9-one phen-
ylhydrazone [10,11] were synthesized according to
literature procedures. All other reagents and solvents
were purchased from Aldrich and used without further
purification.
0.15 g, 64%; m.p. 265–266 °C. H NMR (CD2Cl2, 500
MHz) d 7.61 (s, 2 H, C6H2), 7.37–7.49 (m, 15H, C6H5),
)1.51 (s, 6H, AlCH3); 13C NMR (CD2Cl2, 125 MHz) d
174.7, 142.9, 141.8, 140.5, 139.7, 131.7, 129.2, 128.9,
128.5, 128.4, 128.3, 128.1, 127.4, )12.4. IR (nujol, cmꢀ1
)
1624 (vs), 1598 (vs), 1503 (s), 1418 (s), 1262 (w), 1200 (s),
1192 (s), 1030 (m), 892 (s), 805 (m), 756 (s), 698 (vs).
Elem. Anal. Calc. C54H48Al2O4: C, 79.59; H, 5.94.
Found: C, 79.44; H, 5.71%.
2.1.2. Preparation of [Me2AlN(C6H5)N@C13H8]2 (5)
Fluoren-9-one phenylhydrazone (0.22 g, 0.83 mmol)
was dissolved in ca. 10 mL anhydrous CH2Cl2. To this
solution, trimethylaluminum (2.0 M in hexane, 0.83 mL,
1.6 mmol) was added dropwise. The solvent was evap-
orated slowly at room temperature to give dark purple
crystals. Yield: 0.16 g, 59%; m.p. 207 °C (dec). 1H NMR
(CD2Cl2, 500 MHz) d 8.37 (br s, 1H), 8.21 (d, 7 Hz, 1H),
7.60 (d, J ¼ 7 Hz, 1H), 7.56 (d, J ¼ 7 Hz, 1H), 7.52 (t of
d, J ¼ 7 Hz, 1 Hz, 1H), 7.45 (t of d, J ¼ 7 Hz, 1 Hz,
1H), 7.35 (t of d, J ¼ 7 Hz, 1 Hz, 1H), 7.17 (t of d, J ¼ 7
Hz, 1 Hz, 1H), 6.81 (t, J ¼ 8 Hz, 2H), 6.59 (d, J ¼ 8 Hz,
2.1.1. Preparation of [(CH3)2Al{l-O2CPh(Ph3)}]2 (4)
2,4,6-Triphenylbenzoic acid (0.20 g, 0.57 mmol) was
suspended in 10 mL anhydrous CH2Cl2. To this solu-
tion, trimethylaluminum (2.0 M in toluene, 0.34 mL,
0.68 mmol) was added dropwise. The solvent was
2H), 6.44 (t, J ¼ 7 Hz, 1H), )0.44 (br, 6H, AlCH3); 13
C
NMR (150 MHz, CD2Cl2) d 129.8, 129.7, 128.5, 128.2,
124.7, 122.1, 121.1, 121.0, 113.9, 25.9 (quaternary car-
bon signals not observed due to low solubility). IR
(cmꢀ1) 3378 (w), 3057 (w), 2922 (m), 1602 (s), 1569 (vs),

2188
D.A. Dickie et al. / Journal of Organometallic Chemistry 689 (2004) 2186–2191
1508 (s), 1492 (s), 1447 (s), 1426 (m), 1307 (m), 1260 (vs),
1226 (s), 1195 (s), 1124 (s), 1087 (s), 1070 (m), 1016 (m),
886 (m), 779 (s), 749 (s), 726 (vs), 687 (vs). Elem. Anal.
Calc. C42H38Al2N4: C, 77.28; H, 5.87; N, 8.58. Found:
C, 77.60; H, 6.19; N, 7.92%. UV–Vis (toluene) k 550 nm
(e 1038).
3. Results and discussion
Spectroscopic evidence for dimeric dialkylaluminum
carboxylato complexes was first presented as early as
1970 [12] however it was not until recently that Barron
and co-workers [3] published the first structural char-
acterization of dialkylaluminum carboxylato complexes
as part of their study of alumoxane materials. Amidi-
nates, on the other hand, have been shown to chelate a
single aluminum centre to form monomeric complexes
[2,6,7]. In the case of amides, Lin and co-workers [13]
have shown that changing the steric properties of the
ligand can have a large effect on the coordination mode
adopted at the aluminum centre. As the carboxylate li-
gand 2 has similar steric requirements to amidine 1 [6],
and is bulkier than the carboxylates studied by Barron
and co-workers [3], we expected a monomeric complex,
possibly with a 4-membered AlO2C ring, which, to the
best of our knowledge, remain unreported for dialkyl-
aluminum complexes.
2.2. X-ray crystallography
Crystals of 4 and 5 were grown from concentrated
CH2Cl2 solutions. A colourless chip of 4 was broken
loose from a cluster of crystals and was mounted on a
glass fibre. Data for 4 were collected at room tempera-
ture (21 °C) on a Nonius j-CCD diffractometer with
COLLECT (Nonius B.V., 1998). The unit cell parame-
ters were calculated and refined from the full data set.
Crystal cell refinement and data reduction were carried
out using DENZO (Nonius B.V., 1998). The data were
scaled using SCALEPACK (Nonius B.V., 1998). The
SHELXTL-NT V6.1 (Sheldrick, G.M.) suite of programs
was used to solve the structure by direct methods.
Subsequent difference Fouriers allowed the remaining
atoms to be located. The two methyl groups were each
modeled as six half-hydrogen atoms.
A single crystal of 5 was mounted on a glass fibre.
Data for 5 were collected at )60 °C on a Seimens 1K
SMART/CCD diffractometer using Mo Ka radiation.
Lorentz and polarization corrections were applied and
data were also corrected for absorption using redundant
data and the SADABS program. Direct methods and
Fourier techniques were used to solve the crystal struc-
tures. Refinement was conducted using full-matrix least-
squares calculations and SHELX-TL PC V 5.03.
Addition of trimethylaluminum to a solution of 2,4,6-
triphenylbenzoic acid 2 in dichloromethane results in the
formation of 4 (Scheme 1). Colourless crystals were
formed upon evaporation of solvent or cooling of the
solution. The compound was characterized using 1H
and 13C NMR spectroscopy, IR spectroscopy and X-ray
1
crystallography. The H NMR spectrum shows a dis-
tinctive singlet at )1.51 ppm attributed to the aluminum
methyl groups. The aromatic protons of the flanking
phenyls appear as a multiplet ranging from 7.37–7.49
ppm, while the two protons of the central phenyl appear
as a singlet at 7.61 ppm. In the infrared spectrum, we
observed a diagnostic shift of m(C@O) from 1696 cmꢀ1
in the free ligand to 1624 cmꢀ1 in the complex, as well as
the disappearance of m(O–H) at 3438 cmꢀ1 in the free
ligand.
X-ray crystallographic studies (Table 1) revealed that
4 exists as a symmetrical dimer in the solid state (Fig. 3)
with an 8-membered O–Al–O–C heterocyclic core in
which the benzoate ligands bridge two aluminum cen-
All of the non-hydrogen atoms for 4 and 5 were refined
with anisotropic thermal parameters. The hydrogen atom
positions were calculated geometrically and were in-
cluded as riding on their respective carbon atoms. The
3
ꢀ
largest residual electron density peak for 4 (0.173 e/A )
was associated with one of the phenyl rings. The largest
3
ꢀ
residual electron density peak for 5 (0.223 e/A ) was as-
sociated with Al(2). There was one molecule of methylene
chloride per dimer in the lattice of 5.
ꢀ
tres. Complex 4 has an Alꢁ ꢁ ꢁAl distance of 4.320 A,
ꢀ
which falls in the middle of the range (4.217–4.463 A) of
HO
O
Ph
Ph
Ph
Ph
Ph
O
O
Al
Al
O
O
CH2Cl2
-CH4
Ph
Ph
Al
Ph
Ph
2
4
Scheme 1. Synthesis of 4.

D.A. Dickie et al. / Journal of Organometallic Chemistry 689 (2004) 2186–2191
2189
Table 1
Crystallographic data
Compound
4
5
Empirical formula
Formula weight (g/mol)
Temperature (K)
Crystal system
C54H46Al2O4
812.87
C21:50H20AlClN2
368.83
294(2)
P1
213(2)
Pca2(1)
Space group
ꢀ
Triclinic
9.9702(2)
Orthorhombic
17.560(2)
a (A)
ꢀ
b (A)
10.6813(2)
11.3952(2)
10.1087(12)
21.888(3)
ꢀ
c (A)
a (°)
b (°)
c (°)
77.0130(10)
85.6730(10)
90
90
74.0890(10)
1137.06(4)
90
3885.3(8)
3
ꢀ
Volume (A )
Z
1
1.187
8
1.261
Density (calculated) (Mg/m3)
l (mmꢀ1
F (000)
)
0.109
428
0.248
1544
Crystal size (mm3)
0.58 ꢃ 0.48 ꢃ 0.15
2.79 to 27.50
0.25 ꢃ 0.3 ꢃ 0.3
Theta range for data collection (°)
Index ranges
1.86 to 26.50
ꢀ14 ꢄ h ꢄ 22; ꢀ12 ꢄ k ꢄ 12; ꢀ27 ꢄ l ꢄ 27
21576
ꢀ12 ꢄ h ꢄ 12; ꢀ13 ꢄ k ꢄ 13; ꢀ14 ꢄ l ꢄ 14
20440
Reflections collected
Independent reflections
Absorption correction
Refinement method
5208 [RðintÞ ¼ 0:041]
8008 [RðintÞ ¼ 0:0330]
Integration
None
Full-matrix least-squares on F 2
5208/0/277
1.038
Full-matrix least-squares on F 2
8008/1/460
1.019
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I > 2rðIÞ]
R indices (all data)
R1 ¼ 0:0433; wR2 ¼ 0:1128
R1 ¼ 0:0411; wR2 ¼ 0:0896
R1 ¼ 0:0596; wR2 ¼ 0:1231
R1 ¼ 0:0602; wR2 ¼ 0:0937
ꢀ
Fig. 3. ORTEP diagram of aluminum carboxylato complex 4. Hydrogen atoms have been removed for clarity. Selected bond lengths (A):
Al(4)–O(1) ¼ 1.8095(11), O(1)–C(6) ¼ 1.2482(18), C(6)–O(5) ¼ 1.2502(17), O(5)–Al(4a) ¼ 1.8090(11), Al(4)ꢁ ꢁ ꢁAl(4a) ¼ 4.320. Selected bond angles (°):
Al(4)–O(1)–C(6) ¼ 152.97(10), O(1)–C(6)–O(5) ¼ 124.20(13), C(6)–O(5)–Al(4a) ¼ 152.79(10), O(5a)–Al(4)–O(1) ¼ 107.89(6).
previously characterized aluminum carboxylato dimers
[3]. The C(6)–O(1) and C(6)–O(5) bond lengths of
ꢀ
1.248(2) and 1.250(2) A, respectively, are the same
within experimental error and show that there is com-
plete delocalization within the O–C–O fragment. The
Al(4)–O(1) and Al(4)–O(5) bond lengths of 1.810(1) and
The O–Al–O–C ring of 4 is nearly flat, with a maxi-
ꢀ
mum deviation from the heterocyclic plane of ꢂ0.066 A
for O(5). Due to the steric constraints imposed by the
ortho-phenyls of the ligand, the heterocyclic core of 4 is
twisted out of the plane of the central phenyl with an
O(1)–C(6)–C(7)–C(12) torsion angle of 61.7°. A similar
effect, with a twist angle of 42.6°, was reported by
Barron and co-workers [14] in a gallium–carboxylato
complex based on ortho-toluic acid. No unusual intra-
or intermolecular p-interactions were observed.
ꢀ
1.809(1) A support this conclusion, and are comparable
to literature values for similar compounds [3]. The alu-
minum atoms themselves exhibit the expected tetrahe-
dral geometry, with O–Al–C bond angles ranging from
105.26(8)–107.89(6)° and the C(3)–Al(4)–C(2) bond
angle of 123.8(1)°.
Although they are not used as commonly as the pre-
viously discussed carboxylato ligands, the coordination

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D.A. Dickie et al. / Journal of Organometallic Chemistry 689 (2004) 2186–2191
Ph
HN
Ph
N
Al
N
toluene
-CH4
N
N
Al
Al
N
Ph
3
5
Scheme 2. Synthesis of 5.
chemistry of hydrazone derivatives also promised to be
very interesting. Studies performed by Uhl et al. [4] on
related hydrazines have shown that varying the sterics of
the ligand can lead to the formation of 6-membered
Al2N4, 5-membered Al2N3, or 4-membered Al2N2 het-
erocycles, while Cowley and co-workers [15] described a
ladder-like tetranuclear structure for a 1,1-dimethylhy-
drazine aluminum complex. Studies by Barron’s group
on isoelectronic triazenides have shown that the preferred
coordination geometry is chelation a single four- [16] or
six-coordinate aluminum centre with the number of
triazenido ligands controlled by the presence or absence
of other strong Lewis bases in solution [17,18]. Thus, we
were eager to examine the coordination chemistry of our
hydrazonato ligand 3.
Fig. 4. ORTEP diagram of 5. Hydrogen atoms and solvent have been
ꢀ
Treatment of fluoren-9-one phenylhydrazone 3 with
trimethylaluminum (Scheme 2) results in the immediate
formation of an intense purple solution, with a UV–Vis
absorption at kmax ¼ 550 nm. Dark purple crystals
formed upon slow evaporation of the dichloromethane
removed for clarity. Selected bond lengths (A): Al(1)–N(3) ¼ 1.889(2),
Al(1)–N(2) ¼ 2.036(2), Al(2)–N(1) ¼ 1.899(2), Al(2)–N(4) ¼ 2.043(2),
N(1)–N(2) ¼ 1.445(2), N(3)–N(4) ¼ 1.449(2), Al(1)ꢁ ꢁ ꢁAl(2) ¼ 3.477.
Selected bond angles (°): N(3)–Al(1)–N(2) ¼ 99.30(8), N(1)–Al(2)–
N(4) ¼ 99.08(9), N(2)–N(1)–Al(2) ¼ 108.93(13), N(1)–N(2)–Al(1) ¼
108.37(13), N(4)–N(3)–Al(1) ¼ 110.44(13), N(3)–N(4)–Al(2) ¼ 108.40.
1
solution. Complex 5 was characterized by H and 13C
NMR spectroscopy, IR spectroscopy and X-ray crys-
1
tallography. The H NMR spectrum showed a broad
singlet at )0.44 ppm that was assigned to the methyl
protons bound to the aluminum. Similarly, in the 13C
NMR spectrum, a singlet at 25.9 ppm was identified as
the aluminum methyl carbons.
Analysis of the solid state structure of 5 (Fig. 4) re-
vealed that the aluminum complex exists as a dimer.
Also included in the crystal lattice is a molecule of
dichloromethane. The N–Al bond lengths in 5 vary,
suggesting that the nitrogen attached to the phenyl
forms a covalent bond with the aluminum (N(1)–Al(2)
ꢀ
ꢀ
¼ 1.899(2) A, N(3)–Al(1) ¼ 1.889(2) A), while the other
ꢀ
makes a dative bond (N(2)–Al(1) ¼ 2.036(2) A, N(4)–
Fig. 5. Stick diagram of the twist-boat conformation of the core of 5
and the phenyl substituents.
ꢀ
Al(2) ¼ 2.043(2) A). As is expected in four-coordinate
aluminum, the N–Al covalent bond lengths are slightly
ꢀ
longer than the normal N–Al bond length of 1.78(2) A
while the N–Al dative bond lengths are well within the
range of other N–Al dative bonds in four-coordinate
ꢀ
aluminum complexes (1.94–2.10 A) [19].
The N–Al–N heterocyclic core of 5 adopts a twist-
boat conformation. This conformation was also seen in
Uhl’s dialkylaluminum hydrazone [Me2AlN(CHMe2)-
NCMe2]2 [20]. Chair conformations are found for other
hydrazide derivatives [21,22]. The conformation is likely
determined by steric factors. The two phenyl substitu-
ents in 5 are stacked in an offset face-to-face fashion
ꢀ
with a centroidꢁ ꢁ ꢁcentroid distance of 3.873 A. Typical
ꢀ
p–p stacking distances are in the range of 3.3–3.6 A [23],
so it is unlikely that this contributes to the stabilization

D.A. Dickie et al. / Journal of Organometallic Chemistry 689 (2004) 2186–2191
2191
of the molecule and the alignment is probably coinci-
dental. Similarly, no unusual intermolecular p-interac-
tions were observed (see Fig. 5).
In summary, the synthesis of two dimeric aluminum
complexes was accomplished. The carboxylato complex
4 is one of very few structurally characterized molecules
featuring an 8-membered O–Al–O–C heterocyclic core.
The hydrazonato complex 5 features a 6-membered
Al2N4 heterocycle and expands the use of hydrazones as
ligands in organometallic chemistry.
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Acknowledgements
Funding for this research was provided by the Nat-
ural Sciences and Engineering Research Council of
Canada (NSERC), Simon Fraser University, University
of Western Ontario and Saint Mary’s University. The
authors would like to thank Marcy Tracey (SFU) for the
collection of NMR data.
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