Structure investigations of dichloroaluminum benzoates: An unprecedented example of a monomeric aluminum complex with a chelating carboxylate ligand

Article abstract of DOI:10.1021/ic9016307

Dichloroaluminum benzoate and its adducts with Lewis bases show a large structural variety from molecular complexes to ionic species as indicated by X-ray diffraction, spectroscopic studies, and quantum-chemical calculations.

Full text of DOI:10.1021/ic9016307

10892 Inorg. Chem. 2009, 48, 10892–10894
DOI: 10.1021/ic9016307
Structure Investigations of Dichloroaluminum Benzoates: An Unprecedented
Example of a Monomeric Aluminum Complex with a Chelating Carboxylate Ligand
,†
Zbigniew Florjanczyk,* Wojciech Bury,† Ewa Zygadzo-Monikowska,† Iwona Justyniak,‡ Robert Balawender,‡
ꢀ
,†,‡
ꢀ
and Janusz Lewinski*
†Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland, and
‡Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Received August 14, 2009
Dichloroaluminum benzoate and its adducts with Lewis bases show
a large structural variety from molecular complexes to ionic species
as indicated by X-ray diffraction, spectroscopic studies, and
quantum-chemical calculations.
Barron suggested that a carboxylate-bridged structure II is
preferred and that the chelate mode III is unavailable for
carboxylates on aluminum because of the ring strain asso-
ciated with the AlO2C cycle, and this assumption was
supported by theoretical calculations.5 Worthy of note is
also a number of crystallographically characterized alkyla-
luminum compounds derived from bifunctional carboxylic
acids exhibiting a large structural variation.2c,7
Although the carboxylate anion is one of the most abun-
dant ligands in the coordination chemistry, the chemistry of
aluminum carboxylates is largely an unexplored area. There
have been many attempts to prepare discrete molecular
aluminum carboxylate complexes with well-defined struc-
tures. In part, this is in view of the importance of such single
molecular species as useful precursors in materials science,1
structural models for functional materials based on alumi-
num carboxylates,2 as well as valuable models for elucidation
of the preferred coordination mode of a carboxylate ligand to
the metal center.3 Nevertheless, there is a relative paucity of
structural data for simple aluminum-carboxylate systems.
The first crystallographic evidence for organoaluminum
carboxylate species was provided by Atwood and co-workers
in their structural determinations of the [MeCO2(AlMe3)2]-
anion with the monodentate carboxylate ligand I (Scheme
1).4 More recently, the structural characterization of a
series of di-tert-butylaluminum carboxylates5 and dimethyl-
aluminum derivative of 2,4,6-triphenylbenzoic acid6 con-
firmed dimeric, carboxylate-bridged structures of [R2Al(μ-
O2CR0)]2-type compounds in the solid state. Simultaneously,
All the mentionedeffortshave focused on the synthesis and
characterization of alkylaluminum species. Surprisingly, re-
lated aluminum halogen complexes have not been explored.
Our interest in chloroaluminum carboxylates stems from the
potential use of these derivatives as an anion trap in compo-
site polymer electrolytes applied in lithium or lithium ion
batteries.8 Herein, we report on the solid state and solution
structure investigations of dichloroaluminum benzoate and
its adducts with 4-methylpyridine (py-Me), and provide the
first structurally authenticated aluminum complex with a
chelating carboxylate ligand. The addition of 1 molar equiv.
of MeAlCl2 to benzoic acid in CH2Cl2 results in the quanti-
tative formation of dichloroaluminum benzoate which was
isolated by crystallization from the postreaction solution as
the dimeric adduct [Cl2Al(μ-O2CPh)]2 (1). The molecular
structure of 1 consists of two Cl2Al units bridged by benzoate
groups (Figure 1). The Al-O bond lengths (Al-Oavg
=
˚
1.766 A) are slightly shorter than that in the aluminum alkyl
5
˚
analogues characterized previously (cf. Al-Oavg=1.810 A),
reflecting the different Lewis acidity of the X2Al species
involved. A salient feature of the structure of 1 is the almost
flat central eight-membered Al2O4C2 ring in contrast to
the chairlike conformation of [tBu2Al(μ-O2CPh)]2.5 Barron
*To whom correspondence should be addressed. E-mail: evala@
ch.pw.edu.pl (Z.F.); lewin@ch.pw.edu.pl (J.L.).
(1) Yamane, H.; Kimura, Y. Polymeric Materials Encyclopedia; Sala-
mone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 1, p 202 and references
therein.
(2) See for example: (a) Koide, Y.; Barron, A. R. Organometallics 1995,
14, 4026. (b) Landry, C. C.; Pappe, N.; Mason, M. R.; Apblett, A. W.; Tyler, A. N.;
ꢀ
(7) (a) Lewinski, J.; Zachara, J.; Justyniak, I. Organometallics 1997, 16,
ꢀ
3859. (b) Branch, C. S.; Lewinski, J.; Justyniak, I.; Bott, S. G.; Lipkowski, J.;
Barron, A. R. J. Chem. Soc., Dalton Trans. 2001, 1253. (c) Redshaw, C. C.;
ꢀ
Elsegood, R. J. Chem. Commun. 2001, 2016. (d) Lewinski, J.; Zachara, J.;
ꢀ
MacInnes, A. N.; Barron, A. R. J. Mater. Chem. 1995, 5, 331. (c) Lewinski, J.;
Justyniak, I.; Tratkiewicz, E. Organometallics 2003, 22, 4151. (e) Redshaw, C.
C.; Elsegood, R. J.; Holmes, K. E. Angew. Chem., Int. Ed. 2005, 44, 1850. (f)
Bury, W.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed. 2006, 45, 2872.
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(3) Lewinski, J.; Zachara, J.; Justyniak, I. Inorg. Chem. 1998, 37, 2575.
ꢀ
Ziemkowska, W.; Cyranski, M.; Kunicki, A. Inorg. Chem. 2009, 48, 7006.
(4) Atwood, J. L.; Hunter, W. E.; Crissinger, K. D. J. Organomet. Chem.
ꢀ
(8) (a) Wieczorek, W.; Stevens, J. R.; Florjanczyk, Z. Solid State Ionics
1977, 127, 403.
ꢀ
1996, 85, 67. (b) Wieczorek, W.; Zalewska, A.; Raducha, D.; Florjanczyk, Z.;
(5) Bethley, C. E.; Aitken, C. L.; Harlan, C. J.; Koide, Y.; Bott, S. G.;
Barron, A. R. Organometallics 1997, 16, 329.
Stevens, J. R. J. Phys. Chem. B 1998, 102, 352. (c) Zygadzo-Monikowska, E.;
ꢀ
(6) Dickie, D. A.; Jennings, M. C.; Jenkins, H. A.; Clyburne, J. A. C. J.
Organomet. Chem. 2004, 689, 2186.
Florjanczyk, Z.; Tomaszewska, A.; Pawlicka, M.; Langwald, N.; Kovarsky, R.;
Mazor, H.; Golodnitsky, D.; Peled, E. Electrochim. Acta 2007, 53, 1481.
r
pubs.acs.org/IC
Published on Web 10/30/2009
2009 American Chemical Society

Communication
Inorganic Chemistry, Vol. 48, No. 23, 2009 10893
Scheme 2
of 1 with 2 equiv of py-Me in thf-d8 (i.e., the Al:py-Me molar
ratio equal to 1:1) by NMR and IR spectroscopies reveals
quantitative formation of tentative ionic species 2 of as yet
unidentified structure (Scheme 2, path a). For instance, the
treatment of 1 by py-Me leads to disappearance of the 27Al
resonances due to 1 and to the appearance of a sharp signal at
104 ppm (ω1/2 = 105 Hz) characteristic of the [AlCl4]- anion
in addition to resonance at 0 ppm (ω1/2 = 320 Hz) associated
with a six-coordinate cationic species (Figure 2b).10 Surpris-
ingly, there was a lack of a resonance indicative of the
formation of a five-coordinate molecular adduct [Cl2Al(λ2-
O2CPh)(py-Me)] (20). The IR spectrum of 2 in solution
exhibits a relatively complex pattern in the range of the
Figure 1. Molecular structure of 1 with thermal ellipsoids set at a
40% probability level.
ν
asymm(CO2) with a strong band at 1649 cm-1 and two
weaker bands at 1631 and 1579 cm-1 (only one band is
observed for νsymm(CO2) at 1441 cm-1). These data indicate a
nontrivial structure for 2 in a solution in which various
coordination modes of the benzoate ligand are likely in-
volved. An effort was taken to confirm the structure of 2 by
X-ray crystallography. However, contrary to expectations,
crystals of the unprecedented six-coordinate molecular ad-
duct [Cl2Al(λ2-O2CPh)(py-Me)2] (3) were obtained.
Figure 2. 27Al NMR spectra of: (a) 1 in CD2Cl2, (b, c) 1 with added 2 or
4 equiv. of py-Me in thf-d8, respectively.
Scheme 1
In fact, 3 was isolated essentially quantitatively from the
reaction of 1 with 4 equiv of py-Me (Scheme 2, path b).
The spectroscopic data for 3 are in accordance with the
proposed formulation, which is confirmed by a single-crystal
X-ray diffraction study. The IR spectrum of 3 in Nujol
shows bands at 1627 cm-1 [νasymm(CO2)] and 1451 cm-1
[νsymm(CO2)] consistent with the monomeric chelate struc-
ture. The 27Al NMR spectrum of 3 in thf-d8 revealed the
presence of a single strong resonance at 25 ppm (ω1/2=165
Hz) consistent with a six-coordinate aluminum center,10
along with significantly lower intensity resonances corre-
sponding to 2 (Figure 2c), which indicates that the molecular
adduct 3 is in equilibrium with the ionic form 2 in a solution.
It is interesting to note that dichloroaluminum benzoate and
its reactions with py-Me exhibit a number of features that are
consistent with those observed previously for dichloroalumi-
num acetylacetonate complex, [Cl2Al(acac)],11 i.e., both
compounds form labile molecular species which readily
equilibrate with ionic species in solution,11a and form ionic
species in the presence of Lewis bases11b,c or even molecular
adducts, cf. [Cl2Al(acac)(py-Me)2].11b
observed for a series of [tBu2Al(μ-O2CR)]2 complexes a
correlation between the extent of the puckering of the
Al2O4C2 ring with the steric bulk of the carboxylate alkyl
substituent.5 The observed planar conformation of the cen-
tral ring in 1, i.e., the complex with reduced steric bulk on the
metal centers in comparison to that in [tBu2Al(μ-O2CPh)]2,
indicate that electronic factors may significantly affects con-
formation of the Al2O4C2 cyclic core. The similar IR spectra
of 1 as a Nujol mull and a CH2Cl2 solution with bands at 1602
and 1564 cm-1 [νasymm(CO2)],and 1454 and 1437 cm-1
[νsymm(CO2)] (see Figures S1 and S2) suggested the retention
of the dimeric form in solution. Indeed, the 27Al NMR
spectrum of 1 in noncoordinating solvent (CD2Cl2) is domi-
nated by the single resonance at 77 ppm9 consistent with the
four-coordinate dimeric structure along with a number of
lower intensity resonances in the higher and lower field
(Figure 2a). The presence of the latter signals indicate that
the dimeric form of 1 is unstable in solution and rearrange to
a number of likely neutral and ionic aluminum species with
four- and six-coordinate aluminum centers.10
The molecular structure of 3 consists of a discrete mono-
nuclear unit with the aluminum atom in a distorted octahe-
dral configuration (Figure 3). Strikingly, the carboxylate
group acts as a symmetrical chelating ligand (O-Al-O =
66.4(1)° and O-C-O = 115.8(3)°). The pyridine ligands are
mutually trans (N-Al-N = 174.1(1)°) with coplanar aro-
matic rings, and the Cl1-Al-Cl2 angle of 101.41(5)°). Both
We were also curious as to how the addition of py-Me as a
strong Lewis base might impact on the structure of chlor-
oaluminum carboxylate species. Monitoring of the reaction
˚
˚
the Al-O (1.975(1) A) and the Al-Cl (2.232(1) A) distances
(9) This chemical shift is similar to that observed for the chemically closely
related four-coordinate [Cl2Al(acac)] complex exhibiting the 27Al resonances
at 88 ppm (see refs 10 and 11a).
ꢀ
ꢀ
(11) (a) Lewinski, J.; Pasynkiewicz, S.; Lipkowski, J. Inorg. Chim. Acta
(10) Lewinski, J. Encyclopedia of Spectroscopy and Spectrometry; Lindon,
ꢀ
1990, 178, 113. (b) Lewinski, J.; Pasynkiewicz, S. Inorg. Chim. Acta 1988, 143,
J. C., Tranter, G. E., Holmes, J. L., Eds.; Academic Press: New York, 1999; Vol. 1,
pp 691-703.
ꢀ
39. (c) Lewinski, J.; Pasynkiewicz, S. Inorg. Chim. Acta 1987, 142, 23.

ꢀ
Florjanczyk et al.
10894 Inorganic Chemistry, Vol. 48, No. 23, 2009
Figure 5. DFT calculated geometries of compounds 3, 30, and 300.
Figure 3. Molecular structure of 3 with thermal ellipsoids set at a
40% probability level.
To provide a fuller basis for discussion of the relative
stability of dichloroaluminum carboxylate adducts with
pyridine-type ligands and preferences for monodentate or
chelating carboxylate density functional theory (DFT) cal-
culations were carried out using Gaussian 03 program
package.12 The DFT calculations for model acetate com-
pounds were obtained at B3LYP/cc-pVTZ level of theory
(see Figure S6 in the Supporting Information).12 These
calculations on the corresponding systems show that stability
in these systems can be readily understood in terms of the
relative energetics of the reactants and products. Model
calculations of the reaction of 1 with 2 equiv. of pyridine
(py) (ΔEZPC = -22.7 kJ/mol, Scheme 2 path a and Scheme
S1a in the Supporting Information) demonstrated that this
process is far less exothermic than that of the reaction of 1
with 4 equiv. of py with overall energy gains of ΔEZPC
=
-92.4 kJ/mol (Scheme 2, path b and Scheme S1c in the
Supporting Information). These calculations are in agree-
ment with experimental data and confirm that the formation
of the putative five-coordinate molecular adduct 20 or the
ionic species 2 is less favorable than the formation of the
thermodynamically stable six-coordinate molecular complex
3. The determined total energies for 3 and its cis isomer 30
shows that the former complex is strongly favored (Figure 5).
The comparison of relative energies for 3 and the five-
coordinate adduct 300 featuring a monodentate carboxylate
ligand indicates that the chelating mode for a carboxylate
ligand is stabilized by 13.2 kJ/mol.
In conclusion, the study has revealed a complex nature of
chloroaluminum benzoates featuring both molecular and
ionic forms with different coordination numbers of the metal
center and various coordination modes of the carboxylate
ligand.
Figure 4. Crystal structure of 3: (a) complementary C-Har O (red),
3 3 3
C-Har Cl(green), andC-Haliph π (blue) interactionsinrectangular
3 3 3
3 3 3
meshes, (b) space-filling model of the crystal lattice viewed along the
b axis.
are slightly longer than those observed for the four-coordi-
nate dimer 1, and the axial Al-N distances are 2.057(3) and
˚
2.065(3) A. Compound 3 represents the first monomeric
aluminum carboxylate to be structurally characterized as
well as the first observation of a chelating carboxylate ligand
in an aluminum complex.
Acknowledgment. This work was supported by the
Ministry of Science and Higher Education (Grant PBZ-
KBN-118/T09/2004/04) and the Interdisciplinary Center
for Mathematical and Computational Modeling.
Interestingly, analysis of the crystal structure of 3 revealed
that the monomeric units self-assemble via C-Haliph...π and
C-Har...Cl interactions to produce a 2D grid structure with
Supporting Information Available: Details of syntheses, crys-
tal data, and computational data (PDF and CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
˚
rectangular meshes of dimensions 7.4 ꢀ 9.2 A (Figure 4 and
Figure S5 in the Supporting Information). These 2D grids are
further organized by comple-mentary C-Har O interac-
3 3 3
tions into 3D network featuring open channels directed along
(12) For full details of the theoretical calculations (with additional
B3LYP/6-311þG(d,p) calculations) see the Supporting Information.
the b axis (Figure 4b).