Heterometallic aluminates: Alkali metals trapped by an aluminium aryloxide claw

Article abstract of DOI:10.1039/c4dt00952e

A series of heterometallic aluminium-alkali metal species [AlMMe₂{2,6-(MeO)₂C₆H₃O}₂]_nhave been isolated for lithium, sodium and potassium. These compounds can be generated by the reaction of [AlMe₂{2,6-(MeO)₂C₆H₃O}]₂with the metallated phenol [M{2,6-(MeO)₂C₆H₃O}]_nor through the reaction of the mixture of AlMe₃and the appropriate alkali metal alkyl base with two equivalents of 2,6-dimethoxyphenol. In the heterometallic species obtained, the {AlMe₂{2,6-(MeO)₂C₆H₃O}₂}^-moiety is observed and could be described as a claw which fixes the alkali ion by the phenoxide oxygen atoms while the methoxy groups help to stabilize their coordination sphere. All compounds have been characterized by NMR spectroscopy and X-ray diffraction methods. Catalytic studies reveal that these compounds are active in ring-opening polymerization of l-lactide.

Full text of DOI:10.1039/c4dt00952e

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Heterometallic aluminates: alkali metals trapped
by an aluminium aryloxide claw†
Cite this: Dalton Trans., 2014, 43,
14377
Mª Teresa Muñoz, Tomás Cuenca* and Marta E. G. Mosquera*
A series of heterometallic aluminium–alkali metal species [AlMMe2{2,6-(MeO)2C6H3O}2]n have been iso-
lated for lithium, sodium and potassium. These compounds can be generated by the reaction of
[AlMe2{2,6-(MeO)2C6H3O}]2 with the metallated phenol [M{2,6-(MeO)2C6H3O}]n or through the reaction
of the mixture of AlMe3 and the appropriate alkali metal alkyl base with two equivalents of 2,6-dimethoxy-
phenol. In the heterometallic species obtained, the {AlMe2{2,6-(MeO)2C6H3O}2}− moiety is observed and
could be described as a claw which fixes the alkali ion by the phenoxide oxygen atoms while the methoxy
groups help to stabilize their coordination sphere. All compounds have been characterized by NMR
spectroscopy and X-ray diffraction methods. Catalytic studies reveal that these compounds are active in
ring-opening polymerization of L-lactide.
Received 31st March 2014,
Accepted 6th June 2014
DOI: 10.1039/c4dt00952e
www.rsc.org/dalton
p-block metal, could provide valuable synergic effects, so reac-
tivities and selectivities different from the homometallic
Introduction
Aluminium organometallic compounds are well known for counterparts can be expected.13 In fact, the interest in hetero-
their remarkable structures and unique reactivity.1 These metallic “ate” species has increased exponentially since the
derivatives participate in many organic transformations such beginning of the century due to the remarkable reactivity that
as nucleophilic additions to carbonyl compounds,2 cross coup- they have shown.14 As such, these species have revolutionized
ling reactions with organic halides,3 hydro- and carboalumina- the metallating reactions, outperforming traditional single
tions to unsaturated species,4 conjugative addition to Michael metal metallating agents such as alkyl lithium compounds.15
acceptors5 and many others.6 In addition, they are active in
Although “ate” derivatives have been tested in many
numerous catalytic polymerization processes of a variety of organic transformations, studies on their ability as polymeriz-
substrates such as olefines,7 or functionalized monomers like ation catalysts are scarce.16 Hence it is of interest to evaluate
epoxides8 and cyclic esters.9 In these processes those species the activity of aluminate species in such processes, especially
containing Al–O bonds are particularly active.10
considering that aluminium is a key metal in the preparation
The reactivity of aluminium organometallics stems either of active species for catalytic polymerizations.
from the acidic character of the metal or from the carbanionic
In this context, our research is oriented to the preparation
nature of the organic group bonded to it.11 This dual Lewis of aluminium derivatives active in polymerization processes.17
character confers great versatility to the chemical behaviour of Particularly we are interested in studying derivatives with func-
these compounds. A kind of derivatives in which this multi tionalized aryloxide groups since the additional functionalities
Lewis character is particularly evident is the group 1 alumi- in the ring can influence the structure and reactivity of the
nates, where an alkali metal and the aluminium are placed in species formed.18,19 Surprisingly there are few studies of func-
close proximity.12
For these heterometallic systems the presence of two metals cheap, readily available chemicals.
tionalized phenols as ligand precursors even though they are
of different nature, one very electropositive alkali metal and a
In this study, a phenol bearing donor group in the ortho
position, 2,6-dimethoxyphenol, has been chosen as the ligand
precursor. In our previous investigations we have isolated
heterometallic ate species of different nuclearity, [AlLiMe3{2,6-
(MeO)2C6H3O}]3 and [AlLiMe2{2,6-(MeO)2C6H3O}2]2, from the
reaction of the hexametallic lithium aryloxide cage [Li{2,6-
(MeO)2C6H3O}]6 and different aluminium precursors.19
We report here the extension of our work to potassium and
sodium, using [AlMe2{2,6-(MeO)2C6H3}]2 (2) as the aluminium
precursor. These studies have allowed us to isolate the metal-
Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá,
Campus Universitario, E-28871 Alcalá de Henares, Spain.
E-mail: martaeg.mosquera@uah.es; Fax: +34-918854683; Tel: +34-918854779
†Electronic supplementary information (ESI) available: DOSY experiments for
3b and 3c, 1H NMR data for 2 and [ZpCp2Cl(OAr)], a scaterogram of Al–O dis-
tances and dihedral angles between a Al2O2 plane and a phenyl ring for (AlOAr)2
derivatives. CCDC 993198–993200. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt00952e
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108.64 (m-OAr-H), 150.34 (s, o-OAr), 151.16 (d, i-OAr). Anal.
Calcd (%) for NaC8H9O3 (176.15 g mol−1) C, 54.50; H, 5.11.
Found: C, 53.81; H, 5.05.
Synthesis of [K{2,6-(MeO)2C6H3O}]n (1c). To a solution of
benzyl potassium (0.418 g, 3.21 mmol) in 10 mL of toluene, a
solution of 2,6-dimethoxyphenol (0.50 g, 3.21 mmol) in 10 mL
of toluene was added at −78 °C. The suspension changed from
red to white colour. The mixture was stirred for 10 min at low
temperature and then allowed to reach room temperature, and
then again stirred for 3 hours, and the precipitation of a white
Fig. 1 Aluminium phenoxide claw, M = Li, Na, K.
lated phenols [M{2,6-(MeO)2C6H3}]n (M = Na (1b), K (1c)) and
the two new heterometallic ate compounds [AlMMe2{2,6-
(MeO)2C6H3O}2]n (M = Na (3b), K (3c)). Interestingly, com-
pound 3c is the first potassium–aluminium derivative with
aryloxide ligands bridging the metals. The structure of the
heterometallic derivatives 3b and 3c was determined in the
solid state and both show the structural motif {AlMe2(2,6-
(MeO)2C6H3O)2}−, analogous to the one present in the lithium
species previously described by our group.19 This fragment
acts as a kind of claw that can grasp alkali metals of very
different sizes such as lithium and potassium, without chan-
ging substantially its structural parameters (Fig. 1). The solid
state structure of the species [AlMe2{2,6-(MeO)2C6H3}]2 (2)19
previously reported by us is also described.
1
solid was observed. Yield: 54% (0.33 g, 1.73 mmol). H NMR
(C6D6): δ 3.13 (s, 6H, OCH3), 6.41 (m, 3H, OAr-H). 13C (C6D6):
δ 54.54 (s, OCH3), 104.22 (s, m-OAr-H), 109.38 (s, p-OAr-H),
150.98 (s, OAr). Anal. Calcd (%) for KC8H9O3.0.1C7H8 (201.47 g
mol−1) C, 51.87; H, 4.90. Found: C, 52.71; H, 4.45.
Synthesis of [AlNaMe2{2,6-(MeO)2C6H3O}2]2 (3b). To a mix
of 0.10 g (0.568 mmol) of 1b and 0.12 g (0.284 mmol) of
[AlMe2{2,6-(MeO)2C6H3}]2 was added 10 mL of toluene at
−78 °C. The mixture was stirred for 10 min at low temperature
and then allowed to reach room temperature, and the precipi-
tation of a white solid was observed. The suspension was
stirred for 2 hours at room temperature. The solid was dis-
solved after a brief reflux. Storage of the solution at room
temperature for a day allowed the formation of white crystals
of compound 3b. Yield: 60% (0.13 g, 0.34 mmol). 1H NMR
(C6D6): δ −0.49 (s, 3H, AlCH3), 3.40 (s, 6H, OCH3), 6.29 (d, 2H,
Experimental section
General considerations
3
3JHH = 8 Hz, m-OAr-H), 6.56 (t, 1H, JHH = 8 Hz, p-OAr-H).
13C (C6D6): δ −-10.33 (s, AlCH3), 55.72 (t, OCH3), 105.60 (t,
All manipulations were carried out under an inert atmosphere m-OAr-H), 116.33 (s, p-OAr-H), 140.44 (s, o-OAr), 150.57 (s,
of argon using standard Schlenk and glovebox techniques. All i-OAr). Anal. Calcd (%) for Na2Al2C36H48O12 (772.58 g mol−1
solvents were rigorously dried prior to use following standard C, 55.96; H, 6.26. Found: C, 55.71; H, 6.59.
)
methods. NMR spectra were recorded at 400.13 (1H) and
Synthesis of [AlKMe2{2,6-(MeO)2C6H3O}2]n (3c). A solution
100.62 (13C) MHz on a Bruker AV400. Chemical shifts (δ) are of 2,6-dimethoxyphenol (0.50 g, 3.21 mmol) in toluene (10 mL)
given in ppm using C6D6 and THF-d8 as the solvent. 1H and was added to 0.42 g (3.21 mmol) of KBz in 10 mL of toluene at
13C resonances were measured relative to solvent peaks con- −78 °C, and the mixture was stirred for 10 minutes. Then the
sidering TMS δ = 0 ppm. Elemental analyses were obtained on solution was allowed to reach room temperature. After stirring
a Perkin-Elmer Series II 2400 CHNS/O analyzer. Molecular for 2 hours, a grey solution with solids was formed. 0.68 g
weights of polymers were determined by gel permeation (1.60 mmol) of [AlMe2{2,6-(MeO)2C6H3O}]2 heated to a brief
chromatography (GPC) in a Varian HPL apparatus in THF at reflux in 20 mL of toluene was added to the suspension and
room temperature calibrated with respect to polystyrene stan- stirred for 10 minutes. When the reaction mixture reached
dards and corrected with a factor of 0.58. L-Lactide was pur- room temperature the white solid formed was dissolved by
chased from Aldrich and then purified by recrystallization heating while 25 mL of THF was added, giving a pale yellow
from toluene and sublimation under vacuum. All reagents solution. After one day of storage at room temperature, white
were commercially obtained and used without further purifi- crystals of compound 3c were observed. Yield: 52% (0.68 g,
1
cation. Benzyl sodium,20 benzyl potassium20 and [AlMe2{2,6- 1.68 mmol). H NMR (THF-d8): δ −1.10 (s, 3H, AlCH3), 3.71 (s,
19
(MeO)2C6H3}]2
methods.
were prepared according to reported 6H, OCH3), 6.38 (m, 1H, p-OAr-H), 6.45 (m, 2H, m-OAr-H).
1H NMR (C6D6): δ −0.50 (s, 3H, AlCH3), 3.35 (s, 6H, OCH3),
Synthesis of [Na{2,6-(MeO)2C6H3O}]n (1b). To a solution of 6.30 (m, 2H, m-OAr-H), 6.48 (m, 1H, p-OAr-H). 13C (C6D6):
benzyl sodium (0.37 g, 3.21 mmol) in 10 mL of toluene, a solu- δ −9.00 (s, AlCH3), 55.36 (s, OCH3), 104.17 (s, m-OAr-H), 105.64
tion of 2,6-dimethoxyphenol (0.50 g, 3.21 mmol) in 10 mL of (s, p-OAr-H), 138.10 (s, o-OAr), 151.23 (s, i-OAr). Anal. Calcd
toluene was added at −78 °C. The mixture was stirred for (%) for KAlC18H24O6 (402.46 g mol−1) C, 53.72; H, 6.01. Found:
10 min at low temperature and then allowed to reach room C, 52.93; H, 5.51.
temperature, and then again stirred for 2 hours, and the pre-
cipitation of a white solid was observed. Yield: 53% (0.30 g,
Polymerization of L-lactide
1.70 mmol). 1H NMR (C6D6): δ 3.24 (s, 6H, OCH3), 6.51 (m, 3H, In a glovebox, a Schlenk flask was charged with 26 µmol of the
OAr-H). 13C (C6D6): δ 55.76 (s, OCH3), 105.93 (s, p-OAr-H), desired complex and 26 µmol of benzyl alcohol. 2.5 mL of dry
14378 | Dalton Trans., 2014, 43, 14377–14385
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toluene was added. This solution was added to 2.6 mmol The structures were solved using the WINGX package,22 by
(373 mg) of L-lactide dissolved in 2.5 mL of dry toluene and direct methods (SHELXS-97) and refined using full-matrix
the mixture was heated in an oil bath at 125 °C. Small portions least-squares against F2 (SHELXL-97).23 All non-hydrogen
were removed with a pipette to determine the conversion by atoms were anisotropically refined. Hydrogen atoms were geo-
1H NMR. After 5 h, polymerization was quenched with hexane, metrically placed and left riding on their parent atoms. A dis-
then it was purified with dichloromethane and, finally, the ordered THF molecule per asymmetric unit of 3c is present in
polymer was precipitated with an excess of hexane. The white the unit cell; this solvent molecule was found in the difference
solid was filtrated and dried under vacuum to constant weight. Fourier map but was very disordered and it was not possible to
get a chemically sensible model for it, so the Squeeze pro-
cedure24 was used to remove its contribution to the structure
Circular dichroism (CD)
factors. Full-matrix least-squares refinements were carried out
Measurements were performed using a JASCO-715 spectro-
2
2 2
by minimizing ∑w(Fo − Fc ) with the SHELXL-97 weighting
scheme and stopped at shift/err < 0.001. The final residual
electron density maps showed no remarkable features.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-993198 [2], CCDC-993199 [3b] and
CCDC-993200 [3c·C4H8O].
polarimeter. Recorded spectra were the average of 2 scans
taken at the speed of 50 nm min−1 with a 0.125 s time
response. The sensitivity and resolution were fixed at 20 mdeg
and 0.5 nm respectively. Measurements were carried out in
1 cm path quartz cells at 25 °C. The sample was measured in
toluene at several concentrations. No CD spectrum was
obtained for any of the solutions whatever the concentration
was. Results show the absence of any chirality in solution.
Structure determination of compounds 2, 3b and 3c·C4H8O
Results and discussion
Details of the X-ray experiment, data reduction, and final struc-
ture refinement calculations are summarized in Table 1. Suit- The approach to achieve the formation of heterometallic
able single crystals of 2, 3b and 3c·C4H8O for the X-ray species may be oriented in two ways; one would imply the
diffraction study were selected. Data collection was performed initial reaction of 2,6-dimethoxyphenol with the alkali metal
at 200(2) K, with the crystals covered with perfluorinated ether alkyl base MR and then the treatment of the metallated
oil. The crystals were mounted on a Bruker-Nonius Kappa CCD phenol with the aluminium precursor [AlMe2{2,6-
single crystal diffractometer equipped with a graphite-mono- (MeO)2C6H3O}]2 (2). In the other synthetic approach the initial
chromated Mo-Kα radiation (λ = 0.71073 Å). Multiscan21 step would be to generate an alkali metal–aluminium hetero-
absorption correction procedures were applied to the data. metallic species by mixing AlMe3 and MR that would then
Table 1 Crystallographic data for 2, 3b and 3c·C4H8O
2
3b
3c·C4H8O
Formula
FW
C20H30Al2O6
420.40
C36H48Al2Na2O12
772.68
C18H24AlKO6·C4H8O
474.56
Color/habit
Colourless/block
0.38 0.25 0.21
Orthorhombic
Pbca
15.3154(12)
9.5003(8)
16.2703(17)
90
Colourless/block
0.45 0.43 0.28
Orthorhombic
P21212
13.7778(16)
14.062(3)
10.594(3)
90
Colourless/block
0.49 0.46 0.37
Monoclinic
P21/c
12.4801(13)
7.311(2)
25.671(7)
92.860(17)
2339.5(9)
4
Cryst dimensions (mm3)
Cryst Syst
Space group
a (Å)
b (Å)
c (Å)
β (°)
V (Å3)
Z
2367.3(4)
4
2052.4(8)
2
T (K)
200
1.180
0.152
896
200
1.250
0.149
816
200
1.347
0.304
1008
ρcalcd (g cm−3
)
μ (mm−1
F(000)
)
θ range (°)
3.42–26.60
36 478
2474/0.0979
2474/0/127
0.0523/0.1421
0.1009/0.1636
3.25–27.51
35 554
4697/0.0470
4697/0/236
0.0505/0.1392
0.0838/0.1577
0.0(3)
3.18–27.50
42 907
5351/0.0518
5351/0/235
0.0835/0.2146
0.1083/0.2260
No. of rflns collected
No. of indep rflns/Rint
No. of data/restraints/params
R1/wR2 (I > 2σ(I))a
R1/wR2 (all data)a
Flack parameter
GOF (on F2)a
1.036
0.255/−0.194
1.032
0.325/−0.503
1.103
0.676/−0.767
Diff. peak/hole (e Å−3
)
a R1 = ∑(||Fo| − |Fc||)/∑|Fo|; wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2; GOF = {∑[w(Fo2 − Fc2)2]/(n − p)}1/2
.
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Compounds 3 are air sensitive and should be stored under
argon, but for short periods of time remain unchanged in air.
They were characterized in solution by multinuclear NMR
spectroscopy. In the 1H NMR spectra in C6D6, [AlLiMe2{2,6-
(MeO)2C6H3O}2]2 and compounds 3 show a resonance for the
methyl groups bound to aluminium at similar values (δ
ca. −0.50), indicating an analogous electronic environment for
those methyl groups in the three compounds. The other hydro-
gen atoms in the structure appear also at similar shifts for the
three derivatives (δ 3.35–3.40 (OMe range), 6.23–6.30 (m-OAr-H
range) and 6.48–6.56 (p-OAr-H range)). These data suggest that
the methyl groups have the same acidic character and the
different alkali metals have a little effect on the shielding of
those protons. However, there is an important change in com-
parison to the precursor [AlMe2{2,6-(MeO)2C6H3O}]2 (2), in
that, the signal for the methyl group bonded to the aluminium
moves significantly to higher field values, from −0.27 ppm in
2 to −0.50 ppm in 3 signifying a more shielded environment
for the methyl groups in the aluminates.
Scheme 1 Synthesis of 1.
react with the right stoichiometric amount of 2,6-
dimethoxyphenol.
Starting with the first strategy, in order to prepare the
sodium and potassium derivatives [M(OAr)]n, 2,6-dimethoxy-
phenol was reacted with benzyl sodium and benzyl potassium,
giving the species [M{2,6-(MeO)2C6H3O}]n (M = Na (1b), K (1c))
(see Scheme 1).
Compounds 1b and 1c are air sensitive and should be
stored under argon. They were characterized by analytical and
spectroscopic methods. The formation of 1 was confirmed by
the disappearance of the signal for the alcohol group in the
1H NMR spectra, where similar resonances are observed for
1b–1c in comparison to the previously reported lithium deriva-
tive [Li{2,6-(MeO)2C6H3O}]6.19 As such, we observe one singlet
at 3.24 (1b) and 3.13 (1c) ppm for the methoxy group hydrogen
atoms, being at 3.16 ppm for the lithium derivative. The reson-
ances for the aromatic ring also appear at close shift values for
these compounds, a multiplet at 6.51 (1b) and 6.14 (1c) ppm,
being slightly different in the lithium case, which shows a
doublet at 6.48 ppm and a triplet at 6.56 ppm for meta and
para protons respectively.
The nuclearities in the solid state for compounds 3 were
determined by single-crystal X-ray diffraction studies since
appropriate crystals were isolated. Also, for comparison pur-
poses the study of the structure of the aluminium precursor
[AlMe2{2,6-(MeO)2C6H3O}]2 (2) was also performed.
As shown in Fig. 2, compound 2 is a dinuclear derivative
where the aryloxide group acts as a bridging ligand. In this
compound, the aluminium centres show a typical tetrahedral
environment, being bonded to two aryloxide ligands and two
methyl groups. There is also a non-covalent interaction
between the oxygen atoms from the MeO moieties and the alu-
minium, Al⋯OMe 2.59 Å and 2.54 Å, giving a pseudo-
octahedral environment for the metal. The central core Al2O2
and the two phenyl rings are co-planar, and this disposition is
probably affected by the presence of these Al⋯OMe inter-
actions. In fact, a CSD (Cambridge Structural Database)
search25 showed that the most frequent arrangement for
derivatives bearing the fragment (AlOAr)2 is when the central
Al2O2 plane and the phenyl rings are placed at an angle bigger
than 40°. For those cases where the angle is close to planarity,
in the vast majority of the compounds there is a substituent in
the ortho position that establishes an interaction with the
The reaction of compounds
1
and [AlMe2{2,6-
(MeO)2C6H3O}]2 (2) led to the formation of the heterometallic
derivatives 3b and 3c that were isolated as colourless crystals
(see Scheme 2). The potassium species 3c is barely soluble in
toluene so it was necessary to use a donor solvent such as THF
to dissolve it and achieve the formation of crystals.
When we perform the synthesis of the heterometallic
species using the second strategy, that is, by mixing initially
the metallic precursors AlMe3 and MR and then adding two
equivalents of 2,6-dimethoxyphenol, the corresponding hetero-
metallic compounds 3 were also generated, although the yields
are lower and the formation of other species is observed in the
NMR spectra.
Fig. 2 ORTEP plot of
2 showing thermal ellipsoid plots (30%
Scheme 2 Synthesis of 3.
probability).
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aryloxide oxygen atoms, O1 and O2, are bridging the alu-
minium and sodium atoms composing two four-membered
rings that are attached to the central Na2O2 core by sharing the
Na1–O2 edge. The Na–O distances in this central cycle are
quite different from each other (2.272(4) and 2.466(4) Å). The
disposition of the aryl groups projected above and below the
Na2O2 plane prevents the cycles from stacking26 and the three
four-membered rings associate laterally in a cradle disposition.
In 3b the sodium atoms are pentacoordinated, bonded to
three aryloxide groups and two methoxy moieties. The inter-
actions observed between the sodium and the methoxy groups
are asymmetric, Na⋯OMe 2.310 and 2.903 Å, and help to
stabilise the coordination sphere of the alkali metal. In a
similar way as in 2, the close disposition of two methoxy
groups in relation to the aluminium atoms suggests the pres-
ence of weak interactions Al1⋯O3 and Al1⋯O4 giving a hexa-
coordinated environment for the aluminium. This structure is
analogous to the one observed for the lithium counterpart
[AlLiMe2{2,6-(MeO)2C6H3O}2]2.19 However, in this case the
central rings are bigger, as expected due to the higher ionic
radius of sodium in comparison to lithium.27
Fig. 3 ORTEP plot of 3b asymmetric unit showing thermal ellipsoid
plots (30% probability).
metal directing this co-planar disposition, as in 2. For these
kinds of (AlOAr)2 derivatives a clear tendency in the Al–O dis-
tance is also observed, being shorter for those compounds
with a coplanar arrangement of the Al2O2 plane and the
phenyl rings (see ESI†).
In Fig. 3, an ORTEP view for the asymmetric unit of 3b
along with the atom-labelling schemes is shown. In this unit,
the presence of one aluminium atom and one sodium atom
bridged by two phenoxide groups is observed. The aluminium
center completes its coordination sphere with two methyl
groups. The sodium atom is bonded to
a {AlMe2{2,6-
(MeO)2C6H3O}2}− fragment that grabs the alkali ion.
This {AlMe2(2,6-(MeO)2C6H3O)2}− moiety is also present in
compound 2, although in this case it would be binding a
{Al(CH3)2}+ fragment. When comparing the bond distances
and angles in this fragment for 2 and 3b, a shortening of the
Al–O bonds lengths and an increase of the O–Al–O angle are
observed for the heterometallic derivative (Table 2). However,
the Al–C distances do not change significantly.
When a 3b asymmetric unit is grown, a tetrametallic Na2Al2
derivative is revealed (see Fig. 4a). In this structure the two
Fig. 4 (a) Ball and stick plot of compound 3b. (b) Central core.
Table 2 Selected bond lengths (Å) and angles (°) for 2, [AlLiMe2{2,6-(MeO)2C6H3O}2]2, 3b and 3ca
Bond lengths
2
[AlLi]19
3b
3c
Al1–O1
Al(1)–O(1)#1
O1–C11
C11–C12
C12–O3
1.890(2)
1.898(2)
1.368(3)
1.382(4)
1.358(3)
Al1–O1
Al1–O2
O1–C11
C11–C12
C12–O3
O2–C21
C21–C22
C22–O5
Al1–C1
1.852(1)
1.799(2)
1.359(2)
1.390(3)
1.372(3)
1.346(2)
1.395(3)
1.385(3)
1.960(3)
1.950(2)
1.798(2)
1.833(2)
1.314(4)
1.405(4)
1.372(4)
1.351(3)
1.385(4)
1.386(4)
1.970(3)
1.963(4)
1.810(3)
1.819(3)
1.335(4)
1.388(6)
1.379(5)
1.341(4)
1.385(6)
1.380(5)
1.945(5)
1.945(5)
C11–C13
C13–O2
Al1–C1
Al1–C2
1.388(4)
1.359(3)
1.956(3)
1.945(3)
Al1–C2
Angles
O1–Al1–O#1
O1–C11–C12
C11–C12–O3
O1–C11–C13
C11–C13–O2
76.25(9)
119.9(2)
113.7(2)
120.6(2)
114.0(2)
O1–Al1–O2
86.93(7)
89.06(10)
120.3(3)
114.1(3)
120.8(3)
115.0(3)
86.39(13)
119.3(3)
115.2(3)
119.2(4)
114.5(4)
O1–C11–C12
C11–C12–O3
O2–C21–C22
C21–C22–O5
118.44(19)
113.45(18)
120.80(19)
114.49(18)
a Symmetry transformations: #1 −x + 1, −y, −z + 1.
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Another noteworthy difference is that 3b crystallises in a
chiral space group and it is enantiomerically pure. Considering
that none of the reactants were chiral, the chirality must come
from the central core conformation. As depicted in Fig. 4b, the
three rings that compose the central core are arranged heli-
coidally as a dextro rotatory helix. In solution no chirality is
observed, as shown by a circular dichroism study. In order to
gain further insight into the structure of 3b in solution,
diffusion-ordered spectroscopy (DOSY) NMR experiments were
carried out. Following the methodology applied by several
groups,28 we used three reference compounds and plotted the
diffusion coefficients logarithm in relation to the logarithm of
Fig. 7 ORTEP plot of 3c showing the chain along the a-axis (thermal
ellipsoid plots 30% probability).
the molecular weights (see ESI†). The correlation points (MeO)2C6H3O}2}− nearly coplanar and with
a symmetric
towards a formulation [AlNaMe2{2,6-(MeO)2C6H3O}2] solvated coordination to the alkali metal. As such, the dihedral angles
with one solvent molecule. This result will be in agreement between the central AlO2 core and the phenyl rings are 14.32°
with the lack of chirality in solution since it indicates that the and 15.14°, the Al⋯OMe distances being 2.819 Å and 2.869 Å,
central core Na2O2 is not present when dissolved.
respectively. For the sodium species 3b, the coordination of
The structure in the solid state for compound 3c shows the the alkali metal to the aryloxide fragment distorts this arrange-
same asymmetric unit as 3b where the potassium is again ment and only one phenyl ring is coplanar with the AlO2 core
bonded to a {AlMe2{2,6-(MeO)2C6H3O}2}− fragment (Fig. 5d (distance Al⋯OMe 2.836 Å), the dihedral angle to the other
and 6d). However, in this case, there is a striking difference aromatic ring being 42.51°, which places the OMe group
due to the bigger size of potassium27 and, as shown in further apart from the aluminium (distance Al⋯OMe 3.263 Å).
Fig. 6d, the metal stacks out leaving the moiety {AlMe2{2,6- For the lithium compound this asymmetry is more evident
and the dihedral angles between the central core AlO2 and the
aromatic rings are 12.37° and 50.17°, the Al⋯OMe distances
being 2.838 Å and 3.255 Å, respectively.
In 3c the disposition of the aromatic rings’ substituents
being nearly coplanar to the central core allows the stacking of
the units, and instead of the formation of a discrete cradle
core, the units pile along the a-axis, generating a polymeric
structure where the potassium atoms act as the connecting
nexus (Fig. 7) of the {AlMe2{2,6-(MeO)2C6H3O}2}− fragments
that place themselves in alternating positions above and below
the potassium chain. The K⋯K distance is 3.6673 Å, shorter
than in potassium metal (4.54 Å) and much smaller than
double the van der Waals radius of potassium (rvdW
=
2.75 Å).29 Actually, this distance is quite short for a polymeric
potassium structure being more often observed in dinuclear
derivatives.30
As in the aluminium–lithium and aluminium–sodium
derivatives, the presence of methoxy groups in the ortho posi-
tion seems to have a decisive effect on the stabilization of the
alkali metals through the interactions between the oxygen and
these metals. As such, potassium atoms show an octacoordi-
nated arrangement with four bonds shared with phenoxide
oxygen atoms and four with methoxy groups (Fig. 8 and 9).
This behaviour is different for lithium and sodium species,
which show a pentacoordinated arrangement and can be
attributed to potassium’s bigger size. All the K–O distances in
the compound are very similar, either from the bonds to the
phenoxide groups, 2.921(4) and 2.939(4) Å, or from the
methoxy groups, 2.917(4) and 2.971(4) Å.
Fig. 5 Ball and stick model for (a) molecular structure of 2. (b–d) Asym-
metric units for [AlLiMe2{2,6-(MeO)2C6H3O}2]2, 3b and 3c respectively.
All these interactions saturate the coordination sphere of
the potassium atom and, although the crystallization took
place in the presence of a donor solvent such as THF, no co-
ordinated solvent molecules are present. The structure of this
Fig. 6 ORTEP plot showing thermal ellipsoid plots (30% probability) of
(a) molecular structure of 2. (b–d) Asymmetric units for [AlLiMe2{2,6-
(MeO)2C6H3O}2]2, 3b and 3c respectively.
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framework by the phenoxide oxygen atoms while the methoxy
groups help to stabilize the coordination sphere of the alkali
metals. These bonding features, where the Al–Ophenoxide bonds
show an anchoring nature that provides the framework for the
structure, while the alkali metals attach to it via ancillary
bonds, are in good agreement with the well-established
concept of anchoring/ancillary bonding in heterobimetallic
chemistry.32
Reactivity studies
The reactivity of the [AlMMe2{2,6-(MeO)2C6H3O}2]2 (M = Li, Na
(3b), K (3c)) compounds towards [ZpCp2Cl2] was analysed.
When the reactions for the three compounds were monitored
by 1H NMR spectroscopy, the apparition of two sets of reso-
nances is observed. One group of signals at δ: −0.27 (s, 6H),
3
3
3.38 (s, 6H), 6.24 (d, 2H, JHH = 8 Hz) and 6.65 (t, 1H, JHH
=
Fig. 8 ORTEP plot of 3c along the b-axis (thermal ellipsoid plots 30%
8 Hz) evidences the generation of derivative 2 in the reaction.
probability).
3
The other set of signals at δ: 6.72 (t, 1H, JHH = 8 Hz), 6.40 (d,
3
2H, JHH = 8 Hz), 6.19 (s, 10H) and 3.45 (s, 6H) corresponds to
the compound [ZpCp2Cl(OAr)] indicating that the substitution
of a Cl ligand for an aryloxide ligand has taken place. A clear
tendency of reactivity is observed, as such, after one day, the
reaction had evolved to a greater extent for 3c than for the
sodium and lithium counterparts (see ESI† for NMR spectra).
This behaviour indicates that the reactivity of these hetero-
metallic species [AlMMe2{2,6-(MeO)2C6H3O}2]n is highly influ-
enced by the alkali metal and the ligand coordinated to it is
transferred to the zirconium center, while the aluminum
methyl groups remain unreactive.
Fig. 9 Potassium coordination sphere in 3c.
Initial studies of the conduct of these derivatives
in polymerization processes show that they are active in the
ROP of L-lactide. We performed the experiments with a
100 : 1 monomer–initiator ratio in toluene, at 125 °C, using
BnOH as the initiator and [AlMMe2{2,6-(MeO)2C6H3O}2]2 (M =
Li, Na (3b), K (3c)) as catalysts. For comparison purposes, the
activity of compound 2 was also analyzed. In these conditions,
derivative was also analysed in solution by a diffusion-ordered
spectroscopy (DOSY) NMR experiment in thf-d8. In this
medium the results indicate the presence of the heterobi-
metallic species [AlKMe2{2,6-(MeO)2C6H3O}2].
According to a search in the CSD, heterometallic alu-
minium potassium derivatives are not very numerous; those
with a K–O–Al fragment being scarce.31 In particular, com-
pound 3c is the first potassium–aluminium derivative where
the metals are connected by aryloxide bridges.
the most active catalysts were
2
and [AlLiMe2{2,6-
(MeO)2C6H3O}2]2, with a conversion of 78% and 75% respecti-
vely. The activity of the heterometallic catalysts reduces when
moving from Li to Na and K, with a conversion of 48% for 3b
In conclusion, in the series of compounds [AlMMe2{2,6-
(MeO)2C6H3O}2]2 (M = Li, Na (3b), K (3c)) the asymmetric units
are isostructural. In them, the alkali metals are chelated by a
{AlMe2(2,6-(MeO)2C6H3O)2}− moiety. For the three compounds,
bond distances and angles within this fragment are very
similar (Al–O distances, 1.798(2)–1.852(1) Å range; C–O dis-
tances 1.314(4)–1.359(2) Å range; O–Al–O angles 86.39(13)–
89.06(10)° range) (Table 2). The main changes amongst the
three are the modification of the dihedral angles between the
phenyl groups and the AlO2 central core to accommodate
the different sizes of the alkali ion. Hence, in these structures
it can be considered that the phenoxide groups bind to
aluminium until coordinative saturation, giving the
{AlMe2(2,6-(MeO)2C6H3O)2}− moiety. This anion satisfies the
coordinative needs of the approaching alkali metal cation and
can be described as a claw which fixes the alkali metals to the
and 20% for 3c. For the most active catalysts,
2 and
[AlLiMe2{2,6-(MeO)2C6H3O}2]2, the polymers obtained show
narrow polydispersities,‡ especially when using the lithium–
aluminium derivative. Further studies on these polymerization
processes are underway.
Conclusions
A range of alkali metal [AlMMe2{2,6-(MeO)2C6H3O}2]n deriva-
tives have been isolated. In all the structures the presence
of the {AlMe2(2,6-(MeO)2C6H3O)2}− moiety is observed. This
fragment could be described as a claw which fixes the alkali
‡Compound 2: Mn = 7951, polydispersity = 1.45; [AlLiMe2{2,6-(MeO)2C6H3O}2]2:
Mn = 5653, polydispersity = 1.23.
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metals by the phenoxide oxygen atoms while the methoxy
groups help to stabilize the coordination sphere of these
metals. When moving from Li to Na and K this aryloxide frag-
ment does not change substantially the spectroscopic data or
its structural parameters, only the dihedral angles between the
central AlO2 core and the phenyl rings modify to accommodate
to the different alkali ions sizes. Preliminary polymerization
studies indicate that the aluminium lithium species
[AlLiMe2{2,6-(MeO)2C6H3O}2]2 are active in L-lactide ROP,
giving polymers with narrower polydispersities than the homo-
metallic counterpart 2.
H. Yamaguchi, T. Kanzawa and T. Hirano, Polym. Bull.,
2000, 45, 97; T. A. Zevaco, J. K. Sypien, A. Janssen,
O. Walter and E. Dinjus, J. Organomet. Chem., 2007, 692,
1963; T. A. Zevaco, J. Sypien, A. Janssen, O. Walter and
E. Dinjus, Catal. Today, 2006, 115, 151.
9 M. Normand, T. Roisnel, J.-F. Carpentier and E. Kirillov,
Chem. Commun., 2013, 49, 11692; O. Dechy-Cabaret,
B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104,
6147; N. Spassky, M. Wisniewski, C. Pluta and A. Le
Borgne, Macromol. Chem. Phys., 1996, 197, 2627;
D. Commereuc, H. Olivier-Bourbigou, V. Kruger-Tissot and
L. Saussine, J. Mol. Catal. A: Chem., 2002, 186, 215–222;
M. O. Miranda, Y. DePorre, H. Vazquez-Lima,
M. A. Johnson, D. J. Marell, C. J. Cramer and W. B. Tolman,
Inorg. Chem., 2013, 52, 13692.
Acknowledgements
Financial support from Factoría de Cristalización-Consolider-
Ingenio (CSD2006-00015) and the Universidad de Alcalá
(CCG2013/EXP-061) are gratefully acknowledged. M. T. M.
thanks the Universidad de Alcalá for a research fellowship.
The authors thank Prof. Francisco Mendicuti and Thais
Carmona for the CD measurements.
10 S. Dagorne and C. Fliedel, Top. Organomet. Chem., 2013, 41,
125; S. Milione, F. Grisi, R. Centore and A. Tuzi, Organo-
metallics, 2006, 25, 266–274; A. D. Schwarz, Z. Chu and
P. Mountford, Organometallics, 2010, 29, 1246–1260;
M. H. Chisholm, C.-C. Lin, J. C. Gallucci and B.-T. Ko,
Dalton Trans., 2003, 406; M. Bouyahyi, T. Roisnel and
J.-Fr. Carpentier, Organometallics, 2010, 29, 491;
B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, Dalton
Trans., 2001, 2215; A. Kowalski, A. Duda and S. Penczek,
Macromolecules, 1998, 31, 2114; P. Hormnirun,
E. L. Marshall, V. C. Gibson, A. J. P. White and
D. J. Williams, J. Am. Chem. Soc., 2004, 126, 2688; J. Liu,
N. Iwasa and K. Nomura, Dalton Trans., 2008, 3978;
S. Dagorne, F. Le Bideau, R. Welter, S. Bellemin-Laponnaz
and A. Maisse-François, Chem. – Eur J., 2007, 13, 3202;
C. Zhang and Z.-X. Wang, J. Organomet. Chem., 2008, 693,
3151.
Notes and references
1 T. Taguchi and H. Yanai, Al(III) Lewis acids, in Acid catalysis
in modern organic synthesis, ed. H. Yamamoto and
K. Ishihara, Wiley, Weinheim, 2008; S. Woodward and
S. Dagorne, Modern Organoaluminum Reagents, Springer,
2012; H. Naka, M. Uchiyama, Y. Matsumoto,
A. E. H. Wheatley, M. McPartlin, J. V. Morey and Y. Kondo,
J. Am. Chem. Soc., 2007, 129, 1921; H. Noth, A. Schlegel and
M. Suter, J. Organomet. Chem., 2001, 621, 231; S. Saito and 11 Y. Naganawa and K. Maruoka, Top. Organomet. Chem.,
H. Yamamoto, Chem. Commun., 1997, 1585. 2013, 41, 187.
2 J. Ternel, F. Agbossou-Niedercorn and R. M. Gauvin, Dalton 12 F. Mongin and A. Harrison-Marchand, Chem. Rev., 2013,
Trans., 2014, 43, 4530; P. von Zezschwitz, Synthesis, 2008,
1809; Z. Elkhayat, I. Safir, M. Dakir and S. Arseniyadis,
Tetrahedron: Asymmetry, 2007, 18, 1589.
113, 7563; D. J. Linton, P. Schooler and A. E. H. Wheatley,
Coord. Chem. Rev., 2001, 223, 53; R. E. Mulvey,
D. R. Amstrong, B. Conway, E. Crosbie, A. R. Kennedy and
S. D. Robertson, Inorg. Chem., 2011, 50, 12241.
3 S. Baba and E.-I. Negishi, J. Am. Chem. Soc., 1976, 98, 6729;
E.-i. Negishi and S. Baba, J. Chem. Soc., Chem. Commun., 13 R. E. Mulvey, Dalton Trans., 2013, 42, 6676; S. K. Mandal
1976, 596.
and H. W. Roesky, Acc. Chem. Res., 2010, 43, 248;
R. E. Mulvey, Organometallics, 2006, 25, 1060.
4 U. M. Dezhemilev and V. A. D’yakonov, Top. Organomet.
Chem., 2013, 41, 215; T. Miyoshi, S. Matsuya, M. Tsugawa, 14 F. Soki, J.-M. Neudorfl and B. Goldfuss, J. Organomet.
S. I. Sato, M. Ueda and O. Miyata, Org. Lett., 2013, 15, 3374.
5 O. Pamiés and M. Dieguez, Top. Organomet. Chem., 2013,
41, 277.
6 S. S. Kumar and H. W. Roesky, Dalton Trans., 2004, 3927;
P. von Zezschwitz, Synthesis, 2008, 1809; W. Uhl, Coord.
Chem., 2008, 693, 2139; W. Clegg, E. Lamb, S. T. Liddle,
R. Snaith and A. E. H. Wheatley, J. Organomet. Chem., 1999,
573, 305; R. E. Mulvey, Acc. Chem. Res., 2009, 42, 743;
H. R. Simmonds and D. S. Wright, Chem. Commun., 2012,
48, 8617.
Chem. Rev., 2008, 252, 1540; J. P. Campbell and 15 J. García-Alvarez, E. Hevia, A. R. Kennedy, J. Klett and
W. L. Gladfelter, Inorg. Chem., 1997, 36, 4094.
7 E. Y. X. Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391;
H. Sinn and W. Kaminsky, Adv. Organomet. Chem., 1980,
R. E. Mulvey, Chem. Commun., 2007, 2402; R. E. Mulvey,
F. Mongin, M. Uchiyama and Y. Kondo, Angew. Chem., Int.
Ed., 2007, 46, 3802.
18, 99; H. W. Roesky, M. G. Walawalkar and R. Murugavel, 16 X. Pan, A. Liu, L. Yao, L. Wang, J. Zhang, J. Wu, X. Zhao
Acc. Chem. Res., 2001, 34, 201.
8 M. Miyamoto, Y. Saeki, H. Maeda and Y. Kimura, J. Polym.
Sci., Part A: Polym. Chem., 1999, 37, 435; T. Kitayama,
and C.-C. Lin, Inorg. Chem. Commun., 2011, 14, 763;
L. Wang, J. Zhang, L. Yao, N. Tang and J. Wu, Inorg. Chem.
Commun., 2011, 14, 859; C. Decu, C. Casey, M. C. Case and
14384 | Dalton Trans., 2014, 43, 14377–14385
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
A. J. Shusterman, Organometallics, 2012, 31, 7849; 27 N. N. Greenwood and A. Earnshaw, Chemistry of the
M. J. Harvey, M. Proffitt, P. Wei and D. A. Atwood, Chem.
Commun., 2001, 2094; S. Singh, J. Chai, A. Pal, V. Jancik,
Elements, Pergamon, 1993, p. 86. Ionic radius (6-coordi-
nate) Li 0.76 Å, Na 1.02 Å and K 1.38 Å.
H. W. Roesky and R. Herbst-Irmer, Chem. Commun., 2007, 28 G. A. Morris, Diffusion-Ordered Spectroscopy, in Encyclopedia
4934; C. Gallegos, V. Tabernero, F. M. García-Valle,
M. E. G. Mosquera, T. Cuenca and J. Cano, Organometallics,
2013, 32, 6624.
of Magnetic Resonance, John Wiley & Sons, Ltd, UK, 2009;
D. Li, I. Keresztes, R. Hopson and P. G. Williard, Acc. Chem.
Res., 2009, 42, 270; D. R. Armstrong, P. García-Alvarez,
A. R. Kennedy, R. E. Mulvey and J. A. Parkinson, Angew.
Chem., Int. Ed., 2010, 49, 3185; S. E. Baillie, W. Clegg,
P. Garcia-Alvarez, E. Hevia, A. R. Kennedy and L. Russo,
Chem. Commun., 2011, 47, 388.
17 V. Tabernero, M. E. G. Mosquera and T. Cuenca, Organo-
metallics, 2010, 29, 3642; G. Martínez, J. Chirinos,
M. E. G. Mosquera, T. Cuenca and E. Gómez, Eur. J. Inorg.
Chem., 2010, 1522.
18 G. Martínez, S. Pedrosa, V. Tabernero, M. E. G. Mosquera 29 C. S. Barrett, Acta Crystallogr., 1956, 9, 671; A. Bondi,
and T. Cuenca, Organometallics, 2008, 27, 2300.
19 M. T. Muñoz, C. Urbaneja, M.
J. Chem. Phys., 1964, 68, 441.
Temprado, 30 A CSD search gave 17 hits for potassium polymeric struc-
M. E. G. Mosquera and T. Cuenca, Chem. Commun., 2011,
47, 11757.
tures with a K⋯K distance shorter than 3.6673 Å and
40 hits for dinuclear potassium derivatives.
20 L. Lochmann, J. Pospišil and D. Lim, Tetrahedron Lett., 31 J. A. Meese-Marktscheffel, R. Weimann, H. Schumann and
1966, 2, 257.
J. W. Gilje, Inorg. Chem., 1993, 32, 5894; W. Uhl, A. Vester,
D. Fenske and G. Baum, J. Organomet. Chem., 1994, 464,
23; A. Tapparo, S. L. Heath, P. A. Jordan, G. R. Moore and
A. K. Powell, J. Chem. Soc., Dalton Trans., 1996, 1601;
W. Uhl, R. Gerding and A. Vester, J. Organomet. Chem.,
1996, 513, 163; P. Henke, N. Trapp, C. E. Anson and
H. Schnockel, Angew. Chem., Int. Ed., 2010, 49,
3146.
21 R. H. Blessing, SORTAV, Acta Crystallogr., Sect. A: Fundam.
Crystallogr., 1995, 51, 33.
22 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
23 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
logr., 2008, 64, 112.
24 P. Van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A:
Fundam. Crystallogr., 1990, 46, 194.
25 Search on the CSD, version 5.35 update Nov 2013.
26 D. Barr, W. Clegg, R. E. Mulvey, R. Snaith and K. Wade,
J. Chem. Soc., Chem. Commun., 1986, 295; D. R. Amstrong,
D. Barr, W. Clegg, R. E. Mulvey, D. Reed, R. Snaith and
K. Wade, J. Chem. Soc., Chem. Commun., 1986, 869.
32 R. E. Mulvey, Chem. Commun., 2001, 1049; D. R. Armstrong,
E. Brammer, T. Cadenbach, E. Hevia and A. R. Kennedy,
Organometallics, 2013, 32, 480; S. E. Baillie, W. Clegg,
P. García-Álvarez, E. Hevia, A. R. Kennedy, J. Klett and
L. Russo, Organometallics, 2012, 31, 5131.
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