Steric Effects on the Structures, Reactivity, and Coordination Chemistry of Tris(2-pyridyl)aluminates

Article abstract of DOI:10.1002/chem.201502156

Introducing substituents in the 6-position of the 2-pyridyl rings of tris(pyridyl)aluminate anions, of the type [EtAl(2-py′)₃]^- (py′=a substituted 2-pyridyl group), has a large impact on their metal coordination characteristics. This is seen most remarkably in the desolvation of the THF solvate [EtAl(6-Me-2-py)₃Li·THF] to give the monomer [EtAl(6-Me-2-py)₃Li] (1), containing a pyramidal, three-coordinate Li⁺ cation. Similar monomeric complexes are observed for [EtAl(6-CF₃-2-py)₃Li] (2) and [EtAl(6-Br-2-py)₃Li] (3), which contain CF₃ and Br substituents (R). This steric influence can be exploited in the synthesis of a new class of terminal Al-OH complexes, as is seen in the controlled hydrolysis of 2 and 3 to give [EtAl(OH)(6-R-2-py)₂]^- anions, as in the dimer [EtAl(OH)(6-Br-2-py)₂Li]₂ (5). Attempts to deprotonate the Al-OH group of 5 using Et₂Zn led only to the formation of the zincate complex [LiZn(6-Br-py)₃]₂ (6), while reactions of the 6-Br substituted 3 and the unsubstituted complex [EtAl(2-py)₃Li] with MeOH give [EtAl(OMe)(6-Br-2-py)₂Li]₂ (7) and [EtAl(OMe)(2-py)₂Li]₂ (8), respectively, having similar dimeric arrangements to 5. The combined studies presented provide key synthetic methods for the functionalization and elaboration of tris(pyridyl)aluminate ligands.

Full text of DOI:10.1002/chem.201502156

DOI: 10.1002/chem.201502156
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&
Aluminate Anions
Steric Effects on the Structures, Reactivity, and Coordination
Chemistry of Tris(2-pyridyl)aluminates
Raffll García-Rodríguez* and Dominic S. Wright[a]
Abstract: Introducing substituents in the 6-position of the
2-pyridyl rings of tris(pyridyl)aluminate anions, of the type
[EtAl(2-py’)3]À (py’=a substituted 2-pyridyl group), has
a large impact on their metal coordination characteristics.
This is seen most remarkably in the desolvation of the THF
solvate [EtAl(6-Me-2-py)3Li·THF] to give the monomer
[EtAl(6-Me-2-py)3Li] (1), containing a pyramidal, three-coordi-
nate Li+ cation. Similar monomeric complexes are observed
for [EtAl(6-CF3-2-py)3Li] (2) and [EtAl(6-Br-2-py)3Li] (3), which
contain CF3 and Br substituents (R). This steric influence can
OH complexes, as is seen in the controlled hydrolysis of 2
and 3 to give [EtAl(OH)(6-R-2-py)2]À anions, as in the dimer
[EtAl(OH)(6-Br-2-py)2Li]2 (5). Attempts to deprotonate the AlÀ
OH group of 5 using Et2Zn led only to the formation of the
zincate complex [LiZn(6-Br-py)3]2 (6), while reactions of the
6-Br substituted 3 and the unsubstituted complex [EtAl(2-
py)3Li] with MeOH give [EtAl(OMe)(6-Br-2-py)2Li]2 (7) and
[EtAl(OMe)(2-py)2Li]2 (8), respectively, having similar dimeric
arrangements to 5. The combined studies presented provide
key synthetic methods for the functionalization and elabora-
be exploited in the synthesis of a new class of terminal AlÀ tion of tris(pyridyl)aluminate ligands.
py’)3]À, which are unusual in this area in that they are nega-
tively charged instead of neutral (B; Figure 1).[3a] As one of the
only anionic members of the tris(pyridyl) family,[5] the alumi-
nates are particularly suitable for the coordination of metal cat-
ions. Indeed, aluminate ligands of this type have extensive co-
ordination chemistry with a range of main-group and transi-
tion-metal ions.[6] The coordination of a tris(pyridyl) ligand to
another metal provides a facile route to heterometallic com-
plexes, such as the sandwich compound [{MeAl(2-py)3}2Fe],
which is a highly selective styrene epoxidation catalyst in air.[7]
So far, studies of the coordination chemistry of tris(2-pyridyl)
aluminate anions have focused almost exclusively on arrange-
ments based on the unsubstituted 2-pyridyl ligand.[3a] We have
found recently that the introduction of methyl groups at the
6-position of the 2-pyridyl rings has a large effect on the coor-
dination character of the ligand.[8] For example, the [EtAl(6-Me-
2-Py)3]À ligand is particularly useful in the steric stabilization of
unusual metal oxidation states such as in [{EtAl(6-Me-2-
py)3}2Sm], in which the Sm2+ cation is sterically shielded by
the six Me-groups of the two aluminate ions. This results in an
apparently large stabilization of the complex towards molecu-
lar oxygen as compared with the unsubstituted complex
[{EtAl(2-py)3}2Sm], which rapidly scavenges molecular oxygen,
giving the [EtAl(2-py)2O]2À dianion (C; Figure 1).[9] Relevant to
the formation of a terminal AlÀO ligand framework of this
type, Roesky has shown that hydrolysis or hydroxylation of the
sterically encumbered b-diketiminato AlIII complexes
[HC{(CMe)(NDipp)}2AlR2] (Dipp=2,6-iPr2C6H3, R=Cl, I or alkyl)
produces terminal AlÀOH compounds.[10]
Introduction
Over the last three decades, neutral tris(pyridyl) ligands of the
general type [Y(Py)3] (Py=2-pyridyl, Y=CR, COR, CH, N, P, P=O,
As; A in Figure 1), have emerged as an important family of li-
gands.[1] These ligands, along with the related tris(pyrazolyl)bo-
rates and methanes, have found a vast range of applications in
coordination, organometallic, and bioinorganic chemistry.[2] It is
been only relatively recently, however, that attention has
turned to ligands containing the heavier Group 13 and 14
atoms at the bridgehead.[3] This change from a non-metallic to
a more metallic atom in particular opens up the possibility of
redox activity and variable oxidation states at the bridge-
head.[4] Of particular interest are tris(pyridyl)aluminates [RAl(2-
Figure 1. The framework found in the family of tris(2-pyridyl) ligands (A), the
tris(2-pyridyl)aluminate family of ligands (B), and the oxo–pyridyl ligand set
(C).
[a] Dr. R. García-Rodríguez, Prof. D. S. Wright
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge CB2 1EW (UK)
E-mail: rg489@cam.ac.uk
In the study presented herein, we explore the synthesis and
coordination properties of a series of sterically encumbered
tris(pyridyl)aluminate ligands of the type D (Figure 2), having
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502156.
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Figure 2. The steric effects introducing substituents at the 6-position of the
tris(2-pyridyl) framework (D) and the resulting stabilisation of the hydroxo
and alkoxide aluminate ions (E and F).
different substituents at the 6-position of the pyridyl ring (R=
Me, CF3, Br). We then show that such aluminate ions can be
used as simple precursors for the synthesis of a new family of
terminal AlÀOH (E; Figure 2) and related alkoxide complexes
(F).
Scheme 1. Desolvation of 1·THF, producing the monomeric complex
1 rather than the anticipated dimer [1]2.
Results and Discussion
As noted above, the coordination chemistry of tris(pyridyl)li-
gands has been a major research theme in the past thirty
years or so. Surprisingly, however, in contrast to the tris(pyrazo-
lyl)borate ligands, there have been few systematic studies of
the effects of introducing different substituents on the pyridyl
ring.[8,11] We showed recently that methyl group substitution of
the pyridyl ring units in the aluminate ion of [EtAl(6-Me-2-
py)3Li·THF] (1·THF) provided a sterically demanding metal coor-
dination site that could be used to stabilize unusual oxidation
states.[9] The extreme steric congestion of the aluminate ion is
witnessed in the solid-state structure of 1·THF, which has
a highly distorted Li–THF coordination environment. In light of
this apparently weak coordination of THF to Li+, we decided in
our preliminary studies to explore the desolvation of 1·THF, an-
ticipating the formation of a dimeric complex [1]2, which
would be analogous to the dimeric complex [MeAl(2-py)3Li]2
formed by desolvation of THF using the unsubstituted com-
plex [MeAl(2-py)3Li·THF] (Scheme 1).[6a]
Like the unsubstituted complex, 1·THF has a marked tenden-
cy to lose coordinated THF when placed under vacuum during
isolation or when stored under an inert atmosphere for a pro-
longed period. Complete desolvation can be accomplished
quantitatively by placing 1·THF under vacuum (0.1 mm Hg) for
ca. 1.5 h at 708C, as seen by the absence of THF in solution
1H NMR spectrum of the solid product produced. Crystals of
the desolvated complex were grown from a concentrated tolu-
ene solution. Despite some difficulties with collection and re-
finement of single-crystal X-ray data (see the Experimental Sec-
tion), the connectivity of the structure is determined unambig-
uously. Contrary to our expectations, the X-ray study shows
that 1 is in fact a monomer, containing an unsolvated pyrami-
dal Li+ cation (Figure 3a). The steric protection that the three
CH3 groups in the 6-positions of the pyridyl rings provide to
the Li+ can be seen in the space filling view of the molecule
(Figure 3b). Additional Me CÀH···Li contacts (range ca. 2.88–
2.92 Š) may also contribute to the stabilisation of such an un-
Figure 3. a) Structure of the unsolvated monomer 1. Only one of two crystal-
lographically independent (chemically identical) molecules is shown.
b) space-filling view along the Li···Al axis, illustrating the sterically congested
nature of the Li+ cation. Owing to difficulties with the structure refinement
(see the Experimental Section), no bond lengths or angles are quoted here.
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usual coordination environment
for Li. However, although these
contacts are well below the sum
of the van der Waals radii of H
and Li (ca. 3.02 Š)[12] they are
outside the range accepted for
genuine CÀH···Li agostic interac-
tions (1.80–2.20 Š).[13]
In contrast to the behaviour
of the dimeric unsubstituted
complex [MeAl(2-py)3Li]2,[6a] no
evidence for a monomer/dimer
equilibrium is detected in solu-
tion for desolvated 1. Room-
temperature 1H and 7Li NMR
Scheme 2. Synthesis of the new complexes 2 and 3.
spectroscopic
studies
in
[D8]toluene show the presence
of a single compound in solu-
mation of the dilithiate (2,6-Li2-py), which is subsequently in-
volved in the formation of the monolithiate (2-Li-6-Br-py).[14]
Significantly, even though the syntheses of 2 and 3 are under-
taken in THF and Et2O as the solvents, no coordination of the
Li+ cations in either of the complexes was seen in the analyti-
cal or spectroscopic analyses of the isolated solid products.
This gave us a preliminary indication of the apparently greater
steric influence of the 6-CF3 and 6-Br groups compared to 6-
Me. For 2, this is in line with the greater van der Waals radius
of the CF3 group (2.74 Š for CF3 versus 2.23 Š for CH3).[15] How-
ever, for 3 this appears to be counterintuitive because the van
der Waals radius of Br is smaller than that for a Me group
(1.85 Š).[12,15]
tion, with no variation in the spectra observed on changing
the concentration. The monomeric nature of 1 in solution was
also confirmed by variable-concentration cryoscopic molecular
mass measurements in benzene, which give an association
state (n) for [1]n of 1.03–1.14 over the concentration range
6.6”10À3 to 1.4”10À3 molLÀ1 (Table 1).
Table 1. Cryoscopic molecular mass measurements of compounds 1–3.[a]
Compound
Substituent at
6-position
Concentration
[molLÀ1
Mw
n
]
1
1
1
2
2
3
3
CH3
CH3
CH3
CF3
CF3
Br
1.4”10À3
2.1”10À3
6.6”10À3
3.2”10À3
7.8”10À3
8.95”10À3
1.3”10À2
349Æ9
1.03Æ0.03
1.14Æ0.02
1.03Æ0.06
0.95Æ0.11
1.05Æ0.05
1.15Æ0.05
1.08Æ0.05
Both 2 and 3 were characterized by chemical analysis and
multinuclear NMR spectroscopy. The monomeric nature of
these compounds in solution was further supported by varia-
ble-concentration cryoscopy in benzene (Table 1). The room-
386Æ8
350Æ19
476Æ53
525Æ25
614Æ25
577Æ17
1
7
temperature H and Li NMR spectra also only showed sharp
concentration-independent resonances. Final confirmation of
this is given by the single-crystal X-ray structures of 2 and 3
(Figure 4a and b, respectively). Both complexes feature three-
coordinate, pyramidal Li+ coordination geometries that are
similar to that found in 1. A few salient features of the molecu-
lar structures of both complexes are worth mentioning here. In
particular, the LiÀN bond lengths (range 2.024(5)–2.027(6) in 2
and 2.020(6)–2.016(5) Š in 3) and N-Li-N angles (range
100.2(2)–105.8(2) in 2 and 101.3(2)–107.8(2)8 in 3 are similar in
both complexes, despite the different steric influence of the
CF3 and Br groups in the 6-positions of the 2-py rings.
Br
[a] All measurements were carried out in benzene.
The unusual monomeric behaviour found for 1 in the solid
and solution states motivated us to explore other related steri-
cally constrained tris(pyridyl)aluminate ligands. A further po-
tential issue that we also wanted to explore was the effect of
electron-withdrawing effects on the coordination ability of the
tris(pyridyl) ligand set. The new monomeric complexes [EtAl(6-
CF3-2-py)3Li] (2) and [EtAl(6-Br-2-py)3Li] (3) were prepared in
moderate yields (of 41 and 51%, respectively) using a similar
synthetic procedure to 1·THF, involving the reactions of EtAlCl2
with the corresponding 2-lithiopyridines (Scheme 2). However,
one important modification was introduced in the case of 3 in
regard to the lithiation of 2,6-dibromopyridine. Instead of the
lithiation being accomplished by the addition of nBuLi
(1 equiv) to the bromopyridine at À788C, the lithiation of 2,6-
dibromo-2-pyridine is most effective if the bromopyridine is
added to the solution of nBuLi. Although there is some debate
as to the reasons for this, it is thought that the presence of
excess nBuLi at the beginning of the addition results in the for-
Views of 2 and 3 along the Al···Li axes of each of the mono-
mers are presented in Figure 5, from which it can be seen that
the blocking of the Li+ coordination site appears to be great-
est in the CF3 derivative 2. It can be noted, however, that in
the case of 2 and 3 the low coordination number of Li+ may
also stem from the presence of Li···F and Li···Br interactions. Il-
lustrating this, the Li···F contacts in 2 are in the range 2.50–
2.55 Š, while the Li···Br contacts in 3 are in the range 3.27–
3.38 Š (both well within the sums of the van der Waals radii of
Li and F (ca. 3.29 Š) and Li and Br (ca. 3.67 Š)).[12] The extent to
which these secondary interactions contribute to the coordina-
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tion of Li+ is unclear at this stage. In the case of 3, for exam-
7
ple, variable-temperature 19F and Li NMR spectroscopy (298–
213 K) indicated no Li···F coupling and the presence of a singlet
in the 19F NMR spectrum at all temperatures shows that the
CF3 groups rotate freely.
We next moved on to explore the coordination chemistry of
the new aluminate ligands of 2 and 3. In contrast to the reac-
tion of 1 with FeCl2, which produces a green solution presuma-
bly of the half-sandwich compound [{EtAl(2-py)3}FeCl],[8] nei-
ther 2 nor 3 coordinate FeCl2. Further attempts to coordinate
other metal ions with 2 and 3 also failed. It therefore appears
that the introduction of the CF3 and Br groups into the 6-posi-
tions completely blocks their coordination behaviour. This
steric effect can potentially be turned to an advantage, howev-
er, in the stabilization of unusual AlIII complexes. It has been
suggested that the stabilization of a terminal AlÀOH group
relies on the steric influence of the supporting ligand groups
(L) and their impact on the acidity of the OÀH bond, because
lower steric demands of the ligand set and higher acidity of
the OÀH bond will encourage dimerization (Scheme 3).[16]
Scheme 3. Dimerisation of a terminal AlÀOH complex to give AlÀOÀAl.
With this background in mind we decided to explore the
use of 2 and 3 as scaffolds for the synthesis of terminal AlÀOH
compounds. The reaction of 2 with 1–2 equiv of H2O in
[D8]toluene was investigated in an in situ multinuclear NMR
spectroscopy study. Within 5 min of mixing at room tempera-
7
ture, the Li NMR spectrum showed the presence of unreacted
2 (singlet, d=2.87 ppm) along with a new minor resonance
1
(singlet, d=2.54 ppm). The H NMR spectrum confirms the for-
Figure 4. Solid-state structures of the monomeric complexes a) 2 and b) 3.
Hydrogen atoms and the disorder of the CF3 groups in 2 are omitted for
clarity. Selected bond lengths [Š] and angles [8]: 2: Cpy–Al1 range 2.015(3)–
2.001(3), N1–Li1 2.024(5), N2–Li1 2.028(6), N3–Li1 2.027(6); Cpy-Al1-Cpy range
102.92(12)–103.94(12), Al1-Cpy-N range 114.8(2)–116.5(2), N-Li-N range
100.2(2)–105.8(2), Li···Br range 2.343(18)–2.55(2). 3: Cpy–Al1 range 2.019(3)–
2.034(3), N1–Li1 2.020(6), N2–Li1 2.016(5), N3–Li1 2.016(5); Cpy-Al1-Cpy range
101.78(11)–105.10(11), Al1-Cpy-N range 114.11(19)–116.9(2), N-Li-N range
101.3(2)–107.8(2), Li···Br range 3.272(6)–3.384(6).
mation of a single new species which can be identified as the
[EtAl(OH)(6-CF3-2-py)2]À anion (4; Scheme 4), observed as
minor pyridyl-CÀH and EtÀAl resonances next to the corre-
sponding resonances for 2. Furthermore, free 6-CF3-2-py-H and
ethane (singlet, d=0.81 ppm) are also observed, suggesting
that the hydrolysis of 2 is not selective, that is, that either the
Al-(6-CF3-2-py) or EtÀAl groups can be involved in deprotona-
tion of H2O. The optimal reaction time was found to be about
1.5 h, by which time the resonances for 4 amount to only ca.
1
10% of the total integrated H NMR resonances.
1
Scheme 4. Assignment of the H NMR spectrum of the anion 4 from the in
situ NMR spectrum.
Figure 5. Views of the Li+ cation along the Li···Al axis: a) 2 and b) 3.
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An in situ NMR spectroscopic study of the reaction of 3 with
H2O indicated the formation of a closely related terminal AlÀ
OH complex (5) in solution, but now quantitatively. The reac-
tion is complete after 30 min, giving a mixture of 5, 6-Br-2-py-
H and a small amount of ethane. Compound 5 has a similar
1H NMR spectrum to the anion 4, with 6-Br-pyridyl and EtÀAl
resonances in 1:2 ratio along with a singlet characteristic of
AlÀOH at d=1.12 ppm. The reaction was scaled up, allowing
the preparation of 5 in 25% crystalline yield after workup
(Scheme 5a). The presence of the OH group in solid 5 is
Scheme 5. Synthesis of the terminal AlÀOH complex 5 and the alkoxides 6
and 7.
shown by a sharp OÀH stretching band at 3641 cmÀ1 in the IR
spectrum. This can be compared to values of greater than
3700 cmÀ1 for terminal AlÀOH, indicating weakening of the OÀ
1
Figure 6. a) Structure of the hydroxo bis(pyridyl)aluminate dimer 5 featuring
a terminal AlÀOH group. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Š] and angles [8]: Al1–O1 1.790(3), Cpy–Al1 2.016(4)–2.019(4),
N1–Li1 2.037(7), N2–Li1 2.046(7), O1–Li1A 1.964(7), O1–Li1 1.980(7); Cpy-Al1-
Cpy 104.28(15), Cpy-Al1-O1 101.41(14)–101.65(14), Al1-Cpy-N 113.9(3)–115.0(3),
O-Al1-Li 36.4(2)–36.6(2), O1-Li1-N1A 112.6(4), O1A-Li1-N2 113.7(4), N2-Li1-
N1A 135.3(4), N2-Li1-O1 96.8(3), O1-Li1-O1A 95.7(3), O1-Li1-N2 96.8(3).
b) Space-filling diagram, showing the environment around the OH group.
H bond. Full assignment of the H NMR resonances and confir-
mation of the presence of the Et-Al-OH and Et-Al-Py linkages
1
1
were obtained using 2D NMR H–13C HMQC, H–1H NOESY, and
1H–13C HMBC experiments (see the Supporting Information).
The single-crystal X-ray structure of 5 confirms all of the con-
clusions drawn from spectroscopic data. Molecules of 5 have
a centrosymmetric dimer structure [{EtAl(6-Br-2py)2(OH)}Li]2 in
which the OH groups of the [EtAl(6-Br-2py)2(OH)]À anions
bridge the two Li+ cations together in a central Li2O2 ring unit
(Figure 6a). The H atom of the OH group was located in the
difference Fourier map (then OH was refined as a rigid group).
Overall, the most interesting feature of 5 is the stabilization of
the AlÀOH functionality within a dimeric arrangement of this
type, in which elimination of H2O and the formation of an Al-
O-Al bridge appears to be set up. One explanation for the sta-
bility of the complex is the location of the OH group within
a cleft in the molecular arrangement, bounded by an Al-
bonded Et group and two 6-Br-2-py groups, as seen in the
space-filling diagram shown in Figure 6b. The AlÀO bonds in 5
(1.790(3) Š) are noticeably longer than typically found in mon-
omeric terminal AlÀOH complexes (ca. 1.73 Š).[17] The acute in-
tramolecular Br···HÀO(Al) angles (89.78 and 103.18) and the
Br···H (3.842 Š and 3.664 Š) and Br···O distances (3.965(3) Š and
4.010(3) Š) in 5 argue against the presence of significant stabi-
lizing H···Br H-bonds in the complex.
Roesky and co-workers have shown previously that the ter-
minal AlÀOH retains its Brønsted acidity, reacting for example
with Cp3Ln (Ln=lanthanides) to give heterometallic Al-O-Ln
bridged compounds.[18] To assess the acidity of the AlÀOH
groups in 5 we first explored its thermal behaviour in solution.
A solution of 5 in [D8]toluene was heated at 808C for 3 h, re-
sulting in the elimination of free 6-Br-2-py-H and ethane, pre-
sumably via intramolecular deprotonation of the OH group
1
(see H NMR studies the Supporting Information). In a follow-
up experiment, 5 was reacted with ZnEt2 as an external base
1
(1 equiv). A mixture of compounds is observed by in situ H
and 13C NMR spectroscopy. However, the ca. 20 ppm increase
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Figure 7. Structure of the 2-Br-py zinc complex 6. Hydrogen atoms are omit-
ted for clarity. Selected bond lengths [Š] and angles [8]: Cpy–Zn range
2.019(8)–2.279(8), N–Li range 1.965(14)–2.022(14); N-Li-N range 103.6(6)–
134.0(8), Zn-Cpy-N range 116.7(5)–124.8(6).
in the chemical shifts of C2 of the 2-py group in the 13C NMR
spectrum compared to 5 suggested that at least some transfer
of the 2-py group onto Zn2+ had occurred.[19] This was con-
firmed by the isolation of a few crystals of dimeric [LiZn(6-Br-
py)3]2 (6) from the NMR-scale reaction and their structural char-
acterization (see Figure 7). Compound 6 consists of a dimeric
2À
zincate [Zn(6-Br-2-py)3]2 dianion ion-paired with two Li+ cat-
ions, in which full transfer of the 6-Br-2-py groups has occurred
from Al to Zn.
The use of tris(2-pyridyl)aluminates as general precursors for
the preparation of heteroleptic variants, demonstrated in the
synthesis of the mixed-ligand 2-py/OH complexes 4 and 5, is
an attractive one. To carry this idea further we explored the re-
actions of alcohols (ROH) with aluminates. Reactions of the un-
substituted complex [EtAl(2-py)3Li.THF] or 6-Br substituted 3
with MeOH (1.3 equivalents) in toluene at 08C give the closely
related heteroleptic compounds [{EtAl(6-Br-2-py)2(OMe)}Li]2 (7)
and [{EtAl(2-py)2(OMe)}Li]2 (8) in 20–30% yields of crystalline
product (Scheme 5b). These new compounds were character-
ized by analytical and spectroscopic techniques prior to their
X-ray structural characterization. Both complexes have dimeric
structures that are similar to that of 5 in the solid state, but
now with the OH group replaced by OMe (Figure 8a,b). The re-
tention of these dimeric structures in solution along with the
full assignment of their resonances are supported by extensive
1D and 2D multinuclear NMR investigations (see the Support-
ing Information).
Figure 8. Structure of the heteroleptic alkoxide aluminates a) 7 and b) 8. Hy-
drogen atoms are omitted for clarity. Selected bond lengths [Š] and angles
[8]: 7: Al1–O1 1.808(2), Cpy–Al1 2.012(3) and 2.021(3), N1–Li1 2.048(6), N2–
Li1A 2.045(6), O1–Li1 2.011(6), O1–Li1A 2.010(6); Cpy-Al1-Cpy 105.60(14), Cpy-
Al1-O1 101.92(12) and 102.16(12), Al1-Cpy-N 115.3(2) and 115.6(2), O-Al-Li1
33.08(11)-33.33(11), O1-Li1-N2A 115.8(3), N2A-Li1-N1 132.3(3), O1-Li1-N1
96.7(2), O1A-Li1-O1 95.7(2). 8: Al1–O1 1.8012(11), Cpy–Al1 2.0175(16) and
2.0188(17), N1–Li1 2.029(3), N2–Li1A 2.023(3), O1–Li1 1.999(3), O1–Li1A
1.989(3); Cpy-Al1-Cpy 106.71(6), Cpy-Al1-O1 102.17(6) and 102.34(6), Al1-Cpy-N
116.36(11) and 116.50(11), O1-Al1-Li1 38.50(6), O1-Li1-Al1 34.12(5), O1-Li1-
N2A 114.43(14), O1-Li1-N1 99.24(13), N2A-Li1-N1 127.81(15), O1A-Li1-O1
96.42(12).
readily be transferred to other metals (such as FeII), whereas
the more sterically congested ligand of 8 cannot be.
Conclusion
Both 7 and 8 are much more thermally stable than their AlÀ The introduction of steric congestion at the 6-position of the
OH relatives 4 and 5, presumably because there is now no
longer any possibility of intramolecular deprotonation of the
OH group by the Al-bonded 2-py’ or Et groups. As far as
ligand properties are concerned, our preliminary studies have
so far shown that the unsubstituted aluminate ligand of 7 can
pyridyl ring units of tris(2-pyridyl)aluminate ligands has a pro-
found effect on their coordination properties. This is shown
most dramatically in the current work by the labile loss of co-
ordinated THF from the 6-Me substituted complex [EtAl(6-Me-
2-py)3Li·THF] (1·THF), which gives the highly unusual monomer
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EtAlCl2 (2.95 mL, 2.95 mmol, 1.0m in hexanes) was added dropwise
to the solution over 20 min. The resulting mixture was allowed
slowly to reach ambient temperature overnight. The solvent was
removed in vacuo and toluene (30 mL) and THF (10 mL) were
added and the yellow–brown mixture was stirred at room temper-
ature for 3 h and filtrated over Celite. The solvent was removed in
vacuo until the precipitation of a white solid was observed, which
was redissolved by gentle heating. Storage at À158C afforded col-
ourless crystals of 2, which were dried in a glovebox. Total yield of
isolated crystalline product: 600 mg, 1.2 mmol, 41%. Note: If the
lithiation step is carried out in THF at À788C rather than in Et2O,
1, as opposed to the expected dimeric arrangement that
would have a coordination number of four rather than three
for Li+. The steric effect of substituents at the 6-position is also
seen in the behaviour of the 6-Br substituted ligand frame-
work, which reacts with H2O to give a room-temperature
stable heteroleptic aluminium 2-py/OH complex. This type of
reaction, using the tris(pyridyl) ligand as a scaffold to build het-
eroleptic systems selectively, is an important synthetic step be-
cause it allows for extremely extensive elaboration of the steric
and donor character of the ligands and for the potentially
facile incorporation of chiral alcohols or amines into heterolep-
tic 2-py ligand arrangements. Our studies are continuing in
this area, particularly with a view to obtaining families of
simply prepared chiral aluminates for catalysis.
1
then compound 2 was obtained in lower yield (10–20%). H NMR
(298 K, [D8]toluene, 500 MHz), d=7.74 (d, JHH =7.4 Hz, 3H, H3 py),
6.81 (t, JHH =7.7 Hz, 3H, H4 py), 6.74 (dd, JHH =7.8 and 0.9 Hz, 3H,
H5 py), 1.88 (t, JHH =8.2 Hz, 3H, Al-CH2CH3), 0.99 (q, JHH =8.2 Hz,
3H, Al-CH2). 13C{1H} NMR (298 K, [D8]toluene, 100.6 MHz), d=189.0
2
(br, C2 py), 146.19 (q, JCF =32.5 Hz, C6 py), 135.94 (C3 py), 133.92
(C4 py), 123.89 (q, 1JCF =273 Hz, CF3), 117.35 (C5 py), 10.93 (Al-
CH2CH3), À2.91 (br, Al-CH2). 27Al NMR (298 K, [D8]toluene,130.3 MHz,
Experimental Section
7
ref. solution of AlCl3·6H2O/D2O), d=127.11 (br, s). Li NMR (298 K,
Materials and general methods
[D8]toluene, 194.4 MHz, ref. solution of LiCl/D2O), d=2.87 (s). Ele-
mental analysis(%) calcd for 2: C 47.9, H 2.8, N 8.4; found C 47.6,
H 2.8, N 8.1.
All of the syntheses were carried out on a vacuum line under an
argon atmosphere. Products were isolated and handled with the
1
aid of a N2-filled glove box (Saffron type a). H and 13C NMR spec-
tra were recorded on a Bruker Avance 400 QNP or Bruker Avance
500 MHz Cryo spectrometer. 7Li and 27Al NMR NMR spectra were re-
corded on a Bruker Avance 500 MHz
Synthesis of [EtAl(6-Br-2-py)3Li] (3)
A solution of nBuLi (12.5 mL, 20 mmol, 1.6m in hexanes) in THF
(12 mL) was cooled at À788C. To this solution was added dropwise
over 30 min a solution of 2,6-dibromopyridine (4.74 g, 20 mmol) in
THF (28 mL). The resulting dark green mixture was stirred (40 min
at À788C). EtAlCl2 (6.6 mL, 6.6 mmol, 1.0m in hexanes) was added
to the solution of 2-lithio-6-bromopyridine over 15 min. The result-
ing mixture was allowed slowly to reach ambient temperature
overnight and stirred for a further 36 h. The solvent was removed
in vacuo. The addition of toluene (40 mL) and THF (10 mL) afforded
a pale yellow–brown mixture (dark brown mixtures were associat-
ed with lower yields) which was filtrated over Celite. The solvent
was removed in vacuo until the precipitation of a white solid was
observed, which was redissolved by gentle heating. Storage at am-
bient temperature (24 h) afforded colourless crystals of 3. Further
concentration of the solution and storage at À158C afforded more
colourless crystals of 3. Total yield of isolated product: 1.80 g,
3.37 mmol, 51%. 1H NMR (298 K, [D8]toluene, 500 MHz), d=7.49
(dd, JHH =6.1 and 2.1 Hz, 3H, H3 py), 6.66–6.52 (m, 6H, H4 and H5
py), 1.82 (t, JHH =8.2 Hz, 3H, Al-CH2CH3), 0.89 (q, JHH =8.2 Hz, 3H,
Al-CH2). 13C{1H} NMR (298 K, [D8]toluene, 100.6 MHz), d=191.9 (br,
C2 py), 142.95 (C6 py), 136.07 (C4 py), 131.94 (C3 py), 123.71 (C5
py), 10.93 (Al-CH2CH3), À2.90 (br, Al-CH2). 27Al NMR (298 K,
[D8]toluene, 130.3 MHz, ref. solution of AlCl3·6H2O/D2O), d=125.30
Cryospectrometer. Elemental analysis
was obtained on a PerkinElmer 240
Elemental Analyser. The unambigu-
ous assignment of NMR resonances
was accomplished by additional 2D
1
NMR experiments (1H–1H COSY, H–1H
NOESY, 1H–13C HMQC, and 1H–13C
HMBC experiments (see (Scheme 6)
for atom labelling and the Support-
ing Information for details). [EtAl(6-
Me-2-py)3Li·THF], 1·THF, was synthe-
sized as described previously.[9]
Scheme 6. Atom labelling
used in the NMR studies for
the pyridyl aluminate ligands.
Synthesis of [EtAl(6-Me-2-py)3Li] (1)
[EtAl(6-Me-2-py)3Li·THF] (250 mg, 0.608 mmol) was placed under
vacuum (0.1 mm Hg) for ca. 1.5 h at 708C, affording 1 as a white
solid in quantitative yield: 205 mg, 0.604 mmol, 99%. Colourless
crystals of 1 were obtained from a saturated solution of 1 in tolu-
ene at 208C. 1H NMR (298 K, [D8]toluene, 500 MHz), d=7.69 (d,
J
J
HH =7.3 Hz, 3H, H3 py), 7.00 (t, JHH =7.5 Hz, 3H, H 4py), 6.46 (d,
HH =7.8 Hz, 3H, H5 py), 2.26 (s, 9H, C6 CH3), 1.98 (t, JHH =8.1 Hz,
7
3H, Al-CH2CH3), 1.09 (q, JHH =8.1 Hz, 3H, Al-CH2). 13C{1H} NMR
(298 K, [D8]toluene, 100.6 MHz), d=188.62 (br, C2 py), 154.93 (C6
py), 133.32 (C4 py), 130.61 (C3 py), 119.79 (C5 py), 24.89 (C6 CH3),
11.23 (Al-CH2CH3), À2.54 (br, Al-CH2). 27Al NMR (298 K, [D8]toluene,
130.3 MHz, ref. solution of AlCl3·6H2O/D2O), d=126.55 (br, s). 7Li
NMR (298 K, [D8]toluene, 194.4 MHz, ref. solution of LiCl/D2O), d=
3.53 (s). Elemental analysis(%) calcd. for 1: C 70.8, H 6.8 N, 12.4;
found: C 69.9, H 6.8, N 12.6.
(br, s). Li NMR (298 K, [D8]toluene, 194.4 MHz, ref. solution of LiCl/
D2O), d=1.58 (s). Elemental analysis(%) calcd. for 3: C 38.2, H 2.6,
N 7.9; found: C 38.2, H 2.7, N 7.8.
Synthesis of [EtAl(OH)(6-Br-2-py)2Li]2 (5)
H2O (76 mL, 4.2 mmol, 1.4 equiv) was added at room temperature
to a solution of 3 (1.40 g, 2.62 mmol) in toluene (50 mL). The prog-
ress of the reaction can be monitored by Li NMR. The mixture was
7
stirred at room temperature for 3 h and subsequently filtered over
Celite to afford a colourless solution. The solution was concentrat-
ed under vacuum (ca. 3 mL) and n-pentane was added until turbid-
ity was observed. Storage at À158C afforded colourless crystals of
5 (258 mg, 0.327 mmol, 25% yield of crystalline product). Note:
The compound is highly soluble in toluene; however, if the solvent
Synthesis of [EtAl(6-CF3-2-py)3Li] (2)
2-Bromo-6-(trifluoromethyl)pyridine (2.00 g, 8.85 mmol) was dis-
solved in Et2O (40 mL). nBuLi (5.6 mL, 8.96 mmol, 1.6m in hexanes)
was added to the solution dropwise at À788C over a period of
20 min. The resulting orange solution was stirred at À788C for 3 h.
Chem. Eur. J. 2015, 21, 14949 – 14957
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is evaporated under vacuum and the resulting residue is dried
under prolongated vacuum to remove 2-bromopyridine and H2O,
440 mg of compound 5 (0.558 mmol, 43% yield) containing 5% of
X-ray crystallographic studies
Data were collected for 1 on a Nonius Kappa CCD diffractometer
with graphite-monochromated MoKa radiation, for 3, 5, 7, and 8
on a Bruker D8 QUEST diffractometer with an Incoatec ImS Cu mi-
crofocus source, and for 2 and 6 on a Bruker SMART X2S diffrac-
tometer with a monochromatic MoKa microfocus source. Crystals
were mounted directly from solution using perfuorohydrocarbon
oil to prevent atmospheric oxidation, hydrolysis, and solvent
loss,[20] and the temperature was held between 180 and 250 K
using an Oxford Cryosystems N2 cryostat. Data were collected
using Bruker Apex2 or GIS, processed using SAINT and SADABS
and refined using SHELXL.[21] Details of the data collections and
structural refinements are given in the Supporting Information,
Table S1. Further details of the methods of refinement of the struc-
tures are as follows. 1: After several crystallization attempts, all
crystals obtained for 1 were relatively weakly diffracting and fre-
quently showed multiple spots indicative of several crystalline do-
mains. It is noted that the reported triclinic lattice has approxi-
mately monoclinic metric symmetry, which may indicate a likeli-
hood for twinning, but we were not able to implement any effec-
tive multicomponent integration or refinement. The refinement re-
ported for 1 is the best of five datasets collected from five
different crystals. The molecular geometry is generally satisfactory,
with restraints applied to the ethyl groups to maintain sensible
bond distances. Several of the atoms exhibit relatively prolate dis-
placement ellipdoids. 2: The CF3 groups exhibit rotational disorder
and were modelled over two positions with restrained geometry.
The site occupancy factors were initially refined, then constrained
to the values 0.58:0.42 for the final refinement cycles. 5: the H
atom of the OH group was included in a position taken from the
difference Fourier map, then the OH group was treated as a rigid
body for subsequent refinement, with an individual isotropic dis-
placement parameter refined for H. 7: the toluene solvent mole-
cule is disordered about an inversion centre. It was modelled with
a constrained benzene ring and common isotropic displacement
parameters for the C atoms. Compounds 2 and 3 are isostructural
(that is, the Br atoms in 3 occupy essentially the same space as the
disordered CF3 groups in 2).
3 was obtained. IR (Nujol), n(OH): 3641 cmÀ1 1H NMR (298 K,
.
[D8]toluene, 500 MHz), d=7.47 (dd, JHH =7.0 and 1.1 Hz, 2H, H3
py), 6.82–6.78 (m, 2H, H5 py), 6.78–6.72 (m, 2H, H4 py), 1.46 (t,
J
HH =8.1 Hz, 3H, Al-CH2CH3), 1.07 (s, 1H, Al-OH), 0.50 (q, JHH =
8.1 Hz, 3H, Al-CH2). 13C{1H} NMR (298 K, [D8]toluene, 100.6 MHz),
d=191.8 (br, C2 py), 143.76 (C6 py), 136.02 (C4 py), 131.29 (C3 py),
125.0 (C5 py, overlapped with residual toluene solvent signal),
10.08 (Al-CH2CH3), À0.30 (br, Al-CH2,). 27Al NMR (298 K, [D8]toluene,
130.3 MHz, ref. solution of AlCl3·6H2O/D2O), d=138.0 (br, s). 7Li
NMR (298 K, [D8]toluene, 194.4 MHz, ref. solution of LiCl/D2O), d=
1.94 (s). Elemental analysis(%) calcd. for 5: C 36.6, H 3.1, N 7.1;
found C 35.6, H 3.1, N 6.8.
Synthesis of [EtAl(OMe)(6-Br-2-py)2Li]2 (7)
Methanol (120 mL, 3.04 mmol, 1.25 equiv) was added at 08C to a so-
lution of 3 (1.30 g, 2.435 mmol) in toluene (50 mL). The mixture
was stirred (1.5 h at 08C) and subsequently allowed to reach room
temperature. The resulting cloudy solution was stirred for 1 h at
room temperature and filtered over Celite. The colourless solution
produced was concentrated (to ca. 5 mL) and was layered with n-
pentane. Storage at À158C afforded colourless crystals of 7. Total
yield of isolated crystalline product: 220 mg, 0.27 mmol, 22%.
1H NMR (298 K, [D8]toluene, 500 MHz), d=7.47 (dd, JHH =7.0 and
1.1 Hz, 2H, H3 py), 6.79–6.83 (m, 2H, H5 py), 6.77–6.71 (m, 2H, H4
py), 3.52 (s, 3H, OCH3), 1.54 (t, JHH =8.1 Hz, 3H, Al-CH2CH3), 0.64 (q,
J
HH =8.1 Hz, 3H, Al-CH2). 13C{1H} NMR (298 K, [D8]toluene,
100.6 MHz), d=191.3 (br, C2 py), 143.60 (C6 py), 136.07 (C4 py),
131.62 (C3 py), 125.32 (C5 py, overlapped with residual toluene sol-
vent signal), 51.52 (OCH3,), 10.29 (Al-CH2CH3), À1.25 (br, Al-CH2,).
27Al NMR (298 K, [D8]toluene, 130.3 MHz, ref. solution of
AlCl3·6H2O/D2O), d=139.27 (br, s). 7Li NMR (298 K, [D8]toluene,
194.4 MHz, ref. solution of LiCl/D2O), d=1.98 (s). Elemental analy-
sis(%) calcd. for 5: C 38.3, H 3.5, N 6.9; found: C 38.6, H 3.5, N 6.8.
CCDC 1404218 (1),
1404219 (2),
1404220 (3),
1404222 (5),
1404221 (6), 1404223 (7), and 1404224 (8) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre.
Synthesis of [EtAl(OMe)(2-py)2Li]2 (8)
Methanol (63 mL, 1.56 mmol, 1.25 equiv) was added at À788C to
a solution of [EtAl(2-py)3Li·THF] (480 mg, 1.30 mmol) in toluene
(28 mL). The mixture was stirred at À788C for 5 min and subse-
quently transferred to an ice bath and allowed to reach 08C. The
mixture was slowly allowed to reach room temperature overnight
and the resulting pale yellow cloudy solution was filtered over
Celite. The solution produced was concentrated (to ca. 3 mL). Addi-
tion of n-pentane (ca. 5 mL) and storage at À158C afforded colour-
less crystals of 8. Total yield of isolated crystalline product: 100 mg,
0.20 mmol, 31%. 1H NMR (298 K, [D8]toluene, 500 MHz), d=8.22–
8.19 (m, 2H, H6 py), 7.78–7.74 (m, 2H, H3 py), 7.15 (td, JHH =7.6
and 1.7 Hz, 2H, H4 py), 6.69–6.64 (m, 2H, H5 py), 3.20 (s, 3H,
OCH3), 1.57 (t, JHH =8.1 Hz, 3H, Al-CH2CH3), 0.62 (q, JHH =8.1 Hz, 3H,
Al-CH2). 13C{1H} NMR (298 K, [D8]toluene, 100.6 MHz), d=187.90 (br,
C2 py), 148.80 (C6 py), 133.42 (C3 py), 133.30 (C4 py), 121.29 (C5
py), 51.11 (OCH3,), 10.61 (Al-CH2CH3), À0.48 (br, Al-CH2,). 27Al NMR
(298 K, [D8]toluene, 130.3 MHz, ref. solution of AlCl3·6H2O/D2O), d=
Acknowledgements
We thank the EU for a Marie Curie Intra European Fellowship
within the seventh European Community Framework Pro-
gramme for R.G.R. and an Advanced Investigator Award for
D.S.W. We also thank Dr J. E. Davies for collecting X-ray data
on compound 1 and Dr A. D. Bond for further assistance with
the crystallographic data.
Keywords: aluminates · lithium · N ligands · steric effects ·
terminal hydroxide ligands
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Received: June 2, 2015
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Trans. 2014, 43, 14045–14053.
Revised: July 6, 2015
Published online on August 25, 2015
Chem. Eur. J. 2015, 21, 14949 – 14957
www.chemeurj.org
14957
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