Synthesis and coordination behaviour of aluminate-based quinolyl ligands

Article abstract of DOI:10.1039/d1dt02438h

The effects of moving the donor N-atom from the 2-position in lithium (2-pyridyl)- and (2-quinolyl)aluminates to the more remote position in (8-quinolyl)aluminates have been investigated by solid-state structural and DFT computational studies of the new c

Full text of DOI:10.1039/d1dt02438h

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Synthesis and coordination behaviour of
aluminate-based quinolyl ligands†
Cite this: DOI: 10.1039/d1dt02438h
a
b
a
a
Jessica E. Waters, Schirin Hanf,
Marina Rincón-Nocito, Andrew D. Bond,
c
a
d
Raul García-Rodríguez,
Dominic S. Wright * and Annie L. Colebatch
*
The effects of moving the donor N-atom from the 2-position in lithium (2-pyridyl)- and (2-quinolyl)alu-
minates to the more remote position in (8-quinolyl)aluminates have been investigated by solid-state
structural and DFT computational studies of the new complexes [{EtAl(2-qy)
3
}Li(μ-X)Li(THF)
3
] (X = Cl/Br
−
+
−
+
6
3 3
2 : 38) [(1)Li(μ-X)Li(THF) ], [{(EtAl(2-qy) )Li}
2
(μ-Br)] Li(THF)
4
[{1Li}
2
(μ-Br)] Li(THF)
4
, [{EtAl(2-Me-8-qy)
3
}
Li] [(2)Li], [{Me
2
Al(2-Me-8-qy)
2
}Li(THF)] [(3a)Li(THF)], [{Me
2
Al(6-Me-2-py)
2
}Li(THF)
2
] [(4)Li(THF) ] and
2
[
{{EtAl(2-Me-8-qy)
2
}
2
O}(Li
2
THF)] (5). Increasing the remoteness of the donor N-atom from the bridgehead
+
Received 23rd July 2021,
Accepted 12th August 2021
results in large differences in the coordination of the Li cations by the (8-quinolyl)aluminate anions com-
pared to 2-quinolyl or 2-pyridyl counterparts. The results are of potential interest in understanding how
the coordination sites of ligands of this type can be tuned for the coordination requirements of specific
metal centres.
DOI: 10.1039/d1dt02438h
rsc.li/dalton
1
1
cations similar to their tris(pyrazolyl) counterparts. Until
fairly recently, however, few examples have included bridge-
The development of tripodal, facially-coordinating ligands has head atoms beyond Period 3 of the periodic table.
Introduction
1
2
been an important and on-going challenge over the past three
The incorporation of heavier, more metallic bridgehead
decades. Such ligands have extensive applications in modern atoms into tris(2-pyridyl) ligands provides a means of chan-
coordination, organometallic and biomimetic chemistry, as ging their donor properties systematically by descending a par-
1
–5
−
well as in catalysis. Tris(pyrazolyl)borate (Tp ) ligands, first ticular p-block group. This has been demonstrated by a recent
6
synthesised by Trofimenko and co-workers (Fig. 1a), are one study of the neutral group 15 tris(2-pyridyl) ligands E(6-Me-2-
3
of the most versatile families of tripodal ligands, in part py) , (E = As, Sb, Bi, Fig. 1c) in which it was shown that the
arising from the ability to tune their steric and electronic pro- increase in the Lewis acidity of the bridgehead can be used to
perties by introducing substituents in their pyrazolyl modulate both their coordination behaviour and the catalytic
7
–9
13
moieties. Although they have been studied to a considerably activity of their complexes. In addition, by changing the
lesser extent than their tris(pyrazolyl)borate relatives, tris(2- element or oxidation state of the bridgehead new anionic var-
pyridyl) ligands have emerged as another important class. The iants can be accessed which are directly analogous to tris(pyra-
−
majority of studies in the past four decades have focused on zolyl)borates. Tris(2-pyridyl)aluminates, [RAl(2-py′)
ligands of the type E(2-py) containing lighter, non-metallic
bridgehead atoms (py = pyridyl, E = CR, COR′, CH, N, P, PvO,
3
]
(R =
3
1
0
etc.; Fig. 1b). These ligands have found a variety of appli-
a
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road,
Cambridge CB2 1EW, UK. E-mail: dsw1000@cam.ac.uk
b
Karlsruhe Institut für Technologie (KIT), 76131 Karlsruhe, Baden-Württemberg,
Germany
c
GIR MIOMeT-IU Cinquima-Química, Inorgunica Facultad de Ciencias, Universidad
de Valladolid, Campus Miguel Delibes, 47011 Valladolid, Spain
d
Research School of Chemistry, The Australian National University, Canberra, ACT
2601, Australia. E-mail: annie.colebatch@anu.edu.au
3
Fig. 1 Notable examples of C -symmetric tripodal ligands featuring a
†
Electronic supplementary information (ESI) available: Details of synthesis,
range of bridgeheads and donor groups; (a) tris(pyrazolyl)borate anion,
spectroscopy, calculations and X-ray crystallography. CCDC 2096424–2096428
and 2097862. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1dt02438h
(b) “classical” neutral tris(2-pyridyl) ligands, (c) tris(2-pyridyl) ligands,
E(6-Me-2-py)
3
, containing heaver group 15 elements, (d) tris(2-pyridyl)
aluminate anion, (e) example of a tris(3-pyridyl) ligand, PhSn(3-py)
3
.
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Fig. 2 (a) 2-Quinolyl (2-qy), and (b) 2-Me-8-quinolyl (2-Me-8-qy) var-
iants presented in this study.
alkyl, py′ = substituted pyridyl, e.g., Fig. 1d) are a particularly
interesting class of these anionic ligands, which have found
applications in the formation of lanthanoid(II) sandwich com-
1
4
11
plexes, in the iron-catalysed epoxidation of styrene and as
1
5
chiral discrimination reagents. Closely related anionic tris(2-
pyridyl)borates have also been employed recently in a range of
materials applications.
1
6
Changing the position of the donor N-atoms within the
pyridyl ring units from the 2- to 3-position significantly alters
the coordination characteristics, allowing the coordination of 62 : 38) [(1)Li(μ-X)Li(THF) ], [{(EtAl(2-qy) )Li} (μ-Br)] Li(THF)
Fig. 3 Bis- and tris(quinolyl)-based main group ligands and complexes
explored in the current study: [{EtAl(2-qy)
3
}Li(μ-X)Li(THF)
2
3
] (X = Cl/Br
−
+
3
3
4
[{1Li} (μ-
2
−
+
more than one metal centre. Studies of the group 14 ligands Br)] Li(THF)
4
,
3 2 2
[{EtAl(2-Me-8-qy) }Li] [(2)Li], [{Me Al(2-Me-8-qy) }Li
1
7
18
2 2 2
(THF)] [(3a)Li(THF)], and [{{EtAl(2-Me-8-qy) } O}(Li THF)] (5).
MeSi(3-py)
3
and PhSn(3-py)
3
(Fig. 1e) have shown that
these can form a range of extended and molecular supramole-
cular arrangements with various metal ions. So far, however,
the functionalisation of tris(pyridyl) ligands with polyaromatic
N-donor groups has been largely ignored as a means of modi-
fying the coordination site, the only example of this type con-
EtAlCl
2
(3 : 1 equiv.) only afforded a small crop of crystalline
material, which contained a mixture of the ion-paired complex
[(1)Li(μ-X)Li(THF) ] (X = Cl/Br in a ratio of 62 : 38, respectively)
3
−
+
and ion-separated complex [{1Li}
2
(μ-Br)] Li(THF)
4
(as shown
taining a heavier element bridgehead being MeSi(3-qy)
quinolyl).
3
(qy =
However, an earlier study showed that sequential
replacement of 2-pyridyl groups with 2-quinolyl substituents in
the HC(2-py) ligand can lead to large changes in the geome-
1
7a
by single-crystal X-ray diffraction studies, Scheme 1). The
reason for the low yield appears to be the instability of the
anion 1, presumably to reductive elimination to form 2,2′-
3
1
1
9
biquinoline, which was detected in the H NMR spectroscopic
tries of coordinated metal centres. The quinolyl substituent
is a particularly interesting candidate for ligand modification
since the position of the donor N-atom relative to the bridge-
head atom can be changed readily, making the coordinated
metal centre closer (i.e., 2-qy) or further away (i.e., 8-qy) from
the bridgehead (Fig. 2).
and HR-MS studies of the crude reaction mixture (see ESI†).
Similar reductive elimination processes have been observed
with tris(2-pyridyl) systems involving other main group
1
2,13
element bridgeheads.
The solid-state structures of [(1)Li(μ-X)Li(THF)
{1Li}
(μ-Br)]− anion are shown in Fig. 4. Both complexes have
conceptually similar arrangements, in which one of the Li
3
] and of the
[
2
In this study we investigate the synthesis and coordination
properties of a series of tris(quinolyl) aluminate ligands con-
taining 2-qy and 8-qy substituents.
+
Results and discussion
With the previous background in mind, we set out to obtain a
range of bis- and tris-quinolyl ligands containing main group
bridgehead atoms. The new compounds synthesised in the
current study are shown in Fig. 3, which also shows the num-
bering scheme used throughout the discussion and
Experimental sections.
Our studies started by exploring the synthesis of the alumi-
−
nate anion [EtAl(2-qy)
3
] (1), in which the N-donor functional-
III
ity is in close proximity to the Al bridgehead, providing a
similar environment for metal coordination to the previously
−
explored aluminates [RAl(2-py)
2
3
] . However, lithiation of
n
-bromo-quinoline with BuLi at −78 °C in THF followed by
Scheme 1 Synthesis of [(1)Li(μ-X)Li(THF)
3
] (X = Cl/Br 62 : 38) and ion-
−
+
in situ reaction of the intermediate 2-lithio-quinoline with separated complex [{1Li}
2
(μ-Br)] Li(THF)
4
.
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Scheme 2 Synthesis of (2)Li.
n
halogen exchange of 8-bromo-2-methylquinoline with BuLi
was accomplished at −78 °C in THF, followed by (3 : 1) reaction
2
with EtAlCl (Scheme 2). Crystals of the new complex [{EtAl(2-
Me-8-qy) }Li] [(2)Li] were obtained in 42% yield after workup.
3
1
The room-temperature H NMR spectrum in D –THF shows
8
the expected 1 : 3 ratio of the Et and 2-Me-8-qy groups. The
single-crystal X-ray structure of (2)Li is that of a monomeric
+
arrangement in which the Li cation has a trigonal planar geo-
metry [N–Li–N range 118.97(3)–120.45(3)°]. This appears to
result from a combination of the geometric disposition of the
Fig. 4 (a) Structure of [(1)Li(μ-X)Li(THF)
3
] (X = Cl/Br 62 : 38) and (b) the quinolyl N-donor atoms and the steric shielding of the Li-
(μ-Br)] . H-atoms and lattice solvation have been omitted centre (which is therefore not able to attain a tetrahedral geo-
−
anion [{1Li}
2
for clarity. Selected bond lengths (Å) and angles (°): [(1)Li(μ-X)Li(THF)
Et–Al 1.987(7), Cqy–Al range 2.007(6)–2.021(6), N–Li range 2.164(10)–
.191(10), Al⋯Li 3.060(9), Li–O range 1.880(12)–1.902(11), Li–X range
.32(2)–2.511(15), Cqy–Al–Cqy range 104.4(2)–106.1(2), N–Li–N range
3
]:
metry by the coordination of a THF molecule). It can be noted
in this regard that (albeit weak) coordination of the Li cation
C
2
2
+
by a THF ligand does occur in the analogous lithium tris(2-
−
22
1
00.9(4)–104.8(4), Li–Cl–Li 164.6(9), Li–Br–Li 153.2(8). [{1Li}
2
(μ-Br)] : pyridyl)aluminate complex [{EtAl(6-Me-2-py)
3
}Li(THF)], also
C
Et–Al range 1.978(7)–1.980(7), Cqy–Al range 2.009(7)–2.022(7), N–Li containing Me-groups adjacent to the N-donor atoms (i.e., in a
range 2.145(11)–2.198(12), Al⋯Li range 3.054(11)–3.058(10), Li–Br range
.558(11)–2.568(10), Cqy–Al–Cqy range 101.9(3)–107.6(3), N–Li–N range
7.2(4)–108.8(5), N–Li–Br range 111.0(4)–118.8(5), Li–Br–Li range 126.0
similar position to those present in (2)Li). The trigonal planar
geometry observed in (2)Li is in marked contrast to the three-
2
9
+
coordinate, pyramidal geometry of the Li cation found in the
(
(
5)–129.0(5). Colour code: C (grey), Al (pink), N (blue), Li (magenta), Cl
green), Br (brown), O (red).
2
3
unsolvated complex [{EtAl(6-Me-2-py)
loss of THF from [{EtAl(6-Me-2-py) }Li(THF)].
Unexpectedly, despite the apparently greater remoteness of
3
}Li], resulting from the
3
+
cations is coordinated by the three N-atoms of the aluminate the coordinated Li cation in (2)Li from the bridgehead Al
, with the pseudo-tetrahedral geometry being completed by a centre compared to 2-pyridyl aluminates, the Al⋯Li separation
1
−
−
+
halide (Cl or Br ) anion. The halide ions bridge the two Li
cations (with a Li(THF)
similar to those observed previously for the tris(2-pyridyl)alu- 2-py)
(2.736(5) Å) is significantly shorter than that seen in tris(2-
+
3
or (1)Li fragment). The structures are pyridyl)aluminate complexes (e.g., mean 2.84 Å in [{EtAl(6-Me-
2
3
+
3
}Li] ). The accommodation of the trigonal planar Li
and cation within the coordination site of the aluminate anion 2
2
0
minate complexes [MeAl(3-Me-2-py) Li(μ-Br)Li(THF) ]
3
3
−
21
2
3 2
[{(MeAl(2-py) )Li} (μ-Cl)] . The similarity of the new 2-qy results in noticeable misalignment of the sp lone pairs of the
complexes to these previous examples is perhaps unsurprising, N-atoms, with the planes of the quinolyl groups being tilted by
bearing in mind the similar position of the N-donor atoms, ca. 30° with respect to the N–Li bond axes (Fig. 5b). This
and indicates that the presence of the fused-benzene substitu- suggests that there is significant strain in this arrangement.
ent has little effect on the coordination environment com-
A
similar synthetic procedure involving lithiation of
pared to the parent (2-pyridyl)aluminates.
8-bromo-2-methylquinoline in THF and in situ reaction of the
In light of our initial work on the (2-quinolyl)aluminates, intermediate lithio-quinoline in a 2 : 1 stoichiometric ratio with
we decided to direct our investigation to aluminates incorpor- Me AlCl gave the bis(2-methyl-8-quinolyl)aluminate complex
ating 2-methyl-8-quinolyl substituents. In previous work on [{Me Al(2-Me-8-qy) }Li(THF)] [(3a)Li(THF)] (Scheme 3). This was
2
2
2
tris(2-pyridyl) ligand systems we had shown that the presence isolated in 50% yield after crystallisation from toluene. The
of a Me-substituent adjacent to the donor N-atom increases solid-state structure was determined by single-crystal X-ray ana-
the stability by supressing the elimination of bipyridines. At lysis. The structure is that of a monomer in which the [Me Al(2-
2
−
+
2
the same time, it was reasoned that the greater separation of Me-8-qy) ] anion (3a) coordinates the Li cation via the two qy–
the N-donor atoms from the bridgehead would result in a sig- N atoms (Li–N range 2.046(5)–2.075(5) Å), with the Al⋯Li
nificantly different coordination environment. Lithium- contact (2.808(4) Å) being slightly longer than that seen in (2)Li
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Fig. 6 Solid-state structure of [(3a)Li(THF)] (in the toluene mono-
solvate). H-atoms on the qy groups and the lattice toluene molecule
have been omitted for clarity. Selected bond lengths (Å) and angles (°):
C
Me–Al (terminal) 1.998(2), CMe–Al (bridging) 2.024(3), Cqy–Al range
2.019(2)–2.025(3), N–Li range 2.046(5)–2.075(5), C⋯Li 2.449(5) (C–
H⋯Li 1.84), Li–O 2.002(5), Al⋯Li 2.808(4), Cqy–Al–Cqy 109.03(10), N–
Li–N 113.6(2), O–Li–N range 97.50(19)–135.3(2), Al–C⋯Li 77.1(1). Colour
code: C (grey), Al (pink), N (blue), Li (magenta), O (red), H (white).
Fig. 5 (a) Structure of (2)Li. H-atoms and THF lattice solvent have been
omitted for clarity. Selected bond lengths (Å) and angles (°): CEt–Al
2
.000(3), Cqy–Al range 2.043(3)–2.052(3), N–Li range 1.952(5)–1.971(5),
Al⋯Li 2.736(5), Cqy–Al–Cqy range 109.27(11)–113.97(12), N–Li–N range
18.9(2)–120.6(3). (b) View down the approximate Al⋯Li C -axis of the
molecule, showing the sterically-shielded Li cation and the distorted
Although the two Me–Al groups are inequivalent in the
solid-state structure, the room temperature H NMR spectrum
1
3
1
+
arrangement of the three 2-Me-8-qy substituents. Colour code: C shows only one Me resonance (at δ −0.34 ppm in D
grey), Al (pink), N (blue), Li (magenta), H (white).
–THF),
8
1
(
suggesting fluxionality. However, variable temperature
H
NMR data revealed no splitting of this peak, even at substan-
tially low temperatures (ca. 238 K), suggesting that the acti-
vation energy for this process is very low. Confirmation of the
persistence of the C–(H)⋯Li interactions in solution and the
1
7
maintenance of its molecular structure is seen in the H– Li
HOESY NMR spectrum, which shows two (albeit weak) corre-
+
lations to the Li cation – one to the Al–CH3 peak at δ
−0.34 ppm and another to the 2-Me substituents of the quino-
lyl group at δ 2.58 ppm (see ESI†).
In a background study we also prepared and structurally
Scheme 3 Synthesis of (3a)Li(THF).
characterised the 6-methyl-2-pyridyl analogue of 3a [Me
Me-2-py) ]
2
2
Al(6-
(4), which was isolated as the bis-THF adduct [(4)
Li(THF) ] (see ESI†). Not unexpectedly, bis-THF solvation of
−
2
+
+
(
Fig. 6). Further Li coordination by a THF molecule, and the the Li cation is preferred to the monosolvation and accompa-
presence of an additional Al–Me⋯Li interaction with one of the nying Al–Me⋯Li interaction that is observed for the quinolyl
Me groups of the Al bridgehead, results in a distorted trigonal system. It can therefore be concluded that the unusual
pyramidal geometry at the Li centre (in which the N and O arrangement in [(3a)Li(THF)] arises primarily from the steric
atoms are approximately coplanar). The most significant effects of the 2-Me-8-qy groups on the coordination site of the
Me⋯Li interaction in the complex (C⋯Li 2.449(5) Å; H⋯Li aluminate anion, as well as the change in geometry required to
1
.84 Å) compares to mean C⋯Li and H⋯Li distances of 2.285 accommodate the additional distance between the N-donor
and 2.07 Å, respectively, found in previously reported lithium atom and the Al bridgehead.
24,25
aluminates containing similar interactions.
In addition,
DFT calculations were carried out on [(3a)Li(THF)] to
there are two longer-range C–H⋯Li interactions with the 2-Me explore the nature of the bonding within the Al–Me⋯Li
groups of the quinolyl ligands (2.61 Å). It is worthwhile noting bridge. The data presented here were geometry optimised at
2
7
28,29
that the type of bridgehead alkyl⋯Li interaction found in [(3a) the TPSS /def2-TVZP
level of theory, which produced Li–H
Li(THF)] has not been observed in the few previously reported and Li–C bond lengths that were most consistent with the
26
bis(2-pyridyl) group 13 complexes of Ga and In – [(3a)Li(THF)] X-ray structure. However, the results were also confirmed using
3
0–32
33,34
being the first bis-aluminate of this class.
BP86
and B3LYP
functionals. A single point calcu-
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lation of the optimised structure was used for the population density at the bond critical point of the C–Li interaction is
3
5,36
and NBO analyses.
From these calculations (Fig. 7), it can −0.402 eV, from which the bond energy for this interaction
be seen that the formation of the Al–Me⋯Li interaction results can be estimated to be 2.55 kcal mol− (in line with the NBO
1
−
1 38
in weakening of the Al–C bond compared to the terminal Al–C analysis, with delocalisation energy of 3.72 kcal mol ).
bond and a small net increase in the total negative charge of However, no bond critical point could be found between the H
the Me group (from −1.20e for the terminal CH group to and Li atoms using the AIMS approach. Based on the NBO
1.21e for the bridging CH , −0.59e to −0.61e for their C results and the AIMS analysis, the distinction between a weak
atoms). From the Wiberg bond order and second-order pertur- hydrogen bond or agostic interaction is not trivial and can
3
−
3
3
9
bation theory, the presence of a very weak interaction largely lead to mis-assigned interactions.
However, despite the
involving one of the H-atoms and the C-atom of the bridging reduced charge on H, which is typical for agostic interactions,
Me-group can be assumed (see ESI† for further details). A σ(C– the geometric parameters of the Al–Me⋯Li bridge, such as the
−
1
H)⋯Li delocalisation energy of 3.72 kcal mol was computed. large Li–H⋯C bond angle of 117° and the presence of
To verify the nature of this C–H⋯Li interaction further and to different Li⋯H and Li⋯C distances, point more towards the
classify it either as hydrogen bond or agostic interaction, an existence of a weak hydrogen bond rather than an agostic
3
7
40
Atoms in Molecules (AIMS) analysis was carried out (see interaction. This conclusion is in line with a recent com-
Fig. 7). From this a bond critical point between the bridging bined experimental and theoretical study that suggested that
carbon atom and the lithium was identified. The electron an NBO based σ(C–H)⋯M delocalisation energy of >5.0 kcal
−
1
mol is characteristic of an agostic interaction (i.e., well above
−1
the 2.55 kcal mol calculated for the interaction in [(3a)Li
4
1
(
THF)]).
Repeated attempts to prepare the analogue of [(3a)Li(THF)]
containing an Et Al bridgehead, [{Et Al(2-Me-8-qy) Li(THF)]
(3b)Li(THF)], using the same synthetic procedure but with
2
2
2 2
}
[
Et
2
AlCl in place of Me
2
AlCl, unexpectedly produced the crystal-
O}(Li THF)] (5) in
variable yields from a few crystals up to 23% (with respect to
Et AlCl supplied). The complex contains an [{EtAl(2-Me-8-
qy)
line aluminate complex [{{EtAl(2-Me-8-qy)
2
}
2
2
2
2
−
2
}
2
O]
dianion in which two Al centres are bridged
together by an oxo-ligand. This arrangement notionally results
from reaction of the desired product [(3b)Li(THF)], with adven-
titious H O during crystallisation at −20 °C or present in the
2
solvent used, supported by the observation of ethane for-
mation as one of the by-products (Scheme 4). However,
attempts to obtain the complex by the deliberate addition of a
stoichiometric amount of H O after in situ formation of [(3a)Li
2
(
THF)] led only to a mixture of products (including free quino-
line). Compound 5 proved to be highly moisture sensitive and
was characterised only by single-crystal X-ray analysis and
elemental analysis. The formation of the complex is of interest
with respect to previous studies of the hydrolysis and alcoholy-
sis of lithium tris(2-pyridyl)aluminate complexes, which
showed that the Al–C bonds to the 2-pyridyl groups are con-
siderably more reactive than the Al-bonded alkyl groups of the
1
5,23
bridgehead;
the opposite to the reactivity pattern observed
for the putative intermediate [(3b)Li(THF)]. A potential expla-
Fig. 7 Top: Selected natural charges (in blue) and NBO bond orders (in
2
7
28,29
red) from the TPSS /def2-TVZP
Bottom: Graph of the Laplacian of the electron density of compound
(3a)Li(THF)]. The red lines represent bond critical paths and the blue
optimised structure of [(3a)Li(THF)].
[
points are bond critical points within the Li(2)–C(6)–H(8) plane. The
numbering scheme refers to that applied in the DFT calculations.
2
Scheme 4 The partial hydrolysis of [(3b)Li(THF)] with adventitious H O.
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the closely related complex [{Et
2 2
Al(2-Me-8-qy) }Li(THF)] [(3b)Li
2
−
(
THF)] with H O gives the O-bridged [{EtAl(2-Me-8-qy) } O]
2
2 2
dianion. Interestingly, this reactivity is the opposite of that
−
3
seen in the case of tris(2-pyridyl)aluminates [RAl(2-py) ] , in
which the 2-py groups are more basic than the R-group.
Overall, the results illustrate that functionalisation of the alu-
minate frameworks with polyaromatic N-donor substituents
provides the means of introducing radically different coordi-
nation environments and that this should be a promising area
of study in the future.
Fig. 8 Solid-state structure of 5. H-atoms and the lattice THF molecule
have been omitted for clarity. Selected bond lengths (Å) and angles (°):
Experimental section
C
1
Et–Al 1.995(3)–2.003(3), Cqy–Al range 2.015(3)–2.042(3), Al–O range General experimental methods
.7693(17)–1.7720(17), N–Li range 2.088(5)–2.175(5) (the longest of
Syntheses were carried out on a Schlenk line under a nitrogen
atmosphere using oven-dried glassware, unless otherwise speci-
these bonds is to the μ-N-quinolyl group), μ-Nqy⋯Li 2.467(5), Li–Ooxo
range 1.926(5)–1.947(4), Cqy–Al–Cqy range 105.10(10)–106.90(10), Al–
O–Al 135.47(10), Nqy–Li–Nqy 100.49(19)–107.0(2) (within each subunit), fied. Starting materials were commercially obtained from sup-
Li–(μ-Nqy)⋯Li 76.52(16). Colour code: C (grey), Al (pink), N (blue), Li
pliers and used as received. Lower temperatures in synthesis
(
magenta), O (red).
were achieved using dry ice/acetone (−78 °C) baths. MeCN and
CH Cl were dried over CaH and distilled under nitrogen.
Et O, n-hexanes and THF were dried over Na/benzophenone
2
2
2
2
nation for this is provided by the previously discussed DFT cal-
culations of [(3a)Li(THF)], which showed that bridging of the
Me-group between the Al and Li atoms results in significant
weakening of the Al–C bond compared to the terminal Me–Al
bond (as seen in the reduction in NBO bond order from 0.62
to 0.56).
and distilled under nitrogen. Deuterated solvents were distilled
and/or dried over molecular sieves before use. A nitrogen-filled
glove box (Saffron type α) was used to manipulate solids, includ-
ing room temperature reactions, product recovery and sample
preparation for analysis. Yields are given as isolated yields of
1
7
13
1
solid or crystalline products. Room temperature H, Li, C{ H}
The molecular structure of 5 consists of an [{EtAl(2-Me-8-
27
and Al NMR spectra were recorded on a Bruker 400 MHz
2
−
qy) } O] dianion in which two [EtAl(2-Me-8-qy) ] subunits are
2
2
2
Avance III HD Smart Probe spectrometer and referenced to the
joined together by a bridging O-atom (Fig. 8). There are two
chemically distinct Li environments in this arrangement, with
27
7
residual solvent peaks. For Al and Li NMR, external refer-
ences were used (AlCl ·6H O and 1 M LiCl in D O, respectively).
3
2
2
+
both Li cations being coordinated by the two quinolyl
Unambiguous assignments of NMR resonances were made on
N-atoms from each [EtAl(2-Me-8-qy)
.175(5) Å), but with one THF being solvated while in the other
the THF solvation is replaced by a long-range μ-N⋯Li inter-
action with a quinolyl N-atom of the other [EtAl(2-Me-8-qy)
2
] subunit (range 2.088(5)–
1
1
1
13
the basis of 2D NMR experiments ( H– H COSY, H– C HSQC,
2
1
13
1
13
H– C HMBC and H– C HMQC). Fig. 9 shows the labelling
scheme for NMR assignments. Mass spectra were obtained by
positive ion electrospray ionisation using a Thermo Fisher
Orbitrap mass spectrometer. Elemental analysis for carbon,
hydrogen, and nitrogen was performed using a PerkinElmer
2
]
+
subunit (2.467(5) Å). As a result, both Li cations have dis-
torted tetrahedral geometries.
240 Elemental Analyser. X-ray crystallographic data were col-
lected using either a Nonius KappaCCD (sealed-tube MoKa) or a
Bruker D8-QUEST PHOTON-100 (Incoatec IμS Cu microsource)
diffractometer. The temperature was held at 180(2) K using an
Conclusion
Oxford Cryosystems N cryostat. Structures were solved using
SHELXT and refined using SHELXL.
2
The results of this investigation show that moving the donor
N-atom from the 2-position of 2-pyridyl or 2-quinolyl groups to
the more remote position in 8-quinolyl substituents has a
large effect on the coordination of Li cations in the corres-
ponding aluminate complexes. This is seen perhaps most dra-
4
2
43
Synthesis of new compounds
+
[
{EtAl(2-qy) }Li(μ-X)Li(THF) ] (X = Cl/Br 62 : 38) [(1)Li(μ-X)Li
3
3
−
+
(
THF)
3
]
and [{(EtAl(2-qy)
3
)Li}
2
(μ-Br)] Li(THF)
4
2
[{1Li} (μ-
3
matically in the structure of [{EtAl(2-Me-8-qy) }Li] (2Li), in
+
which the Li cation has an unusual three-coordinate, trigonal
planar arrangement stemming from a combination of steric
effects and the geometric constraints of the donor N-atoms.
The bis(quinolyl)aluminate complex [{Me Al(2-Me-8-qy) }Li
2
2
(
THF)] [(3a)Li(THF)] is of particular interest, in which N-donor
bonding in the tris(2-methyl-8-quinolyl)aluminate is replaced
by a bridging C–H⋯Li interaction. The adventitious reaction of Fig. 9 Labelling scheme for NMR assignments.
Dalton Trans.
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−
+
Br)] Li(THF)
4
4
. A solution of 2-bromoquinoline (900 mg, was stirred at −78 °C for 1 h. Dimethylaluminium chloride
n
.32 mmol) in THF (30 mL) was cooled to −78 °C. BuLi (1.6 (1.0 M in hexanes, 1.5 mL, 1.5 mmol) was added. The solution
M in hexanes, 2.7 mL, 4.32 mmol) was added dropwise and was allowed to warm to room temperature and stirred over-
the red solution was stirred at −78 °C for 2.5 h. In a separate night. The volatiles were removed from the orange solution
flask, ethylaluminium dichloride (1.0 M in hexanes, 1.43 mL, under vacuum. The residue was extracted with toluene (40 mL)
1
.43 mmol) was diluted in 10 mL THF and kept at −78 °C. and filtered through Celite. The solution was concentrated
This solution was transferred to the first flask (containing under vacuum until a precipitate formed. The precipitate was
-lithio-quinoline) dropwise with a cannula. The solution was dissolved by the addition of THF (ca. 5 mL), and the solution
2
allowed to warm to room temperature and stirred overnight. was stored in the freezer at −20 °C. Needle-like colourless crys-
The volatiles were removed from the dark brown solution tals were collected by filtration and dried under vacuum.
1
under vacuum, and toluene (20 mL) and THF (5 mL) were Isolated yield (two batches) 320 mg (0.762 mmol, 50%).
added to the residue. The mixture was gently heated, filtered, NMR (298 K, D
and concentrated under vacuum. Storage at −15 °C afforded a HH 6.7, 1.1), 8.06 (d, 2 H, H4, JHH 8.2), 7.56 (dd, 2 H, H5, JHH
few colourless crystals which were shown to be a mixture of 8.1, 1.4), 7.42 (dd, 2 H, H6, JHH 8.1, 7.5) 7.17 (d, 2 H, H3, JHH
H
8
–THF, 400 MHz), δ [ppm] = 8.25 (dd, 2 H, H7,
J
1
both products by X-ray crystallography. H NMR (298 K, D
8
–
8.3), 2.70 (s, 6 H, qy–CH
3
3
), −0.34 (s, 6 H, Al–CH ) (the THF
THF, 400 MHz), δ [ppm] = 8.79 (d, 3 H, H8, JHH 8.4), 7.89 (d, 3 ligand is partially or completely absent as a result of the labi-
H, H3, JHH 8.1), 7.82 (d, 3 H, H4, JHH 8.1), 7.65 (d, 3 H, H5, JHH lity of the ligand in the complex when placed under vacuum
1
3
1
7
.7), 7.56–7.52 (m, 3 H, H6/H7), 7.32 (dd, 3 H, H6/H7, JHH 7.6), during isolation). C{ H} NMR (298 K, D8–THF, 101 MHz), δ
.56 (t, 3 H, Et–CH , JHH 7.2), 0.78 (q, 2 H, Et–CH , JHH 8.1). [ppm] = 155.9 (C2), 154.6 (C9), 140.8 (C7), 138.7 (C4), 125.9
1
3
2
1
3
1
C{ H} NMR (298 K, D –THF, 126 MHz): δ [ppm] = 194.4 (br, (C10), 125.3 (C6), 124.8 (C5), 120.0 (C3), 24.0 (qy–CH ). C8 and
not observed. Al NMR (298 K, D
C5), 149.8 (C5 or C6), 131.0 (C3), 130.9 (C4), 129.7 (C7), 128.6 [ppm] = 146.9. Li NMR (298 K, D
8
3
1
13
27
C2, detected through H– C HMBC experiment), 151.2 (C6 or Al–CH
3
8
–THF, 104 MHz), δ
7
8
–THF, 155 MHz), δ [ppm] =
(
C10), 128.1 (C8), 124.9 (C9), 11.21 (Et–CH ), −0.2 (br, Et–CH , 3.5. Elemental analysis (%): calcd for C H AlLiN (THF mole-
3
2
22 22
2
1
13
27
detected through H– C HMQC). Al NMR (298 K, D
1
1
8
–THF, cule removed) C 75.8, H 6.4, N 8.0; found C 75.5, H 6.9, N 8.3.
–THF, [{{EtAl(2-Me-8-qy) O}(Li THF)] (5). A solution of 8-bromo-
2-methylquinoline (666 mg, 3.00 mmol) in THF (40 mL) was
7
30 MHz): δ [ppm] = 128 (br, s). Li NMR (298 K, D
94 MHz): δ [ppm] = 1.02.
8
2
}
2
2
n
[
{EtAl(2-Me-8-qy)
3
}Li] [(2)Li]. A solution of 2-bromoquinoline cooled to −78 °C. BuLi (1.6 M in hexanes, 1.9 mL, 3.00 mmol)
(
666 mg, 3.00 mmol) in THF (40 mL) was cooled to −78 °C. was added dropwise and the red solution was stirred at −78 °C
BuLi (1.6 M in hexanes, 1.9 mL, 3.00 mmol) was added drop- for 1 h. Diethylaluminium chloride (1.0 M in hexanes, 1.5 mL,
n
wise and the red solution was stirred at −78 °C for 1 h. 1.5 mmol) was added. The solution was allowed to warm to
Ethylaluminium dichloride (0.9 M in heptane, 1.5 mL, room temperature and stirred overnight. The volatiles were
1
.5 mmol) was added. The solution was allowed to warm to removed from the orange solution under vacuum. The residue
room temperature and stirred overnight. The volatiles were was extracted with toluene (40 mL) and filtered through Celite.
removed from the orange solution under vacuum. The residue The solution was concentrated under vacuum until a precipi-
was extracted with toluene (40 mL) and filtered through Celite. tate formed. The precipitate was dissolved by the addition of
The solution was concentrated under vacuum until a precipi- THF (ca. 5 mL), and the solution was stored in the freezer at
tate formed. The precipitate was dissolved by the addition of −20 °C. Needle-like colourless crystals were collected by fil-
THF (ca. 5 mL), and the solution was stored in the freezer at tration and dried under vacuum. Isolated yield (first reaction)
−20 °C. Needle-like colourless crystals were collected by fil- 0.246 mg (0.346 mmol, 23%), however, further attempts pro-
tration and dried under vacuum. Isolated yield (two batches) duced variable yields lower than this. Elemental analysis (%):
1
4
16 mg (0.850 mmol, 43%). H NMR (298 K, CD Cl , 400 MHz), calcd for C H Al Li N O C 74.3, H 6.0, N 7.9; found C 73.8,
2 2 44 42 2 2 4
δ [ppm] = 8.22 (dd, 3 H, H7, JHH 6.7, 1.4), 8.11 (d, 3 H, H4, JHH H 5.9, N 7.6.
8
.3), 7.63 (dd, 3 H, H5, JHH 8.0, 1.4), 7.42 (dd, 3 H, H6, JHH 7.8,
6
.7), 7.16 (d, 3 H, H3, JHH 8.3), 2.18 (s, 9 H, qy–CH ), 1.40 (t, 3
3
1
3
1
H, Et–CH
3
, JHH 7.9), 0.72 (q, 2 H, Et–CH
Cl , 101 MHz), δ [ppm] = 156.5 (C2), 154.7
C10), 142.2 (C7), 139.6 (C4), 126.4 (C9), 126.3 (C6), 126.2 (C5),
2
, JHH 7.9). C{ H}
Author contributions
NMR (298 K, CD
(
2
2
J. E. W., A. L. C. and M. R.-N. performed the synthetic
studies. S. H. did the DFT calculations. A. D. B. collected and
solved crystal data. D. S. W., A. L. C. and R. G.-R. supervised
the studies. D. S. W. and A. L. C. wrote the paper.
1
20.8 (C3), 24.3 (qy–CH
3
), 11.4 (Et–CH
Cl , 104 MHz), δ [ppm] =
37.2. Li NMR (298 K, CD Cl , 155 MHz), δ [ppm] = 4.9.
3 2
). C8 and Et–CH not
2
7
observed. Al NMR (298 K, CD
1
Elemental analysis (%): calcd for C32H29AlLiN C 78.5, H 6.0, N
3
2
2
7
2
2
8
.6; found C 77.6, H 5.8, N 8.6.
{Me Al(2-Me-8-qy) }Li(THF)] [(3a)Li(THF)]. A solution of
[
2
2
8
-bromo-2-methylquinoline (666 mg, 3.00 mmol) in THF
Conflicts of interest
n
(40 mL) was cooled to −78 °C. BuLi (1.6 M in hexanes,
1
.9 mL, 3.00 mmol) was added dropwise and the red solution The authors declare that there are no conflicts of interest.
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1
7 (a) M. S. Deshmukh, A. Yadav, R. Pant and
R. Boomishankar, Inorg. Chem., 2015, 54, 1337–1345;
(b) M. S. Deshmukh, V. S. Mane, A. S. Kumbhar and
R. Boomishankar, Inorg. Chem., 2017, 56, 13286–
13292.
Acknowledgements
We thank the Walters-Kundert Studentship of Selwyn College
(scholarship for J. E. W.), the Leverhulme Trust (R. G.-R. and
D. S. W.), the Australian Research Council (A. L. C.,
DE200100450), the Young Investigator Group preparation 18 E. S. Yang, A. J. Plajer, A. García-Romero, A. D. Bond,
program at KIT funded by the Federal Ministry of Education
and Research (BMBF) and the Baden-Württemberg Ministry of
Science as part of the Excellence Strategy of the German
T. K. Ronson, C. M. Álvarez, R. García-Rodríguez,
A. L. Colebatch and D. S. Wright, Chem. – Eur. J., 2019, 25,
14003.
Federal and State Governments. (S. H.), the state of Baden- 19 N. Wei, N. N. Murthy and K. D. Karlin, Inorg. Chem., 1994,
Württemberg through bwHPC and the German Research 33, 6093–6100.
Foundation (DFG) through grant no INST 40/575-1 FUGG 20 T. H. Bullock, W. T. K. Chan, D. J. Eisler, M. Streib and
JUSTUS 2 cluster) (S. H.), and the Spanish MINECO-AEI and D. S. Wright, Dalton Trans., 2009, 1046–1054.
The EU (ESF) for a Ramon y Cajal contract (R. G.-R., RYC-2015- 21 T. H. Bullock, W. T. K. Chan and D. S. Wright, Dalton
(
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2 R. García-Rodríguez, T. H. Bullock, M. McPartlin and
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3 R. García-Rodríguez and D. S. Wright, Chem. – Eur. J., 2015,
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This journal is © The Royal Society of Chemistry 2021
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