Monomeric Cp3tAl(i): synthesis, reactivity, and the concept of valence isomerism

Article abstract of DOI:10.1039/c8sc05175e

With the isolation of Cp^3tAl (1), the first monomeric Cp-based Al(i) species could be realized in a pure form via a three-step reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr₃. Due to its monomeric structure, reactions involving 1 were found to proceed more selectively, faster, and under milder conditions than for tetrameric (Cp*Al)₄. Thus, 1 readily formed simple Lewis acid-base adducts with tBuAlCl₂ (6) and AlBr₃ (7), reactions that before have always been interfered with by the presence of aluminum halide bonds. In addition, the 2?:?1 reaction of 1 with AlBr₃ enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N₂O and PhN₃ at room temperature to afford Al₃O₃ and Al₂N₂ heterocycles 9 and 10, respectively. With the structural characterization of products 4 and 5, the reaction of monovalent 1 with Cp^3tAlBr₂ (2) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(i)/Al(iii) Lewis adducts.

Full text of DOI:10.1039/c8sc05175e

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DOI: 10.1039/C8SC05175E
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3
t
Monomeric Cp Al(I): Synthesis, reactivity, and the concept of
valence isomerism
Alexander Hofmann, Tobias Tröster, Thomas Kupfer, and Holger Braunschweig*
Received 00th January 20xx,
Accepted 00th January 20xx
3
t
With the isolation of Cp Al (1), the first monomeric Cp-based Al(I) species could be realized in pure form via a three-step
reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr . Due to its
monomeric structure, reactions involving 1 were found to proceed more selective, faster, and under milder conditions than
for tetrameric (Cp*Al) . Thus, 1 readily formed simple Lewis acid-base adducts with tBuAlCl (6) and AlBr (7), reactions that
before have always been interfered by the presence of aluminum halide bonds. In addition, the 2:1 reaction of 1 with AlBr
enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N O and PhN at room
temperature to afford Al and Al heterocycles 9 and 10, respectively. With the structural characterization of products
and 5, the reaction of monovalent 1 with Cp AlBr
isomerism between dialanes and their Al(I)/Al(III) Lewis adducts.
DOI: 10.1039/x0xx00000x
3
www.rsc.org/
4
2
3
3
2
3
3
O
3
2 2
N
3
t
4
2
(2) provided first experimental evidence for the concept of valence
R
monomeric form (Cp Al) is established, which is highly
R
19-22
Introduction
The organometallic chemistry of aluminum is strongly
dominated by the oxidation state +III, and subvalent aluminum
species are quite rare. Even though currently on the transition
from chemical curiosities to a rather well-established class of
compounds, the synthesis of molecules with aluminum in the
low oxidation states +I and +II is still experimentally challenging
and consistently hampered by their high reactivity and
dependent on the sterics of Cp and the temperature.
For (Cp*Al) , this equilibrium lies almost completely on the
4
side of the tetrameric aggregate at ambient temperatures,
which together with the low solubility of the aggregate in
common organic solvents efficiently masks the high reactivity of
the Al(I) centers. Accordingly, (Cp*Al)
unreactive, and often higher temperatures and/or longer
1
-4
4
appears rather
R
R
Table 1 Equilibrium of (Cp Al)
4
and Cp Al in solution at rt. Species that exclusively form
5
,6
R
pronounced tendency to disproportionation. Thus, only a monomeric Cp Al and have been isolated are highlighted.
handful examples of dialanes with Al(II) centers has been
CpR
Al
reported so far.6
realized by Schnöckel and coworkers with their seminal
discoveries of metastable AlX (X = halide)
-12
Monovalent aluminum has first been
1
3,14
15,16
1/4
and (Cp*Al)
4
Al
Al
Al
Al
These studies have inspired many main group element
chemists, and several other Al(I) species have been made
CpR
CpR
temperature
available,1
7,18
including other Cp-based derivatives,
19-22
β-
sterics of CpR
2
3
diketiminate-stabilized systems, as well as the recently
reported NHC-stabilized dialumene, and aluminyl anion.
Among these, Schnöckel’s (Cp*Al) and derivatives derived
thereof still represent the best-studied subvalent aluminum
systems. It was shown that (Cp Al)
2
4
25
Species in solution
CpR
isolated
Ref.
R
R
(
Cp Al)
4
Cp Al
4
a
C
5
H
5





-
-
-
-
-


-
-
-
20
21
15
22
22
19
20
20
20
5 4
C Me H
R
4
tend to be tetrameric in the
b
Cp*
Me
Me
(CH
tBuc
iPr
solid state. In solution however, an equilibrium with their
C
C
5
4
Prb
iPr
Ph)
-
5
4


-




C
5
2
5
C
5
H
4

-
-
-
a. Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am
Hubland, 97074 Würzburg, Germany. E-mail: h.braunschweig@uni-wuerzburg.de.
C
5
4
H
b.
C
C
H
(SiMe
(SiMe
)
)

-
-
-
Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-
Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
5
3
3
2
H
5 2
3
3
20
†
Electronic Supplementary Information (ESI) available: Synthesis and
C
5 2
H
tBu
3
-

this work
characterization of new compounds, NMR spectra, crystallographic details. CCDC
878104-1878111 and 1879323. For ESI and crystallographic data in CIF or other
1
a
decomposes above -60°C. b Cp Al detectable at 60°C. decomposes above -30°C.
R
c
electronic format see DOI: 10.1039/x0xx00000x.
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reaction times are required. For this reason, much efforts have The solid state structure of 2 nicely reflects the increased
View Article Online
R
3t
DOI: 10.1039/C8SC05175E
been made to develop monomeric Cp Al species by introducing sterical requirements of the Cp ligand with respect to Cp* as
R
27
bulky Cp ligands, which are expected to prevent any evidenced by its monomeric structure (Figure 1). By contrast,
aggregation. As shown in Table 1, increasing the steric demand Cp*AlBr
2
is a halide-bridged dimer in the solid state. However,
R
of Cp to C
5
(CH
2
Ph)
5
, C
5
H
2
(SiMe
3
)
3
R
, and C
5
iPr
was detected in solution distance between the aluminum center and the Cp centroid
via Al NMR spectroscopy. However, the synthetic approaches (Al-cent 1.835 Å) compare very well with the values found in
4
H proved actually both the Al-C bond lengths (2.175(4)-2.239(4) Å), as well as the
successful, and no tetramer (Cp Al)
4
2
7
2
8
2
proved either tedious (long reaction times) and/or required Cp*AlBr (av. Al-C 2.223 Å; Al-cent 1.865 Å).
3
t
3t
special equipment (generation of metastable AlX precursors), With Cp AlBr
2
(2) in hand, we next tried to make Cp Al (1)
R
20
and did not allow for the isolation of Cp Al.
accessible via direct reductive dehalogenation, a route that has
*
Recently, we reported an alternative approach towards the been successfully applied to the synthesis of (Cp Al)
4
, for
R
16
generation of such Cp Al(I) species by Lewis base-induced instance. However, reduction of 2 with alkali metal-based
*
disproportionation of a Cp*-substituted dialane into Cp Al(I) reductants such as KC
8 8
, NaK2.8, Na, Na[C10H ], and Li under
*
26
2
and Cp Al(III)Br L (L = Lewis base). We envisaged that it might various conditions consistently failed, and no tractable
be feasible to transfer this strategy to systems with bulkier Cp materials could be isolated. Thus, we next attempted the
groups, eventually allowing for a straightforward isolation of reductive elimination pathway recently introduced by Fischer
R
*
29
pure monomeric Cp Al. Thus, we initially set out to study the and Frenking for an alternative synthesis of (Cp Al)
4
.
This
3
t
3t
suitability of 1,3,5-tri-tert-butylcyclopentadiene (Cp ) as a approach requires (Cp )
stabilizing ligand for subvalent Cp Al (1).
2
AlH as reagent, though, which upon
heating might be susceptible to Cp H elimination to generate
the desired monovalent Cp Al (1). Again, all our attempts to
AlH remained without success.
Mg and HAlCl always resulted in the
formation of elemental Al among several other undefined
species. According to Al NMR spectroscopy, only trace
amounts of a compound with a chemical shift of  –161,
indicative of the presence of 1, were present in the reaction
mixtures. Thus, we were not able to separate and isolate 1.
After these rather disappointing results we took up our most
recent findings on the related Cp* system, i.e. the Lewis base-
induced cleavage of dialane Cp*(Br)Al-Al(Br)Cp* into (Cp Al)
and Cp Al(III)Br L. Unfortunately, no Cp -substituted dialane
2
R
3t
3
t
3
t
selectively prepare (Cp )
2
Results and discussion
The synthetic protocol used for the generation of Cp Al (1) is
3t
Heating a mixture of (Cp )
2
2
3
t
2
7
outlined in Scheme 1. Accordingly, we first developed a
3
t
convenient synthesis for the precursor Cp AlBr
2
(2). While
simple salt elimination reactions of MCp (M = Li, Na, K) with
AlBr were found to be not efficient showing only poor
conversion and were, in part, accompanied by decomposition
3
t
3
3
t
2
processes, 2 was isolated in good yields by applying (Cp ) Mg in
*
2
7
4
pentane solution (Scheme 1). Due to the high solubility of the
magnesium reagent in hydrocarbon solvents, the reaction
proceeded quickly and quantitatively within 30 minutes.
Characterization of 2 in solution provided no difficulties and
NMR spectroscopic parameters agree very well with the
anticipated structure. Thus, the chemical shift of the 2 Al NMR
resonance of 2 ( –42) strongly resembles that of Cp*AlBr ( –
6). Also, the H NMR spectrum features a signal for the
aromatic protons at  6.64, and two signals for the tBu groups
at  1.41 and 1.24. Colorless crystals of 2 were obtained by
cooling a saturated hexane solution. 2 is highly sensitive
towards water and oxygen, which is evident by an immediate
color change from colorless to slightly brownish.
*
26
3t
was accessible by stoichiometric reduction of 2. However,
keeping in mind that (i) DFT calculations have established an
equilibrium between dialane Cp*(Br)Al-Al(Br)Cp* and the
*
26
7
corresponding Lewis acid-base adduct Cp AlAl(Br)
2
Cp*, which
is in line with the valence isomerism between these two classes
of compounds, as already suggested by Cowley, and (ii) Al-Al
bond formation has also been accomplished by reaction of Al(I)
AlNacNac (NacNac = ArNC(Me)CHC(Me)NAr] ; Ar
2 6 3 3 2 6 3
C H ) and AlX cAAC (X = Cl, I; cAAC = 1-[2,6-iPr C H ]-
,3,5,5-tetramethyl-2-pyrrolidinylidene), we wondered if an
2
3
0
2
8
1
4
–
species with H
2
3
1
=
3
2,6-iPr
3
2
asymmetric dialane is
Figure 1 Molecular structures of 2 (left) and 3 (right) in the solid state. Hydrogen atoms
omitted for clarity. Selected bond lengths (Å): 2 Al1-C1 2.196(4), Al1-C2 2.214(4), Al1-C3
2
2
.175(4), Al1-C4 2.187(4), Al1-C5 2.239(4); 3 Al1-Al2 2.533(1), Al1-C1 2.005(3), Al2-C11
.196(3), Al2-C12 2.180(3), Al2-C13 2.136(3), Al2-C14 2.192(3), Al2-C15 2.220(3).
Scheme 1 Experimental approach towards the synthesis of 1.
2
| Chem. Sci. 2012, 00, 1-3
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available by comproportionation of (Cp Al)
EDGE ARTICLE
*
3t
4
and Cp AlBr
2
, or if
View Article Online
DOI: 10.1039/C8SC05175E
an initial Lewis acid-base adduct is already susceptible to
3
t
release the targeted Cp Al (1) upon treatment with Lewis
bases. To this end, a solution of 2 in toluene was treated with
(
4
Cp*Al) under sonification, whereupon the color of the
2
7
suspension turned from yellow to colorless (Scheme 1). The
progress of the reaction was easily monitored by the
disappearance of the Al NMR signals of 2 ( –46) and (Cp*Al)
2
7
4
1
( –78), while no product resonances were apparent. H NMR
spectroscopic studies on the reaction mixture evidenced the
3
t
selective formation of a single product with Cp* and Cp ligands
in a 1:1 ratio, thus pointing to the formation of either an
asymmetric dialane or the asymmetric Al(I)-Al(III) Lewis acid-
1
base adduct 3. In its H NMR spectrum, the reaction product
features one signal at  1.90 for the Cp* protons, as well as
3
t
signals at  6.25 and  1.38, 1.24 for the Cp ligand. In particular,
3
t
the highfield shift of the aromatic Cp protons in comparison to
is noteworthy and favors a Cp3t ligand as part of an Al(I)
2
Scheme 2 Valence isomerism between adduct 4 and dialane 5 (top). Energy profile of the
isomerisation from 4 to 5 via transition state TS (bottom; M06L/Def2-SVP; free energies
given in kcal/mol).
fragment, thus suggesting the presence of 3 (Scheme 1).
X-ray diffraction studies on suitable single crystals of 3
2
7
eventually verified the adduct formation. As evident from
Figure 1, (Cp*Al) acted as a reductant in its reaction with 2, and
4
essentially pure 1. Even though all attempts to obtain suitable
the Cp* fragment now features a Lewis-acidic Al(III) center
27
single crystals of 1 for X-ray diffraction failed, its Al NMR
containing the two bromides formerly belonging to 2.
resonance (cf. monomeric Cp*Al(I) shows a signal at  –150 at
3
t
Accordingly, it is the Cp fragment that plays the Lewis-basic
20
6
0°C), the absence of characteristic signals for a tetrameric
3
t
5
Al(I) part in adduct 3, while the Cp ligand adopts a 
coordination mode with bonding parameters (Al-C 2.136(3)-
20
aggregate (usually between  –60 and  –110), and the results
of H DOSY NMR spectroscopic studies (single species, diffusion
coefficient D = 8.703 x e m /s; see Figure S32 in the ESI) leaves
no doubt on its constitution and its description as a monomeric,
monovalent aluminum species.
1
2
2
.220(3) Å, Al-cent 1.815 Å) similar to those in 2 (Al-C 2.175(4)-
–10
2
.239(4) Å, Al-cent 1.835 Å). By contrast, the Cp* ligand shows
1
a  coordination mode to the Al(III) center with an Al-C
distance (2.005(3) Å) in the same region as observed before. The
Al-Al separation in 3 (2.533(1) Å) is somewhat shorter than in
the Al(I)-Al(III) adducts Cp AlAltBu
Cp AlAl(C
The isolation of pure 1 also allowed us to evaluate the
concept of valence isomerism between dialanes and their
respective Al(I)-Al(III) adducts, and to probe the possibility of
*
33
3
(2.689(2) Å)
or
*
30
F )
6 5 3
(2.591(2) Å), which indicates a quite strong
3t
generating a Cp -based dialane by simply reacting 1 with one
dative Al-Al interaction in 3. The Al-Al distance in 3 is rather
3t
27
27
equivalent of Cp AlBr
2
(2; Scheme 2). The Al NMR spectrum
reminiscent of the Al-Al bond length found in dialane Cp*(Br)Al-
of the reaction mixture features a single broad resonance at  –
1
2
Al(Br)Cp* (2.530(2) Å), which might be taken as another
evidence for the close relationship between dialanes and the
corresponding Al(I)-Al(III) adducts as valence isomers.
6
4 with a chemical shift intermediate between those of 1 ( –
1
61) and 2 ( –46). Also, only one set of signals is evident in the
1
3t
H NMR spectrum, while the chemical shift of the aromatic Cp
protons ( 6.39) is found again intermediate between the values
of 1 ( 5.94) and 2 ( 6.64). Hence, while the observation of a
single set of NMR spectroscopic parameters would suggest the
formation of a symmetric dialane (5), the observed chemical
shift ( –64) significantly differs from that of dialane Cp*(Br)Al-
Al(Br)Cp* ( –11). On the other hand, for adduct 4, two sets of
signals are to be expected with substantially different Al NMR
chemical shifts as observed in this case (cf. Cp AlAl(C
1
Adduct 3 appears to be an ideal precursor for the release of
3
t
Cp Al (1) by addition of strong Lewis bases, for which reason
we reacted solutions of 3 in C with PMe , IPr (1,3-
D
6 6
3
2
7
diisopropyl-imidazol-2-ylidene), and cAAC. In all cases, Al NMR
spectroscopy showed the formation of a new signal at  -161,
indicative of a monomeric Al(I) species. In addition, the
12
expected resonances for Cp*AlBr
 106) could be found in the respective Al NMR spectra of the
reaction mixtures, while the signal of Cp*AlBr cAAC was not
2
PMe
3
( 48), and Cp*AlBr
2
IPr
27
2
7
(
*
F )
6 5 3
 –
2
30
16, 107). For a related gallium system comprising of
detected, most likely due to its broadness. The formation of all
2
Cp*Gaand GaX Cp* (X = Cl, I) Jutzi and co-workers similarly
1
three adducts was, however, clearly verified by H NMR
reported only one set of NMR signals for the Cp* moieties upon
spectroscopy, which showed the expected signal patterns for
34
mixing of the components, even at –80 °C, which indicates a
one equivalent Cp*AlBr
.20 for monovalent 1. Compound 1 was eventually isolated on
a larger scale by reaction of 3 with cAAC in pentane, making use
of the poor solubility of Cp*AlBr cAAC under these conditions
Scheme 1).27 After filtration and removing the solvent under
2
L, as well as signals at  5.94, 1.36, and
corresponding valence isomerism in the case of gallium. Thus,
our NMR spectroscopic studies rather indicate the presence of
a rapid equilibrium between adduct 4 and dialane 5, which we
tried to verify by VT NMR spectroscopic studies in toluene
solution (–90 °C to +100 °C). Unfortunately, we were not able to
derive a clear picture of this process, most likely because the
1
2
(
reduced pressure, a yellow oil remained, which contained
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Figure 2 Molecular structures of 4 (left) and 5 (right) in the solid stateV.ieHwydArrotgicelenOatnolimnes
exchange process is too fast on the NMR time scale across the
entire temperature range. Nevertheless, a few hints can be
extracted from the measurements. While low temperature Al
NMR spectroscopy was not very helpful in solving this puzzle
omitted for clarity. Selected bond lengths (Å) of 4 Al1-Al2 2.599(3), Al1-C1 2.209(7), Al1-
DOI: 10.1039/C8SC05175E
C2 2.183(7), Al1-C3 2.216(7), Al1-C4 2.192(7), Al1-C5 2.178(7), Al2-C11 2.063(7); 5 Al1-
Al2 2.586(3), Al1-C1 2.231(8), Al1-C2 2.343(8), Al1-C3 2.392(8), Al1-C4 2.247(8), Al1-C5
2
7
2
.172(8).
(
the signal only broadened, eventually disappearing in the
2
7
baseline; see Figures S28-S31 in the ESI), the Al NMR
resonance at  –64 is shifted to higher field ( –90) at increased
temperatures. This chemical shift is reminiscent for Al(I) centers
*
30
6 5 3
in Lewis acid-base adducts (cf. Cp AlAl(C F )  –116). In
addition, a low intensity signal appears at  90, which is found
Scheme 3 Lewis base reactivity of monovalent 1.
in the same region as the tetracoordinate aluminum atom of
*
Cp AlAl(C
F )
6 5 3
( 106) or adducts 6 ( 146) and 7 ( 95; see
mixture. Importantly, we were able to isolate again 4 and 5 from
both of these solutions under the same conditions as described
above, thus providing a definite experimental prove for this
isomerization process. To the best of our knowledge, this
represents the first instance, in which the suggested valence
isomerism between dialanes and their respective Al(I)-Al(III)
adducts could be demonstrated directly within a single
molecular system. It should be noted, however, that Nikonov
and co-workers had previously observed the reversible
disproportionation of a dialane to the corresponding pair of
discrete Al(I) and Al(III) species, which, in contrast to 4, did not
below). These findings suggest that at higher temperatures, the
equilibrium mixture of 4/5 is pushed to the side of adduct 4. By
1
contrast, the H NMR signals of the aromatic C
5
H
2
tBu
3
protons
are gradually shifted to lower field upon cooling the sample
rt 6.39; –60°C 6.59) and finally became chemically non-
equivalent at –80 °C, as evidenced by two low-field signals (
0°C 6.89, 6.64). It should be noted that this decoalescence is not
related to the 4/5 isomerization process, but only to the
(
–
8
3
t
rotation of the Cp rings, which gets frozen at low
temperatures. Thus, we still see only one Cp3t ligand
environment, which in combination with the low-field chemical
shift of these protons (characteristic for three-coordinate
aluminum species such as 2) indicates an equilibrium mixture
shifted to the side of the symmetric dialane 5. In order to
evaluate the thermodynamic stabilities of both species 4 and 5,
we performed DFT calculations on the M06L/Def2-SVP level of
theory (Scheme 2 (bottom)). Accordingly, dialane 5calc is
energetically more stable by 4.0 kcal/mol than the asymmetric
adduct structure 4calc, and both species are separated by a low-
lying transition state TS with an activation barrier of only
3
1
form an Al(I)-Al(III) Lewis pair.
The structural parameter of adduct 4 are comparable to
those of 3, but illustrate the increased sterical demand of the
3
t
Cp ligand. Accordingly, the Al-Al separation of 4 (2.599(3) Å) is
slightly larger than that of the less bulky adduct 3 (2.533(1) Å),
*
30
6 5 3
but in the same region as in Cp AlAl(C F ) (2.591(2) Å). In
3
t
analogy to 3, the Cp ligand of the monovalent aluminum
center of 4 (Al1; Al-C 2.178(7)-2.216(7) Å, Al-cent 1.824 Å)
adopts a  coordination mode with essentially identical
5
bonding parameters as in 3 (Al-C 2.136(3)-2.220(3) Å, Al-cent
5
.6 kcal/mol for the transformation of 4 into 5. These results fit
3
t
1
(
(
.815 Å). Similarly, the Cp ligand of the Al(III) fragment of 4
very well to our experimental findings that only mixtures of
adduct 4/dialane 5 can be generated in solution, and that the
interconversion is possible over the entire temperature range.
Eventually, both isomers could be characterized in the solid
state by X-ray diffraction (Figure 2). Consistent with the
spectroscopic results and the calculated stabilities, we were
able to selectively crystallize dialane 5 at –30 °C and adduct 4 at
1
Al2) is again bound in a  fashion, while the Al2-C11 distance
2.063(7) Å) is slightly elongated with respect to 3 (2.005(3) Å).
3
t
The bulkier nature of the Cp ligand is also evident in the
molecular structure of dialane 5. Thus, the torsion angle
Br1-Al1-Al2-Br2 (–152.71(8)°) and the Al-cent distances of 5
(1.925 Å, 1.930 Å) are significantly larger than in the related
dialane Cp*(Br)Al-Al(Br)Cp* (Br1-Al1-Al2-Br2 102.04(5)°;
2
7
room temperature from the same pentane reaction mixture.
1
2
Al-cent 1.902 Å, 1.904 Å). Other relevant structural features of
In agreement with the equilibrium shown in Scheme 2, both
isolated species showed the same averaged NMR spectra as the
initial reaction
5
strongly resemble those found for Cp*(Br)Al-Al(Br)Cp*, i.e.
5
both species show  -cooridination modes for the Cp rings and
similar Al-Al distances (5: Al-C 2.164(8)-2.392(8) Å, Al-Al
2
2
.586(3) Å; Cp*(Br)Al-Al(Br)Cp*: Al-C 2.169(4)-2.365(4) Å, Al-Al
.530(2) Å).
Reaction of isolated 1 with simple haloalanes aimed at
1
2
elucidating similarities and differences of the reactivity patterns
*
of 1, (Cp Al)
4
, and other monovalent Al(I) species. It has been
shown that halide substituents are usually non-innocent in the
reaction of Al(I) species with haloalanes, leading to ionic or
*
comproportionation products. Thus, reaction of (Cp Al)
4
with
3
AlCl enabled the isolation of the decamethylaluminocenium
4
| Chem. Sci. 2012, 00, 1-3
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EDGE ARTICLE
appears to be more tightly bound to the Al(I) center as in all
View Article Online
DOI: 10.1039/C8SC05175E
other adducts.
A closer inspection of the synthesis of 7 revealed another
highly interesting reactivity of 1. When a solution of 1 was
added dropwise to a solution of AlBr , 7 was formed
3
quantitatively as the single product. By reverse addition
however, a minor amount of a second aluminum-containing
2
7
species was observed by Al NMR spectroscopy ( –71), which
we could eventually isolate as colorless crystals in low yield. This
compound was subsequently identified by X-ray diffraction as
the trialuminum species 8 (Figure 4) with a very rare structural
Figure 3 Molecular structures of 6 (left) and 7 (right) in the solid state. Hydrogen atoms
omitted for clarity. The asymmetric unit of 6 contains two independent molecules with
similar metrical parameters; only one molecule is shown. Selected bond lengths (Å): 6
Al1-Al2 2.621(2), Al1-C1 2.202(3), Al1-C2 2.174(4), Al1-C3 2.203(4), Al1-C4 2.194(4), Al1-
C5 2.176(4), Al2-C11 1.973(4); 7 Al1-Al2 2.554(1), Al1-C1 2.188(2), Al1-C2 2.177(2), Al1- motif. In the solid state, 8 features three distinct aluminum
C3 2.142(2), Al1-C4 2.178(2), Al1-C5 2.172(2).
centers with aluminum in two different oxidation states, being
best described as the Lewis pair of the asymmetric dialane
3
t
2
Br Al-Al(Br)Cp and 1. Thus, the Lewis-basic Al(I) atom Al3 of 1
forms a dative interaction with the sterically less crowded and
more Lewis-acidic Al(II) center Al2 of the dialane fragment
3
t
(
‘
(
Br
2
Al-Al(Br)Cp ), while the second Al(II) center Al3 remains
and covalently bound to Al2
2
Br Al-Al(Br)Cp ). The observed Al-Al bond lengths are
three-coordinate’
Scheme 4 Synthesis of 8.
3
t
consistent with this picture, and Al1-Al2 (2.538(1) Å) and Al2-
Al3 distances (2.601(1) Å) show typical values for covalent and
dative Al-Al bonds, respectively. To the best of our knowledge,
only one related, even though ionic non-cluster-type
trialuminum species has been mentioned in the literature,
+
– 36
[
2 3 2 2 4
Cp* Al I ] [Cp*Al I ] . The selective, large scale synthesis of 8
was accomplished by either reacting adduct 7 with one
equivalent of 1 or by dropwise addition of 0.5 equivalents of
2
7
3
AlBr to a solution of 1 (Scheme 4). The Al NMR spectrum of 8
features only a single resonance for a three-coordinate
aluminum atom ( –71), which clearly substantiates the
formulation of 8 as a dialane adduct. The absence of resonances
for the other two aluminum atoms can be rationalized in
Figure 4 Molecular structures of 8 in the solid state. Hydrogen atoms omitted for clarity.
Selected bond lengths (Å): 8 Al1-Al2 2.538(1), Al2-Al3 2.601(1), Al1-Br1 2.473(1), Al2-Br2
2
.319(1), Al2-Br3 2.319(1).
Al]+,35 while for β-diketiminate-stabilized LAl(I)
combination with
behavior of the bromide atoms. Thus, only one set of signals for
the Cp3t protons is evident in the
H NMR spectrum. These
findings strongly indicate scrambling of the bromide
1
H NMR data, which both suggest fluxional
cation [Cp*
asymmetric dialanes were formed via comproportionation.
2
3
2
3
0
1
Only when reacted with halide-free alanes such as Al(C
F )
6 5 3
or
acts as a Lewis base. In our hands, monovalent
did not show any evidence for reduction chemistry upon substituents between the two Cp Al units. While the exact
treatment with tBuAlCl and AlBr
, but rather afforded adducts mechanism for the formation of 8 is not known, the fluxional
and 7 quantitatively within seconds (Scheme 3). As
3
3
*
3 4
AltBu , (Cp Al)
3
t
1
2
3
2
7
behaviour in solution is consistent with a mechanism
6
1
proceeding either (i) by reversible oxidative addition of one Al-
expected, H NMR spectroscopic parameters of 6 and 7 strongly
resemble each other and those of adducts 3 and 4, both Br bond of 7 to the Al(I) center of 1, (ii) by reversible
featuring the typical set of three signals for the monovalent coordination of the Lewis-basic Al(I) center of 1 to the
3
t
1
hypothetical valence isomer of 7 (i.e. Cp3t(Br)Al-AlBr ), or (iii) by
2
Cp Al part (6:  6.13, 1.30, 1.22; 7:  6.10, 1.13, 0.99). The H
NMR resonance for the tBu group of 6 ( 1.09) integrates very initial formation of the bis(adduct) Cp AlAlBr
well to 9H, thus confirming the presence of a 1:1 ratio of the subsequent bromide migration. So far, we do not have any
3
t
3t
3
AlCp and
2
7
spectroscopic evidence for a favoured reaction pathway.
Al(I) and Al(III) fragments. The Al NMR spectra of 6 ( 146) and
( 95) showed only a single resonance each for the
7
Monovalent aluminum compounds have also been used to
tetracoordinate Al(III) fragments, while the signals of the Al(I) study Al=E multiple bonding (E = N, P, As, O, Se, Te). Thus,
*
centers were not detected presumably because of their large (Cp Al)
4
was shown to form tetrameric heterocubane type
3
7
full widths at half maximum. The molecular structures of 6 and clusters (Cp*AlE)
are unobtrusive and reminiscent of known Al(I)/Al(III) Lewis and Te,16,38 respectively. Similarly, treatment of (tBu
acid-base adducts such as 3 and 4, or the aforementioned molecular oxygen afforded (tBu SiAlO) among other
products.39 Employing a β-diketiminate-stabilized LAl(I) in the
.554(1) Å), as well as the Al-C (6: 2.176(4)-2.203(4) Å; 7: reaction with O
.142(2)-2.188(2) Å), and Al-cent distances (6: 1.820 Å; 7: 1.796 results imply a strong impact of the electronic and steric
4
2 2
when reacted with elemental O , N O, Se,
7
3
SiAl)
4
with
3
4
literature examples.27 The Al-Al separations (6: 2.621(2) Å; 7:
4
0
2
2
2
led to a dimeric product, (LAlO)
2
.
These
3
t
parameters of the Al(I) species on the product structure, while
Å) are found in the expected regions, while the Cp ligand in 7
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Chemical Science
a truly monomeric Al=O compound is still absent in the compounds, the formation of which can be rationalized by the
View Article Online
literature.41 Accordingly, we became curious what kind of stability of the initial iminoalane Cp*Al=NR’ and the nature and
DOI: 10.1039/C8SC05175E
product 1 might generate in view of its larger steric bulk as size of R’. While for R’ = SiiPr
3
, SiPh
3
, and SitBu
core have been
2 3 3
O, which obtained, reaction with Me SiN was more complicated
3
symmetric
*
compared to (Cp Al)
4
. To this end, a degassed solution of 1 in iminoalane dimers with a regular Al
2
N
2
4
2
C D
6 6
was subjected to an atmosphere of N
4
3
instantaneously resulted in the formation of the oxygenated affording an irregular iminoalane dimer. With the bulky azide
2
7
species 9 (Scheme 5). NMR spectroscopic studies suggested MesN
3
, the monomeric iminoalane did not participate in
the presence of a symmetric compound in solution, as oligomerization reactions, but was rather stabilized by a C-H
3
t
1
43
evidenced by a single set of signals for the Cp ligand in the H bond activation/proton migration sequence. Other (RAlNR’)
n
4
4
NMR spectrum of 9 ( 6.49, 1.61, 1.39). However, NMR data structures ranging from monomeric iminoalanes (n = 1) up to
were not sufficient to clarify the identity of 9, particularly species with n = 16 were also described in the literature.
4
5-47
2
7
because no Al NMR resonance was found.
However, either β-diketiminate-stabilized LAl(I) was used as
reagent in the reaction with azides, or other approaches were
applied in their synthesis.
In our hands, the reaction of 1 with PhN
3
proceeded
smoothly at room temperature to afford dimeric iminoalane 10
2
7
3t
in good yields (Scheme 5). Again, the degree of Cp Al=NPh
aggregation is not apparent from NMR spectroscopy in solution,
which pointed to either the presence of a monomeric
1
iminoalane, or a symmetric species with a single set of H NMR
signals for the Cp ligand ( 6.78, 1.57, 1.18) and the phenyl
3 3 2 2
Scheme 5 Synthesis of Al O (9) and Al N (10) heterocycles from 1.
3
t
2
7
group ( 7.35, 7.11, 6.97) in a relative ratio of 1:1. No Al NMR
resonance was observed for 10. X-ray diffraction eventually
established a dimeric iminoalane structure of 10 in the solid
state with the Cp ligands in typical  coordination modes (Al-
C 2.198(3)-2.295(2) Å, Al-cent 1.883 Å; Figure 5). The basic
structural parameters of 10 are very similar to the iminoalane
3
t
5
4
8
(
Mes*AlNPh)
2
described by Power,
while noticeable
differences are found to Roeskys’ actually more related
4
2
iminalane (Cp*AlNSitBu In analogy to (Mes*AlNPh)
3
)
2
.
2
(Al-N
1
.824(2) Å,  360°), the Al N core of 10 is perfectly planar, as
N
2 2
Figure 5 Molecular structures of 9 (left) and 10 (right) in the solid state. Hydrogen atoms
omitted for clarity. Symmetry operation for the conversion of Al1 to Al1’ in 10: –x+1, –
y+1, –z+1. Selected bond lengths (Å): 9 Al1-O1 1.701(2), Al1-O3 1.703(2), Al2-O1
expected for a centrosymmetric heterocycle, with Al-N bond
lengths of 1.809(2) and 1.827(2) Å, and trigonal planar nitrogen
1
.702(2), Al2-O2 1.697(2), Al3-O2 1.705(2), Al3-O3 1.701(4), Al-C 2.216(3)-2.296(3); 10 atoms (
N
359.9°). By contrast, (Cp*AlNSitBu
3 2
) features
Al1-N1 1.809(2), Al1-N1’ 1.827(2), Al1-C11 2.295(2), Al1-C12 2.244(2), Al1-C13 2.216(2),
Al1-C14 2.198(2), Al1-C15 2.249(2), N1-C1 1.396(3).
somewhat elongated Al-N bonds (1.835(2)-1.842(2) Å) and
nitrogen atoms that significantly deviate from planarity (
N
1
3
53.4°, 353.7°), and most importantly  -coordinated Cp*
It was an X-ray diffraction study on suitable single crystals of
ligands. These structural distinctions most likely trace back to
the steric demand of the extremely bulky NSitBu fragments.
9
that eventually helped to assign the exact composition.27 As
3
2
can be seen from Figure 5, oxygenation of 1 with N O goes along
with the formal oligomerization of hypothetical monomeric
3
t
Cp Al=O to afford the six-membered Al
a new member of the [LAlO]
family with n = 3 is accessible Conclusions
making use of the steric demand of the Cp ligand. The central
Al ring of 9 is almost planar with all Al-O distances within a
narrow range (1.697(2)-1.705(2) Å). The Al-O distances of 9 are
3 3
O heterocycle 9. Thus,
n
3
t
3
t
In this contribution, we demonstrated that Cp Al (1) can be
obtained by a three-step protocol involving the Lewis base-
induced release of 1 from an Al(I)/Al(III) adduct as the key step.
Thus, it was possible for the first time to isolate a monomeric
Cp-based Al(I) compound analytically pure, a task that has
consistently been hampered by synthetic difficulties. Detailed
reactivity studies clearly furnished evidence that reactions
involving 1 proceed more selectively (adducts 6 and 7), faster,
3 3
O
4
0
thus significantly shorter than in dimeric (LAlO)
tetrameric (R*AlE) (R = SitBu 1.836 Å; R = Cp*calc 1.845 Å),
4 3
which might indicate larger ionic contributions to the Al-O
bonds in 9. As a consequence, the Al-C distances (2.216(3)-
.295(3) Å) of all three exocyclic  -bound Cp ligands are
somewhat elongated in comparison to all other Cp based
compounds described in this study.
2
and
37
3
9
5
3t
2
3
t
and under milder conditions (heterocylces 9 and 10) than for
*
(
Cp Al)
4
, which is in line with our expectations that the
The related RAl=NR’ chemistry appears even more
monomeric nature of 1 might provide some experimental
benefits. Thus, monovalent 1 is expected to show a rich
chemistry, particularly with respect to the realization of novel
structural motifs (dialane adduct 8), and the activation of small
unpredictable due to the presence of a substituent at the
nitrogen center. Hence, reaction of (Cp Al)
azides R’N was used to realize a variety of Al-N-based
3
*
4
with different
6
| Chem. Sci. 2012, 00, 1-3
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Chemcial Science
EDGE ARTICLE
2
2
2
4 P. Bag, A. Porzelt, P. J. Altmann, S. Inoue, J. AmVi.ewCAhreticmle.OSnolince.
017, 139, 14384.
5 J. Hicks, P. Vasko, J. M. Goicoechea, S. Aldridge, Nature 2018,
molecules, two areas that are part of our current research
efforts. One finding that deserves special attention is the
experimental verification (reaction of 1 with 2) of the concept
of valence isomerism between dialanes and the respective
Al(I)/Al(III) Lewis acid-base adducts (structural characterization
of 4 and 5), which so far has only been proposed on the basis of
computational studies.
2
DOI: 10.1039/C8SC05175E
5
57, 92.
6 A. Hofmann, A. Lamprecht, J. O. C. Jiménez-Halla, T. Tröster,
R. D. Dewhurst, C. Lenczyk, H. Braunschweig, Chem. Eur. J.
2
018, 24, 11795.
2
2
7 Experimental details, and details on the X-ray diffraction
studies are included in the Electronic Supplementary
Information (ESI).
8 M. Schormann, K. S. Klimek, H. Hatop, S. P. Varkey, H. W.
Roesky, C. Lehmann, C. Röpken, R. Herbst-Irmer, M.
moltemeyer, J. Solid State Chem. 2001, 162, 225.
9 C. Ganesamoorthy, S. Loerke, C. Gemel, P. Jerabek, M. Winter,
G. Frenking, R. A. Fischer, Chem. Commun. 2013, 49, 2858.
0 J. D. Gorden, C. L. B. Macdonald, A. H. Cowley, Chem.
Commun. 2001, 75.
Conflicts of interest
The authors declare no conflict of interest.
2
3
3
3
Acknowledgements
Financial support from the Julius-Maximilians-Universität
Würzburg is gratefully acknowledged.
1 T. Chu, I. Korobkov, G. I. Nikonov, J. Am. Chem. Soc. 2014, 136,
9
195.
2 B. Li, S. Kundu, H. Zhu, H. Keil, R. Herbst-Irmer, D. Stalke, G.
Frenking, D. M. Andrada, H. W. Roesky, Chem. Commun. 2017,
5
3, 2543.
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Table of Contents
View Article Online
DOI: 10.1039/C8SC05175E
3
t
With Cp Al, a monomeric Al(I) species was isolated, which
reacted faster, more selectively, and under
4
milder conditions than well-known tetrameric (Cp*Al) .
8
| Chem. Sci. 2012, 00, 1-3
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