Aluminum Amidinates: Insights into Alkyne Hydroboration

Article abstract of DOI:10.1021/acs.inorgchem.1c00619

The mechanism of the aluminum-mediated hydroboration of terminal alkynes was investigated using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. The new aluminum complexes were fully characterized and found to facilitate the formation of the (E)-vinylboronate hydroboration product, with rates and orders of reaction linked to complex size and stability. Kinetic analysis and stoichiometric reactions were used to elucidate the mechanism, which we propose to proceed via the initial formation of an Al-borane adduct. Additionally, the most unstable complex was found to promote decomposition of the pinacolborane substrate to borane (BH3), which can then proceed to catalyze the reaction. This mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in the catalytic cycle.

Full text of DOI:10.1021/acs.inorgchem.1c00619

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Aluminum Amidinates: Insights into Alkyne Hydroboration
Katie Hobson, Claire J. Carmalt,* and Clare Bakewell*
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ABSTRACT: The mechanism of the aluminum-mediated hydro-
boration of terminal alkynes was investigated using a series of novel
aluminum amidinate hydride and alkyl complexes bearing
symmetric and asymmetric ligands. The new aluminum complexes
were fully characterized and found to facilitate the formation of the
(E)-vinylboronate hydroboration product, with rates and orders of
reaction linked to complex size and stability. Kinetic analysis and
stoichiometric reactions were used to elucidate the mechanism,
which we propose to proceed via the initial formation of an Al-
borane adduct. Additionally, the most unstable complex was found
to promote decomposition of the pinacolborane substrate to borane (BH3), which can then proceed to catalyze the reaction. This
mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to
proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in
the catalytic cycle.
INTRODUCTION
■
The application of main-group metals in catalysis has
flourished over recent years, largely driven by the need to
alleviate global demand on conventional precious metal
systems and find more sustainable alternatives.1 Main-group
compounds have been widely shown to mimic transition metal
behavior and, as they usually react via different mechanistic
pathways, can offer divergent reactivity. Areas where such
main-group systems have shown promise include catalytic
dehydrocoupling, hydroamination, hydroboration, and hydro-
silylation reactions, as well as examples of stoichiometric
oxidative addition and reductive eliminations.1−5 Hydro-
boration reactions are of particular interest as organoborane
compounds are widely exploited as synthetic intermediates,
owing to their versatility in a range of carbon−carbon and
carbon−heteroatom bond formation reactions.6 Over the past
decade, main-group systems have been shown to efficiently
catalyze a host of hydroboration reactions.1,7−10 The first
Figure 1. Examples of catalysts or precatalysts for hydroboration
reactions.
example of aluminum-mediated hydroboration dates back to
2000, where a combination of LiAlH4, 1,1′-bi-2-naphthol
(BINOL), and methanol was found to stoichiometrically
the hydroboration of terminal alkynes, nitriles, and alkenes
reduce acetophenone with HBcat.11 However, it was not until
such as conjugated bis-guanidinate-supported aluminum
2015 that Roesky, Parameswaran, and Yang reported the first
dihydrides (B), reported by Nembenna and co-workers
example of aluminum-catalyzed hydroboration.12 Here, alumi-
num β-diketiminate A was found to be proficient at catalyzing
the room temperature hydroboration of aldehydes and ketones
(Figure 1). Catalysis proceeds through an aluminum hydride,
Received: March 1, 2021
Published: July 16, 2021
with initial hydroalumination of carbonyl carbon, generating
aluminum alkoxide, followed by σ-bond metathesis to
regenerate A. In the intervening years, numerous examples of
structurally diverse aluminum catalysts have been reported for
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(Figure 1).13−17 In 2016, the substrate scope of aluminum-
catalyzed hydroboration was expanded by Cowley, Thomas,
and co-workers to include disubstituted alkynes, using the
commercially available bench stable complex triethylalumi-
nium−1,4-diazabicyclo[2.2.2]octane (Et3Al−DABCO) (C,
Figure 1).18 High conversions were achieved in 2 h at 110
°C, with a range of different symmetric and asymmetric,
aromatic, and aliphatic alkynes found to be tolerated.
However, in both transition metal and main-group catalyzed
hydroboration reactions, questions have arisen about the
nature of the “true” catalytic species. It has been proposed that
boranes, formed in situ, may catalyze hydroboration reactions
and that the metal complexes are instead facilitating
pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(HBpin)) decomposition/borane formation,19−22 although
this is more widely documented for HBCat. Recently, a variety
of boranes have been shown to be competent hydroboration
catalysts for alkynes and alkenes.23−25 In 2020, the significance
of hidden borane catalysis was extensively investigated by
Thomas and co-workers, with nucleophile promoted (inc.
LiAlH4) HBpin decomposition investigated.19 Despite the
implications of borane species in catalysis, this had not
previously been investigated for any other aluminum catalysts
in the literature. We became interested in the mechanism of
aluminum-catalyzed hydroboration reactions, and the possi-
bility of in situ borane formation. This was particularly driven
by the prohibitively high activation barriers calculated for the
aluminum dihydride-promoted hydroboration of alkynes (also
noted by Cowley and Thomas), which was proposed to
proceed via an initial dehydrogenation reaction to form an
acetylide in situ.13,26 We aimed to design a series of structurally
related complexes whose reactivity could be used to probe the
mechanism of alkyne hydroboration through stoichiometric
and kinetic analysis. Herein, we present the synthesis of novel
aluminum hydride and alkyl complexes bearing amidinate
ligands and investigate their use in the catalytic hydroboration
of phenylacetylene.
Figure 2. (a) General representation of an amidinate complex (left)
and a β-diketiminate complex (right); (b) pro-ligands L1−L4.
2b) were synthesized in a modular fashion via modified
literature procedures leading to a series of novel symmetric and
asymmetric amidinates (for full experimental details, see the
Supporting Information (SI)).32
The reactivity of the ligands toward aluminum precursors
was then explored. Pro-ligands L1−L4 were added to 1.2 equiv
of trimethylamine alane (AlH3·NMe3) in either benzene-d6 or
toluene at −78 °C (Figure 3a). Hydrogen gas was seen to
1
evolve immediately upon addition, and H nuclear magnetic
resonance (NMR) analysis showed complete consumption of
the ligand starting material in 1 h. The 1H NMR spectra of the
reaction of L1, L3, and L4 with the alane precursor revealed
the presence of a single set of peaks corresponding to the
amidinate ligand and, in all cases, a distinct broad resonance
integrating to two hydrides was observed between 3.6 and 5.1
ppm. This strongly indicates the formation of the mono-ligated
aluminum dihydride complexes, 1, 3, and 4 (Al−H2: 1 5.06
ppm; 3 4.77 ppm; 4 3.63 ppm). Crystals of 3 and 4 suitable for
single-crystal X-ray diffraction were grown from a benzene or
benzene/hexane solution. Complex 3 crystallized as a hydride-
bridged dimer, in an orthorhombic Aea2 space group (Figure 4
and Table S1). The complex, whose hydrides were freely
refined, has a slight asymmetry, with one bridging Al−H longer
than the other (Al1−HA 1.63904(4) Å; Al1−H 1.74959(4) Å)
and is comparable with other related aluminum dihydride
species.33 In contrast, 4 crystallized as a monomeric aluminum
SYNTHESIS AND REACTIVITY STUDIES
■
Complex Synthesis. Amidinates are a ubiquitous class of
ligand commonly employed in organometallic chemistry (for
select examples of group 13 amidinate and related guanidinate
complexes, see detailed review articles by Jones and
27,28
̊
̌
̌
Ruzicka),
with the general structure [R1NC(R2)NR3]
(Figure 2a). Their modular synthesis allows for independent
tuneability of the substituent on the nitrogen atoms and the
substituent at the bridgehead carbon, all of which can be either
aromatic or aliphatic. Upon coordination to a metal center,
they form a four-membered chelate, though other coordination
modes are possible, with a narrow N−M−N bite angle leaving
much of the coordinated metal center exposed. As such, their
use in the stabilization of highly reactive metal species has been
somewhat limited in comparison with the more widely
employed β-diketiminate ligand system (Figure 2a). However,
recent work has shown that it is possible to use sterically
demanding aryl substituents (e.g., 2,6-bis(diphenylmethyl)-
phenyl) to stabilize a range of highly reactive species, including
magnesium and strontium hydrides and an iron(IV) nitride
(related guanidinate ligand).29−31 We chose to target a series
of amidinate ligands with varying degrees of sterically
demanding substituents at the R1 and R3 positions. A 4-
methylphenyl group was chosen as the R2 group as it aided
solubility during ligand synthesis. Pro-ligands L1−L4 (Figure
̅
dihydride, with a distorted tetrahedral geometry in a P1 space
group. A distinct interaction between the hydride ligands and
two of the amidinate phenyl groups is observed, with through
space hydride···phenyl distances of ∼3 Å (Figure S1). Both
Al−N and Al−H bond lengths are near identical, with terminal
Al−H bond lengths of 1.46(2) Å. This is the first example of a
monomeric aluminum dihydride bearing an amidinate ligand
in the solid state, which is undoubtably driven by the sterically
demanding Ar* ligand substituents. Investigation into the
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Figure 3. Synthesis of (a) aluminum dihydrides 1−4 and 2′. (b) Aluminum monohydride 2″. (c) Aluminum dimethyl compounds 5 and 6.
reaction of L2 with AlH3·NMe3 revealed an asymmetric ligand
environment, a broad singlet at 4.54 ppm corresponding to two
Al−H, as well as an additional peak at 1.93 ppm integrating to
nine protons in the 1H NMR spectrum, indicating the presence
of trimethylamine. This was confirmed by the solid-state
structure which revealed a molecule of trimethylamine datively
bound to the aluminum metal center in addition to amidinate
ligand (2′). Compound 2′ crystallized as a monomeric
more sterically demanding 2,6-diisopropylphenyl (dipp) and
2,6-bis(diphenylmethyl)-4-methylphenyl) groups. No evidence
of an amine adduct was observed in the reaction of L1, L3, and
L4 under any conditions investigated. The formation of the
amine free analogue could be achieved by altering the reaction
work-up. After removal of solvent, the crude material was
subjected to heat (40 °C) and vacuum for 4 h, which led to the
elimination of the trimethylamine group and resulted in full
conversion to 2. A distinctive shift in the Al−H signal was
observed (4.37 ppm) as well as a restoration of symmetry to
the ligand environment. Complex 2 crystallized as a dimer in a
C2/c space group, with the C2 axis lying between the two
aluminum centers. The terminal and bridging hydrides were
freely located, with the terminal aluminum hydrides both
facing in the same direction, approximately parallel to one
another. The amidinate ligands on each aluminum center are
̅
aluminum dihydride in the P1 space group. The NMe3
group coordinates in the same plane as amidinate nitrogen
atoms, approximately orthogonal to the N-terminal Al−H
bonds, which are elongated versus the terminal hydrides in 4
(Al−HA 1.47(2) Å; Al−H 1.54(2) Å) and the bound NMe3
group render the Al−N bonds of the amindiate ligand slightly
asymmetric. The formation of the NMe3 adduct is facilitated
by the reduced steric bulk of the mesityl substituent versus the
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Figure 4. Solid-state structures of compounds 2, 2′, 2″, and 3−5. See the SI for a table of key bond lengths and bond angles (Table S1).
1
contorted away from one another on the opposing side of the
molecule. This is an unusual structure as all other examples of
crystallographically characterized bridging aluminum dihy-
drides have terminal hydrides facing in opposite directions to
one another, as in 3. The Al−H bond lengths to the two
bridging hydrides are highly asymmetric, at 1.54(2) and
1.90(2) Å.
trimethylaluminum in toluene at −78 °C. H NMR analysis
of 5 and 6 showed a distinctive upfield resonance
corresponding to the six Al−Me protons at −0.28 and −0.88
ppm, respectively. The single-crystal X-ray structure of 5
showed a distorted tetrahedral geometry and Al−N and Al−C
bond lengths were comparable with other literature com-
pounds.41
Interestingly, a further derivative of L2 could also be formed.
The addition of 2 equiv of L2 to trimethylamine alane in
toluene at −78 °C led to the formation of the bis-ligated
product, 2″. At room temperature, the 1H NMR spectrum was
broad. This is likely due to steric crowding in the N−C−N
region and indicates restricted conformation in solution as well
as slow exchange on the NMR time scale. A 1H NMR
experiment conducted at 70 °C significantly resolved the
spectrum, showing five different CH3 signals each integrating
to six protons. The Al−H resonance was not observed, as is the
case with related bis-ligated amidinate aluminum hy-
drides.34−36 The formation of the bis-ligated product was
somewhat surprising, given the sterically demanding nature of
the 2,6-bis(diphenylmethyl)phenyl substituent. The steric
profile of the ligand is highlighted in the solid-state structure
(Figure 4), where 2″ is seen to crystallize in a C2/c space
group with a severely distorted trigonal pyramidal geometry
and τ value of 0.68 (1 for an ideal trigonal pyramidal
geometry).37 The Al−N bond lengths were unsymmetrical,
with the N−2,4,6-trimethylphenyl (mes) bond length being
significantly shorter than the N−Ar* bond length, 1.89(9)
versus 2.13(9) Å. This is also observed in related complexes
with symmetrical ligands, but to a much lesser extent.36,38−40
The terminal aluminum hydride could be freely located and
has a bond length of 1.46(2) Å. Attempts to form bis-ligated
products with other amidinate ligands discussed were not
possible. However, coordination investigations using amidinate
ligands could be extended to other aluminum precursors, with
the facile formation of the aluminum dimethyl compounds 5
and 6 by reaction of L3 and L4 with 1.2 equiv of
Solution versus Solid-State Structures. Compounds 1−
6, 2′, and 2″ show the range of complexes that can be formed
across the amidinate ligand series. Solid-state structures reveal
that a mixture of monomeric and dimeric structures can form
depending on the nature of the ligand employed. However, it is
the solution-state structure of aluminum hydrides that dictates
their reactivity.42 While the 1H NMR spectra of complexes 1−
4 and 2′ show a range of chemical shifts corresponding to the
aluminum dihydrides, no definitive correlation between
structure and Al−H shift could be made (Table S2). For
instance, the relatively upfield resonance of 4 at 3.63 ppm is
likely due to shielding by the pendant phenyl groups (Figure
S2). Diffusion-ordered NMR spectroscopy (DOSY) was
therefore used to obtain diffusion coefficients and calculate
hydrodynamic radii across the series (Table S2). The
monomeric dimethyl complexes 5 and 6 have hydrodynamic
radii of 5.8 and 6.8 Å, respectively, demonstrating that Ar* has
a significantly greater solution volume than the dipp
substituent. The hydrodynamic radii of 5 and 6 are in good
agreement with structurally related monomeric complexes
reported in the literature.43−45 Comparatively, the dihydride
complexes 3 and 4 both have slightly larger hydrodynamic radii
than their analogous counterparts (1.4× and 1.3× respec-
tively), indicating a solution-based monomer−dimer equili-
brium that lies toward a monomeric structure but with some
dimeric character. In contrast, compound 2 was found to have
the largest radius across the series (7.6 Å), despite having a
smaller ligand than 3 and 4, suggesting a larger degree of
dimeric character in solution. 2′, whose additional NMe3
ligand disfavors dimerization, has a significantly smaller
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hydrodynamic radius. Density functional theory (DFT)
calculations were used to further probe the most stable
solution-based structure of the series of mesityl compounds.
These indicate that it is most thermodynamically favorable for
2 to exist in its dimeric form (Figure 5), which is in line with
Table 1. Catalytic Hydroboration of Phenylacetylene with
HBpin
a
b
cat.
time (h)
conv. (%)
1
2
3
4
5
6
6
22
11
56
12
95
89
89
89
89
90
89
a
Reactions conducted in benzene-d6, 0.25 M [HBpin], and [phenyl-
b
acetylene], 10 mol % cat. at 80 °C. Calculated by 1H NMR,
mesitylene was used as an internal standard.
overall trend 1 > 3 > 2 > 4 (Table 1 and Figures S52 and S53).
There is a general trend linking sterics and the reaction rate,
with the most sterically encumbered compound 4 exhibiting
the slowest initial rate. However, compound 2, which contains
a mesityl substituent in its ligand framework, is significantly
slower than 3 (22 versus 11 h to reach 89% conversion), which
contains a more sterically demanding diisopropylphenyl group
in the same position. The alkyl compounds 5 and 6 were found
to exhibit significantly different reaction kinetics. In both cases,
there was a lag period before any hydroboration product was
observed to form (1 h 5, 7 h 6, Figures S47 and S48),
indicating the slow formation of an active catalytic species.
Also, in neither case was an obvious second regime observed.
Comparison between the hydride and alkyl complexes showed
high conversions were reached on a similar time scale for 3 and
5, but that 6 was significantly slower than 4. The differing
kinetic profiles between the hydride and alkyl complexes
suggest that different reaction mechanisms are in play.
Similarly, the differences in the rate of reaction across the
series indicate that catalyst structure is important and
potentially contravenes the suggestion that a common catalytic
species may be in operation, though it could also suggest that
they are formed at different rates. The activity of AlH3·NMe3
toward the hydroboration reaction was also investigated, with
48% of the (E)-vinylboronate ester observed after 2 h under
the same conditions. However, after this point, the reaction
slowed and did not proceed to completion (13 h, 54%, Figure
S51). This is unsurprising given the temperature-sensitive
nature of AlH3·NMe3 and suggests that the compound
degrades under the reaction conditions.
Previous studies have proposed two plausible reaction
mechanisms for the aluminum-catalyzed hydroboration of
acetylenes; (a) the dehydrogenative formation of the active
aluminum acetylide catalyst, followed by hydroboration and
subsequent protonation of the alkenyl group (acetylide
pathway, Figure 6 top) or (b) the hydroalumination of
acetylene followed by a σ-bond metathesis with HBpin to form
the vinylboronate ester product and regeneration of the
aluminum hydride catalyst (hydroalumination pathway, Figure
6 bottom).13,18 Cowley and co-workers used the latter
mechanism to explain the ability of their aluminum precursors
to hydroborate internal alkynes.18 To help determine the most
likely mechanism(s) in play, a series of stoichiometric reactions
was conducted. First, the stability of the substrates and
catalysts were explored; heating the compounds at 80 °C in
Figure 5. Gibbs free energies of compounds 2-mono, 2′, and 2-dimer.
a
experimental observations. Compounds 2, 2′, and 3 are the
first examples of aluminum hydride complexes bearing
asymmetric amidinate ligands, and the family of compounds
represents a rare series of structurally distinct aluminum
complexes. With this in mind, we were keen to explore the
reactivity of the compounds and discern trends across this
series.
Catalysis. The series of structurally related aluminum alkyl
and hydride compounds 1−6 were used to investigate the
mechanism of aluminum-catalyzed alkyne hydroboration.
Previous work has shown both aluminum alkyls and hydrides
to be effective hydroboration catalysts.14,42,46,47 The reaction of
aluminum hydrides 1−4 and aluminum alkyls 5 and 6 with
phenylacetylene and pinacolborane (HBpin) at 0.25 M
concentration in benzene-d6 was monitored over time using
1H NMR spectroscopy. At room temperature, the slow
formation of a hydroboration product was observed, with
38% conversion obtained in 12 h (3). However, the rates of
reaction were seen to dramatically increase when reactions
were conducted at 80 °C, as such all complexes were tested
under these conditions. All compounds were found to be
catalytically active toward the hydroboration reaction, with the
exclusive formation of the (E)-vinylboronate ester product via
anti-Markovnikov addition observed in all cases (Table 1). The
reactions took between 6 and 95 h to reach high conversion
(>88%), depending on the structure of the aluminum complex
employed. Using compound 1, the reaction went to
completion in 6 h, while 6 was the most sluggish with high
conversions only reached after 95 h. To gain a better
understanding of how the reactions progress over time, they
1
were monitored at regular intervals using H and 11B NMR
spectroscopy.
Analysis of the plot of [HBpin] versus time for hydro-
boration reactions employing aluminum hydride compounds
1−4 revealed two different catalytic regimes, one at low
conversion and a second at high conversion (>75% conversion,
Figures S45−S48). This indicates that at high conversion an
alternative pathway, side reactions, or catalyst decomposition
starts to become more prominent. Comparison of the
aluminum hydrides 1−4 reveals significant differences in the
rate of reaction across the series, with reactivity following the
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was ruled out owing to the observed loss of symmetry of
1
methyl pinacolate groups. H−11B NMR correlation spectros-
copy was attempted to further investigate the structure of this
adduct, but experiments were unsuccessful due to the short-
lived nature of the adduct at room temperature. Heating the
reaction at 80 °C for 30 min led to the formation of a second
species, which was identified as the aluminum pinacolate 7
(Figure 7b), and the disappearance of the resonance at −10
c
ppm in the 11B NMR spectrum (Figure S12). A distinctive
1
signal at 1.45 ppm was observed in the H NMR spectrum,
corresponding to the 12 protons of the pinacolate group. The
solid-state structure shows a puckered pinacolate moiety, with
roughly equal Al−O bond lengths (1.718(1)/1.721(1) Å) and
an O−Al−O angle of 98.5° (Figure 9). A contraction of Al−N
bonds is observed compared to the parent dihydride 4 (Al−N
7: 1.897(1)/1.904(1) Å; Al−N 4: 1.943(10)/1.939(10) Å). A
related β-diketiminate complex revealed nearly identical Al−N
bond lengths (1.897(1)/1.904(1) Å) but possessed longer Al−
O bonds (1.718(1)/1.721(1) Å) and a smaller O−Al−O angle
(92.7°).49 Monitoring the reaction in situ showed a quintet at
−34.6 ppm in the 11B NMR spectrum, corresponding to a
[BH4]− species. Similar reactivity was observed with 3, but the
isolation of the proposed pinacolate was not possible (Figures
S13 and S14).
The transfer of the pinacolate group from boron to
aluminum must occur with the generation of BH3 which
then further reacts to form a [BH4]− species. While BH3
generated could act as a catalytic species, formation of 7 was
not observed during any catalytic reactions. It is also worth
noting that 7 does not react further with HBpin, and it does
not catalyze the reaction. Thus, BH3/[BH4]− formation was
ruled out in the presence of phenylacetylene via this pathway.
A handful of aluminum pinacolates have been previously
reported; however, there are no reported examples of
pinacolate formation from HBpin.49,50 In fact, to the best of
our knowledge, this is the first example of pinacolate transfer
from boron to a metal center, though partial pinacolate transfer
has been observed with a magnesium−magnesium dimer,
magnesium hydride, and a scandium hydride species.51,52
The stoichiometric reactivity of 2 followed a slightly
different pattern. The reaction of 2 with 1 equiv of
phenylacetylene at 80 °C led to the formation of an
unidentified product along with decomposition, as well as
the formation of styrene and trace H2 (Figure S15). The
reaction of 2 with HBpin at 298 K also showed the formation
of an unidentified product (no new pinacolate signal is
observed) in addition to the production of BH3, as confirmed
by a quartet at −7.0 ppm in the 11B NMR spectrum. This
product decomposed slowly at room temperature and at an
increased rate at 80 °C (Figures S16 and S17). Reactions were
also conducted at room temperature with an analogous β-
diketiminate (8, Figure 7d). No reaction was observed
between 8 and phenylacetylene over the course of 24 h
(Figures S18 and S19). As with 4, the reaction of 8 with HBpin
proceeded quickly at 30 °C in both C6D6 and CDCl3. In both
cases, the immediate formation of an intermediate (likely the
adduct) was observed, followed by the slower formation of a
second product, which was identified as the pinacol transfer
product 9, with concomitant formation of a [BH4]− species
Figure 6. Previously proposed mechanisms for aluminum-catalyzed
hydroboration of alkynes: acetylide pathway (top) and hydro-
alumination pathway (bottom).
benzene-d6 for 16 h saw no decomposition observed in any
instance. Similarly, heating the substrates in the absence of an
aluminum complex under the same conditions saw no reaction
or formation of product (Figures S4 and S5). NMR-scale
reactions between 3/4 and phenylacetylene, in benzene-d6,
were used to probe the possible acetylide pathway or
b
hydroalumination pathway (Figure 7a). However, no reaction
was observed at room temperature, and after heating at 80 °C
for 4 h, only a small amount of decomposition was observed
(Figures S6−S8). Heating for longer periods of time (24−120
h) showed further decomposition and trace amounts of
hydrogen formation. This is in contrast to other aluminum
dihydride systems, which report the formation of either the
dehydrogenation or insertion product.26,48 DFT calculations
were conducted to investigate the viability of the two proposed
a
reaction pathways (Figure 8). A simplified aluminum complex
was used, and transition states were located for the initial stage
of both pathways. The insertion (hydroalumination) of
acetylene into the Al−H bond was found to be both kinetically
and thermodynamically favorable, with a ΔG of activation of
28.4 kcal mol−1. However, this is still a significant energy
barrier as the hydroboration reaction does proceed slowly at
room temperature; therefore, we would expect a lower
associated energy. In contrast, addition of HBpin to 4 saw
the immediate formation of a new product, proposed to be a
HBpin adduct 4·HBpin (Figures 7b, S9, and S10). Here, the
Al−H signal is shifted to 3.40 ppm, and the methyl groups of
the pinacolate group resonate as two signals at 1.20 and 1.32
ppm. A broad signal ∼−10 ppm is observed in the 11B NMR
spectrum (Figure S11). The formation of a Lewis acidic adduct
d
(Figures S20−S22). In combination, these results suggest
neither the initial step of the acetylide pathway nor the
hydroalumination pathway accurately describe our system. The
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Figure 7. Stoichiometric reactions with 4−6 and 8.
different reactivity of 2 and 3/4 also hints that these complexes
may catalyze the reaction via different mechanisms.
with 1 equiv. HBpin led to a mixture of products; HBpin was
fully consumed in 2 h (80 °C) with the formation of MeBpin
and the aluminum pinacolate 7, along with unreacted 6
(Figures S26 and S27).53 It is presumed that this reaction
proceeds via the aluminum dihydride intermediate, 4, which
can then react further with HBpin. Indeed, when 6 was reacted
with an excess of HBpin, complete formation of 7 was achieved
after 7 h at 80 °C (Figures S28 and S29).
The alkyl complexes were found to be significantly more
stable at high temperatures; the reaction of 5 with phenyl-
acetylene (80 °C, benzene-d6, 24 h) led to the formation of a
1
major and minor product in the H NMR spectrum (Figures
S23 and S24). Desymmetrization of methine protons and
formation of methane gas suggested the formation of the
aluminum acetylide complex. A second symmetrical product
was also observed and is proposed to be the bisacetylide
complex. In contrast, after 5 days, at 80 °C, 6 only showed
minor reactivity, with a new resonance at −0.92 and methane
formation indicating the slow formation of an analogous
acetylide complex (Figure S25). Attempts to grow single
crystals of any products were unsuccessful. The reaction of 6
MECHANISTIC DISCUSSION
■
The stoichiometric reactions suggest that the acetylide
pathway or hydroalumination pathway pathways previously
proposed by Roesky and Cowley and Thomas, respectively, are
not applicable to our system. There was no evidence for the
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a peak at 22 ppm when 3 is mixed with diphenylacetylene and
HBpin also supports the proposed formation of a “N−Bpin”
bond. Interestingly, the point at which these new species begin
to form is the point at which the reaction kinetics start to
deviate.
The lack of reactivity between the aluminum dihydrides 1, 3,
and 4 and phenylacetylene in the absence of HBpin leads us to
propose that these reactions are more likely to proceed via a
HBpin adduct. Adduct formation has previously been
proposed by Rueping and co-workers for the hydroboration
of alkynes; here, the active species is proposed to be HBpin
bound to BuMg−H via a pinacolate oxygen.55 However, in the
case of our complexes, this would create a coordinatively
saturated aluminum center, thus prohibiting catalytic turnover.
Indeed, attempts to use DFT to calculate such a compound
were unsuccessful. We therefore propose that the HBpin
instead either (a) binds to Al through oxygen and causes the
ligand to partially de-coordinate or (b) coordinates directly to
the ligand framework in some fashion. As the reaction
progresses, these adducts start to degrade to form the proposed
“N−Bpin” species observed in the 11B NMR. The formation of
these “N−Bpin” compound(s) is supported by the rather
complex kinetic profiles observed in these catalytic reactions.
The hydroboration reaction proceeded significantly slower
in the presence of 2 than the more sterically encumbered 3.
While 2 was shown to have a more dimeric solution character,
which could account for this reduced rate, reactions using
compound 2 also showed more complicated reactivity, with a
distinctive quartet observed at −7 ppm in the 11B NMR
spectrum, corresponding to the formation of BH3. The BH3
signal increased in intensity throughout the course of the
reaction and was also observed in the stoichiometric reaction
of 2 with HBpin. It was noted in the stoichiometric reactions
that compound 2 did not follow the same reactivity as 3 and 4.
This hints at the possibility of an alternate reaction mechanism,
whereby BH3, formed in situ via the aluminum hydride
promoted decomposition of HBpin, is contributing to the
catalyst.20,21,56 Borane can then react with the alkyne substrate
(hydroboration) to form an alkenylborane intermediate which
then undergoes transborylation with a further molecule of
HBpin to yield the desired product and regenerate the BH3
catalyst.19 In reality, in the presence of 2, the reaction is
probably proceeding by multiple catalytic species including 2
(assuming the slow formation of BH3), BH3, and any
additional aluminum decomposition products formed. The
observed formation of BH3 from 2, but not 1, 3, and 4 points
toward a lack of stability of the complex, which contains a
smaller mesityl substituent. Although it is possible that BH3
may form at different rates for different catalysts, the marked
difference in the solution and solid-state structure of 2
supports the trend of divergent reactivity. To further rule out
BH3 formation, 4 was reacted with 10 equiv of HBpin to
mirror catalytic reaction conditions. No difference in reactivity
was observed when reacted with 1 or 10 equiv (Figures S29−
S31).
Figure 8. Calculated reaction pathway for the initial steps of the
hydroalumination pathway (left-hand side, LHS) and the acetylide
pathway (right-hand side, RHS).
Figure 9. X-ray structure of 7 with hydrogen atoms omitted for
clarity. Selected bond lengths (Å) and angles (deg): N1−Al1
1.904(1), N2−Al1 1.895(1), Al1−O1 1.721(1), and Al1−O2
1.718(1); N1−Al1−N2 70.59(6), O1−Al1−O2 98.47(6).
reaction between any of the aluminum dihydride complexes
1−4 and phenylacetylene, the first step in each catalytic
pathway, only decomposition of the complex over time. It is
also worth noting that no reaction was observed with
complexes 2, 3, or 6 when phenylacetylene was replaced
with diphenylacetylene. While there is a clear reaction between
compounds 3 and 4 with HBpin, we have seen no evidence for
the formation of the pinacolate species 7 or [BH4]− under
catalytic conditions. This suggests that the formation of the
hydroboration product is more favorable.
Analyzing the catalytic reaction mixture by 11B NMR
provides further insight. In addition to the starting material
(doublet, 28 ppm) and the hydroboration product (broad
singlet, 30 ppm), several other peaks were observed to form
during the reaction. Reactions employing compounds 1, 3, and
4 all contain a small peak at 22 ppm which develops over time,
whereas reactions using 2, 5, and 6 contain peaks at 22 and 24
ppm. The precise identity of these peaks remains unaccounted
for; however, related compounds with “N−Bpin” bonds have
been reported in this region (21−25 ppm).54 The formation of
Analysis of reactions using 5 and 6 shows that they have an
initial lag period before any hydroboration product is formed.
As in stochiometric reactions with HBpin, MeBpin formation
was observed, in addition to a peak at 22 ppm indicating the
presence of a small amount of a “N−Bpin” compound which
grows in gradually through the course of the reactions (vide
supra). “N−Bpin” formation does not appear to coincide with
a drop in the rate of reaction as with dihydride compounds.
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Solvents were obtained from a Grubbs solvent purification system
(SPS), degassed and stored on 3 Å molecular sieves prior to use.
Anhydrous benzene-d6 was obtained from Sigma and was degassed,
and stored on 3 Å molecular sieves. NMR-scale reactions were
conducted in J. Young’s tap tubes and prepared in a glovebox. All
heating of YT NMR tubes was conducted in a DrySyn NMR tube
heating block at the temperature stated. Details of synthesis of ligands
L1−L4 can be found in the SI. Trimethylaluminium, 2 M in toluene
was obtained from Sigma and used without further purification.
Phenylacetylene was purchased from Sigma, distilled using CaH2, and
stored over 3 Å molecular sieves. 4,4,5,5-Tetramethyl-1,3,2-dioxabor-
olane (HBpin) and diphenylacetylene were purchased from Sigma
and used without further purification. Nuclear magnetic resonance
(NMR) spectra were recorded on Bruker Avance 400, 500, and 600
Product formation coincided with MeBpin production,
suggesting that an aluminum hydride generated in situ is
required for catalysis to occur. However, the significantly
different reaction kinetics led us to rule out the formation of
dihydrides 3 and 4. Instead, it is possible that a mixed alkyl-
hydride species is formed, which could then go onto react via
the acetylide pathway. This is supported by the formation of
trace amounts of hydrogen in the reaction mixture, which
indicated the formation of an aluminum acetylide species, but
requires further investigation.
CONCLUSIONS
■
1
In summary, we have reported a series of structurally varied
aluminum hydride and alkyl complexes, including the first
example of an unsupported monomeric aluminum amindinate
dihydride. All compounds have been fully characterized, and
the solid state and solution structures compared. The solution-
state monomer−dimer equilibrium revealed that 1, 3, and 4
exist predominantly as monomers, whereas 2 retains much of
its dimeric character in solution. The aluminum hydrides 1−4
and aluminum alkyls 5 and 6 were all found to mediate the
hydroboration of phenylacetylene, with rates of reaction linked
to structure, size, and stability.
spectrometers operating at 400, 500, and 600 MHz for H NMR,
respectively, and 100, 125, and 150 MHz, respectively, for 13C NMR.
Spectra were processed and analyzed using Mestrenova and Bruker
Topspin software. The following notation system for the ligand
moieties has been implemented below: L = p-toluidine backbone, mes
= 2,4,6-trimethylphenyl substituent, dipp = 2,6-diisopropylphenyl
substituent, and Ar* = 2,6-diphenylmethyl-4-methylphenyl substitu-
ent. In NMR analysis, Ph refers to aromatic. Italicized o, m, and p
refers to the ortho, meta, and para positions, respectively. CIV refers to
quaternary carbons.
Synthesis of 1. A solution of L1 (1.1 mmol, 500 mg) dissolved in
toluene (7 mL) was added dropwise at −78 °C to a solution of
trimethylamine alane (1.54 mmol, 137 mg) in toluene (7 mL) at −78
°C. Hydrogen gas was seen to evolve immediately, and the reaction
was stirred for 1 h at 298 K. The solvent was removed in vacuo before
the product was extracted into hexane (2 × 10 mL) and filtered. The
solvent was removed in vacuo and the product isolated as a white
solid (328 mg, 62%).
Detailed stoichiometric and kinetic analysis revealed differ-
ent reaction mechanisms across the series. The aluminum
dihydrides 1, 3, and 4 all led to the same kinetic profile, and
stoichiometric reactions with phenylacetylene showed no
reaction. It is therefore proposed that these complexes proceed
via the formation of a HBpin adduct, which becomes the active
catalyst. This is in contrast to previous reports using analogous
β-diketiminate complexes, which propose an aluminum
acetylide, formed by a dehydrogenation reaction between an
aluminum dihydride and acetylene, to be the active catalytic
species. Interestingly, complex 2 was found to be less stable
under the reaction conditions (as observed in stoichiometric
reactions) and promoted the degradation of HBpin, with BH3
formation observed during catalysis. This is in agreement with
recent reports of borane-catalyzed hydroboration and high-
lights the importance of complex stability in mechanistic
investigation.19,57 The variation in reaction rate across the
series, combined with in situ 11B NMR analysis of the catalytic
reaction mixtures, indicates that the reactions are mediated by
the aluminum species (with the exception of 2) and not via
hidden borane species. The precise nature of the proposed
aluminum−HBpin active catalyst is currently under inves-
tigation computationally and will be the subject of further
work. We will also continue to probe the complex kinetics
displayed by a seemingly simple reaction system.
The structure of the catalyst has proved critical in
determining the mechanism, even across a closely related
series of compounds. Relatively, subtle differences in ligand
structure have been found to have profound effects on
catalysis. Additionally, different co-ligands Al−H versus Al−
Me have been shown to operate by distinct mechanisms. This
highlights how nuanced complex structure can be in
determining reactivity, but also offers the opportunity to
access divergent reaction pathways through simple synthetic
manipulations.
1H NMR (500 MHz, C6D6, 298 K) δ (ppm): 1.03 (d, 12H,
CH(CH3)2, 3JHH = 6.5 Hz), 1.31 (d, 6H, CH(CH3)2, 3JHH = 7.0 Hz),
1.63 (s, 3H, LCH3), 3.67 (sept, 4H, CH(CH3)2, 3JHH = 7.0 Hz), 5.06
L
3
(s, 2H, AlH2), 6.38 (d, 2H, o-CH, JHH = 8.5 Hz), and 7.03−7.08
(m, 8H, PhCH); 13C{1H} NMR (125 MHz, C6D6, 298 K) δ (ppm):
21.2 (LCH3), 22.4 (CH(CH3)2), 25.9 (CH(CH3)2), 29.3 (CH-
(CH3)2), 123.4 (PhCH), 123.9 (PhCH), 126.0 (PhCH), 128.6 (Lo-
CH), 129.9 (PhCH), 138.7 (CIV), 141.3 (CIV), and 143.5 (CIV). Anal.
Calcd (C32H43N2Al): C, 79.63; H, 8.98; and N, 5.80. Found: C,
79.57; H, 9.08; and N, 5.85.
Synthesis of 2. A solution of L2 (0.30 mmol, 200 mg) dissolved in
toluene (7 mL) was added dropwise at −78 °C to a solution of
trimethylamine alane (0.36 mmol, 31 mg) in toluene (7 mL) at −78
°C. Hydrogen gas was seen to evolve immediately, and the solution
was stirred for 1 h at 298 K. The solvent was removed in vacuo, and
the resultant mixture heated under vaccum (40 °C) for 4 h. The crude
product was washed with hexane, filtered, and isolated to yield a white
solid (49 mg, 24%).
L
1H NMR (600 MHz, C6D6, 298 K) δ (ppm): 1.63 (s, 3H, CH3),
1.78 (Ar*CH3), 2.00 (s, 3H, mesp-CH3), 2.10 (s, 6H, meso-CH3), 5.02
L
3
(s, 2H, AlH2), 6.02 (d, 2H, o-CH, JHH = 7.8 Hz), 6.22 (br d, 2H,
Lm-CH), 6.39 (s, 2H, CH(Ph2)) 6.51 (s, 2H, mesm-CH), 6.87−7.19
3
(m, PhCH), 6.95 (Ar*m-CH), and 7.37 (d, 4H, Pho-CH, JHH = 7.8
Hz); 13C{1H} NMR (150 MHz, C6D6, 298 K) δ (ppm): 20.4 (mesm-
CH3), 21.0 (LCH3), 21.0 (Ar*CH3), 21.1 (mesp-CH3), 21.4 (CIV), 53.5
(CH(Ph2)), 123.5 (PhCH), 126.0 (PhCH), 126.4 (PhCH), 128.5, 128.6
(PhCH), 128.7 (PhCH), 129.0 (PhCH), 129.3 (Lm-CH), 129.6 (mesm-
CH), 130.1 (Lo-CH), 130.1 (Pho-CH), 130.5 (PhCH), 133.9 (CIV),
134.5 (CIV), 137.4 (CIV), 139.1 (CIV), 142.2 (CIV), 143.0 (CIV), 143.8
(CIV), and 146.1 (CIV). IR (solid-state): n = 1872, (solution): n =
1716, 1752 cm−1.
Synthesis of 2′. To a solution of trimethylamine alane (0.028
mmol, 2.5 mg) dissolved in benzene-d6, L2 (0.028 mmol, 18 mg) was
added, and the solution was transferred to a J Young NMR tube; H2
gas was seen to evolve. The mixture was left at room temperature for
1 h. Single crystals suitable for X-ray analysis were grown from
benzene-d6/hexane. Attempts to scale up this reaction were
unsuccessful.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out using
standard Schlenk-line and glovebox techniques under an inert
atmosphere of argon. An MBraun Unilab Pro glovebox was used.
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L
1H NMR (500 MHz, C6D6, 298 K) δ (ppm): 1.67 (s, 3H, CH3),
1.82 (s, 3H, Ar*CH3), 1.99 (s, 18H, N(CH3)3), 2.00 (s, 3H, mesp-
CH3), 2.31 (s, 6H, meso-CH3), 4.10 (s, 4H, AlH2), 5.91 (s, 2H,
CH(Ph)2), 6.07 (d, 2H, Lo-CH, 3JHH = 8.0 Hz), 6.30 (d, 2H, Lm-CH,
3JHH = 8.0 Hz), 6.61 (s, 2H, mesm-CH), 7.03−7.20 (m, 12H, CH),
Synthesis of 5. Ligand L3 (0.28 mmol, 200 mg) was dried under
vacuum for 1 h, dissolved in toluene (10 mL), and cooled to −78 °C.
Trimethylaluminum (0.34 mmol, 0.17 mL) was added dropwise, and
the reaction was stirred at 298 K overnight. The solvent was removed
in vacuo, washed with hot hexane, and recrystallized at −18 °C. The
product was filtered, dried, and isolated as a white solid (160 mg,
74%).
7.33 (d, 4H, Aro-CH, 3JHH = 7.5 Hz), and 7.62 (d, 4H, Aro-CH, 3JHH
=
7.5 Hz); 13C{1H} NMR (125 MHz, C6D6, 298 K) δ (ppm): 18.5
(LCH3), 19.6 (meso-CH3), 20.5 (mesp-CH3), 22.0 (Ar*CH3), 50.5
(PhCH), 125.7 (CIV), 125.9 (PhCH), 128.5 (Lm-CH3), 128.8 (mesm-
CH), 129.1 (CIV), 129.5 (CIV), 130.0 (Pho-CH), 130.5 (Pho-CH),
132.5 (CIV), 133.9 (CIV), 134.9 (CIV), 138.7 (CIV), 143.7 (CIV), and
147.4 (CIV).
1H NMR (500 MHz, C6D6, 298K) δ (ppm): −0.035 (s, 6H,
3
Al(CH3)2), 0.77 (d, 6H, CH(CH3)2, JHH = 7.0 Hz), 1.23 (d, 6H,
3
L
CH(CH3)2, JHH = 6.5 Hz), 1.72 (s, 3H, CH3), 1.81 (s, 3H, Ar*p-
3
L
CH3), 3.43 (sept, 2H, CH(CH3)2, JHH = 7.0 Hz), 6.21 (d, 2H, o-
3
L
CH, JHH = 8.5 Hz), 6.49 (s, 2H, CH(Ph)2), 6.65 (d, 2H, m-CH,
3JHH = 8.5 Hz), and 6.95−7.28 (m, 25H, PhCH); 13C{1H} NMR (500
MHz, C6D6, 298K) δ (ppm): −9.6 (Al(CH3)2), 21.2 (LCH3), 21.2
(Ar*CH3), 23.2 (CH(CH3)2), 25.7 (CH(CH3)2), 28.3 (CH(CH3)2),
51.6 (CH(Ph)2), 128.8 (Lo-CH), 129.7 (Lm-CH), 124.1 (PhCH),
125.9 (PhCH), 126.1 (PhCH), 128.5 (PhCH), 128.6 (PhCH), 129.4
(PhCH), 129.6 (PhCH), 130.1 (CIV), 130.2 (CIV), 131.4 (CIV), 134.0
(CIV), 139.1 (CIV), 140.6 (CIV), 143.5 (CIV), 143.9 (CIV), and 145.2
(CIV). Anal. Calcd (C55H57N2Al): C, 84.47; H, 7.80; and N, 3.94.
Found: C, 84.84; H, 7.34; and N, 3.58.
Synthesis of 2″. A solution of L2 (14.5 mmol, 100 mg) dissolved
in toluene (7 mL) was added dropwise at −78 °C to a solution of
trimethylamine alane (7.3 mmol, 6.5 mg) in toluene (7 mL) at −78
°C. The reaction was stirred for 1 h before the solvent was removed in
vacuo. The crude product was washed with hexane, filtered, and
isolated as a white solid. Single crystals suitable for X-ray analysis were
grown from toluene/hexane (166 mg, 41%).
1H NMR (500 MHz, C6D6, 343 K) δ (ppm): 1.63 (s, 6H, CH3),
1.85 (s, 6H, CH3), 1.90 (s, 6H, CH3), 2.13 (s, 6H, CH3), 2.46 (s, 6H,
CH3), 5.95 (s, 8H, CH), 6.19 (s, 2H, CH), 6.35 (s, 2H, CH), 6.88−
7.12 (bm, 40H, CH), and 7.30 (bm, 8H, CH). It was not possible to
assign 13C NMR spectra at 298 or 343 K. IR (solid-state): n = 1867
cm−1.
Synthesis of 6. Ligand L4 (0.20 mmol, 200 mg) was dried under
vacuum for 1 h, dissolved in toluene (10 mL), and cooled to −78 °C.
Trimethylaluminum (0.26 mmol, 0.13 mL) was added dropwise, and
the reaction was stirred at 298 K overnight. The solvent was removed
in vacuo, and the resultant product was washed with hexane to yield a
white solid after filtration (93 mg, 44%).
Synthesis of 3. A solution of L3 (0.279 mmol, 200 mg) dissolved
in toluene (7 mL) was added dropwise at −78 °C to a solution of
trimethylamine alane (0.297 mmol, 25 mg) in toluene (7 mL) at −78
°C. Hydrogen gas was seen to evolve immediately, and the solution
was stirred for 1 h at 298 K. The crude product was washed with
hexane, filtered, and isolated as a white solid. Single crystals suitable
for X-ray analysis were grown from benzene-d6 at 298 K (120 mg,
58%).
1H NMR (500 MHz, C6D6, 298K) δ (ppm): −0.88 (s, 6H,
L
Al(CH3)2), 1.73 (s, 6H, Ar*CH3), 1.82 (s, 3H, CH3), 6.06 (d, 2H,
3
Lo-CH, JHH = 8.5 Hz), 6.43 (s, 4H, CH(Ph)2), 6.96−7.11 (m, 38H,
PhCH), and 7.23 (d, 8H, Pho-CH, 3JHH = 8 Hz); 13C{1H} NMR (500
MHz, C6D6, 298K) δ (ppm): 21.5 (Ar*CH3), 51.2 (CH(Ph)2), 126.0
(PhCH), 126.5 (PhCH), 128.2 (Lm-CH), 129.0 (Lo-CH), 129.4 (Pho-
CH), 130.3 (PhCH), 131.5 (CIV), 134.4 (CIV), 138.9 (CIV), 139.5
(CIV), 144.2 (CIV), and 145.2 (CIV). Anal. Calcd (C76H65N2Al): C,
88.17; H, 6.52; and N, 2.71. Found: C, 87.78; H, 6.56; and N, 2.72.
Synthesis of 7. Compound 4 (0.0496 mmol, 50 mg) was dissolved
in toluene (5 mL), and HBpin (0.0546 mmol, 8 mL) was added. The
reaction was heated for 30 min at 80 °C before the solvent was
removed in vacuo. The crude product was recrystallized from
toluene/hexane to yield colorless crystals (18 mg, 33%).
1H NMR (500 MHz, C6D6, 298 K) δ (ppm): 0.79 (d, 6H,
CH(CH3)2, 3JHH = 6.5 Hz), 1.26 (d, 6H, CH(CH3)2, 3JHH = 6.5 Hz),
1.69 (s, 3H, LCH3), 1.86 (s, 3H, Ar*CH3), 3.52 (sept, 2H, CH(CH3)2,
3JHH = 6.5 Hz), 6.13 (d, 2H, o-CH, JHH = 8 Hz), 6.53 (d, 2H, m-
L
3
L
3
CH, JHH = 7.5 Hz), 6.79 (s, 2H, CH(Ph)2), 6.98−7.14 (m, 21 H,
PhCH), and 7.47 (d, 4H, PhCH, 3JHH = 7.5 Hz); 13C{1H} NMR (125
MHz, C6D6, 298 K) δ (ppm): 21.3 (Ar*CH3), 21.6 (LCH3), 23.7
(CH(CH3)2), 25.4 (CH(CH3)2), 28.1 (CH(CH3)2), 51.5 (CH-
(Ph)2), 124.6 (PhCH), 128.4 (Lo-CH), 128.5 (PhCH), 128.8 (PhCH),
129.7 (PhCH), 129.8 (Lm-CH), 130.8 (PhCH), 130.0 (CIV), 130.2
(CIV), 133.2 (CIV), 138.9 (CIV), 143.6 (CIV), 144.3 (CIV), and 146.1
(CIV). IR (solid-state): n = 1838, 1879 cm−1, (solution): n = 1813,
1958 cm−1. Anal. Calcd (C53H53N2Al): C, 84.42; H, 7.53; and N,
4.10. Found: C, 83.67; H, 7.07; and N, 3.76.
1H NMR (600 MHz, C6D6, 298 K) δ (ppm): 1.45 (s, 12H, CH3),
1.65 (s, 3H, LCH3), 1.69 (s, 6H, Ar*CH3), 5.64 (d, 2H, Lo-CH, 3JHH
=
8.4 Hz), 6.14 (d, 2H, Lm-CH, 3JHH = 6.0 Hz), 6.36 (s, 4H, CH(Ph2)),
7.00 (s, 4H, Ar*m-CH), 6.80−7.33 (m, PhCH), and 7.67 (d, 8H, Pho-
3
CH, JHH = 8.4 Hz); 13C{1H} NMR (150 MHz, C6D6, 298 K) δ
(ppm): 14.2 (CIV), 20.8 (LCH3), 22.9 (Ar*CH), 28.0 (CH3), 31.0
(CIV), 51.2 (CH(Ph)2), 77.8 (CIV), 125.6 (PhCH), 126.4 (PhCH),
128.2 (Lo-CH), 128.2 (PhCH), 129.4 (PhCH), 129.9 (Lm-CH), 129.9
(Pho-CH), 130.1 (CIV), 130.2 (CIV), 131.2 (PhCH), 131.3 (CIV), 134.8
(CIV), 136.5 (CIV), 139.1 (CIV), 142.2 (CIV), and 146.6 (CIV).
General Procedure for the Catalytic Hydroboration of
Phenylacetylene. Phenylacetylene (0.015 mmol, 0.0165 mL) was
added to a solution of catalyst (0.0015 mmol, 10 mol %), 1,3,5-
trimethylbenzene (0.01 mL), and HBpin (0.015 mmol, 0.0215 mL) in
benzene-d6 (0.60 mL) and transferred to a J Young NMR tube. A t =
Synthesis of 4. A solution of L4 (0.204 mmol, 200 mg) in toluene
(7 mL) was added dropwise at −78 °C to a solution of
trimethylamine alane (0.204 mmol, 18 mg) in toluene (7 mL) at
−78 °C. Hydrogen gas was seen to evolve immediately, and the
solution was stirred for 1 h at 298 K. The crude product was washed
with hexane, filtered, and isolated as a white solid. Single crystals
suitable for X-ray analysis were grown from benzene-d6/hexane at 298
K (145 mg, 70%).
1H NMR (500 MHz, C6D6, 298 K) δ (ppm): 0.79 (d, 6H,
CH(CH3)2, 3JHH = 6.5 Hz), 1.26 (d, 6H, CH(CH3)2, 3JHH = 6.5 Hz),
1.69 (s, 3H, LCH3), 1.86 (s, 3H, Ar*CH3), 3.52 (sept, 2H, CH(CH3)2,
1
0, H NMR spectrum was recorded, and the sample tube was then
1
heated at 80 °C. Each reaction was monitored over time, with H
NMR spectra recorded at regular time points until >88% completion
3JHH = 6.5 Hz), 6.13 (d, 2H, o-CH, JHH = 8 Hz), 6.53 (d, 2H, m-
L
3
L
1
was achieved. The yield was determined by H NMR spectroscopy
3
CH, JHH = 7.5 Hz), 6.79 (s, 2H, CH(Ph)2), 6.98−7.14 (m, 21 H,
using 1,3,5-trimethylbenzene as an internal standard.
PhCH), and 7.47 (d, 4H, PhCH, 3JHH = 7.5 Hz); 13C{1H} NMR (125
MHz, C6D6, 298 K) δ (ppm): 21.3 (Ar*CH3), 21.6 (LCH3), 23.7
(CH(CH3)2), 25.4 (CH(CH3)2), 28.1 (CH(CH3)2), 51.5 (CH-
(Ph)2), 124.6 (PhCH), 128.4 (Lo-CH), 128.5 (PhCH), 128.8 (PhCH),
129.7 (PhCH), 129.8 (Lm-CH), 130.8 (PhCH), 130.0 (CIV), 130.2
(CIV), 133.2 (CIV), 138.9 (CIV), 143.6 (CIV), 144.3 (CIV), and 146.1
(CIV). IR (solid-state): n = 1618 cm−1, (solution): n = 1753 cm−1.
Anal. Calcd (C74H63N2Al): C, 87.68; H, 6.50; and N, 2.96. Found: C,
86.94; H, 6.18; and N, 2.74.
1H NMR (500 MHz, C6D6, 298 K) δ (ppm): 1.13 (s, 12H, CH3),
6.46 (d, 1H, CH, 3JHH = 18.5 Hz), 6.98−7.04 (m, 3H PhCH), 7.32 (d,
3
3JHH = 10.0 Hz), and 7.76 (d, 1H, CH, JHH = 18.5 Hz).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00619.
10967
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Inorg. Chem. 2021, 60, 10958−10969

Inorganic Chemistry
Full details of experiments and analysis; General
pubs.acs.org/IC
Article
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Accession Codes
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CCDC 2054524−2054530 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
(7) Villegas-Escobar, N.; Schaefer, H. F.; Toro-Labbé, A. Formation
of Formic Acid Derivatives through Activation and Hydroboration of
CO2 by Low-Valent Group 14 (Si, Ge, Sn, Pb) Catalysts. J. Phys.
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Corresponding Authors
Claire J. Carmalt − Department of Chemistry, University
College London, London WC1H 0AJ, United Kingdom;
orcid.org/0000-0003-1788-6971; Email: c.j.carmalt@
ucl.ac.uk
Clare Bakewell − Department of Chemistry, University College
London, London WC1H 0AJ, United Kingdom;
orcid.org/0000-0003-4053-8844; Email: c.bakewell@
ucl.ac.uk
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Stabilized by N-Heterocyclic Imines as Catalysts for Hydroborations
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Ligands Leading to Unprecedented Zwitterionic Meisenheimer
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Author
Katie Hobson − Department of Chemistry, University College
London, London WC1H 0AJ, United Kingdom
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00619
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Ramsay Memorial Trust and UCL Chemistry are thanked
for their financial support (C.B.) and UCL Chemistry for a
departmental studentship (K.H.). The EPSRC is thanked for
funding via the Impact Acceleration Account award to UCL
2017-20 (EP/R511638/1) and grant EP/L017709 (C.J.C.).
(16) Bismuto, A.; Cowley, M. J.; Thomas, S. P. Aluminum-Catalyzed
Hydroboration of Alkenes. ACS Catal. 2018, 8, 2001−2005.
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Dialumenes-Aryl: Vs. Silyl Stabilisation for Small Molecule Activation
and Catalysis. Chem. Sci. 2020, 11, 4817−4827.
ADDITIONAL NOTES
■
a
The calculations were performed at a C H N (6-31G**)/Al
(SDDAll), ωB97X level of theory + ΔEsp (ωB97XD) + ΔEsolv
(polarizable continuum model (PCM), benzene). Basis set and
functional were benchmarked by comparison to the solid-state
structure of 4.
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Catalyzed Hydroboration of Alkynes. Angew. Chem., Int. Ed. 2016, 55,
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b
Reaction of 3 with phenylacetylene under the same
conditions saw significantly more decomposition.
The product 7 was also shown to form slowly at room
temperature.
Both reactions also saw the formation of a second
unidentified minor product (approximately 40%).
(19) Bage, A. D.; Hunt, T. A.; Thomas, S. P. Hidden Boron
Catalysis: Nucleophile-Promoted Decomposition of HBpin. Org. Lett.
2020, 22, 4107−4112.
c
d
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