Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation

Article abstract of DOI:10.1021/acs.organomet.0c00226

Alkaline earth (Ae) metal complexes with the alanate anion AlH₄^-have been prepared by salt metathesis between NaAlH₄and AeCl₂in THF and could be isolated as Mg(AlH₄)₂·(THF)₄, Ca(AlH₄)₂·(THF)₄, and Sr(AlH₄)₂·(THF)₅. The previously reported crystal structure of the Mg alanate complex shows bonding of AlH₄^-with one bridging hydride, H₃Al-(μ-H)-Mg, while the Ca and Sr alanates show a combination of H₃Al-(μ-H)-Ae and H₂Al-(μ-H)₂-Ae bridging. The heteroleptic β-diketiminate complexes (^DIPPBDI)Mg(AlH₄)·THF and (^DIPPBDI)Ca(AlH₄)·(THF)₂have been prepared by reaction of the corresponding Ae hydride complexes with AlH₃·(THF)₂[^DIPPBDI = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-diisopropylphenyl]. Crystal structures show H₂Al-(μ-H)₂-Ae bridging. The Ca complex decomposes at room temperature by reduction of the β-diketiminate anion. Density functional theory calculations (B3PW91/def2tzvpp) show that the formation of Ae(AlH₄)₂from AeH₂and AlH₃is exothermic by δH (kilocalories per mole): Be, -68.8; Mg, -66.1; Ca, -95.4; Sr, -100.9; Ba, -112.3. Calculations of NPA charges on LiAlH₄and the Ae alanate complexes (Ae = Mg, Ca, or Sr) show that these are highly ionic salts in which the charge on AlH₄^-of approximately -0.95 is hardly dependent on the countercation. Compared to LiAlH₄, the Ae alanates are very efficient catalysts for imine hydrogenation, clearly extending the substrate scope. In addition to aldimines RC(H)=NR′ (R/R′ = Ph/tBu, tBu/tBu, nPr/tBu, or Ph/Ph), ketimine PhC(Me)=NtBu could be reduced. The salt [Bu₄N⁺][AlH₄^-] is catalytically not active, which shows that the s-block metal is crucial. The highest activities were found for the heterobimetallic Ca and Sr alanates.

Full text of DOI:10.1021/acs.organomet.0c00226

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Article
Alkaline Earth Metal Aluminates as Catalysts for Imine
Hydrogenation
Holger Elsen, Jens Langer, Michael Wiesinger, and Sjoerd Harder*
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ABSTRACT: Alkaline earth (Ae) metal complexes with the alanate anion AlH4 have
been prepared by salt metathesis between NaAlH4 and AeCl2 in THF and could be
isolated as Mg(AlH4)2·(THF)4, Ca(AlH4)2·(THF)4, and Sr(AlH4)2·(THF)5. The
previously reported crystal structure of the Mg alanate complex shows bonding of
−
AlH4 with one bridging hydride, H3Al-(μ-H)-Mg, while the Ca and Sr alanates show a
combination of H3Al-(μ-H)-Ae and H2Al-(μ-H)2-Ae bridging. The heteroleptic β-
diketiminate complexes (DIPPBDI)Mg(AlH4)·THF and (DIPPBDI)Ca(AlH4)·(THF)2
have been prepared by reaction of the corresponding Ae hydride complexes with
AlH3·(THF)2
[
DIPPBDI = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-
diisopropylphenyl]. Crystal structures show H2Al-(μ-H)2-Ae bridging. The Ca complex
decomposes at room temperature by reduction of the β-diketiminate anion. Density functional theory calculations (B3PW91/
def2tzvpp) show that the formation of Ae(AlH4)2 from AeH2 and AlH3 is exothermic by ΔH (kilocalories per mole): Be, −68.8; Mg,
−66.1; Ca, −95.4; Sr, −100.9; Ba, −112.3. Calculations of NPA charges on LiAlH4 and the Ae alanate complexes (Ae = Mg, Ca, or
−
Sr) show that these are highly ionic salts in which the charge on AlH4 of approximately −0.95 is hardly dependent on the
countercation. Compared to LiAlH4, the Ae alanates are very efficient catalysts for imine hydrogenation, clearly extending the
substrate scope. In addition to aldimines RC(H)NR′ (R/R′ = Ph/tBu, tBu/tBu, nPr/tBu, or Ph/Ph), ketimine PhC(Me)NtBu
could be reduced. The salt [Bu4N+][AlH4 ] is catalytically not active, which shows that the s-block metal is crucial. The highest
−
activities were found for the heterobimetallic Ca and Sr alanates.
Ae[N(SiMe3)2]2 (Ae = Mg, Ca, Sr, or Ba). The latter are key
reagents in group 2 metal chemistry and easily accessible at a
larger scale.9 Comprehensive calculations on the hydrogenation
mechanism confirmed the intermediacy of metal hydride
species. This recognition led to follow-up research using the
even simpler, commercially available, bulk commodity LiAlH4.10
Screening a variety of imine substrates for hydrogenation, the
researchers found that LiAlH4 performed not as well as the Ae
amide catalysts but for certain substrates, mild temperatures (85
°C), low catalyst loadings (2.5 mol %), and a conveniently low
pressure of only 1 bar of H2 could be applied (Scheme 1). The
simplicity and mild conditions of this experiment found a broad
interest11 and stand in strong contrast with Slaugh’s early
attempt to hydrogenate alkenes under extreme conditions (35
mol % LiAlH4, 190 °C, and 80 bar of H2).12 While Slaugh’s
experiences did not invite further research in this area, 50 years
later LiAlH4 catalysis is currently experiencing a revival (Scheme
1). The groups of Cowley and Thomas showed the hydro-
INTRODUCTION
■
The simplest metal hydride, (LiH)∞, is a highly insoluble,
essentially inert, salt that is stable in air and reportedly does not
even react with hydrogen chloride.1 This can be explained with
its considerable lattice energy of 920 kJ mol−1, which is the
highest of those of the alkali metal hydrides.2 However, in
combination with AlH3, it becomes soluble in organic ethereal
solvents and represents one of the most powerful and versatile
reducing agents. Schlesinger’s synthesis of Li+AlH4− in the 1940s
should be rated a milestone discovery.3 Key to its fame is its fair
solubility in ethereal solvents, rendering it highly reactive toward
a large array of substrates.4,5 Although old hat, at present LiAlH4
is still one of the most potent reducing agents. This mixed metal
hydride salt developed from a small scale laboratory reagent to
industrial relevant bulk product.6 LiAlH4 is especially in the
synthesis of alcohols or amines from polar double bonds like
CO or CN bonds the reducing agent of choice. This
synthetic approach, however, needs stoichiometric quantities of
LiAlH4 that after hydrolysis inherently generates copious
amounts of Li/Al salts that complicate product workup and
additionally pose fire and explosion hazards.
Special Issue: Organometallic Chemistry of the Main-
Group Elements
As part of our research activities in early main group metal
catalysis,7 we pioneered the reduction of CC and CN
bonds using the simplest bulk reducing agent H2.8 The catalysts
for these transformations can be surprisingly simple alkaline
earth (Ae) metal amide complexes with a formula of
Received: April 1, 2020
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© XXXX American Chemical Society
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Scheme 1. Overview of LiAlH4-Catalyzed Reactions
Scheme 2. Proposed Heterobimetallic Mechanism for
LiAlH4-Catalyzed Imine Hydrogenation
various N-ligands.21 The solubilities of Ae(AlR4)2 compounds
increase with an increase in alkyl chain length but decrease with
an increase in Ae2+ ion size. The Anwander group reported the
synthesis of Ba(AlMe4)2, which is insoluble in aromatic solvents,
and Ba(AlEt4)2, which crystallized from toluene as a three-
dimensional (3D) coordination polymer or toluene solvate.22
Much less is known of Ae metal complexes with the smallest
boration of alkenes (5−10 mol %, 110 °C, and 4 h),13 while the
groups of Mulvey, Hevia, and Okuda converted LiAlH4 into the
well-defined catalyst LiAlH2[N(SiMe3)2]2 for ketone/aldehyde
hydroboration.14 These hydroborations take advantage of the
very polar Bδ+−Hδ− bond that is substantially more reactive than
apolar H2. The LiAlH4-catalyzed alkene hydrosilylation with
SiH4 by the Kobayashi group profits from a similar high
reactivity of the polarized Siδ+−Hδ− bond.15 Further niche
applications of the LiAlH4 catalyst are illustrated by Kim’s
nitroaldol condensation,16 the dehydrogenation of HMe2NBH3
by Wright and co-workers,17 and Itoh’s dehydrogenative
alkyne−silane coupling.18
−
aluminate anion AlH4 , which are expected to show lower
solubilities. Fichtner et al. reported the synthesis of Mg(AlH4)2
and Ca(AlH4)2 from the corresponding AeCl2 and NaAlH4 in
ethereal solvents.23,24 For Sr(AlH4)2, only a ball-milling route
has been documented.25 We prepared these aluminates using
the salt metathesis route mixing AeCl2 (Ae = Mg, Ca, or Sr) and
NaAlH4 in THF in a 1/2 ratio (Scheme 3). The mixtures were
stirred in THF at 60 °C overnight and filtered hot to remove
precipitates. All aluminates crystallize very well from THF and
were isolated in the form of their THF adducts. 1H NMR spectra
of the Mg, Ca, and Sr alanate complexes in THF-d8 show very
broad resonances for AlH4− between 2 and 4 ppm. These signals
are broadened due to substantial Al−H magnetic coupling and
the very large quadrupole moment of 27Al. The 27Al NMR
chemical shifts are in the range of 107−114 ppm. The 1H NMR
spectra also show considerable quantities of THF, which is very
loosely bound and partially lost during crystal drying.
While wet synthetic methods successfully gave the Mg, Ca,
and Sr alanates, BaCl2 did not react with NaAlH4. Alternatively,
reaction of Ba[N(SiMe3)2]2 with PhSiH3, to generate BaH2 in
situ and addition of AlH3 led to an insoluble white powder that
could not be dissolved in various polar solvents. Due to its
complete insolubility, the presumed [Ba(AlH4)2]∞ was not
further characterized or used in catalysis.
There is strong evidence for heterobimetallic cooperativity in
LiAlH4-catalyzed reactions.10,13,14 On the basis of the
importance of both metals, a mechanism for LiAlH4-catalyzed
imine hydrogenation has been proposed (Scheme 2).10 The
heterobimetallic nature of the LiAlH4 catalyst makes this system
perfectly suitable for further optimization. Herein, we present
the synthesis and compare the structures of a series of
homoleptic group 2 metal aluminates Ae(AlH4)2 (Ae = Mg,
Ca, or Sr). In addition, we present heteroleptic β-diketiminate
−
Ae metal complexes with the AlH4 anion. Both homo- and
heteroleptic complexes have been investigated in the catalytic
reduction of various imines by H2, and their performance was
compared to that of LiAlH4.
RESULTS AND DISCUSSION
■
Homoleptic Aluminate Complexes. The chemistry of the
tetraalkylaluminates of group 2 metals is reasonably well
developed. Westerhausen and co-workers describe [Ca2+·
The crystal structure of Mg(AlH4)2·(THF)4 is known and
described as a complex in which both aluminate anions are
bound to Mg2+ in a H3Al-(μ-H)-Mg fashion (Figure 1a and
Table 1).23 The crystal structure of Ca(AlH4)2·(THF)4 is
reported to be isostructural but of very poor quality, and hydride
positions are uncertain.24 A high-quality structure, including
refined hydride positions, is shown in Figure 1b. While one of
19
−
(THF)6][AlMe4 ]2, while the Anwander group reported the
20
structure of the similar Mg salt [Mg2+·(THF)6][AlMe4 ]2.
−
The latter group also published crystal structures of
tetraethylaluminates [Ca(AlEt4)2]∞, Ca(AlEt4)2·(THF)2, and
Ca(AlEt4)2·(phenanthroline).20 Also, Mitzel and co-workers
contributed to the field with structures of Ca(AlEt4)2 solvated by
−
the AlH4 anions is bound in a H3Al-(μ-H)-Ca fashion, the
other should be described as H2Al-(μ-H)2-Ca. Moreover, there
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Scheme 3. Synthesis of Homoleptic and Heterolepic Alkaline
Earth Metal Alanate Complexes
Table 1. Selected Bond Lengths (angstroms) and Angles
a
(degrees) for Ae(AlH4)2 Complexes
Mg(AlH4)2·
23
complex
(THF)4
Ca(AlH4)2·(THF)4 Sr(AlH4)2·(THF)5
Ae-(μ-H)
1.977(18)
H3Al(μ-H):
2.33(2), 2.43(2)
H3Al(μ-H): 2.45(3)
b
H2Al(μ-H)2:
2.44(2), 2.68(3)
H2Al(μ-H)2:
2.60(3), 2.65(3)
Al-(μ-H)
1.573(18)
H3Al(μ-H): 1.54(2) H3Al(μ-H): 1.59(3)
H2Al(μ-H)2:
1.49(2), 1.57(3)
H2Al(μ-H)2:
1.58(3), 1.58(3)
Al-Ht
1.52(2),
1.50(2),
1.55(2)
H3Al(μ-H):
1.50(3), 1.54(3),
1.54(3)
H3Al(μ-H):
1.51(3), 1.52(3),
1.54(3)
H2Al(μ-H)2:
1.49(3), 1.52(3)
H2Al(μ-H)2:
1.50(3), 1.53(3)
Ae···Al
3.5383(5)
H3Al(μ-H):
3.8467(6)
H3Al(μ-H):
4.0044(8)
H2Al(μ-H)2:
3.2705(6)
H2Al(μ-H)2:
3.3467(9)
Al···Ae···Al′ 180
173.41(2)
167.15(2)
a
b
(μ-H) = bridging hydride, and Ht = terminal hydride. Contact to
the neighboring molecule.
as shown by O−Sr−O′ angles that are larger than 72° [range of
72.50(5)−77.81(5)°, average of 75.69°]. The aluminate anions
are bound in axial positions [Al···Sr···Al′, 167.15(2)°] either in a
H3Al-(μ-H)-Sr or in a H2Al-(μ-H)2-Sr fashion. The aluminate
anions in these crystal structure show no deviation from a
tetrahedral geometry, but the Al−H distances for bridging
hydrides are somewhat larger than those for terminal hydrides
(Table 1).
To evaluate the influence of the group 2 metal in catalysis,
additionally an s-block metal-free aluminate complex was
synthesized. Reaction of [Bu4N+][Cl−] with NaAlH4 in THF
gave after workup [Bu4N+][AlH4 ] in 54% yield. This salt
−
−
dissolves well in THF-d8, and the highly symmetric free AlH4
is an additional H3Al-(μ-H)-Ca contact to a neighboring
molecule, and the complex should more appropriately be
described as a dimer; however, due to the weakness of the Ae···
AlH4 and Ae···THF bonds, these complexes will be extremely
dynamic in solution. For strontium alanate, only an inaccurate
powder X-ray diffraction structure for the 3D coordination
polymer of THF-free [Sr(AlH4)2]∞ is known.25 Crystallization
from THF gave the complex Sr(AlH4)2·(THF)5, and the crystal
structure is shown in Figure 1c. The five THF ligands coordinate
approximately in an equatorial plane that deviates from planarity
anion showed in 1H NMR a well-resolved sextet (1/1/1/1/1/1)
at 2.27 ppm (1JAl−H = 174 Hz) and a 27Al NMR signal at 100.0
ppm.
Heteroleptic Aluminate Complexes. Large spectator
ligands like N,N-chelating β-diketiminate ligands will enhance
the solubility of alanate complexes. Jones and co-workers
reported the synthesis of [(DEPBDI)Mg(AlH4)·Et2O]2 [DEPBDI
= DEP-NC(Me)C(H)C(Me)N-DEP, where DEP = 2,6-
diethylphenyl], but due to poor crystal quality, the exact
−
bridging mode of AlH4 ions could not be established.
Figure 1. Crystal structures of (a) Mg(AlH4)2·(THF)4,23 (b) [Ca(AlH4)2·(THF)4]2, and (c) Sr(AlH4)2·(THF)5. H atoms, except those of AlH4 ,
−
have been omitted for the sake of clarity.
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Interestingly, exchanging the Lewis base Et2O for Me3N gave a
product with Al-amine coordination (Scheme 4).
Table 2. Selected Bond Lengths (angstroms) and Angles
(degrees) for (DIPPBDI)Mg(AlH4)·THF and
a
(
DIPPBDI)Ca(AlH4)·(THF)2
Scheme 4. β-Diketiminate Magnesium Alanate Complexes26
complex
(
DIPPBDI)Mg(AlH4)·THF
(
DIPPBDI)Ca(AlH4)·(THF)2
Ae-N
Ae-O
Ae-(μ-H)
Al-(μ-H)
Al-Ht
2.056(2), 2.048(2)
2.036(2)
1.91(3), 2.03(3)
1.54(3), 1.55(3)
1.55(3), 1.57(3)
2.768(1)
2.395(2), 2.388(1)
2.358(1), 2.406(1)
2.39(2), 2.38(2)
1.58(2), 1.59(2)
1.54(2), 1.49(2)
3.1502(7)
Ae···Al
N-Ae···Al
O-Ae···Al
125.57(7), 124.47(7)
99.48(6)
143.60(4), 100.00(3)
76.42(3), 118.04(3)
a
(μ-H) = bridging hydride, and Ht = terminal hydride.
bound aluminate anion (Figure 2b and Table 2). The product
−
dissolves well in C6D6 and shows a broad AlH4 resonance at
We here report the synthesis of Mg and Ca alanate complexes
with the bulkier β-diketiminate ligand DIPPBDI [DIPP-NC-
(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-diisopropylphen-
yl] (Scheme 3). Reaction of [(DIPPBDI)MgH]2 with AlH3·
(THF)2 in toluene gave after recrystallization (DIPPBDI)Mg-
(AlH4)·THF in 69% yield. The complex crystallizes as a
monomer with a H2Al(μ-H)2-bound aluminate anion (Figure 2a
and Table 2). The product dissolves well in C6D6 and shows a
broad AlH4− resonance at 3.20 ppm and a 27Al NMR signal at
97.4 ppm.
The synthesis of the analogue Ca complex was considerably
more problematic due to the instability of the product, which
requires synthesis at a lower temperature. Reaction of
[(DIPPBDI)CaH·THF]2 with AlH3 in toluene at 0 °C gave
after crystallization (DIPPBDI)Ca(AlH4)·(THF)2 in 64% yield.
The complex crystallizes as a monomer with a H2Al(μ-H)2-
2.88 ppm and a 27Al signal at 120.4 ppm. It decomposes slowly at
temperatures above 0 °C. Although we could not isolate higher
yields of a defined decomposition product, we obtained in one
case a few crystals. Due to severe disorder, the crystal structure
was partially determined (see the Supporting Information) and
suggests that decomposition proceeds by reduction of the β-
−
diketiminate anion by the AlH4 anion and a Ca/Al exchange
reaction (Scheme 3).
Density Functional Theory (DFT) Calculations. The
stability of alkaline earth metal aluminates was investigated by
calculation (B3PW91/def2tzvpp) of a simple model reaction in
which AeH2 is reacted with AlH3. While for Be and Mg similar
ΔH values of −68.8 and −66.1 kcal/mol, respectively, have been
calculated, the reaction becomes considerably more exothermic
for the heavier Ae metals (Scheme 5a), i.e., with increasing
ionicity of AeH2.
The DFT-optimized structures of Ca(AlH4)2·(THF)4 and
Sr(AlH4)2·(THF)4 compare fairly well to the crystal structures,
but for Mg(AlH4)2·(THF)4, there is a different minimum in
which one AlH4− ligand binds to Mg2+ in a monodentate fashion
while the other coordinates in a bidentate fashion (see Figures
S37−S39). Upon comparison of the NPA charges of AeH2 with
those in Ae(AlH4)2·(THF)n complexes, the saline nature of the
alkaline earth aluminates becomes apparent (Scheme 5b). For
AeH2, the ionic character of the Ae-H bond increases from 66%
(Mg) to 83% (Sr). However, in the alanate complexes, the
−
negative charge on AlH4 is relatively constant and hardly
dependent on the s-block metal (range of −0.922 to −0.979).
Consequently, there is also hardly any variation in the positive
charge of Mg2+, Ca2+, and Sr2+ ions, and these alanates should be
considered salt-like species with ∼95% ionic bond character. For
LiAlH4, a similar charge distribution is found. The bridging
hydrides are considerably more negatively charged than the
terminal ones. The negative charge on the hydrides increases in
the following order: terminal H− < double bridging H− < single
bridging H−. This is most likely due to the very good
−
polarizability of the AlH4 anion and the hydride anion itself.
This electrostatic polarization can be visualized by the Laplacian
of the electron density according to AIM theory (Scheme 5c).
The spherical electron density in an isolated hydride anion (see
the inset) is strongly polarized toward the hard and highly
charged Al3+ cation (thick green arrow) and only weakly
attracted by Ca2+ (thin green arrow).
Catalytic Imine Hydrogenation. Homo- and heteroleptic
Ae metal alanate complexes have been investigated as catalysts in
imine hydrogenation under standard conditions (neat imine, 20
Figure 2. Crystal structures of (a) (DIPPBDI)Mg(AlH4)·THF and (b)
(
DIPPBDI)Ca(AlH4)·(THF)2. The iPr substituent of DIPPBDI and the H
−
atoms, except those of AlH4 , have been omitted for the sake of clarity.
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Scheme 5. DFT Calculations (B3PW91/def2tzvpp), Including (a) Calculated Enthalpies for Formation of Metal Alanates from
Their Metal Hydrides, (b) Calculated NPA Charges, and (c) the Laplacian of the Electron Density in Ca(AlH4)2·(THF)4 in the
a
Plane of Mg and the Bridging Hydrides
a
THF molecules have been omitted for the sake of clarity, and bond critical points are colored light blue. Green arrows represent polarization of the
hydrides as compared to an unperturbed hydride anion (see the inset).
a
Table 3. Catalytic Hydrogenation of Imines
a
Runs in neat imine (except for entries 21−23, in which case the solid imine was dissolved in THF, 2.8 M) and conversion followed by 1H NMR.
b
1
Due to the severe overlap of product/substrate H NMR signals, the conversion was determined by GC/MS.
1
bar of H2, 85 °C), and conversion was followed by H NMR
(Table 3). To evaluate the metal effects, their performance is
loading was halved in comparison to that of LiAlH4. As catalysis
with these alanates is thought to be a heterobimetallic process,
this comparison is debatable. Therefore, the performances of the
Ae(AlH4)2 catalysts described herein are conservative estimates.
compared to that of LiAlH4 and [Bu4N+][AlH4 ]. Because the
−
Ae(AlH4)2 complexes contain two alanate anions, their catalyst
E
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Screening studies using the benchmark substrate PhC(H)
NtBu show that Mg alanate is clearly less active than LiAlH4 but
activities increase with the size of the Ae metal (Mg < Ca < Sr;
entries 1−4). Using the Sr(AlH4)2·(THF)5 complex as a
catalyst, essentially full conversion was achieved within 2.5 h.
This clearly shows that the nature of the second metal in alanate
complexes is important. This conclusion is especially supported
by attempts to catalyze imine hydrogenation with [Bu4N+]-
be much more challenging than aldimine reduction. While
LiAlH4 is essentially not active, nearly quantitative conversion
could be achieved with the catalyst Sr(AlH4)2·(THF)5 (entries
24−26).
This study shows that Ae metal alanates are considerably
more active in imine hydrogenation than LiAlH4. While LiAlH4
is conveniently commercially available, the catalyst loadings
could not be decreased below 2.5 mol %.10 The more active
catalyst Ca(AlH4)2·(THF)4 is still active at catalyst loadings
down to 0.5 mol %, but a higher H2 pressure was needed (entry
27).
−
[AlH4 ] (entry 5). As there is hardly any conversion, the
presence of a second metal is crucial for catalysis. This
hypothesis was tested by addition of the metal salts. While
−
combinations of [Bu4N+][AlH4 ] with LiI, NaI, or KI are not
CONCLUSIONS
active, the addition of group 2 salts AeI2 (Ae = Mg, Ca, Sr, or Ba)
led to a considerable increase in activities (entries 6−9). A
maximum in activity was found for the addition of CaI2. This
may be due to differences in solubility (in all cases, the AeI2 salt
did not completely dissolve).
In previous studies of LiAlH4-catalyzed imine hydrogenation,
it was found that pressure only slightly affects conversion rates:
decreasing the pressure from 20 to 1 bar led to only a small
decrease in activity (cf. entries 1 and 10).10 Also, imine
hydrogenation with Ca(AlH4)2·(THF)4 could be performed
conveniently at atmospheric pressure (entry 11). A slight
temperature decrease from 85 to 60 °C had larger consequences
(entries 12 and 13). The activities with heteroleptic catalysts
■
Synthetic procedures and crystal structures for homo- and
heteroleptic Ae metal complexes of the alanate anion AlH4− have
been presented. While the complex with the smaller Ae metal
Mg(AlH4)2·(THF)4 shows bonding of the aluminate anion with
one bridging hydride, H3Al-(μ-H)-Mg, complexes with the
larger Ae metals Ca and Sr show a combination of H3Al-(μ-H)-
Ae and H2Al-(μ-H)2-Ae bridging.
−
Heteroleptic β-diketiminate complexes with AlH4 anions
have been isolated for Mg and Ca. The Ca complex is so reactive
that already at room temperature the β-diketiminate anion is
reduced to a dianionic bis-amide ligand.
DFT calculations demonstrate that the formation of Ae-
(AlH4)2 from AeH2 and AlH3 is increasingly exothermic from Be
to Ba. Like LiAlH4, all Ae(AlH4)2 complexes should be
(
DIPPBDI)Mg(AlH4)·THF and (DIPPBDI)Ca(AlH4)·(THF)2
were higher than those of the corresponding homoleptic salts
(entries 14 and 15, respectively), and for the Ca complex, nearly
full conversion could be achieved within 2.5 h. Due to the facile
decomposition of the β-diketiminate Ca alanate complex
(Scheme 3), the nature of the catalyst is not exactly clear.
Earlier studies of LiAlH4-catalyzed imine hydrogenation
showed that slight variations in the substrate led to problems.
Exchanging the Ph group in PhC(H)NtBu for a tBu
substituent impedes conversion by steric hindrance and
electronic effects: the electron-releasing alkyl group makes the
imine C less electrophilic. Hence, for LiAlH4, long reaction
times of ≤4 days gave only incomplete conversion (entry 16).
Using the Ae alanates, however, solved this problem, and full
conversion was obtained within 24 h (entries 17 and 18).
The substrate nPrC(H)NtBu represents an imine that
potentially could be deprotonated to form an aza-allyl anion.
This complication was previously encountered in the mecha-
nistically related imine hydrosilylation with early main group
metal catalysts.27 Although we could not isolate defined
products from stoichiometric reactions between Ca(AlH4)2·
(THF)4 and nPrC(H)NtBu, reaction of the Ca hydride
complex [(DIPPBDI)CaH·THF]2 indeed gave deprotonation
and formation of an aza-allyl product that could be characterized
by NMR and X-ray diffraction (see Figure S35). Such side
reactions potentially impede imine hydrogenation. It could,
however, be shown that the catalyst Ca(AlH4)2·(THF)4 is
clearly superior to LiAlH4 (entries 19 and 20).
−
considered highly ionic salts in which the charge on the AlH4
anions (approximately −0.95) is hardly dependent on the
countercation.
The Ae(AlH4)2 salts are very efficient catalysts for imine
hydrogenation. Compared to LiAlH4, not only higher activities
but also an extended substrate scope could be demonstrated.
The nature of the s-block metal strongly influences catalytic
activity. While Mg aluminate is less active than LiAlH4, the
catalyst performance generally increased with metal size, and for
most substrates, the Sr alanate performed best. As the salt
−
[Bu4N+][AlH4 ] is catalytically not active, the s-block metal is
crucial. This is a strong argument for heterobimetallic catalysis.
A comprehensive, combined experimental and theoretical, study
of the exact mechanisms for imine hydrogenation with
heterobimetallic catalysts will be reported in due course.
EXPERIMENTAL SECTION
■
All experiments were carried out in dry glassware under N2 using
standard Schlenk techniques and freshly dried and degassed solvents
(all solvents were dried over a column except for THF, which was dried
over sodium benzophenone and redistilled). Commercially available
LiAlH4 was extracted with diethyl ether and after removal of all solvents
dried in vacuo to give a white fluffy material. Commercially available
NaAlH4 was purified via a similar method using THF. Ph(H)CNtBu
was obtained commercially from Sigma-Aldrich (97%). The following
imine substrates were prepared according to literature procedures:
PhC(H)NPh,28 tBuC(H)NtBu,29 nPrC(H)NtBu,30 and PhC-
(Me)NtBu.31 AlH3·(THF)2 was prepared according to Smith.32
Complexes Mg(AlH4)2·THF433 and Ca(AlH4)2·THF434 were prepared
according to Fichtner, but both could also be prepared by a similar
procedure as described below for Sr(AlH4)2·THF5. All complexes
decomposed rapidly on contact with air but did not ignite. Caution!
The presumed Ba(AlH4)2 powder was found to be very pyrophoric in air.
Hydrogen of quality N5 has been used for catalytic hydrogenation.
Deuterated solvents were dried over molecular sieves (3 Å) and stored
in a glovebox. All substrates were dried by being stirred over freshly
powdered CaH2 at 60 °C and then distilled. In the case of PhC(H)
NPh, which is a solid, the substrate was first dissolved in hexane and
The imine with Ph substituents at C and N, PhC(H)NPh,
is a challenging substrate. The addition of hydride to this highly
activated (conjugated) CN bond should be unproblematic,
but subsequent reaction of the resulting amide anion
PhCH2(Ph)N− with H2 is difficult due to stabilization of this
anion by charge delocalization. Although more active than
LiAlH4, the Ae alanate catalysts also gave only partial conversion
(entries 21−23).
In contrast, Ae alanates were found to be very effective
catalysts for the hydrogenation of a ketimine, which is known to
F
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then dried over CaH2, and after filtration, all volatiles were removed
under vacuum. Stainless steel pressure reactors were dried by being
heated overnight in an oven at 70 °C. The products from catalytic
reactions have been analyzed by NMR and GC-MS. NMR spectra were
recorded on Bruker Avance III HD 400 MHz and Bruker Avance III
HD 600 MHz spectrometers. Elemental analysis was performed with a
Hekatech Eurovector EA3000 analyzer. Crystal structures have been
measured on a SuperNova (Agilent) diffractometer with dual Cu and
Mo microfocus sources and an Atlas S2 detector. Crystallographic data
have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publications 1993706−1993710.
Synthesis of Sr(AlH4)2·THF5. The salt SrCl2 (500 mg, 3.15 mmol)
and NaAlH4 (341 mg, 6.31 mmol) were suspended in THF (10 mL),
and the mixture was stirred at room temperature for 16 h. The solution
was filtered hot, reduced to half of its volume, and kept at −30 °C to
grow crystals. The product was isolated as colorless crystals (481 mg,
0.942 mmol, 30%). Elemental analysis is complicated by the large
amounts of loosely bound THF that is partially lost during product
isolation but cannot be removed completely under high vacuum.
Similar problems have been described for the isolation and character-
ization of [Ca2+·(THF)6][Me4Al−]2 by Westerhausen et al.:35 1H NMR
(THF-d8, 600 MHz, 298 K) δ 4.0−2.1 extremely broad due to 27Al
coupling, AlH4, 3.58 (m, THF), 1.73 (m, THF); 27Al NMR (THF-d8,
233 MHz, 298 K) δ 109.5.
28.1, 25.1, 24.6, 24.3; 27Al NMR (C6D6, 233 MHz, 298 K) δ 120.4 (br).
Elemental Anal. Calcd for C37H61N2O2AlCa: N, 4.43%; C, 70.21%; H,
9.71%. Found: N, 4.40%; C, 70.55%; H, 9.34%.
Synthesis of (DIPPBDI)Ca[N(tBu)(HCC(H)Et)]·THF. Complex
[(DIPPBDI)CaH·THF]2 (62.6 mg, 0.0590 mmol) was dissolved in 2 mL
of benzene. nPrC(H)NtBu (15.0 mg, 20.0 μL, 0.118 mmol) was
added, and the solution was heated to 60 °C for 1 h. The solution was
concentrated under reduced pressure and held at 8 °C. The product was
isolated as yellow crystals (47.2 mg, 0.0720 mmol, 61%): 1H NMR (600
MHz, C6D6) δ 7.28−7.14 (m, 6H, H-Ph), 4.80 [s, 1H, CC(H)C], 3.53
(m, 4H, THF), 3.28 [m (br), 4H, CH3CHCH3], 3.08 (m, 1H,
CHCHN), 1.69 (s, 6H, CH3), 1.57 (m, 2H, CH3CH2CH), 1.31 (br,
12H, CH3CHCH3), 1.26 (d, 3JH−H = 6.60 Hz, 12H, CH3CHCH3), 1.21
3
(m, 5H, THF and NCHCHCH2), 0.98 (s, 9H, tBu) 0.94 (t, JH−H
=
7.32 Hz, 3H, CH3CH2CH); 13C NMR (151 MHz, C6D6) δ 165.5,
151.1, 147.3, 141.2, 124.1, 123.7, 93.4, 84.2, 68.8, 51.8, 31.7, 29.0, 28.1,
24.9, 24.7, 24.5, 24.33, 23.8, 18.0. Elemental Anal. Calcd for
C41H65N3OCa: C, 75.06%; H, 9.99%; N, 6.40%. Found: C, 74.59%;
H, 10.03%; N, 6.35%.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00226.
■
sı
Synthesis of [nBu4N+][AlH4−]. The salt nBu4N+Cl− (834 mg, 3.00
mmol) and NaAlH4 (162 mg, 3.00 mmol) were stirred in THF (20 mL)
at room temperature for 16 h. The suspension was filtered, and the
solvent was removed under reduced pressure. The residue was washed
twice with cold pentane (8 mL) and dried under vacuum. The product
was isolated as a white powder (444 mg, 1.62 mmol, 54%): 1H NMR
(THF-d8, 600 MHz, 298 K) δ 3.72, 3.44, 3.14, 2.85, 2.56, 2.27 (sextet,
1JAl−H = 174 Hz, 4H, AlH4), 3.36 (m, 8H, N-CH2), 1.72 (m, 8H, NCH2-
CH2), 1.43 (sextet, 3JH−H = 7.5 Hz, 8H, CH3-CH2), 1.00 (t, 3JH−H = 7.5
Hz, 12H, CH3); 13C NMR (THF-d8, 150 MHz, 298 K) δ 58.5, 23.9,
19.6, 13.1; 27Al NMR (THF-d8, 233 MHz, 298 K) δ 100.0 (quint, 1JAl−H
= 258.4 Hz). Elemental Anal. Calcd for C16H40NAl: N, 5.12%; C,
70.27%; H, 14.74%. Found: N, 4.78%; C, 69.60%; H, 14.62%.
Synthesis of (DIPPBDI)Mg(AlH4)·THF. Complex [(DIPPBDI)-
MgH]2 (143 mg, 0.162 mmol) was dissolved in toluene (5 mL) and
cooled to −80 °C. AlH3·(THF)2 (56.2 mg, 0.323 mmol) was dissolved
in toluene (5 mL) and added dropwise to the solution of
[(DIPPBDI)MgH]2. The reaction mixture was allowed to warm to
room temperature. The solvent was removed under reduced pressure.
The product was washed with cold pentane (2 mL) and isolated as a
white powder. Recrystallization from cold toluene yielded crystals
Crystallographic details, including ORTEP plots, selected
1H and 13C NMR spectra, and details for the DFT
calculations (PDF)
XYZ files (XYZ)
Accession Codes
CCDC 1993706−1993710 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Sjoerd Harder − Inorganic and Organometallic Chemistry,
̈
̈
Friedrich-Alexander-Universitat Erlangen-Nurnberg, 91058
Erlangen, Germany; orcid.org/0000-0002-3997-1440;
Email: sjoerd.harder@fau.de
1
suitable for X-ray diffraction (121 mg, 0.222 mmol, 69%): H NMR
(600 MHz, C6D6, 298 K) δ 7.17−7.15 (m, 6H, H-Ph), 4.80 [s, 1H,
CC(H)C], 3.67 (m, 4H, THF), 3.20 (br. 4H, AlH4), 3.16 (septet, 3JH−H
= 6.6 Hz, 4H, CH3CHCH3), 1.63 (s, 6H, CH3), 1.29 (d, J = 6.6 Hz,
12H, CH3CHCH3), 1.21 (d and m, 3JH−H = 6.6 Hz, 16H, CH3CHCH3
and THF); 13C NMR (151 MHz, C6D6, 298 K) δ 169.4, 145.3, 142.5,
125.7, 124.0, 95.1, 70.6, 28.5, 25.3, 25.1, 24.7, 24.5; 27Al NMR (156
MHz, C6D6, 298 K) δ 97.43 (br). Elemental Anal. Calcd for
C33H53N2OAlMg: N, 5.14%; C, 72.72%; H, 9.80%. Found: N, 5.11%;
C, 72.77%; H, 9.60%.
Authors
Holger Elsen − Inorganic and Organometallic Chemistry,
Friedrich-Alexander-Universitat Erlangen-Nurnberg, 91058
Erlangen, Germany
̈
̈
Jens Langer − Inorganic and Organometallic Chemistry,
̈
̈
Friedrich-Alexander-Universitat Erlangen-Nurnberg, 91058
Erlangen, Germany; orcid.org/0000-0002-2895-4979
Michael Wiesinger − Inorganic and Organometallic Chemistry,
Synthesis of (DIPPBDI)Ca(AlH4)·THF2. Complex [(DIPPBDI)CaH·
(THF)]2 (116.4 mg, 0.110 mmol) was dissolved in toluene (7 mL) and
cooled to −80 °C. AlH3·(THF)2 (38.2 mg, 0.219 mmol) was dissolved
in toluene (5 mL) and added dropwise to the solution of
[(DIPPBDI)CaH·(THF)]2. The reaction mixture was allowed to warm
to 0 °C. The solvent was removed under reduced pressure while the
temperature was kept at approximately 0 °C. The product was isolated
as a white powder. The crude product was washed with 3 mL of cold
pentane. Recrystallization from cold toluene yielded crystals suitable for
̈
̈
rnberg, 91058
Friedrich-Alexander-Universitat Erlangen-Nu
Erlangen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00226
Notes
The authors declare no competing financial interest.
1
X-ray diffraction (89.2 mg, 0.141 mmol, 64%): H NMR (600 MHz,
ACKNOWLEDGMENTS
The authors acknowledge Mrs. C. Wronna and Mrs. A. Roth
(University of Erlangen-Nu
■
C6D6, 298 K) δ 7.14 (m, 6H, H-Ph), 4.78 [s, 1H, CC(H)C], 3.59 (m,
8H, THF), 3.20 (septet, 3JH−H = 6.8 Hz, 4H, CH3CHCH3), 2.88 (br,
4H, AlH4), 1.66 (s, 6H, CH3), 1.34 (d and m, 6.8 Hz, 20H, CH3CHCH3
̈
rnberg) for numerous CHN analyses
3
and THF), 1.22 (d, JH−H = 6.8 Hz, 12H, CH3CHCH3); 13C NMR
̈
and J. Schmidt and Dr. C. Farber (University of Erlangen-
(C6D6, 151 MHz, 298 K) δ 165.5, 146.2, 141.4, 124.4, 123.7, 93.9, 68.9,
Nu
̈
rnberg) for assistance with the NMR analyses. The authors
G
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Robertson, S. D.; Okuda, J.; Mulvey, R. E. Donor-influenced Structure-
Activity Correlations in Stoichiometric and Catalytic Reactions of
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2018, 24, 9940−9948.
thank the DFG for supporting this research via a grant to S.H.
(HA3218/10-1).
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Organometallics XXXX, XXX, XXX−XXX