Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings

Article abstract of DOI:10.1002/anie.202001160

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)₂]₂ (1-Ae) and Ae[N(TRIP)(DIPP)]₂ (2-Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr₃, DIPP=2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized. Monomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene.

Full text of DOI:10.1002/anie.202001160

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Accepted Article
Title: Highly Active Superbulky Alkaline Earth Metal Amide Catalysts
for Hydrogenation of Challenging Alkenes and Aromatic Rings
Authors: Sjoerd Harder, Johannes Martin, Christian Knüpfer, Jonathan
Eyselein, Christian Färber, Samuel Grams, Jens Langer,
Katharina Thum, and Michael Wiesinger
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001160
Angew. Chem. 10.1002/ange.202001160
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http://dx.doi.org/10.1002/ange.202001160

10.1002/anie.202001160
Angewandte Chemie International Edition
Highly Active Superbulky Alkaline Earth Metal Amide Catalysts
for Hydrogenation of Challenging Alkenes and Aromatic Rings
Johannes Martin, Christian Knüpfer, Jonathan Eyselein, Christian Färber, Samuel Grams, Jens
Langer, Katharina Thum, Michael Wiesinger and Sjoerd Harder*
J. Martin, C. Knüpfer, J. Eyselein, Dr. C. Färber, S. Grams, Dr. J. Langer, K. Thum, M. Wiesinger, Prof. Dr. S. Harder*
Chair of Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058
Erlangen, Germany
Fax: (+49) 9131-8527387
E-mail: sjoerd.harder@fau.de
Supporting information for this article is available on the WWW under http//:www.angewandte.chemie.org or
from the author.
Keywords: Alkaline Earth Metal – Alkene – Aromatic rings – Hydrogenation – DFT calculations
Table of Contents
Bulk = Hulk: Superbulky alkaline earth metal amide complexes were found to be extremely active
catalysts for alkene hydrogenation, clearly extending the substrate scope. Even various arenes, including
benzene, can be reduced under relatively mild conditions.
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Introduction
Addition of molecular hydrogen to multiple bonds may seem a simple reaction. In reality, however, this
industrially important transformation has been subject of immense research.[1] Since Sabatier’s
milestone discovery,[2] alkene-to-alkane conversion has been fully dominated by transition metal
catalysts. The last decade, however, saw the introduction of s-block metal catalysts,[3-11] or even metal-
free systems like Frustrated Lewis Pairs[12] or boranes,[13] showing that alkene activation by d→*
backbonding is not an absolute requirement.[14,15] Since the first report on alkaline earth (Ae) metal
catalyzed alkene hydrogenation[3] there has been a considerable improvement in catalyst performance,
continuously shifting borders to milder reaction conditions and extending the substrate scope to more
challenging substrates. In general, alkene hydrogenation becomes more difficult with the number of
substituents while conjugation or incorporation in a ring system accelerate reduction (Scheme 1).
Scheme 1. Reactivity order for alkene hydrogenation.
In 2008 we reported the hydrogenation of activated (conjugated) alkenes with K, Ca and Sr benzyl
catalysts or a ß-diketiminate Ca hydride complex.[3] This transformation follows a simple mechanism in
which addition of the alkene to a metal hydride intermediate is a key step. Since styrene or butadiene
type substrates react to resonance-stabilized benzylic or allylic intermediates, hydrogenation of
conjugated double bonds is facile.
Okuda and coworkers introduced cationic calcium hydride catalysts (I-II) and demonstrated
hydrogenation of unactivated alkenes like 1-hexene.[6-8] As the dicationic catalyst (II) was found to be
particularly active, it was concluded that the positive charge is critical in imparting sufficient
electrophilicity to the metal center. The Hill group introduced a THF-free analogue (III)[16] of an earlier
reported ß-diketiminate Ca hydride complex.[17] Unstabilized by a Lewis base, this hydride complex is
extremely reactive, however, at higher temperatures it is also prone to ligand exchange by the Schlenk
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equilibrium: [(DIPPBDI)CaH]2 ⇄ (DIPPBDI)2Ca + CaH2. Since precipitation of (CaH2)∞ results in catalyst
deactivation, alkene hydrogenation was studied at 25 °C. Unactivated alkenes like 1-hexene or
norbornene could be reduced, however, conversion times of 2-3 weeks are needed.[9]
At the same time, we reported the unexpected catalytic activity of well-known Ae[N(SiMe3)2]2 complexes
abbreviated as AeN’’2 (Ae = Ca, Sr, Ba).[4] Metal hydride formation by deprotonation of H2 (pKa ≈ 49)[18]
by this weak base (pKa of N’’H = 25.8)[19] seemed illogical (Scheme 2) and was calculated to be endergonic
by ca. 11 kcal/mol. However, reaction of CaN’’2 with H2 in benzene gave N’’H and undefined clusters
(N’’CaH)x(CaH2)y with a MW up to 7500.[20] The energy released in the aggregation process is the driving
force for this reaction. Addition of PMDTA enabled interception of the smaller cluster
(N’’CaH)3(CaH2)3∙(PMDTA)3 (Scheme 2b); also small Sr and Ba clusters have been isolated.[21] Use of
simple AeN’’2 catalysts turned out to be highly advantageous. First of all, these catalysts fully suppress
alkene oligomerization. This undesired side-reaction is generally observed for activated substrates like
styrene for which oligomerization is competitive with the more difficult H2 deprotonation. However, for
AeN’’2 catalysts the acidic amine N’’H (formed during catalyst initiation) rapidly traps the benzylic
intermediate preventing oligomerization (Scheme 2a). Noteworthy is also the very high thermal stability
of these catalysts. At an operating temperature of 120 °C, decomposition and Schlenk equilibria are not
an issue. The activities of AeN’’2 catalysts strongly increase with metal size: Ca < Sr < Ba. The superb
activity of the Ba catalyst was independently confirmed by the Cheng group who introduced a highly
active barium hydride catalyst (IV) that operates at 30 °C.[10]
Scheme 2. (a) Catalytic cycle for alkene hydrogenation) with AeN’’2 catalysts. (b) Crystal structure of
Ca6H9N’’3·(PMDTA)3.[21a]
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Despite continuous breakthroughs in alkene hydrogenation, early main group metal catalysts are a far
cry from the classical transition metal catalysts. While unactivated terminal alkenes like 1-hexene can
be reduced, internal alkenes or higher substituted alkenes pose a problem: semi-activated norbornene
could be hydrogenated[4,9] but cyclohexene is already fully inert.
By far more challenging than alkene hydrogenation is the reduction of highly stable aromatic arenes.
The catalysts for breaking the ring’s aromaticity are generally based on platinum group metals. Many
homogeneous catalysts react in reality as heterogenous systems,[22] i.e. larger nanoparticles often
operating under harsher conditions, and very few true homogeneous catalysts exist.[23,24] Stephan and
co-workers reported the metal-free catalyst B(C6F5)3 for hydrogenation of N-containing rings or
arenes.[25,26] In these cases the substrate is part of the Frustrated Lewis Pair (FLP) catalyst but the FLP
combination Ph2PC6F5/B(C6F5)3 was able to hydrogenate larger polycyclic aromatic hydrocarbons (PAH’s)
like anthracene and tetracene. Although these extended -systems are generally easier reduced, harsh
conditions were needed (80 °C, 100 bar H2).[27] Stock demonstrated that LiN(iPr2)2 or KN(SiMe3)2 catalyze
the reduction of activated PAH’s like anthracene or naphthalene under drastic conditions (25 mol%
catalyst, 200 °C, 70 bar H2, 18 h).[28] The Hill group very recently demonstrated the stoichiometric
reduction of activated PAH’s with Ca hydride complex IV[29] but the catalytic hydrogenation of aromatic
rings with Ae metal catalysts has hitherto not been reported.
We here introduce a concept that boosts the activity of Ae metal amide catalysts tremendously. We
demonstrate the hydrogenation of most challenging alkenes and, for the first time, also of aromatic
rings.
Results and Discussion
In contrast to the many advantages for hydrogenation with AeN’’2 catalysts, there is one major
drawback: generally high catalyst loadings of 5-10 mol% are needed.[20] We attribute this to the
aforementioned aggregation to larger (N’’AeH)x(AeH2)y clusters resulting in a drastic lowering of the
catalyst concentration. Work by Jones, Ruhlandt-Senge and Mills has shown that superbulky amide
ligands suppress aggregation.[30-33] It was
reasoned that use of superbulky amides will
decrease cluster size and increase catalyst
concentration (Scheme 3).
Scheme 3. Influence of ligand bulk on particle
aggregation and cluster size (red: cluster core,
blue: capping ligand).
Fine-tuning the size of nanoparticles with
capping ligands is a well-known concept:
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bulkier ligands decrease core size while sharpening the particle size distribution.[34] For Pd nanoparticles
capped by dendritic ligands, the activity in alkene hydrogenation increased with dendrimer
generation.[35] The same principle also applies to smaller molecular clusters.[36]
Synthesis and structures
We targeted the syntheses of the superbulky Ae metal amides Ae[N(TRIP)2]2 (1-Ae) and
Ae[N(TRIP)(DIPP)]2 (2-Ae), see Scheme 3 for abbreviations. Complex 1-Ca has been reported but earlier
attempts to isolate 1-Mg or 1-Sr failed.[33] Amines HN(TRIP)2 and HN(TRIP)(DIPP)[33,37,38] were converted
to their potassium salts and subsequent reaction with AeI2 in aromatic solvents gave the bulky amide
complexes 1-Ae (Ae = Mg, Ca, Sr, Ba) in crystallized yields of 57-67%. Similarly, 2-Ae (Ae = Mg, Ca, Sr, Ba)
complexes were isolated in yields of 40-46%. The excellent solubility of these complexes in alkanes
prevents high crystallized yields. All complexes were fully characterized by NMR, CHN analyses and X-
ray diffraction (Figure 1, Table S2).
Figure 1. Selected crystal structures: (a) 1-Mg and 1-Ba; H atoms omitted. (b) Space-filling models for 1-Mg (top)
and 1-Ba (bottom). (c) 2-Mg and 2-Ba; H atoms omitted.
Complexes in the series 1-Ae are all monomeric featuring (nearly) linear two-coordinated metal ions
with N-Ae-N’ angles ranging from 171.9(1)° to 179.7(1)°. The Mg-N bond in 1-Mg (1.997(1) Å) is clearly
longer than that in monomeric MgN’’2 (1.91(3) Å)[39] illustrating the considerable steric stress in the 1-
Ae series. Noteworthy is the linear nature of 1-Ba: N-Ba-N’ = 177.5(1)°. While Sarazin recently reported
the first two-coordinate Ba complex, a boryloxide complex that is strongly bent,[40] 1-Ba is the first
example of a near linear two-coordinate barium compound. All complexes feature numerous short
anagostic interactions with the iPr groups (see Table S2). The Si-N-Si’ angles are very obtuse and increase
with metal size from 132.4(1)° in 1-Mg to 141.3(1)° in 1-Ba. The bis-silyl amide anion (R3Si)2Nˉ is isolobal
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to (R3Si)2O and Si-N-Si’ units have similar to Si-O-Si’ units the tendency to be linear.[41] This originates
from negative hyperconjugation, i.e. partial charge delocalization of the N (or O) lone pairs in empty
*(Si-R) orbitals,[42] which increases along the series 1-Mg < 1-Ca < 1-Sr < 1-Ba (this is indicated by
increasing Si-N-Si’ angles and decreasing Si-N bonds, Table S2).
Complexes 2-Ae are also monomeric but with N-Ae-N’ angles ranging from 143.8(1)° to 169.5(1)° they
are clearly less linear than the 1-Ae series. The Ae-N distances in 2-Ae are consistently shorter than those
in 1-Ae by 0.06-0.08 Å, showing that N(TRIP)2 is bulkier. The N(TRIP)(DIPP) ligand, however, should still
be considered bulky: the Mg-N bonds in 2-Mg (1.934(2)-1.946(2) Å) are significantly longer than those
in Mg[N(SiMe3)(DIPP)]2 (1.919(2) Å).[32b] The larger metal ions in the 2-Ae series have a strong tendency
to interact with the DIPP -system (this is especially the case in 2-Ba). The Ae∙∙∙DIPP interactions are
clearly more important than anagostic Ae∙∙∙TRIP interactions. This is evident from the very acute Ae-N-
C angles (average: 89.1-90.5°) and rather obtuse Ae-N-Si angles (average: 131.8-138.7°).
Alkene hydrogenation
Since deactivating Schlenk equilibria do not play a role for 1-Ae and 2-Ae, these very robust catalysts
were tested at 120 °C but the H2 pressure was kept low at 6 bar (Table 1; see Table S3 for a more
extensive list including additional substrates and different catalysts). As a first test case the
hydrogenation of 1-hexene, an unactivated alkene, was investigated. While the Mg catalysts 1-Mg and
2-Mg are essentially inactive, activities increased considerably down the group: Mg < Ca < Sr < Ba (entries
1-8). The catalysts with the bulkier N(TRIP)2 ligand (1-Ae) clearly show the better performance. The less
active 2-Ae catalysts gave much more isomerisation to internal alkenes which could not be reduced
furthe. The Ba catalyst 1-Ba shows significant activity at 60 °C but is slow at room temperature (entries
9-10). Lowering the H2 pressure to 1 bar gave considerable isomerisation and incomplete reduction
(entry 11) but lowering the catalyst loading to 1 mol% is unproblematic (entry 12). Compared to BaN’’2
the superbulky catalyst 1-Ba is extremely active (cf. entries 8 and 13). Sterically hindered alkenes like
tBuC(H)=CH2 could be reduced within hours (entry 14) while hydrogenation of 1,5-hexadiene led only to
minor cyclization (entry 15). The very high activity of 1-Ba is further supported by rapid reduction of
doubly substituted, conjugated, alkenes which were fully converted within 1-1.5 hours using only 1 mol%
catalyst (entries 16-18); also diphenylacetylene was effectively doubly reduced (entry 19).
One of the shortcomings of these superbulky catalysts became clear while reducing substrates sensitive
towards oligomerization. -Methylstyrene is rapidly converted by 1-Ba but gave considerable quantities
of dimer (entry 20); cf. BaN’’2 gave under similar conditions clean conversion (entry 21). This is explained
by the fact that trapping of the benzylbarium intermediate by N’’H (Scheme 2a) is much faster than by
HN(TRIP)2 which due to steric hindrance is very hard to deprotonate. Especially at low catalyst
concentrations, and thus low free amine concentrations, alkene dimerization is an issue (entry 22).
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However, using the less active Ca catalyst 1-Ca led to full conversion with only traces of dimer (entry 23).
Also for hydrogenation of the semi-activated alkene (Me3Si)C(H)=CH2 dimeric products were observed,
especially at low catalyst loading. In this case dimerization could be fully prevented by lowering the
temperature (entries 24-26).
Norbornene, a semi-activated alkene, is converted twenty times faster to norbornane with 1-Ba than
with BaN’’2 (entry 27). We demonstrate a first case for hydrogenation of the very challenging substrate
cyclohexene with activities rapidly increasing down the group: 1-Ca < 1-Sr < 1-Ba (entries 28-30). For
comparison, BaN’’2[5] and the dicationic Ca hydride complex II[8] gave nill conversion. The catalyst loading
for 1-Ba could be lowered to 5 mol% (entry 31). Also 4-vinylcyclohexene was fully reduced (entry 32).
Ring enlargement to cyclooctene increased the conversion rate (entry 33).
Alkenes with heteroatoms that potentially coordinate to the Ae metal posed problems. The most active
Ba catalyst 1-Ba did not convert 3,4-dihydropyrane but instead formed a stable 1/1 complex (Figure S55)
suggesting that Ba-ether coordination blocks free coordination sites at Ba killing any further reactivity
with H2.
Catalyst 1-Ba even converted acyclic, unactivated, internal alkenes (entries 34-36). As expected,
reduction of cis-3-hexene is faster than trans-3-hexene but 1,1-Et2C=CH2, which should be reduced more
facile, gave partial isomerisation to a trisubstituted alkene which could only be slowly reduced further.
We were, however, able to hydrogenate 1-Ph-cyclohexene (entries 37-38) and, most impressively, also
the cyclic unactivated trisubstituted alkene: 1-Me-cyclohexene (entry 39). The superb performance of
1-Ba is further highlighted by the hydrogenation of tetraphenylethylene (entry 40), a most challenging
substrate (catalyst IV gave stoichiometric conversion).[10] The limitation of the highly active catalyst 1-
Ba is set by the tetraalkylated alkene 2,3-dimethyl-2-butene which was reduced in small quantities
(entry 41). It should, however, be mentioned that reduction of this substrate is even for platinum group
catalysts highly challenging.[43]
Arene hydrogenation
While the dearomatization of arenes by hydrogenation is quite a challenge, first attempts in group 2
metal catalyzed arene hydrogenation focused on anthracene in which particularly the central ring is
activated for reduction (Table 2; see Table S4 for a more extensive list including additional substrates
and different catalysts). Even simple AeN’’2 complexes are able to reduce anthracene, the activity
increasing from Ca to Ba (entries 1-3). Reduction of the central ring is preferred and only slight reduction
in the terminal rings was observed. Again, the bulkier amides 1-Ae (Ca, Sr, Ba) are clearly more active
(entries 4-6); the most active catalyst 1-Ba gave after 2.5 hours nearly quantitative yield. Complex BaN’’2
was also able to reduce the more challenging substrate naphthalene (entry 7). While bulky 1-Ca is
inactive, the 1-Sr and 1-Ba catalysts quantitatively hydrogenated one of the rings in naphthalene with
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conversion times as short as two hours (entries 8-10). The most active catalyst 1-Ba was able to
hydrogenate alkylated PAH’s like 9-Me-anthracene and 1-Me-naphthalene (entries 11-12) which are
deactivated for reduction by electronic effects.[44] The preference for the hydrogenation of non-alkylated
rings is evident from the 80/20 ratio observed for reduction of 1-Me-naphthalene. Hydrogenation of
phenanthrene gave a mixture of four isomers but extending the reaction time reduced this to only two
products (entry 13-14). Using the most active catalyst 1-Ba, biphenyl was hydrogenated within ten hours
leaving the second ring intact. The activity increases with metal size Ca < Sr < Ba (entries 15-17).
Hydrogenation of 1,3,5-triphenylbenzene gave exclusively reduction of the highly activated central ring
producing the two possible diastereomers (entry 18).
These results demonstrate that group 2 metal catalysts, in particular 1-Ba, are well able to hydrogenate
highly stable aromatic substrates provided the rings are conjugated. Although conversion of isolated
aromatic rings was not observed, we found in longer runs with the most active hydrogenation catalyst
1-Ba always minor quantities of cyclohexane. The latter is formed by hydrogenation of the benzene
solvent. Since benzene-to-cyclohexane conversion is an important industrial bulk process, we attempted
benzene hydrogenation under more stringent conditions of 50 bar H2 and 140 °C, i.e. conditions similar
to the IFP process for large-scale cyclohexane production.[45] Using only 2.5 mol% 1-Ba we found that
after three days 18% of the benzene solvent was converted to cyclohexane (entry 19). Although the Ba
catalyst is not as efficient as Ni/Al catalysts, the current result is a most striking demonstration that also
unactivated (non-conjugated) aromatic rings like benzene, which are generally very hard to reduce,[46]
can be hydrogenated with group 2 metal catalysts.
Mechanistic considerations
In contrast to catalytic studies with well-defined Ae metal hydride catalysts,[3,9,10] it is extremely difficult
to identify the “true” catalyst in Ae metal amide catalyzed hydrogenation.[20] Borders between
homogeneous molecular catalysis and heterogeneous nanoparticle catalysis are starting to disappear
and it is often extremely hard, or even impossible, to establish the nature of the catalyst. It is even likely
that numerous catalytically active species operate in concert.[47] Building upon our previous work in
alkene hydrogenation,[4,5] it is reasonable to propose the initial formation of monomeric (amide)AeH
species.
In order to increase our understanding on possible catalytically active species, a solution of 1-Ae (Ca or
Ba) in C6D6 was reacted with H2 (1 bar, 60-90 °C) and monitored by 1H NMR. The appearance of signals
for HN(TRIP)2 and very broad resonances in the amide and hydride regions suggested the formation of
undefined hydride clusters. Reaction of 1-Ca with H2 is much slower than hydrogenolysis of CaN’’2. At 1
bar H2 and 60 °C only 50% conversion was reached in 50 hours. Hydrogenolysis of 1-Ba was considerably
faster (1 bar H2, 90 °C, 100% conversion, 5 hr). As discussed previously, this contrathermodynamic
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reaction is possible due to aggregation which releases energy. All attempts to crystallize defined metal
hydride clusters from reaction of 1-Ae with H2 failed. This is partially due to the extreme solubility of
these species induced by the multiple iPr substituents.
The effect of ligand size on cluster size was investigated by DOSY NMR using the external calibration
method described by Stalke and coworkers (see ESI).[48] For 1-Ca and 1-Ba in benzene we found in both
cases molecular weights lower than calculated for the monomers (1-Ca: calc. 697, found 515; 1-Ba: calc.
795, found 548). Using a different calibration method, Mills and coworkers reported a molecular weight
of 1000 for 1-Ca in benzene.[33] It is possible that the nature of the ligand, which is very rich in H, may
cause systematic errors. DOSY measurements on a solution of 1-Ca converted with H2 gave species with
a molecular weight up to circa 1700 g/mol, For reaction mixtures of 1-Ba with H2 clusters up to a
molecular weight of circa 3300 g/mol have been observed. The higher aggregation for the Ba hydride
species is in line with the fact that larger metal cations tend to form larger aggregates. However, both
clusters are considerably smaller than those obtained from reaction of CaN’’2 with H2 (7500 g/mol),[20]
supporting the idea that bulky amide ligands give rise to smaller catalytic species in higher
concentrations.
Mechanistic understanding was improved by various DFT studies using the B3PW91/def2tzvpp method
including correction for dispersion (GD3BJ) and solvent (PCM=benzene); Ba was described by
pseudopotentials (see ESI for details).
In a first set of calculations we investigated the effect of amide bulk on aggregation by optimization of
(R2NCaH)x species (x = 1, 2 or 4) with increasing amide size Me2N < (Me3Si)2N < (TRIP)2N (Table S9 and
Scheme S4). Monomer→dimer conversion is independent on amide bulk and in all cases exothermic by
circa H = -30 kcal/mol. However, the enthalpies for monomer→tetramer conversion decrease with
increasing amide size;H in kcal/mol: Me2N -87.7, (Me3Si)2N -72.4 and (TRIP)2N -39.6. As aggregation
causes considerable entropy loss the G values are less negative (Me2N -55.8, (Me3Si)2N -21.6) while
formation of [(TRIP)2NCaH]4 is even endergonic by G = +8.2 kcal/mol. This clearly supports the
observation that bulkier amides reduce cluster size leading to increased catalyst concentration.
The second set of calculations aimed at understanding the influence of amide bulk and the metal on
alkene hydrogenation. The pathway for catalytic ethylene hydrogenation with 1-Ca and 1-Ba catalysts is
shown in Scheme 4a. For comparison, we also recalculated the previously reported pathway for CaN’’2
using this method. The barrier for hydride formation by reaction of 1-Ca with H2 is higher (Ca2*: 22.9
kcal/mol) than that for the reaction of CaN’’2 with H2 (17.6 kcal/mol). This is in agreement with
experimental observation and likely related to unusually high steric stress in the superbulky amide
complex. Steric stress in 1-Ca is also reflected by its exothermic conversion to (TRIP)2NCaH (Ca4: -7.0
kcal/mol) while reaction of CaN’’2 to N’’CaH is endothermic (Ca4’: +14.8 kcal/mol).[4] The activation
energy for formation of (TRIP)2NBaH from 1-Ba is higher and not in agreement with the faster reaction
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of 1-Ba with H2. This is likely due to insufficient modeling of solvent effects for the highly unsaturated
species (TRIP)2NBaH by the PCM method.
Scheme 4. Energy profiles (H in kcal/mol) for (a) the hydrogenation of ethylene by catalysts 1-Ca (orange), 1-Ba
(black) and CaN’’2 (red), and (b) benzene hydrogenation by 1-Ba; B3PW91/def2tzvpp including correction for
dispersion (GD3BJ) and solvent (PCM=benzene).
The activation energy for alkene insertion for the Ba catalyst (Ba5 → Ba6*: +2.7 kcal/mol) is lower than
that for the Ca catalyst (Ca5 → Ca6*: +5.2 kcal/mol). Also the reaction of the ethyl metal intermediate
with H2 has a lower barrier for Ba (Ba7 → Ba9*: +6.7 kcal/mol) than for Ca (Ca7 → Ca9*: +9.9 kcal/mol).
The very shallow energy profile for the Ba catalyst is in line with its much higher activity and excellent
performance in alkene hydrogenation. Comparison of the profiles for both Ca catalysts, N’’CaH and
(TRIP)2NCaH, shows that the bulk of the remaining amide ligand has only a minor influence on ethylene
hydrogenation. However, for the less bulky catalyst N’’CaH deactivation by the reverse reaction with
N’’H back to CaN’’2 (Ca4’ → Ca2’*: +2.8 kcal/mol) is easier than ethylene insertion (Ca4’ → Ca6’*: +4.9
kcal/mol). In contrast, reaction of bulky (TRIP)2NCaH with (TRIP)2NH is extremely difficult (Ca4 → Ca2*:
+29.9 kcal/mol) and ethylene insertion is facile. It is therefore unlikely that formation of the catalyst
(TRIP)2NCaH is an equilibrium. This may also explain the much higher activity of 1-Ca compared to CaN’’2.
While the monomer R2NAeH may not be the best model system for larger aggregates, the hydride-
alkene insertion step was additionally calculated for a dimeric catalyst (Scheme S2). In line with the
higher reactivity of monomeric metal hydrides, these dimeric catalysts show somewhat higher activation
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enthalpies for ethylene insertion (H in kcal/mol): [(TRIP)2NCaH]2 +9.9, [(TRIP)2NBaH]2 +9.9 and
[N’’CaH]2 +14.6. In this case the Ca catalyst with the bulkier amide [(TRIP)2N is more reactive than that
with N’’.
In a third set of calculations the pathway for benzene hydrogenation with the most active (TRIP)2NBaH
catalyst has been evaluated (Scheme 4b). The cycle starts with benzene complexation followed by
formation of a Meisenheimer which is surprisingly exothermic (Ar3: -3.5 kcal/mol); breaking benzene’s
aromaticity by formation of the Meisenheimer anion is generally endothermic.[5,29,49] Two possible
transition states for the reaction with H2 have been located (Ar5* and Ar6*). The transition state for
formation of 1,3-cyclohexadiene (Ar5*) is 1.6 kcal/mol lower in energy than that for 1,4-cyclohexadiene
formation (A6*). The activation energies of +18.7 and +20.3 kcal/mol are considerably higher than those
for alkene hydrogenation. This is in line with the experimentally very challenging reduction of aromatic
substrates. Note that the first step, benzene-to-cyclohexadiene reduction, is endothermic by circa +10
kcal/mol but further hydrogenation to cyclohexane is overall exothermic[50] and should be facile.
Conclusions
We introduced two sets of bulky Ae metal amide complexes, 1-Ae and 2-Ae (Ae = Mg, Ca, Sr, Ba) that
are monomeric in the solid state. All 1-Ae complexes with the bulkier N(TRIP)2 ligand show highly linear
coordination geometries while 2-Ae complexes with the N(TRIPP)(DIPP) ligand are bent and reveal
shorter Ae-N bonds.
The bulk of the amide ligand has a tremendous effect on alkene hydrogenation which is likely best
illustrated by entries 8 and 13 in Table 1. The catalyst activities increase with the size of the amide ligand
along the series AeN’’2 < 2-Ae < 1-Ae and with metal size Mg < Ca < Sr < Ba. The most active catalyst 1-
Ba clearly extended the substrate scope for s-block metal catalyzed alkene hydrogenation. Highly active
1-Ba is able to reduce unactivated internal alkenes like cyclohexene or 3-hexene but also hydrogenation
of highly challenging substrates like 1-Me-cyclohexene (a trisubstituted double bond) or
tetraphenylethylene could be achieved. A drawback for the bulky amide catalysts is the enhanced
oligomerization found for activated alkenes like styrene. In this case less bulky AeN’’2 catalysts would be
preferred.
The very high activity of bulky amide catalysts was further demonstrated by high activities in arene
hydrogenation which hitherto was never achieved with group 2 metal catalysts. While PAH’s with
extended -systems could be routinely reduced to products with at least one aromatic ring,
hydrogenation of the remaining aromatic 6-electron system is difficult and also for transition metal
catalysts a challenge.[43] However, during hydrogenation catalysis using our most active catalyst 1-Ba in
benzene, generally traces of cyclohexane were found. Under more forcing conditions benzene could be
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partially hydrogenated to cyclohexane. This first main group metal catalyzed benzene hydrogenation
illustrates the enormous potential of the heavier Ba catalysts.
Since Ae metal hydride species have strong tendencies to aggregate to complicated mixtures of species,
the nature of the catalyst remains unclear. DOSY NMR measurements show that the much higher activity
of superbulky Ae amide catalysts may be explained by the lower aggregation numbers for these bulky
systems. This is supported by DFT calculations which demonstrate that aggregation of R2NCaH species
becomes less favorable with increasing amide size: Me2N < N’’ < (TRIP)2N. Calculations also show that
catalyst generation may play a role: for smaller amides like N’’ the equilibrium, AeN’’2 + H2 ⇄ N’’AeH +
N’’H, lies to the left. The reaction of 1-Ae with H2 to give (TRIP)2NAeH is much slower but once formed
the reverse catalyst deactivation, i.e. deprotonation of sterically hindered (TRIP)2NH, is unlikely.
The herein described catalytic reduction of highly challenging alkenes and arenes by superbulky Ae metal
amides is unrivalled. These results demonstrate that current state-of-the-art group 2 metal
hydrogenation catalysis starts to reach a similar level as traditional transition metal catalysis. It should
be considered highly remarkable that stable aromatic molecules like benzene submit to group 2 metal
power.[51] We currently aim to increase catalyst activities and applications even further.
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Acknowledgments We kindly acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding of
this project (HA 3218/9-1).
Supporting Information Experimental procedures for complex syntheses and catalytic experiments
(including a more extensive selection of substrates and group 2 metal catalysts), selected NMR spectra,
DOSY measurements, details for X-ray diffraction and DFT investigations including XYZ coordinates for
all optimized minima and transition states.
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Table 1. Catalytic alkene hydrogenation.[a]
H2
T
t
Conv.[b]
Entry Catalyst mol% Substrate
Product(s)
[bar] [°C] [h]
[%]
1
2
2-Mg
1-Mg
2-Ca
1-Ca
2-Sr
10
10
10
10
10
10
10
10
10
10
10
1
6
6
6
6
6
6
6
6
6
6
1
6
6
6
120 24
120 24
120 24
11 / 1[c]
4 / 1[c]
45 / 42[c]
99
3
4
120
3
5
120 24
120 0.5
120 24
120 0.5
78 / 19[c]
6
1-Sr
99
98[c]
7
2-Ba
1-Ba
1-Ba
1-Ba
1-Ba
1-Ba
BaN‘‘2
1-Ba
8
99
9
60
25
6
99
10
11
12[d]
13[e]
14
24
10 / 1[c]
10 / 83[c]
99
120 24
120
120 24
4
10
10
42 / 58
99
120
2
15
1-Ba
10
6
120
2
84 / 16[c]
16
17
1-Ba
1-Ba
1
1
6
6
120 1.5
99[c]
120
1
99
18
19
1-Ba
1-Ba
1-Ba
BaN‘‘2
1-Ba
1-Ca
1-Ba
1-Ba
1-Ba
1
10
10
10
1
6
6
6
6
6
6
6
6
6
120
120
1
1
99[c]
99[c]
74 / 26[c]
20
21[e]
120 0.5
120 0.25
120 0.5
99
22
67 / 33[c]
98 / 2[c]
81 / 16[c][f]
90 / 10[c]
99[c]
23
24[d]
1
120
4
1
120 0.5
120 0.5
25
10
10
26
60
2
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Table 1. Continued.
H2
T
t
Conv.[b]
Entry
Catalyst
mol% Substrate
Product(s)
[bar] [°C] [h]
[%]
27
1-Ba
1
6
120 0.5
99
28
29
30
31
1-Ca
1-Sr
1-Ba
1-Ba
10
10
10
5
6
6
6
6
120 24
120 10
51
99
99
26
120
3
120 24
120 3.5
32
33
34
1-Ba
1-Ba
1-Ba
10
10
10
6
6
6
99
99
120 1.5
120 24
87 / 13[c]
35
36
37
1-Ba
1-Ba
1-Ba
10
10
10
6
6
6
120
7
99
99
99
120 22
120
120
1
1
38[g]
39
1-Ba
1-Ba
10
10
6
6
99[c]
81[c]
120 24
120 24
40[g]
1-Ba
10
6
73[c]
41[h]
1-Ba
20
20 120 24
14[c]
[a] General catalytic reaction conditions: [substrate]0 = 0.5 M in C6D6 (1 mL); reaction times for essentially full
conversion (99%) have been optimized in 0.5 h-steps; for non-quantitative reactions, the conversion after 24 h is
1
given. [b] Conversion was determined from a H NMR spectrum of the crude reaction mixture by integration of
significant signals of alkane product and substrate, where applicable; GC/MS was used for further product
identification (see ESI for experimental details). [c] Conversion and product distribution have been determined by
GC/MS analysis. [d] Reaction conducted with [alkene]0 = 1 M in C6D6 (1 mL). [e] Data taken from Ref. [4]. [f] The
product mixture contained traces (<3%) of unidentified species. [g] Reaction run at [alkene]0 = 0.25 M in C6H6 (1
mL). [h] C6H6 (1 mL) used as solvent in order to prevent D incorporation in the product.
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Table 2. Catalytic arene hydrogenation; [arene]0 = 0.5 mmol, Ae metal catalyst (10 mol%), C6H6 (1 mL),
H2 (12 bar), 120 °C.
t
[h]
Conv.[a]
Entry
Catalyst
Substrate
Product(s)
[%]
1[b]
2[b]
3[b]
4[b]
5[b]
6[b]
7
CaN‘‘2
SrN‘‘2
BaN‘‘2
1-Ca
24
24
24
24
24
2.5
24
24
24
2
44 / 1
97 / 2
95 / 3
83 / 2
94 / 3
94 / 2
53
1-Sr
1-Ba
BaN‘‘2
1-Ca
8
7
9
1-Sr
99
10
1-Ba
99
11[b]
1-Ba
1-Ba
24
24
84 / 15
80 / 20
12
42 / 32
15 / 11
13
1-Ba
1-Ba
24
48
51 / 49
0 / 0
14[c]
15
16
17
1-Ca
1-Sr
1-Ba
24
24
10
18
68
99
18[b]
1-Ba
24
99
19[d]
1-Ba
72
18[e]
[a] Conversion and product distribution were determined by GC/MS analysis; NMR spectroscopy was also used to
corroborate the product identification. [b] Reaction conducted with [arene]0 = 0.25 M in C6H6 (1 mL). [c] Reaction
at 20 bar H2. [d] Reaction performed with 2.5 mol% 1-Ba at 140 °C under 50 bar H2. [e] Conversion determined by
quantitative 13C{1H} NMR spectroscopy.
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Abstract:
Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)2]2 (1-
Ae) and Ae[N(TRIP)(DIPP)]2 (2-Ae) (Ae = Mg, Ca, Sr, Ba, TRIP = SiiPr3, DIPP = 2,6-diisopropylphenyl). While
monomeric 1-Ca was already known, the new complexes have been structurally characterized.
Monomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide
complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing
from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly
challenging substrates like 1-Me-cyclohexene or tetraphenylethylene. It is also active in arene
hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as
biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step
in catalytic hydrogenation is formation of an (amide)AeH species which can form larger aggregates.
Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s)
is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics
for formation of the (amide)AeH species. Complex 1-Ba is currently the most powerful Ae metal
hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously
reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging
multi-substituted unactivated alkenes and even to arenes among which benzene.
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