Mononuclear calcium complex as effective catalyst for alkenes hydrogenation

Article abstract of DOI:10.1039/d0cc01745k

Hydrogenolysis of the scorpionate-supported calcium benzyl complex [(Tp^Ad,iPr)Ca(p-CH₂C₆H₄-Me)(THP)] (Tp^Ad,iPr= hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate, THP = tetrahydropyran) (2-THP) afforded the mononuclear calcium hydrido complex [(Tp^Ad,iPr)Ca(H)(THP)] (3). Under mild conditions (40 °C, 10 atm H₂, 5 mol% cat.), complex3effectively catalyzed the hydrogenation of a variety of alkenes, including activated alkenes, semi-activated alkenes, non-activated terminal and internal alkenes. Mononuclear calcium unsubstituted alkyl complex [(Tp^Ad,iPr)Ca{(CH₂)₄Ph}(THP)] (6), proposed as the catalytic hydrogenation intermediate, was isolated and structurally characterized.

Full text of DOI:10.1039/d0cc01745k

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Mononuclear calcium complex as effective
catalyst for alkenes hydrogenation†
Cite this:DOI: 10.1039/d0cc01745k
a
Xianghui Shi,
Jianhua Cheng
Cuiping Hou,ab Lanxiao Zhao,ac Peng Dengac and
*
ac
Received 6th March 2020,
Accepted 24th March 2020
DOI: 10.1039/d0cc01745k
rsc.li/chemcomm
Hydrogenolysis of the scorpionate-supported calcium benzyl complex including hydrogenation and hydrofunctionalization.12 Structurally
[(TpAd,iPr)Ca(p-CH2C6H4-Me)(THP)] (TpAd,iPr = hydrotris(3-adamantyl-5- characterized alkaline-earth metal terminal hydrides are limited to
isopropyl-pyrazolyl)borate, THP = tetrahydropyran) (2-THP) afforded magnesium, with a handful of examples.13 The more reactive
the mononuclear calcium hydrido complex [(TpAd,iPr)Ca(H)(THP)] (3). calcium terminal hydride has never been observed.
Under mild conditions (40 8C, 10 atm H2, 5 mol% cat.), complex 3
Recently, we described the use of the super-bulky scorpionate
effectively catalyzed the hydrogenation of a variety of alkenes, (TpAd,iPr) ligand to afford the dimeric barium hydrides complex
including activated alkenes, semi-activated alkenes, non-activated [(TpAd,iPr)Ba(m-H)]2.7 Under the same conditions, the expected
terminal and internal alkenes. Mononuclear calcium unsubstituted calcium analogue [(TpAd,iPr)Ca(m-H)]2 was not obtained, mainly
alkyl complex [(TpAd,iPr)Ca{(CH2)4Ph}(THP)] (6), proposed as the attributed to the smaller ionic radius of calcium in comparison
catalytic hydrogenation intermediate, was isolated and structurally to barium (Ca2+ 1.00 Å, Ba2+ 1.35 Å; CN = 6).14 Neutral donor
characterized.
solvents play a crucial role in the stabilization of heavy alkaline-
earth metal alkyl and hydride complexes, and might dissociate
Molecular alkaline-earth metal (Mg, Ca, Sr, and Ba) hydrides the aggregated metal hydrides into monomeric species.5c,13f,15
continue to receive growing interest,1 as they are used as low- The coordination of a donor molecule to the calcium center will
cost and nontoxic alternatives to transition-metal catalysts in lower the reactivity and provide the potential to stabilize the
a variety of organic transformations.2 As two of the most mononuclear species. Herein, we report the synthesis and
abundant elements in the Earth’s crust,3 Mg and Ca based efficient structure of mononuclear calcium hydride complex [(TpAd,iPr)-
catalysts are of particular attraction, due to their environmentally Ca(H)(THP)] (3). We also disclose that complex 3 exhibited
benign and bio-compatible nature showing potential applications in superior reactivity and a larger substrate scope than the pre-
both pharmacological and medicinal fields. Among the limited vious dimeric calcium hydrides in the catalytic hydrogenation of
number of well-defined molecular group 2 metal hydride com- alkenes. A series of mononuclear calcium alkyls including the n-alkyl
plexes, magnesium hydrides exhibit twelve different metal hydride derivative [(TpAd,iPr)Ca{(CH2)4Ph}(THP)] (6), regarded as the catalytic
cores with nuclearities up to thirteen.4 Most of the heavier reaction intermediates, are presented as well.
complexes such as calcium,5 strontium5g,6 and barium5g,7
Protonolysis of [Ca(p-CH2C6H4-Me)2(THF)4] (1)16 with the acid
hydrides are dimeric, and other tri-,8 tetra-,9 hexa-10 and hepta- form of the scorpionate ligand (TpAd,iPr)H7 in Et2O easily afforded
nuclear11 metal hydrides are sporadically reported. Terminal the corresponding benzyl complex [(TpAd,iPr)Ca(p-CH2C6H4-Me)-
hydrides of alkaline-earth metals, which are expected to be (THF)] (2) as a pale-yellow solid in excellent yield. Treatment of
extremely reactive, play a key role in various catalytic cycles,2 complex 2 in THP/hexane gave an almost quantitatively THP
coordinated calcium benzyl complex [(TpAd,iPr)Ca(p-CH2C6H4-
Me)(THP)] (2-THP). Complex 2-THP was more stable than
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
complex 2, in agreement with the previously reported calcium
Applied Chemistry, Chinese Academy of Sciences, No. 5625, Renmin Street,
alkyl complexes.15
Changchun 130022, China. E-mail: jhcheng@ciac.ac.cn
b University of Chinese Academy of Sciences, Changchun Branch,
Hydrogenolysis of 2-THP in hexane under 20 atm H2 successfully
delivered the corresponding hydride complex [(TpAd,iPr)Ca(H)(THP)]
(3) as colorless crystals in 81% isolated yield (Scheme 1). The
1H NMR spectrum of complex 3 shows the absence of the
p-CH2C6H4-Me group and the presence of a TpAd,iPr ligand and
a THP molecule. A singlet at 5.51 ppm in d12-cyclohexane, which
Changchun 130022, China
c University of Science and Technology of China, Hefei, Anhui 230029, China
† Electronic supplementary information (ESI) available: Selected 1H and 13C NMR
spectra and crystallographic details. CCDC 1967134 (2), 1967136 (4), 1967137 (5)
and 1967138 (6). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0cc01745k
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Scheme 1 Synthesis of mononuclear calcium hydride [(TpAd,iPr)Ca(H)(THP)] (3).
Fig. 1 ORTEP drawing of [(TpAd,iPr)Ca(PhCHCH2Ph)] (5) with thermal ellipsoids
drawn at the 20% probability level. All the hydrogen atoms in pyrazoles
and phenyl groups are omitted for clarity. Selected interatomic distances [Å]:
Ca1–C2 2.544(3), Ca1–C3 2.768(3), Ca1–C4 2.723(3), C1–C2 1.510(3),
Ca1–N1 2.860(2), Ca1–N2 2.454(2), Ca1–N4 2.400(2), and Ca1–N6 2.401(2).
integrates as the 1H per TpAd,iPr ligand, is assigned to the
hydrido ligand, and further confirmed by deuterated analogous
[(TpAd,iPr)Ca(D)(THP)] (3-D) in 2D NMR. The hydride chemical
shift in 3 is at slightly lower field than those in the dimeric
calcium hydride complexes [(DippBDI)Ca(m-H)(THF)n]2 (DippBDI =
[(DippNCMe)2CH]À, Dipp = 2,6-iPr2C6H3) (n = 1, 4.45 ppm;5a
n = 0, 4.27 ppm5e), [(Me4TACD)2Ca2(m-H)2]2+ (Me4TACD =
1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) (4.70 ppm)5b
and [(CpAr)Ca(m-H)(THF)]2 (CpAr = C5Ar5, Ar = 3,5-iPr2-C6H3)
(5.22 ppm).5g Complex 3 is stable in hexane and toluene at room
temperature overnight without any obvious change. Owing to the
co-crystallization with other calcium species and low-quality data
set, only the mononuclear nature of complex 3 was verified by
X-ray crystallographic analysis. A detailed discussion on bond
lengths and angles is not warranted (see the ESI†).
The reaction of complex 3 with one equivalent of 1,1-diphenyl-
ethylene (DPE) in benzene at room temperature for 2 h afforded
the corresponding 1,1-diphenylethyl derivative [(TpAd,iPr)Ca{CPh2-
(Me)}] (4) as dark-red crystals in 56% isolated yield. The mole-
cular structure of complex 4 (see Fig. S35, ESI†) reveals that the
Ca2+ center is coordinated to a distorted TpAd,iPr ligand with a
side-on bound Z2-pyrazolyl ring, benzylic carbon (C2), and to a
lesser extent of aromatic ring (C3 and C4). This bond pattern is
different from the CaÁ Á ÁPh p interaction observed previously in
[(DippBDI)Ca{(Z6-Ph)C(Ph)Me}(THF)].17
hexane and toluene at low temperature, but slowly decomposed
at room temperature. As shown in Fig. 2, the Ca2+ center is five-
coordinate in a trigonal-bipyramidal fashion, with distortion from
ideal geometry due to the constrains imposed by the TpAd,iPr
ligand. The overall structure of complex 6 could be viewed formally
by replacement of the terminal hydride in complex 3 with the
(CH2)4Ph group. The Ca–C1 distance of 2.515(2) Å, is comparable
to that of the terminal methyl moiety found in [(TptBu,Me)Ca-
(Me)(THP)] (2.485(2) and 2.507(2) Å),19 and to that of the intra-
molecular Ca–C bonds in dimeric calcium n-alkyl complexes
[(DippBDI)Ca(CH2CH2R)]2 (R = H, Et, t-Bu, n-Bu, n-Hex, CH2Ph,
(CH2)2Ph) (2.47–2.51 Å).5e,20 As far as we know, complex 6 is the
first example of the mononuclear heavier alkaline-earth metal
unsubstituted n-alkyl complex.
With these well-established stoichiometric reactions in mind
(Scheme 2), we then attempted the catalytic hydrogenation of a
range of alkenes under mild conditions (40 1C, 10 atm H2,
5 mol% cat.). Complex 2-THP was used as the pre-catalyst, and
the results of this study are summarized in Table 1.
Treatment of complex 3 with one equivalent of trans-stilbene
in benzene at room temperature for 2 h gave the benzyl product
[(TpAd,iPr)Ca(PhCHCH2Ph)] (5) as pale-yellow crystals in 67%
isolated yield. As shown in Fig. 1, the Ca2+ adopts a similar
coordination mode to that in complex 4. The Ca1–C2 distance
of 2.544(3) Å is slightly shorter than that of Ca–C (a-carbon of
the benzylic anion) (2.608(4) Å) found in [(DippBDI)2Ca2-
(m-H)(PhCHCH2Ph)].18 The interatomic distances of 2.768(3) Å
(Ca1–C3) and 2.723(3) Å (Ca1–C4) indicate the intense inter-
actions between the calcium center and the ipso and ortho
carbon atoms of the a-phenyl group. The C1–C2 distance of
1.510(3) Å demonstrates that the CQC bond of trans-stilbene
has been reduced to a C–C single bond.
The reaction of complex 3 with one equivalent of CH2QCHCH2-
CH2Ph (4-phenyl-1-butene) in hexane at room temperature for 0.5 h
easily delivered the corresponding n-alkyl complex [(TpAd,iPr)-
Ca{(CH2)4Ph}(THP)] (6) as colorless crystals in 81% isolated
yield. Complex 6 is stable in hydrocarbon solvents such as
Fig. 2 ORTEP drawing of [(TpAd,iPr)Ca{(CH2)4Ph}(THP)] (6) with thermal
ellipsoids drawn at the 20% probability level. All the hydrogen atoms in
pyrazoles, THP and phenyl group are omitted for clarity. Selected inter-
atomic distances [Å]: Ca1–C1 2.515(2), Ca1–O1 2.415(2), Ca1–N2 2.458(2),
Ca1–N4 2.468(2), Ca1–N6 2.472(2), and C1–C2 1.527(3).
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The hydrogenation of styrene and styrenic derivatives (DPE,
trans-stilbene and a-methyl styrene) was achieved within 1–4 h,
and no oligomeric by-products were observed (entries 1–4). The
slow conversion of 1-phenyl-cyclohexene is mainly attributed to
the steric hindrance (entry 5).
Mononuclear calcium hydride 3 also showed a remarkable
performance in hydrogenation of semi-activated alkene and the
non-activated alkenes. The hydrogenation of Me3SiCHQCH2,
1-hexene and 4-phenyl-1-butene was completed within one hour
(entries 6–8). These results demonstrated that mononuclear
calcium hydride exhibited superior reactivities than the previously
reported dimeric calcium hydrides, including [(DippBDI)Ca(m-H)-
(THF)n]2,5a,e [(Me4TACD)2Ca2(m-H)2]2+,5b and [(CpAr)Ca(m-H)-
(THF)]2.5g
The most striking performance of mononuclear calcium hydride
is the expanded substrate scope. The internal alkenes can be
hydrogenated as well. The conversion of 2-octene to octane
increased gradually (entry 9), with 31% in 24 h and up to 90%
over 72 h. Norbornene was reduced to norbornane in 97%
conversion within 7 h (entry 10). The reaction time for hydro-
genation of cis-cyclooctene was longer (95% in 24 h) (entry 11). In
a short time, only the exocyclic CQC bond in 4-vinylcyclohexene
was selectively hydrogenated to give 4-ethylcyclohexene (entry 12).
It is noteworthy that the hydrogenation of cyclohexene could also
be achieved, albeit the conversion was low (4%, 24 h; 22%, 120 h;
entry 13). No reactivity of disubstituted 2-ethyl-1-butene was
observed (entry 14).
Scheme 2 Reactions of [(TpAd,iPr)Ca(H)(THP)] (3) towards alkenes.
Table 1 Catalytic hydrogenation of alkenes with the pre-catalyst 2-THPa
Entry
1
Substrate
t [h]
Product
% conv.b
99
1
On the basis of the above-mentioned experimental observations,
a possible mechanism for terminal alkene hydrogenation is shown
in Scheme 3. At first, the hydrogenation of complex 2-THP would
afford mononuclear calcium hydride 3. The insertion of a terminal
n-alkene into the Ca–H bond in a 1,2-fashion would give a calcium
n-alkyl complex, such as 6. Subsequent s-bond metathesis with
H2 regenerated 3 and released the final product alkanes. The
dissociative THP enables the Ca2+ center to coordinate bulky
substrates. The maintenance of monomeric calcium species
throughout the transformations might play a key role in the
remarkable catalytic efficiency, in contrast to the dimeric structures
2
3
99
3
4
5
1
99
99
53
4
72
6
7
8
9
1
99
1
99
1
99
24/72
31/90
10
11
7
97
95
24
12
13
1
99c
24/120
24
4/22
0
14
a
Reaction conditions: 0.012 mmol of catalysts, 0.24 mmol of substrate
in 1 mL of C6D6. Determined by 1H NMR spectroscopy. Only the
b
c
Scheme 3 Proposed catalytic cycle for the hydrogenation of terminal
alkenes with mononuclear calcium hydride 3.
hydrogenation of the vinyl double bond was confirmed.
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in calcium hydrides complexes, such as [(DippBDI)Ca(m-H)]2,5e,18,20
[(Me4TACD)2Ca2(m-H)2]2+,5b and [(CpAr)Ca(m-H)(THF)]2.5g
In summary, we have demonstrated for the first time
the isolations of mononuclear calcium terminal hydride and
unsubstituted n-alkyl complexes. Mononuclear calcium hydride
shows a surprisingly outstanding performance in terms of both
catalytic efficiency and substrate scope. As previous literature
studies5g,21 indicated that the activity of heavy alkaline-earth
metal catalysts increases with the metal size (Ca o Sr o Ba),
mononuclear strontium and barium hydride complexes are
expected to exhibit higher catalytic activity and potentially have
more substrate scope.
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Science Foundation of China (no. 21672202, 21971232 and
21901238).
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