Diverse catalytic reactivity of a dearomatized PN3P?-nickel hydride pincer complex towards CO2 reduction

Article abstract of DOI:10.1039/c8cc05948a

A dearomatized PN³P?-nickel hydride complex has been prepared using an oxidative addition process. The first nickel-catalyzed hydrosilylation of CO₂ to methanol has been achieved, with unprecedented turnover numbers. Selective methylation and formylation of amines with CO₂ were demonstrated by such a PN³P?-nickel hydride complex, highlighting its versatile functions in CO₂ reduction.

Full text of DOI:10.1039/c8cc05948a

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DOI: 10.1039/C8CC05948A
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Diverse catalytic reactivity of a dearomatized PN3P*-nickel
hydride pincer complex towards CO2 reduction
Huaifeng Li,a Théo P. Gonçalves,a Qianyi Zhao,a Dirong Gong,a Zhiping Lai,b Zhixiang Wang,c
Received 00th January 20xx,
Accepted 00th January 20xx
Junrong Zheng,d and Kuo-Wei Huang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A dearomatized PN3P*-nickel hydride complex has been prepared N-heterocyclic carbenes with the highest turnover number
by oxidative addition process. The first nickel-catalyzed (TON) of 1840.7 These reports remain the only two
hydrosilylation of CO2 to methanol has been achieved, with homogeneously catalyzed systems to directly reduce CO2 to
unprecedented turnover numbers. Selectvie methylation and silylated methanol in one-step. More recently, the groups of
formylation of amines with CO2 were demonstrated by such PN3P*- Fontaine,8 Oestreich9 and Abu-Omar10 developed the
nickel hydride complex, highlighting its versatile functions in CO2 hydrosilylation of CO2 to silylated formate or silylated formal
reduction.
compounds, rather than silylated methanol. These products
could in turn be reduced to silylated methanol by addition of an
additional reductant, at elevated reaction temperatures or over
a longer reaction time. The selective one-step reduction to
silylated methanol for the hydrosilylation of CO2 to methanol
remains challenging.
CO2 is a renewable, nontoxic and abundant feedstock, and thus
it is an attractive C1 building block for the synthesis of organic
molecules.1 However, the transformation and activation of CO2
is highly challenging due to its inherent thermodynamic and
kinetic stability.1f Considerable efforts have been devoted to
developing efficient catalysts or catalytic systems to overcome
these challenges for the CO2 utilization to produce various
value-added chemicals.2 In particular, the reduction of CO2 to
methanol under mild conditions is a challenging goal.3 While
some homogeneous catalytic systems have been reported for
the hydrogenation4 and the hydroboration of CO2 to methanol,5
examples of hydrosilylation of CO2 to methanol are still rare.
Ir(CN)(CO)dppe (dppe = 1,2-bis(diphenylphosphino)ethane)
was the first reported catalyst for the hydrosilylation of CO2 to
silylated methanol from which methanol could be generated
upon hydrolysis in 1989 by Eisenberg and Eisenschmid,6 but the
catalytic efficiency was very low as the transformation required
weeks to complete. In 2009, Zhang and Ying group reported the
reduction of CO2 to silylated methanol with silanes catalyzed by
On the other hand, efficient hydrosilylation of CO2 to valuable
formamides, methylamines or aminals in the presence of amines has
emerged as a promising methodology recently.11 These novel
catalytic strategies for CO2 utilization feature the formation of the
new carbon-nitrogen bond. Cantat and co-workers first described
the catalytic reductive formylation of amines using CO2 and silanes in
2012.12 The catalytic reduction of CO2 with hydrosilanes and amines
to methylamines was unveiled independently by Cantat and co-
workers and Beller and co-workers in 2013.13 After these initial
reports, rapid progress in this area has led to the discovery of several
amine formylation or methylation catalysts for the hydrosilylation of
CO2.11 However, among these systems, only a few cases have been
reported where not only methylamines but also formamides could
be selectively produced using the same catalyst.14, 15 These catalysts
suffer from either the low reactivity or the poor selectivity, especially
for the methylation reaction. Interestingly, both of formylation and
methylation reactions could be achieved by using and inorganic salt
(e.g. Cs2CO3)14b or an organic inner salt (Betaine)15 as catalysts, but
they only showed moderate reactivity towards the methylation
reaction of primary amines along with the formation of mixtures of
the monomethylated and dimethylated products. Clearly, the
development of a general and highly active catalytic system for the
selective formation of methylamines and formamides derived from
CO2 and amines is still highly desirable.
a.Division of Physical Sciences and Engineering, KAUST Catalysis Center, King
Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
E-mail: hkw@kaust.edu.sa
b.Division of Physical Sciences and Engineering, Advanced Membranes & Porous
Materials Center, King Abdullah University of Science and Technology, Thuwal
23955-6900, Saudi Arabia.
c. College of Chemistry and Chemical Engineering, Graduate University of the
Chinese Academy of Sciences, Beijing 100049, China.
d.College of Chemistry and Molecular Engineering, Peking University, Beijing
100871, China.
As part of our ongoing interest in the PN3-pincer complexes for
their unique kinetic and thermodynamic properties,16 we report
† Electronic Supplementary Information (ESI) available: Experimental details, NMR
spectra of products. CCDC 1505721, 1530059 and 1571043. For crystallographic
data in CIF or other electronic format see DOI:
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DOI: 10.1039/C8CC05948A
Fig. 1 ORTEP diagram of PN3P*-Ni hydride complexes Ni1, Ni2 and Ni3 at 30% ellipsoid probability. Hydrogen atoms (except for pincer arms and Ni-H) are omitted
for clarity.
and CH3CN, albeit with slower reaction rates. No methanol was
formed when the solventwas changed to the nonpolar solvents,
Table 1 Hydrosilylation of CO2 with diphenylsilane catalyzed by PN3P*-Ni hydride
complexes.a
Scheme 1 Synthesis of the dearomatized Ni-H pincer complexes via oxidative
addition.
herein the preparation of well-defined dearomatized PN3P*-nickel
hydride complexes and their applications on hydrosilylation of CO2.
To the best of our knowledge, this catalytic system represents the
first nickel-catalyzed hydrosilylation of CO2 to methanol, with the
highest TON reported to date. To further extend the diverse range of
products accessible from CO2 and silanes, this dearomatized PN3P*-
nickel hydride catalytic system is employed to the reductive
functionalization of CO2 with amines as well to offer a methodology
for the selective formation of formamides and methylamines.
The dearomatized PN3P*-nickel hydride complexes Ni1, Ni2, and
Ni3 were synthesized by oxidative addition of the corresponding
pincer ligands (L1-L3) to bis(1,5-cyclooctadiene)nickel(0) in toluene
in one step (Scheme 1).17 In 1H NMR spectra, the characteristic Ni-H
resonance appears at -15.87, -15.62, and -16.44 ppm for complexes
Ni1, Ni2, and Ni3, respectively. The 31P NMR spectra show two
doublets, corresponding to the two magnetically different
phosphorus atoms of the dearomatized pincer ligands. All Ni1, Ni2
and Ni3 were characterized by single crystal X-ray diffraction,
exhibiting a similar coplanar geometry with shorter C-N bond
distance for the imine arm of the dearomatized pyridine ring (Fig. 1).
The catalytic studies of the PN3P*-Ni hydride complexes
commenced with the reduction of 1 atm of CO2 in DMF using
Ph2SiH2 as a reductant under various conditions (Table 1). The
hydrosilane was fully consumed in 12 hours as monitored by
such as CH2Cl2 and toluene (Table S1). Other hydrosilanes were
also investigated when using the same equivalent of Si−H
groups. Reactions with PhSiH3 resulted in a good yield of
methanol, yet a prolonged reaction time was needed. The
bulkier trisubstituted hydrosilanes did not give methanol as the
final product (Table S2). The influence of the catalyst loadings
was then examined. Catalyst Ni1 showed high activity even with a
catalyst loading as low as 0.02% with the TON up to 4900 (Table 1,
entry 10). To the best of our knowledge, this is the highest TON
reported for the reduction of CO2 with silane to methanol.
We next further explored the reductive functionalization of
CO2 with amines. The reaction of dibenzylamine 1a was chosen
as a model substrate when employing Ph2SiH2 as the reducing
agent and CO2 as a C1 source (Table S4). To our delight, the
notable reactivity afforded methylamine 2a in 85% and
formamide 3a in 11% yield, observed when using 3 mol% Ni1, 5
equiv. of Ph2SiH2 and 2.7 atm of CO2 at 100 °C for 24 h (Table S3,
entry 1). Only small amounts of 3a was detected in the absence
of thecatalyst (Table S3, entry 2). We found that the higher
reaction temperature was favorable for the formation of
methylation product (2a). Finally, the highest activity towards
the formation of 2a was provided at 120 °C under 2.7 atm of
CO2 (Table S3, entry 3). Interestingly, with the increasing of the
CO2 pressure or decreasing of the reaction temperatures, the
improved yields of 3a were achieved (Table S3, entries 4 and 5).
GC-MS when using complex Ni1 as the catalyst (Table 1, entry 1)
.
Gratifyingly, after the CO2-reduction product was subjected to
hydrolysis, methanol was obtained in 91% yield. Either in the
absence of catalyst or using L1 alone, only trace amounts of
methanol was observed, although Ph2SiH2 was completely
consumed (Table 1, entries 2 and 3).8 Relatively lower yields of
methanol were observed when complexes Ni2 and Ni3 were
tested (Table 1, entries 4 and 5; Table S3, entries 2 and 3).
Further optimization revealed that DMF was a better solvent.
The reaction could also work well in polar solvents such as THF,
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Meanwhile, the decreased amount of the reducing agent and afforded the corresponding methylamines in good yields. Meanwhile,
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solvent has a positive influence on the selectivity for the
Table 2 Methylation of various amines using CO2 and Ph2SiH2 catalyzed by Ni1.a
secondary aromatic amines with both electrDoOn-I:d1o0n.1a0t3in9/gC8CC05948A
Table 3 Formylation of various amines using CO2 and Ph2SiH2 catalyzed by Ni1.a
and electron-withdrawing substituents at the para position (1g-1k)
reacted smoothly and the corresponding products were obtained in
good yields. Notably, the more challenging substrate diaryl
secondary amine 1l was found to be reactive as well.14b Several cyclic
aliphatic secondary amines (1m-1o) were employed and excellent
yields of the cyclic methylamines were achieved. In addition, not only
aliphatic primary amines (1p-1u) but also aromatic primary amines
(1v-1ae) proceeded selectively to provide the dimethylated products.
Remarkably, even the methylation of sterically hindered substrates
(1q, and 1ac-1ae) proceeded smoothly to give the corresponding
dimethylated products. It is noteworthy that the methylation
reaction could be performed in the presence of halides, such as F, Cl,
or Br, and reductive dehalogenation was not observed. furthermore,
ester-substituted amine 1af was well tolerated under our reducing
conditions, providing the corresponding dimethylated amine 2af in
good yield.
formylation reaction (Table S3, entry 6). By increasing the CO2
pressure to 8.2 atm, the formylation reaction could be
performed at room temperature with 96% yield of 3a (Table S3,
entry 7). On the basis of the reaction conditions of entry 3 and
7, the use of the other two PN3P*-Ni hydride complexes Ni2 and
Ni3 showed lower selectivities or yields in comparison with Ni1
(Table S3, entries 8-11).
With the optimized reaction conditions in hand, we began to
examine the substrate scope for each transformation (Table 2 and 3).
As shown in Table 2, sterically less sterically hindered aliphatic
secondary amines (1a, 1b, and 1d) showed high reactivity to afford
the corresponding methylamines. Moreover, the more hindered
aliphatic secondary amines, such as N,N-dicyclohexylamine 1c, N-
tert-butylmethylamine 1e, and 2,2,6,6,-tetramethylpiperidine 1f
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Chem. Soc.,2009, 131, 14168-14169; (c) C. Federsel, A. Boddien, R. Jackstell,
Utilizing optimal formylation reaction conditions at room
temperature, various aliphatic and aromatic, secondary and primary
amines were successfully converted into the desired formamides
with good to excellent yields (Table 3). Both of the acyclic (1a, 1d, 1e,
1g-1j, and 1ak) and cyclic (1m-1o, and 1ag-1aj) secondary amines
could be transformed to the corresponding formamides in excellent
yields. Importantly, the hydrosilylation of CO2 to formamides showed
the potential chemoselectivity. The carbonyl and ester groups could
be tolerated under these conditions, as exemplified by the substrate
1ag and 1af. Additionally, primary amines (1p-1z, 1ab-1ad, and 1al-
1an) proceeded in a similar fashion to secondary amines, generating
monoformylated products. The formylation protocol is also
compatible with a variety of halides (1i, 1j, 1r, and 1z). The sterically
hindered amines (1e, 1q, 1ac, and 1ad) were also suitable for this
transformation, giving the desired formamides in good to excellent
yields.
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R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy; M. Beller, Angew. Chem.
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3
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Mechanistic proposals on metal hydride complex catalyzed
hydrosilylation of CO2 usually involve the insertion of CO2 into a
metal-H bond and generate CH4 as the final product.18 However,
this pathway was ruled out since no reaction was observed
6 T. C. Eisenschmid; R. Eisenberg, Organometallics,1989, 8, 1822-1824.
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between complex Ni
suggests that CO2 insertion into the Ni-H bond of Ni
1
with CO2 at 125 °C. This observation
might not
1
8
M.-A. Courtemanche, M.-A. Legare, E. Rochette; F.-G. Fontaine, Chem.
a catalysis related event. Catalyst Ni2 bearing a NMe arm
showed some similar reactivity (Table 1, entry 4; Table S4,
entries 8 and 10;), implying that the N-H gorup of Ni1 was not
necessary for the activation of CO2. Although we have not
identified the active catalyst in the aboved CO2 reduction
reactions, we speculate that an alternative pathway for CO2
activation may involve the nucleophlic attack of CO2 by the
iminic nitrogen of the ligand. The nucleophilicity of the imine
-donating hydride
ligand is introduced. Further mechanistic studies are ongoing.
In summary, we have successfully synthesized and fully
characterized several readily accessible dearomatized PN3P*-
nickel hydride pincer complexes via an oxidative addition
process. The first example of nickel catalyzed hydrosilylation of
CO2 to methanol has been achieved, with an unprecedentedly
high turnover number of 4900. Moreover, these PN3P*-nickel
hydride pincer complexes are capable of selectively catalyzing
reductive methylation and formylation of amines with CO2 with
a very broad substrate scope.
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Conflicts of interest
There are no conflicts to declare.
ACS Catal.,2017, 7, 4446-4450; (f) T. P. Gonçalves; K.-W. Huang, J. Am. Chem.
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Rev.,2004, 248, 2425-2442; (b) R. Tanaka, M. Yamashita; K. Nozaki, J. Am.
4 | J. Name., 2012, 00, 1-3
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