Carbon Dioxide Reduction to Methanol Catalyzed by Mn(I) PNP Pincer Complexes under Mild Reaction Conditions

Article abstract of DOI:10.1021/acscatal.8b04106

Well-defined Mn(I) hydrido carbonyl PNP pincer-type complexes were tested as efficient and selective nonprecious transition metal catalysts for the reduction of CO₂ to MeOH in the presence of hydrosilanes. The choice of reaction temperature and

Full text of DOI:10.1021/acscatal.8b04106

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Article
Carbon Dioxide Reduction to Methanol Catalyzed by Mn(I)
PNP Pincer Complexes under Mild Reaction Conditions
Federica Bertini, Mathias Glatz, Berthold Stöger, Maurizio
Peruzzini, Luis Filipe Veiros, Karl Kirchner, and Luca Gonsalvi
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04106 • Publication Date (Web): 10 Dec 2018
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a
b
c
a,d
e
Federica Bertini, Mathias Glatz, Berthold Stöger, Maurizio Peruzzini, Luis F. Veiros, Karl Kirch-
,b
,a
ner* and Luca Gonsalvi*
a
Consiglio Nazionale delle Ricerche (CNR), Istituto di Chimica dei Composti Organometallici (ICCOM), Via Madonna del
Piano 10, 50019 Sesto Fiorentino (Firenze), Italy.
b
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060 Wien, Aus-
tria.
c
Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060
Wien, Austria.
d
Consiglio Nazionale delle Ricerche, Dipartimento di Scienze Chimiche e Tecnologia dei Materiali (CNR-DSCTM), Via dei
Taurini 19, 00185 Rome, Italy.
e
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lis-
boa, Portugal.
ABSTRACT: Well-defined Mn(I) hydrido carbonyl PNP pincer-type complexes were tested as efficient and selective non-precious
transition metal catalysts for the reduction of CO
2
to MeOH in the presence of hydrosilanes. The choice of reaction temperature and
type of silanes proved to be pivotal to achieve fast reactions and high selectivity to the methoxysilyl- vs. silylformate derivatives in
DMSO. The catalytic data are complemented by DFT calculations, highlighting a stepwise CO
the Mn catalyst without metal-to-ligand cooperation (MLC).
2
reduction mechanism centred on
KEYWORDS . CO
culations.
2
hydrosilylation • manganese pincer complexes • homogeneous catalysis • mechanistic studies • DFT cal-
could be obtained in up to 44% yield by reduction of CO
2
with
H
2
(75 bar total pressure) in the presence of the iron scorpi-
3
onate catalyst [FeCl
solvent- and amine-free mild reaction protocol. Another pro-
tocol, involving at first N-formylation of an amine with CO
and H and subsequent formamide reduction to MeOH, was
described by Surya Prakash and coworkers. By use of a well-
defined Mn(I) PNP pincer complex as catalyst, yields of 84%
under 60 bar total pressure were obtained at 110-120 °C. Natu-
2
{κ -HC(pz)
3
}] (pz = pyrazol-1-yl) with a
Carbon dioxide is a known greenhouse gas, and its increasing
amount in the atmosphere is causing concern. On the other
hand, for small scale applications, it can be an attractive C1
carbon source for chemical synthesis as it is renewable, abun-
dant, non-flammable and inexpensive. The combination of
these two aspects has brought about a renewed interest of
chemists and significant efforts have been devoted towards the
4
b
2
2
5
a
rally, catalysts based on noble metals for CO hydrogenation
2
development of new strategies for CO activation and trans-
2
2,3
1
to MeOH are also known. A recent addition to the literature
was made by Everett and Wass using homogeneous ruthenium
formation into fuels and value-added chemicals. To date, a
number of transition metal-based and organocatalysts have
been developed, which allow the chemical reduction of carbon
dioxide to formic acid, methanol or methane. In particular, the
homogeneous hydrogenation of carbon dioxide to formic acid
5
b
catalysts in the presence of amine auxiliaries. A TON (turno-
ver number) of 8900 and TOF (turnover frequency) of 4500
−
1
h
were achieved using [RuCl
alyst with a diisopropylamine auxiliary.
Apart from H gas, that has the drawbacks of poor solubility in
most solvents, the need of pressure and related safety con-
cerns, other reagents were used to reduce CO , and silanes
have found a renewed attention for this reaction. CO hydrosi-
2
(Ph
2
PCH
2
CH
2
NHMe) ] as cat-
2
has been extensively studied due to its importance as chemical
2
hydrogen storage method. However, CO
2
hydrogenation to
2
methanol, a desired bulk chemical with many different uses in
industry and academia, proved to be more challenging and has
so far been achieved only with a handful of homogeneous sys-
2
2
3
tems. Another important issue is process economic sustaina-
lylation is a thermodynamically favored process due to the
formation of strong Si-O bonds, acting as driving force for the
reaction. As reported in the literature, this reaction is generally
bility, suggesting the use of non-noble metals in catalysis. The
first homogeneous non-noble metal catalyst for the hydrogena-
6
tion of CO
2
to MeOH was described by Beller and coworkers
], triphos (tri-
1,1,1-tris(diphenylphosphinomethyl)ethane) and
.4 The catalytic system works at 100 °C under a total
used to obtain silyl formates, versatile building blocks for or-
using a catalyst formed in situ from [Co(acac)
3
ganic syntheses and polymers. For silyl formate synthesis, one
of the most efficient catalysts to date is the Cu complex
[CuH(L)] [L = 1,2-bis(diisopropylphosphino)benzene] report-
ed by Baba and coworkers. The catalyst, formed in situ by ad-
dition of the ligand to Cu salts under reducing conditions,
promoted the formation of silylformates with TON of ca.
phos
=
a
HNTf
2
pressure of 90 bar (pH2 = 70 bar, pCO2 = 20 bar), with a maxi-
mum TON = 50 after 24 h (TON = turnover number). A more
recent study by Martins and coworkers showed that MeOH
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in the presence of
Page 2 of 9
7
0000 after only 24 h under 1 atm CO
PMHS (PMHS = polymethylhydrosiloxane). CO
reduced, albeit under harsh conditions, using (EtO)
presence of the Zn catalyst [(IMes) Zn (THF)](BPh
ing silylformate as the main product. Room temperature, Zn-
catalysed CO reduction to silylformate was reported in 2018,
using terminal the zinc hydride complex [Tntm]ZnH (Tntm =
2
pincer complex [Re(PNN)(O)
2
][OTf] was capable of CO re-
2
7
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
was also
SiH in the
, giv-
duction to methoxysilyl products using a combination of
2
1
3
PhMe
hexyl)
this reaction.
2
SiH and PhSiH
3
.
The perruthenate salt [N(n-
3
3
H
4
4
)
2
4
][ReO ] was also shown to be an effective catalyst for
4
8
22
2
9
tris(6‐tert‐butyl‐3‐thiopyridazinyl)methanide). CO
2
reduction
to CH with silanes has also been achieved, in most cases in
4
the presence of a strong Lewis acid such as B(C
6
F
5
)
3
as co-
] (M = Zn,
tris[(1-isopropylbenzimidazol-2-
10
PriBenz
catalyst. Complexes {[Tism
]M}[HB(C
6
F
)
5 3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
PriBenz
Mg;
Tism
=
yl)dimethylsilyl] methyl ligand) gave efficient catalytic CO
hydrosilylation in combination with B(C and R SiH to
afford sequentially the bis(silyl)acetal H C(OSiR and CH
SiH = PhSiH , Et SiH, and Ph SiH). The selectivity to the
different possible products could be switched by the choice of
silane, with PhSiH favoring CH , and Ph SiH favoring the
bis(silyl)acetal H C(OSiPh Zr /Hf alkyl/amido com-
plexes containing a tridentate N-ligand based on benzoimidaz-
ole were also used as catalysts precursors for the tandem CO
hydrosilylation to CH in combination with B(C as cocat-
alyst/activator and various hydrosilanes. Iwasawa and
coworkers reported that Pd complexes bearing a Group 13
metalloligand catalyze CO hydrosilylation to formates with a
turnover frequency (TOF) of 19300 h , the highest reported to
2
6
F
5
)
3
3
2
3
)
2
4
(
R
3
3
3
3
3
4
3
1
1
IV
IV
2
3
2
) .
2
4
6
F
)
5 3
Chart 1. Selected examples of transition metal catalysts used
for CO hydrosilylation to MeOH.
12
2
2
We have recently reported that homogeneous CO
tion to formate could be achieved with high TONs under rela-
tively mild reaction conditions (H /CO = 1:1, 80 bar, 80 °C)
hydrogena-
2
-
1
1
3
date for this reaction.
2
2
The selective reduction of CO
2
to methoxysilyl derivatives,
in the presence of Mn(I) hydrido carbonyl complexes bearing
PNP pincer ligands based on the 2,6-diaminopyridine scaffold
giving MeOH by simple hydrolysis, appeared to be more chal-
lenging and, to date, few catalytic systems are capable of pro-
moting such a reaction. The first example was reported by Ei-
senberg et al., using [Ir(CN)(CO)(dppe)] at 40 °C, albeit with
2
3
(complexes 1 and 2 in Chart 1). These complexes were also
used as efficient catalysts for other reactions, including cata-
lytic coupling of alcohols and amines to imines, and for the
2
4
sluggish reactions and low TONs [dppe
=
1,2-
synthesis of substituted quinolines and pirimidines. We were
interested in expanding the scope of these catalysts to more
1
4
bis(diphenylphosphino)ethane]. Ying and coworkers used a
stable NHC organocatalyst at room temperature, reaching
challenging reactions involving the use of CO
block, thus we thought of interest to test them in CO
2
as C1 building
hydrosi-
-1
TOFs as high as 25.5 h (based on Si-H), even by using air as
2
15
CO
2
-containing feedstock. More recently, pyridine-decorated
lylation to MeOH. The results of the catalytic tests, together
with mechanistic studies by DFT calculations, are hereby pre-
sented.
MWCNTs (MWCNT = multi-walled carbon nanotubes) were
efficiently used as heterogeneous organocatalysts for this reac-
1
6
tion, with hydroboranes as reducing agents. Guan reported
the use of a PCP pincer-type NiH(L) complex [L = 2,6-
(
tBuPO)
organoborane as a reducing agent, obtaining a TOF = 495 h
based on B–H units, and studying the reaction mechanism by
2
C
6
H
3
] as efficient catalyst for this reaction, using an
Catalytic Studies.
-1
Catalytic CO
2
hydrosilylation was at first tested at room tem-
perature and atmospheric pressure (1 bar) in the presence of
1
7
DFT methods. In 2018, Huang and coworkers showed very
NH
NMe
[Mn(PNP -iPr)(CO)
2) with PhSiH , using a known excess of silane respect to
CO (5 equiv. of Si-H bonds). It was previously demonstrated
2
H] (1) and [Mn(PNP -iPr)(CO) H]
2
high activity of a dearomatized (pyridyldiamino)diphosphine
(
3
3
PN P–nickel hydride complex for the reduction of CO
2
to
2
MeOH (TONmax = 4900) and selective methylation and
that the choice of solvent can be crucial for this kind of reac-
tions. Li and coworkers recently reported that catalyst-free N-
18
formylation of amines using Ph
2
SiH
2
. Among other pincer-
type complexes, Chirik and coworkers showed that
formylation of amines using CO and silanes may be achieved
2
tBu
tBu
[CoH( PNP)]
butylphosphinomethyl)pyridine] promoted the catalytic hy-
drosilylation of CO with PhSiH to a mixture of silyl for-
[
PNP
=
2,6-bis(di-tert-
under ambient conditions using polar solvents such as DMSO.
It was postulated that the high polarity of this solvent was suf-
2
3
ficient to allow the activation of chemical bonds in CO
2
and
silanes. In our previous work, we demonstrated that the first
step of CO activation in the presence of 1 consisted in its in-
sertion in the Mn-H bond to give the formate complex cis-
1
9
mates, bis(silyl)acetals, and methoxysilyl derivatives. Oes-
25
23
treich reported that tethered Ru−S complexes of general for-
2
mula [Ru(PR
3
)L] (Chart 1) are able to catalyze CO hydrosi-
2
lylation in the presence of monohydrosilanes, with high selec-
tivity either to bis(silyl)acetals or methoxysilyl products, re-
spectively. It was demonstrated that the chemoselectivity of
the reaction depends mainly on the reaction temperature.
However, temperatures as high as 150 °C, long reaction times
NH
[
Mn(PNP -iPr)(CO) {OC(O)H}] (3). Complex 3 was found
2
to be poorly soluble in THF, but significantly more soluble in
DMSO. Accordingly, we chose DMSO as solvent for our ex-
periments. The tests were carried out on NMR tube scale,
monitoring the vatiation of products distribution in time by
2
0
(days) and high catalyst loadings (1% mol) were required.
Abu-Omar and coworkers reported that the oxorhenium PNN
13
12
1
1
13
C{ H} and H NMR spectroscopy. CO
2
was used instead of
CO , in line with other related studies described in the litera-
2
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ture. The expected product distribution is shown in Scheme 1,
however formation of CH was never observed in our tests.
showed two additional signals at 12.74 (s) and 8.43 ppm (brd,
JHC = 217.72 Hz) which were assigned to free formic acid. The
1
2
3
4
5
6
7
8
9
4
1
3
1
corresponding C{ H} NMR signal appeared as a sharp sin-
glet at 162 ppm. The origin of the proton source needed to re-
lease HCOOH from silyl formate is unclear. A plausible hy-
pothesis could be that, under these reaction conditions, partial
silylformate decomposition by dehydration may occur, giving
1
9
H O and CO, as observed by other authors. Indeed, the pres-
2
1
3
ence of traces of CO in solution was detected in some
Scheme 1. Product distribution expected for CO hydrosilyla-
2
13
1
C{ H} NMR spectra at 184.6 ppm (see Supporting Infor-
mation). An additional doublet of low intensity centered at ca.
tion.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
Initially, the catalytic tests were carried out at room tempera-
ture. The main results are summarized in Table 1. In the pres-
9.5 ppm ( JHC = 178.61 Hz) was also observed in the H NMR
spectrum, together with a C{ H} NMR singlet at 197 ppm.
We attribute these signals to traces of formaldehyde, by com-
1
3
1
1
3
ence of 1 and PhSiH
into silylformates (95% based on C{ H} NMR) within the
first 5 min after the addition of 13CO
to the tube containing
the catalytic mixture (entry 1). The formyl units appear as a
3
,
CO was rapidly converted essentially
2
1
3
1
parison with literature data. MnBr
2
and Mn(CO) Br were also
5
2
used as catalysts for room temperature tests under the condi-
tions described in Table 1, to verify the effect of the PNP lig-
and. They both proved to be less efficient than 1 and 2, with
1
3
broad signal in the C NMR spectrum in the range 159-165
ppm, with the corresponding protons showing as a doublet
43% CO
2
conversion to silylformate only and 74% CO con-
2
1
centered at δ
H
= 8.28 ppm in the H NMR spectrum, with a
version to silylformate (64%) and silylacetals (10%) after 3 h,
respectively (see Supporting Information).
1
13
1
large coupling constant due to H- C coupling ( JHC = 224.38
1
Hz). A new H NMR resonance observed at 5.0 ppm was as-
Next, the effect of temperature and type of silane were tested.
26
signed to the silylformate PhSiH
2
(O
2
CH), and it gradually
A small library of silanes was screened for the hydrosilylation
decreased in intensity as the reaction proceeded. Prolonged
reaction times resulted in the formation of methoxysilyl spe-
13
of CO
2
(1 bar) in d -DMSO at 80 °C in the presence of 1,
6
monitoring the reactions by NMR spectroscopy. Selected data
are summarized in Table 2.
cies (δ
H
= 3.0-3.5 ppm; δ
C
= ca. 50 ppm) at the expenses of
= ca. 84
silylformates via bis(silylacetal) intermediates (δ
C
Table 2. Selected results for silane screening in CO
lylation with 1 at 80 °C.
2
hydrosi-
1
3
1
ppm), reaching 84% yield (based on C{ H} NMR) after 24 h
and 93% yield after 46 h. A comparable reactivity was ob-
served with complex 2 (entry 2). In a control experiment in the
a
En
try
Silane
t
%
%
%
Si(OCH
%
absence of catalyst (entry 3), CO
selectively to silylformates, but longer reaction times were
needed to reach nearly quantitative CO conversion. After 24
2
hydrosilylation proceeded
[h] CO
2
Si(O
2
CH)
2
O)Si
Si(OMe)
1
PhSiH
3
1
3
6
1
3
0
0
0
6
3
3
55
8
-
-
4
2
0
8
6
7
5
1
7
5
4
0
0
0
0
0
0
89
93
99
79
84
88
0
0
0
0
0
2
h, only minor amounts of bis(silylacetal) units (2%) and meth-
oxysilyl species (<1%) were detected.
2
Ph
2
SiH
2
Table 1. Selected results for catalyst screening in CO
2
hydros-
a
ilylation with PhSiH
3
at 25 °C.
24
3
3b
4c
5
Ph
Et
PhMe
SiH
SiH
SiH
1
24
1
44
90
-
-
31
72
3
En-
try
Cata-
lyst
t [h]
%
CO
%
%
Si(OCH
%
2
Si(O
2
CH)
2
O)Si
Si(OMe)
3
24
1
2
1
2
<0.1
1
3
3
1
0
0
0
0
0
0
0
95
79
55
6
0
6
12
10
4
2
2
1
69
28
14
33
84
93
14
10
24
81
93
0
24
0
a
Reaction conditions: 13CO
0.014 mmol, catalyst:CO = 1:10); silane (0.70 mmol, 5.0 equiv.
Si-H bonds to CO , 2.0 mol% catalyst to Si-H bonds); d -DMSO
(400 l); mesitylene (10 l) as internal standard; 80 °C. Product
2
(1 bar, 0.14 mmol); catalyst 1
2
4
4
6
(
2
3
2
6
<0.1
1
3
78
76
52
7
8
14
24
12
6
0
0
1
3
1
b
distribution calculated from C{ H} NMR values (error ± 5%).
An insoluble precipitate formed during the reaction, as previously
2
2
4
8
c
reported (ref. 10c). Only traces of silylformate.
0
30
9
1
3
none
1
3
24
70
91
97
Monohydrosilanes such as Ph
active under the reaction conditions described above, giving
only the silylformate product after 24 h. With Et SiH, a bipha-
3
SiH and PhMe
2
SiH were less
0
<1
1
2
3
a
Reaction conditions: 13CO
sic solution formed in the NMR tube, hampering monitoring
of the reaction. NMR analysis of the crude mixture after 24 h
and workup showed only silylformate as product in trace
2
(1 bar, 0.14 mmol); catalyst, if any
= 1:10); PhSiH (0.70 mmol, 5.0
equiv. Si-H bonds respect to CO , 2.0 mol% catalyst to Si-H
bonds); d -DMSO (400 l); mesitylene (10 l) as internal stand-
(0.014 mmol, catalyst:CO
2
3
2
quantities. On the other hand, PhSiH
ly afforded the desired methoxysilyl product in high yields
(80-89%) after only 1 h at 80 °C. With PhSiH as reducing
agent, full CO reduction to methoxysilyl derivatives (>99%)
3
and Ph
2
SiH
2
successful-
6
1
3
1
ard; 25 °C. Product distribution calculated from C{ H} NMR
values (error ± 5%).
3
2
In the latter experiment, i.e. in the absence of a catalyst, soon
was achieved after 6 h at 80 °C (Table 2, entry 1). At the end
of the reaction, unreacted silane was still clearly visible in the
1
after the addition of CO
2
to the tube, the H NMR spectrum
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1
1
H NMR spectrum (Si-H signal at 4.11 ppm), whereas all
agreement with data recently reported by Sortais and cowork-
ers.
3
29
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CO
2
was consumed. The final silane conversion was 62%,
. Considering that essen-
originally placed in the NMR tube (ca. 0.14
corresponding to 0.14 mmol PhSiH
tially all the CO
mmol) was reduced to the desired product, the experimentally
observed conversion of the silane matches perfectly the ex-
pected value. In contrast, using Ph
ance of the Si-H signals was observed in the H NMR spec-
trum after 3h, although not all CO was consumed judging
from the corresponding C{ H} NMR data (Table 2, entry 2).
This result suggests that unproductive side reactions involving
3
Mechanistic Studies.
2
In order to get an insight in the mechanism of CO
lylation catalyzed by 1, NMR experiments and DFT calcula-
tions were carried out. Insertion of CO into a metal-hydrogen
bond constitutes the initial step in many inner-sphere reaction
of CO using transition metal hydrido complexes. Indeed, in
our previous work on CO
demonstrated that the reaction of 1 with CO
at room temperature to give complex cis-[Mn(PNP
iPr)(CO) {OC(O)H}] (3) (Scheme 2a). Crystals of 3 suitable
for X-ray diffraction analysis were now grown by diffusion of
Et O into a THF/dmso (9/1) solution of 3. A structural view of
2
hydrosi-
2
2
SiH
2
, complete disappear-
1
2
2
2
3
13
1
2
hydrogenation to formate, we
proceeds rapidly
2
NH
-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ph
2
SiH
2
may take place, decreasing the overall process effi-
had to be
2
ciency. Thus, we concluded that the stronger PhSiH
preferred, and it was chosen as reducing agent in the following
experiments, in agreement with Garcia and coworkers for the
3
2
1
0b
3 is shown in Figure 1, with selected bond distances and an-
gles given in the caption.
Ru-catalyzed CO
and coworkers for the Fe-catalyzed CO
building block to synthesize formamides and methylamines
from amines.
Encouraged by these results, we then tested the reduction of
CO with 1 using an excess of CO respect to silane. As
silanes are more expensive reagents than CO , and excess
silane or silicon waste by-products are often difficult to re-
move by workup (whereas CO can be simply vented), a pro-
cess with total and selective conversion of silanes could be of
interest for practical applications. The tests were run in
Schlenk tubes using CO
and 80 °C, to check the effect of temperature on the reaction
selectivity and product distribution. For the tests at 25 °C, H
2
hydrosilylation to CH
4
,. and with Cantat
2
utilization as C1
2
7
2
2
2
2
1
2
2
and PhSiH at room temperature
3
Figure 1. Structural view of 3 showing 50% thermal ellipsoids
1
(most H atoms omitted for clarity). Selected bond lengths (Å) and
NMR analysis of the reaction mixture after 24 h revealed full
consumption of PhSiH
s, HCOOH; 12.76, s, HCOOH) as the sole CO
bond angles (°): Mn1-C18 1.779(3), Mn1-C19 1.799(3), Mn1-O3
2.062(2), Mn1-N1 2.082(3), Mn1-P1 2.294(1), Mn1-P2 2.292(1),
P2-Mn1-P1 164.06(4).
3
and formation of formic acid (δ: 8.17,
-reduction
2
1
3
1
product. C{ H} NMR showed, accordingly, a broad signal
centered at 161.23 ppm. The signal broadness may suggest
that the mixture contained both silylformates and free
To establish whether 3 was involved as intermediate in the
Mn-catalyzed CO
2
hydrosilylation mechanism, a Wilmad
HCOOH. Interestingly, the 13C{ H} NMR spectrum showed
1
quick pressure valve NMR tube was charged with 1 (5.0 mg,
3
1
1
0.01 mmol) and d
6
-DMSO (400 L). The P{ H} NMR spec-
also a well-defined heptet at δ
blet at δ = 18.2 ppm. The multiciplity of these signals is at-
tributed to C-D coupling. The C{ H} NMR heptet was at-
tributed to the formation of (CD S by comparison with litera-
ture values (18-25 ppm depending on solvent). (CD
indeed form by reduction of d -DMSO as unproductive side
C
= 16.75 ppm and a broad dou-
trum showed, as expected, a sharp singlet at 160.8 ppm due to
1. The tube was then charged with CO (1 bar) at room tem-
C
1
3
1
3
1
2
perature and transferred to the NMR probe for analysis. Quan-
)
3 2
2
8
titative formation of 3 was confirmed by the appearance of the
3
2
) S may
3
1
1
expected singlet at 136.7 ppm in the P{ H} NMR spectrum.
The tube was then evacuated and the CO atmosphere replaced
with N . PhSiH (10 equiv. to Mn) was then added to the
NMR tube, resulting in partial conversion (ca. 16%) of 3 into
6
2
reaction of silanes under these experimental conditions.
2
3
The test was then repeated at 80 °C under otherwise identical
conditions. After 24 h, NMR analysis revealed the complete
consumption of PhSiH
methoxysilyl species (δ
any signs of undesired DMSO reduction, with TON = 50
based on Si-H consumption, as calculated from H NMR sig-
nal integration. Further optimization was obtained by halving
the catalyst content (0.007 mmol; 1 mol% respect to “Si-H”)
and repeating the Schlenk tube test at 80 °C for 24 h. Also in
this case, the reaction gave the methoxysilyl product with
complete selectivity, reaching TON = 100.
31
1
1 after 2 h. After 24 h, the P{ H} NMR spectrum showed
only the singlet at 160.8 ppm due to the formation of 1
(Scheme 2b).
3
and 100% selective formation of
= 3.36, br.s; δ = 49.81, s), without
H
C
1
We also checked for the fate of the Mn complex after the reac-
tion under excess of CO
°C, to get information about catalyst deactivation. In the H
NMR spectrum, recorded at the end of the catalytic test, the
resonance due to Mn-H hydride had disappeared. The corre-
2
in the presence of PhSiH and 1 at 25
3
1
3
1
1
sponding P{ H} NMR spectrum showed a single P-
Scheme 2. Synthesis of 3 by CO
in 1 (a) and reverse reaction in the presence of PhSiH
2
insertion into the M-H bond
(b).
containing species with a singlet at 133.7 ppm, that we assign
3
to the cationic tricarbonyl complex [Mn(PNPNH-iPr)(CO)
] in
+
3
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ACS Catalysis
The full mechanism was then studied by DFT calculations,30
1
2
3
4
5
6
7
8
9
leading to the proposed catalytic cycle shown in Scheme 3.
The complete free energy profile of the stepwise reduction of
CO to the methoxysilyl level is presented in the Supporting
2
Information (Figures S1-S3). The reactions composing the
catalytic cycle, shown in Scheme 3, correspond to the four ma-
jor steps of the mechanism. First, hydride attack to the C-atom
of CO
2
occurs, resulting in a metal-coordinated formate ligand
in intermediate C. This part of the mechanism is common to
23
the CO hydrogenation pathway previously addressed. For-
2
mation of C is an easy process with a maximum barrier of 8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
kcal mol and represents an exergonic reaction, as the formate
-1
complex C is 9 kcal mol more stable than the initial reac-
tants. From C, the mechanism proceeds first with attack of a
silane molecule on the coordinated O-atom of formate, fol-
lowed by hydride transfer from Si to Mn. The result is the silyl
formate (HCOO)SiH
2
Ph with regeneration of the initial Mn
hydride complex, in intermediate H. This part of the mecha-
-1
nism has the highest barrier of the entire path (29 kcal mol )
corresponding to the step of silane attack, but overall it is al-
-1
most thermoneutral with ΔG = 2 kcal mol . In the next part of
the mechanism, the hydride in the metallic fragment attacks
the carbonyl C-atom of the formate, yielding a metal-
–
Scheme 3. Proposed (simplified) catalytic cycle. The free en-
coordinated silyl hemiacetal, [(OCH
2
O)SiH
2
Ph] , in interme-
-1
ergy values (kcal mol ) refer to the initial reactant (A) and the
values in italics represent barriers. Full details in Supporting
Information.
diate M. The process is comparatively more facile with a bar-
-1
-1
rier of 20 kcal mol and a free energy balance of 3 kcal mol .
The attack of a second silane molecule to the non-coordinated
O-atom of the hemiacetal in M, followed by hydride transfer
from the Si-atom to the carbonyl C-atom, results in the libera-
The mechanism for the formation of the silylformate
(
HCOO)SiH
2
Ph from CO
2
and PhSiH in the absence of cata-
3
tion of silyl ether (PhSiH ) O, and in the formation of a meth-
2
2
lyst (in DMSO) was also studied for comparison. The reaction
proceeds in a single step with concerted formation of the Si–
oxide Mn complex (Q). The barrier associated with this part of
-1
the mechanism is significant (24 kcal mol ) but the process is
C(CO
sponding profile is presented in the Supporting Information
Figure S4). The process is favored from a thermodynamic
2
) bond and H-transfer from Si to the O-atom. The corre-
clearly favored from the thermodynamic point of view as indi-
-1
cated by a free energy balance of ΔG = –18 kcal mol . In the
(
final stage of the catalytic cycle, a third silane molecule at-
tacks the coordinated methoxide in Q and, with a subsequent
hydride transfer from Si to Mn, yields the final product meth-
-
1
point of view (ΔG = –11 kcal mol ) but the barrier involved
(
-1
-1
37 kcal mol ) is 9 kcal mol higher than the one obtained for
the Mn-catalyxed reaction, clearly showing the role of the Mn
catalyst in the process.
oxyphenylsilane (CH
3
O)SiH Ph giving back the initial Mn hy-
2
dride A. This a fairly easy stage of the mechanism with a bar-
-1
rier of only 8 kcal mol , and the closing of the cycle, from Q
-1
back to A, is clearly exergonic with ΔG = –28 kcal mol .
In summary, we have hereby reported the first example of ef-
ficient direct Mn(I)-catalyzed CO
protected MeOH under mild reaction conditions (80 °C, 1 bar
CO ). Catalytic tests showed that the desired product could be
obtained working both in excess of silane and in excess of
CO , respectively. It was also demonstrated that, at room tem-
2
hydrosilylation to silyl-
2
2
perature, the reaction gave quickly silylformates, reaching the
methoxysilyl level only after long reaction times. Alternative-
ly, the selectivity of the process can be switched completely to
the methoxysilyl products by running the reactions at 80 °C.
DFT calculations showed that the catalytic cycle proceeds via
stepwise reduction of CO
cies. The catalyst works in the first step as hydride transfer
agent to CO to afford a formato complex, and in the second
2
to formate, acetal and methoxy spe-
2
step to transfer a hydride to the silylformate to give the silyla-
cetal species. The first reduction step has the highest barrier of
-1
29 kcal mol , but the exoergonic thermodynamic balance
helps to carry the reaction further to the methoxysilyl product.
The latter, upon release from the metal center, regenerates the
initial Mn-H complex that is thus able to start the next turno-
ver of the catalytic cycle.
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CO
2
catalytic reduction under excess silane. In a typical
1. (a) Li, Y.; Cui, X.; Dong, K.; Junge, K.; Beller, M. Utilization
2
TM
of CO as a C1 Building Block for Catalytic Methylation Reactions.
catalytic run, a Wilmad quick pressure valve NMR tube fit-
ted with a Teflon valve (total volume ca. 3.5 mL) was charged
ACS Catal. 2017, 7, 1077-1086; (b) Klankermayer, J.; Wesselbaum,
S.; Beydoun, K.; Leitner, W. Selective Catalytic Synthesis Using the
Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the
Interface of Energy and Chemistry. Angew. Chem. Int. Ed. 2016, 55,
with d
Si-H” respect to 13CO
mol% respect to “Si-H”; catalyst:CO
6
-DMSO (400 L), silane (0.7 mmol “Si-H”, ca. 5 eq
), the catalyst, if any (0.014 mmol, 2
ratio = 1:10) and me-
“
2
2
7
296–7343; (c) Zhang, Z.; Sun, Q.; Xia, C.; Sun, W. CO
2
as a C1
sitylene (10 μL) as internal standard. The NMR tube was then
attached to a high vacuum line, frozen in ice/water, and the
Source: B(C F ) -Catalyzed Cyclization of o-Phenylene-diamines to
6
5 3
Construct Benzimidazoles in the Presence of Hydrosilane. Org. Lett.
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2
version into Chemicals and Fuels: the Distinctive Contribution of
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1
3
headspace evacuated. The tube was then charged with CO
1 bar; ca. 0.14 mmol considering the headspace), thawed and
2
(
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
quickly transferred to the NMR probe kept at 25 °C. For tests
run at 80 °C, the NMR tube was kept in an oil bath set at the
desired temperature, and quickly transferred to the NMR
probe for analysis at the chosen time. The products distribu-
tion and silane conversion were determined by C{ H} NMR
and H NMR spectroscopy, respectively. All experiments were
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Capture, Se-
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1
2
repeated at least twice to check for reproducibility (average
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(total volume = 15 mL) was charged with d -DMSO (400 μL),
PhSiH (29 μL, 0.23 mmol; 0.7 mmol “Si-H”, ca. 1 equiv. “Si-
H” respect to CO ), catalyst 1 (3.0 or 6.0 mg, 0.007 or 0.014
mmol; 1 or 2 mol% respect to “Si-H”), mesitylene (10 μL) as
internal standard. The resulting solution was frozen in
ice/H
2
catalytic reduction under excess CO
2
. In a typical ex-
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O, the tube evacuated and refilled with 1 bar CO (3
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times). The tube containing the reaction mixture was then
placed in an oil bath set at the desired temperature and the re-
action mixture was allowed to stir for 24 h. Then, an aliquot of
the reaction mixture was transferred into an NMR tube. The
products distribution and silane conversion were determined
1
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1
3
1
1
by C{ H} NMR and H NMR spectroscopy, respectively. All
experiments were repeated at least twice to check for repro-
ducibility (average error ca. 5%).
Supporting Information. General methods and materials; select-
ed NMR spectra; additional tables for catalytic tests; computa-
tional details and full reaction pathways; atomic coordinates for
DFT optimized structures (xyz file); crystallographic data for 3
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Hy-
*
*
E-mail for L.G.: l.gonsalvi@iccom.cnr.it.
E-mail for K.K.: karl.kirchner@tuwien.ac.at.
2
1
ergy Future with Formic Acid: a Renewable Chemical Hydrogen
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All authors have given approval to the final version of the manu-
script.
Financial contributions by CRF Foundation through project
ENERGYLAB is gratefully acknowledged. This work was also
supported by COST Action CA15106 CHAOS (C-H Activation in
Organic Synthesis). LFV acknowledges Fundação para a Ciência
e Tecnologia, grant UID/QUI/00100/2013. NG and KK gratefully
acknowledge the Financial support by the Austrian Science Fund
(FWF) (Project No. P29584-N28).
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