Efficient and Mild Carbon Dioxide Hydrogenation to Formate Catalyzed by Fe(II) Hydrido Carbonyl Complexes Bearing 2,6-(Diaminopyridyl)diphosphine Pincer Ligands

Article abstract of DOI:10.1021/acscatal.6b00416

Fe(II) hydrido carbonyl complexes supported by PNP pincer ligands based on the 2,6-diaminopyridine scaffold were found to promote the catalytic hydrogenation of CO₂ and NaHCO₃ to formate in protic solvents in the presence of bases, r

Full text of DOI:10.1021/acscatal.6b00416

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Article
Efficient and Mild Carbon Dioxide Hydrogenation to
Formate Catalyzed by Fe(II)-Hydridocarbonyl Complexes
bearing 2,6-(diaminopyridyl)diphosphine Pincer Ligands
Federica Bertini, Nikolaus Gorgas, Berthold Stöger, Maurizio
Peruzzini, Luis F. Veiros, Karl A. Kirchner, and Luca Gonsalvi
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00416 • Publication Date (Web): 30 Mar 2016
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ACS Catalysis
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Efficient and Mild Carbon Dioxide Hydrogenation to Formate
Catalyzed by Fe(II)-Hydridocarbonyl Complexes bearing 2,6-
(diaminopyridyl)diphosphine Pincer Ligands.
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Federica Bertini,a Nikolaus Gorgas,b Berthold Stöger,c Maurizio Peruzzini,a Luis F. Veiros,d Karl Kirch-
ner*,b and Luca Gonsalvi*,a
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 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lis-
boa, Portugal.
ABSTRACT: Fe(II) hydridocarbonyl complexes supported by PNP pincer ligands based on the 2,6-diaminopyridine scaffold were
found to promote the catalytic hydrogenation of CO2 and NaHCO3 to formate in protic solvents in the presence of bases, reaching
quantitative yields and high TONs under mild reaction conditions, with pressures as low as 8.5 bar and temperature as low as 25 °C.
NMR and DFT studies highlighted the role of dihydrido and hydridoformate complexes in catalysis.
KEYWORDS . CO2 hydrogenation • iron pincer complexes • homogeneous catalysis • mechanistic studies • DFT calcula-
tions.
active catalysts for hydrogenation reactions.5c Mechanistic stud-
ies showed that the N–H spacer of the PNP ligand in 1 can work
INTRODUCTION
The use of CO2 as a C1-source is a matter of great interest
due to its high abundance, availability and low cost. In particu-
lar, its reduction to HCOOH or derivatives has attracted signif-
icant attention in recent years, since it holds the potential for
reversible hydrogen storage.1 The reduction of NaHCO3 is also
of interest, as CO2 can be easily trapped in basic solutions and
reversible hydrogen storage cycles based on bicarbonate and
formate have been proposed.2 The most efficient catalysts for
CO2 hydrogenation are typically based on expensive noble met-
als such as ruthenium or iridium.3 In the quest for cheaper alter-
natives, the preparation of well-defined earth abundant metal
catalysts of comparable activity is highly desirable and im-
portant progresses have been made recently.4 Efficient iron
based catalysts supported by tetraphosphine ligands have been
reported by Beller4a,b and some of us,4c whereas Milstein re-
ported that the iron pincer complex [Fe(PNP)(H)2(CO)] (PNP
as a bifunctional catalyst promoting metal-ligand coopera-
tion,5c,d while the N–Me spacer in complex 2 prevents such a
possibility. In addition, the presence of a labile bromide and
strong σ-donating H and CO ligands, could give the ideal donor
set suitable for catalytic CO2 hydrogenation.4d Thus, we inves-
tigated the activities of these complexes for CO2 and NaHCO3
hydrogenation reactions.
e
M
H
H
H
N
PR2
N
PR2
e
F
N
CO
e
F
N
CO
N
PR2
N
PR2
e
M
r
B
r
B
H
=
i
r
P
2
R
1
Scheme1. Fe-PNP pincer complexes 1 and 2.
RESULTS AND DISCUSSION
=
2,6-bis(di-tert-butylphosphinomethyl)pyridine) catalyzes
Catalytic Studies. Initially, the catalytic activities of 1 and 2
in NaHCO3 hydrogenation were tested in different solvents us-
ing 0.05 mol% catalyst at 80 °C, 90 bar H2, 24 h (Table S1).
The best results were obtained in H2O/THF (4:1) mixtures
which ensure good solubility of both catalysts and substrate,
reaching 98% formate yield and TON = 1964 for 1 and 52%
formate yield and TON = 1036 for 2, respectively. In MeOH,
TONs and yields decrease by ca. 50% with both catalysts,
whereas the reaction does not proceed in neat THF indicating
the need of a protic solvent. In all cases, 1 performed better than
2 under analogous conditions (see SI for details). Based on the
solvent screening results, the hydrogenation of NaHCO3 in
CO2 hydrogenation at low pressure.4d More recently, Hazari
and co-workers achieved impressive catalytic activities in Fe-
catalyzed CO2 hydrogenation reaching turnover numbers
(TONs) up to 79600 using iron PNP pincer complexes in the
presence of Lewis acid (LA) co-catalysts.4e In recent years,
some of us developed a new class of PNP pincer complexes
based on the 2,6-diaminopyridine scaffold where the PR2 moi-
eties of the PNP ligand are connected to the pyridine ring via
NH, N-alkyl or N-aryl spacers.5 Among these, iron hydrido car-
bonyl complexes [Fe(PNPH-iPr)(H)(CO)(Br)] (1) and [Fe(PNP
Me-iPr)(H)(CO)(Br)] (2) shown in Scheme 1 were shown to be
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H2O/THF was then studied with 1 under different conditions of Higher NaOH/catalyst ratios gave worse results regardless of
1
2
3
4
5
6
7
8
temperature, pressure and catalyst loading (Table 1). In the
presence of only 0.005 mol% of 1, TONs up to 4560 could be
achieved at 80 °C, 90 bar H2 after 24h (entry 2). Either higher
or lower temperatures resulted in lower turnover numbers (en-
tries 3,4). Noteworthy, the reaction proceeds even at room tem-
perature giving a TON = 188 after 72h (entry 5). Reducing the
H2 pressure resulted in a drop of TONs (entries 6,7), yet at
higher catalyst loadings (0.1 mol%) sodium formate was ob-
tained (14% yield) with a TON of 140 at only 8.5 bar H2 (Mil-
stein’s conditions)4d after 16h (entry 8).
concentrations (entries 2-4). We then tested the hydrogenation
of CO2 with 1 in EtOH in the presence of different amine bases.
Quite surprisingly, formate was not formed using either DBU
(1,8-diaza-bicyclo[5.4.0]undec-7-ene) or DMOA (N,N-dime-
thyloctylamine), whereas in the presence of NEt3 formate was
obtained only in low yields (entries 5-7). The observation that
complex 1 fails to catalyze the hydrogenation of CO2 in EtOH
in the presence of amine bases such as DBU or DMOA may be
attributed to the fact that EtOH appears to prevent the formation
of dihydrides,5c which are expected to be the catalytically active
species in this reaction. No reaction occurred in EtOH in the
absence of base (entry 8), nor in THF with DBU as base due to
catalyst decomposition (entry 9).
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Table 1. Hydrogenation of NaHCO3 to NaHCO2 with 1 at dif-
ferent catalyst loadings, temperatures and pressures.a
Entrya
Cat.1
T
P
TONb
t[h]
Yieldc
[%]
A complete screening of the effects of catalyst concentration,
nature of base, solvent and temperature for CO2 hydrogenation
in the presence of 2 was then carried out (Table 3).
[mol%]
[°C] [bar]
1
2
3
4
5
6
7
8
0.05
80
80
100
60
25
80
80
80
90
90
90
90
90
60
30
8.5
1964
4560
400
2360
188
640
80
24
24
24
24
72
24
24
16
98
23
2
Table 3. Hydrogenation of CO2 to formate with 2 using differ-
0.005
0.005
0.005
0.05
ent solvents and bases.a
Entry Cat. 2
Base
Solvent
T
[°C]
TONi
Yieldj
[%]
12
9
[mol%]
1
2b
0.08
0.04
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.1
NaOH
NaOH
NaOH
NaOH
DBU
H2O/THF 80
H2O/THF 80
680
372
220
654
1153
452
686
0
54
15
18
53
92
36
55
0
0.005
0.005
0.1
3
<1
14
3
MeOH
EtOH
EtOH
80
80
80
80
80
80
80
80
25
25
80
25
80
80
80
80
80
140
4
a General reaction conditions: 20 mmol NaHCO3, 0.01 - 0.001
mmol catalyst, 25 mL H2O/THF 4:1, 80 °C, 90 bar, 24h. b TON =
(mmol of formate)/(mmol of catalyst). c Yields calculated from the
integration of 1H NMR signals due to NaHCO2, using DMF as in-
ternal standard.
5
6
DMOA EtOH
7
NEt3
-
EtOH
EtOH
THF
8
Next, the hydrogenation of CO2 to formate in H2O/THF (4:1)
in the presence of 1 and NaOH as base was studied (Table 2),
reaching TONs up to 1220 with nearly quantitative yield under
optimized conditions (catalyst/NaOH = 1/1250, CO2/H2
40/40 bar, 80°C, 21h).
9
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
0
0
10c,d
11c
12c,e
13c,f
14
15
16c,g
17c,f,h 0.01
18a
19k
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
480
856
1032
9840
465
48
86
103
98
5
=
0.1
0.1
Table 2. Hydrogenation of CO2 to formate with 1 using differ-
ent solvents and bases.a
0.01
0.01
0.005
0.001
Entry
Cat. 1
Base
Solvent
TONe
Yieldf
[%]
10275 21
[mol%]
5000
620
0
5
1
0.08
0.04
0.008
0.04
0.08
0.08
0.08
0.08
0.08
NaOH
NaOH
NaOH
NaOH
DBU
DMOA
NEt3
H2O/THF
H2O/THF
H2O/THF
H2O/THF
EtOH
1220
608
120
656
0
98
24
1
6
2b
3c
4d
5
-
0
0.08
1163
93
26
0
a General reaction conditions: 12.5 mmol base, 0.01 mmol cata-
lyst, 25.0 mL solvent, 21 h. b 25.0 mmol base. c 10.0 mmol base. d
8.5 bar (CO2/H2 = 1:1) total pressure. e 72 h. f 0.001 mmol catalyst.
g 0.0001 mmol catalyst. h In the presence of LiOTF as Lewis Acid
additive, DBU:LiOTf = 7.5. i TON = (mmol of formate)/(mmol of
catalyst). j Yields calculated from the integration of 1H NMR sig-
6
EtOH
0
0
7
EtOH
288
0
23
0
8
-
EtOH
k
a
nals due to NaHCO2, using DMF as internal standard. As for ,
Hg(0) added.
9
DBU
THF
0
0
a General reaction conditions: 12.5 mmol base, 0.01 mmol cata-
lyst, 25.0 mL solvent, 80 °C, 80 bar total pressure, 21 h. b 25.0
mmol base. c 0.001 mmol catalyst. d 0.005 mmol catalyst. e TON =
(mmol of formate)/(mmol of catalyst). f Yields calculated from the
integration of 1H NMR signals due to NaHCO2, using DMF as in-
ternal standard.
As for NaHCO3 hydrogenation, catalyst 2 showed poorer
performance compared to 1 in the hydrogenation of CO2 in
H2O/THF (4:1) in the presence of NaOH (Table 3, entries 1,2
vs Table 2, entries 1,2). Among alcohols, reactions in EtOH
gave comparable activity to what observed in H2O/THF (entry
4), whereas worse performance was achieved in MeOH (entry
3). Based on the solvent screening results, amine screening was
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ACS Catalysis
H
H
H
Fe
H
Me
Me
Me
then studied for CO2 hydrogenation with 2 in EtOH. To our de-
light, using DBU as base gave nearly quantitative formate yield
H
bar)
(i) 2 (1
N
PR2
CO
N
PR2
N
PR2
CO
1
2
3
4
5
6
7
8
DBU, EtOH
+
N
Fe
Br
N
Fe
H
N
(>90%) with a TON of 1153 at 80 °C under 80 bar total pressure
(entry 5). Using either DMOA or NEt3 instead of DBU resulted
in lower TONs (entries 6,7) and no reaction occurred in the ab-
sence of base (entry 8) or with DBU in THF (entry 9) under
otherwise analogous conditions.
H
+Br-
]
N PR2
N
N PR2
PR2
[DBU-
Me
Me
Me
CO
CO
(ii)
(1
2
2
3a
3b
bar)
H
H
H
Me
Me
Me
EtOH
N
PR2
CO
N
PR2
CO
N
PR2
N
Fe
O
N
Fe
Br
N
Fe
CO
The potential of catalyst 2 was then further explored under
milder reaction conditions. At first, the effect of lower total
pressure was determined. In the presence of 0.1 mol% of 2 a
TON of 480 was reached after 21h at 80 °C under only 8.5 bar
H2/CO2 (1:1) (entry 10), an activity comparable to that of other
known iron pincer catalysts.4d,f Then, temperature effects were
studied. At 25 °C, catalyst 2 manifested a remarkable catalytic
activity, affording sodium formate in high yields6 with a TON
of 856 after 21h and of 1032 after 72h under 80 bar initial pres-
sure (entries 11,12) in the presence of 0.1 mol% catalyst. To the
best of our knowledge, these are the highest TONs obtained for
Fe-catalyzed CO2 hydrogenation at room temperature to date.
Br-
N PR2
N PR2
N PR2
4
Me
Me
Me
O
H
H
5
9
O
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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Scheme 2. Stepwise reaction of 2 with H2 (i) and CO2 (ii) in
the presence of DBU as base in EtOH.
Under these reaction conditions, 4 is in equilibrium with 2
due to the presence of bromide anions in solution. As a result, a
broad signal at 8.65 ppm due to free formate salt appeared in
the corresponding 1H NMR spectrum. In addition, the cationic
hydride complex [Fe(PNPMe-iPr)(H)(CO)(EtOH)]+ (5) was pre-
1
sent (Scheme 2), exhibiting a H NMR triplet resonance at -
Finally, the effect of catalyst loading was studied. At lower
catalyst loading (0.01 mol%) sodium formate was still obtained
in excellent yield (98%) with a TON of 9840 after 21h at 80 °C
(entry 13), whereas a TON of 465 was achieved at 25 °C after
21h at 80 bar (entry 14). Reducing further the catalyst loading
to 0.005 mol%, CO2 formate was still achieved with a high
TON of 10275 albeit in low yield respect to the base (21%, entry
15). Lowering further the catalyst amount (0.001 mol%) under
the same conditions resulted in a lower TON of 5000 (entry 16).
We also tested the effect of additives at high substrate to catalyst
ratios. Surprisingly, in contrast to what observed by Hazari et
al.,4e the use of a LA co-catalyst such as LiOTf affected nega-
tively the performance (entry 17). Colloidal metal catalysis was
ruled out by carrying out Hg poisoning test which gave compa-
rable results to what observed in the original run (entry 19 vs.
5).
25.57 ppm. The 31P{1H} NMR chemical shift of 5 is very close
to that of formate complex 4. However, no signal for the free
1
formate counteranion could be found in the H spectrum of 5.
Noteworthy, complex 5 was independently synthesized by
treatment of 2 with silver salts in EtOH.7 In a separate experi-
ment, stirring a mixture of 2 and sodium formate (4 equiv.) in
EtOH for 1h also affords a mixture of 2, 4 and 5. In another
experiment, this time starting from isolated 3, the reaction with
CO2 in EtOH afforded 5 with minor traces of 4. Evidently, the
formate ligand is easily displaced by an excess of solvent under
these conditions.
Single crystals of 4 suitable for X-ray diffraction analysis
were obtained by slow diffusion of pentane into a concentrated
solution of the complex in THF under an atmosphere of CO2.
The solid state structure of 4 confirms the geometry proposed
on the basis of NMR data. A structural view is depicted in Fig-
ure 1 with selected bond distances given in the caption. Com-
plex 4 adopts a distorted octahedral geometry around the metal
center with the fomate and hydride ligands trans to each other
and in cis position to the CO ligand. The hydride could be un-
ambiguously located in the difference Fourier maps. The Fe-H
distance was refined to 1.46(2) Å.
Mechanistic Studies. In order to gain insights in the reaction
mechanism, the reactivity of complex 2 was investigated in stoi-
chiometric reactions by NMR techniques. Exposure of an EtOH
solution of 2 to H2 (1 bar) in the presence of KOtBu resulted in
the quantitative formation of dihydrides 3a and 3b (cis and
trans isomers).5c,7 The 31P{1H} NMR spectrum exhibits two sin-
glets at 187.5 ppm (3a) and 189.9 ppm (3b), while the 1H NMR
exhibits a triplet at -9.57 ppm for 3a and a broad resonance at -
13.86 ppm for 3b. Upon cooling to -50 °C, the broad signal
starts to split into two separate triplets centered at -8.82 ppm
and -17.64 ppm.7 Using DBU as base, NMR analysis revealed
that only 40% of 2 was converted into the Fe(II) dihydrides 3,
even after prolonged standing under an hydrogen atmosphere,
suggesting that 2 and 3 are in equilibrium with each other
(Scheme 2, step i). This may be explained by the lower pKa
value of [DBU-H]+ in comparison to tBuOH.8
Next, the EtOH/base solution containing in situ formed 3 was
stirred under an atmosphere of CO2 for 30 min. Regardless of
the base used, we observed the formation of the hydridoformate
complex [Fe(PNPMe-iPr)(H)(CO)(η1-O2CH)] (4, Scheme 2,
step ii) characterized by a triplet at -24.71 ppm for the hydride
and a singlet at 7.96 ppm for the proton of the formate ligand,
which both integrate to 1 in the 1H NMR spectrum (see SI).
Figure 1. Structural view of 4 showing 50% thermal ellipsoids
(most H atoms and a second independent complex omitted for clar-
ity). Selected bond lengths (Å) and angles (deg): Fe1-P1 2.1765(7),
Fe1-P2 2.1789(7), Fe1-N1 2.004(2), Fe1-C20 1.737(2), Fe1-O2
2.032(2), Fe1-H1 1.46(2), P1-Fe1-P2 162.57(2), N1-Fe1-C20
175.58 (8).
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Based on the experimental evidence, a catalytic cycle for O-atom. This intermediate represents the potential well of the
1
2
3
4
5
6
7
8
CO2 hydrogenation starting from 2 can be proposed, encom-
passing formation of dihydrides 3, CO2 insertion to give the hy-
drido formate complex 4 followed by hydrogenolysis and for-
mate elimination giving back 3 in the presence of base. Solvent-
assisted formate decoordination in 4 may occur to leave a highly
reactive, unobserved pentacoordinate cationic Fe(II) hydrido-
carbonyl species, which can be stabilized by EtOH coordination
to give 5, observed by NMR (Scheme 3).
mechanism, thus it may be viewed as the catalyst resting state.
Alternatively, the formate ion dissociates from the metal in B to
give C, opening one coordination position that is occupied by a
molecule of H2 in the following step. Both processes are com-
petitive with barriers within 2 kcal/mol. The reaction pathway
proceeds with H2 addition to C yielding the dihydrogen com-
plex F. This process has an energy barrier of 14 kcal/mol, cor-
responding to the highest of the entire mechanism. In the final
step, the formate ion is protonated by F, regenerating the initial
complex 3a and producing formic acid. Giving the excess of
base present in the reaction medium under experimental condi-
tions, formed acid will be then deprotonated in an acid-base re-
action that provides the final driving force for the entire process.
Importantly, the free formate ion (the reaction product under the
experimental conditions) is stabilized by a H-bond with the
nearby water molecule in intermediates E/F.9 This facilitates
the opening of the coordination position that will be used by H2
in the following step of the mechanism, justifying the need of a
protic solvent in the catalytic reaction. A similar reaction mech-
anism was recently proposed for the selective hydrogenation of
aldehydes in EtOH with 3 as catalyst7 and by other authors on
related systems.10
Further mechanistic details on CO2 hydrogenation mecha-
nism were obtained by DFT calculations using 3a as the initial
active species. The free energy profile is shown in Scheme 4.
Computational details are presented as SI. The model used in
the calculations included one explicit water molecule that pro-
vides H-bond stabilization of the intermediates. The highlights
of the calculated mechanism are presented in Scheme 4 with
relevant intermediates and the corresponding free energy val-
ues.9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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57
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H
Me
H
Me
N
PR2
CO
N
PR2
CO
N
Fe
N
Fe
Br
CO2
HCOO-
N PR2
4
N PR2
HCOO-
Me
Me
O
O
2
H
H
Fe
H
H
Me
Me
HCOO- EtOH
PR2
CO
N
PR2
CO
N
DBU, H2
EtOH
CONCLUSIONS
Fe
N
N
EtOH
In conclusion, selected Fe(II) pincer-type complexes of 2,6-
diaminopyridyl-bis(diisopropylphosphine) gave high activities
as catalysts for CO2 and NaHCO3 reduction to formate under
very mild to moderate conditions, even at room temperature.
Mechanistic details were obtained by NMR techniques high-
lighting the role of dihydride and hydridoformate complexes.
DFT calculations indicate an outer sphere mechanism with a
hydridoformate complex as catalyst resting state and suggest
that the overall reaction is pushed forward by the acid-base re-
action between the product (formic acid) and the excess base
present in solution. Protic solvents promote catalysis by stabi-
lizing the reaction intermediates and assisting formate elimina-
tion from the coordination sphere of the metal.
N PR2
3a
N PR2
H
Me
Me
Me
N
PR2
CO
H
Me
N
N
Fe
PR2
H
N PR2
N
Fe
Me
O
5
HCOO- HDBU
][
N PR2
+
H
H
2 / DBU
[
]
Me
CO
3b
Scheme 3. Proposed catalytic cycle for CO2 hydrogenation
with 3a.
.
2 77
.
.
1 71
2 13
TSDE
.
.
2 84
14 0
+
.
H2
D
2 24
.
.
.
1 42
1 70
9 5
.
.
1 78
1 77
TSBH
H
.
H
.
TSBC
ASSOCIATED CONTENT
1 80
2 05
.
.
C
O
1
75
C
9 6
O
.
TSAB
O
H
.
7 9
TSFG
.
H
2 26
6 8
Supporting Information. General materials and methods, syn-
thetic procedures, NMR experiments and spectra, cartesian coordi-
nates of the computed structures, crystallographic data for the X-
ray structure of 4, additional catalytic data. This material is availa-
ble free of charge via the Internet at http://pubs.acs.org.
.
O
H
H
.
7 8
H
7 2
.
.
1 72
5 71
O
G
E/F
O
C
N
PR2
.
.
.
H
B
.
e
F
3 1
N
CO
7 7 6 4
/
.
4 28
N
PR2
H
O
2 6
N
PR2
H
H
A
.
E
H
e
1 72
.
N
F
CO
(
(
)
)
O
(
O
.
1 71
F
1 92
H
H
C
0
N
PR2
O
H
H
C
H
O
.
H
H
O
1 45
.
E
F
2 53
.
H
)
)
O
O
H
.
2 05
H
(
.
2 18
AUTHOR INFORMATION
Corresponding Authors
H
.
O
H
1 83
O
.
1 77
H
H
N
PR2
O
O
O
C
C
H
H
O
C
.
e
F
N
CO
-
.
3 38
O
H
10 2
.
N
PR2
H
2 08
PR2
1 78
N
PR2
e
F
N
CO
N
PR2
N
N
PR2
H
e
F
e
F
N
N
CO
CO
e
F
N
PR2
N
PR2
CO
* E-mail for L.G.: l.gonsalvi@iccom.cnr.it.
* E-mail for K.K.: karl.kirchner@tuwien.ac.at.
H
N
PR2
N PR2
N
H
H
H
Author Contributions
Scheme 4. Free energy profile calculated (DFT) for the hydro-
genation of CO2 catalyzed by 3a (denoted as A). The free en-
ergy values (kcal/mol) are referred to the initial reactants and
relevant distances (Å) are indicated.
All authors have given approval to the final version of the manu-
script.
ACKNOWLEDGMENT
In the first step of the calculated mechanism, from A to B, the
hydride attack from complex 3a to a CO2 molecule results in a
H-bonded formate complex. This is a facile process with a bar-
rier of 7.8 kcal/mol. In the resulting intermediate (B) the for-
mate ion is stabilized by a H-bond with the water molecule.
From B, formate can coordinate the metal giving complex 4 (H
in Scheme 4), where the formate ligand is bonded through the
Financial contributions by CNR and ECRF through projects EFOR
and HYDROLAB-2.0, respectively, are gratefully acknowledged.
This work was also supported by COST Action CM1205
CARISMA (Catalytic Routines for Small Molecule Activation).
LFV acknowledges Fundação para a Ciência e Tecnologia, grant
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E.; Allmaier, G.; Kirchner, K. Dalton Trans 2014, 43, 14669–14679;
UID/QUI/00100/2013. NG and KK gratefully acknowledge the Fi-
c) Gorgas, N.; Stöger, B.; Veiros, L. F.; Pittenauer, E.; Allmaier, G.;
Kirchner, K. Organometallics, 2014, 33, 6905–6914; d) Bichler, B.;
Holzhacker, C.; Stöger, B.; Puchberger, M.; Veiros, L. F.; Kirchner, K.
Organometallics 2013, 32, 4114–4121; e) Benito-Garagorri, D.;
Becker, E.; Wiedermann, J.; Lackner, W.; Pollak, M.; Mereiter, K.;
Kisala, J.; Kirchner, K. Organometallics 2006, 25, 1900–1913; f)
Benito-Garagorri, D.; Wiedermann, J.; Pollak, M.; Mereiter, K.;
Kirchner, K. Organometallics 2007, 26, 217–222; g) Benito-Garagorri,
D.; Puchberger, M.; Mereiter, K.; Kirchner, K. Angew. Chem. Int. Ed.
2008, 47, 9142–9145.
(6) Over-quantitative yields of formate have been reported with
DBU as base, see: Hsu, S.-F.; Rommel, S.; Eversfield, P.; Muller, K.;
Klemm, E.; Thiel, W. R.; Plietker, B. Angew. Chem. Int. Ed. 2014, 53,
7074–7078; see also ref. 4e and references cited therein.
(7) Gorgas, N.; Stöger, B; Veiros, L. F.; Kirchner, K. ACS Catal.
2016, 6, 2664–2672.
(8) pKa references: pKa for DBU is ca. 11.5-11.9, see: Kaupmees,
K.; Trummal, A.; Leito, I. Croat. Chem. Acta 2014, 87, 385–395; pKa
for tBuOH in water is 17.
(9) Free energy values obtained at the B3LYP/VDZP level using
the GAUSSIAN 09 package. All calculations included solvent effects
(THF) using the PCM/SMD model. A full account of the computational
details and a complete list of references are provided as SI.
(10) Yang, X. Inorg. Chem. 2011, 50, 12836-12843.
1
2
3
4
5
6
7
8
nancial support by the Austrian Science Fund (FWF) (Project No.
P24583-N28).
REFERENCES
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