A Diversity of Chemical Strategies to Implement
Homogeneous Carbon Dioxide Hydrogenations
Y. Jiang, R. Kunjanpillai, S. Chakraborty, F. Zhou
O. Blacque, T. Fox and H. Berke
E2C 2013, Budapest
Strategies for Homogeneous Carbon Dioxide
Hydrogenations
‘Non-noble Metals for Noble Tasks’
-
Search for molecular middle transition element catalysts
Cheap catalysts and processes
Processes running at ambient or near ambient conditions
Oxygenates (CnHmOo) as products
Use of molecular hydrogen from ’renewable’, ‘waste’ or CO2-free sources
As for methanol as the simplest oxygenate product
Homogen.cat
CO2 + 3H2  CH3OH + H2O
- MeOH available since the 1920s by heterogenous CO hydrogenation
Pros
Cons
- Potential energy carrier
- Small scale and de-centralized production seems possible
- Facile convertion to other oxygenates ( for instance DME) or hydrocarbons
- Toxic
- Low boiling point
Middle Transition Elements in Homogeneous
Hydrogenation Catalysis
- Middle transition elements particularly rhenium is bordering precious metals  may have
retained some of the „noble“ properties, like high affinity to hydrogen and to unsaturated organic
molecules
- Isoelectronic replacements in precious metal complexes may generate active catalysts; for
instance the Ru-L unit (L= 2e- donor) replaced by the Re-NO unit
- Middle transition element seek stable 18e- configurations; disadvantage for catalysis, which
needs permanently or temporarily vacant coordination sites
- Tuning of the ligand sphere is required to stabilize vacant sites via ligand effects, such as of
ligands with variable electron counts, large-bite-angle-effects of bidentates, large cone angles of
monodentates, cis labilization effect and the trans influence/effect and others
Electrochemical Volcano Plot for Hydrogen
Kinetics
1
Hydrogen evolution
reaction
log(i0) (A/cm-2)
Pd
Which metals are
preferred in hydrogen
catalysis?
5
10
M-H bond strength
(kJ/mol)
125
200
275
350
Thermodynamics
S. Trasatti, J. Electroanal. Chem. 39 (1977) 163
S. Trasatti, Electrochim. Acta 39 (1994) 1739
Stages of CO2 Reduction by H2
Carbon oxidation state from +IV to -II
Carbonic acid, Formic acid or formate stage
CO2 or
(thermodynamically
carbonate stage unstable, but kinetically too
stable
Methanol stage,
Hydroxy carbene or
potential to be
formaldehyde stage
(joint catalytic processes to converted into
other
make ‘formoses’ (CnH2nOn)
oxygenates
seem possible)
Formation of esters via the reaction of carbonic and formic acid and derivatives with alcohols
including the product methanol (catalyzed by protons) may help to overcome the reactivity
deficit of formic acid and derivatives. Esters are more easily reduced than acids or their anions
Hydrogenation of CO2 to Formic Acid and Formates
hydridic

H
C
O
O

H
pKa = 3.8
protonic
As a polar molecule HCOOH would be prone for reactions with heterolysis of H2!
Thermodynamics of the hydrogenation CO2 to formic acid:
CO2(g) + H2(g) → HCOOH(l) ∆Go = 32.9 kJ/mol; ∆Ho = -31.2 kJ/mol; ∆So = -215 J/(mol K)
CO2(g) + H2(g) + NH3 (aq) → HCOO- (aq) + NH4+(aq) ∆Go = -9.5 kJ/mol; ∆Ho = -84.3 kJ/mol;
∆So = -250 J/(mol K)
CO2(aq) + H2(aq) + NH3 (aq) → H(CO)O- (aq) + NH4+(aq) ∆Go = -35.4 kJ/mol; ∆Ho = -59.8
kJ/mol; ∆So = -81 J/(mol K)
Additional driving force by salt formation!
Despite their thermodynamic instability, formic acid or formate salts are kinetically
(too) stable!
Olefin Hydrogenations with Mo and W Nitrosyl Hydride
Catalysts
cat: M = Mo,W
S. Chakraborty, T. Fox,
O. Blacque, 2013
submitted
Cat/
(mol%)
Substrate
Co-catalyst
BCF = B(C6F5)3
Temp
(°C)
H2
(bar)
Time
(h)
TOF
(h-1)
Conv
(%)
Mo/0.05
1-hexene
-
140
60
0.5
912
21
Mo/0.03
1-hexene
Et3SiH/BCF
140
60
2
5253
75
W/0.020
1-hexene
Et3SiH/BCF
140
60
0.5
8200
80
Mo/0.08
1-octene
Et3SiH/BCF
140
60
<2
3500
75
W/0.049
1-octene
Et3SiH/BCF
140
60
0.5
3615
88
Mo/0.01
Styrene
Et3SiH/BCF
140
60
2
2079
84
W/0.037
Styrene
Et3SiH/BCF
140
60
0.5
2064
39
Mo/0.03
1,7-octadiene
Et3SiH/BCF
140
60
2
1056
65
Mo/0.07
Cyclohexene
Et3SiH/BCF
140
60
<10
200
100
Mo/0.07
Cyclooctene
Et3SiH/BCF
140
60
3
1355
55
Mo/0.07
α-methyl
styrene
Et3SiH/BCF
140
60
16
205
60
Mo/0.09
1,5
cyclooctadiene
Et3SiH/BCF
140
60
14
200
74
Hydrogenation of CO2 to Formic Acid Using a
Tungsten Complex
Base = DBU
Exploitation of Ligand Effects to Make the
Hydrogenation of CO2 with a Tungsten Hydride Catalytic
Large cone-angle „on - off“ chelate phosphine!
Tungsten has great
affinity to H2!
Carbynes like nitrosyls (3e donors!) are trans influence and trans effect ligands and
activate trans positions.
Catalytic CO2 Hydrogenation to Formate
cat
Products
[HCOO][DBUH] and
HCOOH•DBU
precipitate from
toluene solution
Low energy
‘Organo-catalytic’
transition state
Rhenium Based Catalytic CO2 Hydrogenation to
Formate and Methanol
FLP Type Activation of CO2 with Rhenium Complexes
and a Lewis Acid
FLP Type Activation of CO2 with a Rhenium Hydride and
B(C6F5)3 as Lewis Acid – Stoichiometric Reactions
Frustrated Lewis pair (FLP)
where the hydride plays the
role of a Lewis base!
Fully characterized
by NMR in solution
Crystal structure
Jiang, Y.; Blacque, O.; Fox, T.; Berke, H., J. Am. Chem. Soc. 2013, 135, 7751-7760
A Stable Formate Dihydrogen Complex and
Subsequent Stoichiometric Reaction with Et3SiH
Isolated and fully
characterized
Jiang, Y.; Blacque, O.; Fox, T.; Berke, H., J. Am. Chem. Soc. 2013, 135, 7751-7760
Catalytic Reduction of CO2 with Et3SiH
Hydrogenation
Hydrosilylation
H2
CH2O
HOMe
H2O
Hydrogenation of CO2 with a Rhenium
Hydride/Lewis Acid System in Presence of TMP
Structure of [TMPH][HCOO]
Proposed Mechanism for the Rhenium Catalyzed CO2
Hydrogenation In Presence of B(C6F5)3 and TMP
Hydrogenations with Rhenium Nitrosyl Complexes
Bearing Large Bite-Angle Xantphos Diphosphines
Reactive site interconversion
Trans NO reactive site
Kinetic ‘isomer’?
Xantphos framework may
‘bite’ bi- or tridentate
Ammonia
complex
(NO/Br
disorder)
Cis NO reactive site
Thermodynamic isomer?
Rhenium Based Catalytic CO2 Hydrosilylation and
Combined Hydrogenation/Hydrosilylation to Methanol
and Derivatives
Hydrogenation/hydrosilylation with TONs up to 330:
Hydrosilylation with TONs up to 410:
1
2
Rhenium Based Catalytic CO2 Combined
Hydrogenation/Hydrosilylation to Formate Derivatives
Rhenium Based Ester, Aldehyde, Ketone and
Bicarbonate Hydrogenation to Alcohols
Substrate
140 °C, 60 bar H2
Benzaldehyde
Acetophenone
Catalyst
Co-
Solvent
Product
TON
Yield (%)
(mol%)
catalyst
0.02
n-Bu4NBr
THF
PhCH2OH
500
100
0.02
-
Toluene
PhCH2OH
4807
96
0.05
-
Toluene
PhCH(OH)
Me
2000
> 99
0.5
-
CH3OH
112
0.5
n-Bu4NBr
EtOH
CH3OH
161
81
0.2
-
MeOH
HCOONa
28
5.6
0.2
-
D2O/THF
HCOONa
68
13.6
EtOH
CH3OH
20
10
H(CO)ONa
24
12
0.5
n-Bu4NBr
THF
56
Rhenium Based and Halide Co-catalytic CO2
Hydrogenation to Methanol
Optimum loading of co-cat.bromide
Improved performance by EtOH add.
No improvement by acid add.
H2 pressure matters
Co-catalyst 1 /Equiv.
Co-catalyst 2
/Equiv./acid
CO2/H2 (bar)
TON
-
-
10/30
07
n-Bu4NBr /5
-
10/30
28
n-Bu4NBr /5
-
10/30
26c
-
-
10/30
13
n-Bu4NI /100
-
10/30
11
NaI/100
-
10/30
05
n-Bu4NBr /2
-
10/30
20
n-Bu4NBr /50
-
10/30
30
n-Bu4NBr /100
-
10/30
30
Et4NBr /100
-
10/30
1.6
n-Bu4NBr /5
-
10/30
0.9
-
EtOH/250
10/30
06
n-Bu4NBr /100
EtOH/250
10/30
33
n-Bu4NBr /5
EtOH/100/p-TsOH
10/30
29g
n-Bu4NBr /5
-
20/60
88
Is formate ester formation crucial to catalysis? What is the influence of the produced water on
ester formation?
Rhenium Based Hydrogenation of Methyl
Formate to Methanol
Methyl formate cycle
Formaldehyde cycle
Remaining Questions on the CO2 Hydrogenation to
Methanol
catalysts optimizations
- Relevance of formic ester formation
- Can ester formation be enhanced by protic additives?
- Role of the water product for the ester formation. Removal of water required?
- Change type of the hydrogenation course and develop two-step
hydrogenation processes?
CO2  Formic acid  formic acid ester  methanol
CO2  carbonic acid  carbonic acid diester  methanol
Acknowledgements
Preparative work
Y. Jiang, R. Kunjanpillai, S. Chakraborty, F. Zhou
X-ray structures and DFT
O. Blacque
NMR
T. Fox
Financial Support
Funds of the University of Zurich
Swiss National Science Foundation
Lanxess AG, Leverkusen, Germany
Utilization of Metal-Ligand Activation of CO2
Stoichiometric CO2 Hydrogenation to Formate
Catalytic CO2 Hydrogenation to Formate
Cat
Base
Time (h)
Yield (%)
Mo
Na[N(SiMe3)2]
15
4
W
Na[N(SiMe3)2]
15
2
Poor catalysis due to
a too stable CO2
complex!
Rhenium Based Catalytic CO2
Hydrogenation/Hydrosilylation to Methanol and
Silyl Derivatives
Reaction Center Mediated Splitting Pathways of
Dihydrogen
Mononuclear and dinuclear dihydrogen complexes prepare the H2 molecule for the splitting
reaction by polarization
Principles of Hydrogen Reactions
Activation of H2 = H2 splitting and H-transfers
•
•
Single-step processes combined splitting and
H-transfers (rare)
Multi-step processes
separate splitting and H-tranfers
Principles of H2 reactivity
H2
Homolytic,
heterolytic
splitting?
Splitting

Homolytic splitting H2  2 H Wilkinson/Osborntype hydrogenation catalyses with oxidative addition of
H2

Heterolytic splitting H2  H- + H+
Ionic hydrogenations (protonic-hydridic reactivity,
bifunctional activation). Usually achieved by
deprotonation of H2 complexes with a base
H
Substrate XY
Sequence of
Transfers?
H
H-transfers:
homopolar,
heteropolar?
HXYH
H-Transfers



Simultaneous (concerted) polar transfers of polarized H atoms (H(+) protic and H(-)
hydridic)
Stepwise transfers: homopolar H atom transfer via insertion into the M-H bond and reductive
elimination
Heteropolar stepwise transfers
H. Berke, ChemPhysChem, (2010), 11, 1837
Interconversion of ‘Isomeric’ Xantphos Nitrosyl Rhenium
Complexes via Anionic Trihalo Intermediates – Basis for
the Catalytic Halide Effect
I(trans) more reactive than I(cis)!
Structure of 3a
Cis PiPr3 ligands: P(1)-Re(1)-P(2), 103.78(2)