681 Seminar
Activation of Dihydrogen by Main Group
Compounds Under Mild Conditions
Haiyan Zhao
Advisor: François P. Gabbaï
March 30th 2009
1
Outline
Introduction
*
Frustrated Lewis Acid-Base Pairs :
*
Bimolecular systems
Unimolecular systems
Reactivity
*
H2 Activation by Heavier Main Group Systems:
Aryl gallium species: ArGa
Diarylstannylene compounds: SnAr2
Group 14 alkyne analogues: ArEEAr, E = Ge, Sn
* Conclusion
2
Dihydrogen Activation - Introduction
H2 activation
Valuable green fuel: Production, storage, transportation
Chemical reactions: Catalysis of hydrogenation
Challenges
Activation of a strong bond ( HB (H2) = 103 kcal/mol)
For storage application, the reaction should be reversible.
3
Kubas, G. J. Science, 2006, 314, 1096.
Developments
Transition metal complex
Disadvantages of transition metal
A low weight percentage of H2
High cost
Environmentally unfriendly
Transition-metal free system
H2
P1
W
H1
P2
transition metals involved: Pt, Rh, Ir,
Ru et al.
Biological system model: Iron-sulfur
cluster-free hydrogenase
base-catalyzed
acid-catalyzed
Lewis acid and Lewis base
Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.,2004, 248, 2201.
Kubas, G. J. et al J. Am. Chem. Soc., 1984, 106, 451.
4
Transition Metal Compounds
Orbital interactions
Lewis base
Lewis acid
Could similar interactions be observed with main group Lewis acid and Lewis base?
5
Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93, 913.
Outline
Introduction
*
Frustrated Lewis Acid-Base Pairs (FLPs) :
*
Bimolecular systems
Unimolecular systems
Reactivity
*
H2 Activation by Heavier Main Group Systems:
Aryl gallium species: ArGa
Diarylstannylene compounds: SnAr2
Group 14 alkyne analogues: ArEEAr, E = Ge, Sn
* Conclusion
6
Frustrated Lewis Pairs: Introduction
Concept
Combination of sterically hindered Lewis acid and Lewis base
(1) Steric effect inhibits the formation of Lewis acid-base adduct.
(2) The Lewis acidity and basicity are retained
Example
Adduct
FLP
7
Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.
FLPs: Bimolecular reaction systems
Reaction:
Reaction Conditions: Toluene at room temperature under one atmosphere
Combinations of Lewis acid and Lewis base toward H2 activation
Acid
BArF3
BArF3
BPh3
BArF3
BtBu3
BArF3
BArF3
Base
PtBu3
PMes3
PtBu3
PArF3
PtBu3
PMe3
PPh3
Reactivity
100%
100%
33%
0
0
adduct adduct
8
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
FLPs: Combination of acidity and basicity
Acid
Base
BArF3
PMe3
BArF3
PPh3
BArF3
PArF3
BtBu3
PtBu3
BPh3
PtBu3
BArF3
PMes3
BArF3
PtBu3
Classical adducts are formed because of the low steric
constraints
No reactivity because i) the basicity of PArF3 is too
weak ii) the acidity of BtBu3 is too weak
Low reactivity because of the moderate acidity of BPh3
High reactivity due to the favorable combination of
strong acidity/basicity and high steric crowding
A combination of favorable electronic and steric conditions
9
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
FLPs: Mechanism of bimolecular reaction system
H
P
F
B
Loose structure
Active
B3LYP/6-31G(d)
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T. Soos; I. Papai, et al Angew. Chem., Int. Ed. 2008, 47, 2435.
FLPs: Bimolecular systems for the irreversible activation of H2
Entry
Acid
Base
H2 uptake
H2 release
1
B(C6F5)3
PtBu3
25 oC, 1 atm
No
2
B(C6F5)3
PMes3
25 oC, 1 atm
No
3
B(C6F5)3
25 oC, 4 atm
No
4
B(C6F5)3
25 oC, 1 atm
No
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
Ramos, A.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009, 1118.
Sumerin, V.; Rieger, B.; et al Angew. Chem., Int. Ed. 2008, 47, 6001.
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FLPs: Bimolecular systems for the reversible activation of H2
Entry
Acid
5
6
7
H2 uptake
H2 release
B(C6F5)3
25 oC, 2 atm
60 oC
B(C6F5)3
25 oC, 1 atm
100 oC
B(p-C6F4H)3
Base
P(o-C6H4Me)3
25
oC,
1 atm
Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.
25 oC,
vacuum
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Factors governing the reversibility: Does the H┈H distance matter?
Reaction:
P1
2.08 Å
P1
H2
2.75 Å
H1
H2
H1
B1
reversibility
B1
P1, B1 50% ellipsoid; H1, H2 ball and stick; other H atoms omitted
Wang, H.; Frohlich, R.; Kehr, G.; Erker, G., Chem Commun (Camb) 2008, 5966-8.
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
13
Factors governing the reversibility: Does the H┈H distance matter?
Acid
Base
H2 release
H┈H Distance Å
B(C6F5)3
PtBu3
No
2.75
B(C6F5)3
No
2.927
B(C6F5)3
No
2.973
B(C6F5)3
60 oC
2.08
B(C6F5)3
100 oC
1.853
Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
Geier, S. J.; Stephan, D. W. J.
Am. Chem. Soc. 2009, 131, 3476.
Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009,
14
131, 52. Sumerin, V.; Rieger, B.; et al Angew. Chem., Int. Ed. 2008, 47, 6001.
Ramos, A.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009, 1118.
FLPs: Bimolecular systems for the reversible activation of H2
Entry
Acid
5
6
7
H2 uptake
H2 release
B(C6F5)3
25 oC, 2 atm
60 oC
B(C6F5)3
25 oC, 1 atm
100 oC
B(p-C6F4H)3
Base
P(o-C6H4Me)3
25
oC,
1 atm
Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.
25 oC,
vacuum
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Factors governing the reversibility: Acidity and Basicity
R1 in B(R1)3
R2 in P(R2)3
H2 uptake
H2 release
25 oC, 1 atm
No
25 oC, 1 atm
No
25
oC,
1 atm
25 oC,
vacuum
The right balance of acidity and basicity allows for reversible H2
binding at 25 oC.
Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.
Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
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FLPs: A special bimolecular system for activation of H2
At the boundary of classical adduct and frustrated Lewis pair reactivity
(NMR)
H = -42(1) kJ mol-1
S = -131(5) J mol-1 K-1
N1
B1
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Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
Outline
Introduction
*
Frustrated Lewis Acid-Base Pairs :
*
Bimolecular systems
Unimolecular systems
Reactivity
*
H2 Activation by Heavier Main Group Systems:
Aryl gallium species: ArGa
Diarylstannylene compounds: SnAr2
Group 14 alkyne analogues: ArEEAr, E = Ge, Sn
* Conclusion
18
FLPs: Unimolecular systems for the activation of H2
Entry
FLP
H2 uptake
H2 release
8
25 oC, 4 atm
No
9
25 oC, 1.5 atm
No
10
25 oC, 1 atm
100 oC
11
20 oC
110 oC
Welch, G. C.; Stephan, D. W.; et al Science 2006, 314, 1124.
Spies, P.; Erker, G.; et al Angew. Chem., Int. Ed. 2008, 47, 7543.
Sumerin, V.; Rieger, B.; et al J. Am. Chem. Soc. 2008, 130, 14117.
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Unimolecular Reaction Systems: Concerted Mechanism
Factors facilitating the formation
of the frustrated complex
interaction
C-H
F-C interaction
Constraint geometry
Soos, T.; Papai, I.; et al Angew. Chem., Int. Ed. 2008, 47, 2435.
Rieger, B.; et al J. Am. Chem. Soc. 2008, 130, 14117.
Erker, G. Chem. Commun. 2007, 5072.
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Unimolecular Reaction Systems: Intramolecular Mechanism
Kinetic data
Method: 31PNMR
First order kinetics
S≠ = -96±1 J mol-1 K-1
H≠ = 90±1 kJ mol-1
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Welch, G. C.; Stephan, D. W.; et al Science 2006, 314, 1124.
Outline
Introduction
*
Frustrated Lewis Acid-Base Pairs :
*
Bimolecular systems
Unimolecular systems
Reactivity
*
H2 Activation by Heavier Main Group Systems:
Aryl gallium species: ArGa
Diarylstannylene compounds: SnAr2
Group 14 alkyne analogues: ArEEAr, E = Ge, Sn
* Conclusion
22
Frustrated Lewis Pairs: Hydrogenation catalysis
Hydrogenation of enamines, silyl enol ethers and imines
Spies, P.; Erker, G.; et al Angew. Chem., Int. Ed. 2008, 47, 7543.
Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.
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Frustrated Lewis Pairs: Hydrogenation catalysis
Hydrogenation of imines
Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050.
Sumerin, V.; Rieger, B.; et al J. Am. Chem. Soc. 2008, 130, 14117.
24
Frustrated Lewis Pairs: Hydrogenation of imines
Entry
Imine
Time
(h)
Amine
Yield(%)
1
24
4
2
12
99
3
12
99a
4
6
99
Sterically demanding
substrates preclude the
formation of classical
adducts.
Bulkier substrates have
higher reaction rates.
a. 8% catalyst
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Sumerin, V.; Rieger, B.; et al J. Am. Chem. Soc. 2008, 130, 14117.
Frustrated Lewis Pairs: Mechanism of imine hydrogenation
A proposed mechanism for the hydrogenation of imines
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Sumerin, V.; Rieger, B.; et al J. Am. Chem. Soc. 2008, 130, 14117.
Frustrated Lewis Pairs: Conclusion
Frustrated Lewis Acid-Base Pairs :
Two reaction systems (bimolecular and unimolecular) activate H2 under
mild conditions and sometimes reversibly.
Mechanistic studies point to a mechanism in which H2 is activated via a
concerted process involving the synergy of the acid and the base.
FLPs have been used to catalyze the hydrogenation of various
substrates.
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