hindered rotation?
DESCRIPTION
Why no -H species into the enzyme? - no thermodynamic stabilization of terminal-H intermediates... - ...terminal-H corresponds to a kinetic product? But if this is true.... Can interconversion from terminal- to -H species take place into the protein?. Hindered rotation?. - PowerPoint PPT PresentationTRANSCRIPT
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FeS
Fe
S
N
OCCO
ONC
CN
H
H
[Fe4S4]
NH3+
Why no Why no -H species into the enzyme?-H species into the enzyme?
- no thermodynamic stabilization of terminal-H intermediates...- no thermodynamic stabilization of terminal-H intermediates...- - ...terminal-H corresponds to a kinetic product?...terminal-H corresponds to a kinetic product?
But if this is true....But if this is true....Can interconversion from terminal- to Can interconversion from terminal- to -H-Hspecies take place into the protein?species take place into the protein?
•Hindered rotation?
Relevance of studies of protonation regiochemistry in synthetic models!- Brest laboratory- Illinois laboratory
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[FeFe]-hydrogenases models and catalysis. [FeFe]-hydrogenases models and catalysis. Formation of synthetic Fe(II)Fe(II)-HFormation of synthetic Fe(II)Fe(II)-H- - speciesspecies
• Terminal hydride species can be transiently formed and are more reactive than corresponding -H species in H2 production.
• Spontaneously convert to -H species
Van der Vlugt J, Whaley C, Wilson S, Rauchfuss T. J. Am. Chem. Soc., 2005, 127, 16012;
Ezzaer S, Capon J-F, Gloaguen F, Petillon F Y, Schollhammer P, Talarmin J. Inorg. Chem., 2007, 46, 3426
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Protonation of synthetic models of the [2Fe]Protonation of synthetic models of the [2Fe]HH
cluster. DFT results.cluster. DFT results.
• (dppv)(CO)Fe(edt)Fe(PMe3)(CO)2, Fe(I)Fe(I) redox state
dppv = cis-1,2-C2H2(PPh2)2
• Stereo-electronic similarity to [2Fe]H
Fe
S
FeS
OC
P
P
OP(CH3)3
CO
H
Fe
S
FeS
OC
Cys-S
O
CO
H
NC
CN
possibility to verify theoretical predictions (Illinois, Brest)
J-F Capon, F Gloaguen, F Y Petillon, P Schollhammer, J Talarmin 2009, 253, 1476-1494
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Protonation regiochemistry
Fe
S
FeS
CO
P(CH3)3
CO
P
POC
Fe
S
FeS
CO
P(CH3)3
CO
P
POC H
Fe
S
FeS
H
P(CH3)3
CO
P
POC
O
Fe
S
FeS
CO
P(CH3)3
CO
H
PP
O
+ CF3SO3H
TS(1a-1Ha+)
TS(1c-1Hc+)
TS(1a-1Hb+)
1Ha+
1Hc+
1Hb+
Reaction with triflic acidin acetonitrile: looking for transition statesand intermediate species
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Protonation of synthetic models of the [2Fe]Protonation of synthetic models of the [2Fe]HH
cluster. DFT results.cluster. DFT results.
Fe
S
Fe
S
P
P
COO
H
P(CH3)3
CO
SO
O O
CF3
Fe
S
Fe
S
P
P
CO
OP(CH3)3
CO
SO
O O
CF3
H
Fe
S
Fe
S
P
P
COCO
P(CH3)3
CO SO
O O
CF3
H
Fe
S
Fe
S
P
P
COCO
P(CH3)3
CO
H
SO
O O
CF3
2
1
3
4 4
-28.5
3
2
-5.8
15.2
-0.610.7
1
Reaction Coordinate
• Kinetic control: terminal-H• Thermodynamic control: -H
E (kcal/mol)
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Protonation of synthetic models of the [2Fe]Protonation of synthetic models of the [2Fe]HH
cluster. DFT results.cluster. DFT results.
• In the protonation of (dppv)(CO)Fe(edt)Fe(PMe3)(CO)2
steric factor plays a key role
Importance of intramolecular proton relay!
Fe
S
Fe
S
P
P
COCO
P(CH3)3
CO
SOH
O O
CF3
Fe
S
Fe
S
P
P
COCO
P(CH3)3
CO
H
SO
O O
CF3
S Ezzaher, J-F Capon, F Gloaguen, F Y Petillon, P Schollhammer, J Talarmin 2009, 48, 2-4
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Fe
S
FeS H
R3POC
R3P
PR3
CO
O
O
FeFe
SSH
R3PR3P
CO
PR3
CO
Fe
S
FeS
R3POC
R3P
PR3
CO
O
O
FeFe
SS
R3PR3P
CO
PR3
CO
Fe
S
FeS
R3POC
R3P
PR3
CO
CO
+
+
H+
H+
Transition state is stabilized when bridging CO moves towards the more electron-rich iron atom
Product is stabilized when the CO ligands are more evenly distributed among the iron atoms
Protonation of synthetic models of the [2Fe]Protonation of synthetic models of the [2Fe]HH
cluster. Proximal or distal protonation?cluster. Proximal or distal protonation?
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-protonation terminal-protonation on Fed
G G≠ G G≠
[(dppv)(CO)Fe(edt)Fe(PMe3)(CO)2] (1) -13.3 15.2
-4.9 6.4
(CO)3Fe(edt)Fe(CO)3 (2) 11.3
17.1 -a -
(dppv)(CO)Fe(edt)Fe(CO)3 (3) -1.0 18.3 9.2 -
(PH3)2(CO)Fe(edt)Fe(CO)3 (4) -2.3 18.9 - a -
(PMe3)2(CO)Fe(edt)(CO)(PMe3)2 (5) -26.3 7.9 -23.5 5.6
(dppv)(CO)Fe(pdt)Fe(dppv)(CO) (6) -19.5 19.6
-15.7 16.6
(PH3)2(CO)Fe(edt)(CO)(PH3)2 (7) -13.3 8.1 -3.4 10.9
(PH3)3Fe(edt)(PH3)(CO)2 (7a) -19.4 6.0 -8.1 0.0
a. The reaction product does not correspond to an energy minimum structure and evolves back to reactant (the FeFe complex + triflic acid).
Protonation of synthetic models of the [2Fe]Protonation of synthetic models of the [2Fe]HH
cluster. Extending the seriescluster. Extending the series
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Brief summary
• Terminal-H species are easily formed but spontaneously convert to (less reactive) mu-H species
• Relevance of the investigation of the mechanism of t-H -> mu-H conversion
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Interconversion from terminal- to Interconversion from terminal- to -H -H
Fe
S
Fe
S
LL
HP
PL
O
Fe
S
Fe
S
HL
LP
PL
O
Fe
S
Fe
S
HL
LP
PL
O
Fe
S
Fe
S
HL
CO
LP
PL
120°
120°
3 Int
Int 4
Pseudo Pseudo CC3 rotations3 rotations
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Interconversion from terminal- to Interconversion from terminal- to -H:-H:Pseudo Pseudo CC33 rotations rotations
Fe
S
Fe
S
LL
HP
PL
O
Fe
S
Fe
S
HL
LP
PL
O
Fe
S
Fe
S
HL
LP
PL
O
Fe
S
Fe
S
HL
CO
LP
PL
4
Reaction Coordinate
-28.5
3
Int
-5.6
6.2
15.83 Int
Int 4
E (kcal/mol)
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Fe Fe
S
S
H
O
P
P
CO
L
L'
Fe Fe
S
SH
O
P
P
CO
FeS
Fe
S
H
P
PCO CO
L
L'
Fe
P
P
SFe
H
S
CO L' CO
Fe
H
FeP
PS
S
L'CO
CO
FeS
Fe
SH
P
P
COOC
Fe Fe
S
S
H
O
P
P
CO
L
FeS
Fe
SP
P HCO
L
L'CO
H
Fe
S
FeS
P
PCO
L'
CO
L
L
L'
L
L
L'
L'
L
trigonal (Bailar) twisttransition state
rhombic (Ray-Dutt) twisttransition state
trigonal (Bailar) twisttransition state
rhombic (Ray-Dutt) twisttransition state
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Design of synthetic catalystsDesign of synthetic catalysts
• Easy H2 formation from Fe(II)Fe(I)-H species (terminal-H)
• In synthetic complexes (and in the isolated cofactor): Isomerization of Fe(II)Fe(II) terminal-H to -H coordination compounds is thermodinamically favoured...
• ... is it always kinetically unhindered?
• Do we really need Fe(I)Fe(I) like this:
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Electrocatalytic HElectrocatalytic H2 2 productionproduction
1 = Fe(I)Fe(I) redox state
Borg S, Behrsing T, Best S, Razavet M, Liu X, Pickett C, J. Am. Chem. Soc., 2004, 126, 16988
kf=104
kf=4
FeS
Fe
S CO
COOC
OC
OC
CO
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Intermediates in the electrocatalytic HIntermediates in the electrocatalytic H22 production production
Fe
FeC
OC
O
CO
CO
SSC
OCO
H
Fe
FeC
OC
O
CO
CO
SSC
OCO
HH?
Transient species
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The DFT structure of the The DFT structure of the -CO species-CO species
Methodology: BP-86/TZVP, vibrational analysis (harmonic approximation)
Fe
HS
Fe
S CO
COOC
CO
OCO
H
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DFT characterization of intermediate catalytic DFT characterization of intermediate catalytic species: 1Hspecies: 1H-- and 1H and 1H22
1-H and 1-H- are -H species:
Protonation of 1-H- leads to an intermediate species featuring two hydrogen atoms coordinated to the two iron centres:
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Another example of a catalyst designed for HAnother example of a catalyst designed for H22
productionproduction
-pdt)Fe2(CO)5P(NC4H8)3
Hou J, Peng X, Zhou Z, Sun S, Zhao X, Gao S, J. Org. Chem., 2006, 71, 4633
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Transient formation of a -CO species during turnover (IR absorption at 1768 cm-1)
Exp. characterization of intermediate speciesExp. characterization of intermediate species
Possible formation of an intermediate species (2B) resembling the structure observed in the enzymatic cofactor?
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DFT characterization of intermediate speciesDFT characterization of intermediate species
b1 and b2 (-CO species) are almost isoenergetic and might coexist in solution. No other isomers could be characterized by DFT
b2b1
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DFT characterization of intermediate speciesDFT characterization of intermediate species
Coexistence of b1 e b2 leads to six non superimposed IR bands (1741, 1846, 1879, 1903, 1914, 1959 cm-1).
(R2 = 0.970)
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DFT characterization of intermediate speciesDFT characterization of intermediate species(protonated intermediates)(protonated intermediates)
a-H a-tH1
Ga-H - a-tH1= 34.7
kJ/mol
b-H1 b-H2Gb-H1 - b-H2= 48.9
kJ/mol
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Therefore…Therefore…
- The P(NC4H8)3 ligand does not lead to -CO species
resembling the H-cluster
- The P(NC4H8)3 does not lead to terminal hydride species
such as those most probably formed in the catalytic cycle of the enzyme
... Because P(NC4H8)3 is too bulky
-pdt)Fe2(CO)5P(NC4H8)3