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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 Presentation
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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
[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
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
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
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)
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
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?
-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
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
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
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)
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
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:
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
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
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
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:
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
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?
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
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)
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
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