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Hindered rotation?

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?.

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Hindered rotation?

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  1. 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 -Hspecies take place into the protein? • Hindered rotation? Relevance of studies of protonation regiochemistry in synthetic models! • Brest laboratory • Illinois laboratory

  2. [FeFe]-hydrogenases models and catalysis. Formation of synthetic Fe(II)Fe(II)-H- species • 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

  3. Protonation of synthetic models of the [2Fe]H cluster. DFT results. • (dppv)(CO)Fe(edt)Fe(PMe3)(CO)2, Fe(I)Fe(I) redox statedppv = cis-1,2-C2H2(PPh2)2 • Stereo-electronic similarity to [2Fe]H • possibility to verify theoretical predictions (Illinois, Brest) J-F Capon, F Gloaguen, F Y Petillon, P Schollhammer, J Talarmin 2009, 253, 1476-1494

  4. Protonation regiochemistry Reaction with triflic acid in acetonitrile: looking for transition states and intermediate species

  5. 15.2 -5.8 -28.5 10.7 -0.6 2 1 3 4 Reaction Coordinate Protonation of synthetic models of the [2Fe]H cluster. DFT results. E (kcal/mol) • Kinetic control: terminal-H • Thermodynamic control: -H

  6. Protonation of synthetic models of the [2Fe]H 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! S Ezzaher, J-F Capon, F Gloaguen, F Y Petillon, P Schollhammer, J Talarmin 2009, 48, 2-4

  7. Protonation of synthetic models of the [2Fe]H cluster. Proximal or distal protonation?

  8. Protonation of synthetic models of the [2Fe]H cluster. Extending the series a. The reaction product does not correspond to an energy minimum structure and evolves back to reactant (the FeFe complex + triflic acid).

  9. 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

  10. Interconversion from terminal- to -H 3 Int Int 4 Pseudo C3 rotations

  11. Interconversion from terminal- to -H:Pseudo C3 rotations E (kcal/mol) -5.6 6.2 3 Int 15.8 -28.5 Int 3 4 Int 4 Reaction Coordinate

  12. Design 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:

  13. Electrocatalytic H2 production kf=4 kf=104 Fe(I)Fe(I) redox state 1 = Borg S, Behrsing T, Best S, Razavet M, Liu X, Pickett C, J. Am. Chem. Soc., 2004, 126, 16988

  14. H CO CO H S S H S S Fe Fe Fe Fe CO CO CO CO CO CO CO CO C O C O Intermediates in the electrocatalytic H2 production Transient species ?

  15. The DFT structure of the -CO species Methodology: BP-86/TZVP, vibrational analysis (harmonic approximation)

  16. DFT characterization of intermediate catalytic species: 1H- and 1H2 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:

  17. Another example of a catalyst designed for H2 production -pdt)Fe2(CO)5P(NC4H8)3 Hou J, Peng X, Zhou Z, Sun S, Zhao X, Gao S, J. Org. Chem., 2006, 71, 4633

  18. Exp. characterization of intermediate species Transient formation of a -CO species during turnover (IR absorption at 1768 cm-1) Possible formation of an intermediate species (2B) resembling the structure observed in the enzymatic cofactor?

  19. DFT characterization of intermediate species b1 b2 b1 and b2 (-CO species) are almost isoenergetic and might coexist in solution. No other isomers could be characterized by DFT

  20. DFT 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)

  21. DFT characterization of intermediate species (protonated intermediates) Gb-H1 - b-H2= 48.9 kJ/mol b-H1 b-H2 a-H a-tH1 Ga-H - a-tH1= 34.7 kJ/mol

  22. 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 -pdt)Fe2(CO)5P(NC4H8)3 ... Because P(NC4H8)3 is too bulky

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