Electrochemical and theoretical investigations of the role of the appended base on the reduction of protons by [Fe2(CO)4(κ2-PNP(R)(μ-S(CH2)3S] (PNP(R) ={Ph2PCH2}2NR, R=Me, Ph).
The behavior of [Fe(2)(CO)(4)(κ(2)-PNP(R))(μ-pdt)] (PNP(R) =(Ph(2)PCH(2))(2)NR, R=Me (1), Ph (2); pdt=S(CH(2))(3)S) in the presence of acids is investigated experimentally and theoretically (using density functional theory) in order to determine the mechanisms of the proton reduction steps supported by these complexes, and to assess the role of the PNP(R) appended base in these processes for different redox states of the metal centers. The nature of the R substituent of the nitrogen base does not substantially affect the course of the protonation of the neutral complex by CF(3)SO(3)H or CH(3)SO(3)H; the cation with a bridging hydride ligand, 1 μH(+) (R=Me) or 2 μH(+) (R=Ph) is obtained rapidly. Only 1 μH(+) can be protonated at the nitrogen atom of the PNP chelate by HBF(4)·Et(2)O or CF(3)SO(3)H, which results in a positive shift of the proton reduction by approximately 0.15 V. The theoretical study demonstrates that in this process, dihydrogen can be released from a η(2)-H(2) species in the Fe(I)Fe(II) state. When R=Ph, the bridging hydride cation 2 μH(+) cannot be protonated at the amine function by HBF(4)·Et(2)O or CF(3)SO(3)H, and protonation at the N atom of the one-electron reduced analogue is also less favored than that of a S atom of the partially de-coordinated dithiolate bridge. In this situation, proton reduction occurs at the potential of the bridging hydride cation, 2 μH(+). The rate constants of the overall proton reduction processes are small for both complexes 1 and 2 (k(obs) ≈4-7 s(-1)) because of the slow intramolecular proton migration and H(2) release steps identified by the theoretical study.
UMR CNRS 6521, Chimie, Electrochimie Moléculaires et Chimie Analytique, Université de Bretagne Occidentale, UFR Sciences et Techniques, CS 93837, Brest, France.
Sondes
S
Lounissi
Giuseppe
G
Zampella
Jean-François
JF
Capon
Luca
L
De Gioia
Fatma
F
Matoussi
Sélim
S
Mahfoudhi
François Y
FY
Pétillon
Philippe
P
Schollhammer
Jean
J
Talarmin
D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D055664:Electrochemical Techniques; D005296:Ferrous Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D006864:Hydrogenase / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D018716:Molecular Mimicry; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics
D005296:Ferrous Compounds; D007506:Iron-Sulfur Proteins; D008024:Ligands; D011522:Protons; D006859:Hydrogen; D006864:Hydrogenase; D009584:Nitrogen