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)

Abstract

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.

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] Related to the Active Site of [FeFe] Hydrogenases

“…All the electrochemical studies were carried out in a conventional three‐electrode cell under an inert atmosphere of either N 2 , Ar, or CO, by using Schlenk techniques for the syntheses. The preparation and the purification of the supporting electrolyte [NBu 4 ][PF 6 ] were as described previously . The working electrode was a vitreous carbon disk 0.3 cm in diameter, polished with alumina prior to use.…”

“…The preparation and the purification of the supporting electrolyte [NBu 4 ][PF 6 ]w ere as described previously. [37] The working electrode was av itreous carbon disk 0.3 cm in diameter, polished with alumina prior to use. Ap latinum wire was used as the counter electrode.…”

Influence of the Dithiolate Bridge on the Oxidative Processes of Diiron Models Related to the Active Site of [FeFe] Hydrogenases

“…The preparation and the purification of the supporting electrolyte [NBu 4 ][PF 6 ]w ere conducted as described previously. [31] The working electrode was av itreous carbon disk of 0.3 cm in diameter,p olished with alumina prior to use. Ap latinum wire was used as counter electrode.…”

H2 Activation in [FeFe]‐Hydrogenase Cofactor Versus Diiron Dithiolate Models: Factors Underlying the Catalytic Success of Nature and Implications for an Improved Biomimicry

“…[11] Sun group, in fact, introduced aP NP ligand (Ph 2 PCH 2 N(nPr)CH 2 PPh 2 )c oordinating as ingle iron atom in k 2 fashion (Scheme 4, right), so that the pendant amine can get in close proximity to the FeÀFe region during the expected stereodynamics of the six-membered ring [Fe-(PR 2 ) 2 -(CH 2 ) 2 -NR], as trategy previously used by Schollhammer and co-workers for catalytic H + reduction. [58] The possibility of coming into contact with the Fe 2 region is important because of the formation of metal-metal-bridging hydrides( m-H) during catalysis. Such hydride formationi su sually detrimentalt oa ne fficient turnover (at least in Fe 2 S 2 models): Therefore, the accessibility of the FeÀFe region by the base provides kinetic advantage, by facilitating the reductivee limination of H + from the Fe 2 core (see Ref.…”