Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)n(CO)5-nX] (n = 0, 2, 4; R = H, Me, Et; X = CH3S-, PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H+ + 2e- 虠 H2 reaction. Our study is focused on the reduction step following protonation of the Fe2(SR)2 core. Fe(H)s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (E°μ-H > E°t-H). The nature and number of PR3 unusually affect ΔE°t-H-μ-H: 4PEt3 models feature a μ-H with a milder E° than t-H, whereas the 4PMe3 analogues behave oppositely. The correlation ΔE°t-H-μ-H 虠 stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt3 derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe2 core. That implies that E°μ-H > E°t-H in 4-PEt3 species but not in 4PMe3 analogues. Also determinant to observe E°μ-H > E°t-H is the presence of a Fe apical σ-donor because its replacement with a CO yields E°μ-H < E°t-H even in 4PEt3 species. Variants with neutral NHC and PMe3 in place of CH3S- still feature E°μ-H > E°t-H. Replacing pdt with (Hadt)+ lowers E° but yields E°μ-H < E°t-H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe2 core influences ΔE°t-H-μ-H, provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.

Filippi, G., Arrigoni, F., Bertini, L., DE GIOIA, L., Zampella, G. (2015). DFT Dissection of the Reduction Step in H2 Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?. INORGANIC CHEMISTRY, 54(19), 9529-9542 [10.1021/acs.inorgchem.5b01495].

DFT Dissection of the Reduction Step in H2 Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?

FILIPPI, GIULIA
Primo
;
ARRIGONI, FEDERICA
Secondo
;
BERTINI, LUCA;DE GIOIA, LUCA
Penultimo
;
ZAMPELLA, GIUSEPPE
2015

Abstract

Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)n(CO)5-nX] (n = 0, 2, 4; R = H, Me, Et; X = CH3S-, PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H+ + 2e- 虠 H2 reaction. Our study is focused on the reduction step following protonation of the Fe2(SR)2 core. Fe(H)s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (E°μ-H > E°t-H). The nature and number of PR3 unusually affect ΔE°t-H-μ-H: 4PEt3 models feature a μ-H with a milder E° than t-H, whereas the 4PMe3 analogues behave oppositely. The correlation ΔE°t-H-μ-H 虠 stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt3 derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe2 core. That implies that E°μ-H > E°t-H in 4-PEt3 species but not in 4PMe3 analogues. Also determinant to observe E°μ-H > E°t-H is the presence of a Fe apical σ-donor because its replacement with a CO yields E°μ-H < E°t-H even in 4PEt3 species. Variants with neutral NHC and PMe3 in place of CH3S- still feature E°μ-H > E°t-H. Replacing pdt with (Hadt)+ lowers E° but yields E°μ-H < E°t-H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe2 core influences ΔE°t-H-μ-H, provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.
Articolo in rivista - Articolo scientifico
Inorganic Chemistry; Physical and Theoretical Chemistry; enzymes; DFT; hydrogen; hydrogenase; iron; complex; reduction potential; dithiolate
English
9529
9542
14
Filippi, G., Arrigoni, F., Bertini, L., DE GIOIA, L., Zampella, G. (2015). DFT Dissection of the Reduction Step in H2 Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?. INORGANIC CHEMISTRY, 54(19), 9529-9542 [10.1021/acs.inorgchem.5b01495].
Filippi, G; Arrigoni, F; Bertini, L; DE GIOIA, L; Zampella, G
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/97059
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