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. 2020 Aug 12;10(50):29855-29867.
doi: 10.1039/d0ra06002j. eCollection 2020 Aug 10.

Revisiting the photochemical synthesis of [FeFe]-hydrogenase mimics: reaction optimization, mechanistic study and electrochemical behaviour

Sergio Aguado  1 , Luis Casarrubios  1 , Carmen Ramírez de Arellano  2 , Miguel A Sierra  1
Affiliations

Revisiting the photochemical synthesis of [FeFe]-hydrogenase mimics: reaction optimization, mechanistic study and electrochemical behaviour

Sergio Aguado et al. RSC Adv. .

Abstract

The photoreaction of [(μ-S)2Fe2(CO)6] and alkenes or alkynes has been optimized to readily obtain functionalized [FeFe]-hydrogenase mimics. Irradiation under low CO pressure in THF produces the corresponding photo-adducts in good/acceptable (alkenes/alkynes) yields, with retention of the starting olefin stereochemistry. DFT-calculations provide plausible reaction pathways in both, singlet and triplet states. The DFT-calculation based in the singlet state is energetically more favorable. The electrochemical behavior of the synthesized compounds is also presented, including studies in acidic media. The electrochemical properties of the products vary in the presence of a double bond (cycloaddition of [(μ-S)2Fe2(CO)6] to alkynes), respect to a single bond (cycloaddition to alkenes).

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Fig. 1
Fig. 1. Schematic representation of [FeFe]-H2ase active site (I) and their synthetic mimics (II and III).
Scheme 1
Scheme 1. Synthesis of [FeFe]-H2ase synthetic models.
Scheme 2
Scheme 2. Photocycloaddition of 2 with 1- and 2-pentene 3a and 3b.
Scheme 3
Scheme 3. Photochemical reaction between [(μ-S)2Fe2(CO)6] 2 and olefins or alkynes 3. Initial optimizations.
Scheme 4
Scheme 4. Photochemical reaction of [(μ-S)2Fe2(CO)6] with olefins and alkynes. Substrate scope.
Fig. 2
Fig. 2. X-ray thermal ellipsoid plot for compound 4d (50% probability level) showing a C–H⋯OC intramolecular interaction. Selected bond lengths (Å) and angles (°): Fe(1)–Fe(2) 2.4966(3), Fe(1)–C(21) 1.7983(15), Fe(1)–C(22) 1.7990(15), Fe(1)–C(23) 1.8102(15), Fe(1)–S(1) 2.2428(4), Fe(1)–S(2) 2.2473(4), Fe(2)–C(24) 1.7995(16), Fe(2)–C(25) 1.8004(16), Fe(2)–C(26) 1.8077(15), Fe(2)–S(1) 2.2521(4), Fe(2)–S(2) 2.2513(4), S(1)–C(4) 1.8289(4), S(2)–C(3) 1.8434(4), C(3)–C(4) 1.5203(18), S(1)–Fe(1)–S(2) 81.488(14), S(1)–Fe(2)–S(2) 81.197(13), Fe(1)–S(1)–Fe(2) 67.478(12), Fe(1)–S(2)–Fe(2) 67.416(12), C(31)–N(1)–C(2) 119.64(10), C(31), C(12)⋯O(23) 3.310(2) Å; H(12)⋯O(23) 2.45 Å; C(12)–H(12)⋯O(23) 150.7°.
Scheme 5
Scheme 5. Photochemical reaction between [(μ-S)2Fe2(CO)6] and olefins or alkynes. Compatibility of the process with functionalized substrates. Compound 4i was a racemic material, one single enantiomer is represented for simplicity.
Scheme 6
Scheme 6. Photochemical reaction between [(μ-S)2Fe2(CO)6] and olefins or alkynes. Synthesis of polymetallic systems. Compounds 4l, 4m were racemic mixtures. Only one enantiomer is depicted for clarity.
Fig. 3
Fig. 3. Species implied in the photolysis of complex 2 according to Bruce King et al.
Fig. 4
Fig. 4. Cyclic voltammograms (focused on reduction) of selected compounds 4c,d (10−3 M in CH3CN), 10−1 M [N(nBu)4]PF6, counter-electrode: Pt; working electrode: glassy carbon; reference electrode: Ag/AgCl; scan rate: 100 mV s−1; values given in V. For full voltammograms see the ESI.
Scheme 7
Scheme 7. Reaction of complex 2 with methyl propiolate 3g.
Scheme 8
Scheme 8. Reaction of complex 2 with methyl acrylate 3e.
Fig. 5
Fig. 5. Cyclic voltammograms (focused on reduction) of compounds 4e and 4g (10−3 M in CH3CN), 10−1 M [N(nBu)4]PF6, counter-electrode: Pt; working electrode: glassy carbon; reference electrode: Ag/AgCl; scan rate: 100 mV s−1; values given in V.
Fig. 6
Fig. 6. LUMO's of complexes 4e, 4e˙−, 4g and 4g˙−.
Fig. 7
Fig. 7. (Top) Cyclic voltammograms of 4e with added HOAc (0–20 eq.). Data (V) obtained from 10−3 M acetonitrile solutions, containing 0.1 M [N(nBu)4]PF6 as supporting electrolyte at 20 °C. Potentials are relative to Ag/AgCl. (Bottom) Cyclic voltammograms of 4g with added HOAc (0–20 eq.). Data (V) obtained from 10−3 M acetonitrile solutions, containing 0.1 M [N(nBu)4]PF6 as supporting electrolyte at 20 °C. Potentials are relative to Ag/AgCl.
Fig. 8
Fig. 8. (Top) Cyclic voltammograms of 4e with added TFA (0–20 eq. of H+). Data (V) obtained from 10−3 M acetonitrile solutions, containing 0.1 M [N(nBu)4]PF6 as supporting electrolyte at 20 °C. Potentials are relative to Ag/AgCl. (Bottom) Cyclic voltammograms of 4g with added TFA (0–20 eq. of H+). Data (V) obtained from 10−3 M acetonitrile solutions, containing 0.1 M [N(nBu)4]PF6 as supporting electrolyte at 20 °C. Potentials are relative to Ag/AgCl.
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