Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties

Abstract

The synthesis and electrochemical properties of tetranuclear [Fe₂S₂]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe₂S₂] complex containing a 5,5'-diisocyanide-2,2'-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe₂S₂] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe₂S₂] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe₂S₂] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe₂S₂] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF₄ salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental-computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

Figure 1

Examples of tetranuclear (1, 3, 4, and 5) and hexanuclear (2) iron–sulfur complexes.

Figure 2

Strategy to incorporate metal complexes into a tetranuclear [Fe₂S₂]₂-complex.

Scheme 1. Synthesis of the Tetranuclear [Fe₂S₂] Complex 6

Figure 3

X-ray thermal ellipsoid plot of 6 (50% probability level) with the labeling scheme. Selected bond lengths (Å) and angles (°): Fe(1)-C(7) 1.786(16), Fe(1)-S(1) 2.2718(15), Fe(1)-S(2) 2.2768(13), Fe(1)-Fe(2) 2.4651(9), Fe(2)-S(1) 2.2625(13), Fe(2)-S(2) 2.2793(14), S(1)-C(21) 1.785(5), S(2)-C(22) 1.789(5), C(7)-N(7) 1.171(18), N(7)-C(5) 1.396(16), C(15)-N(17) 1.406(15), N(17)-C(17) 1.172(17), C(7)-Fe(1)-S(1) 90.9(7), C(7)-Fe(1)-S(2) 152.7(5), S(1)-Fe(1)-S(2) 80.39(5), C(7)-Fe(1)-Fe(2) 96.3(6), S(1)-Fe(2)-S(2) 80.54(5), Fe(2)-S(1)-Fe(1) 65.87(4), Fe(1)-S(2)-Fe(2) 65.51(4), N(7)-C(7)-Fe(1) 177(3), C(7)-N(7)-C(5) 172(3).

Scheme 2. Synthesis of [FeFe]-Pt (10) and [FeFe]-Ni (11) complexes

Scheme 3. Synthesis of the [FeFe]-Ru Compound (12)

Figure 4

Cyclic voltammograms of compound 6 (10^–3 M) in CH₂Cl₂ (blue line) and MeCN (orange line; intensity multiplication factor = 4) solutions containing 10^–1 M [NBu₄]PF₆ as supporting electrolyte at 25 °C. Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc⁺/Fc; scan rate: 100 mV/s.

Figure 5

Consecutive two-electron reductions of 6 showing the structural changes in the [Fe₂S₂] complexes: (a) dianion 6^2–; (b) and (c) HOMO and LUMO orbitals of 6^2–; (d) tetraanion 6^4– (computed at the SMD(CH₂Cl₂)-B3LYP-D3/def2-SVP level). Distances in Å. Isosurface value, 0.04.

Figure 6

Cyclic voltammograms of compounds 10a–10c (10^–3 M) in CH₂Cl₂ solutions containing 10^–1 M [NBu₄]PF₆ as supporting electrolyte at 25 °C. Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc⁺/Fc; scan rate: 100 mV/s.

Figure 7

Cyclic voltammogram of [Pt(dppe)(bpy)][PF₆] (13) (10^–3 M, CH₂Cl₂, 10^–1 M [NBu₄]PF₆ as supporting electrolyte at 25 °C). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Ag/AgCl; scan rate: 100 mV/s.

Figure 8

Changes of the structure of the Pt(II) complex during the reduction of the [FeS₂] moieties in species 10a^2–: (a) 10a^4–; (b) 10a^6–. Computed at the SMD(CH₂Cl₂)-B3LYP-D3/def2-SVP level. Distances in Å. H atoms omitted for clarity.

Figure 9

Cyclic voltammogram of compound 12 (10^–3 M in CH₃CN, containing 10^–1 M [NBu₄]PF₆ as supporting electrolyte at 25 °C). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc⁺/Fc; scan rate: 100 mV/s. The CV of 12 was also recorded in CH₂Cl₂. However, the shorter solvent window associated with this solvent, prevented the observation of all the electrochemical processes. See Figure S2 for more details.

Figure 10

Computed structures of complex 12 (SMD(CH₂Cl₂)-B3LYP-D3/def2-SVP level) showing the changes in the structure of the [Fe₂S₂] complexes upon reduction. (a) 12^4–; (b) 12^6–; (c) 12^8–. Distances in Å. H atoms omitted for clarity.

Figure 11

Electrochemical response of a CH₂Cl₂ solution of 6 (10 ^–3 M) in the presence of increasing amounts of acetic acid (0–20 equiv). Cyclic voltammograms registered at 25 °C. Supporting electrolyte: [NBu₄]PF₆ (10^–1 M). Counter-electrode: Pt; working electrode: glassy carbon; potential given in V vs Fc⁺/Fc; scan rate: 100 mV/s.