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

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.