Surprising Condensation Reactions of the Azadithiolate Cofactor

Azadithiolate, a cofactor found in all [FeFe]-hydrogenases, is shown to undergo acid-catalyzed rearrangement. Fe₂ [(SCH₂ )₂ NH](CO)₆ self-condenses to give Fe₆ [(SCH₂ )₃ N]₂ (CO)₁₇ . The reaction, which is driven by loss of NH₄ ⁺ , illustrates the exchange of the amine group. X-ray crystallography reveals that three Fe₂ (SR)₂ (CO)_x butterfly subunits interconnected by the aminotrithiolate [N(CH₂ S)₃ ]^3- . Mechanistic studies reveal that Fe₂ [(SCH₂ )₂ NR](CO)₆ participate in a range of amine exchange reactions, enabling new methodologies for modifying the adt cofactor. Ru₂ [(SCH₂ )₂ NH](CO)₆ also rearranges, but proceeds further to give derivatives with Ru-alkyl bonds Ru₆ [(SCH₂ )₃ N][(SCH₂ )₂ NCH₂ ]S(CO)₁₇ and [Ru₂ [(SCH₂ )₂ NCH₂ ](CO)₅ ]₂ S.

Diversifying Metal-Ligand Cooperative Catalysis in Semi-Synthetic [Mn]-Hydrogenases

The reconstitution of [Mn]-hydrogenases using a series of MnI complexes is described. These complexes are designed to have an internal base or pro-base that may participate in metal-ligand cooperative catalysis or have no internal base or pro-base. Only MnI complexes with an internal base or pro-base are active for H2 activation; only [Mn]-hydrogenases incorporating such complexes are active for hydrogenase reactions. These results confirm the essential role of metal-ligand cooperation for H2 activation by the MnI complexes alone and by [Mn]-hydrogenases. Owing to the nature and position of the internal base or pro-base, the mode of metal-ligand cooperation in two active [Mn]-hydrogenases is different from that of the native [Fe]-hydrogenase. One [Mn]-hydrogenase has the highest specific activity of semi-synthetic [Mn]- and [Fe]-hydrogenases. This work demonstrates reconstitution of active artificial hydrogenases using synthetic complexes differing greatly from the native active site.

Enhanced Photocatalytic Hydrogen Production by Hybrid Streptavidin-Diiron Catalysts

Hybrid protein-organometallic catalysts are being explored for selective catalysis of a number of reactions, because they utilize the complementary strengths of proteins and of organometallic complex. Herein, we present an artificial hydrogenase, StrepH2, built by incorporating a biotinylated [Fe-Fe] hydrogenase organometallic mimic within streptavidin. This strategy takes advantage of the remarkable strength and specificity of biotin-streptavidin recognition, which drives quantitative incorporation of the biotinylated diironhexacarbonyl center into streptavidin, as confirmed by UV/Vis spectroscopy and X-ray crystallography. FTIR spectra of StrepH2 show characteristic peaks at shift values indicative of interactions between the catalyst and the protein scaffold. StrepH2 catalyzes proton reduction to hydrogen in aqueous media during photo- and electrocatalysis. Under photocatalytic conditions, the protein-embedded catalyst shows enhanced efficiency and prolonged activity compared to the isolated catalyst. Transient absorption spectroscopy data suggest a mechanism for the observed increase in activity underpinned by an observed longer lifetime for the catalytic species Fe^I Fe⁰ when incorporated within streptavidin compared to the biotinylated catalyst in solution.