Bioinspired ligand designs for cobalt, iron and manganese complexes : understanding mono-iron hydrogenase (Hmd)
Mono-iron hydrogenase is one of three types of hydrogenases, catalyzing reversible hydride transfer to the substrate (methenyl-H₄MPT⁺) by heterolytically cleaving molecular hydrogen into a proton and a hydride. The key features of the enzyme’s active site include a pyridone moiety, an acyl unit and facial ligation of C [superscript acyl] N [superscript pyridone] S [superscript Cys] donors. Each feature was independently incorporated into a selected ligand system (Schiff-base N4, pincer and thianthrene scaffold, respectively), in efforts to i) develop possible bioinspired catalysts for H₂ activation using earth abundant metals (iron, cobalt, manganese) and ii) make synthetic models of the enzyme active site for deeper understanding of the architecture of the active site and catalytic mechanism. Synthetic routes for making pyridone-based Schiff-base N4 and NNS type ligands were explored: although not isolated, the ligands were spectroscopically detected. On the other hand, the simpler version—pyridine-based Schiff-base N4 ligands—afforded dinuclear cobalt complexes upon metalations with cobalt(II) precursors. Depending on the length of the diamine-linker as well as the substituents on the pyridine rings, either spontaneous O₂ activation or B–F activation was observed, yielding μ-peroxo dicobalt(III) complexes or μ-fluoride bridged dicobalt(II) complexes, respectively. In particular, two μ-fluoride bridged dicobalt complexes showed antiferromagnetic coupling between the two cobalt(II) centers. From the pincer ligands featuring the unique acyl moiety within C [superscript acyl] N [superscript pyridine] S [superscript thioehter] and C [superscript acyl] N [superscript pyridine] P [superscript Ph2] donor set, the (expected) meridional and an (unexpected) facial iron-acyl complexes were isolated, respectively. Upon deprotonation (pyridine→pyridinate de-aromatization), both complexes showed reactivity towards H₂ activation; no evidence for hydride-transfer was observed. For the facial ligation, thianthrene-scaffolded manganese system was examined as a more flexible version of the anthracene-scaffolded systems. Preliminary results of the kinetic studies support the correlation between the flexibility of the scaffold and the reactivity of the metal complex, without greatly altering the electronic environment of the metal center.