Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra-and intermolecular CÀ H bonds.

Complexes [L 2 Fe][Li(DME)] 2 , 1(DME), {[L 2 Fe][Na 2 (DME) 3 ]} 1 , 2(DME) and [L 2 Fe][K(DME) 2 ] 2 , 3(DME) were synthesized by deprotonation of LH 2 (LH 2 = O(SiPh 2 OH) 2 ) with the respective alkali metal tert-butoxides followed by recrystallization from DME. It turned out that upon crossing over from Li + via Na + to K + counterions the structures of the high-spin iron(II) complexes are increasingly distorted from a square planar towards a tetrahedral structure so that 3(DME) represents a borderline case, as indicated by the τ-values. The distortions are also reflected in the Mössbauer spectra through the quadrupole splittings. The compounds behave inert in contact with O atom transfer reagents but react rapidly with dioxygen. The reaction rates are too high to be determined even by stopped-flow measurements quantitatively, but qualitatively it emerged that the rates increase from Li to Na to K. Using NO as an O 2 surrogate an NO adduct with an S = 3/2 ground state was isolated where NO is coordinated in an end-on binding mode, formally as a NO À ligand, with a significantly weakened NO bond.

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[NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H2 into 2 e– and 2 H+ under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)2(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN– molecules. Herein, we investigated the sequential assembly of the [NiFe]-cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O2-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)2(CO) fragment, a precursor that contained all cofactor components but remained redox-inactive, and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, X-ray absorption, and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.

NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H2 into 2 eand 2 H + under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)2(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CNmolecules. Here, we investigated the sequential assembly of the [NiFe]-cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O2-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)2(CO) fragment, a precursor that contained all cofactor components but remained redox-inactive, and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, Xray absorption, and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.