[FeFe]-hydrogenases are metalloenzymes that reversibly reduce protons to molecular hydrogen at exceptionally high rates. We have characterized the catalytically competent hydride state (Hhyd) in the [FeFe]-hydrogenases from both Chlamydomonas reinhardtii and Desulfovibrio desulfuricans using 57Fe Nuclear Resonance Vibrational Spectroscopy (NRVS) and Density Functional Theory (DFT). H/D exchange identified two Fe-H bending modes originating from the binuclear iron cofactor. DFT calculations show that these spectral features result from an iron-bound terminal hydride, with the Fe-H vibrational frequencies being highly dependent on interactions between the amine base of the catalytic cofactor with both hydride and the conserved cysteine terminating the proton transfer chain to the active site. The results indicate that Hhyd is the catalytic state one step prior H2 formation. The observed vibrational spectrum, therefore, provides mechanistic insight into the reaction coordinate for H2 bond formation by [FeFe]-hydrogenases.

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The intermediacy of a reduced nickel–iron hydride in hydrogen evolution catalyzed by Ni–Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1]0; dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1]0 initially produces unsym-[H1]+, which converts by a first-order pathway to sym-[H1]+. These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1]+ protonates at sulfur. The S = 1/2 hydride [H1]0 was generated by reduction of [H1]+ with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1]0 is best described as a Ni(I)–Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1]0. Whereas [H1]+ does not evolve H2 upon protonation, treatment of [H1]0 with acids gives H2. The redox state of the “remote” metal (Ni) modulates the hydridic character of the Fe(II)–H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1]0 and external acid or from protonation of the Fe–H bond in [H1]0 to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1]0 initially produces [1]+, which is reduced by [H1]0. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

Schiff base derivatives of the biometals (Zn, Mg, Ca) in the presence of anion initiators have been shown to be very effective catalysts for the ring-opening polymerization of trimethylene carbonate (TMC or 1,3-dioxan-2-one) to poly(TMC) devoid of oxetane linkages. The order of catalytic activity as a function of metal was found to be Ca(II) . Mg(II) > Zn(II). Optimization of the calcium system was achieved utilizing a salen ligand with tert-butyl substituents in the 3,5-positions of the phenolate rings and an ethylene backbone for the diimine along with an azide ion initiator. These conditions led to a TOF of 1286 h -1 for a melt polymerization carried out at 86 °C. Solution studies in tetrachloroethane demonstrated the polymerization reaction to proceed via a mechanism first order in [monomer], [(salen)Ca], and [anion initiator] and to involve TMC ring-opening by way of acyl-oxygen bond cleavage. The activation parameters were determined to be ∆H q ) 20.1 kJ/mol and ∆S q ) -128 J/(mol K).

The molybdenum cofactor (Moco) is found in the active site of numerous important enzymes that are critical to biological processes. The bidentate ligand that chelates molybdenum (Mo) in Moco is the pyranopterin dithiolene (molybdopterin, MPT); however, neither the mechanism of molybdate insertion into MPT nor the structure of Moco prior to its insertion into pyranopterin molybdenum enzymes is known. Here we report this final maturation step, where adenylated MPT (MPT-AMP) and molybdate are the substrates. X-ray crystallography of the Arabidopsis thaliana Mo-insertase variant Cnx1E S269D D274S identified adenylated Moco (Moco-AMP) as an unexpected intermediate in this reaction sequence. X-ray absorption spectroscopy revealed the first coordination sphere geometry of Moco trapped in the Cnx1E active site. We have used this structural information to deduce a mechanism for molybdate insertion into MPT-AMP. Given their high degree of structural and sequence similarity, we suggest that this mechanism is employed by all eukaryotic Mo-insertases.