for reversible dihydrogen evolution. H2 turnover involves different catalytic intermediates including a recently characterized hydride state of the active site (H-cluster). Applying cryogenic infrared and electron paramagnetic resonance spectroscopy to an [FeFe] model hydrogenase from Chlamydomonas reinhardtii (CrHydA1), we have discovered two new hydride intermediates and spectroscopic evidence for a bridging CO ligand in two reduced H-cluster states.Our study provides novel insights into these key intermediates, their relevance for the catalytic cycle of [FeFe] hydrogenase, and novel strategies for exploring these aspects in detail.

[FeFe] hydrogenases are the most active H2 converting catalysts in nature, but their extreme oxygen sensitivity limits their use in technological applications. The [FeFe] hydrogenases from sulfate reducing bacteria can be purified in an O2‐stable state called Hinact. To date, the structure and mechanism of formation of Hinact remain unknown. Our 1.65 Å crystal structure of this state reveals a sulfur ligand bound to the open coordination site. Furthermore, in‐depth spectroscopic characterization by X‐ray absorption spectroscopy (XAS), nuclear resonance vibrational spectroscopy (NRVS), resonance Raman (RR) spectroscopy and infrared (IR) spectroscopy, together with hybrid quantum mechanical and molecular mechanical (QM/MM) calculations, provide detailed chemical insight into the Hinact state and its mechanism of formation. This may facilitate the design of O2‐stable hydrogenases and molecular catalysts.

Spectroscopic investigation of an isolated [NiFe]-hydrogenase large subunit enables a unique view of the NiFe(CO)(CN)2 cofactor.

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu P, which does not react with either B(C F ) or silylium ions.

[NiFe] hydrogenases are complex model enzymes for the reversible cleavage of dihydrogen (H2). However, structural determinants of efficient H2 binding to their [NiFe] active site are not properly understood. Here, we present crystallographic and vibrational‐spectroscopic insights into the unexplored structure of the H2‐binding [NiFe] intermediate. Using an F420‐reducing [NiFe]‐hydrogenase from Methanosarcina barkeri as a model enzyme, we show that the protein backbone provides a strained chelating scaffold that tunes the [NiFe] active site for efficient H2 binding and conversion. The protein matrix also directs H2 diffusion to the [NiFe] site via two gas channels and allows the distribution of electrons between functional protomers through a subunit‐bridging FeS cluster. Our findings emphasize the relevance of an atypical Ni coordination, thereby providing a blueprint for the design of bio‐inspired H2‐conversion catalysts.

The catalytic properties of hydrogenases are nature's answer to the seemingly simple reaction H ⇌ 2H + 2e. Members of the phylogenetically diverse subgroup of [NiFe] hydrogenases generally consist of at least two subunits, where the large subunit harbors the H-activating [NiFe] site and the small subunit contains iron-sulfur clusters mediating e transfer. Typically, [NiFe] hydrogenases are susceptible to inhibition by O. Here, we conducted system minimization by isolating and analyzing the large subunit of one of the rare members of the group of O-tolerant [NiFe] hydrogenases, namely the preHoxG protein of the membrane-bound hydrogenase from Ralstonia eutropha. Unlike previous assumptions, preHoxG was able to activate H as it clearly performed catalytic hydrogen/deuterium exchange. However, it did not execute the entire catalytic cycle described for [NiFe] hydrogenases. Remarkably, H activation was performed by preHoxG even in the presence of O, although the unique [4Fe-3S] cluster located in the small subunit and described to be crucial for tolerance toward O was absent. These findings challenge the current understanding of O tolerance of [NiFe] hydrogenases. The applicability of this minimal hydrogenase in basic and applied research is discussed.

Using the chelating C,C′‐bis(silylenyl)‐ortho‐dicarborane ligand, 1,2‐(RSi)2‐1,2‐C2B10H10 [R=PhC(NtBu)2], leads to the monoatomic zero‐valent Ge complex (“germylone”) 3. The redox non‐innocent character of the carborane scaffold has a drastic influence on the reactivity of 3 towards reductants and oxidants. Reduction of 3 with one molar equivalent of potassium naphthalenide (KC10H8) causes facile oxidation of Ge0 to GeI along with a two‐electron reduction of the C2B10 cluster core and subsequent GeI‐GeI coupling to form the dianionic bis(silylene)‐supported Ge2 complex 4. In contrast, oxidation of 3 with one molar equivalent of [Cp2Fe][B{C6H3(CF3)2}4] as a one‐electron oxidant furnishes the dicationic bis(silylene)‐supported Ge2 complex 5. The Ge0 atom in 3 acts as donor towards GeCl2 to form the trinuclear mixed‐valent Ge0→GeII←Ge0 complex 6, from which dechlorination with KC10H8 affords the neutral Ge2 complex 7 as a diradical species.

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi) 2 C 2 B 10 H 10 ; L = PhC(N t Bu) 2 ], reaction with adamantyl azide (AdN 3) affords the terminal N-m 2-bridged zwitterionic carborane-1,2-bis(silylium) AdN 3 adduct 2 with an open-cage dianionic nido-C 2 B 10 cluster core. Remarkably, upon one-electron reduction of 2 with C 8 K and liberation of N 2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized N I complex as an anion of 3 with the nido-C 2 B 10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) N I complex 4 with the closo-C 2 B 10 cluster core. Moreover, the corresponding neutral N I radical complex 5 results from single-electron transfer from 3 to 4.