Selected Research Highlights 2017

Stereoselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins with N-Heterocyclic Silylene–Manganese Catalysts

Yu-Peng Zhou, Zhenbo Mo, Marcel-Philip Luecke, Matthias Driess

Highly stereoselective transfer semi-hydrogenation of alkynes to E-olefins has been achieved by applying N-heter-ocyclic silylene (NHSi)–manganese(II) complexes as superior pre-catalysts, bearing a bis-NHSi pyridine pincer ligand.

The cover describes the “catchy performance” conducted by the efficient silylene crab. It is holding a manganese atom which mediates the magic transformation from a fish (alkyne) to a seahorse (E-olefin).

Chem. Eur. J. 2017 | DOI: 10.1002/chem.201705745

O2 Activation on ceria catalysts –
the importance of substrate crystallographic orientation

Photo © Alessandro Trovarelli/University of Udine

Chengwu Yang, Xiaojuan Yu, Stefan Heissler, Peter Weidler, Alexei Nefedov, Yuemin Wang, Christof Woll, Thomas Kropp, Joachim Paier, and Joachim Sauer

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces.

Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O-vacancies on (111) substrates, but not on the less closely packed surfaces. These observations resolve a long standing debate concerning the location of O-vacancies on ceria surfaces and the activation of O2 on ceria powders.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201709199

A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand

Ines Monte Perez, Xenia Engelmann, Yong-Min Lee, Mi Yoo, Elumalai Kumaran, Erik R. Farquhar, Eckhard Bill, Jason England, Wonwoo Nam, Marcel Swart, and Kallol Ray

The sluggish oxidants [FeIV(O)(TMC)(CH3CN)]2+ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeIV(O)(TMCN-d12)(OTf)]+ (TMCN-d12=1,4,7,11-tetra(methyl-d3)-1,4,7,11-tetraazacyclotetradecane) are transformed into the highly reactive oxidant [FeIV(O)(TMCO)(OTf)]+ (1; TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom.

A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1. This rationalizes nature's preference for using O-rich ligand environments for the hydroxylation of strong C−H bonds in enzymatic reactions.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201707872

Temperature dependence of the catalytic two- versus four-electron reduction of dioxygen by a hexanuclear cobalt complex

Ines Monte Perez, Subrata Kundu, Anirban Chandra, Kathryn E. Craigo, Petko Chernev, Uwe Kuhlmann, Holger Dau, Peter Hildebrandt, Claudio Greco, Casey Van Stappen, Nicolai Lehnert, and Kallol Ray

The synthesis and characterization of a hexanuclear cobalt complex involving a nonheme ligand system, supported on a Sn6O6 stannoxane core are reported. This complex acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e/4H+ dioxygen-reduction (to water) to a 2e/2H+ process (to hydrogen peroxide) only by increasing the temperature from −50 to 25 °C.

A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV–vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of this complex.

The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to this complex an entropically unfavorable process.

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J. Am. Chem. Soc. 2017, 139, 15033-042 | DOI: 10.1021/jacs.7b07127

Structural insights into the light-driven auto-assembly process of the water-oxidizing Mn4CaO5-cluster in photosystem II

Mn-depleted site. Apo-WOC: Distances (Å) between O/N atoms which are likely connected by H-bonds.

Miao Zhang, Martin Bommer, Ruchira Chatterjee, Rana Hussein, Junko Yano, Holger Dau, Jan Kern, Holger Dobbek, Athina Zouni

In plants, algae and cyanobacteria, Photosystem II (PSII) catalyzes the light-driven splitting of water at a protein-bound Mn4CaO5-cluster, the water-oxidizing complex (WOC). In the photosynthetic organisms, the light-driven formation of the WOC from dissolved metal ions is a key process because it is essential in both initial activation and continuous repair of PSII. Structural information is required for understanding of this chaperone-free metal-cluster assembly. For the first time, we obtained a structure of PSII from Thermosynechococcus elongatus without the Mn4CaO5-cluster.

Surprisingly, cluster-removal leaves the positions of all coordinating amino acid residues and most nearby water molecules largely unaffected, resulting in a pre-organized ligand shell for kinetically competent and error-free photo-assembly of the Mn4CaO5-cluster. First experiments initiating (i) partial disassembly and (ii) partial re-assembly after complete depletion of the Mn4CaO5-cluster agree with a specific bi-manganese cluster, likely a di-µ-oxo bridged pair of Mn(III) ions, as an assembly intermediate.

eLife 2017; 6:e26933 | DOI: 10.7554/eLife.26933

From a Molecular 2Fe-2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting

Chakadola Panda, Prashanth W. Menezes, Carsten Walter, Shenglai Yao, Matthias E. Miehlich, Vitaly Gutkin, Karsten Meyer, Matthias Driess

A highly active FeSe2 electrocatalyst for durable overall water splitting was prepared from a molecular 2Fe-2Se precursor. The as-synthesized FeSe2 was electrophoretically deposited on nickel foam and applied to the oxygen and hydrogen evolution reactions (OER and HER, respectively) in alkaline media.

When used as an oxygen-evolution electrode, a low 245 mV overpotential was achieved at a current density of 10 mA cm−2, representing outstanding catalytic activity and stability because of Fe(OH)2/FeOOH active sites formed at the surface of FeSe2.

Remarkably, the system is also favorable for the HER. Moreover, an overall water-splitting setup was fabricated using a two-electrode cell, which displayed a low cell voltage and high stability.

In summary, the first iron selenide material is reported that can be used as a bifunctional electrocatalyst for the OER and HER, as well as overall water splitting.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201706196
Hot paper, first published online on 19 July 2017.

Redox-dependent substrate-cofactor interactions in the Michaelis-complex of a flavin-dependent oxidoreductase

Tobias Werther, Stefan Wahlefeld, Johannes Salewski, Uwe Kuhlmann, Ingo Zebger, Peter Hildebrandt & Holger Dobbek

How an enzyme activates its substrate for turnover is fundamental for catalysis but incompletely understood on a structural level. With redox enzymes one typically analyses structures of enzyme–substrate complexes in the unreactive oxidation state of the cofactor, assuming that the interaction between enzyme and substrate is independent of the cofactors oxidation state.

Here, we investigate the Michaelis complex of the flavoenzyme xenobiotic reductase A with the reactive reduced cofactor bound to its substrates by X-ray crystallography and resonance Raman spectroscopy and compare it to the non-reactive oxidized Michaelis complex mimics. We find that substrates bind in different orientations to the oxidized and reduced flavin, in both cases flattening its structure. But only authentic Michaelis complexes display an unexpected rich vibrational band pattern uncovering a strong donor–acceptor complex between reduced flavin and substrate. This interaction likely activates the catalytic ground state of the reduced flavin, accelerating the reaction within a compressed cofactor–substrate complex.

Nature Communications 2017, 8, 16084 | DOI: 10.1038/ncomms16084

Insights into trans-Ligand and Spin-Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition-Metal Complexes

Anja H. Greif, Peter Hrobárik, Martin Kaupp

Surprisingly general effects of trans ligands L on the ligand NMR shifts in third-row transition-metal complexes have been found by quasi-relativistic computations, encompassing 5d10, 5d8, and to some extent even 5d6 situations. Closer analysis, with emphasis on 1H shieldings in a series of linear HAulLq complexes, reveals a dominance of spin-orbit (SO) effects, which can change sign from appreciably shielding for weak trans ligands to appreciably deshielding for ligands with strong trans influence.

This may be traced back to increasing destabilization of a σ-type MO at scalar relativistic level, which translates into very different σ-/ π-mixing if SO coupling is included. For the strongest trans ligands, the σ-MO may move above the highest occupied π-type MOs, thereby dramatically reducing strongly shielding contributions from predominantly π-type spinors. The effects of SO-mixing are in turn related to angular momentum admixture from atomic spinors at the metal center. These SOinduced trends hold for other nuclei and may also be used to qualitatively predict shifts in unknown complexes.

Hot Paper in Chemistry A European Journal 2017, 23, 1-15 | DOI: 10.1002/chem.201700844

Reversible light-dependent molecular switches on Ag/AgCl nanostructures

W. Song, C. J. Querebillo, R. Götz, S. Katz, U. Kuhlmann, U. Gernert, I. M. Weidinger, P. Hildebrandt

Nanostructured Ag/AgCl substrates were used to generate reversible and highly efficient light-dependent chemical switches based on adsorbed 4,4’-dimercaptoazobenzene (DMAB). DMAB was formed in situ via laser-induced dimerization either from 4-nitrothiophenol (4-NTP) or 4-aminothiophenol (4-ATP). The subsequent reaction pathways of DMAB, however, were quite different as monitored by surface enhanced Raman spectroscopy. In the 4-NTP/DMAB system, AgCl catalyses the reversal of the dimerization.

Conversely, irradiation of adsorbed 4-ATP first generated cis-DMAB attached to the surface via two Ag–S bonds, followed by AgCl-catalysed cleavage of one Ag–Sbondand cis → trans photoisomerisation of DMAB. In the dark, the trans-isomer thermally reverts to cis-DMAB. The here presented light–dark chemical switches, which work without changing other parameters (e.g.,pH, anaerobic vs. aerobic), are based on the (photo)catalytic properties of the Ag/AgCl substrate and do not function on pure metal surfaces.

Nanoscale 2017, 24, 8380-8387 | DOI: 10.1039/C7NR02760E

Carbon Monoxide Dehydrogenase Reduces Cyanate to Cyanide

Alexandre Ciaccafava, Daria Tombolelli, Lilith Domnik, Jae-Hun Jeoung, Holger Dobbek, Maria-Andrea Mroginski, Ingo Zebger, Peter Hildebrandt

The biocatalytic function of carbon monoxide dehydrogenase (CODH) has a high environmental relevance owing to its ability to reduce CO2. Despite numerous studies on CODH over the past decades, its catalytic mechanism is not yet fully understood. In the present combined spectroscopic and theoretical study, we report first evidences for a cyanate (NCO-) to cyanide (CN-) reduction at the C-cluster.

The adduct remains bound to the catalytic center to form the socalled CN--inhibited state. Notably, this conversion does not occur in crystals of the Carboxydothermus hydrogenoformans CODH enzyme (CODHIICh), as indicated by the lack of the corresponding CN- stretching mode.

The transformation of NCO-, which also acts as an inhibitor of the two-electronreduced Cred2 state of CODH, could thus mimic CO2 turnover and open new perspectives for elucidation of the detailed catalytic mechanism of CODH.

Angew. Chem. Int. Ed. 2017, 56, 7398-7401 | DOI: 10.1002/anie.201703225

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal-Free Dehydrogenation of Water

Zhenbo Mo, Tibor Szilvási, Yu-Peng Zhou, Shenglai Yao, Matthias Driess

The first single-component N-heterocyclic silylene borane 1 (LSi-R-BMes2; L=PhC(NtBu)2; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me3C6H2), acting as a frustrated Lewis pair (FLP) in small-molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character.

In small-molecule activation 1 allows access to the intramolecular silanone–borane 3 featuring a Si=O→B interaction through reaction with O2, N2O, or CO2, and formation of silanethione borane 4 from reaction with S8. The SiII center in 1 undergoes immediate hydrogenation if exposed to H2 at 1 atm pressure in benzene, affording the silane borane 5-H2, L(H2)Si-R-BMes2.

Remarkably, no H2 activation occurs if the single silylene LSiPh and Mes3B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre-organized Si–B separation in 1 enables a metal-free dehydrogenation of H2O to give the silanone–borane 3 as reactive intermediate.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201700625
Published online on 27 February 2017.

Activation of Dioxygen at a Lewis Acidic Nickel(II) Complex: Characterization of a Metastable Organoperoxide Complex

Patrick Holze, Teresa Corona, Nicolas Frank, Beatrice Braun-Cula, Christian Herwig, Anna CompanyChristian Limberg

In metal-mediated O2 activation, nickel(II) compounds hardly play a role, but recently it has been shown that enzymes can use nickel(II) for O2 activation. Now a low-coordinate Lewis acidic nickel(II) complex has been synthesized that reacts with O2 to give a nickel(II) organoperoxide, as proposed for the enzymatic system. Its formation was studied further by UV/Vis absorption spectroscopy, leading to the observation of a short-lived intermediate that proved to be reactive in both oxygen atom transfer and hydrogen abstraction reactions, while the peroxide efficiently transfers O atoms. Both for the enzyme and for the functional model, the key to O2 activation is proposed to represent a concomitant electron shift from the substrate/co-ligand.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201609526
Published online on 23 January 2017.

Discovery and Investigation of Natural Editing Function against Artificial Amino Acids in Protein Translation

Jan-Stefan Völler, Morana Dulic, Ulla I. M. Gerling-Driessen, Hernan Biava, Tobias Baumann, Nediljko Budisa, Ita Gruic-Sovulj, Beate Koksch

Fluorine being not substantially present in the chemistry of living beings is an attractive element in tailoring novel chemical, biophysical, and pharmacokinetic properties of peptides and proteins. The hallmark of ribosome-mediated artificial amino acid incorporation into peptides and proteins is a broad substrate tolerance, which is assumed to rely on the absence of evolutionary pressure for efficient editing of artificial amino acids.

We used the well-characterized editing proficient isoleucyl-tRNA synthetase (IleRS) from Escherichia coli to investigate the crosstalk of aminoacylation and editing activities against fluorinated amino acids. We show that translation of trifluoroethylglycine (TfeGly) into proteins is prevented by hydrolysis of TfeGly-tRNAIle in the IleRS post-transfer editing domain. The remarkable observation is that dissociation of TfeGly-tRNAIle from IleRS is significantly slowed down. This finding is in sharp contrast to natural editing reactions by tRNA synthetases wherein fast editing rates for the noncognate substrates are essential to outcompete fast aa-tRNA dissociation rates.

Using a post-transfer editing deficient mutant of IleRS (IleRSAla10), we were able to achieve ribosomal incorporation of TfeGly in vivo. Our work expands the knowledge of ribosome-mediated artificial amino acid translation with detailed analysis of natural editing function against an artificial amino acid providing an impulse for further systematic investigations and engineering of the translation and editing of unusual amino acids.

ACS Cent. Sci. 2017, 3 (1), 73-80 | DOI: 10.1021/acscentsci.6b00339

Published online on 23 December 2016.

An S-Oxygenated [NiFe] Complex Modelling Sulfenate Intermediates of an O2-Tolerant Hydrogenase

Nils J. Lindenmaier, Stefan Wahlefeld, Eckhard Bill, Tribor Szilvási, Christopher Eberle, Shenglai Yao, Peter Hildebrandt, Marius Horch, Ingo Zebger, Matthias Driess

To understand the molecular details of O2-tolerant hydrogen cycling by a soluble NAD+-reducing [NiFe] hydrogenase, we herein present the first bioinspired heterobimetallic S-oxygenated [NiFe] complex as a structural and vibrational spectroscopic model for the oxygen-inhibited [NiFe] active site.

This compound and its non-S-oxygenated congener were fully characterized, and their electronic structures were elucidated in a combined experimental and theoretical study with emphasis on the bridging sulfenato moiety. Based on the vibrational spectroscopic properties of these complexes, we also propose novel strategies for exploring S-oxygenated intermediates in hydrogenases and similar enzymes.

Angew. Chem. Int. Ed. 2017 | DOI: 10.1002/anie.201611069