Publications by authors named "Inke Siewert"

26 Publications

  • Page 1 of 1

Renewable resources for sustainable metallaelectro-catalysed C-H activation.

Chem Sci 2020 Jul 31;11(33):8657-8670. Epub 2020 Jul 31.

Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen Tammannstraße 2 37077 Göttingen Germany

The necessity for more sustainable industrial chemical processes has internationally been agreed upon. During the last decade, the scientific community has responded to this urgent need by developing novel sustainable methodologies targeted at molecular transformations that not only produce reduced amounts of byproducts, but also by the use of cleaner and renewable energy sources. A prime example is the electrochemical functionalization of organic molecules, by which toxic and costly chemicals can be replaced by renewable electricity. Unrivalled levels of resource economy can thereby be achieved the merger of metal-catalyzed C-H activation with electrosynthesis. This perspective aims at highlighting the most relevant advances in metallaelectro-catalysed C-H activations, with a particular focus on the use of green solvents and sustainable wind power and solar energy until June 2020.
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http://dx.doi.org/10.1039/d0sc03578eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8163351PMC
July 2020

A Bioinspired Disulfide/Dithiol Redox Switch in a Rhenium Complex as Proton, H Atom, and Hydride Transfer Reagent.

J Am Chem Soc 2021 Apr 16;143(16):6238-6247. Epub 2021 Apr 16.

Universität Göttingen, Institut für Anorganische Chemie, Tammannstraße 4, D-37077 Göttingen, Germany.

The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e/2H disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH···S moiety in the backbone. The disulfide bond in -[Re(bpy)(CO)Cl] (, bpy = [1,2]dithiino[4,3-:5,6-']dipyridine) undergoes two successive reductions at equal potentials of -1.16 V vs Fc at room temperature forming [Re(bpy)(CO)Cl] (, bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···S unit, H. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (Δ° = 34 kcal mol, p = 24.7), an H atom (Δ° = 59 kcal mol), or a hydride ion (Δ° = 60 kcal mol) as demonstrated in the reactivity with various organic test substrates.
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http://dx.doi.org/10.1021/jacs.1c01763DOI Listing
April 2021

A Stable Homoleptic Divinyl Tetrelene Series.

Chemistry 2021 Jun 14;27(33):8572-8579. Epub 2021 May 14.

Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr., Edmonton, Alberta, T6G 2G2, Canada.

The synthesis of the new bulky vinyllithium reagent ( IPr=CH)Li, ( IPr=[(MeCNDipp) C]; Dipp=2,6-iPr C H ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [ IPr=CH] . Furthermore, a high-yielding route to the degradation-resistant Si precursor IPr⋅SiBr is presented. The efficacy of ( IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series ( IPrCH) E: (E=Si to Pb). ( IPrCH) Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the Si center noted. Cyclic voltammetry shows this electron-rich silylene to be a potent reducing agent, rivalling the reducing power of the 19-electron complex cobaltocene (Cp Co).
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http://dx.doi.org/10.1002/chem.202100969DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252546PMC
June 2021

A dinuclear rhenium complex in the electrochemically driven homogeneous and heterogeneous H/CO-reduction.

Dalton Trans 2020 Jun;49(24):8367-8374

Universität Göttingen, Institut für Anorganische Chemie, Tammannstr. 4, 37077 Göttingen, Germany.

A dinculear Re(CO)3 complex with a proton responsive phenol unit and a pyrene anchor in the ligand backbone was investigated in the electrochemical CO2/H+ conversion in solution and adsorbed on multi walled carbon nanotubes (MWCNT) on an GC electrode surface. The pyrene group unit is introduced at the end of the ligand synthesis via a coupling reaction, which allows for a versatile ligand modification in order to tune the electronic properties or to introduce various anchor groups for heterogenisation at a late stage. The redox chemistry of the pyrene-α-diimine-Re(CO)3 complex, 1, was investigated in N,N-dimethylformamide (dmf), including IR-spectroelectrochemical (IR-SEC) characterisation of the short lived, reduced species. Subsequently, the electrochemical H+/CO2-reduction catalysis in dmf/water was investigated. The complex catalyses syngas formation yielding CO and H2 with similar rates, namely in Faraday yields of 45% and 35%, respectively. Since the similar complex without the pyrene anchor in the backbone, I, prefers CO2 over H+ reduction, the formation of syngas was rationalised by the small differences in the redox properties and pKa values of the phenol-pyrene unit in regard to phenol unit as in I. Subsequently, the complex was adsorbed on multi walled carbon nanotubes (MWCNT) on a GC electrode surface. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed coating of the electrode. The immobilised complex was utilised in the electrochemical CO2/H+ reduction in dmf/water, however, the complex quickly desorbed under reductive conditions, likely due to the good solubility of the reduced species. Water as a solvent prevents desorption as confirmed by XPS, however, then a preference for H2 formation over syngas formation was observed under electrocatalytic conditions. Thus, these experiments show, that the results obtained in aqueous organic solution are not easily transferable to the heterogeneous systems operating in water due to changes in the reaction rates for competing pathways.
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http://dx.doi.org/10.1039/d0dt00381fDOI Listing
June 2020

Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well-Defined Base-Metal Catalyst.

Chemistry 2020 Nov 4;26(62):14137-14143. Epub 2020 Oct 4.

Institut für Anorganische Chemie, Universität Göttingen, Tammannstr. 4, 37077, Göttingen, Germany.

Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Here, we introduce an electrochemical method for the hydrogenation of ketones and aldehydes by in situ formation of a Mn-H species. We utilise protons and electric current as surrogate for H and a base-metal complex to form selectively the alcohols. The method is chemoselective for the hydrogenation of C=O bonds over C=C bonds. Mechanistic studies revealed initial 3 e reduction of the catalyst forming the steady state species [Mn (H L)(CO) ] . Subsequently, we assume protonation, reduction and internal proton shift forming the hydride species. Finally, the transfer of the hydride and a proton to the ketone yields the alcohol and the steady state species is regenerated via reduction. The interplay of two manganese centres and the internal proton relay represent the key features for ketone and aldehyde reduction as the respective mononuclear complex and the complex without the proton relay are barely active.
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http://dx.doi.org/10.1002/chem.202002075DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7702145PMC
November 2020

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex.

Eur J Inorg Chem 2020 Apr 31;2020(15-16):1402-1410. Epub 2020 Jan 31.

Institute of Inorganic Chemistry University of Goettingen Tammannstraße 4 37077 Goettingen Germany.

The splitting of N into well-defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl(2)] {2 = N(CHCHPBu) } is reported. The electrochemical characterization of [ReCl(2)] and comparison with our previously reported platform [ReCl(1)] {1 = N(CHCHPBu) } provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N derived nitride complex [Re(N)Cl(2)] with electrophiles is presented.
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http://dx.doi.org/10.1002/ejic.201901278DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7217231PMC
April 2020

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO Reduction.

Inorg Chem 2020 Apr 10;59(7):4215-4227. Epub 2020 Mar 10.

Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34-36, Berlin 14195, Germany.

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three Re complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The Re compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive Re complexes and for electrocatalytic CO reduction.
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http://dx.doi.org/10.1021/acs.inorgchem.9b02591DOI Listing
April 2020

Electrochemical and Photophysical Properties of Ruthenium(II) Complexes Equipped with Sulfurated Bipyridine Ligands.

Inorg Chem 2020 Apr 6;59(7):4972-4984. Epub 2020 Mar 6.

University of Göttingen, Institute of Inorganic Chemistry, Tammannstrasse 4, D-37077 Göttingen, Germany.

The development of new solar-to-fuel scenarios is of great importance, but the construction of molecular systems that convert sunlight into chemical energy represents a challenge. One specific issue is that the molecular systems have to be able to accumulate redox equivalents to mediate the photodriven transformation of relevant small molecules, which mostly involves the orchestrated transfer of multiple electrons and protons. Disulfide/dithiol interconversions are prominent 2e/2H couples and can play an important role for redox control and charge storage. With this background in mind, a new photosensitizer [Ru(bpy)(bpy)] () equipped with a disulfide functionalized bpy ligand (bpy, bpy = 2,2'-bipyridine) was synthesized and has been comprehensively studied, including structural characterization by X-ray diffraction. In-depth electrochemical studies show that the bpy ligand in can be reduced twice at moderate potentials (around -1.1 V vs Fc), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion ( > ) and allowed to derive kinetic parameters for the sequential electron-transfer processes. However, reduction at room temperature also triggers the ejection of one sulfur atom from , leading to the formation of [Ru(bpy)(bpy)](). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy)], features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the bpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of on the basis of ground-state Wigner sampling and using charge-transfer descriptors has shown that bpy modification with a peripheral disulfide moiety leads to an energy splitting between charge-transfer excitations to the bpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound is photostable and shows an emission from a MLCT state in deoxygenated acetonitrile with a lifetime of 109 ns. This work demonstrates a rationally designed system that enables future studies of photoinduced multielectron, multiproton PCET chemistry.
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http://dx.doi.org/10.1021/acs.inorgchem.0c00220DOI Listing
April 2020

The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO) Complexes on the CO Reduction Catalysis.

Inorg Chem 2019 Aug 3;58(16):10444-10453. Epub 2019 Jul 3.

Universität Göttingen , Institut für Anorganische Chemie , Tammannstr. 4 , 37077 Göttingen , Germany.

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO) complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, , a binuclear Mn complex with methoxyphenol unit instead, , and the mononuclear analogue with a phenol unit, . Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that and undergo a reductive H formation forming [Mn(HL)(CO)Br] and [Mn(HL)(CO)], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn-H species. dimerizes during reduction, forming [Mn(L)(CO)], but and do not. Reduction of , , and is accompanied by bromide loss, and the final species represent [Mn(HL)(CO)], [Mn(L)(CO)], and [Mn(HL)(CO)], respectively. and are active catalysts in the electrochemical CO reduction reaction, whereas decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in alters the kinetics for the 2H/2e reduced products of CO in dmf/water mixtures. For , CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of . Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO insertion as well as the reaction rate of the reduced metal center with CO forming CO are similar. The overpotential with regard to the standard redox potential of CO to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s are higher with than with , that is, the OH group is beneficial for catalysis due to the internal proton transfer.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00992DOI Listing
August 2019

Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO) Complexes with Proton-Responsive Ligands in CO Reduction Catalysis.

Chemistry 2019 Apr 20;25(21):5555-5564. Epub 2019 Mar 20.

Institut für Anorganische Chemie, Universität Göttingen, Tammannstr. 4, 37077, Göttingen, Germany.

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO) complexes with proton responsive ligands and their application in the electrochemically-driven CO reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3, and two mononuclear, one having a central phenol unit, 1, and one with a methoxy unit, 2, were utilised. All complexes are active in the CO -to-CO conversion and CO is always the major product. The catalytic rate constant k for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO binding and this step is around ten times faster in the presence of water [2: k (DMF)≈1.7 s ; k (DMF/H O)≈20 s ]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k for 3 is four times higher than of 2, in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO -to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO activation. In DMF, O-H bond breaking and Cl ion loss have similar rate constants [1: k (DMF)≈2 s , k ≈1.5 s ], in water/DMF Cl loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.
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http://dx.doi.org/10.1002/chem.201806398DOI Listing
April 2019

Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction.

J Am Chem Soc 2018 11 25;140(45):15419-15424. Epub 2018 Oct 25.

Institute of Chemistry , Carl von Ossietzky University of Oldenburg , Carl von Ossietzky-Str. 9-11 , D-26129 Oldenburg , Federal Republic of Germany, European Union.

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(CF)) to give the conventional B-Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.
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http://dx.doi.org/10.1021/jacs.8b09214DOI Listing
November 2018

Electrochemical water oxidation using a copper complex.

Dalton Trans 2018 Aug 28;47(31):10737-10741. Epub 2018 Jun 28.

Universität Göttingen, Institut für Anorganische Chemie, Tammannstr. 4, 37077 Göttingen, Germany.

Herein, we report the application of the mononuclear copper complex 1, [Cu(L)], in electrochemical water oxidation catalysis (L = 1,3-bis(((1-methyl-1H-imidazol-2-yl)methyl)amino)propan-2-ol). The complex exhibits a N donor set consisting of two amine and two imidazole units and a dangling OH unit in close proximity to the copper ion. 1 exhibits a moderate apparent rate constant k of 0.12 s in catalysis and operates at an overpotential of 0.83 V. Detailed investigations allowed us to derive a mechanism for water oxidation. The catalysis proceeds only under basic conditions, where [Cu(L)(OH)], 1H, is the main solution species, which indicates that a negatively charged ligand is necessary to drive the catalysis. Initial oxidation of 1H is coupled to proton loss forming a copper(iii) species and further oxidation initiates oxygen evolution. Initial oxidation of 1 under neutral, i.e. non-catalytic, conditions is pH independent, highlighting the importance of PCET steps during catalysis. We collected reasonable evidence that catalysis proceeds via a water nucleophilic attack mechanism. The electrolyte presumably acts as a proton acceptor in catalysis as the onset potential depends on the buffer employed.
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http://dx.doi.org/10.1039/c8dt01323cDOI Listing
August 2018

Mechanism of Chemical and Electrochemical N Splitting by a Rhenium Pincer Complex.

J Am Chem Soc 2018 06 19;140(25):7922-7935. Epub 2018 Jun 19.

University of Goettingen , Institute of Inorganic Chemistry , Tammannstrasse 4 , 37077 Goettingen , Germany.

A comprehensive mechanistic study of N activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl(PNP)] is presented (PNP = N(CHCHP tBu)). Low-temperature studies using chemical reductants enabled full characterization of the N-bridged intermediate [{(PNP)ClRe}(N)] and kinetic analysis of the N-N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N activation to form the N-bridged intermediate. CV data was acquired under Ar and N, and with varying chloride concentration, rhenium concentration, and N pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where "E" is an electrochemical step and "C" is a chemical step) for N activation that proceeds via initial reduction to Re, N binding, chloride dissociation, and further reduction to Re before formation of the N-bridged, dinuclear intermediate by comproportionation with the Re precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N splitting in the tetragonal frameworks enforced by rigid pincer ligands.
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http://dx.doi.org/10.1021/jacs.8b03755DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6026835PMC
June 2018

2,2'-Bipyridine Equipped with a Disulfide/Dithiol Switch for Coupled Two-Electron and Two-Proton Transfer.

Chemistry 2018 Apr 6;24(19):4864-4870. Epub 2018 Mar 6.

Institut für Anorganische Chemie, Universität Göttingen, Tammannstr. 4, 37077, Göttingen, Germany.

[1,2]Dithiino[4,3-b:5,6-b']dipyridine (1) and its protonated open form 3,3'-dithiol-2,2'-bipyridine (2) were synthesised and their interconversion investigated. The X-ray structure of 2 revealed an anti orientation of the two pyridine units and a zwitterionic form. In depth electrochemical studies in combination with DFT calculations lead to a comprehensive picture of the redox chemistry of 1 in the absence and presence of protons. Initial one-electron reduction at E =-1.20 V results in the formation of the radical anion 1 with much elongated S-S bond, which readily undergoes further reduction at E =-1.38 V. Water triggers a potential inversion (E≥-1.13 V for the second reduction) as the radical anion 1 is protonated at its basic N atom. DFT studies revealed that S-S bond breaking and twisting of the pyridine units generally occurs after the second reduction step, whereas the potential inversion induced by protonation is a result of charge compensation. The CV data were simulated to derive rate constants for the individual chemical and electrochemical reactions for both scenarios in the absence and presence of protons.
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http://dx.doi.org/10.1002/chem.201705022DOI Listing
April 2018

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex.

Chemistry 2017 Dec 12;23(69):17438-17443. Epub 2017 Oct 12.

Homogeneous and Supramolecular Catalysis van't Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam, Amsterdam, The Netherlands.

One-electron oxidation of the rhodium(I) azido complex [Rh(N )(PNP)] (5), bearing the neutral, pyridine-based PNP ligand 2,6-bis(di-tert-butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N )(PNP)] (9 ). Interestingly, complex 5 also acts as a catalyst for electrochemical N oxidation (E ≈-0.23 V vs. Fc ) in the presence of excess azide. This is of potential relevance for the design of azide-based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas.
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http://dx.doi.org/10.1002/chem.201702938DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5765409PMC
December 2017

Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO Reduction.

Inorg Chem 2017 Apr 20;56(7):4176-4185. Epub 2017 Mar 20.

Universität Göttingen, , Institut für Anorganische Chemie, Tammannstraße 4, D-37077 Göttingen, Germany.

Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, 1, [Re(L)(CO)Cl], with a proton responsive ligand and its application as a catalyst in the electrochemical CO reduction reaction (L = 4-tert-butyl-2,6-bis(6-(1H-imidazol-2-yl)-pyridin-2-yl)phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as α-diimine donors. The complex is an active catalyst for the electrochemical CO reduction reaction. CO is the main product after catalysis, and only small amounts of H were observed, which can be related to the ligand reactivity. The i/i ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)Cl] in MeCN/HO, albeit 1 requires a slightly larger overpotential (bpy = 2,2'-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of 1. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO. The first 2e reduction is ligand based and leads to homolytic N-H bond cleavage reactions at the imidazole units of 1, which is equal to a net double proton removal from 1 forming [Re(LH)(CO)Cl]. The second 2e reduction process has been identified as an O-H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re(LH)(CO)Cl]. In the presence of CO, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO instead of O-H bond cleavage.
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http://dx.doi.org/10.1021/acs.inorgchem.7b00178DOI Listing
April 2017

Copper complexes as catalyst precursors in the electrochemical hydrogen evolution reaction.

Dalton Trans 2016 Apr;45(16):6974-82

Georg-August-University Göttingen, Institute of Inorganic Chemistry, Tammannstrasse 4, D-37077 Göttingen, Germany.

Herein, we report the synthesis and species distribution of copper(ii) complexes based on two different ligand scaffolds and the application of the two complexes in the electrochemical proton reduction catalysis. The ligands bind to one or two copper(II) ions and the pH-dependent mono/dinuclear equilibrium depends on the steric bulk of the ligands. The two water soluble copper(II) complexes were investigated for their activities in the electrochemical hydrogen evolution reaction (HER). In both complexes the copper(ii) ions have a N4-coordination environment composed of N-heterocycles, although in different coordination geometries (SPY-5 and TBPY-5). The solutions of the complexes were highly active catalysts in water at acidic pH but the complexes decompose under catalytic conditions. They act as precursors for highly active copper(0) and Cu2O deposits at the electrode surface, which are in turn the active catalysts. The absence or presence of the ligands has neither an influence on the catalytic activity of the solutions nor an influence on the activity of the deposit formed during controlled potential electrolysis. Finally, we can draw some conclusions on the stability of copper catalysts in the aqueous electrochemical HER.
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http://dx.doi.org/10.1039/c6dt00082gDOI Listing
April 2016

Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies.

Inorg Chem 2016 Feb 20;55(3):1061-8. Epub 2016 Jan 20.

Georg-August-University Göttingen , Tammannstr. 4, D-37077 Göttingen, Germany.

We synthesized two dinuclear copper complexes, which have ionizable N imidazole and N pyrazole protons in the ligand, respectively, and determined the BDFE of the hypothetical H atom transfer reactions Cu(II)(LH(-1)) + H(•) ↔ Cu(I)(L) in MeOH/H2O (BDFE: bond dissociation Gibbs (free) energy). The ligands have two adjacent N,N',O-binding pockets, which differ in one N-heterocycle: L(a) has an imidazole unit and L(c), a pyrazole unit. The copper(II) complexes of L(a) and L(c) have been characterized, and the substitution pattern has only little influence on the structural properties. The BDFEs of the hypothetical PCET reactions have been determined by means of the species distribution and the redox potentials of the involved species in MeOH/H2O (80/20 by weight). The pyrazole copper complex 3 exhibits a lower BDFE than the isoelectronic imidazole copper complex 1 (1, 292(3) kJ mol(-1); 3, 279(1) kJ mol(-1)). The difference is mainly caused by the higher acidity of the N pyrazole proton of 3 compared to the N imidazole proton of 1. The redox potentials of 1 and 3 are very similar.
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http://dx.doi.org/10.1021/acs.inorgchem.5b02084DOI Listing
February 2016

Electrocatalytic Dihydrogen Production with a Robust Mesoionic Pyridylcarbene Cobalt Catalyst.

Angew Chem Int Ed Engl 2015 Nov 30;54(46):13792-5. Epub 2015 Sep 30.

Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34-36, 14195, Berlin (Germany).

A Co(III) complex with a mesoionic pyridylcarbene ligand is presented. This complex is an efficient electrocatalyst for H2 production at very low overpotential and high turnovers when using a (glassy carbon) GC electrode. The corresponding triazole complexes display no catalytic activity whatsoever under identical conditions. The remarkable robustness of the Co-C(carbene) bond towards acids is likely responsible for the high efficiency of this catalyst. The present results thus open new avenues for carbene-based ligands for generating functional models for hydrogenases.
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http://dx.doi.org/10.1002/anie.201506061DOI Listing
November 2015

Proton-Coupled Electron Transfer Reactions Catalysed by 3 d Metal Complexes.

Authors:
Inke Siewert

Chemistry 2015 Oct 6;21(43):15078-91. Epub 2015 Aug 6.

Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstr. 4, 37077 Göttingen (Germany).

Proton-coupled electron transfer (PCET) reactions are essential for a wide range of natural energy-conversion reactions and recently, the impact of PCET pathways has been exploited in artificial systems, too. The Minireview highlights PCET reactions catalysed by first-row transition-metal complexes, with a focus on the water oxidation, the oxygen reduction, the hydrogen evolution, and the CO2 reduction reaction. Special attention will be paid to systems in which the impact of such pathways is deduced by comparison to systems with "electron-only"-transfer pathways.
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http://dx.doi.org/10.1002/chem.201501684DOI Listing
October 2015

Cobalt catalyst with a proton-responsive ligand for water oxidation.

Chemistry 2015 Feb 18;21(7):2780-4. Epub 2014 Dec 18.

Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstrasse 4, 37077 Göttingen (Germany), Fax: (+49) 551-3933373.

Herein, we report the synthesis, the thermochemical data, and the catalytic reactivity of a new mononuclear cobalt complex, which has four NH protons in the ligand sphere. The combination of the redox-active metal ion and NH units enabled the coupling of proton and electron-transfer steps, which we exploited in the electrocatalytic water oxidation.
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http://dx.doi.org/10.1002/chem.201405020DOI Listing
February 2015

A trispyrazolylborato iron cysteinato complex as a functional model for the cysteine dioxygenase.

Angew Chem Int Ed Engl 2012 Feb 27;51(9):2234-7. Epub 2012 Jan 27.

Humboldt-Universität zu Berlin, Institut für Chemie, Berlin, Germany.

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http://dx.doi.org/10.1002/anie.201107345DOI Listing
February 2012

Probing the influence of steric bulk on anion binding by triarylboranes: comparative studies of FcB(o-Tol)2, FcB(o-Xyl)2 and FcBMes2.

Dalton Trans 2011 Oct 15;40(40):10345-50. Epub 2011 Mar 15.

Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.

Steric crowding brought about on pyramidalization at boron has been predicted computationally to be of central importance to the strength and selectivity of anion binding by triarylboranes. The role of steric factors in systems containing a ferrocenyl reporter unit has been systematically probed in the current study by comparison of the F(-)/CN(-) binding properties of FcB(o-Tol)(2) (1, o-Tol = C(6)H(4)Me-2), FcB(o-Xyl)(2) (2, o-Xyl = C(6)H(3)Me(2)-2,6) and FcBMes(2) (3, Mes = C(6)H(2)Me(3)-2,4,6)), both in solution and in the solid state. Somewhat surprisingly, the inclusion of an extra ortho-methyl aryl substituent (e.g. for 2/3vs.1) is found to have a relatively small effect on the binding affinities of these boranes (e.g. log(10)K(CN) = 5.94(0.02), 4.73(0.01), 5.56(0.02), for 1, 2 and 3 respectively). Consistent with this observation, the degree of pyramidalization at boron determined for the cyanide adducts [1·CN](-), [2·CN](-) and [3·CN](-) in the solid state is also found to be essentially invariant (∠C(aryl)-B-C(aryl) = 338, 337, 337°, respectively), as are the B-CN and mean B-C(aryl) distances. In the solid state at least, it is apparent that the adverse steric effects potentially brought about by increasing ortho substitution are mitigated by a greater degree of synchronous rotation of the aryl substituents about the B-C(aryl) bonds. Thus a mean inter-plane angle of 71° is observed for [1·CN](-) while the corresponding values for [2·CN](-) and [3·CN](-) are 78° and 79°.
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http://dx.doi.org/10.1039/c1dt10185dDOI Listing
October 2011

Low-molecular-weight analogues of the soluble methane monooxygenase (sMMO): from the structural mimicking of resting states and intermediates to functional models.

Chemistry 2009 Oct;15(40):10316-28

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor Str. 2, 12489 Berlin, Germany.

The active centre of sMMO contains a diiron core ligated by histidine and glutamate residues, which is capable of catalysing a remarkable reaction: the oxidation of methane with O(2) yielding methanol. This review describes the results of efforts to prepare low-molecular-weight analogues of this active site directing towards 1) the assignment of the spectroscopic signatures identified for certain intermediates of the sMMO catalytic cycle to structural features and 2) the synthesis of molecular compounds that can mimic the reactivity. The historical development of the model chemistry, which is subdivided into structural and functional mimicking, is illustrated and achievements reached so far are highlighted.
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http://dx.doi.org/10.1002/chem.200901910DOI Listing
October 2009

A dinuclear iron complex based on parallel malonate binding sites: cooperative activation of dioxygen and biomimetic ligand oxidation.

Chemistry 2008 ;14(30):9377-88

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 10829 Berlin, Germany.

A ligand that offers two parallel malonate binding sites linked by a xanthene backbone, namely, Xanthmal2-, has been utilised to synthesise dinuclear FeII complex [Fe2(Xanthmal)2] (1). The reactivity of 1 in contact with O2 was investigated at -40 degrees C and room temperature. After activation of O2 through interaction with both iron centres the ligand is oxidised: at the Calpha position monooxygenation and peroxide formation occur, partially accompanied by C-C bond cleavage to yield alpha-keto ester groups. To reveal mechanistic details investigations concerning 1) peroxide decomposition, 2) the reactivity of a corresponding mononuclear complex, 3) the influence of monooxygenation of the ligand on the reactivity and 4) product formation in dependence on time were carried out. The results can be explained by postulating formation of high-valent Fe intermediates and ligand-to-metal electron transfer, and the mechanistic scheme derived includes several steps that mimic the (suggested) functioning of non-heme iron enzymes. In agreement with this proposal, ligand oxidation can also be performed catalytically. Furthermore, we show that via a competitive route [(Xanthmal)2Fe2O] (2) is formed, which is unreactive towards O2 and thus is a dead end with respect to ligand oxidation. Both compounds 1 and 2 were fully characterised, and their properties are discussed.
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http://dx.doi.org/10.1002/chem.200800955DOI Listing
December 2008

A trispyrazolylborato iron Malonato complex as a functional model for the acetylacetone dioxygenase.

Angew Chem Int Ed Engl 2008 ;47(41):7953-6

Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Strasse 2, 10829 Berlin, Germany.

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http://dx.doi.org/10.1002/anie.200802955DOI Listing
November 2008
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