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A light-driven dual and ternary catalytic aza-Wacker protocol for the construction of 3-pyrrolines by partially disulfide-assisted selenium-π-acid multicatalysis is reported. A structurally diverse array of sulfonamides possessing homopolar mono-, di- and trisubstituted olefinic double bonds is selectively converted to the corresponding 3-pyrrolines in up to 95 % isolated yield and with good functional group tolerance. Advanced electrochemical mechanistic investigations of the protocol suggest a dual role of the disulfide co-catalyst. On the one hand, the disulfide serves as an electron hole shuttle between the excited photoredox catalyst and the selenium co-catalyst. On the other hand, the sulfur species engages in the final, product releasing step of the catalytic cycle by accelerating the β-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields., (© 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N 2 -to-NH 3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N 2 splitting ([ L MoCl 3 ] - , 1 - , L H = 2,6-bis((di- tert -butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N 2 activation. The catalyst is indeed active in N 2 -to-NH 3 conversion utilizing the proton-coupled electron transfer reagent SmI 2 /ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3 - is also accessible by electrochemical reduction of 1 - under a N 2 atmosphere. IR- and UV/vis-SEC measurements suggest that N 2 splitting occurs via formation of an "overreduced" but more stable [( L (N 2 ) 2 Mo 0 ) 2 μ-N 2 ] 2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The activation of N 2 , CO 2 or H 2 O to energy-rich products relies on multi-electron transfer reactions, and consequently it seems desirable to understand the basics of light-driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four-electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible-light irradiation using the organic super-electron donor tetrakis(dimethylamino)ethylene, a cascade of light-induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Nanoconfinement of catalytically active molecules is a powerful strategy to control their chemical activity; however, the atomic-scale mechanisms are challenging to identify. In the present study, the site-specific reactivity of a model rhenium catalyst is studied on the subnanometer scale for complexes confined within quasi-one-dimensional molecular chains on the Ag(001) surface by scanning tunneling microscopy. Injection of tunneling electrons causes ligand dissociation in single molecules. Unexpectedly, while half of the complexes show only the dissociation, the confined molecules show also the reverse reaction. On the basis of density functional theory calculations, this drastic difference can be attributed to the limited space in confinement. That is, the split-off ligand adsorbs closer to the molecule and the dissociation causes less structural disruption. Both of these facilitate the reverse reaction. We demonstrate formation and disruption of single chemical bonds of nanoconfined molecules with potential application in molecular data storage.

Molecular systems combining light harvesting and charge storage are receiving great attention in the context of, for example, artificial photosynthesis and solar fuel generation. As part of ongoing efforts to develop new concepts for photoinduced proton-coupled electron transfer (PCET) reactivities, we report a cyclometallated iridium(III) complex [Ir(ppy) 2 ( S-S bpy)](PF 6 ) ( [1] PF 6 ) equipped with our previously developed sulfurated bipyridine ligand S-S bpy. A new one-step synthetic protocol for S-S bpy is developed, starting from commercially available 2,2'-bipyridine, which significantly facilitates the use of this ligand. [1] + features a two-electron reduction with potential inversion (| E 1 | > | E 2 |) at moderate potentials ( E 1 = -1.12, E 2 = -1.11 V versus . Fc +/0 at 253 K), leading to a dithiolate species [1] - . Protonation with weak acids allows for determination of p K a = 23.5 in MeCN for the S-H···S - unit of [1H]. The driving forces for both the H atom and the hydride transfer are calculated to be ∼60 kcal mol -1 and verified experimentally by reaction with a suitable H atom and a hydride acceptor, demonstrating the ability of [1] + to serve as a versatile PCET reagent, albeit with limited thermal stability. In MeCN solution, an orange emission for [1] PF 6 from a triplet-excited state was found. Density functional calculations and ultrafast absorption spectroscopy are used to give insight into the excited-state dynamics of the complex and suggest a significantly stretched S-S bond for the lowest triplet-state T 1 . The structural responsiveness of the disulfide unit is proposed to open an effective relaxation channel toward the ground state, explaining the unexpectedly short lifetime of [1] + . These insights as well as the quantitative ground-state thermochemistry data provide valuable information for the use of S-S bpy-functionalized complexes and their disulfide-/dithiol-directed PCET reactivity.

Metal complexes with ligands that coordinate via the nitrogen atom of azo (N═N) or imino (C═N) groups are of interest due to their π-acceptor properties and redox-active nature, which leads to interesting (opto)electronic properties and reactivity. Here, we describe the synthesis and characterization of rhenium(I) tricarbonyl complexes with neutral N , N -bidentate formazans, which possess both N═N and C═N fragments within the ligand backbone (Ar 1 -NH-N═C(R 3 )-N═N-Ar 5 ). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO) 5 ] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure fac -[ReBr(CO) 3 (κ 2 - N 2 ,N 4 (Ar 1 -N 1 H-N 2 ═C( R 3 )-N 3 ═N 4 -Ar 5 ))]. The formazan ligand coordinates the metal center in the ' open ' form, generating a five-membered chelate ring with a pendant NH arm. The electronic absorption and emission properties of these complexes are governed by the presence of low-lying π*-orbitals on the ligand as shown by DFT calculations. The high orbital mixing between the metal and ligand results in photophysical properties that contrast to those observed in fac -[ReBr(CO) 3 (L,L)] species with α-diimine ligands.

Transition metal halide complexes are used as precursors for reductive N 2 activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N 2 splitting with the rhenium(III) complexes [ReX 2 (PNP)] (PNP = N(CH 2 CH 2 P t Bu 2 ) 2 and X = Cl, Br, I) is presented. As previously reported for the chloride precursor ( J. Am. Chem. Soc. 2018, 140, 7922), the heavier halides give rhenium(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N 2 -bridged complexes, [{ReX(PNP)} 2 (μ-N 2 )], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and Re III /Re I comproportionation, the iodide system also reacts via an alternative Re II /Re II -dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N 2 activation that was observed for the chloride precursor. Computational analysis of the relevant Re III/II and Re II/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re-X bonding interactions over contributions from charge transfer.

The synthesis of certain commodity chemicals, e.g., methanol and acetic acid, relies on CO, which is currently mainly produced by the combustion of carbon or natural gas. Photo- or electrochemical conversion of atmospheric CO 2 to CO represents an attractive alternative strategy as this approach is carbon-neutral. Such photo- or electrochemically formed CO can also be used in the Fischer-Tropsch process forming liquid hydrocarbons for energy storage applications. The multiple electroreduction of CO 2 is preferably coupled with proton transfer steps as this requires less energy than the single outer-sphere 1e - reduction of CO 2 .In 1984 and 2011, it was shown that [(L bpy )Re(CO) 3 Cl] ( 1 ) and [(L bpy )Mn(CO) 3 Br] ( 2 ), respectively, mediate the electrochemical 2e - /2H + reduction of CO 2 forming CO and water (L bpy = 2,2'-bipyridine). Since proton management is crucial for catalysis, recently the impact of internal proton sources close to the axial position in such complexes has been investigated. However, binuclear complexes have been used rarely as mediators although it has been shown very early for 1 that electron management is also important: the 2e - /2H + reduction pathway with 1 exhibits a higher reaction rate than going via the singly reduced species, though the pathway requires a higher overpotential. In this Account, we focus on recent developments of binuclear LMn 2 (CO) 6 and LRe 2 (CO) 6 mediators with an internal phenol group in the electroreduction of CO 2 . In contrast to mononuclear derivatives, for which the impact of the internal proton source on catalysis is very diverse, we always observed a higher reaction rate and for the Mn complexes also a lower overpotential with the binuclear complexes compared to the mononuclear variants. Spectroscopic, electrochemical, and computational studies on the mono- and binuclear complexes shed light on their reactivity under reductive conditions, elucidated the structure of reduced species, unraveled the kinetics for catalytically productive and unproductive (side) reactions, and allowed us to derive some hypothesis on the CO 2 reduction mechanism. Finally, I emphasize that the electrohydrogenation of the polar double bonds by the binuclear complex LMn 2 (CO) 6 with a central phenol unit is not restricted to CO 2 but is also applicable to organic compounds with C═O bonds.

The discovery of metallic conductivity in polyacetylene [-HC=CH-] n upon doping represents a landmark achievement. However, the insolubility of polyacetylene and a dearth of methods for its chemical modification have limited its widespread use. Here, we employ a ring-opening metathesis polymerization (ROMP) protocol to prepare functionalized polyacetylenes (fPAs) bearing: (1) electron-deficient boryl (-BR 2 ) and phosphoryl (-P(O)R 2 ) side chains; (2) electron-donating amino (-NR 2 ) groups, and (3) ring-fused 1,2,3-triazolium units via strain-promoted Click chemistry. These functional groups render most of the fPAs soluble and can lead to intense light absorption across the visible to near-IR region. Also, the presence of redox-active boryl and amino groups leads to opposing near-IR optical responses upon (electro)chemical reduction or oxidation. Some of the resulting fPAs show greatly enhanced air stability when compared to known polyacetylenes. Lastly, these fPAs can be cross-linked to yield network materials with the full retention of optical properties., (© 2021 Wiley-VCH GmbH.)

The synthesis of the new bulky vinyllithium reagent ( Me IPr=CH)Li, ( Me IPr=[(MeCNDipp) 2 C]; Dipp=2,6-iPr 2 C 6 H 3 ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [ Me IPr=CH] - . Furthermore, a high-yielding route to the degradation-resistant Si II precursor Me IPr⋅SiBr 2 is presented. The efficacy of ( Me IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series ( Me IPrCH) 2 E: (E=Si to Pb). ( Me IPrCH) 2 Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the Si II 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 2 Co)., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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 fac -[Re( S-S bpy)(CO) 3 Cl] ( 1 , S-S bpy = [1,2]dithiino[4,3- b :5,6- b ']dipyridine) undergoes two successive reductions at equal potentials of -1.16 V vs Fc +|0 at room temperature forming [Re( S2 bpy)(CO) 3 Cl] 2- ( 1 2- , S2 bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). 1 2- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H··· - S unit, 1 H - . The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (Δ G ° H + = 34 kcal mol -1 , p K a = 24.7), an H atom (Δ G ° H • = 59 kcal mol -1 ), or a hydride ion (Δ G ° H - = 60 kcal mol -1 ) as demonstrated in the reactivity with various organic test substrates.

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 2 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 2 (H -1 L)(CO) 6 ] - . 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., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

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 via 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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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.

The splitting of N 2 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 2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl 2 ( L 2)] { L 2 = N(CHCHP t Bu 2 ) 2 - } is reported. The electrochemical characterization of [ReCl 2 ( L 2)] and comparison with our previously reported platform [ReCl 2 ( L 1)] { L 1 = N(CH 2 CH 2 P t Bu 2 ) 2 - } provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N 2 derived nitride complex [Re(N)Cl( L 2)] with electrophiles is presented., (© 2020 The Authors. Published by Wiley‐VCH Verlag GmbH & Co. KGaA.)

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( S-S bpy)(bpy) 2 ] 2+ ( 1 2+ ) equipped with a disulfide functionalized bpy ligand ( S-S 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 S-S bpy ligand in 1 2+ can be reduced twice at moderate potentials (around -1.1 V vs Fc +/0 ), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion ( E 2 > E 1 ) 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 1 2+ , leading to the formation of [Ru( S bpy)(bpy) 2 ] 2+ ( 2 2+ ). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy) 3 ] 2+ , 1 2+ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the S-S bpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 1 2+ 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 S-S bpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 1 2+ is photostable and shows an emission from a 3 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.

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 I 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 I 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 2 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 I complexes and for electrocatalytic CO 2 reduction.

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO) 3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO 2 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, 1 , a binuclear Mn complex with methoxyphenol unit instead, 2 , and the mononuclear analogue with a phenol unit, 3 . Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H 2 formation forming [Mn 2 (H -1 L 1 )(CO) 6 Br] and [Mn(H -1 L 3 )(CO) 3 ], 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. 2 dimerizes during reduction, forming [Mn 2 (L 2 )(CO) 6 ] 2 , but 1 and 3 do not. Reduction of 1 , 2 , and 3 is accompanied by bromide loss, and the final species represent [Mn 2 (H -1 L 1 )(CO) 6 ] 3- , [Mn 2 (L 2 )(CO) 6 ] 2- , and [Mn(H -1 L 3 )(CO) 3 ] 2- , respectively. 1 and 2 are active catalysts in the electrochemical CO 2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H + /2e - reduced products of CO 2 in dmf/water mixtures. For 2 , CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1 . 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 2 insertion as well as the reaction rate of the reduced metal center with CO 2 forming CO are similar. The overpotential with regard to the standard redox potential of CO 2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s -1 are higher with 1 than with 2 , that is, the OH group is beneficial for catalysis due to the internal proton transfer.

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO) 3 complexes with proton responsive ligands and their application in the electrochemically-driven CO 2 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 2 -to-CO conversion and CO is always the major product. The catalytic rate constant k cat 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 2 binding and this step is around ten times faster in the presence of water [2: k Cl (DMF)≈1.7 s -1 ; k Cl (DMF/H 2 O)≈20 s -1 ]. 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 obs 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 2 -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 2 activation. In DMF, O-H bond breaking and Cl - ion loss have similar rate constants [1: k Cl (DMF)≈2 s -1 , k OH ≈1.5 s -1 ], 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C 6 F 5 ) 3 ) 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.

Herein, we report the application of the mononuclear copper complex 1, [Cu II (L)] 2+ , 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 4 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 cat of 0.12 s -1 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 II (L)(OH)] + , 1H -1 , is the main solution species, which indicates that a negatively charged ligand is necessary to drive the catalysis. Initial oxidation of 1H -1 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.

A comprehensive mechanistic study of N 2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl 2 (PNP)] is presented (PNP - = N(CH 2 CH 2 P tBu 2 ) 2 - ). Low-temperature studies using chemical reductants enabled full characterization of the N 2 -bridged intermediate [{(PNP)ClRe} 2 (N 2 )] 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 2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N 2 activation to form the N 2 -bridged intermediate. CV data was acquired under Ar and N 2 , and with varying chloride concentration, rhenium concentration, and N 2 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 2 activation that proceeds via initial reduction to Re II , N 2 binding, chloride dissociation, and further reduction to Re I before formation of the N 2 -bridged, dinuclear intermediate by comproportionation with the Re III 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 2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

[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 =-1.20 V results in the formation of the radical anion 1 red with much elongated S-S bond, which readily undergoes further reduction at E 2 =-1.38 V. Water triggers a potential inversion (E≥-1.13 V for the second reduction) as the radical anion 1 red 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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

One-electron oxidation of the rhodium(I) azido complex [Rh(N 3 )(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 2 )(PNP)] + (9 + ). Interestingly, complex 5 also acts as a catalyst for electrochemical N 3 - oxidation (E p ≈-0.23 V vs. Fc +/0 ) 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., (© 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, 1, [Re 2 (L)(CO) 6 Cl 2 ], with a proton responsive ligand and its application as a catalyst in the electrochemical CO 2 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 2 reduction reaction. CO is the main product after catalysis, and only small amounts of H 2 were observed, which can be related to the ligand reactivity. The i c /i p ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO) 3 Cl] in MeCN/H 2 O, 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 2 . 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 2 (LH -2 )(CO) 6 Cl 2 ] 2- . 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 2 (LH -3 )(CO) 6 Cl] 2- . In the presence of CO 2 , the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO 2 instead of O-H bond cleavage.

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.

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.

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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°.

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.

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.