(Electro)chemical N2 Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand.

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]-, 1 -, LH = 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 N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/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 N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an "overreduced" but more stable [(L(N2)2Mo0)2µ-N2]2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials.

Mechanistic Analysis Reveals Key Role of Interchalcogen Multicatalysis in Photo-Aerobic 3-Pyrroline Syntheses by Aza-Wacker Cyclizations.

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 ß-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

Accumulation of Four Electrons on a Terphenyl (Bis)disulfide.

The activation of N2 , CO2 or H2 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.

Making and breaking of chemical bonds in single nanoconfined molecules.

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.

Neutral Formazan Ligands Bound to the fac-(CO)3Re(I) Fragment: Structural, Spectroscopic, and Computational Studies.

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 (Ar1-NH-N═C(R3)-N═N-Ar5). 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(κ2-N2,N4(Ar1-N1H-N2═C(R3)-N3═N4-Ar5))]. 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.

Luminescent Iridium Complexes with a Sulfurated Bipyridine Ligand: PCET Thermochemistry of the Disulfide Unit and Photophysical Properties.

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-Sbpy)](PF6) ([1]PF6) equipped with our previously developed sulfurated bipyridine ligand S-Sbpy. A new one-step synthetic protocol for S-Sbpy 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 (|E1| > |E2|) at moderate potentials (E1 = -1.12, E2 = -1.11 V versus. Fc+/0 at 253 K), leading to a dithiolate species [1]-. Protonation with weak acids allows for determination of pKa = 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]PF6 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 T1. 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-Sbpy-functionalized complexes and their disulfide-/dithiol-directed PCET reactivity.

Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes.

Transition metal halide complexes are used as precursors for reductive N2 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 N2 splitting with the rhenium(III) complexes [ReX2(PNP)] (PNP = N(CH2CH2PtBu2)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 N2-bridged complexes, [{ReX(PNP)}2(µ-N2)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and ReIII/ReI comproportionation, the iodide system also reacts via an alternative ReII/ReII-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N2 activation that was observed for the chloride precursor. Computational analysis of the relevant ReIII/II and ReII/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.

Electrochemical CO2 Reduction Catalyzed by Binuclear LRe2(CO)6Cl2 and LMn2(CO)6Br2 Complexes with an Internal Proton Source.

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 CO2 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 CO2 is preferably coupled with proton transfer steps as this requires less energy than the single outer-sphere 1e- reduction of CO2.In 1984 and 2011, it was shown that [(Lbpy)Re(CO)3Cl] (1) and [(Lbpy)Mn(CO)3Br] (2), respectively, mediate the electrochemical 2e-/2H+ reduction of CO2 forming CO and water (Lbpy = 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 LMn2(CO)6 and LRe2(CO)6 mediators with an internal phenol group in the electroreduction of CO2. 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 CO2 reduction mechanism. Finally, I emphasize that the electrohydrogenation of the polar double bonds by the binuclear complex LMn2(CO)6 with a central phenol unit is not restricted to CO2 but is also applicable to organic compounds with C═O bonds.

Redox-Active Heteroatom-Functionalized Polyacetylenes.

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 (-BR2 ) and phosphoryl (-P(O)R2 ) side chains; (2) electron-donating amino (-NR2 ) 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.

Ordering a rhenium catalyst on Ag(001) through molecule-surface step interaction.

Atomic scale studies of the anchoring of catalytically active complexes to surfaces may provide valuable insights for the design of new catalytically active hybrid systems. In this work, the self-assembly of 1D, 2D and 3D structures of the complex fac-Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine), a CO2 reduction catalyst, on the Ag(001) surface are studied by a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. Infrared and sum frequency generation spectroscopy confirm that the complex remains chemically intact under sublimation. Deposition of the complexes onto the silver surface at 300 K leads to strong local variations in the resulting surface coverage on the nanometer scale, indicating that in the initial phase of deposition a large fraction of the molecules is desorbing from the surface. Low coverage regions show a decoration of step edges aligned along the crystal's symmetry axes <110>. These crystallographic directions are found to be of major importance to the binding of the complexes to the surface. Moreover, the interaction between the molecules and the substrate promotes the restructuring of surface steps along these directions. Well-aligned and decorated steps are found to act as nucleation point for monolayer growth (2D) before 3D growth starts.

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

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-Sbpy)(CO)3Cl] (1, S-Sbpy = [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(S2bpy)(CO)3Cl]2- (12-, S2bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). 12- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···-S unit, 1H-. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (ΔG°H+ = 34 kcal mol-1, pKa = 24.7), an H atom (ΔG°H• = 59 kcal mol-1), or a hydride ion (ΔG°H- = 60 kcal mol-1) as demonstrated in the reactivity with various organic test substrates.

A Stable Homoleptic Divinyl Tetrelene Series.

The synthesis of the new bulky vinyllithium reagent (Me IPr=CH)Li, (Me IPr=[(MeCNDipp)2 C]; Dipp=2,6-iPr2 C6 H3 ) 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 SiII precursor Me IPr⋅SiBr2 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 SiII 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 (Cp2 Co).

A dinuclear rhenium complex in the electrochemically driven homogeneous and heterogeneous H+/CO2-reduction.

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.

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

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 H2 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 [Mn2 (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.

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

The splitting of N2 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 N2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl2(L2)] {L2 = N(CHCHPtBu2)2 -} is reported. The electrochemical characterization of [ReCl2(L2)] and comparison with our previously reported platform [ReCl2(L1)] {L1 = N(CH2CH2PtBu2)2 -} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N2 derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.

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

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-Sbpy)(bpy)2]2+ (12+) equipped with a disulfide functionalized bpy ligand (S-Sbpy, 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-Sbpy ligand in 12+ 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 (E2 > E1) 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 12+, leading to the formation of [Ru(Sbpy)(bpy)2]2+(22+). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy)3]2+, 12+ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the S-Sbpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 12+ 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-Sbpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 12+ is photostable and shows an emission from a 3MLCT 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.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction.

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 ReI 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 ReI 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 CO2 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 ReI complexes and for electrocatalytic CO2 reduction.

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

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.

The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis.

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 CO2 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 H2 formation forming [Mn2(H-1L1)(CO)6Br] and [Mn(H-1L3)(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 [Mn2(L2)(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 [Mn2(H-1L1)(CO)6]3-, [Mn2(L2)(CO)6]2-, and [Mn(H-1L3)(CO)3]2-, respectively. 1 and 2 are active catalysts in the electrochemical CO2 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 CO2 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 CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 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.

Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis.

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 CO2 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 CO2 -to-CO conversion and CO is always the major product. The catalytic rate constant kcat 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 CO2 binding and this step is around ten times faster in the presence of water [2: kCl (DMF)≈1.7 s-1 ; kCl (DMF/H2 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 kobs 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 CO2 -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 CO2 activation. In DMF, O-H bond breaking and Cl- ion loss have similar rate constants [1: kCl (DMF)≈2 s-1 , kOH ≈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.