Coordination Sphere Effects Research Articles

CO2 Reduction at a Borane-Modified Iron Complex: A Secondary Coordination Sphere Strategy.

This work addresses fundamental questions that deepen our understanding of secondary coordination sphere effects on carbon dioxide (CO2) reduction using derivatized hydride analogues of the type, [Cp*Fe(diphosphine)H] (Cp* = C5Me5-) - a well-studied family of organometallic complex - as models. More precisely, we describe the general reactivity of [(Cp*-BR2)Fe(diphosphine)H], which contains an intramolecularly positioned Lewis acid, and its cooperative reactivity with CO2. Control experiments underscore the critical nature of borane incorporation for CO2to reduced products, a reaction that does not occur for unfunctionalized [Cp*Fe(diphosphine)H]). Additional experiments highlight relevance of borane hybridization and substituent effects. Mechanistic studies performed in the presence and absence of CO2emphasize the significance of carbonyl substrate to catalyst longevity. Lessons from these reactions were also transferable - with such borane-containing complexes enabling the chemoselective reduction of aldehydes in the presence of alkenes. These findings provide valuable insights into metal-ligand cooperative design strategies for carbonyl reduction and illustrate the versatility of intramolecularly positioned Lewis acids for otherwise challenging chemical transformations.

Selective diesterase-like activity of bioinspired FeIIINiII and FeIIIZnII complexes and their 3-aminopropyl silica composites: A homo- and heterogeneous catalytic study

In this study, the preparation and characterization of two new mixed-valence heterodinuclear complexes [FeIIINiII(BPPAMFF)(μ-OAc)2(H2O)]ClO4 (1) and [FeIIIZnII(BPPAMFF)(μ-OAc)2(H2O)]ClO4 (2) with the bioinspired ligand 2-[(N-benzyl-N-2-pyridylmethylamine)]-4-methyl-6-[N-(2-pyridylmethyl)aminomethyl)])-4-methyl-6-formylphenol (H2BPPAMFF) are reported. These compounds were immobilized in 3-aminopropyl silica (APS) to afford composites APS-1 and APS-2 successfully. The aldehyde-containing ligand provided a reactive functional group, which could serve as a cross-linking group to bind the complexes to the directly amino-modified SiO2 surface. The complexes’ chemical integrity on the APS inorganic platform were probed by spectroscopical techniques, such as FTIR, UV–Vis and EPR. Potentiometric and spectrophotometric titrations allowed the chemical species present in solution to be rationalized, and identified which of them were potentially active in the hydrolytic cleavage of phosphodiester 2,4-BDNPP. Kinetic studies showed that FeIIINiII species (1 and APS-1) presented higher catalytic efficiency (E = kcat/KM) than FeIIIZnII species (2 and APS-2). Catalytic mechanisms were proposed based on a series of kinetic experiments, in which all the catalysts tested behaved as selective phosphodiesterases. In addition, it was also demonstrated that the hydrolase activity of the immobilized catalytic centers, APS-1 and APS-2, was better than the homogeneous processes, where second coordination sphere effects may be involved in directing and stabilizing the transition state.

Understanding the Role of Second Coordination Sphere Effects on Electrochemical CO2 Reduction with Iron Porphyrins

Enzymes achieve fast kinetics and high selectivity by carefully controlling the functional groups in the vicinity of the active site. Collectively, these peripheral groups are termed the Second Coordination Sphere (SCS). Synthetic chemists have long been inspired by the biological importance of the SCS and have demonstrated that the SCS can play an important role in a number of transformations promoted by molecular catalysts. In particular, it is well-known that the kinetics of electrochemical CO2 reduction depend on the presence of SCS functional groups capable of proton transfer, hydrogen bonding, or electrostatic interactions. However, many aspects still remain incompletely understood, such as the pKa requirements for protic SCS groups, the positional dependence of the SCS group with respect to the active site, and the role(s) of the SCS group in the catalytic reaction mechanism. This talk will showcase various SCS modifications made to iron porphyrins and, relying on a combination of molecular synthesis, electrochemistry, spectroscopy, and computational insights, will outline the various ways that the SCS can perturb reaction mechanisms and outcomes of electrochemical CO2 reduction.

Ynone Co-Coordination at a Nickel Borane Complex: An Assessment of Secondary Coordination Sphere Effects.

Secondary coordination sphere ligand effects can be used to direct or organize small molecule substrates at a metal center. Herein, we assess the bifunctional ambiphilic diphosphine, tri-tert-butylboranyldiphosphinoethane (ttbbpe) and its ability to influence stereoselective substrate coordination, while appended to nickel. This report takes a synthetic/computational approach to test the impacts and limitations associated with ligand-directed substrate coordination using [Ni(ttbbpe)(η2:η2-COD)] (COD = 1,5-cyclooctadiene) and ynones (alkynes having an α-carbonyl group at the propargylic position) as model substrates.

Origin of Spin‐State Precise Modulation for Enhanced Oxygen Evolution Activity: Effect of Secondary Coordination Sphere

AbstractThe process of oxygen evolution reaction (OER) is crucial for energy storage and conversion, and the spin electronic structure of catalyst significantly influences its catalytic activity. Precisely regulating the spin electronic structures of metal active centers with intermediate spin (IS) states is challenging but important. This study presents a general method for achieving spin‐state precise modulation by altering the secondary coordination sphere (SCS) in Fe‐substituted LaCo1‐xFexO3 perovskites, denoted as Co6‐y−[Co]−Fey (y = 0–6). The concentration‐dependent SCSs can precisely regulate the spin state of Co3+ from high‐spin (HS) to IS and low‐spin (LS) state by tuning the Co─O binding energy of primary coordination sphere (PCS) to ≈567 KJ mol−1. The binding energy demonstrates a strong negative correlation with the spin state of Co3+, serving as a quantitative descriptor for precise spin‐state modulation. Furthermore, a universal optimal doping concentration is proposed for generating IS‐state Co3+ with the best OER activity, ranging from 1/(m+1) to 2/(m+1) in M‐doped ACo1‐xMxOy system with the coordination number of m. As a proof‐of‐concept, the LaCo7/9Fe2/9O3 with IS Co3+ exhibits significantly enhanced OER activity, almost six times higher than the control samples (without IS Co3+). These findings provide new insights into spin‐state modulation for effective OER catalysts.

Repurposing a supramolecular iridium catalyst via secondary Zn⋯O[double bond, length as m-dash]C weak interactions between the ligand and substrate leads to ortho-selective C(sp2)-H borylation of benzamides with unusual kinetics.

The iridium-catalyzed C-H borylation of benzamides typically leads to meta and para selectivities using state-of-the-art iridium-based N,N-chelating bipyridine ligands. However, reaching ortho selectivity patterns requires extensive trial-and-error screening via molecular design at the ligand first coordination sphere. Herein, we demonstrate that triazolylpyridines are excellent ligands for the selective iridium-catalyzed ortho C-H borylation of tertiary benzamides and, importantly, we demonstrate the almost negligible effect of the first coordination sphere in the selectivity, which is so far unprecedented in iridium C-H bond borylations. Remarkably, the activity is dramatically enhanced by exploiting a remote Zn⋯O[double bond, length as m-dash]C weak interaction between the substrate and a rationally designed molecular-recognition site in the catalyst. Kinetic studies and DFT calculations indicate that the iridium-catalyzed C-H activation step is not rate-determining, this being unique for remotely controlled C-H functionalizations. Consequently, a previously established supramolecular iridium catalyst designed for meta-borylation of pyridines is now compatible with the ortho-borylation of benzamides, a regioselectivity switch that is counter-intuitive regarding precedents in the literature. In addition, we highlight the role of the cyclohexene additive in avoiding the formation of undesired side-products as well as accelerating the HBpin release event that precedes the catalyst regeneration step, which is highly relevant for the design of powerful and selective iridium borylating catalysts.

Combining Electrochemistry and Protein Engineering to Elucidate Outer-Sphere Effects in Hydrogenase Catalysis

Hydrogenases, the bacterial enzymes that oxidize and produce H2, come in two flavors, “NiFe hydrogenases” and “FeFe hydrogenases”, depending on the structure of their active site.In each of these two families of enzymes, the active site and its immediate environment are fully conserved, and yet the investigations of these enzymes using direct electrochemistry have shown that their catalytic properties vary greatly: this is true regarding catalytic bias and reversibility1, resistance to oxygen, and other properties which define whether or not a particular hydrogenase could be used under the operating conditions of a particular device.These functional variations within each family also tell us that structural features that are remote from the active site, the so-called “outer-sphere” effects, define, at least in part, the catalytic properties.2 In the FeFe hydrogenase from Clostridium beijerinckii, we have shown that oxygen resistance is related to a conformational change that depends on non-conserved residues that are up to 18 Å away from the active site. This conformational change occurs in two steps that can be resolved by combining site-directed mutagenesis and detailed protein film voltammetry experiments.3 These remote residues define an emerging family of O2-resistant FeFe hydrogenases. (a) A. Fasano, H. Land, V. Fourmond, G. Berggren, and C. Léger, « Reversible or irreversible catalysis of H+/H2 conversion by FeFe hydrogenases », J. Am. Chem. Soc. 143, 48, 20320-20325 (2021) (b) V. Fourmond, N. Plumeré, C. Léger, « Reversible catalysis », Nature Reviews Chemistry, 5, 348-360 (2021)S. Stripp, B. Duffus, V. Fourmond, C. Léger, S. Leimkühler, S. Hirota, Y. Hu, A. Jasniewski, H. Ogata, M. Ribbe, « Second and Outer Coordination Sphere Effects in Low Valent Metalloenzymes », Chemical Reviews, 122 11900 (2022) (a) M. Winkler, J. Duan, A. Rutz, C. Felbek, L. Scholtysek, O. Lampret, J. Jaenecke, U.-P.r Apfel, G. Gilardi, F. Valetti, V. Fourmond, E. Hofmann, C. Léger, T. Happe « A safety cap protects hydrogenase from oxygen attack » Nature Communications 12, 756 (2021) (b) A. Rutz, C. Das, A. Fasano, J. Jaenecke, S. Yadav, U.P. Apfel, V. Engelbrecht, V. Fourmond, C. Léger, L. Schäfer, T. Happe « Increasing the O2 resistance of the [FeFe]-hydrogenase CbA5H through enhanced protein flexibility », ACS Catalysis 13, 2, 856–865 (2023) Figure 1

Increasing Reduction Potentials of Type 1 Copper Center and Catalytic Efficiency of Small Laccase from Streptomyces coelicolor through Secondary Coordination Sphere Mutations.

The key to type 1 copper (T1Cu) function lies in the fine tuning of the CuII/I reduction potential (E°'T1Cu ) to match those of its redox partners, enabling efficient electron transfer in a wide range of biological systems. While the secondary coordination sphere (SCS) effects have been used to tune E°'T1Cu in azurin over a wide range, these principles are yet to be generalized to other T1Cu-containing proteins to tune catalytic properties. To this end, we have examined the effects of Y229F, V290N and S292F mutations around the T1Cu of small laccase (SLAC) from Streptomyces coelicolor to match the high E°'T1Cu of fungal laccases. Using ultraviolet-visible absorption and electron paramagnetic resonance spectroscopies, together with X-ray crystallography and redox titrations, we have probed the influence of SCS mutations on the T1Cu and corresponding E°'T1Cu . While minimal and small E°'T1Cu increases are observed in Y229F- and S292F-SLAC, the V290N mutant exhibits a major E°'T1Cu increase. Moreover, the influence of these mutations on E°'T1Cu is additive, culminating in a triple mutant Y229F/V290N/S292F-SLAC with the highest E°'T1Cu of 556 mV vs. SHE reported to date. Further activity assays indicate that all mutants retain oxygen reduction reaction activity, and display improved catalytic efficiencies (kcat /KM ) relative to WT-SLAC.

Increasing Reduction Potentials of Type 1 Copper Center and Catalytic Efficiency of Small Laccase from Streptomyces coelicolor through Secondary Coordination Sphere Mutations

AbstractThe key to type 1 copper (T1Cu) function lies in the fine tuning of the CuII/I reduction potential (E°′T1Cu) to match those of its redox partners, enabling efficient electron transfer in a wide range of biological systems. While the secondary coordination sphere (SCS) effects have been used to tune E°′T1Cu in azurin over a wide range, these principles are yet to be generalized to other T1Cu‐containing proteins to tune catalytic properties. To this end, we have examined the effects of Y229F, V290N and S292F mutations around the T1Cu of small laccase (SLAC) from Streptomyces coelicolor to match the high E°′T1Cu of fungal laccases. Using ultraviolet‐visible absorption and electron paramagnetic resonance spectroscopies, together with X‐ray crystallography and redox titrations, we have probed the influence of SCS mutations on the T1Cu and corresponding E°′T1Cu. While minimal and small E°′T1Cu increases are observed in Y229F‐ and S292F‐SLAC, the V290N mutant exhibits a major E°′T1Cu increase. Moreover, the influence of these mutations on E°′T1Cu is additive, culminating in a triple mutant Y229F/V290N/S292F‐SLAC with the highest E°′T1Cu of 556 mV vs. SHE reported to date. Further activity assays indicate that all mutants retain oxygen reduction reaction activity, and display improved catalytic efficiencies (kcat/KM) relative to WT‐SLAC.

Proton Activation in the Presence of a Weak Acid Facilitated by Second Coordination Sphere Effects in Iron Porphyrins**

AbstractThe catalytic activity of two iron‐based porphyrin complexes containing pyridine‐functionalized second coordination spheres, referred to as Py2XPFe and CuPy2XPFe, have been investigated for the hydrogen evolution reaction (HER) and compared with the unsubstituted analog TMPFe in acetonitrile. The CuPy2XPFe incorporates a second metal center within the pyridine residues and represents a heterodinuclear system, while the structurally analogous Py2XPFe lacks an additional metal in the second coordination sphere. Both the Py2XFe and CuPy2XPFe complexes are observed to activate the weak acid, acetic acid (AcOH) at the FeII/I couple rather than at the more energy intensive FeI/0 couple observed for the unfunctionalized TMPFe complex at low acid concentrations. The ability of the monometallic Py2XPFe to activate the weak proton source at the FeII/I couple manifests as an ECEC(E)′ type electrochemical mechanism, rather than an EECC(E)′ type mechanism as observed for TMPFe. The CuPy2XPFe displays improved reactivity compared with the Py2XPFe and TMPFe under the same substrate conditions, however the mechanistic nuances into bimetallic CuPy2XPFe system are currently unclear. The concentration of AcOH was incrementally increased and the rate constants of the initial protonation step i. e. formation of a hydridic species (k1,app), as well as of the protonation of the hydridic species to produce dihydrogen (kglobal) were calculated using the Foot‐of‐the‐wave and plateau current rate equation methodologies, respectively. The kinetic analysis indicates that the activation of protons through the ECEC(E)′ type mechanism for the Py2XPFe results in a system in which k1,app< kglobal. As both the monometallic Py2XPFe and bimetallic CuPy2XPFe activate the AcOH at less cathodic potentials than TMPFe under identical conditions, the overpotentials (ηTOF/2) for these complexes are thus dramatically lower. The reactivity difference of the Py2XPFe when compared to non‐substituted iron porphyrin analogs is due to the hanging group's influence, which is believed to either act as hydrogen bond promoters for the active site or aids to stabilize a catalytic intermediate species during catalysis.

Tailoring Secondary Coordination Sphere Effects in Single-metal-site Catalysts by Surface Immobilization of Supramolecular Cages.

Controlling the coordination sphere of heterogeneous single-metal-site catalysts is a powerful strategy for fine-tuning their catalytic properties but is fairly difficult to achieve. To address this problem, we immobilized supramolecular cages where the primary- and secondary coordination sphere are controlled by ligand design. The kinetics of these catalysts were studied in a model reaction, the hydrolysis of ammonia borane, over a temperature range using fast and precise online measurements generating high-precision Arrhenius plots. The results show how catalytic properties can be enhanced by placing a well-defined reaction pocket around the active site. Our fine-tuning yielded a catalyst with such performance that the reaction kinetics are diffusion-controlled rather than chemically controlled.

Primary and Secondary Coordination Sphere Effects on the Structure and Function of S-Nitrosylating Azurin.

Much progress has been made in understanding the roles of the secondary coordination sphere (SCS) in tuning redox potentials of metalloproteins. In contrast, the impact of SCS on reactivity is much less understood. A primary example is how copper proteins can promote S-nitrosylation (SNO), which is one of the most important dynamic post-translational modifications, and is crucial in regulating nitric oxide storage and transportation. Specifically, the factors that instill CuII with S-nitrosylating capabilities and modulate activity are not well understood. To address this issue, we investigated the influence of the primary and secondary coordination sphere on CuII-catalyzed S-nitrosylation by developing a series of azurin variants with varying catalytic capabilities. We have employed a multidimensional approach involving electronic absorption, S and Cu K-edge XAS, EPR, and resonance Raman spectroscopies together with QM/MM computational analysis to examine the relationships between structure and molecular mechanism in this reaction. Our findings have revealed that kinetic competency is correlated with three balancing factors, namely Cu-S bond strength, Cu spin localization, and relative S(ps) vs S(pp) contributions to the ground state. Together, these results support a reaction pathway that proceeds through the attack of the Cu-S bond rather than electrophilic addition to CuII or radical attack of SCys. The insights gained from this work provide not only a deeper understanding of SNO in biology but also a basis for designing artificial and tunable SNO enzymes to regulate NO and prevent diseases due to SNO dysregulation.

Structural Basis for the Effects of Phenylalanine on Tuning the Reduction Potential of Type 1 Copper in Azurin.

In order to investigate the effects of the secondary coordination sphere in fine-tuning redox potentials (E°') of type 1 blue copper (T1Cu) in cupredoxins, we have introduced M13F, M44F, and G116F mutations both individually and in combination in the secondary coordination sphere of the T1Cu center of azurin (Az) from Pseudomonas aeruginosa. These variants were found to differentially influence the E°' of T1Cu, with M13F Az decreasing E°', M44F Az increasing E°', and G116F Az showing a negligible effect. In addition, combining the M13F and M44F mutations increases E°' by 26 mV relative to WT-Az, which is very close to the combined effect of E°' by each mutation. Furthermore, combining G116F with either M13F or M44F mutation resulted in negative and positive cooperative effects, respectively. Crystal structures of M13F/M44F-Az, M13F/G116F-Az, and M44F/G116F-Az combined with that of G116F-Az reveal these changes arise from steric effects and fine-tuning of hydrogen bond networks around the copper-binding His117 residue. The insights gained from this study would provide another step toward the development of redox-active proteins with tunable redox properties for many biological and biotechnological applications.

An Artificial [Fe4S4]-Containing Metalloenzyme for the Reduction of CO2 to Hydrocarbons.

Iron-sulfur clusters have been reported to catalyze various redox transformations, including the multielectron reduction of CO2 to hydrocarbons. Herein, we report the design and assembly of an artificial [Fe4S4]-containing Fischer-Tropschase relying on the biotin-streptavidin technology. For this purpose, we synthesized a bis-biotinylated [Fe4S4] cofactor with marked aqueous stability and incorporated it in streptavidin. The effect of the second coordination sphere provided by the protein environment was scrutinized by cyclic voltammetry, highlighting the accessibility of the doubly reduced [Fe4S4] cluster. The Fischer-Tropschase activity was improved by chemo-genetic means for the reduction of CO2 to hydrocarbons with up to 14 turnovers.

A Polymeric Unimolecular CO Dehydrogenase Mimic with both Inner and Outer Spheres for Enhanced Photocatalytic CO2 Reduction in Aqueous Solution.

Inspired by carbon monoxide dehydrogenase (CODH), mimicking its inner and outer spheres is a promising strategy in CO2 reduction catalyst design. However, artificialCODH-like catalysts are generally limited to the inner sphere effect and only applicable in organic solvents or for electrocatalysis. Herein, we report an aqueous CODH mimic with both inner and outer spheres for photocatalysis. In this polymeric unimolecular catalyst, the inner sphere is composed of cobalt porphyrin with four appended amido groups and the outer sphere consists of four poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) arms. Upon visible light irradiation (λ > 420 nm), the as-prepared photocatalyst exhibits a high turnover number (TONCO = 1731.2) in the reduction of CO2 into CO, which is superior to most reported molecular photocatalysts in aqueous solution. The mechanism studies indicate that, in this water-dispersible and structurally well-defined CODH mimic, the cobalt porphyrin core serves as the catalysis center, amido groups function as hydrogen-bonding pillars helping to stabilize the CO2 adduct intermediate, while PDMAEMA shell renders both the water solubility and a CO2 reservoir through reversibly capturing of CO2. The present work has clarified the significance of inner and outer coordination sphere effects for improving the aqueous photocatalytic CO2 reduction performance of CODH mimics.

Surface‐Functionalized Nanoparticles as Catalysts for Artificial Photosynthesis

AbstractAnalogously to enzymatic catalysis, where the active metal sites and their environment are controlled by protein residues, the catalytic properties of metal nanoparticles (NPs) can be tuned by carefully selecting their surface‐coordinated species. In artificial photosynthesis, surface‐functionalization emerged in the last decade, grounded on the development of reliable methods for tailored synthesis, advanced characterization and theoretical modeling of metal NPs, altogether with the aim of transferring to the nanoscale the mechanistic knowledge acquired from molecular complexes. Metal NPs surface‐functionalization modulates the energetics of key catalytic intermediates, introduces second coordination sphere effects, influences the catalyst‐electrolyte interface, and determines the metal NPs surface coverage and, accordingly, the number of accessible active sites. In photoactivated systems, metal NPs surface‐functionalization may play a key role in modulating the charge transfers and recombination processes between the light absorber and the active sites and in the light absorber itself. Thus, after a presentation of the most relevant synthetic methods to produce well‐defined surface‐functionalized metal NPs, a critical analysis of why the above effects are the cornerstone in enhancing their catalytic performance in the key processes of artificial photosynthesis, namely the oxygen evolution reaction, the hydrogen evolution reaction, and the CO2 reduction reaction, is given.

Metallocavitins as Advanced Enzyme Mimics and Promising Chemical Catalysts

The supramolecular approach is becoming increasingly dominant in biomimetics and chemical catalysis due to the expansion of the enzyme active center idea, which now includes binding cavities (hydrophobic pockets), channels and canals for transporting substrates and products. For a long time, the mimetic strategy was mainly focused on the first coordination sphere of the metal ion. Understanding that a highly organized cavity-like enzymatic pocket plays a key role in the sophisticated functionality of enzymes and that the activity and selectivity of natural metalloenzymes are due to the effects of the second coordination sphere, created by the protein framework, opens up new perspectives in biomimetic chemistry and catalysis. There are two main goals of mimicking enzymatic catalysis: (1) scientific curiosity to gain insight into the mysterious nature of enzymes, and (2) practical tasks of mankind: to learn from nature and adopt from its many years of evolutionary experience. Understanding the chemistry within the enzyme nanocavity (confinement effect) requires the use of relatively simple model systems. The performance of the transition metal catalyst increases due to its retention in molecular nanocontainers (cavitins). Given the greater potential of chemical synthesis, it is hoped that these promising bioinspired catalysts will achieve catalytic efficiency and selectivity comparable to and even superior to the creations of nature. Now it is obvious that the cavity structure of molecular nanocontainers and the real possibility of modifying their cavities provide unlimited possibilities for simulating the active centers of metalloenzymes. This review will focus on how chemical reactivity is controlled in a well-defined cavitin nanospace. The author also intends to discuss advanced metal–cavitin catalysts related to the study of the main stages of artificial photosynthesis, including energy transfer and storage, water oxidation and proton reduction, as well as highlight the current challenges of activating small molecules, such as H2O, CO2, N2, O2, H2, and CH4.

The acceleration of BODIPY dye-sensitized photocatalytic hydrogen production in aqueous ascorbic acid solutions using alkyl-chain formed second coordination sphere effects

Optimizing hydrogen production in ascorbic acid solutions: enhancing BODIPY dye-sensitized processes through alkyl-chain-enhanced second coordination sphere effects.

Exploring second coordination sphere effects in flavodiiron nitric oxide reductase model complexes.

Flavodiiron nitric oxide reductases (FNORs) equip pathogens with resistance to nitric oxide (NO), an important immune defense agent in mammals, allowing these pathogens to proliferate in the human body, potentially causing chronic infections. Understanding the mechanism of how FNORs mediate the reduction of NO contributes to the greater goal of developing new therapeutic approaches against drug-resistant strains. Recent density functional theory calculations suggest that a second coordination sphere (SCS) tyrosine residue provides a hydrogen bond that is critical for the reduction of NO to N2O at the active site of FNORs [J. Lu, B. Bi, W. Lai and H. Chen, Origin of Nitric Oxide Reduction Activity in Flavo-Diiron NO Reductase: Key Roles of the Second Coordination Sphere, Angew. Chem., Int. Ed., 2019, 58, 3795-3799]. Specifically, this H-bond stabilizes the hyponitrite intermediate and reduces the energetic barrier for the N-N coupling step. At the same time, the role of the Fe⋯Fe distance and its effect on the N-N coupling step has not been fully investigated. In this study, we equipped the H[BPMP] (= 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol) ligand with SCS amide groups and investigated the corresponding diiron complexes with 0-2 bridging acetate ligands. These amide groups can form hydrogen bonds with the bridging acetate ligand(s) and potentially the coordinated NO groups in these model complexes. At the same time, by changing the number of bridging acetate ligands, we can systematically vary the Fe⋯Fe distance. The reactivity of these complexes with NO was then investigated, and the formation of stable iron(II)-NO complexes was observed. Upon one-electron reduction, these NO complexes form Dinitrosyl Iron Complexes (DNICs), which were further characterized using IR and EPR spectroscopy.

Coordination effects on the binding of late 3d single metal species to cyanographene.

Anchoring single metal atoms on suitable substrates is a convenient route towards materials with unique electronic and magnetic properties exploitable in a wide range of applications including sensors, data storage, and single atom catalysis (SAC). Among a large portfolio of available substrates, carbon-based materials derived from graphene and its derivatives have received growing concern due to their high affinity to metals combined with biocompatibility, low toxicity, and accessibility. Cyanographene (GCN) as highly functionalized graphene containing homogeneously distributed nitrile groups perpendicular to the surface offers exceptionally favourable arrangement for anchoring metal atoms enabling efficient charge exchange between the metal and the substrate. However, the binding characteristics of metal species can be significantly affected by the coordination effects. Here we employed density functional theory (DFT) calculations to analyse the role of coordination in the binding of late 3d cations (Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Cu+, and Zn2+) to GCN in aqueous solutions. The inspection of several plausible coordination types revealed the most favourable arrangements. Among the studied species, copper cations were found to be the most tightly bonded to GCN, which was also confirmed by the X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), and isothermal titration calorimetry (ITC) measurements. In general, the inclusion of coordination effects significantly reduced the binding affinities predicted by implicit solvation models. Clearly, to build-up reliable models of SAC architectures in the environments enabling the formation of a coordination sphere, such effects need to be properly taken into account.