A Conserved Binding Pocket in HydF is Essential for Biological Assembly and Coordination of the Diiron Site of [FeFe]-Hydrogenases.

The active site cofactor of [FeFe]-hydrogenases consists of a cubane [4Fe-4S]-cluster and a unique [2Fe-2S]-cluster, harboring unusual CO- and CN--ligands. The biosynthesis of the [2Fe-2S]-cluster requires three dedicated maturation enzymes called HydG, HydE and HydF. HydG and HydE are both involved in synthesizing a [2Fe-2S]-precursor, still lacking parts of the azadithiolate (adt) moiety that bridge the two iron atoms. This [2Fe-2S]-precursor is then finalized within the scaffold protein HydF, which binds and transfers the [2Fe-2S]-precursor to the hydrogenase. However, its exact binding mode within HydF is still elusive. Herein, we identified the binding location of the [2Fe-2S]-precursor by altering size and charge of a highly conserved protein pocket via site directed mutagenesis (SDM). Moreover, we identified two serine residues that are essential for binding and assembling the [2Fe-2S]-precursor. By combining SDM and molecular docking simulations, we provide a new model on how the [2Fe-2S]-cluster is bound to HydF and demonstrate the important role of one highly conserved aspartate residue, presumably during the bioassembly of the adt moiety.

FeNi2S4-A Potent Bifunctional Efficient Electrocatalyst for the Overall Electrochemical Water Splitting in Alkaline Electrolyte.

For a carbon-neutral society, the production of hydrogen as a clean fuel through water electrolysis is currently of great interest. Since water electrolysis is a laborious energetic reaction, it requires high energy to maintain efficient and sustainable production of hydrogen. Catalytic electrodes can reduce the required energy and minimize production costs. In this context, herein, a bifunctional electrocatalyst made from iron nickel sulfide (FeNi2S4 [FNS]) for the overall electrochemical water splitting is introduced. Compared to Fe2NiO4 (FNO), FNS shows a significantly improved performance toward both OER and HER in alkaline electrolytes. At the same time, the FNS electrode exhibits high activity toward the overall electrochemical water splitting, achieving a current density of 10 mA cm-2 at 1.63 V, which is favourable compared to previously published nonprecious electrocatalysts for overall water splitting. The long-term chronopotentiometry test reveals an activation followed by a subsequent stable overall cell potential at around 2.12 V for 20 h at 100 mA cm-2.

Acid-catalyzed Transformation of Nitrite to Nitric Oxide on Copper(II)-Cobalt(II) Centers in a Bimetallic Complex.

Nitrite (NO2-) serves as a pool of nitric oxide (NO) in biological systems under hypoxic conditions, and it is transformed to NO by nitrite reductase (NiR) enzyme in the presence of acid. However, NO synthases generate NO in normoxic conditions. Previously, acid-induced NO2- reduction chemistry was modeled on mono-metallic 3d-metals, generating metal-nitrosyls or NO(g) with H2O or H2O2 products. Herein, to understand the relative potency of a bimetallic system, we report the acid-induced reductive conversion of η2-bound NO2- to NO on CuII-CoII centers of a hetero-bimetallic CuII­nitrito-CoII complex, [(LN8H)CuII­NO2-­CoII]3+ (CuII-NO2--CoII, 2) bearing an octadentate N8-cryptand ligand (LN8H). The CuII-NO2--CoII generates [CuII(LN8H)CoII]4+ (1) upon reaction with one equiv. acid (HClO4, H+ ions source) with NO(g) via a presumed transient nitrousacid (ONOH) intermediate species. Likewise, this NO2- reduction was found to form H2O, which is believed to be from the decomposition of H2O2, an intermediate species. In addition, complex 2, in the presence of more than one equiv. H+ ions also showed the formation of NO(g) with H2O. Mechanistic investigations, using 15N-labeled-15NO2-, 18O-labeled-18O14N16O- and 2H-labeled-DClO4 (D+ source), revealed that the N-atom and O-atom in the 14/15NO and 14N18O gases are derived from NO2- ligand and H-atom in H2O derived from H+-source, respectively.

Electroplated Electrodes for Continuous and Mass-Efficient Electrochemical Hydrogenation.

Electrocatalytic hydrogenations (ECH) enable the reduction of organic substrates upon usage of electric current and present a sustainable alternative to conventional processes if green electricity is used. Opposed to most current protocols for electrode preparation, this work presents a one-step binder- and additive-free production of silver- and copper-electroplated electrodes. Controlled adjustment of the preparation parameters allows for the tuning of catalyst morphology and its electrochemical properties. Upon optimization of the deposition protocol and carbon support, high faradaic efficiencies of 93 % for the ECH of the Vitamin A- and E-synthon 2-methyl-3-butyn-2-ol (MBY) are achieved that can be maintained at current densities of 240 mA cm-2 and minimal catalyst loadings of 0.2 mg cm-2, corresponding to an unmatched production rate of 1.47 kgMBE gcat -1 h-1. For a continuous hydrogenation process, the protocol can be directly transferred into a single-pass operation mode giving a production rate of 1.38 kgMBE gcat -1 h-1. Subsequently, the substrate spectrum was extended to a total of 17 different C-C-, C-O- and N-O-unsaturated compounds revealing the general applicability of the reported process. Our results lay an important groundwork for the development of electrochemical reactors and electrodes able to directly compete with the palladium-based thermocatalytic state of the art.

Insights into the Electrochemical CO2RR Performance and Binding of Small Molecules on Quaternary Thiospinels Ag2FeSn3S8 and Cu2FeSn3S8.

Using a mechanical synthesis method in the form of ball milling and an additional annealing step, a novel and accelerated route for the synthesis of the thiospinels toyohaite (Ag2FeSn3S8) and rhodostannite (Cu2FeSn3S8) was discovered. Both thiospinels display faradaic efficiencies of up to 73% for CO2 reduction to CO using an organic electrolyte in an H-type cell. The materials were furthermore implemented in a zero-gap electrolyzer, with toyohaite producing 22% CO and 52% H2 at 100 mA cm-2 and rhodostannite 28% CO and 37% H2. The catalytically active sites are studied using density functional theory, revealing strong CO binding interactions on both Ag and Cu, whereas Sn is found to contribute to the decomposition of Ag2FeSn3S8 and Cu2FeSn3S8 by coordination with oxygen. Postmortem analysis of the thiospinel-based electrodes by means of SEM-EDX, XRD, XPS, and Mössbauer spectroscopy showed sulfur leaching from the catalysts after applying 100 mA cm-2. These spectroscopic results-in conjunction with DFT calculations of the oxidized surfaces-suggest that the catalytically active species consists of metal oxides. As a conversion of the metal sulfides into the corresponding metallic species was observed via XRD, the decomposition pathways of both catalysts were also computed using DFT; thus, elucidating the energetically most favorable decomposition products and expanding the possible composition of the catalysts postelectrolysis.

Oxygen sensitivity of [FeFe]-hydrogenase: a comparative study of active site mimics inside vs. outside the enzyme.

[FeFe]-hydrogenase is nature's most efficient proton reducing and H2-oxidizing enzyme. However, biotechnological applications are hampered by the O2 sensitivity of this metalloenzyme, and the mechanism of aerobic deactivation is not well understood. Here, we explore the oxygen sensitivity of four mimics of the organometallic active site cofactor of [FeFe]-hydrogenase, [Fe2(adt)(CO)6-x(CN)x]x- and [Fe2(pdt)(CO)6-x(CN)x]x- (x = 1, 2) as well as the corresponding cofactor variants of the enzyme by means of infrared, Mössbauer, and NMR spectroscopy. Additionally, we describe a straightforward synthetic recipe for the active site precursor complex Fe2(adt)(CO)6. Our data indicate that the aminodithiolate (adt) complex, which is the synthetic precursor of the natural active site cofactor, is most oxygen sensitive. This observation highlights the significance of proton transfer in aerobic deactivation, and supported by DFT calculations facilitates an identification of the responsible reactive oxygen species (ROS). Moreover, we show that the ligand environment of the iron ions critically influences the reactivity with O2 and ROS like superoxide and H2O2 as the oxygen sensitivity increases with the exchange of ligands from CO to CN-. The trends in aerobic deactivation observed for the model complexes are in line with the respective enzyme variants. Based on experimental and computational data, a model for the initial reaction of [FeFe]-hydrogenase with O2 is developed. Our study underscores the relevance of model systems in understanding biocatalysis and validates their potential as important tools for elucidating the chemistry of oxygen-induced deactivation of [FeFe]-hydrogenase.

Developing electrochemical hydrogenation towards industrial application.

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Cobalt(III)-Macrocyclic Scaffolds with Anti-Cancer Stem Cell Activity.

Cobalt(III) compounds with tetradentate ligands have been widely employed to deliver cytotoxic and imaging agents into cells. A large body of work has focused on using cobalt(III)-cyclam scaffolds for this purpose. Here, we investigate the cytotoxic properties of cobalt(III) complexes containing 14-membered macrocycles related to cyclam. A breast cancer stem cell (CSC) in vitro model was used to gauge efficacy. Specifically, [Co(1,4,7,11-tetraazacyclotetradecane)Cl2]+ (1) and [Co(1-oxa-4,8,12-triazacyclotetradecane)Cl2]+ (2) were synthesised and characterised, and their breast CSC activity was determined. The cobalt(III) complexes 1 and 2 displayed micromolar potency towards bulk breast cancer cells and breast CSCs grown in monolayers. Notably, 1 and 2 displayed selective potency towards breast CSCs over bulk breast cancer cells (up to 4.5-fold), which was similar to salinomycin (an established breast CSC-selective agent). The cobalt(III) complexes 1 and 2 were also able to inhibit mammosphere formation at low micromolar doses (with respect to size and number). The mammopshere inhibitory effect of 2 was similar to that of salinomycin. Our studies show that cobalt(III) complexes with 1,4,7,11-tetraazacyclotetradecane and 1-oxa-4,8,12-triazacyclotetradecane macrocycles could be useful starting points for the development of new cobalt-based delivery systems that can transport cytotoxic and imaging agents into breast CSCs.

The "Pull Effect" of a Hanging ZnII on Improving the Four-Electron Oxygen Reduction Selectivity with Co Porphyrin.

Due to the challenge of cleaving O-O bonds at single Co sites, mononuclear Co complexes typically show poor selectivity for the four-electron (4e-) oxygen reduction reaction (ORR). Herein, we report on selective 4e- ORR catalyzed by a Co porphyrin with a hanged ZnII ion. Inspired by Cu/Zn-superoxide dismutase, we designed and synthesized 1-CoZn with a hanging ZnII at the second sphere of a Co porphyrin. Complex 1-CoZn is much more effective than its Zn-lacking analogues to catalyze the 4e- ORR in neutral aqueous solutions, giving an electron number of 3.91 per O2 reduction. With spectroscopic studies, the hanging ZnII was demonstrated to be able to facilitate the electron transfer from CoII to O2, through an electronic "pull effect", to give CoIII-superoxo. Theoretical studies further suggested that this "pull effect" plays crucial roles in assisting O-O bond cleavage. This work is significant to present a new strategy of hanging a ZnII to improve O2 activation and O-O bond cleavage.

Improving Active Site Local Proton Transfer in Porous Organic Polymers for Boosted Oxygen Electrocatalysis.

Improving proton transfer is vital for electrocatalysis with porous materials. Although several strategies are reported to assist proton transfer in channels, few studies are dedicated to improving proton transfer at the local environments of active sites in porous materials. Herein, we report on new Co-corrole-based porous organic polymers (POPs) with improved proton transfer for electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By tuning the pore sizes and installing proton relays at Co corrole sites, we designed and synthesized POP-2-OH with improved proton transfer both in channels and at local Co active sites. This POP shows remarkable activity for both electrocatalytic ORR with E1/2 = 0.91 V vs RHE and OER with h10 = 255 mV. Therefore, this work is significant to present a strategy to improve active site local proton transfer in porous materials and highlight the key role of such structural functionalization in boosting oxygen electrocatalysis.

Switching Electrocatalytic Hydrogen Evolution Pathways through Electronic Tuning of Copper Porphyrins.

The electronic structure of metal complexes plays key roles in determining their catalytic features. However, controlling electronic structures to regulate reaction mechanisms is of fundamental interest but has been rarely presented. Herein, we report electronic tuning of Cu porphyrins to switch pathways of the hydrogen evolution reaction (HER). Through controllable and regioselective ß-oxidation of Cu porphyrin 1, we synthesized analogues 2-4 with one or two ß-lactone groups in either a cis or trans configuration. Complexes 1-4 have the same Cu-N4 core site but different electronic structures. Although ß-oxidation led to large anodic shifts of reductions, 1-4 displayed similar HER activities in terms of close overpotentials. With electrochemical, chemical and theoretical results, we show that the catalytically active species switches from a CuI species for 1 to a Cu0 species for 4. This work is thus significant to present mechanism-controllable HER via electronic tuning of catalysts.

Insights into the Molecular Mechanism of Formaldehyde Inhibition of [FeFe]-Hydrogenases.

[FeFe]-hydrogenases are efficient H2 converting biocatalysts that are inhibited by formaldehyde (HCHO). The molecular mechanism of this inhibition has so far not been experimentally solved. Here, we obtained high-resolution crystal structures of the HCHO-treated [FeFe]-hydrogenase CpI from Clostridium pasteurianum, showing HCHO reacts with the secondary amine base of the catalytic cofactor and the cysteine C299 of the proton transfer pathway which both are very important for catalytic turnover. Kinetic assays via protein film electrochemistry show the CpI variant C299D is significantly less inhibited by HCHO, corroborating the structural results. By combining our data from protein crystallography, site-directed mutagenesis and protein film electrochemistry, a reaction mechanism involving the cofactor's amine base, the thiol group of C299 and HCHO can be deduced. In addition to the specific case of [FeFe]-hydrogenases, our study provides additional insights into the reactions between HCHO and protein molecules.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion.

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(µ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(µ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(µ-MePyr)4(µ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(µ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(µ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(µ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

The [4Fe-4S]-Cluster of HydF is not Required for the Binding and Transfer of the Diiron Site of [FeFe]-Hydrogenases.

The active site of [FeFe]-hydrogenases contains a cubane [4Fe-4S]-cluster and a unique diiron cluster with biologically unusual CO and CN- ligands. The biogenesis of this diiron site, termed [2FeH ], requires the maturation proteins HydE, HydF and HydG. During the maturation process HydF serves as a scaffold protein for the final assembly steps and the subsequent transfer of the [2FeH ] precursor, termed [2FeP ], to the [FeFe]-hydrogenase. The binding site of [2FeP ] in HydF has not been elucidated, however, the [4Fe-4S]-cluster of HydF was considered as a possible binding partner of [2FeP ]. By targeting individual amino acids in HydF from Thermosipho melanesiensis using site directed mutagenesis, we examined the postulated binding mechanism as well as the importance and putative involvement of the [4Fe-4S]-cluster for binding and transferring [2FeP ]. Surprisingly, our results suggest that binding or transfer of [2FeP ] does not involve the proposed binding mechanism or the presence of a [4Fe-4S]-cluster at all.

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine.

In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ4 mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations─bis(triflato), bis(acetonitrile), and mixed coligand species─could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment─all of the {N6}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

Substrate-Gated Transformation of a Pre-Catalyst into an Iron-Hydride Intermediate [(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2]- for Catalytic Dehydrogenation of Dimethylamine Borane.

Continued efforts are made on the development of earth-abundant metal catalysts for dehydrogenation/hydrolysis of amine boranes. In this study, complex [K-18-crown-6-ether][(NO)2Fe(µ-MePyr)(µ-CO)Fe(NO)2] (3-K-crown, MePyr = 3-methylpyrazolate) was explored as a pre-catalyst for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution of H2(g) from DMAB triggered by 3-K-crown, parallel conversion of 3-K-crown into [(NO)2Fe(N,N'-MePyrBH2NMe2)]- (5) and an iron-hydride intermediate [(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2]- (A) was evidenced by X-ray diffraction/nuclear magnetic resonance/infrared/nuclear resonance vibrational spectroscopy experiments and supported by density functional theory calculations. Subsequent transformation of A into complex [(NO)2Fe(µ-CO)2Fe(NO)2]- (6) is synchronized with the deactivated generation of H2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)2Fe(η2-BH4)] (4-Na-crown) with CO(g) as an alternative synthetic route, isolated intermediate [Na-18-crown-6-ether][(NO)2(CO)Fe(µ-H)Fe(CO)(NO)2] (A-Na-crown) featuring catalytic reactivity toward dehydrogenation of DMAB supports a substrate-gated transformation of a pre-catalyst [(NO)2Fe(µ-MePyr)(µ-CO)Fe(NO)2]- (3) into the iron-hydride species A as an intermediate during the generation of H2(g).

Adapting Synthetic Models of Heme/Cu Sites to Energy-Efficient Electrocatalytic Oxygen Reduction Reaction.

In nature, cytochrome c oxidases catalyze the 4e- oxygen reduction reaction (ORR) at the heme/Cu site, in which CuI is used to assist O2 activation. Because of the thermodynamic barrier to generate CuI , synthetic Fe-porphyrin/Cu complexes usually show moderate electrocatalytic ORR activity. We herein report on a Co-corrole/Co complex 1-Co for energy-efficient electrocatalytic ORR. By hanging a CoII ion over Co corrole, 1-Co realizes electrocatalytic 4e- ORR with a half-wave potential of 0.89 V versus RHE, which is outstanding among corrole-based electrocatalysts. Notably, 1-Co outperforms Co corrole hanged with CuII or ZnII . We revealed that the hanging CoII ion can provide an electron to improve O2 binding thermodynamically and dynamically, a function represented by the biological CuI ion of the heme/Cu site. This work is significant to present a remarkable ORR electrocatalyst and to show the vital role of a second-sphere redox-active metal ion in promoting O2 binding and activation.

Evidence of Sulfur Non-Innocence in [CoII (dithiacyclam)]2+ -Mediated Catalytic Oxygen Reduction Reactions.

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)]2+ (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S2 N2 -cyclam)]2+ (2; S2 N2 -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the CoII centers in 1 and 2 point to the involvement of sulfur in the O2 reduction process as the key for the improved catalytic ORR capabilities of 2.

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction.

The nucleophilic attack of water or hydroxide on metal-oxo units forms an O-O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1-M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1-M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal-oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1-M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150-mV overpotential to achieve 10 mA/cm2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal-oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal-oxo units bearing trans axial ligands.

Synthesis and Characterization of Phosphorus-Containing Isocyclam Macrocycles and Their Nickel Complexes.

The tetradentate azamacrocycle cyclam (=1,4,8,11-tetraazacyclotetradecane) was studied profoundly for the coordination of transition metal ions, and the resulting complexes were investigated extensively for their catalytic performance in, e.g., O2 activation and electrocatalytic CO2 reduction. Although the successful synthesis of analogous P4 macrocycles was described earlier, no tetradentate N,P mixed 14-membered macrocycles have been prepared to date and their chemistry remains elusive. Thus, in this work, we showcase the synthesis of phospha-aza mixed cyclam-based macrocycles by selectively "exchanging" one or two secondary amines in the macrocycle isocyclam (=1,4,7,11-tetraazacyclotetradecane) with tertiary phosphines. In addition, we herein present the preparation of the corresponding nickel complexes along with their complex chemical and structural characterization to provide first coordination studies.