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

Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity

The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and Triphos^Si and report on their different coordination properties. Whereas reaction of Triphos^Si and FeX₂ (X = Cl, Br) exclusively afforded (Triphos^Si)­FeX₂ with a κ²-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)­Fe­(CH₃CN)₃]­[Fe₂Cl₆] or [(Triphos)­Fe­(CH₃CN)₃]­[FeBr₄], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (Triphos^Si)­FeCl₂ and (Triphos)­FeCl₂ to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes

Redox Induced Configurational Isomerization of Bisphosphine–Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes

The tricarbonyliron (TCFe) complexes Fe­(CO)₃(dppf) and Fe­(CO)₃(dppp), where dppf = 1,1′-bis­(diphenylphosphino)­ferrocene and dppp = 1,3-bis­(diphenylphosphino)­propane, exhibit redox activity that induces configurational isomerization. The presence of the ferrocenyl (Fc) group stabilizes higher oxidized forms of TCFe. Using spectroelectrochemistry (IR, UV–vis, Mössbauer, and EPR) and computational analysis, we can show that the Fc in the backbone of the dppf ligand tends to form a weak dative bond to the electrophilic TCFe^I and TCFe^II species. The open shell TCFe^I intermediate was stabilized by the distribution of spin between the two Fe centers (Fc and TCFe), whereas lacking the Fc moiety resulted in highly reactive TCFe^I species. The [Fe­(CO)₃(dppf)]⁺ cation adopts two possible configurations, square-pyramidal (without an Fe–Fe interaction) and trigonal-bipyramidal (containing an Fe–Fe interaction). The two configurations are in equilibrium with the trigonal-bipyramidal configuration being enthalpically favored (Δ<i>H</i> = −7 kJ mol^–1). There is an entropic penalty (Δ<i>S</i> = −20 J mol^–1) due to tilting of the Cp (cyclopentadienide) rings of the dppf moieties by ∼8°. Additionally, the terminal iron hydride [FeH­(CO)₃(dppf)]­BF₄ was formed by protonation with a strong acid (HBF₄·Et₂O)

Aging-Associated Enzyme Human Clock-1: Substrate-Mediated Reduction of the Diiron Center for 5‑Demethoxyubiquinone Hydroxylation

The mitochondrial membrane-bound enzyme Clock-1 (CLK-1) extends the average longevity of mice and <i>Caenorhabditis elegans</i>, as demonstrated for Δ<i>clk-1</i> constructs for both organisms. Such an apparent impact on aging and the presence of a carboxylate-bridged diiron center in the enzyme inspired this work. We expressed a soluble human CLK-1 (hCLK-1) fusion protein with an N-terminal immunoglobulin binding domain of protein G (GB1). Inclusion of the solubility tag allowed for thorough characterization of the carboxylate-bridged diiron active site of the resulting GB1-hCLK-1 by spectroscopic and kinetic methods. Both UV–visible and Mössbauer experiments provide unambiguous evidence that GB1-hCLK-1 functions as a 5-demethoxyubiquinone-hydroxylase, utilizing its carboxylate-bridged diiron center. The binding of DMQ_<i>n</i> (<i>n</i> = 0 or 2) to GB1-hCLK-1 mediates reduction of the diiron center by nicotinamide adenine dinucleotide (NADH) and initiates O₂ activation for subsequent DMQ hydroxylation. Deployment of DMQ to mediate reduction of the diiron center in GB1-hCLK-1 improves substrate specificity and diminishes consumption of NADH that is uncoupled from substrate oxidation. Both <i>V</i>_max and <i>k</i>_cat/<i>K</i>_M for DMQ hydroxylation increase when DMQ₀ is replaced by DMQ₂ as the substrate, which demonstrates that an isoprenoid side chain enhances enzymatic hydroxylation and improves catalytic efficiency

Cobalt-Rich Multimetallic Selenides-Exploring Relationships between Chemical Composition, Temperature Treatment, and Electrocatalytic Performance of Solid Electrodes

Multicomponent, transition-metal selenides characterized by TM3Se4 stoichiometry, and monoclinic pseudospinel structure were recently reported as promising catalysts for water-splitting processes. However, the initial data indicate that the simple increase in the number of composing elements might not be sufficient to maximize their performance, with the systematic screening of the different regions of multicomponent phase diagrams proving to be the most effective approach. Thus, in this work, a series of highly conductive bimetallic and trimetallic selenides were synthesized using a high-temperature synthesis and inductive hot-pressing method. Their electrocatalytic activity toward hydrogen evolution reaction was studied and correlated with the chemical composition and corresponding electronic structure, as well as temperature treatment and related microstructure, on both theoretical and experimental grounds. A clear dependence between the composition of the material, its processing, and catalytic activity was established, allowing for a better understanding and more efficient design of catalysts belonging to this material group

Influence of the ZnCrAl Oxide Composition on the Formation of Hydrocarbons from Syngas

The conversion of syngas into value-added hydrocarbons gains increasing attention due to its potential to produce sustainable platform chemicals from simple starting materials. Along this line, the “OX-ZEO” process that combines a methanol synthesis catalyst with a zeolite, capable of catalyzing the methanol-to-hydrocarbon reaction, was found to be a suitable alternative to the classical Fischer–Tropsch synthesis. Hitherto, understanding the mechanism of the OX-ZEO process and simultaneously optimizing the CO conversion and the selectivity toward a specific hydrocarbon remains challenging. Herein, we present a comparison of a variety of ZnCrAl oxides with different metal ratios combined with a H-ZSM-5 zeolite for the conversion of syngas to hydrocarbons. The effect of aluminum on the catalytic activity was investigated for ZnCrAl oxides with a Zn/Cr ratio of 4:1, 1:1, and 1:2. The product distribution and CO conversion were found to be strongly influenced by the Zn/Cr/Al ratio. Although a ratio of Zn/Cr of 1:2 was best to produce lower olefins and aromatics, with aromatic selectivities of up to 37%, catalysts with a 4:1 ratio revealed high paraffin selectivity up to 52%. Notably, a distinct effect of aluminum in the oxide lattice on the catalytic activity and product selectivity was observed, as a higher Al content leads to a lower CO conversion and a changed product spectrum. We provide additional understanding of the influence of different compositions of ZnCrAl oxides on their surface properties and the catalytic activity in the OX-ZEO process. Furthermore, the variation of the zeolite component supports the important role of the channel topology of the porous support material for the hydrocarbon production. In addition, variation of the gas hourly space velocity showed a correlation of contact time, CO conversion, and hydrocarbon selectivity. At a gas hourly space velocity of 4200 mL/gcat h, CO conversion as high as 44% along with a CO2 selectivity of 42% and a lower paraffin (C20–C40) selectivity of 41% was observed

A Novel [FeFe] Hydrogenase Model with a (SCH₂)₂PO Moiety

A novel [FeFe]-hydrogenase model complex containing phosphine oxide in the dithiolato ligand, namely [Fe₂(CO)₆]­[(μ-SCH₂)₂(Ph)­PO] (<b>1</b>), has been synthesized and characterized. Complex <b>1</b> was prepared via the reaction of equimolar quantities of (μ-LiS)₂Fe₂(CO)₆ and OP­(Ph)­(CH₂Cl)₂. The protonation properties of complex <b>1</b> have been investigated by monitoring the changes in IR (in the ν­(CO) region) and ³¹P­{¹H} NMR spectra upon addition of pyridinium tetrafluoroborate, [HPy]­[BF₄], and HBF₄·Et₂O, suggesting protonation of the PO functionality. In addition, high-level DFT calculations on the protonation sites of complex <b>1</b> in CH₂Cl₂ have been performed and support our experimental observations that the PO unit is protonated by HBF₄·Et₂O. Cyclic voltammetric experiments on complex <b>1</b> showed an anodic shift of the oxidation peak upon addition of HBF₄·Et₂O, suggesting a CE process

A Novel [FeFe] Hydrogenase Model with a (SCH₂)₂PO Moiety

A novel [FeFe]-hydrogenase model complex containing phosphine oxide in the dithiolato ligand, namely [Fe₂(CO)₆]­[(μ-SCH₂)₂(Ph)­PO] (<b>1</b>), has been synthesized and characterized. Complex <b>1</b> was prepared via the reaction of equimolar quantities of (μ-LiS)₂Fe₂(CO)₆ and OP­(Ph)­(CH₂Cl)₂. The protonation properties of complex <b>1</b> have been investigated by monitoring the changes in IR (in the ν­(CO) region) and ³¹P­{¹H} NMR spectra upon addition of pyridinium tetrafluoroborate, [HPy]­[BF₄], and HBF₄·Et₂O, suggesting protonation of the PO functionality. In addition, high-level DFT calculations on the protonation sites of complex <b>1</b> in CH₂Cl₂ have been performed and support our experimental observations that the PO unit is protonated by HBF₄·Et₂O. Cyclic voltammetric experiments on complex <b>1</b> showed an anodic shift of the oxidation peak upon addition of HBF₄·Et₂O, suggesting a CE process

Interplay between CN^– Ligands and the Secondary Coordination Sphere of the H‑Cluster in [FeFe]-Hydrogenases

The catalytic cofactor of [FeFe]-hydrogenses (H-cluster) is composed of a generic cubane [4Fe-4S]-cluster (4Fe_H) linked to a binuclear iron–sulfur cluster (2Fe_H) that has an open coordination site at which the reversible conversion of protons to molecular hydrogen occurs. The (2Fe_H) subsite features a diatomic coordination sphere composed of three CO and two CN^– ligands affecting its redox properties and providing excellent probes for FTIR spectroscopy. The CO stretch vibrations are very sensitive to the redox changes within the H-cluster occurring during the catalytic cycle, whereas the CN^– signals seem to be relatively inert to these effects. This could be due to the more structural role of the CN^– ligands tightly anchoring the (2Fe_H) unit to the protein environment through hydrogen bonding. In this work we explore the effects of structural changes within the secondary ligand sphere affecting the CN^– ligands on FTIR spectroscopy and catalysis. By comparing the FTIR spectra of wild-type enzyme and two mutagenesis variants, we are able to assign the IR signals of the individual CN^– ligands of the (2Fe_H) site for different redox states of the H-cluster. Moreover, protein film electrochemistry reveals that targeted manipulation of the secondary coordination sphere of the proximal CN^– ligand (i.e., closest to the (4Fe_H) site) can affect the catalytic bias. These findings highlight the importance of the protein environment for re-adjusting the catalytic features of the H-cluster in individual enzymes and provide valuable information for the design of artificial hydrogenase mimics

Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (Fe_<i>x</i>Ni_1–<i>x</i>)₉S₈ Electrocatalysts Applied in the Hydrogen Evolution Reaction

Inspired by our recent finding that Fe_4.5Ni_4.5S₈ rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (Fe_<i>x</i>Ni_1–<i>x</i>)₉S₈ (<i>x</i> = 0–1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal–metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts