N2O Reductase Activity of a [Cu4S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

The model complex [Cu₄(μ₄‐S)(dppa)₄]²⁺ ( 1 , dppa=μ₂‐(Ph₂P)₂NH) has N₂O reductase activity in methanol solvent, mediating 2 H⁺/2 e^? reduction of N₂O to N₂+H₂O in the presence of an exogenous electron donor (CoCp₂). A stoichiometric product with two deprotonated dppa ligands was characterized, indicating a key role of second‐sphere N?H residues as proton donors during N₂O reduction. The activity of 1 towards N₂O was suppressed in solvents that are unable to provide hydrogen bonding to the second‐sphere N?H groups. Structural and computational data indicate that second‐sphere hydrogen bonding induces structural distortion of the [Cu₄S] active site, accessing a strained geometry with enhanced reactivity due to localization of electron density along a dicopper edge site. The behavior of 1 mimics aspects of the Cu_Z catalytic site of nitrous oxide reductase: activity in the 4Cu^I:1S redox state, use of a second‐sphere proton donor, and reactivity dependence on both primary and secondary sphere effects.     

Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties

A strategy is presented to improve the excited state reactivity of homoleptic copper–bis(diimine) complexes CuL₂⁺ by increasing the steric bulk around Cu^I whereas preserving their stability. Substituting the phenanthroline at the 2-position by a phenyl group allows the implementation of stabilizing intramolecular π stacking within the copper complex, whereas tethering a branched alkyl chain at the 9-position provides enough steric bulk to rise the excited state energy E⁰⁰. Two novel complexes are studied and compared to symmetrical models. The impact of breaking the symmetry of phenanthroline ligands on the photophysical properties of the complexes is analyzed and rationalized thanks to a combined theoretical and experimental study. The importance of fine-tuning the steric bulk of the N–N chelate in order to stabilize the coordination sphere is demonstrated. Importantly, the excited state reactivity of the newly developed complexes is improved as demonstrated in the frame of a reductive quenching step, evidencing the relevance of our strategy.     

Ultrafast Intramolecular Relaxation and Wave‐Packet Motion in a Ruthenium‐Based Supramolecular Photocatalyst

The hydrogen‐evolving photocatalyst [(tbbpy)₂Ru(tpphz)Pd(Cl)₂]²⁺ (tbbpy=4,4′‐di‐tert‐butyl‐2,2′‐bipyridine, tpphz=tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine) shows excitation‐wavelength‐dependent catalytic activity, which has been correlated to the localization of the initial excitation within the coordination sphere. In this contribution the excitation‐wavelength dependence of the early excited‐state relaxation and the occurrence of vibrational coherences are investigated by sub‐20 fs transient absorption spectroscopy and DFT/TDDFT calculations. The comparison with the mononuclear precursor [(tbbpy)₂Ru(tpphz)]²⁺ highlights the influence of the catalytic center on these ultrafast processes. Only in the presence of the second metal center, does the excitation of a ¹MLCT state localized on the central part of the tpphz bridge lead to coherent wave‐packet motion in the excited state.     

Enhanced Electrocatalytic Activity of a Zinc Porphyrin for CO2 Reduction: Cooperative Effects of Triazole Units in the Second Coordination Sphere

The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO₂) to carbon monoxide (CO). This electrocatalyst converted CO₂ to CO with a Faradaic efficiency of 99 % and a current density of −6.2 mA cm⁻² at −2.4 V vs. Fc/Fc⁺ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO₂-catalyst adduct in order to facilitate intramolecular proton transfer. Our findings demonstrate that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO₂ conversion.     

Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

To develop highly efficient molecular photocatalysts for visible light‐driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X‐ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl₂ (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.     

Origin of Nitric Oxide Reduction Activity in Flavo–Diiron NO Reductase: Key Roles of the Second Coordination Sphere

The second coordination sphere constitutes a distinguishing factor in the active site to modulate enzymatic reactivity. To unravel the origin of NO‐to‐N₂O reduction activity of non‐heme diiron enzymes, herein we report a strong second‐coordination‐sphere interaction between a conserved Tyr₁₉₇ and the key iron–nitrosyl intermediate of Tm FDP (flavo–diiron protein), which leads to decreased reaction barriers towards N–N formation and N–O cleavage in NO reduction. This finding supports the direct coupling of diiron dinitrosyl as the N–N formation mode in our QM/MM modeling, and reconciles the mechanistic controversy of external reduction between FDPs and synthetic biomimetics of the iron–nitrosyls. This work highlights the application of QM/MM ⁵⁷Fe Mössbauer modeling in elucidating the structural features of not only first, but also second coordination spheres of the key transient species involved in NO/O₂ activation by non‐heme diiron enzymes.     

Cu(Ⅱ)金属有机骨架中配位环境对类Fenton反应活性的影响

以一水合醋酸铜为铜源,以2,6-二（4''-吡啶基）-4-甲基苯胺（L）为桥联吡啶配体、间苯二甲酸（H₂IPA）为共配体分别合成了一维链状配位聚合物{[（Cu （OAc）₂）₂（L）]·3CH₃CN}_n（1,OAc^-=CH₃CO₂^-）和二维网状配位聚合物{[Cu （IPA）（L）（H₂O）]₂·H₂IPA·H₂O}_n（2）。通过二者的单晶结构分析可以看出,配合物1中的铜原子位于[CuNO₄]₂簇中的四面体配位环境中心,配合物2中的铜原子处于[CuNO₃]变形六面体配位环境的中心,不同的配位环境导致2个配合物具有差异化的光催化降解有机物的活性。通过以亚甲基蓝为底物的类Fenton光催化降解对比实验表明,含有Cu—N、Cu—O配位环境的配合物1的催化效果优于具有相同四面体构型配位环境的HKUST-1,且配合物1和2催化性能的对比也证明了开放性单核铜配位中心的光催化降解活性优于簇合物中的铜配位中心。得益于配体的稳定和框架结构的存在,与相同条件下无配体约束的醋酸铜盐的催化性能相比,2个配合物均具有更高的催化活性和可循环利用特性。通过UV-Vis光谱计算了二者的带隙,光催化降解前后的粉末X射线衍射及电感耦合等离子质谱证明了配合物的稳定性。通过添加自由基捕捉剂苯醌、叔丁醇和三乙醇胺,证实了该催化过程为羟基自由基过程的类Fenton反应机理。     

Homoleptic,bis‐ligated magnesium complexes for ring‐opening polymerization of lactide and lactones: Synthesis,structure, polymerization behavior and mechanism studies

Bis‐ligated, homoleptic magnesium complexes 1–3 were synthesized through the reaction of 1 equiv. dibutyl magnesium with 2 equiv. β‐ketiminato ligands bearing different substituents on the nitrogen atom and 8 position on benzocyclohexanone. All of the complexes were identified by nuclear magnetic resonance (NMR) and X‐ray crystallography. Complexes 2 and 3 adopted distorted tetrahedral geometry around Mg, by chelating of two ancillary ligands, while complex 1 adopted a dimeric structure with penta‐coordination around Mg. These complexes can be used as efficient catalysts for the ring‐opening polymerization of L‐lactide, ε‐caprolactone, δ‐valerolactone (δ‐VL) and trimethylene carbonate in the presence of alcohol as a co‐initiator. With the increasing steric bulk of the ancillary ligands, the catalytic activity of Mg complexes was improved significantly. Particularly, complex 3 having the largest steric hindrance showed excellent catalytic performance for the polymerization of δ‐VL. It could polymerize 800 equiv. δ‐VL in 10 min, and produce polyvalerolactone with narrow molecular weight distributions (M_w/M_n < 1.2) at 35°C or higher temperature. No transesterification side reaction was observed. Moreover, complex 3 exhibited good tolerance to excessive alcohol and an immortal polymerization characteristic. The mechanism studies by in situ NMR demonstrated a coordination‐insertion process. Besides, it revealed that the steric bulky substituents in the active species derived from the complex and alcohol prevented the metal center from deactivation.     

Carbon‐Nanotube‐Supported Bio‐Inspired Nickel Catalyst and Its Integration in Hybrid Hydrogen/Air Fuel Cells

A biomimetic nickel bis‐diphosphine complex incorporating the amino acid arginine in the outer coordination sphere was immobilized on modified carbon nanotubes (CNTs) through electrostatic interactions. The functionalized redox nanomaterial exhibits reversible electrocatalytic activity for the H₂/2 H⁺ interconversion from pH 0 to 9, with catalytic preference for H₂ oxidation at all pH values. The high activity of the complex over a wide pH range allows us to integrate this bio‐inspired nanomaterial either in an enzymatic fuel cell together with a multicopper oxidase at the cathode, or in a proton exchange membrane fuel cell (PEMFC) using Pt/C at the cathode. The Ni‐based PEMFC reaches 14 mW cm⁻², only six‐times‐less as compared to full‐Pt conventional PEMFC. The Pt‐free enzyme‐based fuel cell delivers ≈2 mW cm⁻², a new efficiency record for a hydrogen biofuel cell with base metal catalysts.     

Steric control of coordination: Unusual coordination mode of dimethylpyridine-2,6-dicarboxylate in a new dinuclear copper(II) complex [(dmpc)(Cl)(μ-Cl)Cu2(μ-Cl)(Cl)(dmpc)] and reversal of the coordination mode in [Cu(dmpc) (H2O)3] (ClO4)2

A series of dinuclear copper(II) complexes (1), (2), (3) and (4) containing dimethylpyridine-2,6-dicarboxylate were synthesised and structurally characterised to delineate the role of the steric demand in deciding the observed coordination mode. In contrast to the bulky isopropyl derivative with symmetrical O-sp² carbonylic coordination, the less sterically demanding methyl derivative adopts an unusual asymmetric coordination involving both O-sp² carbonylic and O-sp³ alkoxy atoms of the ester moiety. However, the symmetric O-sp² carbonylic coordination is reestablished upon dissolution in MeOH. In the absence of a sterically encumbered [Cu₂(-Cl)₂] core in (2), (3) and (4), the coordination reverts to the symmetrical carbonylic oxygen coordinating mode, underscoring the importance of overall space availability (steric demand) in and around the coordination sphere of the metal atom. Associated with these differential coordination modes, interesting structural dissimilarities are encountered in both the coordinating ligand and the coordination sphere of the copper centre. Besides X-ray crystallographic investigations on (1) and (2), culminating evidence is gathered through i.r., u.v.–vis. spectra and magnetic susceptibility measurements.     

Formation of High‐Valent Iron–Oxo Species in Superoxide Reductase: Characterization by Resonance Raman Spectroscopy

Superoxide reductase (SOR), a non‐heme mononuclear iron protein that is involved in superoxide detoxification in microorganisms, can be used as an unprecedented model to study the mechanisms of O₂ activation and of the formation of high‐valent iron–oxo species in metalloenzymes. By using resonance Raman spectroscopy, it was shown that the mutation of two residues in the second coordination sphere of the SOR iron active site, K₄₈ and I₁₁₈, led to the formation of a high‐valent iron–oxo species when the mutant proteins were reacted with H₂O₂. These data demonstrate that these residues in the second coordination sphere tightly control the evolution and the cleavage of the O? O bond of the ferric iron hydroperoxide intermediate that is formed in the SOR active site.     

catena‐Poly[[aquacopper(II)]‐μ‐hydroxido‐μ‐naphthalene‐1‐carboxylato‐κ2O:O′]: effect of a bulky aromatic skeleton in self‐assembly

In the title coordination polymer, [Cu(C₁₁H₇O₂)(OH)(H₂O)]_n, the Cu^II center is five‐coordinated by two O atoms from two different naphthalene‐1‐carboxylate (L) ligands, one O atom from one coordinated water molecule and two O atoms from two hydroxide anions. L ligands and hydroxide anions jointly bridge adjacent Cu^II centers to generate a one‐dimensional chain along the b‐axis direction. The results reveal that the steric bulk of the naphthalene ring system in L may play an important role in the formation of the title complex.     

Crystal structure,characterization and catalytic activities of Cu(II) coordination complexes with 8‐hydroxyquinoline and pyrazine‐2‐carboxylic acid

Green and blue crystals of the coordination complexes [Cu(8‐hquin)₂(H₂O)₂], 1 and [Cu(pyzca)₂(H₂O)₂], 2 were obtained by the reaction of copper chloride with 8‐hydroxyquinoline (8‐hquinH) or pyrazine‐2‐carboxylic acid (pyzcaH) as ligands. The structures of 1 and 2 were characterized by elemental analyses, electronic absorption, Infrared (IR) and thermal studies. The luminescent behavior complexes 1 and 2 was also discussed. The coordination environment of copper(II) center displays distorted octahedral coordination geometry. The structure of the complexes 1 and 2 is constructed by an infinite number of discrete mononuclear molecules extending along the a‐axis to form a 1D‐chain via H‐bonds. The extensive hydrogen bonds and short contacts develop the structures of 1 and 2 to 3D‐network. The catalytic behavior of the complexes 1 and 2 was utilized for degradation of methylene blue dye (MB). The kinetic data indicated that the complexes 1 and 2 are effective catalysts for degradation of MB dye. Photoluminescence probing technology was used as a sensitive probe for detecting ^?OH radicals.     

The Glutathione/Metallothionein System Challenges the Design of Efficient O2‐Activating Copper Complexes

Copper complexes are of medicinal and biological interest, including as anticancer drugs designed to cleave intracellular biomolecules by O₂ activation. To exhibit such activity, the copper complex must be redox active and resistant to dissociation. Metallothioneins (MTs) and glutathione (GSH) are abundant in the cytosol and nucleus. Because they are thiol‐rich reducing molecules with high Cu^I affinity, they are potential competitors for a copper ion bound in a copper drug. Herein, we report the investigation of a panel of Cu^I/Cu^II complexes often used as drugs, with diverse coordination chemistries and redox potentials. We evaluated their catalytic activity in ascorbate oxidation based on redox cycling between Cu^I and Cu^II, as well as their resistance to dissociation or inactivation under cytosolically relevant concentrations of GSH and MT. O₂‐activating Cu^I/Cu^II complexes for cytosolic/nuclear targets are generally not stable against the GSH/MT system, which creates a challenge for their future design.     

The Ortho Effect on the Acidic and Alkaline Hydrolysis of Substituted Formanilides

The kinetics of formanilides hydrolysis were determined under first‐order conditions in hydrochloric acid (0.01–8 M, 20–60°C) and in hydroxide solutions (0.01–3 M, 25 and 40°C). Under acidic conditions, second‐order specific acid catalytic constants were used to construct Hammett plots. The ortho effect was analyzed using the Fujita–Nishioka method. In alkaline solutions, hydrolysis displayed both first‐ and second‐order dependence in the hydroxide concentration. The specific base catalytic constants were used to construct Hammett plots. Ortho effects were evaluated for the first‐order dependence on the hydroxide concentration. Formanilide hydrolyzes in acidic solutions by specific acid catalysis, and the kinetic study results were consistent with the A_AC2 mechanism. Ortho substitution led to a decrease in the rates of reaction due to steric inhibition of resonance, retardation due to steric bulk, and through space interactions. The primary hydrolytic pathway in alkaline solutions was consistent with a modified B_AC2 mechanism. The Hammett plots for hydrolysis of meta‐ and para‐substituted formanilides in 0.10 M sodium hydroxide solutions did not show substituent effects; however, ortho substitution led to a decrease in rate constants proportional to the steric bulk of the substituent.     

α‐olefin homopolymerization and ethylene/1‐hexene copolymerization catalysed by novel ansa‐group IV complexes/MAO system

The catalytic properties of a set of ansa‐complexes (R‐Ph)₂C(Cp)(Ind)MCl₂ [R = ^tBu, M = Ti ( 3 ), Zr ( 4 ) or Hf ( 5 ); R = MeO, M = Zr ( 6 ), Hf ( 7 )] in α‐olefin homopolymerization and ethylene/1‐hexene copolymerization were explored in the presence of MAO (methylaluminoxane). Complex 4 with steric bulk ^tBu group on phenyl exhibited remarkable catalytic activity for ethylene polymerization. It was 1.6‐fold more active than complex 11 [Ph₂C(Cp)(Ind)ZrCl₂] at 11 atm ethylene pressure and was 4.8‐fold more active at 1 atm pressure. The introduction of bulk substituent ^tBu into phenyl groups not only increased the catalytic activity greatly but also enhanced the content of 1‐hexene in ethylene/1‐hexene copolymerization. The highest 1‐hexene incorporation was 25.4%. In addition, 4 was also active for propylene and 1‐hexene homopolymerization, respectively, and low isotactic polypropylene (mmmm = 11.3%) and isotactic polyhexene (mmmm = 31.6%) were obtained. Copyright © 2007 John Wiley & Sons, Ltd.     

The Role of a Dipeptide Outer‐Coordination Sphere on H2‐Production Catalysts: Influence on Catalytic Rates and Electron Transfer

The outer‐coordination sphere of enzymes acts to fine‐tune the active site reactivity and control catalytic rates, suggesting that incorporation of analogous structural elements into molecular catalysts may be necessary to achieve rates comparable to those observed in enzyme systems at low overpotentials. In this work, we evaluate the effect of an amino acid and dipeptide outer‐coordination sphere on [Ni(P^Ph₂N^Ph‐R₂)₂]²⁺ hydrogen production catalysts. A series of 12 new complexes containing non‐natural amino acids or dipeptides was prepared to test the effects of positioning, size, polarity and aromaticity on catalytic activity. The non‐natural amino acid was either 3‐(meta‐ or para‐aminophenyl)propionic acid terminated as an acid, an ester or an amide. Dipeptides consisted of one of the non‐natural amino acids coupled to one of four amino acid esters: alanine, serine, phenylalanine or tyrosine. All of the catalysts are active for hydrogen production, with rates averaging ～1000 s^?1, 40 % faster than the unmodified catalyst. Structure and polarity of the aliphatic or aromatic side chains of the C‐terminal peptide do not strongly influence rates. However, the presence of an amide bond increases rates, suggesting a role for the amide in assisting catalysis. Overpotentials were lower with substituents at the N‐phenyl meta position. This is consistent with slower electron transfer in the less compact, para‐substituted complexes, as shown in digital simulations of catalyst cyclic voltammograms and computational modeling of the complexes. Combining the current results with insights from previous results, we propose a mechanism for the role of the amino acid and dipeptide based outer‐coordination sphere in molecular hydrogen production catalysts.     

Photosystem II Inspired NiFe-Based Electrocatalysts for Efficient Water Oxidation via Second Coordination Sphere Effect

The mismatched fast-electron-slow-proton process in the electrocatalytic oxygen evolution reaction (OER) severely restricts the catalytic efficiency. To overcome these issues, accelerating the proton transfer and elucidating the kinetic mechanism are highly sought after. Herein, inspired by photosystem II, we develop a family of OER electrocatalysts with FeO₆/NiO₆ units and carboxylate anions (TA²⁻) in the first and second coordination sphere, respectively. Benefiting from the synergistic effect of the metal units and TA²⁻, the optimized catalyst delivers superior activity with a low overpotential of 270 mV at 200 mA cm⁻² and excellent cycling stability over 300 h. A proton-transfer-promotion mechanism is proposed by in situ Raman, catalytic tests, and theoretical calculations. The TA²⁻ (proton acceptor) can mediate proton transfer pathways by preferentially accepting protons, which optimizes the O−H adsorption/activation process and reduces the kinetic barrier for O−O bond formation.     

Coordination Booster‐Catalyst Assembly: Remote Osmium Outperforming Ruthenium in Boosting Catalytic Activity

Presented herein is a set of bimetallic and trimetallic “coordination booster‐catalyst” assemblies in which the coordination complexes [Ru^II(terpy)₂] and [Os^II(terpy)₂] acted as boosters for enhancement of the catalytic activity of [Ru^II(NHC)(para‐cymene)]‐based catalytic site. The boosters accelerated the oxidative loss of para‐cymene from the catalytic site to generate the active catalyst during the oxidation of alkenes and alkynes into corresponding aldehydes, ketones and diketones. It was found that the boosting efficiency of the [Os^II(terpy)₂] units was considerably higher than its congener [Ru^II(terpy)₂] unit in these assemblies. Mechanistic studies were conducted to understand this unique improvement.     

Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square‐Planar Copper(I) Complex

6,6′′‐Bis(2,4,6‐trimethylanilido)terpyridine (H₂Tpy^NMes) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H₂Tpy^NMes)copper(I)‐halide (Cl, Br and I) complexes is dictated by the strength of the NH–halide hydrogen bond. The Cu^ICl and Cu^IICl complexes are nearly isostructural, the former presenting a highly unusual square‐planar geometry about Cu^I. The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid Cu^I/Cu^II electron‐transfer self‐exchange rates. Cu(H₂Tpy^NMes)Cl shows similar fast electron transfer (≈10⁵ m ^?1 s^?1) which is the same order of magnitude as biological systems.