Molecular Heme-Cyanide-Copper Bridged Assemblies: Linkage Isomerism, Trends in nu(CN) Values, and Relation to the Heme-a(3)/Cu(B) Site in Cyanide-Inhibited Heme-Copper Oxidases

A Selective Copper Based Oxygen Reduction Catalyst for the Electrochemical Synthesis of H2O2 at Neutral pH

H2O2 is a bulk chemical used as "green" alternative in a variety of applications, but has an energy and waste intensive production method. The electrochemical O2 reduction to H2O2 is viable alternative with examples of the direct production of up to 20% H2O2 solutions. In that respect, we found that the dinuclear complex Cu2(btmpa) (6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine) reduces O2 to H2O2 with a selectivity up to 90 % according to single linear sweep rotating ring disk electrode measurements. Microbalance experiments showed that complex reduction leads to surface adsorption thereby increasing the catalytic current. More importantly, we kept a high Faradaic efficiency for H2O2 between 60 and 70 % over the course of 2 h of amperometry by introducing high potential intervals to strip deposited copper (depCu). This is the first example of extensive studies into the long term electrochemical O2 to H2O2 reduction by a molecular complex which allowed to retain the high intrinsic selectivity of Cu2(btmpa) towards H2O2 production leading to relevant levels of H2O2.

End-On Copper(I) Superoxo and Cu(II) Peroxo and Hydroperoxo Complexes Generated by Cryoreduction/Annealing and Characterized by EPR/ENDOR Spectroscopy

In this report, we investigate the physical and chemical properties of monocopper Cu(I) superoxo and Cu(II) peroxo and hydroperoxo complexes. These are prepared by cryoreduction/annealing of the parent [LCuI(O2)]+ Cu(I) dioxygen adducts with the tripodal, N4-coordinating, tetradentate ligands L = PVtmpa, DMMtmpa, TMG3tren and are best described as [LCuII(O2•-)]+ Cu(II) complexes that possess end-on (η1-O2•-) superoxo coordination. Cryogenic γ-irradiation (77 K) of the EPR-silent parent complexes generates mobile electrons from the solvent that reduce the [LCuII(O2•-)]+ within the frozen matrix, trapping the reduced form fixed in the structure of the parent complex. Cryoannealing, namely progressively raising the temperature of a frozen sample in stages and then cooling back to low temperature at each stage for examination, tracks the reduced product as it relaxes its structure and undergoes chemical transformations. We employ EPR and ENDOR (electron-nuclear double resonance) as powerful spectroscopic tools for examining the properties of the states that form. Surprisingly, the primary products of reduction of the Cu(II) superoxo species are metastable cuprous superoxo [LCuI(O2•-)]+ complexes. During annealing to higher temperatures this state first undergoes internal electron transfer (IET) to form the end-on Cu(II) peroxo state, which is then protonated to form Cu(II)-OOH species. This is the first time these methods, which have been used to determine key details of metalloenzyme catalytic cycles and are a powerful tools for tracking PCET reactions, have been applied to copper coordination compounds.

Solid-state and solution characterizations of [(TMPA)Cu(II)(SO3)] and [(TMPA)Cu(II)(S2O3)] complexes: Application to sulfite and thiosulfate fast detection

Sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) ions are used as food preservative and antichlor agent respectively. To detect low levels of such anions we used Cu(II) complex of the Tris-Methyl Pyridine Amine (TMPA) ligand, denoted L. Formation of [LCu(SO₃)] (1) and [LCu(S₂O₃)] (2) in solution were monitored using UV-Vis, EPR and cyclic voltammetry, while the solid-state X-ray structures of both complexes were solved. In addition, we also evaluated the pH range in which the complexes are stable, and the anions binding affinity values for the [LCu(solvent)]²⁺ (3) parent complex. As a matter of illustration, we determined the sulfite content in a commercial crystal sugar.

Influence of Ligand Denticity and Flexibility on the Molecular Copper Mediated Oxygen Reduction Reaction

To date, the copper complex with the tris(2-pyridylmethyl)amine (tmpa) ligand (Cu-tmpa) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the tmpa ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2′:6′,2″-terpyridine (terpy) and bis(2-pyridylmethyl)amine (bmpa) (Cu-terpy and Cu-bmpa, respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the terpy and bmpa ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for H₂O₂ production. The lower catalytic activity is shown to be the result of a stabilization of the Cu^I state of the complex compared to the earlier reported Cu-tmpa catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate terpy and bmpa ligands compared to the tetradentate tmpa ligand. The Laviron plots of the redox behavior of Cu-terpy and Cu-bmpa indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of Cu-terpy and Cu-bmpa to form the preferred tetrahedral coordination geometry for a Cu^I complex easily. Our study illustrates that both the tetradentate nature of the tmpa ligand and the ability of Cu-tmpa to form the preferred tetrahedral coordination geometry for a Cu^I complex are of utmost importance for ORR catalysis with very high catalytic rates.

Redox and electrocatalytic ORR behavior of the mononuclear copper complexes of 2,2′:6′,2″-terpyridine (terpy) and bis(2-pyridylmethyl)amine (bmpa) in neutral aqueous buffer solution: High Faradaic efficiencies for H₂O₂ production were revealed along the ORR active potential window using the rotating ring disk electrode (RRDE), and the foot-of-the-wave analysis (FOWA) was applied to describe the catalytic activity quantitatively. Additionally, the stability of the catalysts under operating conditions receives considerable attention.

Five-coordinate transition metal complexes and the value of τ5: observations and caveats

The τ5 parameter, first proposed by Addison and coworkers, is the principal measure of the geometry of five-coordinate transition metal complexes, with τ5 = 0 said to describe a perfect square pyramidal geometry and τ5 = 1 a perfect trigonal pyramidal geometry. Therefore, the geometries of all five-coordinate complexes are assumed to lie on a continuum between these two extremes. Herein we show that there are a significant number of examples of transition metal complexes having τ5 > 1, leading to an equatorially distorted trigonal bipyramidal geometry with the transition metal ion lying out of the plane of the equatorial donor atoms. We also show that complexes having τ5 = 0 and displaying perfect square pyramidal geometry are very much the exception, and that the majority of complexes for which τ5 = 0 have the metal ion sitting above the mean plane of the donor atoms in the square plane, in a basally distorted square pyramidal geometry. Density functional theory computations on a number of these complexes show that the structural distortions are inherent features of the complexes, and not merely the result of intermolecular interactions.

Intermolecular Interactions and Intramolecular Couplings of Binuclear Porphyrin Models for Cytochrome c Oxidase

Cytochrome c oxidase (CcO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu_B). By using two different porphyrin models derived by Gunter (H₂TPyPP) and us (H₂TImPP), we have isolated several mono- and binuclear complexes including one carbonyl and three chloride derivatives which are determined by 100 K single-crystal X-ray. Low-temperature (4 K) EPR and multitemperature (295-25 K) Mössbauer investigations on the products not only confirmed the spin states of the two metal ions (S = 5/2 Fe³⁺ and S = 1/2 Cu²⁺) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

Commencing an Acidic Battery Based on a Copper Anode with Ultrafast Proton-Regulated Kinetics and Superior Dendrite-Free Property

Building aqueous acidic batteries is in its infancy. There are several sporadic attempts that show desirable electrochemical performance, especially rate stability and high power density. The direct use of a metal anode is regarded as the best protocol for fabricating metal-based batteries. However, introducing an acid-tolerant and electrochemically reversible metal anode into an acidic aqueous battery system remains a considerable challenge. In this work, copper (Cu) metal is used as a reversible metal anode to match acidic regimes with a nearly 100% deposition-dissolution efficiency. The reaction kinetics and mechanism of the Cu anode can be regulated by protons with 400% kinetic acceleration compared with a mild electrolyte. In addition, the anode exhibits a dendrite-free morphology after cycling due to the surface roughening effect, which is different from the morphologies of widely used Zn- and Li-metal anodes. When coupled with the Prussian blue analog as cathodes, the battery delivers ultrafast kinetics of 1830 W kg^-1 at 75 C, which is comparable to the power performance of supercapacitors. Long-term cyclic stability is evaluated, where the capacity retention is 85.6% after 5000 cycles. Finally, flexible fiber-shaped acidic Cu-based batteries are demonstrated for potential wearable applications.

Perspective on the Interfacial Reduction Reaction

Chemical reactions involving oxidation and reduction processes at interfaces may vary from those in conventional liquid-phase or solid-phase reactions and could influence the overall outcome. This article primarily features a study on metal-ligand coordination at the solid-liquid interface. Of particular mention is the spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface without the need of any extraneous reducing agent, unlike in the liquid-phase reaction whereby no reduction of Cu(II) to Cu(I) took place. As a consequence of the interfacial reduction reaction (IRR), thin films of Cu-TCNQ (tetracyanoquinodimethane) and Cu-HCF (hexacyanoferrate) were successfully deposited onto a thiol-functionalized Au substrate via a layer-by-layer (LbL) method. IRR is anticipated to be useful in generating new functional and stimuli-responsive materials, which are otherwise difficult to achieve via conventional liquid-phase reactions.

The fate of copper catalysts in atom transfer radical chemistry

The pathway of atom transfer radical polymerisation (ATRP) is influenced by the nature of the alkyl bromide initiator (RBr) to the extent that reactions between the radical R˙ and the original copper(i) catalyst can divert the reaction toward different products.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Spontaneous Reduction of Copper(II) to Copper(I) at Solid-Liquid Interface

Oxidation and reduction reactions are of central importance in chemistry as well as vital to the basic functions of life and such chemical processes are generally brought about by oxidizing and reducing agents, respectively. Herein, we report the discovery of an interfacial reduction reaction (IRR) - without the use of any external reducing agent. In course of metal-ligand coordination, spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface was observed-unlike in a liquid-phase reaction where no reduction of Cu(II) to Cu(I) was occurred. High-quality thin films of a new coordination network compound bearing a Fe(II)-CN-Cu(I) link were fabricated by IRR and employed for efficient electro-catalysis in the form of oxygen reduction reaction. Also, thermally activated reversible structural phase transition modulated the electron transport property in thin film. This work unveils the importance of chemical reactions at solid-liquid interfaces that can lead to the development of new functional thin film materials.

From protein engineering to artificial enzymes - biological and biomimetic approaches towards sustainable hydrogen production

Hydrogen gas is used extensively in industry today and is often put forward as a suitable energy carrier due its high energy density. Currently, the main source of molecular hydrogen is fossil fuels via steam reforming. Consequently, novel production methods are required to improve the sustainability of hydrogen gas for industrial processes, as well as paving the way for its implementation as a future solar fuel. Nature has already developed an elaborate hydrogen economy, where the production and consumption of hydrogen gas is catalysed by hydrogenase enzymes. In this review we summarize efforts on engineering and optimizing these enzymes for biological hydrogen gas production, with an emphasis on their inorganic cofactors. Moreover, we will describe how our understanding of these enzymes has been applied for the preparation of bio-inspired/-mimetic systems for efficient and sustainable hydrogen production.

Bridging cyanides from cyanoiron metalloligands to redox-active dinitrosyl iron units

Cyanide, as an ambidentate ligand, plays a pivotal role in providing a simple diatomic building-block motif, for controlled metal aggregation, M–CN–M′.

Exploring the boundaries of direct detection and characterization of labile isomers - a case study of copper(ii)-dipeptide systems

The investigation of the linkage isomers of biologically essential and kinetically labile metal complexes in aqueous solutions poses a challenge, as these microspecies cannot be separately studied. Therefore, derivatives are commonly used to initially determine the stability or spectral characteristics of at least one of the isomers. Here we directly detect the isomers, describe the metal ion coordination sphere, speciation and thermodynamic parameters by a synergistic application of temperature dependent EPR and CD spectroscopic measurements in copper(ii)-dipeptide systems including His-Gly and His-Ala ligands. The ΔH = (-23 ± 4) kJ mol^-1 value of the standard enthalpy change corresponding to the peptide-type to histamine-type isomerisation equilibrium of the [CuL]⁺ complex was corroborated by several techniques. The preferential coordination of the side-chains was observed at lower temperatures, whereas, metal-binding of the backbone atoms became favourable upon increasing temperature. This study exemplifies the necessity of using temperature dependent multiple methodologies for a reliable description of similar systems for upstream applications.

Cyanide-bridged iron complexes as biomimetics of tri-iron arrangements in maturases of the H cluster of the di-iron hydrogenase

Concepts from organometallic chemistry are used to define possibilities of cyanide as a docking unit for bioassembly processes.

Developing from certain catalytic processes required for ancient life forms, the H₂ processing enzymes [NiFe]- and [FeFe]-hydrogenase (H₂ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H₂ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent Fe^IFe^I unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.

Insights into water coordination associated with the CuII/CuI electron transfer at a biomimetic Cu centre

The coordination chemistry of an aqua Cu complex was investigated in non-coordinating solvents and in ionic liquids.

(N,N-diethyldithiocarbamato)[tris(3,5-dimethylpyrazol-1-yl)hydroborato]copper(II): a new copper(II) dithiocarbamate compound with the classic Tp(Me2) scorpionate

The title complex, [Cu(C5H10NS2)(C15H22BN6)] or (Tp(Me2))Cu(S2CNEt2), incorporating the classic Tp(Me2) scorpionate, is relevant to blue copper protein models and to Cu extraction from waste treatment and mine-tailing leachate. The IR and UV-Vis spectra are consistent with the crystal structure.

Synthesis of binucleating macrocycles and their nickel(II) hydroxo- and cyano-bridged complexes with divalent ions: anatomical variation of ligand features

The planar NNN-pincer complexes [M(II)(pyN(2)(Me2))(OH)](1-) (M(II) = Ni, Cu) fix CO(2) in η(1)-OCO(2)H complexes; results for the copper system are described. Mn(II), Fe(II), Co(II), and Zn(II) behave differently, forming [M(II)(pyN(2)(Me2))(2)](2-) with N(4)O(2) coordination. Incorporation of the Ni(II) pincer into binucleating macrocycle 2 containing a triamino M(II) locus connected by two 1,3-biphenylene groups affords proximal Ni(II) and M(II) sites for investigation of the synthesis, structure, and reactivity of Ni-X-M bridge units. This ligand structure is taken as a reference for variations in M(II) atoms and binding sites and bridges X = OH(-) and CN(-) to produce additional members of the macrocyclic family with improved properties. Macrocycle 2 with a 22-membered ring is shown to bind M(II) = Mn, Fe, and Cu with hydroxo bridges. Introduction of the 4-Bu(i)O group (macrocycle 3) improves the solubility of neutral complexes such as those with Ni(II)-OH-Cu(II) and Ni(II)-CN-Fe(II) bridges. Syntheses of macrocycle 5 with a 7-Me-[12]aneSN(3) and macrocycle 6 with a 1,8-Me(2)-[14]aneN(4) M(II) binding site are described together with hydoxo-bridged Ni/Cu and cyano-bridged Ni/Fe complexes. This work was motivated by the presence of a Ni···(HO)-Fe bridge grouping in a reactive state of carbon monoxide dehydrogenase. Attempted decrease in Ni-(OH)-M distances (3.70-3.87 Å) to smaller values observed in the enzyme by use of macrocycle 4 having 1,2-biphenylene connectors led to a mononuclear octahedral Ni(II) complex. Bridge structural units are summarized, and the structures of 14 macrocyclic complexes including 8 with bridges are described.

Observation of ligand transfer in ba3 oxidase from Thermus thermophilus: simultaneous FTIR detection of photolabile heme a3(2+)-CN and transient Cu(B)(2+)-CN complexes

FTIR and light-minus-dark FTIR spectroscopy have been employed to investigate the reaction of oxidized and fully reduced ba(3) oxidase with cyanide. The characterization of the structures of the bound CN(-) in the binuclear heme Fe-Cu(B) center is essential, given that a central issue in the function of ba(3) oxidase is the extent to which the partially reduced substrates interact with the two metals. In the reaction of oxidized ba(3) oxidase with cyanide the initially formed heme a(3)(3+)-C≡N-Cu(B)(2+) species with ν(CN) frequency at 2152 cm(-1) was replaced by a photolabile complex with a frequency at 2075 cm(-1) characteristic of heme a(3)(2+)-CN(-). Photolysis of the heme a(3)(2+)-CN(-) adduct produced a band at 2146 cm(-1) attributed to the formation of a transient Cu(B)(2+)-CN(-) complex. All forms are pH independent between pH 5.5-9.5 and at pD 7.5 indicating the absence of ionizable groups that influence the properties of the cyanide complexes. In contrast to previous reports, our results show that CN(-) does not bind simultaneously to both heme a(3)(2+) and Cu(B)(2+) to form the mixed valence a(3)(2+)-CN·Cu(B)(2+)CN species. The photolysis products of the heme a(3)(2+)-CN(-)/Cu(B)(2+) and heme a(3)(2+)-CN(-)/Cu(B)(1+) species are different suggesting that relaxation dynamics in the binuclear center following ligand photodissociation are dependent on the oxidation state of Cu(B).

A tris(2-quinolylmethyl)amine scaffold that promotes hydrogen bonding within the secondary coordination sphere

A new quinolyl-based ligand presents three amide functionalities to act as hydrogen-bond accepting groups to a metal-bound substrate at a well-defined distance. As a confirmation of the design strategy, CH(3)CN coordinated to copper(II) participates in CH-O interactions in the solid state and in solution.

Nonprecious metal catalysts for fuel cell applications: electrochemical dioxygen activation by a series of first row transition metal tris(2-pyridylmethyl)amine complexes

A series of divalent first row triflate complexes supported by the ligand tris(2-pyridylmethyl)amine (TPA) have been investigated as oxygen reduction catalysts for fuel cell applications. [(TPA)M(2+)](n+) (M = Mn, Fe, Co, Ni, and Cu) derivatives were synthesized and characterized by X-ray crystallography, cyclic voltammetry, NMR spectroscopy, magnetic susceptibility, IR spectroscopy, and conductance measurements. The stoichiometric and electrochemical O(2) reactivities of the series were examined. Rotating-ring disk electrode (RRDE) voltammetry was used to examine the catalytic activity of the complexes on a carbon support in acidic media, emulating fuel cell performance. The iron complex displayed a selectivity of 89% for four-electron conversion and demonstrated the fastest reaction kinetics, as determined by a kinetic current of 7.6 mA. Additionally, the Mn, Co, and Cu complexes all showed selective four-electron oxygen reduction (<28% H(2)O(2)) at onset potentials (~0.44 V vs RHE) comparable to state of the art molecular catalysts, while being straightforward to access synthetically and derived from nonprecious metals.

CO and O2 binding to pseudo-tetradentate ligand-copper(I) complexes with a variable N-donor moiety: kinetic/thermodynamic investigation reveals ligand-induced changes in reaction mechanism

The kinetics, thermodynamics, and coordination dynamics are reported for O(2) and CO 1:1 binding to a series of pseudo-tetradentate ligand-copper(I) complexes ((D)LCu(I)) to give Cu(I)/O(2) and Cu(I)/CO product species. Members of the (D)LCu(I) series possess an identical tridentate core structure where the cuprous ion binds to the bispicolylamine (L) fragment. (D)L also contains a fourth variable N-donor moiety {D = benzyl (Bz); pyridyl (Py); imidazolyl (Im); dimethylamino (NMe(2)); (tert-butylphenyl)pyridyl (TBP); quinolyl (Q)}. The structural characteristics of (D)LCu(I)-CO and (D)LCu(I) are detailed, with X-ray crystal structures reported for (TBP)LCu(I)-CO, (Bz)LCu(I)-CO, and (Q)LCu(I). Infrared studies (solution and solid-state) confirm that (D)LCu(I)-CO possess the same four-coordinate core structure in solution with the variable D moiety "dangling", i.e., not coordinated to the copper(I) ion. Other trends observed for the present series appear to derive from the degree to which the D-group interacts with the cuprous ion center. Electrochemical studies reveal close similarities of behavior for (Im)LCu(I) and (NMe(2))LCu(I) (as well as for (TBP)LCu(I) and (Q)LCu(I)), which relate to the O(2) binding kinetics and thermodynamics. Equilibrium CO binding data (K(CO), ΔH°, ΔS°) were obtained by conducting UV-visible spectrophotometric CO titrations, while CO binding kinetics and thermodynamics (k(CO), ΔH(double dagger), ΔS(double dagger)) were measured through variable-temperature (193-293 K) transient absorbance laser flash photolysis experiments, λ(ex) = 355 nm. Carbon monoxide dissociation rate constants (k(-CO)) and corresponding activation parameters (ΔH(double dagger), ΔS(double dagger)) have also been obtained. CO binding to (D)LCu(I) follows an associative mechanism, with the increased donation from D leading to higher k(CO) values. Unlike observations from previous work, the K(CO) values increased as the k(CO) and k(-CO) values declined; the latter decreased at a faster rate. By using the "flash-and-trap" method (λ(ex) = 355 nm, 188-218 K), the kinetics and thermodynamics (k(O(2)), ΔH(double dagger), ΔS(double dagger)) for O(2) binding to (NMe(2))LCu(I) and (Im)LCu(I) were measured and compared to those for (Py)LCu(I). A surprising change in the O(2) binding mechanism was deduced from the thermodynamic ΔS(double dagger) values observed, associative for (Py)LCu(I) but dissociative for (NMe(2))LCu(I) and (Im)LCu(I); these results are interpreted as arising from a difference in the timing of electron transfer from copper(I) to O(2) as this molecule coordinates and a tetrahydrofuran (THF) solvent molecule dissociates. The change in mechanism was not simply related to alterations in (D)LCu(II/I) geometries or the order in which O(2) and THF coordinate. The equilibrium O(2) binding constant (K(O(2)), ΔH°, ΔS°) and O(2) dissociation rate constants (k(-O(2)), ΔH(double dagger), ΔS(double dagger)) were also determined. Overall the results demonstrate that subtle changes in the coordination environment, as occur over time through evolution in nature or through controlled ligand design in synthetic systems, dictate to a critically detailed level the observed chemistry in terms of reaction kinetics, structure, and reactivity, and thus function. Results reported here are also compared to relevant copper and/or iron biological systems and analogous synthetic ligand-copper systems.

Toluene and ethylbenzene aliphatic C-H bond oxidations initiated by a dicopper(II)-mu-1,2-peroxo complex

With an anisole-containing polypyridylamine potential tetradentate ligand (O)L, a mu-1,2-peroxo-dicopper(II) complex [{(O)LCu(II)}(2)(O(2)(2-))](2+) forms from the reaction of the mononuclear compound [Cu(I)((O)L)(MeCN)]B(C(6)F(5))(4) ((O)LCu(I)) with O(2) in noncoordinating solvents at -80 degrees C. Thermal decay of this peroxo complex in the presence of toluene or ethylbenzene leads to rarely seen C-H activation chemistry; benzaldehyde and acetophenone/1-phenylethanol mixtures, respectively, are formed. Experiments with (18)O(2) confirm that the oxygen source in the products is molecular O(2) and deuterium labeling experiments indicate k(H)/k(D) = 7.5 +/- 1 for the toluene oxygenation. The O(2)-reaction of [Cu(I)((Bz)L)(CH(3)CN)](+) ((Bz)LCu(I)) leads to a dicopper(III)-bis-mu-oxo species [{(Bz)LCu(III)}(2)(mu-O(2-))(2)](2+) at -80 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand (Bz)L is a tridentate chelate, possessing the same moiety found in (O)L, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(O)LCu(II)}(2)(O(2)(2-))](2+) is discussed and likely mechanisms of reaction initiated by toluene H-atom abstraction chemistry are detailed. To confirm the structural formulations of the dioxygen-adducts, UV-vis and resonance Raman spectroscopic studies have been carried out and these results are reported and compared to previously described systems including [{Cu(II)((Py)L)}(2)(O(2))](2+) ((Py)L = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu(I), CO-binding properties (i.e., nu(C-O) values) along with electrochemical property comparisons, the relative donor abilities of (O)L, (Bz)L, and (Py)L are assessed.

Reactions of the terminal Ni(II)-OH group in substitution and electrophilic reactions with carbon dioxide and other substrates: structural definition of binding modes in an intramolecular Ni(II)...Fe(II) bridged site

A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged Ni...Fe locus where substrate is bound and transformed in the reversible reaction CO + H(2)O right harpoon over left harpoon CO(2) + 2H(+) + 2e(-). A similar structure has been sought in this work. Mononuclear planar Ni(II) complexes [Ni(pyN(2)(Me2))L](1-) (pyN(2)(Me2) = bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2-)) derived from a NNN pincer ligand have been prepared including L = OH(-) (1) and CN(-) (7). Complex 1 reacts with ethyl formate and CO(2) to form unidentate L = HCO(2)(-) (5) and HCO(3)(-) (6) products. A binucleating macrocycle was prepared which specifically binds Ni(II) at a NNN pincer site and five-coordinate Fe(II) at a triamine site. The Ni(II) macrocyle forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl(2) alone and with ethyl formate and 15 with FeCl(2) affords molecules with the Ni(II)-L-Fe(II) bridge unit in which L = mu(2):eta(1)-OH(-) (17) and mu(2):eta(2)-HCO(2)(-) (18) and -CN(-) (19). All bridges are nonlinear (17, 140.0 degrees ; 18, M-O-C 135.9 degrees (Ni), 120.2 degrees (Fe); 19, Ni-C-N 170.3 degrees , Fe-N-C 141.8 degrees ) with Ni...Fe separations of 3.7-4.8 A. The Ni(II)Fe(II) complexes, lacking appropriate Ni-Fe-S cluster structures, are not site analogues, but their synthesis and reactivity provide the first demonstration that molecular Ni(II)...Fe(II) sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.

Copper(I)/O2 chemistry with imidazole containing tripodal tetradentate ligands leading to mu-1,2-peroxo-dicopper(II) species

Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L(MIm), (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine, and L(EIm), 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethanamine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry, compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching frequencies obtained from carbon monoxide adducts of [(L(MIm))Cu(I)](+) (1a) and [(L(EIm))Cu(I)](+) (2a) show that the imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant of Cu(II) to L(EIm) compared to TMPA and L(MIm), reflecting geometric changes. Oxygenation of [(L(MIm))Cu(I)](+) (1a) in 2-methyltetrahydrofuran (MeTHF) solvent at -128 degrees C leads to an intensely purple colored species with a UV-vis spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(L(MIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (1b(P)) {lambda(max) = 528 nm}, very similar to the previously well characterized complex [{(TMPA)Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) {lambda(max) = 520 nm (epsilon = 12 000 M(-1) cm(-1)), in MeTHF; resonance Raman (rR) spectroscopy: nu(O-O) = 832 (Delta((18)O(2)) = -44) cm(-1)}. In the low-temperature oxygenation of 2a, benchtop (-128 degrees C) and stopped-flow (-90 degrees C) experiments reveal the formation of an initial superoxo-Cu(II) species [(L(EIm))Cu(II)(O(2)(*-))](+) (2b(S)), lambda(max) = 431 nm in THF) . This converts to the low-temperature stable peroxo complex [{(L(EIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (2b(P)) {rR spectroscopy: nu(O-O) = 822 (Delta((18)O(2)) = -46) cm(-1)}. Complex 2b(P) possess distinctly reduced Cu-O and O-O stretching frequencies and a red-shifted UV-vis feature {to lambda(max) = 535 nm (epsilon = 11 000 M(-1) cm(-1))} compared to the TMPA analogue due to a distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(mu(2)-L(EIm-))(4)(Cu(II))(4)](4+), obtained from thermal decomposition of 2b(P) (with formation of H(2)O(2)), also exhibits a distorted square pyramidal Cu(II) ion geometry as does the copper(II) complex [(L(EIm))Cu(II)(CH(3)CN)](2+) (2c), characterized by X-ray crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.

Copper-Carbon Bonds in Mechanistic and Structural Probing of Proteins as well as in Situations where Copper is a Catalytic or Receptor Site

While copper-carbon bonds are well appreciated in organometallic synthetic chemistry, such occurrences are less known in biological settings. By far, the greatest incidence of copper-carbon moieties is in bioinorganic research aimed at probing copper protein active site structure and mechanism; for example, carbon monoxide (CO) binding as a surrogate for O2. Using infrared (IR) spectroscopy, CO coordination to cuprous sites has proven to be an extremely useful tool for determining active site copper ligation (e.g., donor atom number and type). The coupled (hemocyanin, tyrosinase, catechol oxidase) and non-coupled (peptidylglycine α-hydroxylating monooxygenase, dopamine β-monooxygenase) binuclear copper proteins as well as the heme-copper oxidases (HCOs) have been studied extensively via this method. In addition, environmental changes within the vicinity of the active site have been determined based on shifts in the CO stretching frequencies, such as for copper amine oxidases, nitrite reductases and again in the binuclear proteins and HCOs. In many situations, spectroscopic monitoring has provided kinetic and thermodynamic data on CuI-CO formation and CO dissociation from copper(I); recently, processes occurring on a femtosecond timescale have been reported. Copper-cyano moieties have also been useful for obtaining insights into the active site structure and mechanisms of copper-zinc superoxide dismutase, azurin, nitrous oxide reductase, and multi-copper oxidases. Cyanide is a good ligand for both copper(I) and copper(II), therefore multiple physical-spectroscopic techniques can be applied. A more obvious occurrence of a “Cu-C” moiety was recently described for a CO dehydrogenase which contains a novel molybdenum-copper catalytic site. A bacterial copper chaperone (CusF) was recently established to have a novel d-π interaction comprised of copper(I) with the arene containing side-chain of a tryptophan amino acid residue. Meanwhile, good evidence exists that a plant receptor site (ETR1) utilizes copper(I) to sense ethylene, a growth hormone. A copper olfactory receptor has also been suggested. All of the above mentioned occurrences or uses of carbon-containing substrates and/or probes are reviewed and discussed within the framework of copper proteins and other relevant systems.