BOX Ligands in Biomimetic Copper-Mediated Dioxygen Activation: A Hemocyanin Model

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

Coupled dinuclear copper oxygen cores (Cu₂ O₂ ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O₂ transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (^C P) have been proposed as key structures in the early stages of O₂ binding in these proteins; their reversible isomerization to other Cu₂ O₂ cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic ^C P cores in biological and synthetic systems, the properties and reactivity of ^C P Cu₂ O₂ species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (^T P) and ^C P dicopper cores. Ca^II mediates this process by reversible binding at the Cu₂ O₂ core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O₂ activation in biomimetic systems.

On the metal-ligand bonding in dinuclear complexes with redox-active guanidine ligands

Coordination compounds with redox-active ligands are currently intensively studied. Within this research theme, redox-active guanidines have been established as a new, eminent class of redox-active ligands. In this work the variation of metal-guanidine bonding in dinuclear transition metal complexes with bridging redox-active tetrakisguanidine ligands is analysed. A series of dinuclear complexes with different metals (Mn, Fe, Co, Ni, Cu and Zn) is synthesized, using either newly prepared redox-active tetrakisguanidino-dioxine or previously reported tetrakisguanidino-benzene ligands. The discussion of the bond properties in this work is predominantly based on the trends of structural parameters, derived from determination of single-crystal structures by X-ray diffraction and quantum chemical calculations. In addition, the trends in the redox potentials and magnetometric (SQUID) measurements on some of the complexes are included. Due to their combined σ- and π-electron donor capability, redox-active guanidine ligands are weak-field ligands; the σ- and π-bonding contributions vary with the metal. The results highlight the peculiarity of copper-guanidine bonding with a high π-bond contribution to metal-guanidine bonding, enabled by structural distortion of the coordination mode from tetrahedral in the direction of square-planar, short copper-guanidine bonds and minor displacement of the copper atoms from the ligand aromatic plane.

Catalytic Asymmetric Fluorination of Copper Carbene Complexes: Preparative Advances and a Mechanistic Rationale

The Cu-catalyzed reaction of substituted α-diazoesters with fluoride gives α-fluoroesters with ee values of up to 95 %, provided that chiral indane-derived bis(oxazoline) ligands are used that carry bulky benzyl substituents at the bridge and moderately bulky isopropyl groups on their core. The apparently homogeneous solution of CsF in C6 F6 /hexafluoroisopropanol (HFIP) is the best reaction medium, but CsF in the biphasic mixture CH2 Cl2 /HFIP also provides good results. DFT studies suggest that fluoride initially attacks the Cu- rather than the C-atom of the transient donor/acceptor carbene intermediate. This unusual step is followed by 1,2-fluoride shift; for this migratory insertion to occur, the carbene must rotate about the Cu-C bond to ensure orbital overlap. The directionality of this rotatory movement within the C2 -symmetric binding site determines the sense of induction. This model is in excellent accord with the absolute configuration of the resulting product as determined by X-ray diffraction using single crystals of this a priori wax-like material grown by capillary crystallization.

Record Broken: A Copper Peroxide Complex with Enhanced Stability and Faster Hydroxylation Catalysis

Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl)methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3-tBuPz)₂ (4-CO₂ MePy) which stabilises a room-temperature stable μ-η² :η² -peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning.

Acid/base triggered interconversion of μ-η2:η2-peroxido and bis(μ-oxido) dicopper intermediates capped by proton-responsive ligands

CuII2(μ-η2:η2-peroxido) and CuIII2(μ-oxido)2 cores represent key intermediates in copper/dioxygen chemistry, and they are mechanistically important for biological hydroxylation and oxidation reactions mediated by dinuclear (type III) copper metalloenzymes. While the exact nature of the active species in different enzymes is still under debate, shifting equilibria between Cu x /O2 species is increasingly recognized as a means of switching between distinct reactivity patterns of these intermediates. Herein we report comprehensive spectroscopic, crystallographic and computational analysis of a family of synthetic CuII2(μ-η2:η2-peroxido) and CuIII2(μ-oxido)2 dicopper complexes with a bis(oxazoline) (BOX) capping ligand. In particular, we demonstrate that a reversible peroxido/bis(μ-oxido) interconversion of the [Cu2O2] core can be triggered by peripheral (de)protonation events on the ligand backbone. As the copper ions in the enzymes are typically supported by histidine imidazoles that offer a backside N atom amenable to potential (de)protonation, it is well conceivable that the shifting of equilibria between the [Cu2O2] species in response to changes in local pH is biologically relevant.

Activation of dioxygen by copper metalloproteins and insights from model complexes

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

High-valent copper in biomimetic and biological oxidations

A long-standing debate in the Cu-O₂ field has revolved around the relevance of the Cu(III) oxidation state in biological redox processes. The proposal of Cu(III) in biology is generally challenged as no spectroscopic or structural evidence exists currently for its presence. The reaction of synthetic Cu(I) complexes with O₂ at low temperature in aprotic solvents provides the opportunity to investigate and define the chemical landscape of Cu-O₂ species at a small-molecule level of detail; eight different types are characterized structurally, three of which contain at least one Cu(III) center. Simple imidazole or histamine ligands are competent in these oxygenation reactions to form Cu(III) complexes. The combination of synthetic structural and reactivity data suggests (1) that Cu(I) should be considered as either a one or two electron reductant reacting with O₂, (2) that Cu(III) reduction potentials of these formed complexes are modest and well within the limits of a protein matrix and (3) that primary amine and imidazole ligands are surprisingly good at stabilizing Cu(III) centers. These Cu(III) complexes are efficient oxidants for hydroxylating phenolate substrates with reaction hallmarks similar to that performed in biological systems. The remarkable ligation similarity of the synthetic and biological systems makes it difficult to continue to exclude Cu(III) from biological discussions.

Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.