Modeling the Peroxide/Superoxide Continuum in 1:1 Side-on Adducts of O2 with Cu

Redox Isomerism in the S3 State of the Oxygen-Evolving Complex Resolved by Coupled Cluster Theory

The electronic and geometric structures of the water-oxidizing complex of photosystem II in the steps of the catalytic cycle that precede dioxygen evolution remain hotly debated. Recent structural and spectroscopic investigations support contradictory redox formulations for the active-site Mn4 CaOx cofactor in the final metastable S3 state. These range from the widely accepted MnIV 4 oxo-hydroxo model, which presumes that O-O bond formation occurs in the ultimate transient intermediate (S4 ) of the catalytic cycle, to a MnIII 2 MnIV 2 peroxo model representative of the contrasting "early-onset" O-O bond formation hypothesis. Density functional theory energetics of suggested S3 redox isomers are inconclusive because of extreme functional dependence. Here, we use the power of the domain-based local pair natural orbital approach to coupled cluster theory, DLPNO-CCSD(T), to present the first correlated wave function theory calculations of relative stabilities for distinct redox-isomeric forms of the S3 state. Our results enabled us to evaluate conflicting models for the S3 state of the oxygen-evolving complex (OEC) and to quantify the accuracy of lower-level theoretical approaches. Our assessment of the relevance of distinct redox-isomeric forms for the mechanism of biological water oxidation strongly disfavors the scenario of early-onset O-O formation advanced by literal interpretations of certain crystallographic models. This work serves as a case study in the application of modern coupled cluster implementations to redox isomerism problems in oligonuclear transition metal systems.

A theoretical investigation into the first-row transition metal–O2 adducts

Effects of both metal center and ligand ring size on the properties of metal–O₂ adducts.

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.

Re-evaluating the Cu K pre-edge XAS transition in complexes with covalent metal-ligand interactions

Three [Me2NN]Cu(η2-L2) complexes (Me2NN = HC[C(Me)NAr]2; L2 = PhNO (2), (3), PhCH[double bond, length as m-dash]CH2 (4); Ar = 2,6-Me2-C6H3; ArF = 3,5-(CF3)2-C6H3) have been studied by Cu K-edge X-ray absorption spectroscopy, as well as single- and multi-reference computational methods (DFT, TD-DFT, CASSCF, MRCI, and OVB). The study was extended to a range of both known and theoretical compounds bearing 2p-element donors as a means of deriving a consistent view of how the pre-edge transition energy responds in systems with significant ground state covalency. The ground state electronic structures of many of the compounds under investigation were found to be strongly influenced by correlation effects, resulting in ground state descriptions with majority contributions from a configuration comprised of a Cu(ii) metal center anti-ferromagentically coupled to radical anion O2, PhNO, and ligands. In contrast, the styrene complex 4, which displays a Cu K pre-edge transition despite its formal d10 electron configuration, exhibits what can best be described as a Cu(i):(styrene)0 ground state with strong π-backbonding. The Cu K pre-edge features for these complexes increase in energy from 1 to 4, a trend that was tracked to the percent Cu(ii)-character in the ground state. The unexpected shift to higher pre-edge transition energies with decreasing charge on copper (Q Cu) contributed to an assignment of the pre-edge features for these species as arising from metal-to-ligand charge transfer instead of the traditional Cu1s → Cu3d designation.

Mononuclear nickel(II)-superoxo and nickel(III)-peroxo complexes bearing a common macrocyclic TMC ligand

Mononuclear metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are generated as key intermediates in the catalytic cycles of dioxygen activation by heme and non-heme metalloenzymes. We have shown recently that the geometric and electronic structure of the Ni-O2 core in [Ni(n-TMC)(O2)]+ (n = 12 and 14) varies depending on the ring size of the supporting TMC ligand. In this study, mononuclear Ni(II)-superoxo and Ni(III)-peroxo complexes bearing a common macrocylic 13-TMC ligand, such as [NiII(13-TMC)(O2)]+ and [NiIII(13-TMC)(O2)]+, were synthesized in the reaction of [NiII(13-TMC)(CH3CN)]2+ and H2O2 in the presence of tetramethylammonium hydroxide (TMAH) and triethylamine (TEA), respectively. The Ni(II)-superoxo and Ni(III)-peroxo complexes bearing the common 13-TMC ligand were successfully characterized by various spectroscopic methods, X-ray crystallography, and DFT calculations. Based on the combined experimental and theoretical studies, we conclude that the superoxo ligand in [NiII(13-TMC)(O2)]+ is bound in an end-on fashion to the nickel(II) center, whereas the peroxo ligand in [NiIII(13-TMC)(O2)]+ is bound in a side-on fashion to the nickel(III) center. Reactivity studies performed with the Ni(II)-superoxo and Ni(III)-peroxo complexes toward organic substrates reveal that the former possesses an electrophilic character, whereas the latter is an active oxidant in nucleophilic reaction.

Dioxygen reactivity of new bispidine-copper complexes

The reactivity of copper complexes of three different second-generation bispidine-based ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane; mono- and bis-tetradentate; exclusively tertiary amine donors) with dioxygen [(reversible) binding of dioxygen by copper(I)] is reported. The UV-vis, electrospray ionization mass spectrometry, electron paramagnetic resonance, and vibrational spectra (resonance Raman) of the dioxygen adducts indicate that, depending on the ligand and reaction conditions, several different species (mono- and dinuclear, superoxo, peroxo, and hydroperoxo), partially in equilibrium with each other, are formed. Minor changes in the ligand structure and/or experimental conditions (solvent, temperature, relative concentrations) allow switching between the different forms. With one of the ligands, an end-on peroxodicopper(II) complex and a mononuclear hydroperoxocopper(II) complex could be characterized. With another ligand, reversible dioxygen binding was observed, leading to a metastable superoxocopper(II) complex. The amount of dioxygen involved in the reversible binding to Cu(I) was determined quantitatively. The mechanism of dioxygen binding as well as the preference of each of the three ligands for a particular dioxygen adduct is discussed on the basis of a computational (density functional theory) analysis.

X-ray absorption spectroscopic and computational investigation of a possible S···S interaction in the [Cu3S2]3+ core

The electronic structure of the [Cu(3)S(2)](3+) core of [(LCu)(3)(S)(2)](3+) (L = N,N,N',N'-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu(3)S(2)](3+) core is best described as having all copper centers close to but more oxidized than Cu(2+), while the charge on the S(2) fragment is between that of a sulfide (S(2-)) and a subsulfide (S(2)(3-)) species. The [Cu(3)S(2)](3+) core thus is different from a previously described, analogous [Cu(3)O(2)](3+) core, which has a localized [(Cu(3+)Cu(2+)Cu(2+))(O(2-))(2)](3+) electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S-S σ* to the Cu and result in some bonding interaction between the two S atoms at ~2.69 Å in [Cu(3)S(2)](3+), stabilizing a delocalized S = 1 ground state.

Geometric and electronic structure and reactivity of a mononuclear "side-on" nickel(III)-peroxo complex

Metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. The synthesis and spectroscopic characterization of an end-on nickel(II)-superoxo complex with a 14-membered macrocyclic ligand was reported previously. Here we report the isolation, spectroscopic characterization, and high-resolution crystal structure of a mononuclear side-on nickel(III)-peroxo complex with a 12-membered macrocyclic ligand, [Ni(12-TMC)(O(2))](+) (1) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Different from the end-on Ni(II)-superoxo complex, the Ni(III)-peroxo complex is not reactive in electrophilic reactions, but is capable of conducting nucleophilic reactions. The Ni(III)-peroxo complex transfers the bound dioxygen to manganese(II) complexes, thus affording the corresponding nickel(II) and manganese(III)-peroxo complexes. The present results demonstrate the significance of supporting ligands in tuning the geometric and electronic structures and reactivities of metal-O(2) intermediates that have been shown to have biological as well as synthetic usefulness in biomimetic reactions.

Synthesis, structural, and spectroscopic characterization and reactivities of mononuclear cobalt(III)-peroxo complexes

Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+), were synthesized by reacting [Co(12-TMC)(CH(3)CN)](2+) and [Co(13-TMC)(CH(3)CN)](2+), respectively, with H(2)O(2) in the presence of triethylamine. The mononuclear cobalt(III)-peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on η(2) fashion. The O-O bond stretching frequency of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) was determined to be 902 cm(-1) by resonance Raman spectroscopy. The structural properties of the CoO(2) core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) were 1.4389(17) Å and 1.438(6) Å, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O(2)-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt-peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O(2))](+) > [Co(12-TMC)(O(2))](+). In the O(2)-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O(2)-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O(2)-transfer reactions was the same as that observed in the aldehyde oxidation reactions.

Metal-dioxygen and metal-dinitrogen complexes: where are the electrons?

Transition-metal complexes of O(2) and N(2) play an important role in the environment, chemical industry, and metalloenzymes. This Perspective compares and contrasts the binding modes, reduction levels, and electronic influences on the nature of the bound O(2) or N(2) group in these complexes. The charge distribution between the metal and the diatomic ligand is variable, and different models for describing the adducts have evolved. In some cases, single resonance structures (e.g. M-superoxide = M-O(2)(-)) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N(2)(2-) complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O(2) and N(2) complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.

The restricted active space followed by second-order perturbation theory method: theory and application to the study of CuO2 and Cu2O2 systems

A multireference second-order perturbation theory using a restricted active space self-consistent field wave function as reference (RASPT2/RASSCF) is described. This model is particularly effective for cases where a chemical system requires a balanced orbital active space that is too large to be addressed by the complete active space self-consistent field model with or without second-order perturbation theory (CASPT2 or CASSCF, respectively). Rather than permitting all possible electronic configurations of the electrons in the active space to appear in the reference wave function, certain orbitals are sequestered into two subspaces that permit a maximum number of occupations or holes, respectively, in any given configuration, thereby reducing the total number of possible configurations. Subsequent second-order perturbation theory captures additional dynamical correlation effects. Applications of the theory to the electronic structure of complexes involved in the activation of molecular oxygen by mono- and binuclear copper complexes are presented. In the mononuclear case, RASPT2 and CASPT2 provide very similar results. In the binuclear cases, however, only RASPT2 proves quantitatively useful, owing to the very large size of the necessary active space.

Isotopic probing of molecular oxygen activation at copper(I) sites

Copper-dioxygen (CuO2) adducts are frequently proposed as intermediates in enzymes, yet their electronic and vibrational structures have not always been understood. [Cu(eta1-O2)TMG3tren]+ (TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) features end-on (eta1) O2 coordination in the solid state. Described here is an investigation of the compound's solution properties by nuclear magnetic resonance spectroscopy, density functional calculations, and oxygen isotope effects. The study yields two major findings. First, [Cu(eta1-O2)TMG3tren]+ is paramagnetic due to a triplet electronic structure; this is in contrast to other copper compounds where O2 is bound in a side-on manner. Second, the oxygen equilibrium isotope effect upon O2 binding to copper(I) (18O EIE [triple bond] K(16O16O)/K(16O18O) = 1.0148 +/- 0.0012) is significantly larger than those determined for iron and cobalt eta1-O2 adducts. This result is suggested to reflect greater ionic (CuII-O2-I) character within the valence bond description. A revised interpretation of the physical origins of the 18O EIEs upon O2 binding to redox metals is also advanced along with experimental data that should be used as benchmarks for interpreting 18O kinetic isotope effects upon enzyme reactions.

Copper(II)-Mediated Aromaticortho-Hydroxylation: A Hybrid DFT and Ab Initio Exploration

Mechanistic pathways for the aromatic hydroxylation by [CuII(L1)(TMAO)(O)](-) (L1=hippuric acid, TMAO=trimethylamine N-oxide), derived from the O--N bond homolysis of its [CuII(L1)(TMAO)2] precursor, were explored by using hybrid density functional theory (B3LYP) and highly correlated ab initio methods (QCISD and CCSD). Published experimental studies suggest that the catalytic reaction is triggered by a terminal copper-oxo species, and a detailed study of electronic structures, bonding, and energetics of the corresponding electromers is presented. Two pathways, a stepwise and a concerted reaction, were considered for the hydroxylation process. The results reveal a clear preference for the concerted pathway, in which the terminal oxygen atom directly attacks the carbon atom of the benzene ring, leading to the ortho-selectively hydroxylated product. Solvent effects were probed by using the PCM and CPCM solvation models, and the PCM model was found to perform better in the present case. Excellent agreement between the experimental and computational results was found, in particular also for changes in reactivity with derivatives of L1.

X-ray absorption edge spectroscopy and computational studies on LCuO2 species: Superoxide-Cu(II) versus peroxide-Cu(III) bonding

The geometric and electronic structures of two mononuclear CuO2 complexes, [Cu(O2){HB(3-Ad-5-(i)Prpz)3}] (1) and [Cu(O2)(beta-diketiminate)] (2), have been evaluated using Cu K- and L-edge X-ray absorption spectroscopy (XAS) studies in combination with valence bond configuration interaction (VBCI) simulations and spin-unrestricted broken symmetry density functional theory (DFT) calculations. Cu K- and L-edge XAS data indicate the Cu(II) and Cu(III) nature of 1 and 2, respectively. The total integrated intensity under the L-edges shows that the 's in 1 and 2 contain 20% and 28% Cu character, respectively, indicative of very covalent ground states in both complexes, although more so in 1. Two-state VBCI simulations also indicate that the ground state in 2 has more Cu (/3d8) character. DFT calculations show that the in both complexes is dominated by O2(n-) character, although the O2(n-) character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O2 bonding in these LCuO2 systems and tunes the ground states of 1 and 2 to have dominant Cu(II)-superoxide-like and Cu(III)-peroxide-like character, respectively. The contributions of ligand field (LF) and the charge on the absorbing atom in the molecule (Q(mol)M) to L- and K-edge energy shifts are evaluated using DFT and time-dependent DFT calculations. It is found that LF makes a dominant contribution to the edge energy shift, while the effect of Q(mol)M is minor. The charge on the Cu in the Cu(III) complex is found to be similar to that in Cu(II) complexes, which indicates a much stronger interaction with the ligand, leading to extensive charge transfer.

Mononuclear Cu-O2 complexes: geometries, spectroscopic properties, electronic structures, and reactivity

Using interwoven experimental and theoretical methods, detailed studies of several structurally defined 1:1 Cu-O 2 complexes have provided important fundamental chemical information useful for understanding the nature of intermediates involved in aerobic oxidations in synthetic and enzymatic copper-mediated catalysis. In particular, these studies have shed new light on the factors that influence the mode of O 2 coordination (end-on vs side-on) and the electronic structure, which can vary between Cu(II)-superoxo and Cu(III)-peroxo extremes.

Singlet-triplet gaps in large multireference systems: Spin-flip-driven alternatives for bioinorganic modeling

The proper description of low-spin states of open-shell systems, which are commonly encountered in the field of bioinorganic chemistry, rigorously requires using multireference ab initio methodologies. Such approaches are unfortunately very CPU-time consuming as dynamic correlation effects also have to be taken into account. The broken-symmetry unrestricted (spin-polarized) density functional theory (DFT) technique has been widely employed up to now to bypass that drawback, but despite a number of relative successes in the determination of singlet-triplet gaps, this framework cannot be considered as entirely satisfactory. In this contribution, we investigate some alternative ways relying on the spin-flip time-dependent DFT approach [Y. Shao et al. J. Chem. Phys. 118, 4807 (2003)]. Taking a few well-documented copper-dioxygen adducts as examples, we show that spin-flip (SF)-DFT computed singlet-triplet gaps compare very favorably to either experimental results or large-scale CASMP2 computations. Moreover, it is shown that this approach can be used to optimize geometries at a DFT level including some multireference effects. Finally, a clear-cut added value of the SF-DFT computations is drawn: if pure ab initio data are required, then the electronic excitations revealed by SF-DFT can be considered in designing dramatically reduced zeroth-order variational spaces to be used in subsequent multireference configuration interaction or multireference perturbation treatments.

Characterization of the structure and reactivity of monocopper-oxygen complexes supported by β-diketiminate and anilido-imine ligands

Copper-oxygen complexes supported by beta-diketiminate and anilido-imine ligands have recently been reported (Aboelella et al., J Am Chem Soc 2004, 126, 16896; Reynolds et al., Inorg Chem 2005, 44, 6989) as potential biomimetic models for dopamine beta-monooxygenase (DbetaM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM). However, in contrast to the enzymatic systems, these complexes fail to exhibit C--H hydroxylation activity (Reynolds et al., Chem Commun 2005, 2014). Quantum chemical characterization of the 1:1 Cu-O(2) model adducts and related species (Cu(III)-hydroperoxide, Cu(III)-oxo, and Cu(III)-hydroxide) indicates that the 1:1 Cu-O(2) adducts are unreactive toward substrates because of the weakness of the O--H bond that would be formed upon hydrogen-atom abstraction. This in turn is ascribed to the 1:1 adducts having both low reduction potentials and basicities. Cu(III)-oxo species on the other hand, determined to be intermediate between Cu(III)-oxo and Cu(II)-oxyl in character, are shown to be far more reactive toward substrates. Based on these results, design strategies for new DbetaM and PHM biomimetic ligands are proposed: new ligands should be made less electron rich so as to favor end-on dioxygen coordination in the 1:1 Cu-O(2) adducts. Comparison of the relative reactivities of the various copper-oxygen complexes as hydroxylating agents provides support for a Cu(II)-superoxide species as the intermediate responsible for substrate hydroxylation in DbetaM and PHM, and suggests that a Cu(III)-oxo intermediate would be competent in this process as well.

Identification of an “End-on” Nickel−Superoxo Adduct, [Ni(tmc)(O2)]+

An "end-on" Ni2+-superoxo adduct has been prepared via two independent synthetic routes and its structure ascertained by spectroscopic and computational methods. The new structure type in nickel coordination chemistry is supported by resonance Raman and EPR spectroscopic features, the former displaying a high frequency nu (O-O) mode (1131 cm-1) consistent with significant superoxo character. The Ni2+-superoxo adduct oxidizes PPh3 to OPPh3 in quantitative yield.

Modeling enzymatic reactions involving transition metals

High-accuracy quantum chemistry has now been applied for almost 10 years to biological problems involving transition metal active sites. The leading theoretical method is hybrid density functional theory (DFT), usually with the B3LYP functional. The chemical models vary in size, commonly from 30 to 100 atoms treated fully quantum mechanically. Two schools exist, one using the smallest possible adequate models and the other using as large models as possible and sometimes including the entire enzyme by combining quantum mechanics with molecular mechanics. In our group, we have found that the latter approach, which is much more time-consuming and error prone, is seldom needed. In this Account, methods and models will be described and examples of recent applications given. The examples are chosen to illustrate trends and to show cases where theory has predicted new mechanisms not suggested previously.

The performance of hybrid DFT for mechanisms involving transition metal complexes in enzymes

The accuracy of density functional theory with the B3LYP functional is reviewed for systems of relevance to transition-metal-containing enzymes. Calculated energies are commonly within 3-5 kcal/mol of the correct values; however, some exceptions have appeared in the literature and are discussed here. For example, the binding of NO and that of O(2) to metal centers have for some time been known to be underestimated. Most barriers for chemical reactions are overestimated except those involving hydrogen (or proton) transfer, which instead tend to be underestimated. A minor general improvement of the accuracy can probably be obtained by slightly reducing the amount of exact exchange in the B3LYP functional.

Theoretical modelling of tripodal CuN3 and CuN4 cuprous complexes interacting with O2, CO or CH3CN

Dioxygen binding at copper enzymatic sites is a fundamental aspect of the catalytic activity observed in many biological systems such as the monooxygenases, especially peptidylglycine alpha-hydroxylating monooxygenase (PHM), in which two mononuclear Cu(I) sites are involved. Biomimetic models have been developed: dipods, tripods, and, more recently, functionalized calixarenes. The modelling of calixarene systems, although not unreachable for theory yet, requires, however, a number of preliminary investigations to ensure proper calibrations if relevant description of the metal-ligand interaction at the hybrid quantum mechanical/molecular mechanics levels of theory is the aim. In this paper, we report quantum chemistry investigations on a coherent series of representative cuprous tripodal species characterized by (1) monodentate ligands [Cu(ImH)3]+ (where ImH is imidazole), [Cu(MeNH2)3]+ and [Cu(MeNH2)4]+ , (2) neutral tripodal ligands [CuCH(ImH)3]+, [Cu(tren)]+ [where tren is tris(2-aminoethyl)amine], and [Cu(trenMe3)]+ [where trenMe3 is tris(2-methylaminoethyl)amine] and (3) a hydrido-tris(pyrazolyl)borate [CuBH(Pyra)3]. The structures of these complexes, the coordination mode (eta(2) side-on or eta(1) end-on) of O2 to Cu(I) and the charge transfer from the metal to dioxygen have been computed. For some systems, the coordination by CH3CN and CO is also reported. Beyond results relative to structural properties, an interesting feature is that it is possible to build from computational results only a set of abacuses linking the nu(16O-16O) vibrational frequency of the coordinated O2 molecule to the O-O bond length or to the net charge of the O2 moiety. Such abacuses may help experimentalists in distinguishing between the four possible ways of binding O2 to CuN3 and CuN4 cuprous centres, namely (1) end-on triplet states, (2) side-on triplet states, (3) end-on singlet states and (4) side-on singlet states. These abacuses are extended to three tripods obtained by the substitution of one nitrogen atom by either a phosphorus or a sulphur atom. Moreover, it is shown that any factor favouring pyramidalization at copper favours charge transfer and thus coordination of the incoming O2 moiety. All these allow insight into the coordination mode of O2 and into the charge transfer from Cu(I) in site Cu(M) of PHM.

Mononuclear copper active-oxygen complexes

There have been significant advances in the understanding of the dioxygen-activation chemistry of mononuclear copper monooxygenases such as peptidylglycine alpha-amidating monooxygenase and dopamine beta-monooxygenase (DbetaM). Recent structural and spectroscopic studies on a series of biomimetic model compounds have provided new and valuable insights into the key reactive intermediates involved in the dioxygen processing at the mononuclear copper reaction centers in biological systems.

Effects of thioether substituents on the O2 reactivity of beta-diketiminate-Cu(I) complexes: probing the role of the methionine ligand in copper monooxygenases

The activation of dioxygen by dopamine beta-monooxygenase (DbetaM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM) is postulated to occur at a copper site ligated by two histidine imidazoles and a methionine thioether, which is unusual because such thioether ligation is not present in other O2-activating copper proteins. To assess the possible role of the thioether ligand in O2 activation by DbetaM and PHM, two new ligands comprising beta-diketiminates with thioether substituents were synthesized and Cu(I) and Cu(II) complexes were isolated. The Cu(II) compounds are monomeric and exhibit intramolecular thioether coordination. While the Cu(I) complexes exhibit a multinuclear topology in the solid state, variable-temperature 1H NMR studies implicate equilibria in solution, possibly including monomers with intramolecular thioether coordination that are structurally defined by DFT calculations. Low-temperature oxygenation of solutions of the Cu(I) complexes generates stable 1:1 Cu/O2 adducts, which on the basis of combined experimental and theoretical studies adopt side-on "eta(2)" structures with negligible Cu-thioether bonding and significant peroxo-Cu(III) character. In contrast to previously reported findings with related ligands lacking the thioether group, however (cf., Aboelella; et al. J. Am. Chem. Soc. 2004, 126, 16896), purging the solutions of the thioether-containing adducts with argon results in conversion to bis(mu-oxo)dicopper(III) species. A role for the thioether in promoting loss of O2 from the 1:1 Cu/O2 adduct and facilitating trapping of the resulting Cu(I) complex to yield the bis(mu-oxo) species is proposed, and the possible relevance of this role to that of the methionine in the active sites of DbetaM and PHM is discussed.

Evidence for Cu−O2 Intermediates in Superoxide Oxidations by Biomimetic Copper(II) Complexes

The mechanism by which [Cu(II)(L)](OTf)2 and [Cu(II)N3(L)](OTf) (L = TEPA: tris(2-pyridylethyl)amine or TMPA: tris(2-pyridylmethyl)amine; OTf = trifluoromethanesulfonate) react with superoxide (O2*-) to form [Cu(I)(L)](OTf) and O2 is described. Evidence for a CuO2 intermediate is presented based on stopped-flow experiments and competitive oxygen (18O) kinetic isotope effects on the bimolecular reactions of (16,16)O2*- and (18,16)O2*- ((16,16)k/(18,16)k). The (16,16)k/(18,16)k fall within a narrow range from 0.9836 +/- 0.0043 to 0.9886 +/- 0.0078 for reactions of copper(II) complexes with different coordination geometries and redox potentials that span a 0.67 V range. The results are inconsistent with a mechanism that involves either rate-determining O2*- binding or one-step electron transfer. Rather a mechanism involving formation of a CuO2 intermediate prior to the loss of O2 in the rate-determining step is proposed. Calculations of similar inverse isotope effects, using stretching frequencies of CuO2 adducts generated from copper(I) complexes and O2, suggest that the intermediate has a superoxo structure. The use of 18O isotope effects to relate activated oxygen intermediates in enzymes to those derived from inorganic compounds is discussed.

Using synthetic chemistry to understand copper protein active sites: a personal perspective

The results of studies performed in the author's laboratory are surveyed, with particular emphasis on demonstrating the value of a multidisciplinary synthetic modeling approach for discovering new and unusual chemistry helpful for understanding the properties of the active sites of copper proteins or assessing the feasibility of mechanistic pathways they might follow during catalysis. The discussion focuses on the progress made to date toward comprehending the nitrite reductase catalytic site and mechanism, the electronic structures of copper thiolate electron transfer centers, the sulfido-bridged "CuZ" site in nitrous oxide reductase, and the processes of dioxygen binding and activation by mono- and dicopper centers in oxidases and oxygenases.

Can an ancillary ligand lead to a thermodynamically stable end-on 1 : 1 Cu–O2adduct supported by a β-diketiminate ligand?

The finding that dioxygen binds end-on to the Cu(B) site in the crystal structure of a precatalytic complex of peptidylglycine alpha-hydroxylating monooxygenase has spurred the search for biomimetic model complexes exhibiting the same dioxygen coordination. Recent work has not only indicated that sterically hindered beta-diketiminate ligands (L(1)) could support side-on 1 : 1 Cu-O(2) adducts, but also that an end-on L(1)Cu(THF)O(2) structure occurs as an unstable intermediate in the oxygenation mechanism of the Cu(I) complex. In this work, density functional theory and multireference methods are used to determine the potential of ancillary ligands, X, other than THF to yield thermodynamically stable end-on L(1)CuXO(2) species. A diverse set of ligands X, comprising phosphines, thiophene, cyclic ethers, acetonitrile, para-substituted pyridines, N-heterocyclic carbenes, and ligands bearing hydrogen bond donors, has been considered in order to identify ligand characteristics which energetically favor end-on L(1)CuXO(2) over: a) reversion to the Cu(I) complex and dioxygen, b) isomerization to side-on L(1)CuXO(2), and c) decay to L(1)CuO(2) and X. Ancillary ligands with judiciously chosen degrees and orientation of steric bulk and which bear potential hydrogen bond donors to an end-on bound dioxygen moiety most favor oxygenation of L(1)CuX to yield end-on L(1)CuXO(2). Conversion to the side-on isomer can be deterred through the use of a sufficiently bulky ligand X, such as one that is at least the size of a 5-membered ring. Loss of X to give L(1)CuO(2) can be made prohibitively endergonic by employing ligands X which are highly electron donating and which backbond strongly with and sigma-donate significantly to copper.

Electronic tuning of β-diketiminate ligands with fluorinated substituents: effects on the O2-reactivity of mononuclear Cu(i) complexes

Copper(i) complexes with the beta-diketiminate ligands HC{C(R)N(Dipp)}{C(R')N(Dipp)}(-) (Dipp = C(6)H(3)(i)Pr(2-)2,6; L(1), R = CF(3), R' = CH(3); L(2), R = R' = CF(3)) have been isolated and fully characterized. On the basis of X-ray structural comparisons with the previously reported complex LCu(CH(3)CN) (L = HC{C(CH(3))N(Dipp)}(2)(-)), the ligand environments at the copper centers in the analogous nitrile adducts with L(1) and L(2) impose similar steric demands. L(1)Cu(CH(3)CN) reacts instantaneously at low temperature with O(2) to form a thermally-unstable intermediate with an isotope-sensitive vibration at 977 cm(-1) (928 cm(-1) with (18)O(2)), in accord with the peroxo O-O stretch associated with side-on coordination for LCu(O(2)). However, L(2)Cu(CH(3)CN) is unreactive toward O(2) even at room temperature. Evaluation of the redox potentials of the nitrile adducts and the CO stretching frequencies of the carbon monoxide adducts revealed an incremental adjustment of the electronic environment at the copper center that correlated with the extent of ligand fluorination. Furthermore, theoretical calculations (DFT, CASPT2) predicted that an increasing extent of Cu(ii)-superoxo character and end-on coordination of the O(2) moiety in the Cu/O(2) product (L(2) > L(1) > L) are accompanied by increases in the free energy for the oxygenation reaction, with L(2) unable to support a Cu/O(2) intermediate. Calculations also predict the 1 : 1 Cu/O(2) adducts to be unreactive with respect to hydrogen atom abstraction from hydrocarbon substrates on the basis of their stability towards both reduction and protonation.

Models for dioxygen activation by the CuB site of dopamine β-monooxygenase and peptidylglycine α-hydroxylating monooxygenase

On the basis of spectroscopic and crystallographic data for dopamine beta-monooxygenase and peptidylglycine alpha-hydroxylating monooxygenase (PHM), a variety of ligand sets have been used to model the oxygen-binding Cu site in these enzymes. Calculations which employed a combination of density functional and multireference second-order perturbation theory methods provided insights into the optimal ligand set for supporting eta (1) superoxo coordination as seen in a crystal structure of a precatalytic Cu/O(2) complex for PHM (Prigge et al. in Science 304:864-867, 2004). Anionic ligand sets stabilized eta (2) dioxygen coordination and were found to lead to more peroxo-like Cu-O(2) complexes with relatively exergonic binding free energies, suggesting that these adducts may be unreactive towards substrates. Neutral ligand sets (including a set of two imidazoles and a thioether), on the other hand, energetically favored eta (1) dioxygen coordination and exhibited limited dioxygen reduction. Binding free energies for the 1:1 adducts with Cu supported by the neutral ligand sets were also higher than with their anionic counterparts. Deviations between the geometry and energetics of the most analogous models and the PHM crystal structures suggest that the protein environment influences the coordination geometry at the Cu(B) site and increases the lability of water bound to the preoxygenated reduced form. Another implication is that a neutral ligand set will be critical in biomimetic models in order to stabilize eta (1) dioxygen coordination.

How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

The tendency for mixed-isotope O2 fragments to exhibit different stretching frequencies in asymmetric environments is examined with various levels of electronic structure theory for simple peroxides and peroxyl radicals, as well as for a variety of monocopper-O2 complexes. The study of the monocopper species is motivated by their relevance to the active site of galactose oxidase. Extensive theoretical work with an experimental model characterized by Jazdzewski et al. (J. Biol. Inorg. Chem. 8:381-393, 2003) suggests that the failure to observe a splitting between 16O18O and 18O16O isotopomers cannot be taken as evidence against end-on O2 coordination. Conformational analysis on an energetic basis, however, is complicated by biradical character inherent in all of the copper-O2 singlet structures.

Characterization of a 1:1 Cu−O2 Adduct Supported by an Anilido Imine Ligand

Copper(I) complexes of sterically hindered anilido imine ligands o-C6H4{N(C6H3(i)Pr2)}{C(R)=NC6H3(i)Pr2}- (L(1), R = H; L(2), R = CH3) have been prepared and characterized by spectroscopic and X-ray crystallographic methods. These complexes are highly reactive with O2, and in the case of L2 the product of low-temperature oxygenation was fully characterized by spectroscopic, X-ray crystallographic, and computational methods. The resonance Raman spectrum features an isotope-sensitive vibration at 974 cm(-1) (Delta(18O) = 66 cm(-1)), consistent with assignment as an O-O stretch. Despite the asymmetric coordination environment provided by the supporting anilido imine ligand, the X-ray crystal structure confirms rather symmetric side-on binding of the O2 moiety to the copper center, and the O-O bond length of 1.392(2) Angstroms indicates that this intermediate has significant Cu(III)-peroxo character. Theoretical calculations support this interpretation and predict that while a fully optimized end-on singlet geometry can be obtained, it is higher in energy than the side-on isomer by 3.5 kcal mol(-1) at the CASPT2/TZP level.

Dioxygen Activation at a Single Copper Site: Structure, Bonding, and Mechanism of Formation of 1:1 Cu−O2 Adducts

To evaluate the fundamental process of O(2) activation at a single copper site that occurs in biological and catalytic systems, a detailed study of O(2) binding to Cu(I) complexes of beta-diketiminate ligands L (L(1) = backbone Me; L(2) = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for L(2)CuO(2) was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(III)-peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of L(1)Cu(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of L(1)Cu(THF) and (b) direct, bimolecular oxygenation of L(1)Cu(RCN) via an associative process.