Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen.

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a "noninnocent" or "redox active" ligand to transition metal ions to give metal-NO (M-NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M-NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni-NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3- denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated 3NO-. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni-NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound 3NO-, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O2 yielded [Ni(κ2-O2N)(L1″)2](I3), with the oxidation of both 3NO- and the I- ion to yield NO2- and I3-. Both complexes were characterized by X-ray crystallography, IR, and UV-Vis spectroscopy and theoretical calculations.

Silver(I) Coordination Polymer Ligated by Bipyrazole Me4bpzH2, [Ag(X)(Me4bpzH2)] (X = CF3CO2- and CF3SO3-, Me4bpzH2 = 3,3',5,5'-Tetramethyl-4,4'-bipyrazole): Anion Dependent Structures and Photoluminescence Properties.

Coordination polymers of transition metal ions are fascinating and important to coordination chemistry. One of the ligands known to form particularly interesting coordination polymers is 3,3',5,5'-tetramethyl-4,4'-bipyrazole (Me4bpzH2). Group 11 metal(I) ion coordination polymers, other than those of copper(I), are relatively easy to handle because of their low reactivity towards dioxygen and moisture. However, the known silver(I) coordination polymers often have poor solubility in common solvents and so cannot be easily analyzed in solution. By using a tetramethyl substituted bipyrazole ligand, we have synthesized more soluble silver(I) complexes that contain the trifluoromethyl group in the coordinated ions CF3CO2- and CF3SO3- in [Ag(CF3CO2)(Me4bpzH2)] and [Ag(CF3SO3)(Me4bpzH2)]. We determined both structures by single-crystal X-ray analysis at low temperatures and compared them in detail. Moreover, we investigated the solution behavior of these coordination polymers by 1H-NMR, IR, Raman, UV-Vis spectroscopies, and their low-temperature, solid-state photoluminescence. The high-energy band at ~330 nm corresponded to ligand-centered (bipyrazole) fluorescence, and the low-energy band at ~400 nm to ligand-centered phosphorescence resulting from the heavy atom effect.

Superoxide Dismutase-like Activity of Zeolitic Imidazolate Framework Nanoparticles Comprising Biomimetic Imidazolato-bridged CuZn Units.

Nanoparticles exhibiting enzymatic functions have garnered considerable attention due to their structural robustness and the profusion of active sites that can be introduced to a single nanosized particle. Here we report that nanosized mixed-metal zeolitic imidazolate frameworks (ZIFs) show a superoxide dismutase (SOD)-like catalytic activity. We chose a ZIF composed of copper and zinc ions and 2-methylimidazole, CuZn-ZIF-8, in which the Cu and Zn ions are bridged by an imidazolato ligand. This coordination geometry closely mimics the active site of CuZn superoxide dismutase (CuZnSOD). The CuZn-ZIF-8 nanoparticles exhibit potent SOD-like activity, attributed to their porous nature and numerous copper active sites, and also possess exceptional recyclability.

The Influence of Aryl Substituents on the Supramolecular Structures and Photoluminescence of Cyclic Trinuclear Pyrazolato Copper(I) Complexes.

Cyclic trinuclear complexes with group 11 metal(I) ions are fascinating and important to coordination chemistry. One of the ligands known to form these cyclic trinuclear complexes is pyrazolate, which is a bridging ligand that coordinates many transition metal ions in a Npz-M-Npz linear mode (Npz = pyrazolyl nitrogen atom). In these group 11 metal(I) ions, copper is the most abundant metal. Therefore, polynuclear copper(I) complexes are very important in this field. The cyclic trinuclear copper(I) complex [Cu(3,5-Ph2pz)]3 (3,5-Ph2pz- = 3,5-diphenyl-1-pyrazolate anion) was reported in 1988 as a landmark complex, but its photoluminescence properties have hitherto not been described. In this study, we report the photoluminescence and two different polymorphs of [Cu(3,5-Ph2pz)]3 and its derivative [Cu(3-Me-5-Phpz)]3 (3-Me-5-Phpz- = 3-metyl-5-phenyl-1-pyrazale anion). The substituents in [Cu(3-Me-5-Phpz)]3 cause smaller distortions in the solid-state structure and a red-shift in photoluminescence due to the presence of intermolecular cuprophilic interactions.

Thermally stable manganese(III) peroxido complexes with hindered N3 tripodal ligands: Structures and their physicochemical properties.

Mononuclear manganese(III) peroxido complexes are candidates for the reaction intermediates in manganese containing proteins, such as manganese superoxide dismutase (Mn-SOD) etc. In this study, manganese(III) peroxido complexes [Mn(O2)(L3)] and [Mn(O2)(L10)] ligated by anionic N3 type ligands with sterically hindered substituents, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (L3-) and hydrotris(3-adamantyl-5-isopropyl-1-pyrazolyl)borate (L10-), respectively, were structurally characterized. These complexes are the first examples of structurally characterized five-coordinate manganese(III) peroxido complexes. Their characteristic ν(OO) and ν(MnO) stretchings were determined by using H218O2 for the first time. Theoretical calculations were performed to obtain further insight into their structural parameters. The decomposed products were obtained as [{MnIII(µ-O)(L3)}2MnIV] and [MnIII(OH){L10(O)}] from [Mn(O2)(L3)] and [Mn(O2)(L10)], respectively.

Synthesis and Structural Characterization of a Silver(I) Pyrazolato Coordination Polymer.

Coinage metal(I)···metal(I) interactions are widely of interest in fields such as supramolecular assembly and unique luminescent properties, etc. Only two types of polynuclear silver(I) pyrazolato complexes have been reported, however, and no detailed spectroscopic characterizations have been reported. An unexpected synthetic method yielded a polynuclear silver(I) complex [Ag(µ-L1Clpz)]n (L1Clpz- = 4-chloride-3,5-diisopropyl-1-pyrazolate anion) by the reaction of {[Ag(µ-L1Clpz)]3}2 with (nBu4N)[Ag(CN)2]. The obtained structure was compared with the known hexanuclear silver(I) complex {[Ag(µ-L1Clpz)]3}2. The Ag···Ag distances in [Ag(µ-L1Clpz)]n are slightly shorter than twice Bondi's van der Waals radius, indicating some Ag···Ag argentophilic interactions. Two Ag-N distances in [Ag(µ-L1Clpz)]n were found: 2.0760(13) and 2.0716(13) Å, and their N-Ag-N bond angles of 180.00(7)° and 179.83(5)° indicate that each silver(I) ion is coordinated by two pyrazolyl nitrogen atoms with an almost linear coordination. Every five pyrazoles point in the same direction to form a 1-D zig-zag structure. Some spectroscopic properties of [Ag(µ-L1Clpz)]n in the solid-state are different from those of {[Ag(µ-L1Clpz)]3}2 (especially in the absorption and emission spectra), presumably attributable to this zig-zag structure having longer but differently arranged intramolecular Ag···Ag interactions of 3.39171(17) Å. This result clearly demonstrates the different physicochemical properties in the solid-state between 1-D coordination polymer and metalacyclic trinuclear (hexanuclear) or tetranuclear silver(I) pyrazolate complexes.

Effect of Oxygen-Donor Charge on Adjacent Nitrogen-Donor Interactions in Eu3+ Complexes of Mixed N,O-Donor Ligands Demonstrated on a 10-Fold [Eu(TPAMEN)]3+ Chelate Complex.

To reduce high-level radiotoxic waste generated by nuclear power plants, highly selective separation agents for minor actinides are mandatory. The mixed N,O-donor ligand N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H4TPAEN; 1) has shown good performance as a masking agent in Am3+/Eu3+ separation studies. Adjustments on the pyridyl backbone to raise the hydrophilicity led to a decrease in selectivity and a decrease in M3+-Nam interactions. An enhanced basicity of the pyridyl N-donors was given as a cause. In this work, we examine whether a decrease in O-donor basicity can promote the M3+-Nam interactions. Therefore, we replace the deprotonated "charged" carboxylic acid groups of TPAEN4- by neutral amide groups and introduce N,N,N',N'-tetrakis[(6-N″,N''-diethylcarbamoylpyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2) as a new ligand. TPAMEN was crystallized with Eu(OTf)3 and Eu(NO3)3·6H2O to form positively charged 1:1 [Eu(TPAMEN)]3+ complexes in the solid state. Alterations in the M-O/N bond distances are compared to [Eu(TPAEN)]- and investigated by DFT calculations to expose the differences in charge/energy density distributions at europium(III) and the donor functionalities of the TPAEN4- and TPAMEN. On the basis of estimations of the bond orders, atomic charges spin populations, and density of states in the Eu and potential Am and Cm complexes, the specific contributions of the donor-metal interaction are analyzed. The prediction of complex formation energy differences for the [M(TPAEN)]- and [M(TPAMEN)]3+ (M3+ = Eu3+, Am3+) complexes provide an outlook on the potential performance of TPAMEN in Am3+/Eu3+ separation.

Stable Ferrous Mononitroxyl {FeNO}8 Complex with a Hindered Hydrotris(pyrazolyl)borate Coligand: Structure, Spectroscopic Characterization, and Reactivity Toward NO and O2.

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3- = a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO}8 complex in the Enemark-Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound 3NO- ligand where the spins are antiferromagnetic coupled ( St = 1). Vibrational data show that this complex has a very strong Fe-NO bond. DFT calculations support this result and link it to very strong π-donation from the 3NO- ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO}8 complex and a dinitrosyl iron complex (DNIC) of {Fe(NO)2}9 type is observed. The O2 reactivity of the complex is finally reported.

Syntheses and Structures of d10 Coinage Metal Complexes of Electron-Accepting Phosphine Ligands Featuring a 3,3,4,4,5,5-Hexafluorocyclopentene Framework.

Cu(I), Ag(I), and Au(I) complexes of monophosphine or bisphosphine ligands based on the 3,3,4,4,5,5-hexafluorocyclopentene skeleton were synthesized and structurally characterized by X-ray crystallographic analysis. The electron-withdrawing nature of these polyfluorinated phosphines was experimentally revealed via UV/vis absorption studies and crystal structure analysis. Successful catalytic application of the Au(I) complex for alkyne hydration reactions was investigated.

A cobalt-nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen.

The cobalt-nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3-), and shows a linear Co-N-O unit. This complex was prepared by the reaction of the potassium salt of L3- with the cobalt-nitrosyl precursor [Co(NO)2(tmeda)](BPh4) (tmeda = N,N,N,'N'-tetramethylethylenediamine). The obtained cobalt-nitrosyl complex as well as the corresponding products from the reaction with dioxygen, [Co(η2-O2N)(L3)] and [Co(η2-O2NO)(L3)], were characterised by X-ray crystallography and a number of spectroscopic methods including IR/far-IR, UV-Vis, and NMR spectroscopy. We also performed MCD measurements and DFT calculations to further elucidate the electronic structure of [Co(NO)(L3)] and the optical properties of the complex. The MCD spectra reveal two NO-to-Co charge-transfer transitions with strong excited state displacements that give rise to vibrational progressions in the MCD spectra, indicative of a very covalent Co-NO bond. These results provide new insight into the properties of the Co-NO bond and the electron distribution in the complex, which is best described as [CoII(NO-)(L3)].

A personal perspective on the discovery of dioxygen adducts of copper and iron by Nobumasa Kitajima.

Transition metal-dioxygen complexes have fascinated bioinorganic and inorganic chemists for over half a century. The late Nobumasa Kitajima was one of the very successful researchers in this field. Despite his short career (40 years old), he made many important contributions. This Commentary highlights his important accomplishments and how they have impacted subsequent work in this area.

Mono- and binuclear tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate bismuth(III) dichloride complexes: a soft scorpionate ligand can coordinate to p-block elements.

Tris(pyrazolyl)hydroborate ligands have been utilized in the fields of inorganic and coordination chemistry due to the ease of introduction of steric and electronic substitutions at the pyrazole rings. The development and use of the tris(pyrazolyl)hydroborate ligand, called a `scorpionate', were pioneered by the late Professor Swiatoslaw Trofimenko. He developed a second generation for his ligand system by the introduction of 3-tert-butyl and 3-phenyl substituents and this new ligand system accounted for many remarkable developments in inorganic and coordination chemistry in stabilizing monomeric species while maintaining an open coordination site. Bismuth is remarkably harmless among the toxic heavy metal p-block elements and is now becoming popular as a replacement for highly toxic metal elements, such as lead. Two bismuth(III) complexes of the anionic sulfur-containing tripod tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate ligand were prepared. By recrystallization from MeOH/CH2Cl2, orange crystals of dichlorido(methanol-κO)[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III), [Bi(C21H34BN6S3)Cl2(CH4O)], (I), were obtained, manifesting a mononuclear structure. By using a noncoordinating solvent, red crystals of the binuclear structure with bridging Cl atoms were obtained, namely di-µ-chlorido-bis{chlorido[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III)}, [Bi2(C21H34BN6S3)2Cl4], (II). These complexes show {BiIIIS3Cl2O} and {BiIIIS3Cl3} coordination geometries with average BiIII-S bond lengths of 2.73 and 2.78 Šin (I) and (II), respectively. The overall BiIII coordination geometry is distorted octahedral due to stereochemically active lone pairs. The three BiIII-S bond lengths are almost equal in (I) but show considerable differences in (II), with one long and two shorter distances that also correlate with changes in the UV-Vis and 1H NMR spectra. For direct measurements of the Bi-S/Cl coordination, ligand K-edge X-ray absorption measurements were carried out in combination with ground and excited-state electronic structure analyses. For p-block elements, these sulfur-containing ligands are useful for preparing the appropriate complexes due to their flexible coordination geometry.

Comparison of thallium(I) complexes with mesityl-substituted tris(pyrazolyl)hydroborate ligands, [Tl{HB(3-Ms-5-Mepz)3}] and [Tl{HB(3-Ms-5-Mepz)2(3-Me-5-Mspz)}].

Tris(pyrazolyl)borate (scorpionate) ligands can be considered as the most prolific ligands in contemporary coordination chemistry due to the availability of various steric and electronic substituents at the pyrazolyl rings that allow fine-tuning of the open-coordination site for metal centres. The thallium(I) complexes of anionic tridentate-chelating scorpionate ligands, namely [tris(3-mesityl-5-methyl-1H-pyrazol-1-yl-κN2)hydroborato]thallium(I) monohydrate, [Tl(C39H46BN6)]·H2O, (I), and [bis(3-mesityl-5-methyl-1H-pyrazol-1-yl-κN2)(5-mesityl-3-methyl-1H-pyrazol-1-yl-κN2)hydroborato]thallium(I), [Tl(C39H46BN6)], (II), show a {TlIN3} coordination, with average TlI-N bond lengths of 2.53 and 2.55 Šin (I) and (II), respectively. The overall TlI coordination geometry is distorted trigonal pyramidal, with the average N-TlI-N angle being approximately 73° for both. The dihedral angle between the planes of the pyrazolyl and benzene rings of the mesityl group is 82° in (I), while the corresponding angles in (II) are in the range 64-104°. The structural differences between the two ligands are expected to contribute to the different reactivities of the transition metal coordination complexes towards activation of small molecules such as dioxygen and ethylene.

Distorted tetrahedral nickel-nitrosyl complexes: spectroscopic characterization and electronic structure.

The linear nickel-nitrosyl complex [Ni(NO)(L3)] supported by a highly hindered tridentate nitrogen-based ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3), was prepared by the reaction of the potassium salt of the ligand with the nickel-nitrosyl precursor [Ni(NO)(Br)(PPh 3 ) 2 ]. The obtained nitrosyl complexes as well as the corresponding chlorido complexes [Ni(NO)(Cl)(PPh 3 ) 2 ] and [Ni(Cl)(L3)] were characterized by X-ray crystallography and different spectroscopic methods including IR/far-IR, UV-Vis, NMR, and multi-edge X-ray absorption spectroscopy at the Ni K-, Ni L-, Cl K-, and P K-edges. For comparative electronic structure analysis we also performed DFT calculations to further elucidate the electronic structure of [Ni(NO)(L3)]. These results provide the nickel oxidation state and the character of the Ni-NO bond. The complex [Ni(NO)(L3)] is best described as [Ni (II) (NO (-) )(L3)], and the spectroscopic results indicate that the phosphane complexes have a similar [Ni (II) (NO (-) )(X)(PPh 3 ) 2 ] ground state.

[A case of hepatic cryptococcosis complicated by adult T-cell leukemia].

A woman in her 80s was admitted to our hospital on account of jaundice, abnormal liver function tests, and leukocytosis. She was diagnosed with adult T-cell leukemia on the basis of the presence of anti-human T-cell leukemia virus type I (HTLV-I) and the results of flow cytometric analysis of peripheral blood. She also showed lung consolidation and cavitation, and a sputum smear and culture revealed cryptococcal infection. Therefore, she was diagnosed with pulmonary cryptococcosis. However, the cause of the abnormal liver function tests and jaundice remained unclear, and the patient subsequently died. On autopsy, multiple granulomas were observed throughout the liver, consistent with cryptococcal bodies. Herein we report this rare case of hepatic cryptococcosis with predominant hepatobiliary complaints.

Structure and photoluminescence of silver(i) trinuclear halopyrazolato complexes.

Five halogen substituted pyrazolates, 4-chloro-3,5-diisopropylpyrazole (4-Cl-3,5-iPr2pzH), 4-bromo-3,5-diisopropylpyrazole (4-Br-3,5-iPr2pzH), 4-iodo-3,5-diisopropylpyrazole (4-I-3,5-iPr2pzH), 4-chloro-3,5-diphenylpyrazole (4-Cl-3,5-Ph2pzH), and 4-bromo-3,5-diphenylpyrazole (4-Br-3,5-Ph2pzH), were conveniently prepared by halogenation of the appropriate pyrazoles with N-halosuccinimides (NXS) (X = Cl, Br, and I) followed by complexation of the pyrazolate anions with silver(i) nitrate. Single crystal X-ray analysis revealed either dimeric trinuclear {[Ag(µ-4-X-3,5-R2pz)]3}2 (R = iPr, X = Cl, Br, and I) or trinuclear [Ag(µ-4-X 3,5-R2pz)]3 (R = iPr, X = I; R = Ph, X = Cl, R = Ph, X = Br) structures, the latter held together with argentophilic interactions (AgAg interactions) that could also be observed in the Raman spectra. The electronegativity of the halogen substituent could be correlated with the strength of the AgAg interaction and the wavelength of solid-state photoluminescence. All complexes were emissive on UV irradiation at low temperatures, with the colour of emission from the diisopropyl substituted analogues red shifted by the halogens in the order Cl (red) > Br (orange) > I (yellow). Emission from the diphenyl substituted analogues was dominated by the extended aromatic system and was largely invariant to the halogens.

Conversion of carbon dioxide to oxalate by α-ketocarboxylatocopper(II) complexes.

The α-ketocarboxylatocopper(II) complex [{Cu(L1)}{O2CC(O)CH(CH3)2}] can be spontaneously converted into the binuclear oxalatocopper(II) complex [{Cu(L1)}2(µ-C2O4)] upon exposure to O2/CO2 gas. (13)C-labeling experiments revealed that oxalate ions partially incorporated (13)CO2 molecules. Furthermore, the bicarbonatocopper(I) complex (NEt4)[Cu(L1){O2C(OH)}] in an Ar atmosphere and the α-ketocarboxylatocopper(I) complex Na[Cu(L1){O2CC(O)CH(CH3)2}] in an O2 atmosphere were also transformed spontaneously into the oxalato complex [{Cu(L1)}2(µ-C2O4)].

[Composite lymphoma consisting of adult T-cell leukemia-lymphoma and diffuse large B-cell lymphoma].

A 68-year-old man was admitted to our hospital because of left back pain and systemic lymphadenopathy with hypercalcemia. Serum anti-HTLV-1 antibody was positive. Left cervical lymph node (LN) biopsy revealed proliferation of medium-sized to large CD4-positive atypical cells with modest infiltration of CD20 and Epstein-Barr virus (EBV)-encoded RNA dual-positive atypical large cells. Monoclonal integration of HTLV-1 proviral DNA, plus clonal rearrangement of the T-cell receptor chain gene and the immunoglobulin heavy chain gene, were detected in the same LN specimen. Composite lymphoma consisting of adult T-cell leukemia/lymphoma (ATL) and EBV positive diffuse large B-cell lymphoma (DLBCL) was diagnosed. He was successfully treated with aggressive chemotherapy including rituximab and attained remission. However, eight months later, he developed right shoulder pain due to multiple bone invasions with bilateral cervical lymphadenopathy. Biopsies of a bone lesion and cervical LN revealed recurrence of ATL alone. The patient died despite salvage chemoradiotherapy. These findings suggest that ATL-related immunodeficiency might induce EBV-associated DLBCL.

L-edge X-ray absorption spectroscopy and DFT calculations on Cu2O2 species: direct electrophilic aromatic attack by side-on peroxo bridged dicopper(II) complexes.

The hydroxylation of aromatic substrates catalyzed by coupled binuclear copper enzymes has been observed with side-on-peroxo-dicopper(II) (P) and bis-µ-oxo-dicopper(III) (O) model complexes. The substrate-bound-O intermediate in [Cu(II)2(DBED)2(O)2](2+) (DBED = N,N'-di-tert-butyl-ethylenediamine) was shown to perform aromatic hydroxylation. For the [Cu(II)2(NO2-XYL)(O2)](2+) complex, only a P species was spectroscopically observed. However, it was not clear whether this O-O bond cleaves to proceed through an O-type structure along the reaction coordinate for hydroxylation of the aromatic xylyl linker. Accurate evaluation of these reaction coordinates requires reasonable quantitative descriptions of the electronic structures of the P and O species. We have performed Cu L-edge XAS on two well-characterized P and O species to experimentally quantify the Cu 3d character in their ground state wave functions. The lower per-hole Cu character (40 ± 6%) corresponding to higher covalency in the O species compared to the P species (52 ± 4%) reflects a stronger bonding interaction of the bis-µ-oxo core with the Cu(III) centers. DFT calculations show that 10-20% Hartree-Fock (HF) mixing for P and ~38% for O species are required to reproduce the Cu-O bonding; for the P species this HF mixing is also required for an antiferromagnetically coupled description of the two Cu(II) centers. B3LYP (with 20% HF) was, therefore, used to calculate the hydroxylation reaction coordinate of P in [Cu(II)2(NO2-XYL)(O2)](2+). These experimentally calibrated calculations indicate that the electrophilic attack on the aromatic ring does not involve formation of a Cu(III)2(O(2-))2 species. Rather, there is direct electron donation from the aromatic ring into the peroxo σ* orbital of the Cu(II)2(O2(2-)) species, leading to concerted C-O bond formation with O-O bond cleavage. Thus, species P is capable of direct hydroxylation of aromatic substrates without the intermediacy of an O-type species.