The selective one-electron reduction of C60 to C60 •- is attained through photoinduced electron transfer from an NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer analogue [(BNA)2] to the triplet excited state of C60. The limiting quantum yield for formation of C60 •- in the case of (BNA)2 exceeds unity; Φ∞ = 1.3. In this case, the initial electron transfer from (BNA)2 to the triplet excited state (3C60*) is followed by fast C−C bond cleavage in the resulting (BNA)2 •+ to give BNA• and BNA+ and the second electron transfer from BNA• to C60 yields BNA+ and C60 •-, when (BNA)2 acts as a two-electron donor to produce 2 equiv of C60 •-. When BNAH is replaced by 4-tert-butylated BNAH (t-BuBNAH), the photochemical reaction with C60 yields not C60 •- but instead the tert-butylated anion (t-BuC60 -) selectively. In this case, the initial electron transfer from t-BuBNAH to 3C60* is also followed by fast C−C bond cleavage in t-BuBNAH•+ to give t-Bu•, which is coupled with C60 •- produced in the electron transfer to yield t-BuC60 -. The selective two-electron reduction of C60 to 1,2-dihydro[60]fullerene (1,2-C60H2) is also attained with the use of another NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), under visible light irradiation in deaerated benzonitrile solution containing trifluoroacetic acid. The studies on the quantum yields, the kinetic deuterium isotope effects, and the quenching of the triplet−triplet absorption of C60 by AcrH2 have revealed that the photochemical reduction proceeds via photoinduced electron transfer from 10-methyl-9,10-dihydroacridine to the triplet excited state of C60, which is followed by proton transfer from AcrH2 •+ to C60 •- and a second electron transfer from the deprotonated acridinyl radical (AcrH•) to C60H• in the presence of trifluoroacetic acid to yield the final products 10-methylacridinium ion (AcrH+) and 1,2-C60H2. The transient spectra of the radical ion pair formed in the photoinduced electron transfer have been detected successfully in laser flash photolysis of each NADH analogue−C60 system. The mechanistic difference between the selective one- and two-electron reductions of C60 is discussed on the basis of the difference in the redox and acid−base properties of NADH and the dimer analogues.

The first systematic studies on the oxidation of neutral phenols (ArOH) by the mu-eta(2):eta(2)-peroxo)dicopper(II) complex (A) and the bis(mu-oxo)dicopper(III) complex (B) supported by the 2-(2-pyridyl)ethylamine tridentate and didentate ligands L(Py2) and L(Py1), respectively, have been carried out in order to get insight into the phenolic O-H bond activation mechanism by metal-oxo species. In both cases (A and B), the C-C coupling dimer was obtained as a solely isolable product in approximately 50% yield base on the dicopper-dioxygen (Cu(2)/O(2)) complexes, suggesting that both A and B act as electron-transfer oxidants for the phenol oxidation. The rate-dependence in the oxidation of phenols by the Cu(2)/O(2) complexes on the one-electron oxidation potentials of the phenol substrates as well as the kinetic deuterium isotope effects obtained using ArOD have indicated that the reaction involves a proton-coupled electron transfer (PCET) mechanism. The reactivity of phenols for net hydrogen atom transfer reactions to cumylperoxyl radical (C) has also been investigated to demonstrate that the rate-dependence of the reaction on the one-electron oxidation potentials of the phenols is significantly smaller than that of the reaction with the Cu(2)O(2) complexes, indicative of the direct hydrogen atom transfer mechanism (HAT). Thus, the results unambiguously confirmed that the oxidation of phenols by the Cu(2)O(2) complex proceeds via the PCET mechanism rather than the HAT mechanism involved in the cumylperoxyl radical system. The reactivity difference between A and B has also been discussed by taking account of the existed fast equilibrium between A and B.

Tyrosinase is a copper monooxygenase that catalyzes oxygenation of phenols to catechols (phenolase activity) and the subsequent two-electron oxidation of catechols to the corresponding o-quinones (catecholase activity). 1 Chemical and spectroscopic studies have indicated that the enzyme has a dinuclear copper active site nearly identical to that found in hemocyanin, 1,2 where a side-on type (µ-η 2 :η 2 ) peroxo species 3 is generated by the reaction of the reduced dicopper(I) form and O 2 . 1 As a pioneering work by Karlin and co-workers in Cu/O 2 chemistry, aromatic ligand hydroxylation in a dinuclear Cu(I) complex by O 2 was first reported in early 1980s. 4 The mechanistic studies have indicated that the aromatic ligand hydroxylation reaction involves an electrophilic attack on the arene ring by a (µ-η 2 :η 2 -peroxo)-dicopper(II) intermediate. 5 After their finding, several examples of the aromatic ligand hydroxylation have been reported using similar type of m-xylyl dinucleating ligands. 6 With respect to the intermolecular reactions between phenols and the peroxo intermediate, however, most of the reactions so far reported afford a C-C coupling dimer as a major product. 7 Casella and co-workers have recently reported the first synthetic (µ-η 2 :η 2 -peroxo)dicopper-(II) complex which can react with an exogenous phenolate to yield the corresponding catechol. 8,9 Unfortunately, the low yield of the product (20% based on the dicopper complex) has precluded the kinetic and mechanistic investigation on the reaction between the peroxo intermediate and the phenolate. 8 As such, the mechanism for the catechol formation via intermolecular reactions between the peroxo intermediate and phenol derivatives has yet to be clarified. 10 We report herein that efficient conversion of phenol derivatives to the corresponding catechols is achieved for the first time by intermolecular reactions of a (µ-η 2 :η 2 -peroxo)dicopper(II) complex, supported by tridentate ligand L Py2Bz (N,N-bis[2-(2-pyridyl)-ethyl]-R,R-dideuteriobenzylamine), 11 with lithium salts of phenols. The mechanistic studies on the catechol formation have been performed to provide valuable mechanistic insight into the phenolase activity of the enzyme.Treatment of the copper(I) complex, [Cu I (L Py2Bz )](PF 6 ), with dioxygen in anhydrous acetone at -94°C afforded a brown color solution which exhibited a strong absorption band at 364 nm ( ) 26400 M -1 cm -1 ) together with a small one at 530 nm (1500 M -1 cm -1 ) and a resonance Raman band at 737 cm -1 that shifted to 697 cm -1 upon 18 O-substitution. 12,13 The frozen acetone solution of the intermediate was ESR silent at 77 K, and a Cu:O 2 ) 2:1 stoichiometry was obtained for formation of the intermediate by manometry. These results unambiguously indicate that the oxygenated intermediate is a (µ-η 2 :η 2 -peroxo)dicopper(II) complex as suggested previously by Karlin et al. 11 This compound is quite stable (no self-decomposition) at the low-temperature enabling us to examine the reaction with external subst...

Mononuclear copper(II)-superoxo complexes 2(X)-OO(*) having triplet (S = 1) ground states were obtained via reaction of O(2) with the copper(I) starting materials 1(X) supported by tridentate ligands L(X) [1-(2-p-X-phenethyl)-5-(2-pyridin-2-ylethyl)-1,5-diazacyclooctane; X = CH(3), H, NO(2)] in various solvents. The superoxo complexes 2(X)-OO(*) mimic the structure [tetrahedral geometry with an end-on (eta(1))-bound O(2)(*-)] and the aliphatic C-H bond activation chemistry of peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase.

A series of Cu(I) and Cu(II) complexes of a variety of beta-diketiminate ligands (L(-)) with a range of substitution patterns were prepared and characterized by spectroscopic, electrochemical, and, in several cases, X-ray crystallographic methods. Specifically, complexes of the general formula [LCuCl](2) were structurally characterized and their magnetic properties assessed through EPR spectroscopy of solutions and, in one instance, by variable-temperature SQUID magnetization measurements on a powder sample. UV-vis spectra indicated reversible dissociation to 3-coordinate monomers LCuCl in solution at temperatures above -55 degrees C. The Cu(I) complexes LCu(MeCN) exhibited reversible Cu(I)/Cu(II) redox couples with E(1/2) values between +300 and +520 mV versus NHE (cyclic voltammetry, MeCN solutions). These complexes were highly reactive with O(2), yielding intermediates that were identified as rare examples of neutral bis(mu-oxo)dicopper complexes on the basis of their EPR silence, diagnostic UV-vis absorption data, and O-isotope-sensitive resonance Raman spectroscopic features. The structural features of the compounds [LCuCl](2) and LCu(MeCN) as well as the proclivity to form bis(mu-oxo)dicopper products upon oxygenation of the Cu(I) complexes are compared to data previously reported for complexes of more sterically hindered beta-diketiminate ligands (Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel, W. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10600. Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108. Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270). The observed structural and reactivity differences are rationalized by considering the steric influences of both the substituents on the flanking aromatic rings and those present on the beta-diketiminate backbone.

Mechanistic studies on the aliphatic ligand hydroxylation in a copper complex of tridentate ligand 1a {N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine} by O2 have been performed in order to shed light on the structure and reactivity of the active oxygen species of our functional model for copper monooxygenases (Itoh, S.; et al. J. Am. Chem. Soc. 1995, 117, 4714). When the copper complex [CuII(1a)(ClO4)2] was treated with an equimolar amount of benzoin and triethylamine in CH2Cl2 under O2 atmosphere, efficient hydroxylation occurred selectively at the benzylic position of the ligand to provide oxygenated product 2a {N,N-bis[2-(2-pyridyl)ethyl]-2-phenyl-2-hydroxyethylamine} quantitatively. An isotope labeling experiment using 18O2 confirms that the oxygen atom of the OH group in 2a originates from molecular oxygen. Spectroscopic analyses using UV−vis, resonance Raman, and ESR on the reaction of [CuI(1a)]+ and O2 at low temperature show that a μ-η2:η2-peroxodicopper(II) complex is an initially formed intermediate. Kinetic analysis on the peroxo complex formation indicates that the reaction of the Cu(I) complex and the monomeric superoxocopper(II) species is rate-determining for the formation of the μ-η2:η2-peroxodicopper(II) intermediate. When ligand 1a is replaced by 1,1,2,2-tetradeuterated phenethylamine derivative 1a- d 4, a relatively small kinetic deuterium isotope effect (k H/k D = 1.8 at −40 °C) is observed for the ligand hydroxylation step. The rate of the hydroxylation step is rather insensitive to the p-substituent of the ligand [(PyCH2CH2)2NCH2CH2Ar, 1a Ar = C6H5; 1b Ar = p-CH3C6H4, 1c Ar = p-ClC6H4, and 1d Ar = p-NO2C6H4)], but it varies depending on the solvent (THF > acetone > CH3OH > CH2Cl2). The p-substituent, the solvent, and the kinetic deuterium isotope effects suggest that O−O bond homolysis of the μ-η2:η2-peroxodicopper(II) intermediate is involved as a rate-determining step in the aliphatic ligand hydroxylation process. Based on the results of the kinetics and the crossover experiments, we propose a mechanism involving intramolecular C−H bond activation in a bis-μ-oxodicopper(III) type intermediate for the ligand hydroxylation reaction.

Selective hydroxylation of benzene to phenol has been achieved using H2O2 in the presence of a catalytic amount of the nickel complex [Ni(II)(tepa)](2+) (2) (tepa = tris[2-(pyridin-2-yl)ethyl]amine) at 60 °C. The maximum yield of phenol was 21% based on benzene without the formation of quinone or diphenol. In an endurance test of the catalyst, complex 2 showed a turnover number (TON) of 749, which is the highest value reported to date for molecular catalysts in benzene hydroxylation with H2O2. When toluene was employed as a substrate instead of benzene, cresol was obtained as the major product with 90% selectivity. When H2(18)O2 was utilized as the oxidant, (18)O-labeled phenol was predominantly obtained. The reaction rate for fully deuterated benzene was nearly identical to that of benzene (kinetic isotope effect = 1.0). On the basis of these results, the reaction mechanism is discussed.

Copper(I)-dioxygen reactivity has been examined using a series of 2-(2-pyridyl)ethylamine bidentate ligands (R1)Py1(R2,R3). The bidentate ligand with the methyl substituent on the pyridine nucleus (Me)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(6-methylpyridin-2-yl)ethylamine) predominantly provided a (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, while the bidentate ligand without the 6-methyl group (H)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(2-pyridyl)ethylamine) afforded a bis(mu-oxo)dicopper(III) complex under the same experimental conditions. Both Cu(2)O(2) complexes gradually decompose, leading to oxidative N-dealkylation reaction of the benzyl group. Detailed kinetic analysis has revealed that the bis(mu-oxo)dicopper(III) complex is the common reactive intermediate in both cases and that O[bond]O bond homolysis of the peroxo complex is the rate-determining step in the former case with (Me)Py1(Et,Bz). On the other hand, the copper(I) complex supported by the bidentate ligand with the smallest N-alkyl group ((H)Py1(Me,Me), N,N-dimethyl-2-(2-pyridyl)ethylamine) reacts with molecular oxygen in a 3:1 ratio in acetone at a low temperature to give a mixed-valence trinuclear copper(II, II, III) complex with two mu(3)-oxo bridges, the UV-vis spectrum of which is very close to that of an active oxygen intermediate of lacase. Detailed spectroscopic analysis on the oxygenation reaction at different concentrations has indicated that a bis(mu-oxo)dicopper(III) complex is the precursor for the formation of trinuclear copper complex. In the reaction with 2,4-di-tert-butylphenol (DBP), the trinuclear copper(II, II, III) complex acts as a two-electron oxidant to produce an equimolar amount of the C[bond]C coupling dimer of DBP (3,5,3',5'-tetra-tert-butyl-biphenyl-2,2'-diol) and a bis(mu-hydroxo)dicopper(II) complex. Kinetic analysis has shown that the reaction consists of two distinct steps, where the first step involves a binding of DBP to the trinuclear complex to give a certain intermediate that further reacts with the second molecule of DBP to give another intermediate, from which the final products are released. Steric and/or electronic effects of the 6-methyl group and the N-alkyl substituents of the bidentate ligands on the copper(I)-dioxygen reactivity have been discussed.