Theoretical study of aromatic hydroxylation of the [Cu2(H-XYL)O2]2+ complex mediated by a side-on peroxo dicopper core and Cu-ligand effects

In this work, the aromatic hydroxylation mechanism of the [Cu₂(H-XYL)O₂]²⁺ complex mediated by a peroxo dicopper core and Cu-ligand effects are investigated by using hybrid density functional theory (DFT) and the broken symmetry B3LYP method. Based on the calculated free-energy profiles, we proposed two available mechanisms. The first reaction steps of both mechanisms involve concerted O-O bond cleavage and C-O bond formation and the second step involves the Wagner-Meerwein rearrangement of the substrate by a [1,2] H shift (H_A shift from C_A to C_C) or (H_A shift from C_A to O_A) across the phenyl ring to form stable dienone intermediates, and this is followed by the protonation of bridging oxygen atoms to produce the final hydroxylated dicopper(ii) product. The H_A shift from C_A to C_C mechanism is the energetically most favorable, in which the first reaction step is the rate-limiting reaction, with a calculated free-energy barrier of 19.0 kcal mol^-1 and a deuterium kinetic isotope effect of 1.0, in agreement with experimental observations. The calculation also shows that the reaction started from the P-type species of [Cu₂(H-XYL)O₂]²⁺ which is capable of mediating the direct hydroxylation of aromatic substrates without the intermediacy of an O-type species. Finally, we designed some new complexes with different Cu-ligands and found the complex that computationally possesses a higher activity in mediating the hydroxylation of the ligand based aromatic substrate; here, Cu loses a pyridyl ligand donor by dissociation, compared to the [Cu₂(H-XYL)O₂]²⁺ complex.

Designing antiferromagnetically coupled mono-, di- and tri-bridged copper(ii)-based catecholase models by varying the ‘Auxiliary Parts’ of the ligand and anionic co-ligand

The role of the auxiliary part of ligands towards the architecture of copper(ii)-based catecholase models were unveiled with the help of DFT study.

Dual oxidase/oxygenase reactivity and resonance Raman spectra of {Cu3O2} moiety with perfluoro-t-butoxide ligands

A mechanism for the formation of O-donor trinuclear {Cu₃O₂} moiety is reported.

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.

Pathways for Arene Oxidation in Non-Heme Diiron Enzymes: Lessons from Computational Studies on Benzoyl Coenzyme A Epoxidase

Oxygenation of aromatic rings using O₂ is catalyzed by several non-heme carboxylate-bridged diiron enzymes. In order to provide a general mechanistic description for these reactions, computational studies were carried out at the ONIOM(B3LYP/BP86/Amber) level on the non-heme diiron enzyme benzoyl coenzyme A epoxidase, BoxB. The calculations revealed four possible pathways for attacking the aromatic ring: (a) electrophilic (2e^-) attack by a bis(μ-oxo)-diiron(IV) species (Q pathway); (b) electrophilic (2e^-) attack via the σ* orbital of a μ-η²:η²-peroxo-diiron(III) intermediate (Pσ* pathway); (c) radical (1e^-) attack via the π*-orbital of a superoxo-diiron(II,III) species (Pπ* pathway); (d) radical (1e^-) attack of a partially quenched bis(μ-oxo)-diiron(IV) intermediate (Q' pathway). The results allowed earlier work of de Visser on olefin epoxidation by diiron complexes and QM-cluster studies of Liao and Siegbahn on BoxB to be put into a broader perspective. Parallels with epoxidation using organic peracids were also examined. Specifically for the BoxB enzyme, the Q pathway was found to be the most preferred, but the corresponding bis(μ-oxo)-diiron(IV) species is significantly destabilized and not expected to be directly observable. Epoxidation via the Pσ* pathway represents an energetically somewhat higher lying alternative; possible strategies for experimental discrimination are discussed. The selectivity toward epoxidation is shown to stem from a combination of inherent electronic properties of the thioacyl substituent and enzymatic constraints. Possible implications of the results for toluene monooxygenases are considered as well.

Laser-Induced Dynamics of Peroxodicopper(II) Complexes Vary with the Ligand Architecture. One-Photon Two-Electron O2 Ejection and Formation of Mixed-Valent Cu(I)Cu(II)-Superoxide Intermediates

Photoexcitation of end-on trans-μ-1,2-peroxodicopper(II) complex [(tmpa)2Cu(II)2(O2)](2+) (1) (λmax = 525 and 600 nm) and side-on μ-η(2):η(2)-peroxodicopper(II) complexes [(N5)Cu(II)2(O2)](2+) (2) and [(N3)Cu(II)2(O2)](2+) (3) at -80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O2(2-) + hν → O2 + 2e(-)). Interestingly, light excitation of 2 and 3 (having side-on μ-η(2):η(2)-peroxo ligation) led to release of dioxygen, while photoexcitation of 1 (having an end-on trans-1,2-peroxo geometry) did not, even though spectroscopic studies revealed that both reactions proceeded through previously unknown mixed-valent superoxide species: [Cu(II)(O2(•-))Cu(I)](2+) (λmax = 685-740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an "O2-caged" dicopper(I) adduct, Cu(I)2-O2, and a barrierless stepwise back electron transfer to regenerate 1 occurred. Femtosecond laser excitation of 2 and 3 under the same conditions still led to [Cu(II)(O2(•-))Cu(I)](2+) intermediates that, instead, underwent O2 release with a quantum yield of 0.14 ± 0.1 for 3. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of 2 and 3 vs 1 indicated by ligand-Cu(II/I) redox potentials; (N5)Cu(I) and (N3)Cu(I) complexes are far more stable than (tmpa)Cu(I) species. The fast Cu(I)2/O2 rebinding kinetics was also measured after photoexcitation of 2 and 3, with the results closely tracking those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu(I)2](2+) and [(N3)Cu(I)2](2+) and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.

Dinuclear copper complexes: coordination of Group 14 heteroborates

The dinuclear copper(i) complex [Cu₂(μ-dmapm)(CH₃CN)₂][BF₄]₂ was reacted with Group 14 heteroborates. Four different coordination motives have been characterized.

Complete σ* intramolecular aromatic hydroxylation mechanism through O2 activation by a Schiff base macrocyclic dicopper(I) complex

In this work we analyze the whole molecular mechanism for intramolecular aromatic hydroxylation through O2 activation by a Schiff hexaazamacrocyclic dicopper(I) complex, [Cu(I) 2(bsH2m)](2+). Assisted by DFT calculations, we unravel the reaction pathway for the overall intramolecular aromatic hydroxylation, i.e., from the initial O2 reaction with the dicopper(I) species to first form a Cu(I)Cu(II)-superoxo species, the subsequent reaction with the second Cu(I) center to form a μ-η(2):η(2)-peroxo-Cu(II) 2 intermediate, the concerted peroxide O-O bond cleavage and C-O bond formation, followed finally by a proton transfer to an alpha aromatic carbon that immediately yields the product [Cu(II) 2(bsH2m-O)(μ-OH)](2+).

Modeling tyrosinase and catecholase activity using new m-Xylyl-based ligands with bidentate alkylamine terminal coordination

Chemical model systems possessing the reactivity aspects of both tyrosinase and catechol oxidase are presented. Using two m-xylyl-based ligands providing bidentate alkylamine terminal coordination, 1,3-bis[(N,N-dimethylaminoethyl)aminomethyl]benzene (L(H,H)) and 1,3-bis[(N,N,N'-trimethylaminoethyl)aminomethyl]benzene (L(Me,Me)), four new dicopper(I) complexes, [Cu(I)(2)(L(H,H))(MeCN)(4)][ClO(4)](2) (1), [Cu(I)(2)(L(H,H))(PPh(3))(2)(MeCN)(2)][ClO(4)](2) (2), [Cu(I)(2)(L(Me,Me))(MeCN)(2)][ClO(4)](2) (3), and [Cu(I)(2)(L(Me,Me))(PPh(3))(2)][ClO(4)](2) (4), have been synthesized and characterized. Complex 2 has been structurally characterized. Reaction of the dicopper(I) complex 3(2+) with dioxygen at 183 K generates putative bis(μ-oxo)dicopper(III) intermediate (absorption spectroscopy). Oxygenation of 1 and 3 brings about m-xylyl-ring hydroxylation (monooxygenase-like activity), with a noticeable color change from pale-yellow to dark green. The presence of phenoxo- and hydroxo-bridges in the end products [Cu(II)(2)(L(H,H)-O)(OH)(MeCN)(2)][ClO(4)](2) (5) and [Cu(II)(2)(L(Me,Me)-O)(OH)(OClO(3))][ClO(4)]·MeCN(6) has been authenticated by structural characterization. Oxygenation of 3 afforded not only the green complex 6 isolation but also a blue complex [Cu(II)(2)(L(Me,Me))(OH)(2)][ClO(4)](2) (7). Variable temperature magnetic susceptibility measurements on 5 and 6 establish that the Cu(II) centers are strongly antiferromagnetically coupled [singlet-triplet energy gap (J) = -528 cm(-1) (5) and -505 cm(-1) (6)]. The abilities of phenoxo- and hydroxo-bridged dicopper(II) complexes 5 and 6, the previously reported complex [Cu(II)(2)(L(1)-O)(OH)(OClO(3))(2)]·1.5H(2)O (8) (L(1)-OH = 1,3-bis[(2-dimethylaminoethyl)iminomethyl]phenol), and [Cu(II)(2)(L(2)-O)(OH)(OClO(3))()][ClO(4)]() (9) (L(2)-OH = 1,3-[(2-dimethylaminoethyl)iminomethyl][(N,N,N'-trimethyl)aminoethyl]-4-methylphenol) have been examined to catalyze the oxidation of catechol to quinone (catecholase activity of tyrosinase and catechol oxidase-like activity) by employing the model substrate 3,5-di-tert-butylcatechol. Saturation kinetic studies have been performed on these systems to arrive at the following reactivity order [k(cat)/K(M) (catalytic efficiency) × 10(-3) (M(-1) h(-1))]: 470 (6) > 367 (5) > 128 (9) > 90 (8).

Reversible dioxygen binding and arene hydroxylation reactions: Kinetic and thermodynamic studies involving ligand electronic and structural variations

Copper-dioxygen interactions are of intrinsic importance in a wide range of biological and industrial processes. Here, we present detailed kinetic/thermodynamic studies on the O(2)-binding and arene hydroxylation reactions of a series of xylyl-bridged binuclear copper(I) complexes, where the effects of ligand electronic and structural elements on these reactions are investigated. Ligand 4-pyridyl substituents influence the reversible formation of side-on bound μ-η(2):η(2)-peroxodicopper(II) complexes, with stronger donors leading to more rapid formation and greater thermodynamic stability of product complexes [Cu(II) (2)((R)XYL)(O(2) (2-))](2+). An interaction of the latter with the xylyl π-system is indicated. Subsequent peroxo electrophilic attack on the arene leads to C-H activation and oxygenation with hydroxylated products [Cu(II) (2)((R)XYLO(2-))((-)OH)](2+) being formed. A related unsymmetrical binucleating ligand was also employed. Its corresponding O(2)-adduct [Cu(II) (2)(UN)(O(2) (2-))](2+) is more stable, but primarily because the subsequent decay by hydroxylation is in a relative sense slower. The study emphasizes how ligand electronic effects can and do influence and tune copper(I)-dioxygen complex formation and subsequent reactivity.

Endogenous arene hydroxylation promoted by copper(I) cluster helicates

A novel neutral triple-stranded hexanuclear copper(I) cluster helicate [Cu(I)(6)L(3)]·2CH(3)CN derived from a thiosemicarbazone ligand could be synthesized and crystallographically characterized. The MALDI mass spectrum of this complex suggests that the tetranuclear copper(I) cluster helicate [Cu(I)(4)L(2)] is also present in solution. These copper(I) cluster helicates are capable, in the presence of O(2), of hydroxylating the arene linker of their supporting ligand strands. The resulting dinuclear complex [Cu(II)(2)L'(OH)] is formed by two copper(II) centers, a new ligand arising from the hydroxylation reaction, and one hydroxide group. The magnetic investigation of this compound shows a strong antiferromagnetic coupling between the two Cu(II) centers. The kinetic studies for the hydroxylation process show values of ΔH(≠)=-70 kJ mol(-1), similar to those mediated by the tyrosinase enzymes.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.