A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity

Although the reactivity of five-coordinate end-on superoxocopper(II) complexes, CuII(η1-O2•-), is dominated by hydrogen atom transfer, the majority of four-coordinate CuII(η1-O2•-) complexes published thus far display nucleophilic reactivity. To investigate the origin of this difference, we have developed a four-coordinate end-on superoxocopper(II) complex supported by a sterically encumbered bis(2-pyridylmethyl)amine ligand, dpb2-MeBPA (1), and compared its substrate reactivity with that of a five-coordinate end-on superoxocopper(II) complex ligated by a similarly substituted tris(2-pyridylmethyl)amine, dpb3-TMPA (2). Kinetic isotope effect (KIE) measurements and correlation of second-order rate constants (k2's) versus oxidation potentials (Eox) for a range of phenols indicates that the complex [CuII(η1-O2•-)(1)]+ reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [CuII(η1-O2•-)(2)]+. However, [CuII(η1-O2•-)(1)]+ performs HAT much more quickly, with its k2 for reaction with 2,6-di-tert-butyl-4-methoxyphenol (MeO-ArOH) being >100 times greater. Furthermore, [CuII(η1-O2•-)(1)]+ can oxidize C-H bond substrates possessing stronger bonds than [CuII(η1-O2•-)(2)]+ is able to, and it reacts with N-methyl-9,10-dihydroacridine (MeAcrH2) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [CuII(η1-O2•-)(1)]+ is attributed to it possessing higher inherent electrophilicity than [CuII(η1-O2•-)(2)]+, which is a direct consequence of its lower coordination number. These observations are of relevance to enzymes in which four-coordinate end-on superoxocopper(II) intermediates, rather than their five-coordinate congeners, are routinely invoked as the active oxidants responsible for substrate oxidation.

Cu4Ti4-oxo clusters functionalized by in situ - generated 2,2'-biphenolate ligands from the oxidative coupling of phenols

Herein, we report a twisted cubic Cu(I)₄Ti(IV)₄-oxo cluster stabilized by in situ - formed 2,2'-biphenolate ligands from the oxidative coupling of phenols. The 2,2'-biphenolate-functionalized Cu(I)₄Ti(IV)₄O₄ cluster shows short Cu⋯C contacts and exhibits smaller HOMO-LUMO gaps than those of reported Ti(IV)₄O₄.

Phenolate-bonded bis(μ-oxido)-bis-copper(III) intermediates: hydroxylation and dehalogenation reactivities

Exogenous phenolate ortho-hydroxylation by copper oxidants formed from dioxygen is generally thought to occur through one of two limiting mechanisms defined by the structure of the active oxidant: an electrophilic μ-η²:η²-peroxo-bis-copper(II) species as found in the oxygenated form of the binuclear copper enzyme tyrosinase (oxyTyr), or an isomeric bis(μ-oxido)-bis-copper(III) species (O) with ligated phenolate(s) as evidenced by most synthetic systems. The characterization of the latter is limited due to their limited thermal stability. This study expands the scope of an O species with ligated phenolate(s) using N,N'-di-tert-butyl-1,3-propanediamine (DBPD), a flexible secondary diamine ligand. Oxygenation of the [(DBPD)Cu(I)]¹⁺ complex at low temperatures (e.g., 153 K) forms a spectroscopically and structurally faithful model to oxyTyr, a side-on peroxide intermediate, which reacts with added phenolates to form a bis(μ-oxido)-bis-copper(III) species with ligated phenolates, designated as an A species. The proposed stoichiometry of A is best understood as possessing 2 rather than 1 bonded phenolate. Thermal decomposition of A results in regiospecific phenolate ortho-hydroxylation with the ortho-substituent as either a C-H or C-X (Cl, Br) group, though the halogen displacement is significantly slower. DFT and experimental studies support an electrophilic attack of an oxide ligand into the π-system of a ligated phenolate. This study supports a hydroxylation mechanism in which O-O bond cleavage of the initially formed peroxide by phenolate ligation, which precedes phenolate aromatic hydroxylation.

N,N-bis(2-quinolinylmethyl)benzylamine

N,N-bis(2-quinolinylmethyl)benzylamine (1) was synthesized under basic conditions from a pseudo-three-component reaction between benzylamine and two molecules of 2-(quinolinylmethyl)chloride, resulting in the formation of two N–C bonds in a single step. Compound (1) crystallizes in the triclinic system of the P-1 space group. The unit cell comprises a dimer of 1, in which the monomers are linked by two complementary hydrogen bonds between N1 and H′1–C′1 of another molecule. The dimers form chains along the a-axis through intermolecular interactions between the N′2 acceptor atoms and C″17 donors from molecules in the nearest neighboring dimer. These interactions form extended sheets of the dimers of 1, along the ab plane. The quinolinylmeth-2-yl groups of 1 lie in almost orthogonal planes and their N1/2(q) donor atoms are away from the apical amino N3 atom.

Theoretical rationalization for the equilibrium between (μ-η2:η2-peroxido)CuIICuII and bis(μ-oxido)CuIIICuIII complexes: perturbational effects from ligand frameworks

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ-η2:η2-peroxido)CuIICuII complex 1 and the bis(μ-oxido)CuIIICuIII complex 2. The N3-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N3-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding CuI complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786-8794]. Calculations at the B3LYP*-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 Å to 3.40 Å of the N-N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol-1 at the distance of 3.40 Å. Calculated potential energies indicate that the decrease in the N-N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N-N distance of 2.60 Å. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ* orbital of the O-O bond and the dx2-y2 orbitals of the CuII ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O-O bond cleavage from 1 to 2.

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.

Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate

Mononuclear copper complexes, [(tmpa)Cu(II)(CH3CN)](ClO4)2 (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)Cu(II)(H2O)2](ClO4)2 (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)3) in acetone, whereas 1 catalyzes the four-electron reduction of O2 by the same reductant in the presence of Brønsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex [(tmpa)Cu(II)(O2)Cu(II)(tmpa)](2+), the two-electron reduced product of O2 with Sc(3+) is observed to be scandium peroxide ([Sc(III)(O2(2-))](+)). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), [Sc(III)(O2(2-))](+) was oxidized by [Fe(bpy)3](3+) (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)3Sc(III)(O2(•-))](2+) as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex which is highly reactive toward O2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O2 following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O2-reaction stoichiometry from four-electron with Brønsted acids to two-electron reduction in the presence of Sc(3+) and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc(3+) on copper-oxygen intermediates.

Significant differences in the biological activity of mononuclear Cu(ii) and Ni(ii) complexes with the polyquinolinyl ligand

The difference in the redox activity of metal ions Cu(ii) and Ni(ii) results in some discrepancy in the biological activity of their complexes.

Synthesis and Characterization of New Trinuclear Copper Complexes

This report describes our approach towards modelling the copper cluster active sites of nitrous oxide reductase and the multicopper oxidases/oxygenases. We have synthesized two mesitylene-based trinucleating ligands, MesPY1 and MesPY2, which employ bis(2-picolyl)amine (PY1) and bis(2-pyridylethyl)amine (PY2) tridentate copper chelates, respectively. Addition of cuprous salts to these ligands leads to the isolation of tricopper(I) complexes [(Mes-PY1)Cu(I) (3)(CH(3)CN)(3)](ClO(4))(3)·0.25Et(2)O (1) and [(Mes-PY2)Cu(I) (3)](PF(6))(3) (3) Each of the three copper centers in 1 is most likely four-coordinate, with ligated acetonitrile as the fourth ligand; by contrast, the copper centers in 3 are three-coordinate, as determined by X-ray crystallography The synthesis of [(Mes-PY1)Cu(II) (3)(CH(3)CN)(2)(CH(3)OH)(2)](ClO(4))(6)·(CH(3)OH) (2) was accomplished by addition of three equivalents of the copper(II) salt, Cu(ClO(4))(2)·6H(2)O, to the ligand. The structure of 2 shows that two of the copper centers are tetracoordinate (with MeCN solvent ligation), but have additional weak axial (fifth ligand) interactions with the perchlorate anions; the third copper is unique in that it is coordinated by two MeOH solvent molecules, making it overall five-coordinate. For complexes 2 and 3, one copper ion center is located on the opposite side of the mesitylene plane as the other two. These observations, although in the solid state, must be taken into account for future studies where intramolecular tricopper(I)/O(2) (or other small molecules of interest) interactions in solution are desirable.

Sulfur donor atom effects on copper(I)/O(2) chemistry with thioanisole containing tetradentate N(3)S ligand leading to μ-1,2-peroxo-dicopper(II) species

To better understand the effect of thioether coordination in copper-O(2) chemistry, the tetradentate N(3)S ligand L(ASM) (2-(methylthio)-N,N-bis((pyridin-2-yl)methyl)benzenamine) and related alkylether ligand L(EOE) (2-ethoxy-N,N-bis((pyridin-2-yl)methyl)ethanamine) have been studied. The corresponding copper(I) complexes, [(L(ASM))Cu(I)](+) (1a) and [(L(EOE))Cu(I)](+) (3a), were studied as were the related compound [(L(ESE))Cu(I)](+) (2a, L(ESE) = (2-ethylthio-N,N-bis((pyridin-2-yl)methyl)ethanamine). The X-ray structure of 1a and its solution conductivity reveal a monomeric molecular structure possessing thioether coordination which persists in solution. In contrast, the C-O stretching frequencies of the derivative Cu(I)-CO complexes reveal that for these complexes, the modulated ligand arms, whether arylthioether, alkylthioether, or ether, are not coordinated to the cuprous ion. Electrochemical data for 1a and 2a in CH(3)CN and N,N-dimethylformamide (DMF) show the thioanisole moiety to be a poor electron donor compared to alkylthioether (1a is ∼200 mV more positive than 2a). The structures of [(L(ASM))Cu(II)(CH(3)OH)](2+) (1c) and [(L(ESE))Cu(II)(CH(3)OH)](2+) (2c) have also been obtained and indicate nearly identical copper coordination environments. Oxygenation of 1a at reduced temperature gives a characteristic deep blue intermediate [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) with absorption features at 442 (1,500 M(-1) cm(-1)), 530 (8,600 M(-1) cm(-1)), and 605 nm (10,400 M(-1) cm(-1)); these values compare well to the ligand-to-metal charge-transfer (LMCT) transitions previously reported for [{(L(ESE))Cu(II)}(2)(O(2)(2-))](2+) (2b(P)). Resonance Raman data for [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) support the formation of μ-1,2-peroxo species ν(O-O) = 828 cm(-1)(Δ((18)O(2)) = 48), ν(sym)(Cu-O) = 547 cm(-1) (Δ((18)O(2)) = 23), and ν(asym)(Cu-O) = 497 cm(-1) (Δ((18)O(2)) = 22) and suggest the L(ASM) ligand is a poorer electron donor to copper than is L(ESE). In contrast, the oxygenation of [(L(EOE))Cu(I)](+) (3a), possessing an ether donor as an analogue of the thioether in L(ESE), led to the formation of a bis(μ-oxo) species [{(L(EOE))Cu(III)}(2)(O(2-))(2)](2+) (3b(O); 380 nm, ε ∼ 10,000 M(-1) cm(-1)). This result provides further support for the sulfur influence in 1b(P) and 2b(P), in particular coordination of the sulfur to the Cu. Thermal decomposition of 1b(P) is accompanied by ligand sulfoxidation. The structure of [{(L(EOE))Cu(II)(Cl)}(2)](+) (3c) generated from the reductive dehalogenation of organic chlorides suggests that the ether moiety is weakly bound to the cupric ion. A detailed discussion of the spectroscopic and structural characteristics of 1b(P), 2b(P), and 3b(O) is presented.

Toluene and ethylbenzene aliphatic C-H bond oxidations initiated by a dicopper(II)-mu-1,2-peroxo complex

With an anisole-containing polypyridylamine potential tetradentate ligand (O)L, a mu-1,2-peroxo-dicopper(II) complex [{(O)LCu(II)}(2)(O(2)(2-))](2+) forms from the reaction of the mononuclear compound [Cu(I)((O)L)(MeCN)]B(C(6)F(5))(4) ((O)LCu(I)) with O(2) in noncoordinating solvents at -80 degrees C. Thermal decay of this peroxo complex in the presence of toluene or ethylbenzene leads to rarely seen C-H activation chemistry; benzaldehyde and acetophenone/1-phenylethanol mixtures, respectively, are formed. Experiments with (18)O(2) confirm that the oxygen source in the products is molecular O(2) and deuterium labeling experiments indicate k(H)/k(D) = 7.5 +/- 1 for the toluene oxygenation. The O(2)-reaction of [Cu(I)((Bz)L)(CH(3)CN)](+) ((Bz)LCu(I)) leads to a dicopper(III)-bis-mu-oxo species [{(Bz)LCu(III)}(2)(mu-O(2-))(2)](2+) at -80 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand (Bz)L is a tridentate chelate, possessing the same moiety found in (O)L, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(O)LCu(II)}(2)(O(2)(2-))](2+) is discussed and likely mechanisms of reaction initiated by toluene H-atom abstraction chemistry are detailed. To confirm the structural formulations of the dioxygen-adducts, UV-vis and resonance Raman spectroscopic studies have been carried out and these results are reported and compared to previously described systems including [{Cu(II)((Py)L)}(2)(O(2))](2+) ((Py)L = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu(I), CO-binding properties (i.e., nu(C-O) values) along with electrochemical property comparisons, the relative donor abilities of (O)L, (Bz)L, and (Py)L are assessed.

Ni(II)/H(2)O(2) reactivity in bis[(pyridin-2-yl)methyl]amine tridentate ligand system. aromatic hydroxylation reaction by bis(mu-oxo)dinickel(III) complex

The nickel(II) complexes 1(X) supported by bis[(pyridin-2-yl)methyl]benzylamine tridentate ligands carrying m-substituted phenyl groups (X = OMe, Me, H, Cl, NO(2)) at the 6-position of pyridine donor groups (L(X), N,N-bis[(6-m-substituted-phenylpyridin-2-yl)methyl]benzylamine) have been synthesized and characterized. The X-ray crystallographic analyses have revealed that [Ni(II)(L(H))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(H)), [Ni(II)(L(OMe))(CH(3)CN)(MeOH)](ClO(4))(2) (1(OMe)), [Ni(II)(L(Me))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(Me)), and [Ni(II)(L(Cl))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(Cl)) have a five-coordinate square pyramidal geometry, whereas [Ni(II)(L(NO(2)))(CH(3)CN)(2)(H(2)O)](ClO(4))(2) (1(NO(2))) exhibits a six-coordinate octahedral geometry having an additional CH(3)CN co-ligand. (1)H NMR spectra of the nickel(II) complexes 1(X) in CD(3)CN have indicated that all the complexes have a high spin ground state. The nickel(II) complexes 1(X) react with hydrogen peroxide (H(2)O(2)) in acetone to give bis(mu-oxo)dinickel(III) complexes 2(X) exhibiting a characteristic UV-vis absorption band at approximately 420 nm. In the case of 2(H), a resonance Raman band ascribable to a Ni(2)O(2) core vibration was observed at 611 cm(-1) that shifted to 586 cm(-1) upon H(2)(18)O(2). The bis(mu-oxo)dinickel(III) intermediates 2(X) undergo an efficient aromatic ligand hydroxylation reaction, producing a mononuclear nickel(II)-phenolate complexes 4(X) via a putative intermediate (mu-phenoxo)(mu-hydroxo)dinickel(II) (3(X)). The kinetic studies on the aromatic ligand hydroxylation process including m-substituent effects (Hammett analysis) and kinetic deuterium isotope effects (KIE) have indicated that the reaction of 2(X) to 3(X) involves an electrophilic aromatic substitution mechanism, where C-O bond formation and C-H bond cleavage occur in a concerted manner. Intermediate 3(H) was detected by ESI-MS during the course of the reaction, and the final product 4(H) was characterized by elemental analysis, ESI-MS, and X-ray crystallographic analysis.

Formation of {Cu2(III)(mu-O)2}2+ core due to dioxygen reactivity of a copper(I) complex supported by a new hybrid tridentate ligand: reaction with exogenous substrates

Reaction of a Cu(I) complex of a hybrid tridentate ligand, encompassing [2-(pyridin-2-yl)ethyl]amine and dimethyl-substituted ethylalkylamine with dioxygen, generates in acetone at -80 degrees putative bis(mu-oxo)dicopper(III) intermediate. Structural characterization of a PPh(3)-adduct of a mononuclear Cu(I) complex of this new ligand has been achieved. This ligand coordinates in a facial mode utilizing three N-atoms (-CH(2)CH(2)-Py, -CH(2)CH(2)NMe(2), and -NCH(2)Ph). Reactivity of bis(mu-oxo)dicopper(III) intermediate toward exogenous substrates (2,4-di(tert-butyl)phenol and 2,4,6-tri(tert-butyl)phenol) has also been investigated.