Di-, Tetra-, and Polynuclear Copper(II) Complexes: Active Catalysts for Oxidation of Toluene and Benzene

Cu(I) complexes obtained via spontaneous reduction of Cu(II) complexes supported by designed bidentate ligands: bioinspired Cu(I) based catalysts for aromatic hydroxylation

Copper(i) complexes [Cu(L1-7)2](ClO4) (1-7) of bidentate ligands (L1-L7) have been synthesized via spontaneous reduction and characterized as catalysts for aromatic C-H activation using H2O2 as the oxidant. The single crystal X-ray structure of 1 exhibited a distorted tetrahedral geometry. All the copper(i) complexes catalyzed direct hydroxylation of benzene to form phenol with good selectivity up to 98%. The determined kinetic isotope effect (KIE) values, 1.69-1.71, support the involvement of a radical type mechanism. The isotope-labeling experiments using H218O2 showed 92% incorporation of 18O into phenol and confirm that H2O2 is the key oxygen supplier. Overall, the catalytic efficiencies of the complexes are strongly influenced by the electronic and steric factor of the ligand, which is fine-tuned by the ligand architecture. The benzene hydroxylation reaction possibly proceeded via a radical mechanism, which was confirmed by the addition of radical scavengers (TEMPO) to the catalytic reaction that showed a reduction in phenol formation.

Reaching the south: metal-catalyzed transformation of the aromatic para-position

Regioselective functionalization of aromatic arenes has created a rapid insurgence in the modern era of organic chemistry. While the last few years have witnessed significant developments on site-selective ortho- and meta-C-H transformations, there existed very few reports on para-C-H functionalization. Recent advancements on template assisted protocols in para-C-H activation has emerged as a popular and convenient feat in this area. This review highlights the various protocols developed over the years for selective installation of suitable functional groups at the para-position of arenes thereby transforming them into value-added organic cores.

Highly Selective and Efficient Ring Hydroxylation of Alkylbenzenes with Hydrogen Peroxide and an Osmium(VI) Nitrido Catalyst

The Os^VI nitrido complex, Os^VI (N)(quin)₂ (OTs) (1, quin=2-quinaldinate, OTs=tosylate), is a highly selective and efficient catalyst for the ring hydroxylation of alkylbenzenes with H₂ O₂ at room temperature. Oxidation of various alkylbenzenes occurs with ring/chain oxidation ratios ranging from 96.7/3.3 to 99.9/0.1, and total product yields from 93 % to 98 %. Moreover, turnover numbers up to 6360, 5670, and 3880 can be achieved for the oxidation of p-xylene, ethylbenzene, and mesitylene, respectively. Density functional theory calculations suggest that the active intermediate is an Os^VIII nitrido oxo species.

Syntheses, crystal structures, magnetochemistry and catechol oxidase activity of a tetracopper(ii) compound and a new type of dicopper(ii)-based 1D coordination polymer

One phenoxo–hydroxo bridged tetranuclear cluster and one phenoxo-µ_1,1-azido-µ_1,3-azido bridged one-dimensional coordination polymer of copper(ii) derived from a Schiff base ligand are described.

Syntheses and crystal structures of benzene-sulfonate and -carboxylate copper polymers and their application in the oxidation of cyclohexane in ionic liquid under mild conditions

Enhanced catalytic activities of copper polymers for the peroxidative oxidation of cyclohexane are observed in an ionic liquid medium.

Novel tetranuclear Ni(II) Schiff base complex containing Ni4O4 cubane core: synthesis, X-ray structure, spectra and magnetic properties

Novel tetranuclear nickel(II) Schiff base complex having symmetric Ni4O4 cubane-core, [Ni4O2(OAc)2(L)2] (1) has been synthesized. Single crystal of the complex exhibits four nickel atoms in the alternate corner of the cubane and other four sites are occupied by phenolate-O and μ3-O(2-). Variable temperature magnetic moment data suggests the Ni centres are weakly antiferromagnetically coupled with J1=-4.82cm(-1) and J2=-4.83cm(-1). The electronic spectra, emission properties and life time measurement of ligand, HL and complex 1 have been studied.

Ligand effects on the structure and magnetic properties of alternating copper(II) chains with 2,2'-bipyrimidine- and polymethyl-substituted pyrazolates as bridging ligands

A novel series of heteroleptic copper(II) compounds of formulas {[Cu2(μ-H2O)(μ-pz)2(μ-bpm)(ClO4)(H2O)]ClO4·2H2O}n (1), {[Cu2(μ-H2O)(μ-3-Mepz)2(μ-bpm)](ClO4)2·2H2O}n (2), and {[Cu2(μ-OH)(μ-3,5-Me2pz)(μ-bpm)(H-3,5-Me2pz)2](ClO4)2}n (3) [bpm = 2,2'-bipyrimidine, Hpz = pyrazole, H-3-Mepz = 3-methylpyrazole, and H-3,5-Me2pz = 3,5-dimethylpyrazole] have been synthesized and structurally characterized by X-ray diffraction methods. The crystal structures of 1 and 2 consist of copper(II) chains with regular alternating bpm and bis(pyrazolate)(aqua) bridges, whereas that of 3 is made up of copper(II) chains with regular alternating bpm and (pyrazolate)(hydroxo) bridges. The copper centers are six- (1) or five-coordinate (2) in axially elongated, octahedral (1) or square-pyramidal (2) environments in 1 and 2, whereas they are five-coordinate in distorted trigonal-bipyramidal surroundings in 3. The values of the copper-copper separations across the bpm/pyrazolate bridges are 5.5442(7)/3.3131(6) (1), 5.538(1)/3.235(1) (2), and 5.7673(7)/3.3220(6) Å (3). The magnetic properties of 1-3 have been investigated in the temperature range of 25-300 K. The analysis of their magnetic susceptibility data through the isotropic Hamiltonian for an alternating antiferromagnetic copper(II) chain model [H = -J∑i=1-n/2 (S2i·S2i-1 + αS2i·S2i+1), with α = J'/J and Si = SCu = 1/2] reveals the presence of a strong to moderate antiferromagnetic coupling through the bis(pyrazolate)(aqua) [-J = 217 (1) and 215 cm(-1) (2)] and (pyrazolate)(hydroxo) bridges [-J = 153 cm(-1) (3)], respectively, whereas a strong to weak antiferromagnetic coupling occurs through the bis-bidentate bpm [-J' = 211 (1), 213 (2), and 44 cm(-1) (3)]. A simple orbital analysis of the magnetic exchange interaction within the bpm- and pyrazolate-bridged dicopper(II) fragments of 1-3 visualizes the σ-type pathways involving the (dx(2)-y(2)) (1 and 2) or d(z(2)) (3) magnetic orbitals on each metal ion, which account for the variation of the magnetic properties in these three novel examples of one-dimensional copper(II) compounds with regular alternating intrachain antiferromagnetic interactions.

Molecular Cu(II)-O-Cu(II) complexes: still waters run deep

Research on O2 activation at ligated Cu(I) is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu(II) -O-Cu(II) species, which is more special than it appears at first sight: a single oxo ligand between two Cu(II) centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu(II) -O-Cu(II) cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.

Copper(ii) induced oxidative modification and complexation of a schiff base ligand: synthesis, crystal structure, catalytic oxidation of aromatic hydrocarbons and DFT calculation

A mononuclear square planar complex [Cu^II(L^2f)], formed from a Schiff baseH2Laand Cu²⁺with the concomitant oxidation of two –CH₂OH groups to aldehyde, displayed catalytic oxidation of aromatic hydrocarbons.

Nonelectrochemical synthesis, crystal structure, and physical properties of the radical salt [ET]2[CuCl4] (ET = bis(ethylenedithio)tetrathiafulvalene)

The radical salt [ET]2[CuCl4] was obtained by chemical oxidation of bis(ethylenedithio)tetrathiafulvalene (ET) with the tetranuclear copper(II) halide cluster [Cu4OCl10](4-). Although a complex mixture of anions forms in solution during the redox reaction, only this product is obtained as large (>3 mm) single crystals. X-ray diffraction analysis determined that the ET molecules stack in the solid state forming dimerized 1D chains along the a axis, interleaved by [CuCl4](2-) anions. The ET dimers show very short S···S contacts (<3.41 Å). The physical properties are dominated by these intradimer ET interactions. The magnetic behavior shows antiferromagnetic coupling with a singlet-triplet gap >620 K (430 cm(-1)). The Cu(2+) (S = 1/2) centers are magnetically isolated and yield a narrow EPR line in the X-band at g = 2.01. The ET moieties are EPR silent.

Copper-dioxygen complex mediated C-H bond oxygenation: relevance for particulate methane monooxygenase (pMMO)

Particulate methane monooxygenase (pMMO), an integral membrane protein found in methanotrophic bacteria, catalyzes the oxidation of methane to methanol. Expression and greater activity of the enzyme in the presence of copper ion suggest that pMMO is a cuprous metalloenzyme. Recent advances - especially the first crystal structures of pMMO - have energized the field, but the nature of the active site(s) and the mechanism of methane oxidation remain poorly understood-yet hotly contested. Herein the authors briefly review the current understanding of the pMMO metal sites and discuss advances in small molecule Cu-O(2) chemistry that may contribute to an understanding of copper-ion mediated hydrocarbon oxidation chemistry.