Molecular structure and catechol oxidase activity of a new copper(I) complex with sterically crowded monodentate N-donor ligand

The attempted alkylation of 1,3-bis(2′-pyridylimino)isoindoline (indH) by the use of n-BuLi and subsequent alkyl halides led to quaternization of the pyridine nitrogens and the zwitterionic monodentate N-ligand (Me₂ind)I was formed. By the use of the ligand the copper(I) complex [Cu^I(Me₂ind)I₂] was prepared and its structure determined. It was found to be good catalyst for the oxidation of 3,5-di-tert-butylcatechol (DTBCH₂) to 3,5-di-tert-butyl-1,2-benzoquinone (DTBQ) and H₂O₂ by dioxygen. Detailed kinetic studies revealed first-order dependence on the catalyst and dioxygen concentration and saturation type behavior with respect to the substrate.     

Mechanistic insight into the catechol oxidase activity by a biomimetic dinuclear copper complex

The biomimetic catalytic oxidation of 3,5-di-tert-butylcatechol by the dicopper(II) complex of the ligand ,-bis{bis[1-(1-methyl-2-benzimidazolyl)methyl]amino}-m-xylene in the presence of dioxygen has been investigated as a function of temperature and pH in a mixed aqueous/organic solvent. The catalytic cycle occurs in two steps, the first step being faster than the second step. In the first step, one molecule of catechol is oxidized by the dicopper(II) complex, and the copper(II) centers are reduced. From the pH dependence, it is deduced that the active species of the process is the monohydroxo form of the dinuclear complex. In the second step, the second molecule of catechol is oxidized by the dicopper(I)-dioxygen complex formed upon oxygenation of the reduced complex. In both cases, catechol oxidation is an inner-sphere electron transfer process involving binding of the catechol to the active species. The binary catechol-dicopper(II) complex formed in the first step could be characterized at very low temperature (–90 °C), where substrate oxidation is blocked. On the contrary, the ternary complex of dicopper(I)-O₂-catechol relevant to the second step does not accumulate in solution and could not be characterized, even at low temperature. The investigation of the biphasic kinetics of the catalytic reaction over a range of temperatures allowed the thermodynamic (H° and S°) and activation parameters (H and S) connected with the key steps of the catecholase process to be obtained.     

The catalytic cycle of catechol oxidase

Hybrid density functional theory with the B3LYP functional has been used to investigate the catalytic mechanism of catechol oxidase. Catechol oxidase belongs to a class of enzymes that has a copper dimer with histidine ligands at the active site. Another member of this class is tyrosinase, which has been studied by similar methods previously. An important advantage for the present study compared to the one for tyrosinase is that X-ray crystal structures exist for catechol oxidase. The most critical step in the mechanism for catechol oxidase is where the peroxide O–O bond is cleaved. In the suggested mechanism this cleavage occurs in concert with a proton transfer from the substrate. Shortly after the transition state is passed there is another proton transfer from the substrate, which completes the formation of a water molecule. An important feature of the mechanism, like the one for tyrosinase, is that no proton transfers to or from residues outside the metal complex are needed. The calculated energetics is in reasonable agreement with experiments. Comparisons are made to other similar enzymes studied previously.     

Characterization and catechole oxidase activity of a family of copper complexes coordinated by tripodal pyrazole-based ligands

A family of tripodal pyrazole-based ligands has been synthesized by a condensation reaction between 1-hydroxypyrazoles and aminoalcohols. The diversity was introduced both on the substituents of the pyrazole ring and on the side chain. The corresponding copper(II) complexes have been prepared by reaction with CuCl₂ in tetrahydrofuran. They have been characterized by EPR, UV spectroscopy and cyclic voltammetry. The absence of the half-field splitting signals in EPR suggests that the complex exists in solution as mononuclear species. The influence of substituents and side chain of the tripodal ligand on the catecholase activity of the complexes was studied. The reaction rate depends on two factors. First, the presence of an oxygen atom in the third position of the side chain should be avoided to keep the effectiveness of the reaction. Second, the electronic and steric effects of substituents on the pyrazole ring strongly affect the catalytic activity of the complex. Thus, best results were obtained with complexes containing unsubstituted pyrazole based-ligands. Kinetic investigations with the best catalyst based on the Michaelis–Menten model show that the catalytic activity of the mononuclear complex is close to that of some dicopper complexes described in literature.     

Theoretical study of the catalytic mechanism of catechol oxidase

The mechanism for the oxidation of catechol by catechol oxidase has been studied using B3LYP hybrid density functional theory. On the basis of the X-ray structure of the enzyme, the molecular system investigated includes the first-shell protein ligands of the two metal centers as well as the second-shell ligand Cys92. The cycle starts out with the oxidized, open-shell singlet complex with oxidation states Cu₂(II,II) with a μ-η²:η² bridging peroxide, as suggested experimentally, which is obtained from the oxidation of Cu₂(I,I) by dioxygen. The substrate of each half-reaction is a catechol molecule approaching the dicopper complex: the first half-reaction involves Cu(I) oxidation by peroxide and the second one Cu(II) reduction. The quantitative potential energy profile of the reaction is discussed in connection with experimental data. Since no protons leave or enter the active site during the catalytic cycle, no external base is required. Unlike the previous density functional theory study, the dicopper complex has a charge of +2.     

Catecholase activity of dicopper(II)-bispidine complexes: stabilities and structures of intermediates,kinetics and reaction mechanism

A mechanism for the oxidation of 3,5-di-tert-butylcatechol (dtbc) with dioxygen to the corresponding quinone (dtbq), catalyzed by bispidine-dicopper complexes (bispidines are various mono- and dinucleating derivatives of 3,7-diazabicyclo[3.3.1]nonane with bis-tertiary-amine–bispyridyl or bis-tertiary-amine–trispyridyl donor sets), is proposed on the basis of (1) the stoichiometry of the reaction as well as the stabilities and structures [X-ray, density functional theory (B3LYP, TZV)] of the bispidine-dicopper(II)–3,4,5,6-tetrachlorcatechol intermediates, (2) formation kinetics and structures (molecular mechanics, MOMEC) of the end-on peroxo–dicopper(II) complexes and (3) kinetics of the stoichiometric (anaerobic) and catalytic (aerobic) copper-complex-assisted oxidation of dtbc. This involves (1) the oxidation of the dicopper(I) complexes with dioxygen to the corresponding end-on peroxo–dicopper(II) complexes, (2) coordination of dtbc as a bridging ligand upon liberation of H₂O₂ and (3) intramolecular electron transfer to produce dtbq, which is liberated, and the dicopper(I) catalyst. Although the bispidine complexes have reactivities comparable to those of recently published catalysts with macrocyclic ligands, which seem to reproduce the enzyme-catalyzed process in various reaction sequences, a strikingly different oxidation mechanism is derived from the bispidine–dicopper-catalyzed reaction. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Structural and spectroscopic studies of a model for catechol oxidase

A binuclear copper complex, [Cu₂(BPMP)(OAc)₂][ClO₄]·H₂O, has been prepared using the binucleating ligand 2,6-bis[bis(pyridin-2-ylmethylamino)methyl]-4-methylphenol (H-BPMP). The X-ray crystal structure reveals the copper centers to have a five-coordinate square pyramidal geometry, with the acetate ligands bound terminally. The bridging phenolate occupies the apical position of the square-based pyramids and magnetic susceptibility, electron paramagnetic resonance (EPR) and variable-temperature variable-field magnetic circular dichroism (MCD) measurements indicate that the two centers are very weakly antiferromagnetically coupled (J = −0.6 cm⁻¹). Simulation of the dipole–dipole-coupled EPR spectrum showed that in solution the Cu–O–Cu angle was increased from 126° to 160° and that the internuclear distance was larger than that observed crystallographically. The high-resolution spectroscopic information obtained has been correlated with a detailed ligand-field analysis to gain insight into the electronic structure of the complex. Symmetry arguments have been used to demonstrate that the sign of the MCD is characteristic of the tetragonally elongated environment. The complex also displays catecholase activity (k _cat = 15 ± 1.5 min⁻¹, K _M = 6.4 ± 1.8 mM), which is compared with other dicopper catechol oxidase models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Long-distance magnetic coupling in dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands

Two novel dinuclear copper(II) complexes of formulae [Cu₂(tren)₂(bpda)](ClO₄)₄ (2) and [Cu₂(tren)₂(tpda)](ClO₄)₄ (3) containing the tripodal tris(2-aminoethyl)amine (tren) terminal ligand and the 4,4′-biphenylenediamine (bpda) and 4,4″-p-terphenylenediamine (tpda) bridging ligands have been synthesized and structurally, spectroscopically, and magnetically characterized. Their experimentally available electronic spectroscopic and magnetic properties have been reasonably reproduced by DFT and TDDFT calculations. Single crystal X-ray diffraction analysis of 2 shows the presence of dicopper(II) cations where the bpda bridging ligand adopts a bismonodentate coordination mode toward two [Cu(tren)]²⁺ units with an overall non-planar, orthogonal anti configuration of the N-Cu-N threefold axis of the trigonal bipyramidal Cu^II ions and the biphenylene group. The electronic absorption spectra of 2 and 3 in acetonitrile reveal the presence of four moderately weak d-d transitions characteristic of a slightly distorted trigonal bipyramid stereochemistry of the Cu^II ions. TDDFT calculations on 2 identify these transitions as those taking place between the four lower-lying, doubly occupied a₂ (d_yz)², b₂ (d_xz)², b₁ (d_xy)², and a₁ (d_x²-y²)² orbitals and the upper, singly occupied a₁ (d_z²)¹ orbital of each trigonal bipyramidal Cu^II ion. Variable-temperature magnetic susceptibility measurements of 2 and 3 show the occurrence of moderate (J = −8.5 cm⁻¹) to weak intramolecular antiferromagnetic couplings (J = −2.0 cm^-1) [H = −JS₁·S₂ with S₁ = S₂ = S_Cu = ½] inspite of the relatively large copper-copper separation across the para-substituted biphenylene- (r = 12.3 Å) and terphenylenediamine (r = 16.4 Å) bridges, respectively. DFT calculations on 2 and 3 support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the d_z² orbital of each trigonal bipyramidal Cu^II ion through the predominantly π-type orbital pathway of the oligo-p-phenylenediamine bridges, as reported earlier for the parent compound [Cu₂(tren)₂(ppda)](ClO₄)₄·2H₂O (1) with the 1,4-phenylenediamine (ppda) bridging ligand. Finally, a rather slow exponential decay of the antiferromagnetic coupling (-J) with the number of phenylene repeat units, -(C₆H₄)_n- (n = 1-3), has been found both experimentally and theoretically along this series of oligo-p-phenylenediamine-bridged dicopper(II) complexes. These results further support the ability of linear π-conjugated oligo-p-phenylene spacers to transmit the exchange interaction between the unpaired electrons of the two Cu^II centers with intermetallic distances in the range of 7.5-16.4 Å.     

Oxidation of diiron and triiron sulfurdithiolato complexes: mimics for the active site of [FeFe]-hydrogenase

The oxidation of the hexacarbonyl(1,3-dithiolato-S,S')diiron complexes 4a-4c with varying amounts of dimethyldioxirane (DMD) was systematically studied. The chemoselectivity of the oxidation products depended upon the substituent R (R=H, Me, 1/2 (CH2)(5)). For R=H, four oxidation products, 6a-6d, have been obtained. In the case of R=Me, three products, 7a-7c, were formed, and for R=1/2 (CH2)(5), only complex 8 was observed. These observations are due to steric and electronic effects caused by the substituent R. Additionally, oxidation of the triiron complex 5 with DMD was performed to yield the products 9a and 9b. X-Ray diffraction analyses were performed for 6a-6d, 7a, and 7c, as well as for 9a and 9b. The electronic properties were determined by density-functional theory (DFT) calculations.     

How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

The tendency for mixed-isotope O₂ fragments to exhibit different stretching frequencies in asymmetric environments is examined with various levels of electronic structure theory for simple peroxides and peroxyl radicals, as well as for a variety of monocopper–O₂ complexes. The study of the monocopper species is motivated by their relevance to the active site of galactose oxidase. Extensive theoretical work with an experimental model characterized by Jazdzewski et al. (J. Biol. Inorg. Chem. 8:381–393, 2003) suggests that the failure to observe a splitting between ¹⁶O¹⁸O and ¹⁸O¹⁶O isotopomers cannot be taken as evidence against end-on O₂ coordination. Conformational analysis on an energetic basis, however, is complicated by biradical character inherent in all of the copper–O₂ singlet structures. Electronic Supplementary Material Supplementary material is available for this article at .     

Topological control of the spin coupling in dinuclear copper(II) complexes with meta- and para-phenylenediamine bridging ligands

A novel series of copper(II) complexes of formula [Cu(tren)(mpda)](ClO₄)₂ · 1/2H₂O (1), [Cu₂(tren)₂(mpda)](ClO₄)₄ · 2H₂O (2), and [Cu₂(tren)₂(ppda)](ClO₄)₄ · 2H₂O (3) containing the tetradentate tris(2-aminoethyl)amine (tren) terminal ligand and the potentially bridging 1,n-phenylenediamine [n = 3 (mpda) and 4 (ppda)] ligand have been prepared and spectroscopically characterized. X-ray diffraction on single crystals of 1 and 3 show the presence of mono- (1) and dinuclear (3) copper(II) units where the mpda (1) and ppda (3) ligands adopt terminal monodentate (1) and bridging bis(monodentate) (3) coordination modes toward [Cu(tren)]²⁺ cations with an overall non-planar, orthogonal disposition of the phenylene group and the N-Cu-N threefold axis of the trigonal bipyramid of each copper(II) ion [values of the Cu-N-C-C torsion angle (?) in the range of 50.8(3)-79.2(2) (1) and 80.9(2)-86.5(2)° (3)]. Variable-temperature magnetic susceptibility measurements on the dinuclear complexes 2 and 3 show the occurrence of moderate ferromagnetic (J = +8.3 cm⁻¹, 2) and strong antiferromagnetic (J = −51.4 cm⁻¹, 3) couplings between the two copper(II) ions across the meta- and para-phenylenediamine bridges, leading to S = 1 (2) and S = 0 (3) ground spin states [H = −JS₁ · S₂ with S₁ = S₂ = S_Cu = 1/2]. Density functional theory (DFT) calculations on the triplet (2) and broken-symmetry (BS) singlet (3) ground spin states, support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction through the predominantly π-type orbital pathway of the 1,n-phenylenediamine bridge. Finally, a new magneto-structural correlation between the magnitude of the magnetic coupling (J) and the Cu-N-C-C torsion angle (?) has been found which reveals the role of σ- versus π-type orbital pathways in the modulation of the magnetic coupling for m- and p-phenylenediamine-bridged dicopper(II) complexes.     

Copper(II) complexes of a new N-picolylated bis benzimidazolyl diamide ligand: Synthesis, crystal structure and catechol oxidase studies

Copper(II) complexes of a new bis benzimidazole diamide ligand N-picolyl-N,N′-bis(2-methylbenzimidazolyl)hexanediamide [Pic-GBHA = L₂] have been synthesized and characterized. One of the compound [Cu(L₂)(NO₃)₂] has been structurally characterized. The copper atom is bound to two benzimidazolyl nitrogen atoms, two amide carbonyl oxygen atoms and a bidentate nitrate ion, resulting in a distorted octahedral geometry. EPR spectra obtained at low temperature indicate a tetragonal geometry in the solution state. Complexes display a quasi-reversible redox wave due to the Cu(II)/Cu(I) reduction process having fairly cathodic E_1/2. These Cu(II) complexes were utilized to carry out oxidation of ditertbutylcatechol (DTBC) in methanol using molecular oxygen as the oxidant in. Low temperature EPR study of the oxidation reaction implicates the formation of an active copper species with fairly low A_‖ value. The presence of picolyl groups on the ligand also serve as a proton sponge giving 2-3 times higher rates of reaction in comparison to the non-picolylated ligand, implying a role of free basic groups in the pH control of enzymatic oxidation of catechols by catechol oxidase and tyrosinase.     

Unusual reactivity of copper(I) complexes of functionalised calix[4]arenes

Two functionalised calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)calix[4]arene (L¹) and 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)-26,28-dibutoxycalix[4]arene (L³), were prepared and characterised. The copper(I) complexes of both calix[4]arenes were synthesised and their reactivities were analysed and compared. The presence of the metal induced a radical in the case of L¹ whereas no such radical was observed in the metal complex of ligand L³.     

Monomeric, oligomeric, and polymeric copper(II) complexes of calix[4]arene-derived ligands

Deprotonation of the p-tert-butylcalix[4]arene disubstituted at alternate phenolic positions with picolyl groups 2 was achieved with alkali metal hydrides LiH, NaH, and KH. The dianionic calixarene derivatives were subjected to complete substitution at the phenolic rim with allyl bromide, providing the tetraalkylated derivatives in cone 3a and partial-cone conformations 3b; both compounds were crystallographically characterized. Compound 2, as well as 3a and 3b were tested as ligands towards CuCl₂, affording Cu²⁺ complexes in the first two cases. Polymeric [2·CuCl₂] was obtained from 2 and CuCl₂ in MeOH/CH₂Cl₂ solutions, and consists of chains of the ditopic calixarene acting as an N-donor towards Cu²⁺ ions outside the macrocyclic cavity. Employment of EtOH/CH₂Cl₂ mixtures results in the tricopper complex [(2)₂Cu₃Cl₆(EtOH)₂]. In contrast, reactions of ligand 3a with CuCl₂ afforded monomeric [3a·CuCl₂], while no Cu²⁺ complexes could be obtained when 3b was employed. The presence of intramolecular hydrogen bonds in 2 appears to control the formation of oligomeric or polymeric copper complexes, while the lack of such hydrogen bonds allows the proper alignment of N-donors to coordinate Cu²⁺ directly above the macrocyclic cavity.     

Structures, spectroscopy and modeling of a rare set of isomeric copper(II) complexes

The structures and spectroscopic properties of various conformations of two diasteromeric pairs of enantiomers of pentacoordinate Cu^II bispidine complexes with chiral tetradentate ligands are reported. With one of the ligands an interesting type of distortional isomerism was observed experimentally, and this was studied in detail on the basis of the experimental structural and spectroscopic data (UV-Vis-NIR, EPR) and a DFT-, MM- and ligand-field-theory-based analysis.     

Inhibition of the catecholase activity of biomimetic dinuclear copper complexes by kojic acid

The inhibition of the catechol oxidase activity exhibited by three dinuclear copper(II) complexes, derived from different diaminotetrabenzimidazole ligands, by kojic acid [5-hydroxy-2-(hydroxymethyl)-γ-pyrone] has been studied. The catalytic mechanism of the catecholase reaction proceeds in two steps and for both of these inhibition by kojic acid is of competitive type. The inhibitor binds strongly to the dicopper(II) complex in the first step and to the dicopper-dioxygen adduct in the second step, preventing in both cases the binding of the catechol substrate. Binding studies of kojic acid to the dinuclear copper(II) complexes and a series of mononuclear analogs, carried out spectrophotometrically and by NMR, enable us to propose that the inhibitor acts as a bridging ligand between the metal centers in the dicopper(II) catalysts. Received: 23 August 1999 / Accepted: 20 January 2000     

On the oxidation of alkyl and aryl sulfides by [(Me3TACN)MnO(OH)2]: A density functional study

Density functional theory suggests that the formal 2-electron oxidation of sulfides, RR′S, to sulfoxides, by the model Mn^VO catalyst, [(TACN)Mn^V O(OH)₂]⁺, proceeds in two quite distinct 1-electron steps. Transfer of the first electron is barrierless and generates a sulfur radical cation, antiferromagnetically coupled to a Mn^IV centre via a covalent μ-oxo bridge. The second electron-transfer step is accompanied by migration of the oxygen atom to the sulfur centre, and is rate-determining. The absence of a barrier in the first step, where a sulfur radical is formed, means that the presence of electron-donating or withdrawing substituents on the sulfide has only a minor impact on the rate of reaction.     

Systematic development of computational models for the catalytic site in galactose oxidase: impact of outer-sphere residues on the geometric and electronic structures

A systematic in silico approach has been employed to generate sound, experimentally validated active-site models for galactose oxidase (GO) using a hybrid density functional, B(38HF)P86. GO displays three distinct oxidation states: oxidized [Cu(II)-Y*]; semireduced [Cu(II)-Y]; and reduced [Cu(I)-Y]. Only the [Cu(II)-Y*] and the [Cu(I)-Y] states are assumed to be involved in the catalytic cycle, but their structures have not yet been determined. We have developed several models (1-7) for the [Cu(II)-Y*] state that were evaluated by comparison of our computational results with experimental data. An extended model system (6) that includes solvent molecules and second coordination sphere residues (R330, Y405, and W290) is essential to obtain an experimentally correct electronic structure of the active site. The optimized structure of 6 resulted in a five-coordinate Cu site with a protein radical centered on the Tyr-Cys cofactor. We further validated our converged model with the largest model (7) that included additional outer-sphere residues (Q406, H334, Y329, G513, and T580) and water molecules. Adding these residues did not affect significantly the active site's electronic and geometric structures. Using both 6 and 7, we explored the redox dependence of the active-site structure. We obtained four- and three-coordinate Cu sites for [Cu(II)-Y] and [Cu(I)-Y] states, respectively, that corroborate well with the experimental data. The relative energies of these states were validated by a comparison with experimental redox potentials. Collectively, our computational GO models well reproduce the physicochemical characteristics of the individual states, including their redox behaviors.     

Proton pumping in cytochrome c oxidase: Energetic requirements and the role of two proton channels

Cytochrome c oxidase is a superfamily of membrane bound enzymes catalyzing the exergonic reduction of molecular oxygen to water, producing an electrochemical gradient across the membrane. The gradient is formed both by the electrogenic chemistry, taking electrons and protons from opposite sides of the membrane, and by proton pumping across the entire membrane. In the most efficient subfamily, the A-family of oxidases, one proton is pumped in each reduction step, which is surprising considering the fact that two of the reduction steps most likely are only weakly exergonic. Based on a combination of quantum chemical calculations and experimental information, it is here shown that from both a thermodynamic and a kinetic point of view, it should be possible to pump one proton per electron also with such an uneven distribution of the free energy release over the reduction steps, at least up to half the maximum gradient. A previously suggested pumping mechanism is developed further to suggest a reason for the use of two proton transfer channels in the A-family. Since the rate of proton transfer to the binuclear center through the D-channel is redox dependent, it might become too slow for the steps with low exergonicity. Therefore, a second channel, the K-channel, where the rate is redox-independent is needed. A redox-dependent leakage possibility is also suggested, which might be important for efficient energy conservation at a high gradient. A mechanism for the variation in proton pumping stoichiometry over the different subfamilies of cytochrome oxidase is also suggested. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.     

Synthesis, structures, spectral and electrochemical properties of copper(II) complexes of sterically hindered Schiff base ligands

The Schiff base ligands 2-(2,6-diisopropylphenyliminomethyl)phenol H(L1), 5-diethylamino-2-(2,6-diisopropylphenyliminomethyl)phenol H(L2), 2,4-di-tert-butyl-6-(2,6-diisopropylphenyliminomethyl)phenol H(L3), 3-(2,6-diisopropylphenyliminomethyl)naphthalen-2-ol H(L4) and 4-(2,6-diisopropylphenyliminomethyl)-5-hydroxymethyl-2-methylpyridin-3-ol H(L5) have been synthesized by the condensation, respectively, of salicylaldehyde, 4-(diethylamino)salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, 2-hydroxy-1-napthaldehyde and pyridoxal with 2,6-diisopropylaniline. The copper(II) bis-ligand complexes [Cu(L1)₂] 1, [Cu(L2)₂] 2, [Cu(L3)₂] 3, [Cu(L4)₂] 4 and [Cu(L5)₂] · CH₃OH 5 of these ligands have been isolated and characterized. The X-ray crystal structures of two of the complexes [Cu(L1)₂] 1 and [Cu(L5)₂] · CH₃OH 5 have been successfully determined, and the centrosymmetric complexes possess a CuN₂O₂ chromophore with square planar coordination geometry. The frozen solution EPR spectra of the complexes reveal a square-based CuN₂O₂ chromophore, and the values of g_‖ and g_‖/A_‖ index reveal enhanced electron delocalization by incorporating the strongly electron-releasing -NEt₂ group (2) and fusing a benzene ring on sal-ring (4). The Cu(II)/Cu(I) redox potentials of the Cu(II) complexes reveal that the incorporation of electron-releasing -NEt₂ group and fusion of a benzene ring lead to enhanced stabilization of Cu(II) oxidation state supporting the EPR spectral results. The hydrogen bonding interactions between the two molecules present in the unit cell of 5a generate an interesting two-dimensional hydrogen-bonded network topology.