Subtle Structural Changes in (CuIIL)2MnII Complexes To Induce Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic Substrates (H2L = Salen Type Unsymmetrical Schiff Base)

Tetranuclear NiII-Mannich base complex with oxygenase, water splitting and ferromagnetic and antiferromagnetic coupling properties

A new Mannich base (2-(4-(2-hydroxy-3-methoxy-5-methylbenzyl)-piperazin-1-yl)methyl)-6-methoxy-4-ethylphenol (H2L) and its tetranuclear NiII complex [Ni4L2(μ1,1-Cl)2(H2O)4]Cl2 (compound 1) are characterised using single-crystal X-ray diffraction measurements. Compound 1 contains four different NiII centres in a rhombus-like structure. Two Ni atoms (Ni1 and Ni2) have a NiN2O4 coordination sphere, while the other two (Ni3 and Ni4) have a NiO4Cl2 coordination environment and Ni-Cl-Ni bridges connect them. Ni1 and Ni2 are linked to Ni3 and Ni4 by phenoxide bridges. Variable temperature magnetic susceptibility measurements of 1 indicate the presence of alternating antiferromagnetic coupling (J1 = -6.6(1) cm-1) through the phenoxide bridge along the sides of the rhombus and ferromagnetic coupling (J2 = 8.4(1) cm-1) through the double Cl bridge along the short diagonal of the rhombus with a zero-field splitting of |D| = 2.7(1) cm-1. Compound 1 shows oxidase (catecholase-like and phenoxazinone synthase-like) activity. The oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) gives 3,5-di-tert-butylquinone (3,5-DTBQ) and H2O2 and the oxidation of o-aminophenol (OAP) produces 2-aminophenoxazin-3-one (APX) and H2O with turnover numbers of 28.32 and 17.52 h-1, respectively, under aerobic conditions. A mechanism for the oxidase activity catalysed by compound 1 is proposed in line with ESI-mass spectrometry, EPR spectroscopy, and electrochemical data. The reaction involves the cleavage of the tetranuclear Ni4-core to form a mononuclear NiII complex in the presence of the substrate (3,5-DTBC/OAP). This NiII complex is reduced to NiI with the concomitant oxidation of the substrate (3,5-DTBQ/APX). Formation of a radical intermediate is confirmed using EPR. In the catecholase-like activity, O2 is reduced to H2O2 while in the phenoxazinone synthase-like activity O2 produces H2O. Compound 1 participated in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in a strongly basic medium with an onset potential of 418 mV and a Tafel slope of 121 mV dec-1 for OER and an onset potential of 477 mV and Tafel slope of 146 mV dec-1 for HER.

Cu(II)-Ln(III) (Ln = Gd, Tb and Dy) complexes of an unsymmetrical N2O3 donor ligand: field induced SMM behaviour of Cu(II)-Tb(III) complexes

Three new hetero-metallic CuII-LnIII complexes [(CuL)Gd(NO3)3(CH3OH)]n (1), [(CuL)Tb(NO3)3(H2O)]·[CuL] (2) and [(CuL)Dy(NO3)3(H2O)]·[CuL] (3) have been synthesized using a mono-nuclear Cu(II) complex, [CuL], of an unsymmetrically di-condensed N2O3 donor Schiff base ligand, N-(3-methoxysalicylidene)-N-(salicylidene)-1,2-ethylenediamine (H2L). Single crystal X-ray crystallography revealed that complex 1 is a nitrate bridged 1D chain of dinuclear Cu(II)-Gd(III) units whereas in 2 and 3, the dinuclear Cu(II)-Ln(III) units are co-crystallized with a [CuL] unit. The Ln(III) centers are nine coordinated with the geometry of a spherical capped square antiprism for Gd and spherical tricapped trigonal prism for Tb and Dy. The geometry of the Cu(II) center is distorted octahedral for complex 1 and distorted square planar for complexes 2 and 3. Temperature-dependent molar magnetic susceptibility measurements in 1-3 revealed the presence of overall ferromagnetic coupling between the Cu(II) and Ln(III) centers. Notably, field induced single-molecule magnet behavior was witnessed in the Tb(III) derivative (2). The ab initio calculations indicated that upon application of an external magnetic field, the tunneling in the ground state of complex 2 gets reduced and thereby field-induced SMM behaviour is observed. Besides, in the case of complex 1, BS-DFT calculations were carried out to gain further insights into the magnetic exchange coupling interactions between the Cu(II) and Gd(III) centers.

Monomeric copper(II) complexes with unsymmetrical salen environment: Synthesis, characterization and study of biological activities

Three new ONNO-donor tetradentate unsymmetrical salen ligands were synthesized by using o-phenyl diamine with substituted salicylaldehydes followed by a two-step reaction methodology. These three ligands by reaction with Cu(OAc)2.4H2O produced three new monomeric Cu(II) complexes, [CuII(L1-3)] (1-3). Elemental analysis, IR, UV-vis, NMR, and HR-ESI-MS techniques were used to analyze and characterize all the synthesized ligands and their corresponding metal complexes. Molecular structures of 1-3 were confirmed by the single-crystal-XRD analysis. Furthermore, the DNA binding ability of these complexes was checked through UV-vis, fluorescence spectroscopy, and also by circular dichroism studies. All the complexes were found to show an intercalation mode of binding with the Kb value in the range of 104-105 M-1. Finally, 1-3 was tested against two malignant (HeLa and A549) and non-cancerous (NIH-3T3) cell lines to check their in vitro antiproliferative activities. Among all, 1 is the most cytotoxic of the series having IC50 values of 5.7 ± 0.9 and 6.0 ± 0.3 μM against HeLa and A549 cell lines, respectively. This result is also consistent with the DNA binding order. Furthermore, the apoptotic mode of cell death of all the complexes was also evaluated by DAPI, AO/EB, and Annexin V-FITC/PI double staining assays.

Isolation of monomeric copper(II) phenolate selenoether complexes using chelating ortho-bisphenylselenide-phenolate ligands and their electrocatalytic hydrogen gas evolution activity

A series of novel copper(II) phenolate selenoether complexes have been synthesized and structurally characterized for the first time from copper(I) phenanthroline and various substituted ortho-bisphenylselenide-phenol chelating ligands. The synthesized complexes exhibit Jahn-Teller distortion in their geometry and varied from distorted square planar to distorted octahedral by varying the substituent in the bis-selenophenolate ligand. The synthesized complexes electrocatalyze the hydrogen evolution reaction (HER) with a faradaic efficiency of up to 89%, and it was observed that the distorted square pyramidal geometry is the optimum geometry for the maximum efficiency of these copper complexes.

Synthesis and crystal structures of two tri- and tetra-heterometallic Ni(ii)–Mn(ii)/Ni(ii)–Co(iii) complexes from two different Ni(ii)-containing metalloligands: effective catalytic oxidase activity and Schottky device approach

The development of tri- and tetra-nuclear heterometallic Ni(ii)–Mn(ii)/Ni(ii)–Co(iii) complexes with effective catalytic oxidase activity and the Schottky device approach.

Pentanuclear MII–MnII (M = Ni and Cu) complexes of N2O2 donor ligands with a variation of carboxylate anions: syntheses, structures, magnetic properties and catecholase-like activities

One NiII2MnII3 and two CuII2MnII3 complexes have been synthesized using N2O2 donor ligands. The former complex exhibits spin crossover at 2 K temperature. All the complexes exhibit catecholase-like activities.

Robotic Stepwise Synthesis of Hetero-Multinuclear Metal Oxo Clusters as Single-Molecule Magnets

An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn₄}Lu₂A₂ in TBA₅[(A-α-SiW₉O₃₄)₂FeMn₄O₂{Lu(acac)₂}₂A₂] (II^A; A = Ag, Na, K; TBA = tetra-n-butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A⁺ into a paramagnetic {FeMn₄}Lu₂ unit in TBA₇[(A-α-SiW₉O₃₄)₂FeMn₄O₂{Lu(acac)₂}₂] (I). POMs II^A exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal (II^Ag, 40.0 K; II^Na, 40.3 K; II^K, 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.

Influences of polarizability effect of alkyl group and homoring competition effect of substituents on the NMR spectra of salen-type Schiff base

Salen-type Schiff bases are a kind of important compounds and are widely used. In order to explore the effect of alkyl groups and substituents attached to aromatic ring on the chemical shifts, 63 title compounds were synthesized. Their ¹ H NMR and ¹³ C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δ_H (CHN), δ_C (CHN), δ_H (OH), and δ_C (COH)) were studied. The results show that (1) the alkyl polarizability effect index (PEI) has an important influence on the chemical shifts of the above four atoms, with the increase of PEI, the values of δ_H (CHN) and δc(CHN) decrease, and the values of δ_H (OH) and δ_C (COH) increase. (2) The influence of substituent X attached to aromatic ring on the chemical shift is related to its position by taking OH or CHN as reference. As for the effect of substituent on the chemical shifts, the effect of Hammett constant σ(X)_OH and excited-state substituent parameter σ CC ex X  OH with OH as reference are different from that ofσ(X)_CHN and σ CC ex X  CH  N with CHN as reference, and there is a "homoring competition effect" of the substituent. (3) The effect of the cross-interaction between X and OH on the chemical shift is also significantly different due to the different position of X. Quantitative correlation equations against chemical shifts were built for the four atoms, and the stability and prediction ability of the obtained equations were confirmed by leave-one-out cross validation.

Synthesis of Ni(ii)-Mn(ii) complexes using a new mononuclear Ni(ii) complex of an unsymmetrical N2O3 donor ligand: structures, magnetic properties and catalytic oxidase activity

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed N₂O₃ donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (H₂L), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL)₂Mn(NCS)₂(CH₃OH)₂]·CH₃OH (2) [(NiL)₂Mn(N(CN)₂)₂(CH₃OH)₂]·CH₃OH (3) and [(NiL)₂Mn₂(N₃)₂(μ_1,1-N₃)₂(CH₃OH)₂] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O₃ donor (NiL) units are coordinated to the central octahedral Mn(ii) centre, whereas complex 4 has a centrosymmetric tetranuclear structure where two binuclear (NiL)Mn units are linked via two phenoxido and two μ_1,1-N₃ bridges. Among the heterometallic complexes (2-4), only 4 is active towards the catalytic oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone. The turnover number for the aerobic oxidation of 3,5-DTBC is 935 h^-1. ESI-mass spectra have been recorded to scrutinize the mechanistic pathway of this catalytic reaction. Variable temperature magnetic susceptibility measurements suggest that complexes 2-4 are antiferromagnetically coupled with coupling constants (J) of -4.84 and -5.23 cm^-1 for complexes 2 and 3, respectively and J₁ = -2.20 cm^-1, J₂ = 1.13 cm^-1 and J₃ = -1.12 cm^-1 for complex 4. DFT calculations have been used to rationalize the magnetic super-exchange in complexes 2-4, by computing the theoretical coupling constants and analyzing the spin density plots.

Modulation of Nuclearity in CuII -MnII Complexes of a N2 O2 Donor Ligand Depending upon Carboxylate Anions: Structures, Magnetic Properties and Catalytic Oxidase Activities

Three new hetero-metallic copper(II)-manganese(II) complexes, [(CuL)₂ Mn₃ (C₆ H₅ CO₂ )₆ ] (1), [(CuL)₂ Mn(CH₃ CO₂ )₂ ] (2), and {[(CuL)₂ Mn(C₆ H₅ CH₂ CO₂ )₂ ] ⋅ 2CH₃ CN} (3), have been synthesized using [CuL] as ''metalloligand'' (where H₂ L=N,N'-bis(2-hydroxynaphthyl-methylidene)-1,3-propanediamine). Single-crystal structural analyses show an almost linear penta-nuclear structure for complex 1 where a square planar [CuL] unit is connected to each of the two terminal Mn^II ions of a linear, centrosymmetric [Mn₃ (benzoate)₆ ] unit through the double phenoxido bridges. Both complexes 2 and 3 possess a linear tri-nuclear structure where two terminal square-pyramidal [CuL] units are bonded to the central Mn^II ion through double phenoxido oxygen atoms along with a syn-syn bridging acetate (for 2)/phenyl acetate (for 3). All three complexes exhibit catecholase, and phenoxazinone synthase-like activities under aerial conditions. For catecholase like activity, the turnover numbers (k_cat ) are 595, 40, and 205 h^-1 whereas, for phenoxazinone synthase like activity, the turnover numbers are 25, 4, and 11 h^-1 for complexes 1-3, respectively. The mechanism of both catalytic oxidase activities is proposed on the basis of mass spectral evidences. Variable-temperature (2-300 K) dc molar magnetic susceptibility measurements of 1 reveal antiferromagnetic interactions between the Cu-Mn centres (J₁ =-29.3 cm^-1 ), and also between the Mn-Mn centres of the [Mn₃ (benzoate)₆ ] unit (J₂ =-0.68 cm^-1 ). On increasing the magnetic field at 2 K, its ground spin state changes from S=3/2 to S=5/2 at 4 T, attributable to the low value of J₂ which makes the excited spin states close in energy with the ground spin state. Complexes 2 and 3 show antiferromagnetic coupling interactions between the Cu-Mn pairs with J values of -9.51, and -5.32 cm^-1 , respectively.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO₃] (1-9) of newer N₃O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-O_Phenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag⁺ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g_||, 2.243-2.270; A_||, 164-179 × 10^-4 cm^-1) that corresponds to square pyramidal geometry. The bonding parameters α², 0.760-0.825; β², 0.761-0.994; γ², 0.504-0.856 and K_||, 0.698-0.954 and K_⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10^-3 s^-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k_cat) 9.11 × 10⁵ h^-1 for 1 and 4.66 × 10⁵ h^-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

Reaction of Cu(II) Chelates with Uranyl Nitrate to Form a Coordination Complex or H-Bonded Adduct: Experimental Observations and Rationalization by Theoretical Calculations

Four new heterometallic Cu(II)-U(VI) species, [{(CuL¹)(CH₃CN)}UO₂(NO₃)₂] (1), [{(CuL²)(CH₃CN)}UO₂(NO₃)₂] (2), [{(CuL³)(H₂O)}UO₂(NO₃)₂] (3), and [UO₂(NO₃)₂(H₂O)₂]·2[CuL⁴]·H₂O (4), were synthesized using four different metalloligands ([CuL¹], [CuL²], [CuL³], and [CuL⁴], respectively) derived from four unsymmetrically dicondensed N,O-donor Schiff bases. Single-crystal structural analyses revealed that complexes 1, 2, and 3 have a discrete dinuclear [Cu-UO₂] core in which one metalloligand, [CuL], is connected to the uranyl moiety via a double phenoxido bridge. Two chelating nitrate ions complete the octa-coordination around uranium. Species 4 is a cocrystal, where a uranyl nitrate dihydrate is sandwiched between two metalloligands [CuL⁴] by the formation of strong hydrogen bonds between the H atoms of the coordinated water molecules to U(VI) and the O atoms of [CuL⁴]. Spectrophotometric titrations of these four metalloligands with uranyl nitrate dihydrate in acetonitrile showed a well-anchored isosbestic point between 300 and 500 nm in all cases, conforming with the coordination of [CuL¹], [CuL²], [CuL³], and the H-bonding interaction of [CuL⁴] with UO₂(NO₃)₂. This behavior of [CuL⁴] was utilized to selectively bind metal ions (e.g., Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and La³⁺) in the presence of UO₂(NO₃)₂·2H₂O in acetonitrile. The formation of these Cu(II)-U(VI) species in solution was also evaluated by steady-state fluorescence quenching experiments. The difference in the coordination behavior of these metalloligands toward [UO₂(NO₃)₂(H₂O)₂] was studied by density functional theory calculations. The lower flexibility of the ethylenediamine ring and a large negative binding energy obtained from the evaluation of H bonds and supramolecular interactions between [CuL⁴] and [UO₂(NO₃)₂(H₂O)₂] corroborate the formation of cocrystal 4. A very good linear correlation (r² = 0.9949) was observed between the experimental U═O stretching frequencies and the strength of the equatorial bonds that connect the U atom to the metalloligand.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

Joining of Trinuclear Heterometallic CuII2-MII (M = Mn, Cd) Nodes by Nicotinate to Form 1D Chains: Magnetic Properties and Catalytic Activities

In the present work, four new heterometallic coordination complexes, {[(CuL)₂Mn(nic)(H₂O)₂](ClO₄)(0.5H₂O)}_n (1), {[(CuL)₂Cd(nic)(H₂O)₂](ClO₄)(H₂O)}_n (2), [(CuL)₂Mn(nic)₂]·2CH₃OH (3), and [(CuL)₂Cd(nic)₂]·2CH₃OH (4) (where H₂L = N,N'-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2, the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4, the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the Cu^II and Mn^II ions are antiferromagnetically coupled in both 1 and 3, with exchange coupling constants (J_Mn-Cu) of -20.57 ± 0.08 and -9.38 ± 0.08 cm^-1, respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers (k_cat) of complexes 1 and 3 for catecholase activity are 1121 and 720 h^-1, respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h^-1, respectively, at an optimum pH of 9.7. The higher k_cat values of 1 for both reactions are attributable to a water molecule coordinated to the central Mn^II atom that facilitates the substrate-catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL)₂Mn(nic)(H₂O)]⁺ for 1 and [(CuL)₂Mn(nic)]⁺ for 3, are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.

Roles of basicity and steric crowding of anionic coligands in catechol oxidase-like activity of Cu(ii)-Mn(ii) complexes

Five new heterometallic Cu(ii)-Mn(ii) discrete trinuclear complexes, [(CuL)2Mn(CH3COO)2] (1), [(CuL)2Mn(NO3)2] (2), [(CuL)2Mn(C6H5COO)(H2O)]Cl (3), [(CuL)2Mn((p-OH)C6H5COO)(H2O)]ClO4 (4) and [(CuL)2Mn(HCOO)(H2O)]ClO4 (5), have been synthesized using a metalloligand, CuL derived from an N2O2 donor Schiff base, H2L (N,N'-bis(α-methylsalicylidene)-1,3-propanediamine). Single-crystal structural analyses reveal that all five complexes have a common [(CuL)2Mn] core, where two terminal metalloligands, CuL, are connected to the central metal ion, Mn(ii), via double phenoxido bridges. Among the complexes, 1 and 2 possess linear structures where the terminal Cu(ii) atoms are bridged to the central Mn(ii) atoms by acetate and nitrate ions, respectively along with the double phenoxido bridges, whereas 3, 4 and 5 have bent structures in which the respective anionic coligands, benzoate, p-hydroxybenzoate and formate ions are coordinated only to central Mn(ii) in monodentate fashion along with a water molecule that completes its hexa-coordinated geometry. Among the complexes, 1, 3, 4 and 5 show quite high bio-mimicking catecholase-like activity for the aerial oxidation of 3,5-di-tert-butylcatechol with turnover numbers (Kcat) of 139 h-1, 439 h-1, 348 h-1 and 730 h-1, respectively, whereas complex 2 is practically inactive towards this reaction. The presence of the coordinated water molecule to Mn(ii) in the bent complexes, 3-5, appears to be responsible for their high catalytic activity and the difference in their activity may be attributed to steric crowding due to the anionic coligand, whereas the inactivity of 2 seems to be associated with the low basicity of the nitrate ion. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 1-5 are antiferromagnetically coupled with the exchange coupling constants (J) = -8.54 cm-1, -11.50 cm-1, -19.83 cm-1, -10.65 cm-1 and -10.27 cm-1 for 1, 2, 3, 4 and 5 respectively as is expected from the Cu-O-Mn bridging angles.

Structures and esterolytic reactivity of novel binuclear copper(ii) complexes with reduced l-serine Schiff bases as mimic carboxylesterases

Three novel binuclear copper(ii) complexes with reduced l-serine Schiff bases were synthesized and their structures were analyzed with single-crystal X-ray diffraction and DFT calculations. The crystal data revealed that all of these binuclear complexes are chiral. Both 5-halogenated (bromo- and chloro-) binuclear complexes exhibit right-handed helix structural character. Interestingly, the 5-methyl-containing analogue has a two-dimensional pore structure. In this paper, the esterolysis reactivity of the as-prepared complexes shows that in the hydrolysis of p-nitrophenyl acetate (PNPA) these three complexes provide 26, 18, 40-fold rate acceleration as compared to the spontaneous hydrolysis of PNPA at pH 7.0, respectively. Under selected conditions, in excess buffered aqueous solution a rate enhancement by three orders of magnitude was observed for the catalytic hydrolysis of another carboxylic ester, p-nitrophenyl picolinate (PNPP). These complexes efficiently promoted PNPP hydrolysis in a micellar solution of cetyltrimethylammonium bromide (CTAB), giving rise to a rate enhancement in excess of four orders of magnitude, which is approximately 2.0-3.2 times higher than that in the buffer.

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.