Binuclear Nickel Complexes Research Articles

Structural, electrochemical, phosphate-hydrolysis, DNA binding and cleavage studies of new macrocyclic binuclear nickel(ii) complexes

New macrocyclic binuclear nickel(ii) complexes have been synthesized by using the bicompartmental mononuclear complex [NiL] [3,30-((1E,7E)-3,6-dioxa-2,7-diazaocta-1,7-diene-1,8-diyl)bis(3-formyl-5-methyl-2-diolato)nickel(II)] with various diamines like 1,2-bis(aminooxy)ethane (L(1)), 1,2-diamino ethane (L(2)), 1,3-diamino propane (L(3)), 1,4-diamino butane (L(4)), 1,2-diamino benzene (L(5)), and 1,8-diamino naphthalene (L(6)). The complexes were characterized by elemental analysis and spectroscopic methods. The molecular structures of the symmetrical binuclear complex [Ni(2)L(1)(H(2)O)(4)](ClO(4))(2) (1) and unsymmetrical binuclear complex [Ni(2)L(3)(H(2)O)(4)](ClO(4))(2).(H(2)O)(4) (3) were determined by single-crystal X-ray diffraction. The geometry around both the nickel(II) ions in each molecule is a slightly distorted octahedral. The distance between the Ni...Ni centers for complex 1 is 3.039 A and for complex 3 is 3.059 A. The influence of the coordination geometry and the ring size of the binucleating ligands on the electronic, redox, phosphate hydrolysis, DNA binding and cleavage properties have been studied. Electrochemical studies of the complexes show two quasi-reversible one electron reduction processes between -0.49 to -1.69 V. The reduction potential of the binuclear Ni(II) complexes shifts towards anodically upon increasing the macrocyclic ring size. The observed first order rate constant values for the hydrolysis of 4-nitrophenyl phosphate reaction are in the range from 8.69 x 10(-3) to 1.85 x 10(-2) s(-1). The complexes show good binding propensity to calf thymus DNA giving binding constant values in the range from 1.4 x 10(4) to 17.5 x 10(4) M(-1). The absorption, fluorescence and CD spectral data suggests that the complexes are strongly interacting with DNA. These complexes display hydrolytic cleavage of supercoiled pBR322DNA in the presence of H(2)O(2) at pH 7.2 and 37 degrees C. The hydrolytic cleavage of DNA by the complexes is supported by the evidence from free radical quenching and T4 ligase ligation. The pseudo-Michaelis-Menten kinetic parameters k(cat) = 1.27 +/- 0.4 h(-1) and K(M) = 7.7 x 10(-2) M for naphthalene diimine containing macrocyclic binuclear nickel(II) complex, (6) were obtained.

Sensor for fisetin based on gold nanoparticles in ionic liquid and binuclear nickel complex immobilized in silica

Gold nanoparticles dispersed in an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (Au-BMI.PF(6)) and a binuclear nickel(II) complex ([Ni(2)(HBPPAMFF)mu-(OAc)(2)(H(2)O)]BPh(4)) immobilized on functionalized silica were successfully applied in the construction of a novel sensor for the determination of fisetin by square-wave voltammetry. Under optimized conditions, the analytical curve showed two linear ranges for fisetin concentrations from 0.28 to 1.39 microM and 2.77 to 19.50 microM with a detection limit of 0.05 microM. This sensor demonstrated suitable stability (ca. 150 days; at least 500 determinations) and good repeatability and reproducibility, with relative standard deviations of 2.91 and 5.11%, respectively. The recovery study of fisetin in apple juice samples gave values from 96.4 to 106.4%. The efficient analytical performance of the proposed sensor can be attributed to the effective immobilization of the Ni(ii)Ni(ii) complex on silica and the Au-BMI.PF(6) contribution to the electrode response.

Electrochemical, catalytic and antimicrobial activities of N-functionalized cyclam based unsymmetrical dicompartmental binuclear nickel(II) complexes

Five binuclear nickel(II) complexes have been prepared by simple Schiff base condensation of the compound 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-l,4,8,11-tetraazacyclotetradecane (L) with appropriate aliphatic or aromatic diamine, nickel(II) perchlorate and triethylamine. All the complexes were characterized by elemental and spectral analysis. Positive ion FAB mass spectra show the presence of dinickel core in the complexes. The electronic spectra of the complexes show red shift in the d–d transition. Electrochemical studies of the complexes show two irreversible one electron reduction processes in the range of 0 to −1.4 V. The reduction potential of the complexes shifts towards anodically upon increasing chain length of the macrocyclic ring. All the nickel(II) complexes show two irreversible one electron oxidation waves in the range 0.4–1.6 V. The observed rate constant values for catalysis of the hydrolysis of 4-nitrophenyl phosphate are in the range of 1.36 × 10 −2–9.14 × 10 −2 min −1. The rate constant values for the complexes containing aliphatic diimines are found to be higher than the complexes containing aromatic diimines. Spectral, electrochemical and catalytic studies of the complexes were compared on the basis of increasing chain length of the imine compartment. All the complexes show higher antimicrobial activity than the ligand and metal salt.

Structure and chromotropic properties of mono- and binuclear mixed-ligand nickel(II) complexes containing 1,3-diketonate and polyamine ligands

Mixed-ligand Ni II complexes have been syn- thesized with 1,3-diketonate and bulky tri- or tetra-amine ligands. Their structures were determined by X-ray crys- tallography, and solvatochromic behavior was examined by UV-vis-NIR spectroscopy. Steric effects of the bulky ligands on the coordination distances and spectral proper- ties are discussed.

Electronic Structure of a Binuclear Nickel Complex of Relevance to [NiFe] Hydrogenase

The binuclear complex [Ni(2)(L)(MeCN)(2)](3+) (L(2-) = compartmental macrocycle incorporating imine N and thiolate S donors) has a Ni(III) center bridged via two thiolate S-donors to a diamagnetic Ni(II) center. The ground-state has dominant 3d(z)(1)(2) character similar to that observed for [NiFe] hydrogenases in which Ni(III) is bridged via two thiolate donors to a diamagnetic center (Fe(II)). The system has been studied by X-ray crystallography and pulse EPR, ESEEM, and ENDOR spectroscopy in order to determine the extent of spin-delocalization onto the macrocycle L(2-). The hyperfine coupling constants of six nitrogen atoms have been identified and divided into three sets of two equivalent nitrogens. The most strongly coupled nitrogen atoms (a(iso) approximately 53 MHz) stem from axially bound solvent acetonitrile molecules. The two macrocycle nitrogens on the Ni(III) side have a coupling of a(iso) approximately 11 MHz, and those on the Ni(II) side have a coupling of a(iso) approximately 1-2 MHz. Density functional theory (DFT) calculations confirm this assignment, while comparison of the calculated and experimental (14)N hyperfine coupling constants yields a complete picture of the electron-spin density distribution. In total, 91% spin density is found at the Ni(III) of which 72% is in the 3d(z)(2) orbital and 16% in the 3d(xy) orbital. The Ni(II) contains -3.5% spin density, and 7.5% spin density is found at the axial MeCN ligands. In analogy to hydrogenases, it becomes apparent that binding of a substrate to Ni at the axial positions causes a redistribution of the electron charge and spin density, and this redistribution polarizes the chemical bonds of the axial ligand. For [NiFe] hydrogenases this implies that the H(2) bond becomes polarized upon binding of the substrate, which may facilitate its heterolytic splitting.

Synthesis, electrochemical, catalytic and antimicrobial activities of novel unsymmetrical macrocyclic dicompartmental binuclear nickel(II) complexes

A series of novel unsymmetrical dicompartmental binuclear nickel(II) complexes have been prepared by simple Schiff base condensation of the compound 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-l,4,8,11-tetraazacyclotetradecane L with appropriate aliphatic or aromatic diamine, nickel(II) perchlorate and triethylamine. All the complexes were characterized by elemental and spectral analysis. Positive ion FAB mass spectra show the presence of dinickel core in the complexes. The electronic spectra of the complexes show the d–d transition in the range of 550–1040 nm. Electrochemical studies of the complexes show two irreversible one electron reduction process around E pc 1 = - 0.79 to − 1.27 V and E pc 2 = - 1.28 to − 1.43 V . The reduction potential of the binuclear nickel(II) complexes shifts towards anodically upon increasing chain length of the macrocyclic ring. All the nickel(II) complexes show two irreversible oxidation waves around 0.72 to +1.52 V. The observed rate constant values for catalysis of the hydrolysis of 4-nitrophenyl phosphate are in the range of 9.20 × 10 −3–16.81 × 10 −3 min −1. The rate constant values for the complexes containing aliphatic diimines are found to be higher than that of the complexes containing aromatic diimines. Spectral, electrochemical and catalytic studies of the complexes were compared on the basis of increasing chain length of the imine compartment. All the complexes were screened for antifungal and anti bacterial activity.

Syntheses and structures of mononuclear and binuclear transition metal complexes (Cu, Zn, Ni) with (salicylideneglycine and imidazole)

Four transition metal (Cu(II), Zn(II) and Ni(II)) complexes with a Schiff-base ligand (salicylideneglycine) have been synthesized. All complexes have been characterized by elemental analysis, IR spectra and UV-vis spectroscopy. Single-crystal analyses were performed with (C9H7NO3)Cu(C3H4N2) (1), (C9H7NO3)Zn(C3H4N2)2 (2), (C9H7NO3)2Ni2(C3H4N2)4 (3) and (C9H7NO3)Ni(C3H4N2)2(C4H5N2O) · CH3OH · 0.5H2O (4) and fluorescence spectra and thermogravimetric analyses were also carried out. Structural analyses show that 1, 2 and 4 have similar coordinated modes with the tridentate amino-Schiff-base ligand, but differ from the binuclear nickel complex 3. The tridentate amino-Schiff-base ligand contains aliphatic nitrogen, phenoxy, and carboxylic oxygen as three donor atoms. In addition, inter- and intra-molecular hydrogen bonds are also discussed.

Tetra- and Dinuclear Nickel(II)−Vanadium(IV/V) Heterometal Complexes of a Phenol-Based N2O2Ligand: Synthesis, Structures, and Magnetic and Redox Properties

The tetra- and binuclear heterometallic complexes of nickel(II)-vanadium(IV/V) combinations involving a phenol-based primary ligand, viz., N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H2L1), are reported in this work. Carboxylates and beta-diketonates have been used as ancillary ligands to obtain the tetranuclear complexes [Ni(II)(2)V(V)(2)(RCOO)(2)(L(1))(2)O(4)] (R = Ph, 1; R = Me(3)C, 2) and the binuclear types [(beta-diket)Ni(II)L(1)V(IV)O(beta-diket)] (3 and 4), respectively. X-ray crystallography shows that the tetranuclear complexes are constructed about an unprecedented heterometallic eight-membered Ni(2)V(2)O(4) core in which the (L(1))(2)- ligands are bound to the Ni center in a N(2)O(2) mode and simultaneously bridge a V atom via the phenoxide O atoms. The cis-N(2)O(4) coordination geometry for Ni is completed by an O atom derived from the bridging carboxylate ligand and an oxo O atom. The latter two atoms, along with a terminal oxide group, complete the O5 square-pyramidal coordination geometry for V. Each of the dinuclear compounds, [(acac)Ni(II)L(1)V(IV)O(acac)] (3) and [(dbm)Ni(II)L(1)V(IV)O(dbm)] (4) [Hdbm = dibenzoylmethane], also features a tetradentate (L(1))(2)- ligand, Ni in an octahedral cis-N(2)O(4) coordination geometry, and V in an O(5) square-pyramidal geometry. In 3 and 4, the bridges between the Ni and V atoms are provided by the (L(1))(2)- ligand. The Ni...V separations in the structures lie in the narrow range of 2.9222(4) A (3) to 2.9637(5) A (4). The paramagnetic Ni centers (S = 1) in 1 and 2 are widely separated (Ni...Ni separations are 5.423 and 5.403 A) by the double V(V)O(4) bridge that leads to weak antiferromagnetic interactions (J = -3.6 and -3.9 cm-1) and thus an ST = 0 ground state for these systems. In 3 and 4, the interactions between paramagnetic centers (Ni(II) and V(IV)) are also antiferromagnetic (J = -8.9 and -10.0 cm-1), leading to an S(T) = 1/2 ground state. Compound 4 undergoes two one-electron redox processes at E(1/2) = +0.66 and -1.34 V vs Ag/AgCl reference due to a V(IV/V) oxidation and a Ni(II)/I reduction, respectively, as indicated by cyclic and differential pulse voltammetry.

Bis[μ-4-amino-3,5-bis(hydroxymethyl)-1,2,4-triazole]bis{bis[4-amino-3,5-bis(hydroxymethyl)-1,2,4-triazole]nickel(II)} tetranitrate methanol disolvate

The title complex, [Ni2(C4H8N4O2)6](NO3)4·2CH4O, contains a centrosymmetric binuclear nickel(II) complex bridged by a pair of 4-amino-3,5-bis­(hydroxy­meth­yl)-1,2,4-triazole ligands. The separation between the NiII atoms is 3.962 (1) Å. The Ni atoms are in a slightly distorted octa­hedral coordination. Inter­molecular N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonds connect the ligands, solvent mol­ecules and nitrate ions.

X-ray photoelectron spectra and structure of polynuclear nickel complexes

Mono- and binuclear nickel complexes of different stoichiometry have been studied by X-ray photoelectron spectroscopy (XPS). The Ni2p, Ni3p, and N1s X-ray photoelectron spectra have been examined, and the role of a ligand in their formation has been determined. As distinct from a low-spin Ni(II) complex, the Ni2p spectra of high-spin Ni(II) compounds show strong satellite lines. For high-spin Ni(II) complexes, which have unpaired 3d electrons, the Ni2p 1/2-Ni2p 3/2 spin-orbit splitting is larger than that for a low-spin Ni(II) compound. The presence or absence of the satellite structure has made it possible to classify these complexes with regard to their magnetic properties. The difference between the Ni2p 3/2 and N1s binding energies has made it possible to estimate the covalence of the metal-ligand bond. The XPS results are consistent with X-ray crystallography data.

Synthesis, crystal structure and magnetic properties of N -benzesulfonyl-L-glutamic acid-bridged binuclear nickel(II) complex

N-benzesulfonyl-L-glutamic acid-bridged binuclear nickel(II) complex, [Ni2(Bs-Glu)2(bipy)2 (H2O)2] · 3H2O (Bs-Glu = N-benzesulfonyl-L-glutamic acid dianion, bipy = 2,2′-bipyridine), has been synthesized and characterized structurally and magnetically. It crystallizes in the triclinic space group P 1. The α-carboxyl group coordinates to Ni(II) in a monodentate mode, while the γ-carboxyl group coordinates in a chelating mode. In the temperature range 2–300 K, magnetic measurements show that the exchange interaction of the two Ni(II) ions is antiferromagnetic, and the values for J, g are −2.47 cm−1 and 2.18, respectively.

Metal complexes derived from hydrazoneoxime ligands: III – Synthesis, characterization and electrospray ionization mass spectra of some nickel(II) complexes with aroylhydrazoneoximes

Five binuclear nickel(II) complexes [{Ni(L-R)} 2] derived from diacetylmonooxime aroylhydrazones (H 2L-R) have been prepared and characterized. In these complexes the oximate group functions as a bridge between the two nickel(II) ions. The corresponding octahedral bisligand complexes [Ni(HL-R) 2] have been also prepared and characterized. Reaction of [{Ni(L-R)} 2] with excess pyrazole (HPyr) afforded square planar [Ni(L-R)HPyr] complexes. The structure of [Ni(L-H)HPyr] ( 11), [Ni(L-CH 3O)HPyr] ( 13) and [Ni(L-Cl)HPyr] ( 14) have been determined by single-crystal X-ray diffraction. In these complexes, the Ni(II) is coordinated to the oximate nitrogen, hydrazone nitrogen and enolimine oxygen of the aroylhydrazoneoxime ligand, the fourth coordination site is occupied by pyrazole nitrogen. In 11 and 13 the molecular units are linked together by intramolecular NH⋯O hydrogen bonds forming a helical chain propagating along a and b axis respectively. The helical structure is stabilized by CH⋯O, CH⋯π and π⋯π noncovalent interactions. The structure of 14 consists of two independently crystallographic molecular units. The Ni1 molecular units are assembled together by CH⋯O and CH⋯Cl1 interactions forming a two dimensional sheet, while the Ni2 units are jointed together by CH⋯O and CH⋯C interactions forming a chain extending along b-axis which are enclosed between sheets of Ni1 units.

Synthesis, spectral, magnetic, electrochemical and kinetic studies of copper(II), nickel(II) and zinc(II) acetate complexes derived from phenol based ‘end-off’ ligands: Effect of p-substituents

A new class of symmetric, end-off, N-methyl piperazine armed binucleating ligands 2,6-bis(4-methyl piperazin-1-yl-methyl)-4-acetyl phenol (HL 1) and 2,6-bis[(4-methyl piperazin-1-yl-methyl)]-(4-methylcarboxy) phenol (HL 2) were synthesized by the Mannich reaction. Their mononuclear and binuclear Cu(II), Ni(II) and Zn(II) complexes have been synthesized. These complexes were characterized by elemental analysis, infra-red and electronic spectral analysis. In the electronic spectra, the lower electron withdrawing nature of the C(O)CH 3 p-substituent (HL 1) compared with the C(O)OCH 3 p-substituent (HL 2) of the phenolic ring causes a red shift in the LMCT-charge transfer band. The mononuclear Cu(II) complexes 1 and 7 have a magnetic moment value close to the spin only value with four hyperfine EPR signals. The binuclear Cu(II) complexes 4 and 10 illustrate an antiferromagnetic interaction ( μ eff 1.56 and 1.55 BM) at 298 K with a broad EPR signal. A variable temperature magnetic moment study of the binuclear copper(II) complexes shows that the extent of antiferromagnetic coupling increases in the order: CHO [K. Shanmuga Bharathi, A. Kalilur Rahiman, K. Rajesh, S. Sreedaran, P.G. Aravindan, D. Velmurugan, V. Narayanan, Polyhedron 25 (2006) 2859] < C(O)CH 3 < C(O)OCH 3 (−2 J values 134 [Shanmuga Bharathi et al., mentioned above], 149 and 158 cm −1, respectively). The mononuclear Ni(II) complexes 2 and 8 are square planar and diamagnetic. The six coordinated binuclear Ni(II) complexes 5 and 11 show a magnetic moment value of 2.96 and 2.95 BM, respectively. Electrochemical studies of the complexes reveal that all the mononuclear complexes show a single irreversible one-electron transfer reduction wave and the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an anodic shift in the reduction of the metal centres when the electron withdrawing nature of the p-substituent of the phenolic ring increases. The catecholase activity of the mono and binuclear copper(II) complexes, using pyrocatechol as a model substrate, and the hydrolysis of 4-nitrophenyl phosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts showed that the binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. A comparison of the spectral, electrochemical and magnetic behaviour of the complexes derived from the ligands is discussed on the basis of the substituent at the para position of the phenolic ring.

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

The syntheses, characterisation and complexation reactions of a series of binucleating Schiff-base calixpyrrole macrocycles are described. The acid-templated [2+2] condensations between meso-disubstituted diformyldipyrromethanes and o-phenylenediamines generate the Schiff-base pyrrolic macrocycles H(4)L(1) to H(4)L(6) upon basic workup. The single-crystal X-ray structures of both H(4)L(3).2 EtOH and H(4)L(6).H2O confirm that [2+2] cyclisation has occurred, with either EtOH or H2O hydrogen-bonded within the macrocyclic cleft. A series of complexation reactions generate the dipalladium [Pd2(L)] (L=L(1) to L(5)), dinickel [Ni2(L(1))] and dicopper [Cu2(L)] (L=L(1) to L(3)) complexes. All of these complexes have been structurally characterised in the solid state and are found to adopt wedged structures that are enforced by the rigidity of the aryl backbone to give a cleft reminiscent of the structures of Pacman porphyrins. The binuclear nickel complexes [Ni2(mu-OMe)2Cl2(HOMe)2(H(4)L(1))] and [Ni2(mu-OH)2Cl2(HOMe)(H(4)L(5))] have also been prepared, although in these cases the solid-state structures show that the macrocyclic ligand remains protonated at the pyrrolic nitrogen atoms, and the Ni(II) cations are therefore co-ordinated by the imine nitrogen atoms only to give an open conformation for the complex. The dicopper complex [Cu2(L(3))] was crystallised in the presence of pyridine to form the adduct [Cu2(py)(L(3))], in which, in the solid state, the pyridine ligand is bound within the binuclear molecular cleft. Reaction between H(4)L(1) and [Mn(thf){N(SiMe(3))2}2] results in clean formation of the dimanganese complex [Mn2(L(1))], which, upon crystallisation, formed the mixed-valent complex [Mn2(mu-OH)(L(1))] in which the hydroxo ligand bridges the metal centres within the molecular cleft.

Spectroscopic studies of bridged binuclear complexes of Co(II), Ni(II), Cu(II) and Zn(II)

Binuclear cobalt(II), nickel(II), copper(II) and zinc(II) complexes of general composition [M2L1-2(μ-Cl)Cl2] · nH2O with the Schiff-base ligands (where L1H and L2H are the potential pentadentate ligands derived by condensing 2,6-diformyl-4-methylphenol with 4-amino-3-antipyrine and 2-hydroxy-3-hydrazinoquinoxiline, respectively) have been synthesized and characterized. Analytical and spectral studies support the above formulation. 1H-NMR and IR spectra of the complexes suggest they have an endogenous phenoxide bridge, with chloride as the exogenous bridge atom. The electronic spectra of all the complexes are well characterized by broad d–d and a high intensity charge-transfer transitions. The complexes are chloro-bridged as evidenced by two intense far-IR bands centered around 270–280 cm−1. Magnetic susceptibility measurements show that complexes are antiferromagnetic in nature. The compounds show significant growth inhibitory activity against fungi Aspergillus niger and Candida albicans and moderate activity against bacteria Bacillus cirroflagellosus and Pseudomonas auresenosa.

Electrochemistry and catalytic behavior of immobilized binuclear complexes of copper(II) and nickel(II) with Robson type ligand

Various systems containing immobilized binuclear copper(II) and nickel(II) complexes with Robson type ligand ([M2L]Cl2) are studied and compared in relation to catalysis of hydrogen peroxide reduction. Molecular complexes adsorbed on mercury and gold, crystalline complexes immobilized in the carbon paste electrode, and complex species entrapped into polyphenol-modified gold electrodes are considered. Electrocatalysis is assumed to result from the formation of H2O2–[M2L]Cl2 adduct, not from mediating redox transformations. Possible geometry of the formed reaction layers supporting adduct formation is discussed.

Mechanochemical Control of Synthesis and Structures of Aqua‐Bridged Binuclear Nickel(II) Benzoate Complexes

AbstractSynthetic and structural aspects of a few binuclear nickel(II) complexes, having two bridging carboxylate groups and aqua groups are reported. Intramolecular O–H···O hydrogen bonding interactions between the bridging aqua group and the monodentate benzoate groups stabilises the bridging structures. Aqua‐bridged nickel(II) benzoate complex is stable irrespective of solvent system and also can include solvent molecules such as toluene and benzene. A mixed chloro, aqua and 4‐chloro‐benzoato‐bridged NiII binuclear complex was synthesised and structurally characterised. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Bis[μ-2-(2,4-difluorophenyl)-1,3-bis(1,2,4-triazol-1-yl)propan-2-olato]-κ8N2,O:O,N2′-bis[(acetato-κ2O,O′)nickel(II)

In the title centrosymmetric binuclear nickel(II) complex, [Ni2(C13H11F2N6O)2(C2H3O2)2], the two Ni atoms, each attached to an acetate ligand, are linked by two fluconazole mol­ecules. Each Ni atom is six-coordinated by two N atoms from the triazole groups and two bridging O atoms from the deprotonated hydroxyl groups of two different fluconazole ligands and two O atoms from the bidentate acetic anion, exhibiting a distorted octa­hedral geometry.

Synthesis and characterization of novel unsymmetrical macrobicyclic tricompartmental mono and binuclear nickel(II) complexes: Electrochemical and kinetic studies of phosphate hydrolysis

A series of macrobicyclic mono and binuclear nickel(II) complexes of type [NiL](ClO 4) and [Ni 2L](ClO 4) 2, where L is macroyclic ligand derived from the precursor compound 3,4:10,11-dibenzo-1,13[ N, N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,9-dioxocyclopentadecane, have been synthesized in order to examine electrochemical and catalytic studies on the basis of macrocyclic ring size. The macrocycle consists of three dissimilar compartments arising from ether oxygen, tertiary nitrogen and imine nitrogen atoms. Electrochemical studies have shown that the mononuclear nickel(II) complexes undergo quasireversible single step one electron reduction and oxidation and binuclear nickel(II) complexes undergo two quasireversible one electron reduction and oxidation. The EPR silent nature is ascribed to Ni(II) state and all the nickel(II) complexes have square planar geometry and are diamagnetic in nature. The complexes were subjected to hydrolysis of 4-nitrophenyl phosphate and the catalytic activities of the complexes are found to increase with macrocyclic ring size of the complexes. As the macrocyclic ring size of the complexes increases, the spectral, electrochemical and catalytic studies of the complexes show remarkable variation due to distortion in the geometry around the nickel(II) centre.

Synthesis of new ‘end-off’ μ-phenoxo and bis-μ-acetato tri-bridged copper(II), nickel(II) and zinc(II) complexes: Spectral, magnetic, electrochemical and catalytic studies

A new end-off type acyclic ligand with two N-methyl piperazine arms, 2,6-bis[(4-methyl piperazin-1-yl)]-4-formyl phenol [L], has been synthesized by a simple Mannich reaction. The mono and binuclear complexes of Cu(II), Ni(II) and Zn(II) have been prepared. The complexes were characterized by elemental and spectral analysis. The EPR spectrum of the mononuclear copper complex shows four hyperfine splittings and the binuclear complex shows a broad signal due to an anti-ferromagnetic interaction. The room temperature magnetic moment of the mono and binuclear copper complexes are found to be 1.72 μ B ( μ eff) and 1.58 μ B ( μ eff), respectively. The mononuclear Ni(II) complex is square planar and diamagnetic, whereas the six-coordinated binuclear Ni(II) complex shows a magnetic moment value of 2.98 μ B ( μ eff). Cyclic voltammetric studies evidenced one irreversible reduction wave for all the mononuclear complexes and two irreversible one-electron reduction waves for the binuclear complexes are obtained in the cathodic region. In the anodic region, one irreversible oxidation wave for the mononuclear nickel(II) complex and two irreversible oxidation waves for the binuclear nickel(II) complexes are obtained. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the mono and binuclear copper(II) complexes as catalysts and on the hydrolysis of 4-nitrophenylphosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts were carried out. The binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. The rate constant values for the complexes for the hydrolysis are in the order nickel(II) > copper(II) > zinc(II).