The solution state of nickel(II) and nickel(I) in the presence of diphosphines in acetonitrile. A combined electroanalytical and spectrophotometric approach

The equilibria involving nickel(II), nickel(I) and nickel(0) in the presence of bidentate phosphine ligands in acetonitrile solution have been investigated by cyclic voltammetry, controlled-potential electrolysis, and spectrophotometry. The obtained results show that when both cone angles and PP distance make the diphosphine well-suited to act as a chelating ligand, only bis-chelate complexes of the type NiL₂ are formed by nickel in all the three achieved oxidation states. Longer methylene chains interposed between the phosphorus atoms make the diphosphine less suited to act in a bidentate fashion, thus allowing lower-order complexes, e.g. NiLS₂ (S = solvent), to be obtained for nickel in the +2 and +1 oxidation states. Evidence for the poor stability of the [Ni^ILS₂]⁺ complexes and for a higher-order complex of the type NiL₃ for nickel(II) have also been obtained. The dependence of the reduction potentials on the nature of the diphosphine employed is discussed.     

Stabilization of monovalent nickel in aqueous solutions by complexation with the β-isomer of C-5,12-racemic-1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11-tetraazacyclotetradecane

The monovalent nickel complex formed by the reduction of the β-isomer of the complex of C-5,12- racemic-1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11- tetraazacyclotetradecane nickel(II), NiL₁²⁺ in 0.1 M HCO₂Na, pH 7.6, has a half-life longer than 90 h. The redox potential of the couple NiL₁⁺/NiL₁²⁺ is −0.94 V vs. Ag/AgCl. The absorption spectrum of NiL₁⁺ consists of a band with λ_max = 335 nm and ϵ_max = 2200 M⁻¹ cm⁻¹. For the analogous complex with C-5,12-racemic-5,7,7,12,14,14-hexamethyl-1,4, 8,11-tetraazacyclotetradecane, L₂, the half-life time of NIL₂⁺ is less than 1 min and the redox potential is −1.44 V vs. Ag/AgCl. These results are similar to those reported earlier for the analogous nickel complexes with the meso-isomers of the ligands. The results thus indicate that both the kinetic and thermodynamic stabilization of monovalent nickel by N-methylation of tetraazamacrocyclic ligands is not significantly affected by the configuration of the ligand.     

Solution properties of the nickel(II,III) and copper(II,III) complexes of trans-dioxocyclam (trans-dioxocyclam=5,12-dioxo-1,4,8,11-tetraazacyclotetradecane) and the X-ray crystal structure of the N-rac-isomer of the nickel(II) complex

The crystal and molecular structures of the N-rac-isomer of the nickel(II) complex of 14-membered amide-containing macrocycle [NiL¹] · 4H₂O (H₂L¹=5,12-dioxo-1,4,8,11-tetraazacyclotetradecane) have been determined. Two deprotonated amide and two amine donors co-ordinate to the nickel(II) in nearly square planar manner with Ni-N_amine bonds longer than Ni-N_amide ones (1.930 vs. 1.898 Å). Water molecules do not co-ordinate and form hydrogen bond bridges between macrocyclic units in the crystal lattice. The analysis of ¹H NMR data confirmed that the solid-state conformation of the macrocycle in N-rac[NiL¹] is retained in aqueous solution though equilibrated with some amount of N-meso isomer. The comparison of the spectroscopic characteristics of the M(II) and M(III) complexes and the redox potentials of M(III/II) couples (M=Ni and Cu) for ML¹ with those for ML²(H₂L²=5,7-dioxo-1,4,8,11-tetraazacyclotetradecane) revealed a rather small influence of the trans- vs. cis-arrangement of amide donors in co-ordination spheres of the metal ions.     

Synthesis, characterization and X-ray crystal structures of copper(II) and nickel(II) complexes with potentially hexadentate Schiff base ligands

Copper(II) and nickel(II) complexes of potentially N₂O₄ Schiff base ligands 2-({[2-(2-{2-[(1-{2-hydroxy-5-[2-phenyl-1-diazenyl]phenyl}methylidene)amino] phenoxy}ethoxy) phenyl]imino}methyl)4-[2-phenyl-1-diazenyl]phenol (H₂L¹) and 2-({[2-(4-{2-[(1-{2-hydroxy-5-[2-phenyl-1-diazenyl]phenyl}methylidene)amino] phenoxy}butoxy) phenyl]imino}methyl)4-[2-phenyl-1-diazenyl]phenol (H₂L²) prepared of 5-phenylazo salicylaldehyde (1) and two various diamines 2-[2-(2-aminophenoxy)ethoxy]aniline (2) and 2-[4-(2-aminophenoxy)butoxy]aniline (3) were synthesized and characterized by a variety of physico-chemical techniques. The single-crystal X-ray diffractions are reported for CuL¹ and NiL². The CuL¹ complex contains copper(II) in a near square-planar environment of N₂O₂ donors. The NiL² complex contains nickel(II) in a distorted octahedral geometry coordination of N₂O₄ donors. In all complexes, H₂L¹ behaves as a tetradentate and H₂L² acts as a hexadentate ligand. Cyclic voltammetry of copper(II) complexes indicate a quasi-reversible redox wave in the negative potential range.     

A trigonal-prismatic Ag(I) complex and an octahedral Cu(II) complex incorporating with oxamide Ni(II) complex ligand: Syntheses, crystal structures and oxidative nuclease activities

A mononuclear macrocyclic complex Ni^IIL^3a (L^3a = dianion of 2,3-dioxo-5,6:13,14-dibenzo-9,10-cyclohexyl-7,12-bis(methoxycarbonyl)-1,4,8,11-tetraazacyclotetradeca-7,11-diene), which shows high DNA cleavage activity in the presence of H₂O₂, was reported in our previous work. Considering that many systems for natural enzyme-mediated DNA cleavage contain two or more metal active sites, two new trinuclear complexes [Cu(NiL^3a)₂(dca)₂]·2CH₃OH (abbreviated as Cu(NiL^3a)₂) and [Ag(NiL^3a)₂(NO₃)]·2CH₃OH·0.5H₂O (abbreviated as Ag(NiL^3a)₂) were synthesized in this work, where dca is the dicyanamide. The complexes were structurally characterized by single crystal X-ray analysis. The central Cu(II) or Ag(I) atom is linked to two [NiL^3a] ligands by oxamido bridges forming a trinuclear structure. In Cu(NiL^3a)₂, the central Cu(II) ion is in an octahedral coordination geometry. Whereas in Ag(NiL^3a)₂, the central Ag(I) ion is in a rarely reported trigonal-prismatic coordination geometry. The DNA cleavage behavior of the complexes in the presence of H₂O₂ was studied in detail. Comparing with the NiL^3a, the trinuclear complex Ag(NiL^3a)₂ nearly has no ability to cleave DNA, whereas Cu(NiL^3a)₂ is a much better DNA cleavage agent. Cu(NiL^3a)₂ can efficiently convert supercoiled DNA to nicked DNA with a rate constant of 0.074 ± 0.002 min⁻¹ when 40 μM Cu(NiL^3a)₂ and 0.6 mM H₂O₂ are used. The cleavage mechanism between the complex Cu(NiL^3a)₂ and plasmid DNA is likely to involve singlet oxygen as reactive oxygen species. Circular dichroism (CD) and fluorescence spectroscopy indicate that both Cu(NiL^3a)₂ and NiL^3a bind to DNA by a groove binding mode, and the binding between Cu(NiL^3a)₂ and DNA is much stronger than that between NiL^3a and DNA. The present results may provide some information for the design of efficient multinuclear artificial nucleases.     

Preparation of new tetraaza macrocyclic nickel(II) and copper(II) complexes bearing N-propionic methyl ester groups

A 14-membered tetraaza macrocycle, 2,13-bis(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.^1.180^7.12]docosane (L²) bearing two N-CH₂CH₂COOMe groups, and its nickel(II) and copper(II) complexes have been prepared and characterized. The nickel(II) and copper(II) complexes of 2-(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.^1.180^7.12]docosane (L³) containing one N-CH₂CH₂COOMe group have also been prepared. The crystal structure of [NiL²](ClO₄)₂ shows that the complex has a slightly distorted trans-octahedral coordination geometry with two relatively short axial Ni-O (N-CH₂CH₂COOMe group) bonds (2.136(3) Å). In various solvents, however, a considerable proportion of [NiL²]²⁺ exists as a square-planar form, in which the functional pendant arms are not involved in coordination. The proportion of the square-planar isomer varies with solvents in the order of nitromethane ? acetonitrile < H₂O < DMF ? DMSO. In the case of [CuL²](ClO₄)₂, only one N-CH₂CH₂COOMe group is involved in coordination. The N-CH₂CH₂COOMe group of [NiL³](ClO₄)₂ is not directly involved in coordination even in the solid state, though the functional group of [CuL³](ClO₄)₂ is coordinated to the metal ion.     

Spectroscopic and Structural Characterization,Enzyme Inhibitions,and Antioxidant Effects of New Ru(II) and Ni(II) Complexes of Schiff Base

The new complex compounds [RuLCl(p‐cymene)] ? 3H₂O and [NiL₂(H₂O)₂] ? 3H₂O (L: 1‐{4‐[(2‐hydroxy‐3‐methoxybenzylidene)amino]phenyl}ethanone) were prepared and characterized using FT‐IR, ¹H‐ and ¹³C‐NMR, mass spectroscopy, TGA, elemental analysis, X‐ray powder diffraction and magnetic moment techniques. Octahedral geometry for new Ni(II) and Ru(II) complexes was proposed. Thermal decomposition confirmed the existence of lattice and coordinated water molecule in the complexes. To determine the antioxidant properties of Schiff base ligand and its Ni(II), Ru(II) metal complexes, FRAP, CUPRAC, ABTS and DPPH methods of antioxidant assays were used. Moreover, enzyme inhibition of complexes was evaluated against carbonic anhydrase I and II isoenzymes (CA I and CA II) and acetylcholinesterase (AChE). For CA I and CA II, the best inhibition enzymes, was the Ni(II) complex with 62.98±18.41, 86.17±23.62 Ki values, whereas this inhibition effect showed ligand with 24.53±2.66 Ki value for the AChE enzyme.     

Synthesis and characterization of transition metal fluoride complexes with imidazole: X-ray crystal structure reveals short hydrogen bonds between lattice water and lattice fluoride

A new series of complexes of cobalt(II) fluoride, nickel(II) fluoride, copper(II) fluoride and zinc(II) fluoride with imidazole were synthesized and characterized by elemental analysis, molar conductance, magnetic moments, IR and electronic absorption measurements. Based on elemental and spectral data, the complexes were found to be of [M(im)₆]F₂ · XH₂O type, where M is Co(II), Ni(II), Cu(II) and Zn(II) and X 4-5. The magnetic moments and spectral data suggested that all the complexes possessed an octahedral geometry. The crystal structure of the nickel complex, [Ni(im)₆]F₂ · 5H₂O, is also reported in which nickel atom is surrounded by six nitrogen atoms of imidazole. Strong intra- and inter-molecular hydrogen bonding exists between fluoride ions (uncoordinated), nitrogen of imidazole and the -OH of water molecules.     

Mesogenic and magnetic properties of nickel(II) complexes formed from 2′-(4-alkyloxyphenyl)malondialdehydes

The synthesis, characterization, thermal behavior and magnetic properties of a number of bis[2-(4-alkyloxyphenyl)malondialdehyde] nickel(II) complexes are reported. X-ray studies of the nickel(II) complexes were performed. It was found that the nickel(II) complexes show liquid-crystalline smectic A phases over a broad temperature range with low melting points. The temperature-dependent magnetic susceptibility measurements of the bis[2-(4-decyloxyphenyl)malondialdehyde] nickel(II) complex were carried out in the range of 4.2–480 K. The temperature dependent magnetic susceptibilities and magnetic moments of this compound (_eff=3.27 _B at 300 K) indicate that the nickel centers are octahedrally coordinated. Models for the molecular arrangement in the crystalline and liquid-crystalline phases are discussed on the basis of the magnetic data. In spite of the oxygen bridge between the nickel centers, no exchange interactions were found in the crystalline and liquid-crystalline phases.     

New acyclic and cyclie Schiff bases derived from 2,6-diformyl-4-chlorophenol and their interaction with uranyl(Vl), copper(II) and nickel(Il) ions

A new heptadentate compartmental ligand has been synthesized by condensation of 3-formylsalicylic acid and 1,5-diamino-3-thiapentane in methanol (H₄L_a). This Schiff base contains an inner N₂SO₂ and an outer O₂O₂ site and gives, by reaction with copper(II), nickel(II) and uranyl(VI) diacetate, mononuclear, homo- and heterobinuclear complexes. In the mononuclear copper and nickel complexes, the metal ion is in the inner N₂SO₂ site, while it is in the outer O₂O₂ for uranyl; a solvent molecule fills the fifth equatorial coordination position in this last complex. The physico-chemical properties of the compounds are discusscd on the basis of infrared, electronic and magnetic data and by comparison with the analogous complexes with the ligand obtained by reaction of 3- formylsalicylic acid and diethylenetriamine (H₄L_b). The mononuclear copper and the heterodinuclear copper-uranyl complexes show anomalously low magnetic moments.     

Uranyl(VI), copper(II) and nickel(II) complexes derived from a new compartmental ligand

A new heptadentate compartmental ligand has been synthesized by condensation of 3-formylsalicylic acid and 1,5-diamino-3-thiapentane in methanol (H₄L_a). This Schiff base contains an inner N₂SO₂ and an outer O₂O₂ site and gives, by reaction with copper(II), nickel(II) and uranyl(VI) diacetate, mononuclear, homo- and heterobinuclear complexes. In the mononuclear copper and nickel complexes, the metal ion is in the inner N₂SO₂ site, while it is in the outer O₂O₂ for uranyl; a solvent molecule fills the fifth equatorial coordination position in this last complex. The physico-chemical properties of the compounds are discusscd on the basis of infrared, electronic and magnetic data and by comparison with the analogous complexes with the ligand obtained by reaction of 3- formylsalicylic acid and diethylenetriamine (H₄L_b). The mononuclear copper and the heterodinuclear copper-uranyl complexes show anomalously low magnetic moments.     

Syntheses, characterization and X-ray crystal structures of a mono- and a penta-nuclear nickel(II) complex with oximato Schiff base ligands

A mononuclear octahedral nickel(II) complex [Ni(HL¹)₂](SCN)₂ (1) and an unusual penta-nuclear complex [{(NiL²)(μ-SCN)}₄Ni(NCS)₂]·2CH₃CN (2) where HL¹ = 3-(2-aminoethylimino)butan-2-one oxime and HL² = 3-(hydroxyimino)butan-2-ylidene)amino)propylimino)butan-2-one oxime have been prepared and characterized by X-ray crystallography. The mono-condensed ligand, HL¹, was prepared by the 1:1 condensation of the 1,2-diaminoethane with diacetylmonoxime in methanol under high dilution. Complex 1 is found to be a mer isomer and the amine hydrogen atoms are involved in extensive hydrogen bonding with the thiocyanate anions. The dicondensed ligand, HL², was prepared by the 1:2 condensation of the 1,3-diaminopropane with diacetylmonoxime in methanol. The central nickel(II) in 2 is coordinated by six nitrogen atoms of six thiocyanate groups, four of which utilize their sulphur atoms to connect four NiL² moieties to form a penta-nuclear complex and it is unique in the sense that this is the first thiocyanato bridged penta-nuclear nickel(II) compound with Schiff base ligands.     

Coordination chemistry of the metalloligand [Pt2(μ-S)2(PPh3)4] with nickel(II) complexes - an electrospray mass spectrometry directed synthetic study

The reactivity of [Pt₂(μ-S)₂(PPh₃)₄] towards a range of nickel(II) complexes has been probed using electrospray ionisation mass spectrometry coupled with synthesis and characterisation in selected systems. Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with [Ni(NCS)₂(PPh₃)₂] gives [Pt₂(μ-S)₂(PPh₃)₄Ni(NCS)(PPh₃)]⁺, isolated as its BPh₄ ⁻ salt; the same product is obtained in the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with [NiBr₂(PPh₃)₂] and KNCS. An X-ray structure determination reveals the expected sulfide-bridged structure, with an N-bonded thiocyanate ligand and a square-planar coordination geometry about nickel. A range of nickel(II) complexes NiL₂, containing β-diketonate, 8-hydroxyquinolinate, or salicylaldehyde oximate ligands react similarly, giving [Pt₂(μ-S)₂(PPh₃)₄NiL]⁺ cations.     

A study on mono-, bis- and tris-N-functionalized 1,4,7-triazacyclononane with benzimidazolyl groups and their nickel(II) complexes

Three new ligands, 1-(benzimidazolyl-2-methyl)-1,4,7-triazacyclononane L¹, 1,4-bis(benzimidazolyl-2-methyl)-1,4,7-triazacyclonone L², and 1,4,7-tris-(benzimidazolyl-2-methyl)-1,4,7-triazacyclonane L³, were synthesized by a straightforward one-pot method. Their nickel(II) complexes , [NiL²CH₃CN](ClO₄)₂ · 2CH₃OH · H₂O (or [NiL²Cl] · ClO₄) and [Ni(H₋₂L³)] · H₂O were obtained and characterized by electrospray mass spectrum, ¹H NMR, CV and other physical methods. Their crystal structures were determined by X-ray analyses. The crystal structure of the nickel(II) complex of L¹ shows that two Ni(II) atoms are bridged by two Cl⁻ anions. A ferromagnetic exchange coupling and zero-field splitting effect exist in complex 1.     

Synthesis of copper(II) and nickel(II) complexes of N-(hydroxyalkylaminoalkyl)salicylamides

Copper(II) and nickel(II) complexes are prepared of potentially quadridentate ligands (LH₃), N-{2-(2- hydroxyethylamino)ethyl}- (seeH₃), N-{3-(2-hydroxy- ethylamino)propyl}- (steH₃), and N-{2-(3-hydroxypropylamino)ethyl}-salicylamide (setH₃). The nickel complexes Na[Ni(see)] and Na[Ni(set)]·1/2H₂O are diamagnetic and square-planar, in which the ligands act as a quadridentate one coordinated through secondary amino-N, and deprotonated phenolic-O, alcoholic-O, and amido-N atoms. The three copper complexes Na [CuL]·H₂O (L = see, ste, set) with a normal magnetic moment have a similar square-planar structure. In another type complexes Cu(LH)·H₂O (LH = seeH, setH) an alcohol group is not deprotonated. Two isomers are present in Cu(seeH)·H₂O: one has a normal and the other a subnormal magnetic moment. The difficulty of complex formation of steH₃ may be attributed to an unfavourably fused 6-6-5 membered chelate ring with strain.     

Metal complexes with schiff bases obtained from salicylaldehyde derivatives and 2-aminoethylpyridine

Cobalt(II), cobalt(III), nickel(II), copper(II) and palladium(II) complexes with N-2-(2-pyridyl)ethylring-substituted salicylideneiminates (abbreviated as X-Sal-2-Epy) were synthesized. In addition to Co^III (H-Sal-2-Epy)₃, the complexes of the formula M^II(X-Sal-2-Epy)₂·nH₂O were obtained in crystals. The cobalt(III) complex is diamagnetic and has an electronic absorption spectrum typical of the six-coordinate, octahedral cobalt(III) complex. The cobalt(II) complexes in the solid state show electronic spectra typical of the six-coordinate cobalt(II) complexes. Electronic spectra also indicate that the nickel(II) complexes in the solid state and in non-donor solvents are six-coordinate, octahedral. In the cobalt(II) and nickel(II) complexes, the ligand X-Sal-2-Epy functions as terdentates, while in the cobalt(III) complex it acts as a bidentate ligand. The results are compared with those reported previously for related ligands.     

Nickel(II) and copper(II) complexes of Schiff base ligands containing N4O2 and N4S2 donors with pyrrole terminal binding groups: Synthesis, characterization, X-ray structures, DFT and electrochemical studies

A series of hexadentate ligands, H₂L^m (m = 1−4), [1H-pyrrol-2-ylmethylene]{2-[2-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy)ethoxy]phenyl}amine (H₂L¹), [1H-pyrrol-2-ylmethylene]{2-[4-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy)butoxy]phenyl}amine (H₂L²), [1H-pyrrol-2-ylmethylene][2-({2-[(2-{[1H-pyrrol-2-ylmethylene]amino}phenyl)thio]ethyl}thio)phenyl]amine (H₂L³) and [1H-pyrrol-2-ylmethylene][2-({4-[(2-{[1H-pyrrol-2-lmethylene]amino}phenyl)thio]butyl}thio) phenyl]amine (H₂L⁴) were prepared by condensation reaction of pyrrol-2-carboxaldehyde with {2-[2-(2-aminophenoxy)ethoxy]phenyl}amine, {2-[4-(2-aminophenoxy)butoxy]phenyl}amine, [2-({2-[(2-aminophenyl)thio]ethyl}thio)phenyl]amine and [2-({4-[(2-aminophenyl)thio]butyl}thio)phenyl]amine respectively. Reaction of these ligands with nickel(II) and copper(II) acetate gave complexes of the form ML^m (m = 1−4), and the synthesized ligands and their complexes have been characterized by a variety of physico-chemical techniques. The solid and solution states investigations show that the complexes are neutral. The molecular structures of NiL³ and CuL², which have been determined by single crystal X-ray diffraction, indicate that the NiL³ complex has a distorted octahedral coordination environment around the metal while the CuL² complex has a seesaw coordination geometry. DFT calculations were used to analyse the electronic structure and simulation of the electronic absorption spectrum of the CuL² complex using TDDFT gives results that are consistent with the measured spectroscopic behavior of the complex. Cyclic voltammetry indicates that all copper complexes are electrochemically inactive but the nickel complexes with softer thioethers are more easily oxidized than their oxygen analogs.     

Synthesis and characterization of 5,5,7,12,12,14-hexamethyl- 1,4,8,11-tetraazacyclotetradecane-N′,N‴-diacetic acid and its transition metal complexes; crystal structure of the nickel(II) monohydrogen bromide monohydrate complex of this macrocycle

The synthesis and characterization of a new 14- membered tetraazamacrocylic ligand 5,5,7,12,12,14- hexamethyl-1,4,8,11-tetraazacyclotetradecane-N′,N‴- diacetic acid (H₂L²) are reported. Cobalt(III),nickel(II) and copper(II) complexes with this ligand were prepared and their IR and visible spectra studied.The molecular structure of the complex NiL²· HBr·H₂O was proved by single-crystal X-ray analysis. The compound crystallizes in the space group P2₁/n, with cell parameters a = 16.7109(13), b = 9.0539(9), c = 17.0277(13) Å, β = 107.46(6)° and Z = 4. The structure was solved by direct and Fourier methods and refined by full matrix least square calculation to R = 0.042 for 4355 observed reflections (I > 3σ(I)). The complex shows a cis-octahedral geometry with the macrocycle coordinated in a folded configuration to four sites around the central nickel atom. The two carboxylate groups of the side chain are on the same side of the approximate plane of the macrocycle.     

New template reactions of salicylaldehyde S-methyl-isothiosemicarbazone with 2-formylpyridine promoted by Ni(II) or Cu(II) metal ions

The template reaction between salicylaldehyde S-methyl-isothiosemicarbazone and 2-formylpyridine in presence of nickel(II) or copper(II) salts yields two new coordination compounds with general formula [NiL¹]₂(1) and [CuL²]₂(2) (L¹ = the dianionic (N¹-salicylidene)(N⁴-(hydroxy(pyridin-2-yl)methyl) S-methyl-isothiosemicarbazide) ligand and L² = the doubly deprotonated (N¹-salicylidene)(N⁴-(picolinoyl) S-methyl-isothiosemicarbazide) ligand). In the complex 1, the formed L¹ ligand appears as result of an addition reaction of the precursors, while for 2 a redox mechanism is implicated in the formation of L². Despite the fact that the initial organic precursors are the same, the resulting ligands obtained in the template reaction are different. In 1, the Ni(II) metal ion adopts a square-planar geometry and the [NiL¹] units are forming dimerized chains through weak Ni···Ni interactions (3.336 and 3.632 Å). In 2, the Cu(II) metal ions adopt a square-pyramidal geometry and form dinuclear species through weak Cu···O (phenoxo) interactions. The magnetic susceptibility measurements of the complexes reveal the diamagnetic nature of 1 as expected for a square planar Ni(II) complex and a paramagnetic behavior for 2 with weak intra-dimer antiferromagnetic interaction (J/k_B = −2.1(1) K).     

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](ClO₄) and [Ni₂L](ClO₄)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.