Kinetics and mechanism of formation of square-planar copper(II) and nickel(II) complexes of ethylenebisbiguanide in aqueous acetate buffer media

Summary The kinetics of formation of square-planar Cu^II and Ni^II complexes of the quadridentate ligand, ethylenebisbiguanide, have been studied spectrophotometrically in aqueous HOAc–NaOAc buffer, at ionic strength 0.2 mol dm^–3, in the 25–35°C temperature range. The observed rate constants for the formation reactions are independent of pH (and of OAc^– concentration) in the pH range used (3.6–4.8 for Cu^II and 5.0–5.8 for Ni^II) where the product complexes form stoichiometrically, but show first-order dependence on the ligand concentration;i.e. k_obs=k_f[L]_total. At 25°C k_f values (dm³ mol^–1s^–1) are 35.2±0.4 for Cu^II and (8.4±0.1)×10^–3 for Ni^II. The mechanism of the reactions is discussed.     

Encapsulation of nickel(II) and copper(II) by a tetraazamacrocyclic ligand

A tetraazamacrocyclic ligand, L, containing six non-equivalent benzene rings, derived from the condensation of benzil with 1,2- diaminobenzene, has been isolated and its complexes [MLCl₂] (M = Ni²⁺ and Cu²⁺) prepared and characterized by elemental analysis, i.r., u.v.–vis., e.p.r. spectral studies, magnetic moments, redox potentials and conductivity measurements. The complexes have axially elongated octahedral geometries with two axial chlorines, and adopt the trans-configuration. These studies also indicate the covalent nature and the high-spin octahedral structure for these complexes. A cyclic voltammetric investigation reveals that the complexes exhibit a single one-electron redox couple, as anticipated for a copper(II) complex (Cu²⁺/Cu⁺) and a single two-electron redox couple for a nickel(II) complex (Ni²⁺/Ni⁰). The electrochemical processes are considered quasi-reversible. Antimicrobial activities of the ligand and the complexes have been tested against Bacillus megaterium and Candida tropicallis.     

Kinetics of the destruction of cyclic complexes of nickel(III) and their participation in oscillating chemical processes

A spectrophotometric method has been used to study in sulfuric acid solutions the chelate reduction of nickel(III) compounds Ni^IIIL, where L are l4-membered tetraazamacrocyclic ligands with a different number of substitutents (from 0 to 0) and double bonds (from U to 4). Based on the comparison of the kinetic characteristics of the redox reactions of the nickel complexes with data on the participation of such compounds in oscillating chemical reactions in the systems NiLBrO^– ₃-H₂SO₄ it was concluded that a necessary condition for the creation of OCR in this case is, besides the autocatalytic regime of the oxidation of the [NiL]²⁺ complex with bromate, the ability of nickel(III) to undergo rapid reduction (k > 10^–4 sec^–1) by the oxidation of the coordinated macrocyclic ligand.L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of Ukrainian SSR, Kiev. Translated from Teoretichesakaya i ÉksperimentaI'naya Khimiya, Vol. 27, No. l pp. 51–55, January–February, 1991. Original article submitted May 29, 1990.     

Synthesis,characterization and electrochemical behavior of mononuclear,binuclear nickel(II) and heterobinuclear zinc(II)nickel(II) complexes derived from novel macrobicyclic tricompartmental ligands: kinetics of the catalytic hydrolysis of 4-nitrophenyl phosphate by the complexes

Mononuclear, binuclear Ni^II and heterobinuclear Zn^IINi^II complexes have been derived from lateral macrobicyclic tricompartmental ligands embracing three different donor sets: (i) O₂N₂-donor set, derived from ether oxygens and tertiary amine nitrogens; (ii) N₂O₂-donor set, derived from tertiary amine nitrogens and phenolic oxygens; (iii) O₂N₂-donor set, derived from phenolic oxygens and azomethine nitrogens. Cyclic voltammograms of the mononuclear Ni^II complexes showed irreversible one-electron reduction processes in the –1.2 to –1.3 V region and an irreversible oxidation process in the range +0.8 V potential region. The binuclear complexes showed quasireversible two-step single electron reduction processes around the –1.3 and –1.7 V potential regions. The anodic potential region showed an irreversible oxidation process at +1.0 V. The heterobinuclear Zn^IINi^II complex showed an irreversible reduction of the Ni^II species at –1.55 V. The catalytic hydrolysis towards 4-nitrophenyl phosphate by the mononuclear, binuclear Ni^II complexes and the heterobinuclear complex were found to be appreciable. The pseudo-first order rate constant for the catalytic hydrolysis catalyzed by the binuclear and heterobinuclear complexes were found to be higher (9.8 × 10^–4 s^–1) than that of the corresponding mononuclear complexes (1.3 × 10^–5 s^–1), which ascertain the requirement of two metal ions in close proximity for the binding of the nucleophilic OH and the phosphate group.     

Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media

Summary Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML²⁺ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (k_obs) showing acid dependence; for the nickel(II) complex (k_obs)=k_O+k_H[H⁺], the k_o path is however absent with the copper(II) complex. At 60°C (I=0.1M) the k_H values areca 10^–4 M^–1 s^–1 for both complexes; k _H ^Cu /k _H ^Ni =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to H values [copper(II) complex, 29.4±0.5 kJ mol^–1; nickel(II) complex, 35.6±1.5 kJ mol^–1] with highly negative S values [for copper(II), –215.5 ±6.1 JK^–1 mol^–1 and for nickel(II), –208.1 ±5.6 JK^–1 mol^–1] which are much higher than the entropy of solvation of Ni²⁺ (ca. –160 JK^–1 mol^–1) and Cu²⁺ (ca. –99 JK^–1 mol^–1) ions; significant solvation of the released metal ions and the ligand is indicated.     

Chelated complexes of cadmium(II), cobalt(II), copper(II), mercury(II), nickel(II), uranyl(II) and zinc(II) with benzil bis(4-phenylthiosemicarbazone)

Summary Complexes of Co^II, Ni^II, Cu^II, Zn^II, Cd^II, Hg^II and UO ₂ ^II with benzil bis(4-phenylthiosemicarbazone), H₂BPT, have been synthesized and their structures assigned based on elemental analysis, molar conductivity, magnetic susceptibility and spectroscopic measurements. The i.r. spectra suggest that the ligand behaves as a binegative quadridentate (NSSN) (Co^II, Cu^II, Hg^II and UO ₂ ^II complexes) or as a binegative quadridentate-neutral bidentate chelating agent (Ni^II, Zn^II and Cd^II complexes). Octahedral structures for the Co^II and Ni^II complexes and square-planar structure for the Cu^II complex are suggested on the basis of magnetic and spectral evidence. The crystal field parameters (Dq, B and _B) for the Co^II complex are calculated and agree fairly well with the values reported for known octahedral complexes. The ligand can be used for the microdetermination of Ni^II ions of concentration in the 0.4–6×10^–4 mol l^–1 range and the apparent formation constant for the species generated in solution has also been calculated.     

Ternary complexes of copper(II), nickel(II) and zinc(II) with nitrilotriacetic acid as a primary ligand and some phenolic acids as secondary ligands

Summary The formation of ternary complexes of the MAL^3– type [where M = Cu^II, Ni^II and Zn^II ; A = nitrilotriacetic acid (NTA); L = 1-hydroxy-2-naphthoic acid (1,2 HNA) and 2-hydroxy-1-naphthoic acid (2,1 HNA)] have been studied potentiometrically in 50% v/v aqueous — ethanol (25° and µ = 0.1). Under identical conditions the binary complexes of the 1,2- and 2,1-HNA ligands have also been examined. The values of mixed ligand formation constants K_MAL have been found to be lower than K_ML (first step formation constant of binary complexes) and even less than (second step formation constant of binary complexes).     

Ion association studies in aqueous NiSO4 solutions by positron lifetime spectroscopy

Ion association has been studied by positron lifetime spectroscopy in aqueous solutions containing the Ni²⁺ and SO ₄ ^2– ions at 294 K with the double aim of assessing the reliability of the method for quantitative determination of complex formation constants and of probing the validity of various expressions to calculate single-ion activity coefficients at high ionic strength. The existence of two complexes, identified as NiSO₄ and Ni₂SO ₄ ²⁺ , is shown by the data analysis. Considering the formation constant of the former, K_I=(196±10)M^–1, determined in previous works leads to discarding several of the expressions commonly used for activity corrections. Two possible values are retained for K_I, (193±20)M^–1 and (179±20)M^–1, while K_II related to Ni₂SO ₄ ²⁺ is better defined, as (2.57±0.14)M^–1.     

Electrochemical analysis of the coordination sphere of ruthenium(ii) as electron transfer mediator in glucose oxidase catalysis in aqueous solutions

The redox potentials of the cis-[Ru(LL)₂XY]ⁿ⁺ complexes (LL = 2,2"-bipyridyl (bpy), 1,10-phenanthroline (phen), and 4,4"-dimethyl-2,2"-bipyridyl (Me₂bpy); X, Y = Cl^–, Br^–, CO₃ ^2–, NO₂ ^–, SCN^–, N₃ ^–, H₂O, and DMSO) in aqueous buffer solutions were measured and analyzed in the framework of the Lever theory on the additivity of contributions of ligands (E _L) to the apparent redox potential of the complex (E ^o"). The complexes manifest the properties of reversible or quasireversible redox systems, whose formal redox potentials lie in the 0.2—0.5 V range. The complexes are efficient electron transfer mediators between the active center of glucose oxidase (GO) from Aspergillus niger and an electrode.     

Mixed benzohydroxamato-acetylacetonato metal complexes: spectral,thermal and photochemical behaviour

Summary Bis(acetylacetonato)VO^II,–Co^II,–Ni^II,–Cu^II,–Zn^II, –UO ₂ ^II and tris(acetylacetonato)Fe^III react with benzohydroxamic acid to yield the corresponding mixed ligand complexes as a result of displacement of one acetylacetone molecule. Intermolecular association may be the reason for six-coordination geometry around the metal ions. A t.g.a. study of the complexes shows, in most cases, initial loss of alcohol and water molecules associated with the complexes; subsequent decomposition steps are characterised by very sharp weight loss. The photochemical stability of the complexes has been studied. Intraligand excitation causes a decomposition in the case of Fe^III and VO^II-complexes but no detectable effect for Co^II, Ni^II, Cu^II, Zn^II, or UO ₂ ^II -complexes.     

Ruthenium(II)/(III) complexes of bidentate acetyl hydrazide Schiff bases

A series of octahedral Ru^II/Ru^III complexes of the type [Ru(Y)(CO)(BAX)(PPh₃)₂] and [RuCl₂(BAX)(PPh₃)₂] (Y = H or Cl; BAX = benzaldehydeacetylhydrazone anion; X = H, Me, OMe, OH, Cl or NO₂) have been prepared and characterised by spectral, magnetic and cyclic voltammetric studies. The Ru^II complexes are low spin diamagnetic (S = 0) whereas the Ru^III complexes are low spin and paramagnetic (S = 1/2). These Ru^II and Ru^III complexes absorb in the visible region respectively at ca. 16,000 and 28,000 cm^–1 which bands are assigned to the MLCT. The correlation of the _max values of the Ru^III complexes with the ⁺ Hammett parameter, is linear, indicating the profound effect of substituents on the electron density of the central metal. I.r. spectral data reveals that the hydrazone is chelated to ruthenium through the hydrazinic nitrogen and the deprotonated enolic oxygen. The rhombic nature of the e.s.r. spectra of the Ru^III complexes indicates an asymmetry in the electronic environment around the Ru atom. Ru^II complexes in CH₂Cl₂ show an irreversible Ru^II/III redox couple at ca. 0.9–0.5 V, while the Ru^III complexes show two reversible redox couples in the –0.1–0.1 and 0.8–0.6 V range, indicating that the higher oxidation state of ruthenium is stabilised by hydrazones.     

Synthesis,spectroscopic, magnetic,conductometric and electro-chemical investigations of nickel(II)-1-phenyl-4,6-dimethylpyrimidine-2-thione complexes

Summary Tris-, bis- and mono-ligand complexes of Ni^II with 1-phenyl-4, 6-dimethylpyrimidine-2-thione (L) having the general formulae NiL₃X₂·2H₂O (X = ClO _inf4 ^p– , BF _inf4 ^p– ), NiL₂X₂ (X = Cl^–, Br^–, SCN^– or NO _inf3 ^p– ), NiL₂X₂·EtOAc (X = Br^– or I^–), NiL₂X₂·H₂O·EtOH (X = I^– or NO _inf3 ^p– ) and NiLCl₂·3H₂O, were synthesized and their structures deduced from i.r. and electronic spectra, and magnetic properties. The combined evidence is consistent with an octahedral coordination for the Ni^II ion in all the complexes, with the ligand acting as a bidentate N,S-chelating agent. Spectral evidence, conductivity data and electro-chemical results in DMF solution show that the complexes undergo solvolysis readily. Polarographic and c.v. data for the [NiL₃](ClO₄)₂·2H₂O complex and for the [Ni-(DMF)₆](ClO₄)₂-L systems, at increasing ligand concentrations, have shown that in DMF solution the [Ni(DMF)₆]²⁺ cation prevails and that the thiopyrimidine-containing species, [NiL(DMF)₅]²⁺ (L = N-monodentate ligand) , can be formed only in the presence of a large excess of free ligand.Author to whom all correspondence should be directed.     

Kinetics and mechanism of ligand substitution reactions of NiL and Ni2L with cyanide ion (L = hexamethylenediaminetetraacetic acid)

Summary The kinetics and mechanism of reactions of cyanide ion with [NiL] and [Ni₂L] (L = hexamethylenediaminetetraacetic acid) have been studied spectrophotometrically at 25 ±0.1 °C, with pH=11.0±0.02, and I=0.1 M(NaClO₄). In both reactions the final product was [Ni(CN)₄]^2–. The order with respect to [CN^–] was found to be one over a wide range of cyanide ion concentrations for both the systems.In the Ni₂L-CN^– system, however, the reaction becomes zero order with respect to cyanide when [CN^–]<6×10^–4 M.     

Electrochemical synthesis and magnetic studies of neutral transition metal imidazolates and pyrazolates

Summary The single-step electrochemical synthesis of neutral transition metal complexes of imidazole, pyrazole and their derivatives has been achieved at ambient temperature. The metal was oxidized in an Me₂CO solution of the diazole to yield complexes of the general formula: [M(Iz)₂] (where M = Co, Ni, Cu, Zn; Iz = imidazolate); [M(MeIz)₂] (where M = Co, Ni, Cu, Zn; MeIz = 4-methylimidazolate); [M(PrⁱIz)₂] (where M = Co, Ni, Cu, Zn; PrⁱIz = 2-isopropylimidazolate); [M(pyIz)_n] (where M = Co^III, Cu^II, Zn^II; pyIz = 2-(2-pyridyl)imidazolate); [M(Pz)_n] (where M = Co^III, Ni^II, Cu^II, Zn^II; Pz = pyrazolate); [M(ClPz)_n] and [M(IPz)_n] (where M = Co^III, Ni^II, Cu^II, Zn^II; ClPz = 4-chloropyrazolate; IPz = 4-iodopyrazolate); [M(Me₂Pz)_n] (where M = Co^II, Cu^I, Zn^II; Me₂Pz = 3,5-dimethylpyrazolate) and [M(BrMe₂Pz)_n] (where M = Co^II, Ni^II, Cu^I, Zn^II; BrMe₂Pz = 3,5-dimethyl-4-bromopyrazolate). Vibrational spectra verified the presence of the anionic diazole and electronic spectra confirmed the stereochemistry about the metal centre. Variable temperature (360-90 K) magnetic measurements of the cobalt and copper chelates revealed strong antiferromagnetic interaction between the metal ions in the lattice. Data for the copper complexes were fitted to a Heisenberg (S= ) model for an infinite one-dimensional linear chain, yielding best fit values of J=–62––65cm^–1 andg = 2.02–2.18. Data for the cobalt complexes were fitted to an Ising (S= ) model with J=–4.62––11.7cm^–1 andg = 2.06–2.49.     

Homo- and heterodinuclear 2,2′-bipyrimidine-bridged complexes

[M(hfacac)₂(bpym)] complexes, where M = Co^II, Ni^II or Zn^II, hfacac = hexafluoroacetylacetonate and bpym = 2,2bipyrimidine; and [Cl₂M(bpym)M(hfacac)₂] complexes, where M = Co^II, Ni^II Mn^II or ZnII M = Ni^II; M = NiII or Zn^II and M = Zn^II; M = Ni^II and M = Co^II have been prepared and characterized by chemical analysis, conductance measurements, IR and electronic spectroscopies and magnetic susceptibility measurements (4.2–292K range). The dinuclear NiII–Ni^II, Co^II–Ni^II and Mn^II–Ni^II complexes are antiferromagnetic, with an intramolecular exchange parameter, J, of –2.3–8.9cm–1. Co^II and Mn^II are in a high spin state. The low temperature effect observed in monomers and in Ni^II–Zn^II dimers is considered a consequence of either an intermolecular antiferromagnetic interaction or the zero-field splitting in NiII.     

Synthesis,thermodynamic properties and kinetics of the acid-catalyzed dissociation of nickel(II) diamido polyamino complexes

The ligands 1,10-N,N-bis(2-hydroxymethylbenzoyl)-1,4,7,10-tetraazadecane (L₁) and 1,11-N,N-bis(2-hydroxymethylbenzoyl)-1,4,8,11-tetraazaundecane (L₂) have been synthesized. The stability constants of Ni^II complexes of ligands L₁ and L₂ have been studied at 25 °C using pH titrations. The kinetics of general acid (HCl, 0.04–2.34 mol dm^–3) or buffer (DEPP or DESPEN, 0.05 mol dm^–3, pH 4.83–5.72)-catalyzed dissociation of these Ni^II complexes have been investigated at 25 °C using a stopped-flow spectrophotometer. The ionic strength of solution was controlled at I = 2.34 mol dm^–3 (KCl + HCl) and I = 0.1 mol dm^–3 (KNO₃, buffer), respectively. The kinetic dissociation of Ni^II complexes catalyzed by HCl obeys the equilibrium k _obs = k _1d + k _2H[H⁺], whereas in buffer solution the observed rate constant k _obs = k _d + k _1H[H⁺]. At pH < 1.5, both the proton-assisted and direct protonation pathways contribute to the rates, whereas solvation is the dominant pathway at pH > 6. In the 4.8–5.7 pH range, the complexes dissociate mainly through a proton-assisted pathway.     

Synthesis and characteristics of the redox behavior of a ditopic compound contaiing tetraaza macrocyclic and crown ether fragments

A new ditopic compound has been obtained from monocyclic fragments by the reaction of formylbenzo-15-crown-5 with a macrocyclic complex of nickel(II) that contains a pendant amino group and it is characterized by methods of electron and IR spectroscopy, FAB mass spectrometry, and cyclic voltammetry. It was established that binding of protonated amines (methylamine, -alanine) to crown ether receptor centers causes a cathodic shift of the redox potential (by 100–160 mV) of the Ni^IIIL/ Ni^IIL pair of the ditopic complex. Ditopic and monocyclic complexes of nickel(III) were prepared by preparative electrolysis in acetonitrile solutions. The kinetics of the reduction of nickel(III) by bifunctional substrates capable of binding with the crown ether was investigated. The rate of reduction of Ni(III) in the reaction with m-aminophenol in the ditopic complex is 50 times less than in the monocyclic complex and 6 times greater in the reaction with -alanine hydrazide.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 162–167, March–April, 1992.     

Nicke(II) halide complexes with hypoxanthine and xanthine

Summary Ni(LH)₃LX complexes (LH=hypoxanthine or xanthine; X=Cl, Br or I) are formed by boiling under reflux 2:1 molar mixtures of LH and hydrated NiX₂ in HC(OEt)₃–MeCO₂Et. The new complexes appear to be linear chain-like polymers, characterized by bidentate monoanionic L^– ligands singly bidging between adjacent Ni²⁺ ions. A coordination number six is attained by the presence of three terminal unidentate LH and one X ligand in the first coordination sphere of each Ni²⁺ ion. The neutral LH and monoanionic L^– ligands bind exclusivelyvia ring nitrogens to Ni^II. The probable binding sites of the uni- and bi-dentate hypoxanthine and ligands in the new complexes are discussed.Presented in part at the 3rd Chem. Congress of North America (LH=xanthine) and the XXVI ICCC (LH=hypoxanthine), see refs. 1 and 2, respectively.     

Preparation and electrical conductivity of organoheterobimetallic-maleonitriledithiolates

The new complexes [PhHg]₂[M(mnt)₂] [M = Ni^II, Cu^II, Zn^II or Pd^II; mnt^2– = 1,2-dicyano-1,2-ethylenedithiolate (maleonitriledithiolate)] have been characterized spectroscopically and magnetically and their solid phase conductivity measured. All compounds exhibit solid phase _rt in the 1.29 × 10^–12–5.68 × 10^–10 S cm^–1 range and semiconduct in the 313–383 K range.     

Electrochemical and spectroscopic studies of metal cyclam complexes with thiocyanate. Evidence for mixed ligand complex formation

Complexes of Ni^II, Co^II and Cu^II containing the macrocyclic ligand, 1,4,8,11-tetraazacyclotetradecane (cyclam), and their ability to form mixed ligand complexes with thiocyanate have been studied. These complexes in a 1:2 mole ratio, exhibit new absorption peaks at 450, 538 and 512 nm respectively. Addition of thiocyanate to the nickel–cyclam complex (1:2:5 mole ratio) led to the formation of a purple complex, exhibiting three distinct new absorption peaks at 330, 455 and 662 nm. A purple complex (1:2:10 mole ratio) separated, having absorption peaks at 352, 503 and 693 nm in CHCl₃. The Co^II–cyclam complex with thiocyanate in the same mole ratio exhibits two absorption peaks at 437 and 519 nm without appearance of any precipitate. The Cu^II–cyclam complex with thiocyanate did not form a mixed ligand complex. Electrochemical studies also confirmed the complex formation of Ni^II–cyclam with the thiocyanate with the appearance of two new oxidation peaks close to 1.25 and 1.60 V versus Ag/AgCl in H₂O and CHCl₃. The Co^II–cyclam complex with thiocyanate exhibited an oxidation peak at 1.2 V versus Ag/AgCl, while no peak was observed for the Cu^II–cyclam complex with thiocyanate. Based on spectroscopic and electrochemical studies the geometry of the complex has been evaluated.