Importance of π-bonding in the stabilisation of ternary complexes

Summary Stability constants (K _MAL ^MA ) together with other thermodynamic parameters measuring the stabilities and the contribution of astatistical factors governing the stability of ternary complexes, MAL [M=Co^II, Ni^II, Cu^II or Zn^II; A=2, 2-dipyridylamine (A³), 5-nitro-o-phenanthroline (A⁵), 5-methyl-o-phenanthroline (A⁶); LH=benzohydroxamic acid] have been determined at 25°CC, at ionic strength 0.1 M KNO₃. The results are compared with those for the complexes containing polyaminocarboxylic acids such as iminodiacetic acid (A¹) and other heteroaromaticN-bases such as 2, 2-bipyridine (A²),o-phenanthroline (A⁴) and the stability constants are found in the sequence K_{MA^4 L}^{MA^4 } > K_{MA^2 L}^{MA^3 } = ca.K_{MA^3 L}^{MA^3 } \gg K_{MA^1 L}^{MA^1 } .$$ " align="middle" border="0"> For all heteroaromaticN-bases, the sequence K_{ML_2 }^{ML} $$ " align="middle" border="0"> has been found. The sequences are explained in terms of electrostatic interaction, change of electrophilicity of the bound metal and the -acidic character of the primary ligand.     

Synthesis of flexible ligands for assembling two metals in close proximity. Magnetic,electrochemical and spectral properties of binuclear copper (II) complexes with different exogenous bridging motifs

Binuclear Cu^II complexes having new flexible heptadentate ligands 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-bromophenol [HL¹], 2,6-bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methylphenol [HL²], and 2,6-bis{[bis(3,3-N,N-dimethylaminopropyl)amino]methyl}-4-methoxyphenol [HL³], capable of assembling two copper ions in close proximity have been synthesized. Comparisons of the charge-transfer (CT) features, observed in electronic spectra of these complexes, are correlated with the electronic effect on the aromatic ring of the ligand systems. Cyclic voltammetry has revealed the existence of two reduction couples,

Studies on mixed ligand complexes of cobalt(II), nickel(II), copper(II) and zinc(II) involving 8-hydroxyquinoline-5-sulphonic acid as a primary ligand and substituted catechols as secondary ligands

Summary Stability constants (K _MAL ^MA ) and other thermodynamic parameters of the MAL complexes (charges omitted) [M=Co^II, Ni^II, Cu^II or Zn^II; AH₂=8-hydroxyquinoline-5-sulphonic acid; LH₂=catechol (L¹H₂), 1,2-dihydroxybenzene-sulphonate (L²H₂), 1,2-dihydroxybenzene-3, 5-disulphonate (L³H₂), 4-nitro-1,2-dihydroxybenzene (L⁴H₂)] have been determined at 25°C and at an 0.1 M KNO₃ ionic strength by the extended Irying-Rossotti technique. The stability constants lie in the sequences: K_{MAL}^{MA} ?K_{ML_2 }^{ML} ; K_{MAL}^{MA_1 } > K_{MAL}^{MA_2 } > K_{MAL}^{MA_3 } \gg K_{MAL}^{MA_4 } $$ " align="middle" border="0"> and all follow the Irving-Williams stability order. These observations can be explained in terms of electrostatic interaction, change of electrophilicity of the bound metal and -acidic character of the primary ligand.     

New unsymmetrical μ-phenoxo bridged binuclear copper(II) complexes

A series of binuclear Cu^II complexes [Cu₂XL] ⁿ⁺ having two copper(II) ions bridged by different motifs (X = OH^–, MeCO₂ ^–, or Cl^–) have been prepared using the ligands: H₂L¹ = 4-methyl-2-[N-(2-{dimethylamino}ethyl-N-methyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol, H₂L² = 4-nitro-2-[N-(2-{dimethylamino}ethyl-N-methyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol, H₂L³ = 4-methyl-2-[N-(2-{diethylamino}ethyl-N-ethyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol and H₂L⁴ = 4-nitro-2-[N-(2-{diethylamino}ethyl-N-ethyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol. The complexes have been characterized by spectroscopic, analytical, magnetic and electrochemical measurements. Cryomagnetic investigations (80–300 K) revealed anti-ferromagnetic exchange between the Cu^II ions (–2J in the range –50 to –182 cm^–1). The strength of anti-ferromagnetic coupling lies in the order: OAc > OH > Cl. Cyclic voltammetry revealed the presence of two redox couples, assigned to Cu^II/Cu^II/Cu^II/Cu^I/Cu^I/Cu^I. The first reduction potential is sensitive to electronic effects from the aromatic ring substituents and steric effect on the donor nitrogens (side arm) of the ligand systems.     

Equilibrium study of ternary complexes involving heteroaromaticN-bases and acetohydroxamic acid

Summary Stability constants (K _MAL ^MA ) and other thermodynamic parameters, including statistical factors relating to the ternary complexes (MAL) [M=Co^II, Ni^II, Cu^II or Zn^II; A=2, 2-dipyridylamine (A³), 5-nitro-o-phenanthroline (A⁵), 5-methyl-o-phenanthroline (A⁶); LH=acetohydroxamic acid] have been determined at 25° C, at ionic strength 0.1 M KNO₃. The results are compared with data on aminopolycarboxylic acids, such as iminodiacetic acid (A¹), and other unsubstituted heteroaromaticN-bases:e.g. 2, 2-bipyridine (A²),o-phenanthroline (A⁴). The resulting stability sequence is: , and for heteroaromaticN-bases: . The results have been explained in the light of electrostatic interactions, -acidities of the primary ligands (A) andcistrans equilibria for MAL.     

N-Phthaloylglycinate ternary complexes with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) metal ions and aromatic mono,bi and tridentate amines

Complexes of N-phthaloylglycinate (N-phthgly) and Co^II, Ni^II, Cu^II, Zn^II and Cd^II containing imidazole (imi), N-methylimidazole (mimi), 2,2-bipyridyl (bipy) and 1,10-phenanthroline (phen), and tridentate amines such as 2,2,2-terpyridine (terpy) and 2,4,6-(2-pyridyl)s-triazine (tptz), were prepared and characterized by conventional methods, i.r. spectra and by thermogravimetric analysis. For imi and mimi ternary complexes, the general formula [M(imi/mimi)₂(N-phthgly)₂]·nH₂O, where M = Co^II, Ni^II, Cu^II and Zn^II applies. For Cd^II ternary complexes with imi, [Cd(imi)₃(N-phthgly)₂]·2H₂O applies. For the bi and tridentate ligands, ternary complexes of the formula [M(L)(N-phthgly)₂]·nH₂O were obtained, where M = Co^II, Ni^II, Cu^II and Zn^II; L = bipy, phen, tptz and terpy. In all complexes, N-phthgly acts as a monodentate ligand, coordinating metal ions through the carboxylate oxygen, except for the ternary complexes of Co^II, Ni^II and Cu^II with mimi and Cu^II and Zn^II with imi, where the N-phthgly acts as a bidentate ligand, coordinating the metal ions through both carboxylate oxygen atoms.     

Synthesis and Characterization of Two vic-Dioximes Containing the 1,3-Dioxolane Ring and 1,4-Diaminobutane and Their Cobalt(II), Nickel(II), Copper(II) and Zinc(II) Metal Complexes

Two new vic-dioxime ligands, (E,E)-N-{4-[(1,4-dioxaspiro[4.4]non-2-ylmethyl)amino]butyl}-N-hydroxy-2-(hydroxyimino)ethanimidamide (L¹H₂) and (E,E)-N-{4-[(1,4-dioxaspiro[4.5]dec-2-ylmethyl)amino]butyl}-N-hydroxy-2-(hydroxyimino)ethanimidamide (L²H₂) containing two different heteroatoms (N,O) have been prepared from anti-chloroglyoxime, N-(1,4-dioxaspiro[4.4]non-2-ylmethyl)butane-1,4-diamine (3) and N-(1,4-dioxaspiro[4.5]dec-2-ylmethyl)butane-1,4-diamine (4). Co^II, Ni^II and Cu^II complexes of the ligands have a metal:ligand ratio of 1:2 and the ligands coordinate through the two N atoms, as do most of the vic-dioximes. However, Zn^II complexes of the ligands have a metal:ligand ratio of 1:1 and the ligands are coordinated only by the N, O atoms of the vic-dioximes. In the Co^II complexes two water molecules, and in the Zn^II complexes a chloride ion and a water molecule, are also coordinated to the metal ion. The structures of the compounds were determined by a combination of elemental analysis, magnetic moments, molar conductances, thermogravimetric analysis (t.g.a.) and spectroscopic (u.v.–vis., i.r., ¹H- and ¹³C-n.m.r.) data.     

Synthesis,Characterization, and an ab initio Study of a Manganese(II) Macrocyclic Schiff-Base Complex with Two 2-Aminoethyl Pendant Arms

The cyclocondensation of 2,6-diformylpyridine with N,N,N,N-tetrakis(2-aminoethyl)ethane-1,2-diamine (pentene) in the presence of Mn^II forms the [1 + 1] pendant arm Schiff-base macrocyclic complex, [MnL³]²⁺. The ligand is a 15-membered pentaaza macrocycle having two 2-aminoethyl pendant arms {L³= 6,9-bis(aminoethyl)-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentene}. The complex, investigated by analytical, spectroscopic and magnetic techniques, supports the formation of a highly symmetrical pentagonal bipyramid complex with the Mn^II ion located within a pentaaza macrocycle and two pendant amines coordinating on opposite sides of a plane defined by the macrocycle and the metal ion. The structure of the complex was also verified by ab initio HF-MO calculations using a standard 3-21G basis set.     

Electron Delocalization in 3d Complexes Caused by Dimethylurea Checked by Positron Annihilation

The rate constants, k _CR, of ortho- into para-positronium conversion reactions promoted by paramagnetic 3d complexes are linearly correlated with the electron delocalization, , of unpaired metal electrons caused by ligands, being the ratio between the inter-electronic repulsion parameters in complexes and in the free gaseous ions. By applying a procedure previously described the values of Mn^II, Co^II, Ni^II complexes with dimethylurea were deduced from the mentioned correlations and compared with those of complexes with urea obtained both by the method of Ps reactions here applied and that based on UV-Vis absorption spectroscopy.     

Metal-ion directed synthesis of binuclear octaazamacrocyclic complexes of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) and their physico-chemical studies

A novel series of 16-membered binuclear complexes of octaazatetraimine ligand, [M = Mn^II, Co^II, Ni^II, Cu^II and Zn^II; X = Cl or NO₃] have been synthesized by metal template condensation reactions of o-phenylenediamine with N,N′-diacetylhydrazine in 1:1:1 molar ratio in methanol. The proposed stoichiometry and the bonding of the macrocyclic moiety to metal ions along with the overall stereochemistry have been derived from the results of elemental analyses, magnetic susceptibility, conductivity data and the spectral data revealed from FT-IR, , ESI mass, UV–visible studies. An octahedral geometry has been envisaged for Mn^II, Co^II, and Ni^II complexes while a slight distortion in octahedral geometry has been noticed for Cu^II complexes. The low conductivity data of all the complexes suggest their non-ionic nature.     

Synthesis and characterization of new 13 and 14-membered macrocycles and their transition metal complexes

Two new macrocyclic ligands 1,4,7,9,12-pentaaza-10,11-dioxo-8,9,12,13-bis-(1-oxo-3-thio-2-hydropyrimidine)-trideca-7,13-diene, (L¹) and 1,4,7,9,12-pentaaza-10,12-dioxo-8,9,13,14-bis-(1-oxo-3-thio-2-hydropyrimidine)-tetradeca-7,14-diene, (L²) and their complexes with Cr^III, Mn^II, Fe^III, Co^II, Ni^II, Cu^II and Zn^II have been synthesized, and characterized by elemental analysis, i.r., ¹H-n.m.r., e.p.r., u.v.–vis. spectroscopy, magnetic susceptibility and conductance measurements. The conductivity measurements suggest that the complexes of divalent metal ions are 1:1 electrolytes whereas the trivalent metal ions are non-electrolytes. On the basis of electronic spectra and magnetic moment measurements the Cr^III and Fe^III complexes are octahedral, while the divalent metal complexes are tetrahedral except for the Ni^II and Cu^II complexes which are proposed to have square planar geometry. All the ligands and their complexes have been screened against gram-positive bacteria Staphylococcus aureus and gram-negative bacteria E. coli. The results show that they inhibit the growth of bacteria.     

Mononuclear species LnL and LnLH<Subscript>−1</Subscript> based on 1,10-phenanthroline dibenzoimidazole derivatives as catalysts for hydrolysis of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP)

Two novel multidentate ligands: 2,9-bis- -1,10-phenanthroline(L₁) and 2,9-bis- -1,10-phenanthroline(L₂) were synthesized and characterized by elemental analysis and ¹H-n.m.r. spectroscopy. Protonation of the ligands and the stability of complexes of the ligands with rare earth metal ions were investigated. The mononuclear metal complexes [Gd^III and Sm^III] of the ligands were studied as catalysts for the transphosphorylation of the RNA-model substrate 2-hydroxypropyl-p-nitrophenylphosphate(HPNP). Kinetic studies show the second-order rate constants of HPNP hydrolysis catalyzed by complexes LnL and LnLH₋₁, respectively. We found that both LnL and LnLH₋₁ have catalytic activity, but GdL₁H₋₁ was the most efficient catalyst of them, which indicated that the structure of the ligands has obviously influence on the activity of corresponding complexes. A new mechanism was proposed for HPNP hydrolysis reaction catalyzed by LnL and LnLH₋₁.     

Kinetics of the oxidation of [N-(2-hydroxyethyl)-ethylene-diamine-N,N′,N′-triacetato]cobalt(II) by N-bromosuccinimide

The kinetics of the oxidation of [N-(2-hydroxyethyl)-ethylene-diamine-N,N,N-triacetato] cobalt(II), [Co^II-(HEDTA)]^–, by N-bromosuccinimide, NBS, have been studied in aqueous solutions and water-methanol solvent mixtures under various conditions. The reaction stoichiometry indicates that one mole of NBS reacts with one mole of [Co^II(HEDTA)]^–. In aqueous solutions the reaction obeys the following rate law:

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.     

Synthesis, characterization and redox properties of macrocyclic Schiff base by reaction of 2,6-diaminopyridine and 1,3- bis (2-carboxyaldehyde phenoxy)propane and its CuII, NiII, PbII, CoIII and LaIII complexes

A new macrocyclic ligand, 1,3,5-triaza-2,4:7,8:,14,15-tribenzo-9,13-dioxacyclohexadeca-1,5-diene (L) was synthesized by reaction of 2,6-diaminopyridine and 1,3-bis(2-carboxyaldehyde phenoxy)propane. Then, its Cu^II, Ni^II, Pb^II, Co^III and La^III complexes were synthesized by a template effect by reaction of 2,6-diaminopyridine and 1,3-bis (2-carboxyaldehyde phenoxy)propane and Cu O, Ni O, Co O, La O, respectively. The ligand and its metal complexes have been characterized by elemental analysis, IR, ¹H- and ¹³C-NMR-, UV-vis spectra, magnetic susceptibility, conductivity measurements, mass spectra and cyclic voltammetry. All complexes are diamagnetic and the Cu^II complex is binuclear. The diamagnetic behaviour of the binuclear complex may be explained by a very strong anti-ferromagnetic interaction in the Cu–Cu pair. The Co^II was oxidised to Co^III.     

Transition metal complexes of potassium dihydridobis-, hydridotris- and tetrakis-(thiophenolyl)borate anions

Summary The three new potential chelating ligands dihydridobis-, hydridotris- and tetrakis-(thiophenolyl)borate anions, and their chelates with first row transition metals have been synthesised. The divalent and trivalent metal ions form complexes in 12 and 13 (metal:ligand) ratios respectively. The number of ligands coordinated correspond to the number of anions replaced in the metal salis. The compounds were characterized by elemental analysis, i.r. spectra, magnetic susceptibility measurements and electronic spectral studies. The Cr^III and Fe^III complexes of dihydridobis- and hydridotris-(thiophenolyl)borates appear to be octahedral, and those of Cu^II are proposed to be square planar. Tetrahedral geometry is suggested for the Mn^II, Co^II and Ni^II complexes. The tetrakis-(thiophenolyl)borate yielded octahedral complexes with all the metal ions except for Cu^II which is square planar. The ligand field parameters 10Dq, B and have also been calculated wherever possible. The ligands may be placed in the vicinity of EDTA in the nephelauxetic series.     

Potentiometric Studies on Binary and Ternary Complexes of Di- and Trivalent Metal Ions Involving Some Hydroxamic Acids, Amino Acids, and Nucleic Acid Components

Summary. Formation of binary and ternary complexes of Cu^II, Co^II, Ni^II, Zn^II, Fe^III, Al^III, and Cr^III metal ions with some selected aliphatic and aromatic hydroxamic acids and some biologically important amino acids or nucleic acid components was investigated using the potentiometric technique at 25°C and I=0.10moldm^–3 NaNO₃. The acid-base properties of the ligands were investigated and discussed. The acidity constants of the ligands were determined and used for determining the stability constants of the complexes formed in aqueous medium under the experimental conditions. The ternary complex formation was found to occur in a stepwise manner. The stability constants of these binary and ternary systems were calculated. The order of stability of the ternary complexes in terms of the nature of hydroxamic acid, amino acid, nucleic acid component and metal ions was investigated and discussed as well as the values of log K and log X for the ternary systems. The concentration distribution of the various complex species in solution was evaluated. In addition, evaluation of the effect of temperature of the medium on the stability of the ternary system M^III – benzohydroxamic acid – L-histidine or adenine (M^III=Fe^III, Al^III, and Cr^III) has been studied. The thermodynamic parameters were calculated and discussed.     

Coordination Properties of Sterically Stressed Zincporphyrins

Spectrophotometric titration and computer simulation were used to study how the nature of porphyrin and extra ligand affect the formation of extra complexes of zincporphyrins in o-xylene. The compounds under study were zincporphyrins (ZnP) with different substituents and phenyl radicals in meso-positions (zinc-5,15-(p-butyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetraethylporphyrin (ZnP¹), zinc-5,15-(p-butyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrin (ZnP²), zinctetraphenylporphine (ZnP³), and zinc complexes with overlapped porphyrin (ZnP⁴). N-Methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, and dimethylformamide were used as extra ligands (L). The strength of Zn–L bonding was found to decrease in extra complexes (L)ZnP in the series of ZnP as follows: ZnP⁴> ZnP¹> ZnP²> ZnP³. It was established that the stability constant (logK _st) for sterically nonstressed complexes (L)ZnP⁴linearly increases with growth in the extra ligand basicity (log ) and is proportional to the shift of the main absorption bands () in the electronic spectra of extra complexes of zinctetraphenylporphine. For spatially distorted (L)ZnP¹, (L)ZnP², and (L)ZnP³, the values of logK _stand log , as well as logK _stand , change symbatically. The geometric structure and energy characteristics of pentacoordinated zincporphyrins were calculated by quantum-chemical methods. Correlations were established between the calculated values of the energy of the interaction of the central metal atom with the extra ligand molecule and the stability of the extra complexes of zincporphyrins.     

Positronium Spin-Conversion Reactions Promoted by 3d Complexes in Highand Low-Spin States

Continuing in our work on the correlations between the rate constants, k _CR, of the ortho into para-positronium conversion reactions, CR, promoted by complexes of various 3d ions and the metal electron delocalization caused by the ligands, the relationship between the parameters of the correlation lines pertinent to highand low-spin complexes of Cr^II, Mn^II, and Co^II ions was ascertained. Moreover, it was experimentally verified, for the first time, that the statistical probability of the CR promoted by paramagnetic compounds with S = 1/2 is three times larger than that of the CR caused by compounds with S > 1/2.     

Metal chelates with some cellulose derivatives; part IV. Structural chemistry of HEC complexes

Reactions of hydroxyethyl cellulose (HEC) with Cr ^III, Ni^II, Co^II, or Cu^II chlorides in aqueous medium yielded complexes with formulae [M(HEC)Cl _m .n H ₂O], wherem =1 or 2 and n=2 or 3. HEC acted as a uninegatively charged bidentate ligand in the case of Cr^III and Ni^II, and as a neutral ligand in the case of Co^II and Cu^II complexes. The spectra showed that the binding sites in Cr^III and Ni^II complexes were the ether oxygen between two ethoxyl groups and the oxygen of the hydroxyl group; while in the Co^II and Cu^II complexes the binding sites were the oxygen of ethoxyl groups and the primary alcoholic O atom of glucopyranose rings. These complexes would most likely exhibit octahedral geometry with Cr^III, Ni^II, and Co^II, but square planar configuration in the case of the Cu^II complex. The ligand parameters of the Cr^III, Ni^II, and Co^II metal chelates were calculated in different solvents and at different temperatures. The thermal stability of the above complexes was investigated and the overall thermodynamics functions G⁰, H⁰, and S⁰, associated with complex formation, were estimated.