Nickel(II) ion-catalyzed hydrolysis of acetyl esters of 2-pyridineoxime derivatives: Effects of geometry around central metal atom on the efficiency of metal ion catalysis

The kinetics of the Ni(II)-catalyzed ester hydrolysis of O-acetyl-2-pyridine-carboxaldoxime, O-acetyl-2-acetylpyridineketoxime, and O-acetyl-6-carboxy-2-pyridine-carboxaldoxime are measured and the values of various k_cat parameters are calculated for reaction paths involving one metal ion (k_cat^W and k_cat^OH) and two metal ions k_cat^A and k_cat^B). Examination of the kinetic data reveals that the k_cat^W and k_cat^OH paths for the Ni(II)-catalyzed reactions involve the same mechanism as those for the previously reported Cu(II)-catalyzed reactions. For the k_cat^A and k_cat^B paths, the mechanism involving binuclear Ni(II) ions is preferred by analogy with the previously reported Zn(II)-catalyzed reactions. Comparison of k_cat^OH values for the Cu(II)- and Ni(II)-catalyzed hydrolysis of 1–3 indicates that markedly different steric effects are exerted by the substituents of 2 and 3 on the catalytic behavior of the two metal ions. This is explained in terms of differences in the fit of the metal ions in the metal complexes of 1–3. Present results demonstrate that slight changes in the geometry around the central metal atom can affect the catalytic outcome significantly. The implications of the present results on metal substitution in metalloenzymes are also discussed.     

Metal-ion-catalyzed hydrolysis of monomethyl and dimethyl esters of 3-(2-imidazoleazo)phthalic acid: Bifunctional catalysis by carboxyl group and the metal ion in the hydrolysis of an alkyl ester

The Cu(II) or Ni(II) ion-catalyzed hydrolysis of methyl 2-carboxy-6-(2-imidazoleazo)benzoate (1) and the corresponding dimethyl ester (2) was studied kinetically at various pH values. For 2, the ester group located at the o position to the azo substiuent was hydrolyzed. From the rate data obtained at various metal concentrations, the values of k_cat and K_f were estimated at each pH value. For the Ni(II)-catalyzed hydrolysis of 1 at pH < 4, k_cat increases as pH is lowered, indicating bifunctional catalysis by the carboxyl group and the metal ion. For most of the reactions investigated under other conditions, the ester hydrolysis was subjected to sole catalysis by the metal ions. Detailed analysis of kinetic data obtained for these reactions indicated that the metal-ion catalysis involves the rate-determining breakdown of the tetrahedral intermediates formed by the addition of a water molecule or hydroxide ion. The bifunctional catalysis by the carboxyl group and Ni(II) ion can be considered as a model for carboxypeptidase A. The kinetic data indicate that the bifunctional catalysis proceeds through the nucleophilic attack of the carboxylate ion at the Ni(II)-coordinated carbonyl group.     

Preparation and spectroscopic properties of cobalt(III), nickel(II), copper(II) and zinc(II) complexes with a novel saturated macrocyclic ligand: 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22-docosahydrodibenzo-[b,i][1,4,8,11]tetraazacyclotetradecine

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19, 20,21,22-Docosahydrodibenzo[b,i] [1,4,8,11] tetraazacyclotetradecine was prepared by hydrogenation of the benzo-analogue. Five isomers are feasible as a result of this hydrogenation but only two have been isolated: isomer A (melting point 158.5– 161.0 °C) and isomer B (melting point 194.5– 196.0 °C). The ¹³C NMR study was initiated to clear up the conformational differences between isomers. The cobalt(III), nickel(II), copper(II) and zinc(II) complexes of isomers A and B were prepared and investigated by near-ultraviolet, visible, infrared, NMR and ESR measurements. The ligand-field band in the 15 000-30 000 cm⁻¹ region for the cobalt(III), nickel(II) and copper(II) complexes provided information on their geometry around the central metal atom. That is to say, the cobalt(III) complexes are subjected to the octahedral ligand-field with axial elongation. The copper(II) complexes and the nickel- (II) complex of isomer A are subjected to the square- planar ligand-field in these complexes. The ligand- field bands for the nickel(II)complex of isomer B display the square-planar-distorted octahedral equilibrium in the coordinating solvent. ESR measurements for the copper(II) complexes also presented the spin Hamiltonian parameters in accord with the square- planar coordination. A strong band appearing at ca. 3200 cm⁻¹ was assigned to the N-H stretching mode and this band was slightly shifted to lower frequency upon metal coordination. The vibrational spectra and the conductance data provided evidences for the formation of the complexes with perchlorate ion as the counter ion. ¹³C NMR suggest that the complexes of isomer A are the cis-syn-cis form and the complexes of isomer B are the cis-anti-cis form.     

Multifunctional catalysis by metal complexes: 1. Ester hydrolysis catalyzed by oximinatozinc(II) ions

Hydrolysis of p-nitrophenyl acetate catalyzed by a Zn(II) complex of 2-acetylpyridineketoxime or 2-pyridinecarboxaldoxime was studied as a model of multifunctional catalysis by metalloproteases. The reaction proceeded exclusively through the formation of an acylcatalyst intermediate under the experimental conditions, and both the formation and the breakdown of the acyl intermediate were much faster than the spontaneous reaction. The metal ion, the metal-bound water molecule or hydroxide ion, the oximate ion, and general bases contributed to the multifunctional catalysis in ester hydrolysis by the oximinatozinc(II) ions.     

The bioremediator glycerophosphodiesterase employs a non-processive mechanism for hydrolysis

Glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes is a binuclear metallohydrolase that catalyzes the breakdown of a broad range of phosphate ester substrates, and it is of interest for its potential application in the destruction of organophosphate nerve agents and pesticides. The reaction mechanism of GpdQ has been proposed to involve a nucleophilic attack by a terminally bound hydroxide molecule. The hydroxide species bridging the two metal ions is suggested to activate the nucleophile, thus favoring a sequential rather than a processive mechanism of action. Here, the hydrolysis of the two ester bonds in the substrate bis(para-nitrophenyl) phosphate (bpNPP) is probed using ³¹P NMR. The kinetic rates measured compare well with those determined spectrophotometrically. Furthermore, the data indicate that the diester bonds are cleaved in two separate (non-processive) reactions, indicating that only a single nucleophile (the terminal hydroxide molecule) is likely to be employed as a nucleophile for GpdQ.     

Metal Ion Catalysis in the Hydrolysis of Esters of 2-Hydroxy-1,10-phenanthroline: The Effects of Metal Ions on Intramolecular Carboxyl Group Participation

Rate constants have been determined for hydrolysis of the acetate, glutarate, and phthalate monoesters of 2-hydroxy-1,10-phenanthroline in water at 30°C and μ = 0.1 M with KCl. The hydrolysis reactions of the esters are hydroxide ion catalyzed at pH > 9. The phthalate and glutarate monoesters have in addition pH-independent reactions from pH 5.5 to 9 that involve intramolecular participation by the neighboring carboxylate anion. The pH-independent reaction of the glutarate monoester is 5-fold faster than that of the phthalate monoester. The plots of log k_obsd vs pH for hydrolysis of the carboxyl substituted esters are bell shaped at pH < 5, which indicates a rapid reaction of the zwitterionic species (carboxyl anion and protonated phenanthroline nitrogen). The divalent metal ions, Cu²⁺, Ni²⁺, Zn²⁺, and Co²⁺, complex strongly with the esters; saturation occurs at metal ion concentrations less than 0.01 M. The 1:1 metal ion complexes have greatly enhanced rates of hydrolysis; the second-order rate constants for the OH⁻ reactions are increased by factors of 10⁵ to 10⁸ by the metal ion. The pH-rate constant profiles for the phthalate and glutarate ester metal ion complexes have a sigmoidal region below pH 6 that can be attributed to a metal ion-promoted carboxylate anion nucleophilic reaction. The carboxyl group reactions are enhanced 10² - to 10³ -fold by the metal ions, which allows the neighboring group reaction to be competitive with the favorable metal ion-promoted OH⁻ reaction at pH < 6, but not at pH > 6. The half-lives of the pH-independent neighboring carboxyl group reactions of the Cu(II) complexes at 30°C are l2 s. The other metal ion complexes are only slightly less reactive (half-lives vary from 2.5 to 40 s). These are the most rapid neighboring carboxyl group reactions that have been observed in ester hydrolysis.     

Crystal structure and properties of the copper(II) complex of sodium monensin A

The preparation and structural characterization of a new copper(II) complex of the polyether ionophorous antibiotic sodium monensin A (MonNa) are described. Sodium monensin A binds Cu(II) to produce a heterometallic complex of composition [Cu(MonNa)₂Cl₂]·H₂O, 1. The crystallographic data of 1 show that the complex crystallizes in monoclinic space group C2 with Cu(II) ion adopting a distorted square-planar geometry. Copper(II) coordinates two anionic sodium monensin ligands and two chloride anions producing a neutral compound. The sodium ion remains in the inner cavity of the ligand retaining its sixfold coordination with oxygen atoms. Replacement of crystallization water by acetonitrile is observed in the crystal structure of the complex 1. Copper(I) salt of the methyl ester of MonNa, 2, was identified by X-ray crystallography as a side product of the reaction of MonNa with Cu(II). Compound 2, [Me-MonNa][H-MonNa][CuCl₂]Cl, crystallizes in monoclinic space group C2 with the same coordination pattern of the sodium cation but contains a chlorocuprate(I) counter [CuCl₂]⁻, which is linear and not coordinated by sodium monensin A. The antibacterial and antioxidant properties as two independent activities of 1 were studied. Compound 1 is effective against aerobic Gram(+)-microorganisms Bacillus subtilis, Bacillus mycoides and Sarcina lutea. Complex 1 shows SOD-like activity comparable with that of the copper(II) ion.     

2-Benzoylpyridine and copper(II) ion in basic medium: Hydroxide nucleophilic addition stabilized by metal complexation, reactivity, crystal structure, DNA binding study and magnetic behavior

On reaction of 2-benzoylpyridine (Bzpy) with copper(II) ion, different types of copper(II) complexes have been isolated in pure form depending upon the counter anion of the copper(II) salts used as reactant and the pH of the medium. Mono-nuclear copper(II) complexes of formula [Cu(Bzpy)₂(ClO₄)₂] (1) and [Cu(Bzpy)₂(H₂O)₂](NO₃)₂ (2) were formed with copper(II) perchlorate and nitrate, respectively. On the other hand, following a similar reaction type in presence of alkali, we obtained the dinuclear copper(II) complex [Cu₂(Bzpy)₂{BzOpy}₂(H₂O)](ClO₄)₂ (3) containing the hydroxy-2-pyridylphenylmethanolato (BzOpy)⁻ anion, achieved through the nucleophilic addition of the hydroxide to the carbonyl group of Bzpy, which is stabilized by metal complexation. However, this behavior was not recorded with copper(II) nitrate. The complexes were characterized by physicochemical and spectroscopic tools along with structural characterization by single crystal X-ray diffraction analysis. The interaction of dinuclear copper(II) complex 3 with calf thymus DNA (CT-DNA) has been investigated by using absorption and emission spectral studies and the binding constant (K_b) and the linear Stern-Volmer quenching constant (K_sv) have been determined. Complex 3 was active to oxidize the catechol to the corresponding quinone in MeCN medium via complex-catechol intermediate. Magnetic behavior for 3 is typical for uncorrelated spins down even up to 2 K.     

The palladium(II) promoted hydrolysis of the methyl esters of glycyl-L-leucine,glycyl-L-alanine and L-alanylglycine

The palladium(II)-promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine have been studied at 25 °C and I=0.1 M in the pH range 4–5. At a 1:1 metal to ligand ratio the peptide esters act as tridentate ligands, donation occurring via the terminal amino group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Due to the high Lewis acidity of Pd(II) rapid hydrolysis of the ester function by water and hydroxide ion occurs. Rate constants k_OH and k_H2O have been obtained for base hydrolysis and water hydrolysis of the coordinated peptide esters at 25 °C. The rate constants for base hydrolysis are 3.4 X 10⁶ M⁻¹ s⁻¹ (L-alaglyOMe), 6.4 X 10⁶ M⁻¹ s⁻¹ (gly-L-alaOMe) and 2.3 X 10⁷ M⁻¹ s⁻¹ (gly-L-leuOMe). Base hydrolysis of the coordinated peptide esters is at least 10⁶ times that of the free unprotonated ligand. Activation parameters have been obtained for both water and base hydrolysis of the Pd(II) complex of methyl L-alanylglycinate and possible mechanisms for the hydrolyses are considered.     

Reactions of Tp–Os nitrido complexes with the nucleophiles hydroxide and thiosulfate

The reaction between TpOs(N)Cl₂ (1) [Tp = hydrotris(1-pyrazolyl)borate] and aqueous (ⁿBu₄N)(OH) in THF-d₈ forms the nitrosyl complex TpOs(NO)Cl₂ (5) among other products, suggesting an initial hydroxide attack at the nitrido ligand. In contrast, the reaction of the acetate complex TpOs(N)(OAc)₂ (2) with NaOH in Me₂CO/H₂O yields the osmium bis-hydroxide complex TpOs(N)(OH)₂ (3), which has been structurally characterized by single-crystal X-ray diffraction. Acetate for hydroxide exchange could occur by ligand substitution or by nucleophilic attack at the carbonyl carbon of the acetate ligands (saponification). Reacting 2 with Na¹⁸OH in H₂¹⁸O/CD₃CN yields predominantly doubly ¹⁸O-labeled TpOs(N)(¹⁸OH)₂ (3-¹⁸O₂) and unlabeled acetate, by ESI/MS and ¹³C{¹H} NMR. This indicates that hydroxide reacts by substitution rather than by attack at the ligand. The reaction of 2 with the softer nucleophile thiosulfate occurs at the nitrido ligand, giving the thionitrosyl complex TpOs(NS)(OAc)₂ (4). Reacting 4 with NaOH in (CD₃)₂CO/D₂O also generates the bis-hydroxide complex 3.     

Amine-catalyzed elimination of beta-phosphoric esters from aldehydes derived from ribonucleic acid and model substrates.

The kinetics have been measured for several steps of the diamine-catalyzed elimination of the terminal nucleoside from periodate-oxidized RNA and from several model substrates. The general-acid-catalyzed, rate-determining step has a k_HA of 0.13 M^?1 min^?1 (HA = RNH₃⁺) for primary amines, and the specific-base-catalyzed reaction has a k_HH of 0.35 min^?1 (0.2 mm RNA) with ornithine catalysis and a k_HH of 0.077 min^? (0.2 mm RNA) with lysine catalysis. Lysine has a third catalysis component, with a k_AH of 12 min^?1 M^?2. The diamino acid α,γ-diaminobutyrate is not effective as a catalyst, due to cyclic gem diamine formation. Substituents on the 5′-phosphoric ester group do not affect the kinetics unless the substituent is a proton (e.g., as in AMP); thus, AMP is not an accurate model for this type of sequential degradation of RNA.There are two degradative pathways, the β-elimination path and a route that involves cleavage of the C-1′-0-C-4′ ether linkage before the phosphoric ester is eliminated. The direct β-elimination path predominates below pH 7.5, with a maximum near pH 6, and yields only one set of end products. Because of its rapid and predictable course, the latter reaction is preferred for sequential degradation of RNA. The structure of the catalytically active intermediate (general-acid-catalyzed reaction series) involves the primary amino group of ornithine (lysine) condensed with the dialdehyde terminus to form the carbinolamine, aldimine, and enamine intermediates leading to the elimination.The ether cleavage path is controlled by a specific-base (k_HB) intramolecular catalysis above pH 7, and a side reaction leads to lowered yields of phosphoric ester cleavage. A primary amine group is required, since 3-dimethylamino propylamine does not catalyze the ether cleavage.     

Synthesis and in vitro anti-tumor activity of carboranyl levodopa

Reactions of closo-1-Me-2-Iodobutyl-1,2-closo-dicarborane, 1-Me-2-I(CH₂)₄-C₂B₁₀H₁₀, with l-dopa methyl ester can produce carboranyl l-dopa methyl esters in 54% yield in the presence of sodium hydroxide. The appended closo-carboranes can be decapitated with sodium hydroxide in a mixed solvent of ethanol and deionized water to produce highly water-soluble carboranyl levodopa in 64% yield. All the new compounds were characterized by ¹H, ¹³C, ¹¹B NMR, FT-IR spectroscopy and elemental analysis. The highly water soluble carboranyl levodopa 4 shows promising efficacy of anti-tumors in vitro in the presence of slow neutron beams.     

Synthesis, crystal structure, spectroscopic and magnetic properties of copper(II) complexes with 2-(2-pyridyl)imino-N-(2-thiazolin-2-yl)thiazolidine (PyTT)

The copper(II) complexes [Cu(PyTT)₂(H₂O)](NO₃)₂ (A) and [CuCl₂(μ-PyTT)₂CuCl(H₂O)]Cl · 3H₂O (B) were synthesized and characterized by single crystal X-ray diffraction, IR spectroscopy, UV-Vis-NIR diffuse reflectance and magnetic susceptibility measurements. In the mononuclear compound A the copper ion is in a distorted square pyramidal geometry, with the equatorial plane formed by two thiazoline nitrogen atoms, one imino nitrogen atom and one water molecule, whereas the axial site is occupied by one imino nitrogen atom. The compound B is dinuclear and both Cu(II) centres present environments that can be described as slightly distorted square pyramidal geometries. The observed molar magnetic susceptibility for A (μ=2.13 BM) allows to exclude metal-metal interactions, supporting a monomeric structural formulation for this compound. In compound B, magnetic susceptibility measurements in the temperature range 6.2-288 K show an intradimer antiferromagnetic interaction (J=−11.8 cm⁻¹).     

Chlorido-bridged polymeric and dinuclear copper(II) complexes with tridentate Schiff base: Synthesis, crystal structure and magnetic properties

Two copper(II) complexes, [Cu(qsal)Cl](DMF) (1) and [Cu₂(qsalBr)₂Cl₂](DMF) (2), with tridentate Schiff base ligands, 8-(salicylideneamino)quinoline (Hqsal) and 8-(5-bromo-salicylideneamino)quinoline (HqsalBr), respectively, were synthesised and structurally characterized. Each copper(II) ion in the two complexes is in a distorted square pyramidal N₂OCl₂ environment. Complex 1 exists as a polymeric species via equatorial-apical chloride bridges, whereas 2 is a di-chlorido-bridged dinuclear complex, where each bridging chloride simultaneously occupies an in-plane coordination site on one copper(II) ion and an apical site on the other copper(II) ion. Variable-temperature magnetical susceptibility measurements on the two complexes in the temperature range 2-300 K indicate the occurrence of intrachain ferromagnetic (J = +6.58 cm⁻¹) and intramolecular antiferromagnetical (J = −6.91 cm⁻¹) interactions.     

Efficient microwave-assisted direct radiosynthesis of [18F]PR04.MZ and [18F]LBT999: Selective dopamine transporter ligands for quantitative molecular imaging by means of PET

PR04.MZ 8-(4-fluoro-but-2-ynyl)-3-p-tolyl-8-aza-bicyclo[3.2.1]octane-2-carboxylic acid methyl ester (1) and LBT999 8-((E)-4-fluoro-but-2-enyl)-3b-p-tolyl-8-aza-bicyclo[3.2.1]octane-2β-carboxylic acid methyl ester (2) are selective dopamine reuptake inhibitors, derived from cocaine. Compounds 1 and 2 were labelled with fluorine-18 at their terminally fluorinated N-substituents employing microwave enhanced direct nucleophilic fluorination. K[¹⁸F]F^? Kryptofix^®222 cryptate, tetrabutyl ammonium [¹⁸F]fluoride and caesium [¹⁸F]fluoride were compared as fluoride sources under conventional and microwave enhanced conditions. Fluorination yields were remarkably increased under microwave irradiation for all three fluoride salts. Radiochemically pure (>98%) [¹⁸F]PR04.MZ (0.95–1.09 GBq, 42–135 GBq/μmol) was obtained within 34–40 min starting from 3.0 GBq [¹⁸F]fluoride ion in 32–36% non-decay-corrected overall yield using K[¹⁸F]F^?Kryptofix^®222 cryptate in MeCN.     

The palladium(II) promoted hydrolysis of methyl,ethyl and isopropyl glycylglycylglycinate

The palladium(II) promoted hydrolysis of the ester function of methyl, ethyl and isopropyl glycylglycylglycinate has been studied in detail in the pH range 4–5. The tripeptide esters interact with [PdCl₄]²⁻ at a 1:1 metal-to-ligand ratio to give the complex I in which the α-amino group, two deprotonated amide nitrogen atoms and the alkoxy carbonyl group of the ester act as donors. Rate constants have been determined for hydrolysis of the ester function by water and hydroxide ion and activation parameters obtained for the hydrolysis of the methyl ester. At 25 °C base hydrolysis of the coordinated ester is 10⁶ fold that of the free ester ligand. Mechanisms for the reactions are considered.     

Über die orthoesterbildung als konkurrenzreaktion zur glykosylierung

1,2-(Alkyl orthoacetates) (orthoesters) were obtained in good yields from acetylated cis-glycosyl halides and alcohols in tetrahydrofuran in the presence of silver salicylate. The scope and the possible mechanism of the reaction were investigated. The presence of an intermediate oxonium ion (13) derived from the sugar derivative and tetrahydrofuran was deduced, from the occurrence of 4-bromobutyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside as a by-product of the reaction, and from semi-quantitative kinetic investigations. After an intramolecular reaction of this ion to form an acetoxonium ion (14), the reaction with an alcohol yields the orthoester (e.g.1-7). In a similar manner, and in concurrence with direct glycosylation, ortho esters and products arising from orthoesters may be formed in the usual glycoside synthesis, using the reaction just described with the solvent or other nucleophiles.     

Metal-directed synthesis of a chiral acyclic pentaamine and pendant-arm macrocyclic hexaamine derived from an amino acid

l-3-Phenylpropane-1,2-diamine (dapp) was prepared by a three-step synthesis based on l-phenylalanine and characterized, including determination of stability constants with M²⁺ ions (Ni, Cu, Zn, Cd). The reaction of L-3-phenylpropane-1,2-diamine as the [Cu(dapp)₂]²⁺ complex ion with formaldehyde and nitroethane in basic solution yields the acyclic (5-methyl-5-nitro-1,9-diphenyl-3,7-diazanonane-1,9-diamine)copper(II) complex ion, [Cu(1)]²⁺, as the major product. In addition, small amounts of the macrocyclic complex ion (2,10-diphenyl-6,13-dimethyl-6,13-dinitro-1,4,8,11-tetraazacyclotetradecane)copper(II), [Cu(2)]²⁺, form. Reduction of the [Cu(1)]²⁺ ion with zinc in aqueous acid yields the acyclic polyamine 5-methyl-1,9-diphenyl-3,7-diazanonane-1,5,9-triamine (3), an analogue of the previously reported pentaamine 5-methyl-3,7-diazanonane-1,5,9-triamine. Using the bis(l-3-phenylpropane-1,2-diamine)palladium(II) as precursor and an excess of other reagents, the macrocyclization reaction to produce [Pd(2)]²⁺ proved more successful. Reduction and recomplexation to copper(II) allowed isolation of the 2,9-dibenzyl-6,13-diammonio-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane)copper(II) ion, [Cu(4H₂²⁺)]⁴⁺. The acyclic complex [Cu(1)]²⁺ promotes the hydrolytic cleavage of plasmid DNA modestly; a mechanism to support this observation is presented.     

The copper-molybdenum antagonism in ruminants. III. Reaction of copper(II) with tetrathiomolybdate(VI)

The stoichiometry of the reaction between Cu(II) and MoS₄²⁻ in neutral aqueous solution was observed to proceed with a 1.5:1 Cu:Mo ratio. The reaction results in the reduction of Cu(II) and the quantitative formation of an insoluble solid. The results contrast with an earlier report of a 1:1 stoichiometric ratio, this latter ratio was, however, observed with Cu^II(albumin) as reactant, in this case no precipitate was observed. The insoluble products were examined by a number of spectroscopic techniques and by X-ray power diffraction and elemental analysis. Two products were identified. Solid A is isostructural with the known (NH₄)Cu^IMoV^VIS₄, i.e. has the composition M^ICuMoS₄, M^I= NH₄⁺, Na⁺ or Et₄N⁺. Solid B has the approximate composition CuMoS₄O_x, x=2−3 and contains Cu(I) and Mo(V) centres. Formation of compound B therefore involves an unusual internal two-electron redox process. The reaction and products are of particular biological significance.     

Comparative study of the influence of the metal centres: Fe(III), Cu(II) and Zn(II), on the ring opening and oxidative dehydrogenation reactions occurring in a coordinated imidazolidine ligand

Condensation of 2-pyridinecarboxaldehyde and 1,9-bis-(2′-pyridyl)-2,5,8-triazanonane, L¹, yields 1-[3-aza-4-(2-pyridyl)butyl]-2-(2-pyridyl)-3-[(2-pyridyl)methyl]imidazolidine, L², as proven by NMR solution spectra. When L² is reacted with Fe(III) in different alcohols, an imidazolidine ring opening and an oxidative dehydrogenation reaction occur resulting in new complexes of the type: [Fe^IIL^n′]²⁺. Compound 1 with a coordinated L^3′ ligand was obtained in n-propanol as a solvent. Compounds 2, 3 and 4 were obtained with L^4′, L^5′ and L^6′ when iso-propanol, n-butanol and iso-butanol were used as solvent, respectively. The structures for 1, 2, 3 and 4 were determined by NMR solution spectra and additionally by X-ray crystallography in the case of the n-butoxy derivative 3. When Cu(II) was used, the hexadentate ligand L² undergoes also an imidazolidine ring opening reaction on complex formation, however, now generating the well-known pentadentate ligand L¹ that is coordinated to the metal ion, 7. Evidence is again provided by the corresponding X-ray structure. With Zn(II) the initial structure of L² is maintained and in this case L² functions as a tetradentate, 5, or bis-tridentate ligand, 6, depending on whether the stoichiometric ratio M:L was 1:1 or 2:1, respectively. This has been proven by a solid-state X-ray structure analysis as well as by NMR solution spectra. The ring opening reaction in the presence of Fe(III) can be explained as a result of a higher Lewis acidity of this metal centre, which decreases the electronic density on the nitrogen atom of the imidazolidinic cycle, thus weakening the nitrogen-carbon bond, favouring the nucleophilic attack on the carbon atom by alcohols and producing a more stable hexacoordinated species. Electrochemical evidence is provided in order to support this reaction mechanism.