Reduction of [(FeTPP)2O]+SbF6− by iodide ion in dichloromethane

The oxidation in dichloromethane of the iodide ion of cetyltrimethyl ammonium iodide to iodine by the compound consisting of the hexafluoroantimonate anion and the dimeric iron tetraphenylporphyrin (bridged by oxygen) cation, [Fe(TPP)₂O]⁺SbF₆⁻, has been studied spectrophotometrically using the stopped-flow method. At 273 K, d[Fe(TPP)₂O]/dt is equal to 8 × 10⁴[[Fe(TPP)₂O]⁺]-[1⁻] mol dm⁻³ s⁻¹; from this and other measurements E_a is estimated to be 13 kJ mol⁻¹. These results will be compared with other relevant kinetic data, and a possible reaction mechanism will be considered.     

Oxidation of copper(II)-coordinated diethylenetriamine by hexacyanoferrate(III) ion. A kinetic investigation

The stoichiometry and kinetics of the reaction between [Cu(dien)(OH)]⁺ and [Fe(CN)₆]³⁻ in aqueous alkaline medium are described. The rate equation − (d[Fe(III)]/dt = {k₁[OH⁻]²[[Cu(dien)(OH)]⁺] + k₂[OH⁻] × [[Cu(dien)(OH)]⁺]²}([Fe(III)]/[Fe(II)]) (Fe(III) = [Fe(CN)₆]³⁻; Fe(II) = [Fe(CN)₆]⁴⁻, the 4:4:1 OH⁻/Fe(III)/[Cu(dien)(OH)]⁺ stoichiometric ratio and the nature of the ultimate products identified in the reaction solution suggest the fast formation of a doubly deprotonated Cu(III)-diamido complex which slowly undergoes an internal redox process where the ligand is oxidised to the Schiff base H₂NCH₂CH₂NCHCHNH.The [[Cu(dien)(OH)]⁺]² term in the rate equation is explained with the formation of a transient μ-hydroxo mixed-valence Cu dimer. A two-electron internal reduction of the Cu(III) complex yielding a Cu(I) intermediate is suggested to account for the presence of monovalent copper in a precipitate which forms at relatively high reactant concentrations and in the absence of dioxygen.     

Low temperature electrochemistry of metalloporphyrins in dichloromethane: characterization of transient species

Cyclic voltammetry in dichloromethane at temperatures down to −90 °C has been used to characterize Fe(TPP)Cl (TPP=tetraphenylporphyrin dianion) and the transient high-spin six-coordinate complex [Fe(TPPXMeIm)Cl] (MeIm=N-methylimidazole). It is shown that low temperature cyclic voltammetry (LTCV) in dichloromethane can give high quality results using standard equipment and electrode systems; IR drop is not a serious problem. At −85 °C the anion Fe(TPP)Cl⁻ slowly dissociates chloride and separate waves can be seen for the subsequent reduction of Fe(TPP)Cl⁻ and Fe(TPP); this is not observed at room temperature. In the presence of excess MeIm, the transient species [Fe(TPP)(MeIm)Cl] decays with a half life of ca. 10 ms at room temperature, but at −90 °C is sufficiently persistent to allow electrochemical characterization. Its reduction occurs at a potential ca. 130 mV negative of that for Fe(TPP)Cl and is chemically irreversible, rapidly converting to Fe(TPP)(MeIm)₂. The utility of low temperature electrochemistry for investigating unstable metalloporphyrins in relatively nonpolar solvents is discussed.     

Synthesis,characterization and antitumor activity of some metal complexes of 3- and 5-substituted salicylaldehyde o-hydroxybenzoylhydrazones

Complexes of Mn(II), Fe(III), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Pt(II) with 3- and 5-substituted salicylaldehyde o-hydroxybenzoylhydrazones (XSBH, X = H, 3-NO₂, 3-CH₃O, 5-Cl, 5-Br, 5-CH₃ or 5-NO₂) have been prepared and characterized by elemental analysis, conductance measurements, magnetic susceptibilities (from room temperature down to liquid nitrogen temperature) and spectral studies. These studies indicate the following structures: monomeric, high-spin, distorted octahedral for Mn(XSBH)₂; monomeric, high-spin, five-coordinate for Fe(XSBH)SO₄·H₂O; dimeric, high-spin phenoxide bridged, five-coordinate for Fe(XSBH)Cl; dimeric, high-spin five-coordinate for Co(XSBH)Cl·2H₂O; dimeric low-spin, five-coordinate for Ni(XSBH)Cl·2H₂O; dimeric, four-coordinate for Zn(XSBH); and a square-planar structure for M(XSBH)Cl·H₂O (M = Cu(II) or Pt(II).Intermolecular antiferromagnetic exchange interactions are present in Fe(III) complexes, where the exchange parameter (J) is ca. −8.0 cm⁻¹ for these complexes. The Fe(III) complexes exhibit asymmetric quadrupole split doublets in their ⁵⁷Fe Mössbauer spectra. The asymmetry is found to be temperature dependent with relatively symmetrical doublets seen at low temperature. The polycrystalline ESR spectra of Cu(II) complexes are isotropic and indicate a d_x²−y² ground state in square-planar stereo-chemistry. All these metal complexes have been screened for their antitumor activity against the P 388 lymphocytic leukaemia test system in mice and enhanced antitumor activity relative to the free ligand was found but no significant activity at the dosages used.     

Mixed ligand complexes of lanthanides with macrocyclic and open-chained polyaminopolycarboxylic acids and acetylacetone

Studies of mixed ligand complex formation stabilities and dissociation kinetics have been performed on lanthanide ions with macrocyclic and open- chained polyaminopolycarboxylic acids (i.e. DAPDA, DACDA, EDDA, and EDTA) and acetylacetone (acac). From UV spectroscopic evidence, it was found that Ln(DACDA)⁺ and Ln(EDTA)⁻ complexes do not form mixed ligand complexes with acac under the set conditions, i.e. pH = 7.2 and complex concentration of 1 x 10⁻⁴ M. On the other hand, formation of Ln(DAPDA)(acac) and Ln(EDDA)(acac)₂⁻ complexes were readily detectable. The mixed complex forma- tion constants,β₁, for the equilibrium Ln(L)⁺ + acac⁻ ⇌ Ln(L)(acac), and β₂, for the equilibrium Ln(L)⁺ + 2 acac⁻ ⇌ Ln(L)(acac)₂⁻ were determined by potentiometric titration technique where possible. It was found that β₁ values were in general greater for Ln(EDDA)⁺ complexes than for Ln(DAPDA)⁺ complexes indicating the resulting reduced charge density at the lanthanide ion of Ln(DAPDA)⁺ and that less space is available for the acetylacetone moiety to coordinate to the Ln(DAPDA)⁺ complexes due to the large size and the greater number of coordination atoms of DAPDA. The hydrolysis constants of Ln(EDDA)(H₂O)_n⁺ species were also determined and were found to be increasing with increasing atomic number of Ln. Attempts to measure the acid assisted mixed ligand complex dissociation rates by a stopped-flow spectrophotometer were not fruitful due to the much faster rates.     

Correlated behavior of the EPR signal of cytochrome b-559 heme Fe(III) ligated by OH− and the multiline signal of the Mn cluster in PS-II membrane fragments

EPR signals of Cyt b-559 heme Fe(III) ligated by OH⁻ and the multiline signal of the Mn cluster in PS-II membrane fragments have been investigated. In 2,3-dicyano-5,6-dichloro-p-benzoquinone-oxidized PS-II membrane fragments the light-induced decrease of the EPR signal of the heme Fe(III)-OH⁻ is accompanied by the appearance of the EPR multiline signal of the Mn cluster. Addition of F⁻ ions, which act as a stronger ligand for heme Fe(III) than OH⁻, decreases to the same extent the dark- and light-induced signal of the heme Fe(III)-OH⁻ and the light-induced multiline signal of the Mn cluster. These results are discussed in terms of the light-induced formation of a bound OH′ radical shared between the Cyt b-559 heme Fe and the Mn cluster as a first step of water oxidation.     

The interaction of D-galactonic acid with CrVI and CrIII. Structure,stability and physical properties of CrIII—aldonate complexes

The redox reaction between D-galactonic acid and potassium chromate yields ((lyxonateH₋₁)(galactonateH₋₁)Cr(OH₂))K·H₂O_n, with both aldonate molecules acting as bidentate ligands with the carboxylate and one alkoxo function as the donor sites. The shift of the CO₂^{poststaggered−} stretching vibration towards lower frequencies upon coordination and the high value of Δv indicate that the carboxylate acts as a monodentate donor site. Magnetic susceptibility data for the compound in the temperature range 3–300 K exhibit a drop in the effective magnetic moment with temperature below 70 K, which is indicative of antiferromagnetic interactions between the Cr^III centres. The molar magnetic susceptibility versus temperature plot could be fitted with the Fisher Hamiltonian for the case of infinite chains, equation-modified for the presence of monomeric species. The EPR and UV-Vis spectroscopic studies reveal that, in solution, the complex retains the distorted octahedral local coordination geometry. The ((lyxonateH₋₁)(galactonateH₋₁)Cr(OH₂))K_n dissociates slowly in aqueous solution but faster at high [H⁺], because of the rapid protonation of the alkoxo bridges linking the monomeric units. The potentiometric evaluation of the closely related binary system Cr^III-d-galactonate shows that the (Cr(galactonateH_−n)₂)^{1 − 2n} complexes are the major species in the 4–12 pH range, when a 1:2 Cr^III:ligand ratio is used. ¹³C NMR reveals that theCO₂^{poststaggered−} group is one of the coordination sites of the ligand.     

Synthesis and characterization of tetraphenylporphyrin iron(III) complexes with substituted phenylcyanamide ligands

Several five coordinate complexes of [(TPP)Fe^III(L)] in which TPP is the dianion of tetraphenylporphyrin and L is the monoanion of phenylcyanamide (pcyd) (1), 2,5-dichlorophenylcyanamide (2,5-Cl₂pcyd) (2), 2,6-dichlorophenylcyanamide (2,6-Cl₂pcyd) (3), and 2,3,4,6-tetrachlorophenylcyanamide (2,3,4,6-Cl₄pcyd) (4) have been prepared by the reaction of [(TPP)Fe^IIICl] with appropriate thallium salt of phenylcyanamide. Each of the complexes has been characterized by IR, UV-Vis and ¹H NMR spectroscopic data. Dark red-brown needles of [(TPP)Fe^III(2,6-Cl₂pcyd)] (C₅₁H₃₁Cl₂FeN₆ · CHCl₃) crystallize in the triclinic system. The crystal structure of Fe(III) compound shows a slight distortion from square pyramidal coordination with the 2,6-dichlorophenylcyanamide anion in the axial position through nitrile nitrogen atom. Iron atom is 0.47(1) Å out of plane of the porphyrin toward phenylcyanamide ligand. In non-coordinating solvents, such as benzene or chloroform, these complexes exhibit ¹H NMR spectra that are characteristic of high-spin (S = 5/2) species. The X-ray crystal structure parameters are also consistent with high-spin iron(III) complexes. The iron(III) phenylcyanamide complexes are not reactive toward molecular oxygen; however, these complexes react with HCl and produce TPPFe^IIICl.     

Synthesis,photophysical properties,and photodynamic activity of positional isomers of TFPP-glucose conjugates

The synthesis and characterization of a ‘complete set’ of positional isomers of tetrakis(perfluorophenyl)porphyrins (TFPP)-glucose conjugates (1OH, 2OH, 3OH, 4OH, and 6OH) are reported herein. The cellular uptake and photocytotoxicity of these conjugates were examined in order to investigate the influence of location of the TFPP moiety on the d-glucose molecule on the biological activity of the conjugates. An In vitro biological evaluation revealed that the certain of these isomers have a greater effect on cellular uptake and cytotoxicity than others. The TFPP-glucose conjugates 1OH, 3OH, and 4OH were found to exert exceptional photocytotoxicity in several types of cancer cells compared to 2OH and 6OH substituted isomers.     

The uncatalysed and the copper(II) promoted hydrolysis of 4-nitrophenyl L-leucinate

The uncatalysed hydrolysis of 4-nitrophenyl L-leucinate has been studied in detail over a range of pH and temperature at I=0.1 M (KNO₃). Base hydrolysis of the ester is strongly promoted by copper(II) ions. Rate constants have been obtained for the following reactions (where EH⁺ is the N- protonated ester and E is the free base form) EH⁺ + OH⁻ → products E + OH⁻ → products E + H₂O → products CuE²⁺ + OH⁻ → products Base hydrolysis of the copper(II) complex CuE²⁺ is 3.8 × 10⁵ times faster than that of E and 75 times faster than that of EH⁺ at 25 °C and I=0.1 M. Activation parameters for these reactions have been determined and possible mechanisms are considered.     

The hemoglobin of the crossopterygian fish, Latimeria chalumnae (Smith). Subunit structure and oxygen equilibria

There are five oxidation-reduction states of horseradish peroxidase which are interconvertible. These states are ferrous, ferric, Compound II (ferryl), Compound I (primary compound of peroxidase and H₂O₂), and Compound III (oxy-ferrous). The presence of heme-linked ionization groups was confirmed in the ferrous enzyme by spectrophotometric and pH stat titration experiments. The values of pK were 5.87 for isoenzyme A and 7.17 for isoenzymes (B + C). The proton was released when the ferrous enzyme was oxidized to the ferric enzyme while the uptake of the proton occurred when the ferrous enzyme reacted with oxygen to form Compound III. The results could be explained by assuming that the heme-linked ionization group is in the vicinity of the sixth ligand and forms a stable hydrogen bond with the ligand.The measurements of uptake and release of protons in various reactions also yielded the following stoichiometries: Ferric peroxidase + H₂O₂ → Compound I, Compound I + e^? + H⁺ → Compound II, Compound II + e^? + H⁺ → ferric peroxidase, Compound II + H₂O₂ → Compound III, Compound III + 3e^? + 3H⁺ → ferric peroxidase.Based on the above stoichiometries and assuming the interaction between the sixth ligand and heme-linked ionization group of the protein, it was possible to picture simple models showing structural relations between five oxidation-reduction states of peroxidase. Tentative formulae are as follows: [Pr·Po·Fe-(II) $\text{\$}\text{?}$PrH⁺·Po·Fe(II)] is for the ferrous enzyme, Pr·Po·Fe(III)OH₂ for the ferric one, Pr·Po·Fe(IV)OH^? for Compound II, Pr(OH^?)·Po⁺·Fe(IV)OH^? for Compound I, and PrH⁺·Po·Fe(III)O₂^? for Compound III, in which Pr stands for protein and Po for porphyrin. And by Fe(IV)OH^?, for instance, is meant that OH^? is coordinated at the sixth position of the heme iron and the formal oxidation state of the iron is four.     

The thermodynamics of the complexation of lanthanides by 1-hydroxy-4,7-disulfo-2-naphthoic acid

The thermodynamic parameters, log β, ΔH and ΔS, for formation of lanthanide-1-hydroxy-4,7- disulfo-2-naphthoic acid complexes have been determined at 25 °C in 0.10 M NaClO₄ solutions by potentiometric and calorimetric titrations. Under the experimental conditions, the data can be explained with the formation of LnL⁻, LnL₂⁵⁻ and LnHL complexes (H₂L²⁻ = 1-hydroxy-4,7-disulfo-2- naphthoic acid anion). At pH < 3 the LnHL complex is the major species, whereas by increasing pH the formation of LnL_n³⁻⁴ⁿ complexes becomes more important. The data are compared to the comparable data for complexing by aromatic carboxylic acids.     

Dissociation rates of axially coordinated imidazoles and formation constants of low spin ferric complexes derived from tetraphenylporphyrin and tetramesitylporphyrin

The dissociation rates of axially coordinated imidazole in bis-ligated low spin ferric complexes of synthetic porphyrins such as tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP) were measured by NMR method. In both TPP and TMP complexes, the axial lability of imidazoles increased in the order 1-methylimidazole < 2-methylirnidazole < 2-ethylimidazole ∼ 1,2-dimethylimidazole. The results were explained in terms of the steric repulsion between the 2-alkyl group of imidazole and the porphyrin ring. The dissociation rates of TPP complexes were then compared with those of TMP complexes carrying the same axial ligands. In every case examined, imidazole dissociated faster from the TPP complex than from the TMP complex. The results were ascribed to the stability of the bis-ligated TMP complex relative to the corresponding TPP complex; the formation constant of the TMP complex having 2-Melm as axial ligand was larger than that of the corresponding TPP complex by a factor of c. 600. A hypothesis has been proposed to explain the stability of the sterically hindered porphyrin complex relative to the less hindered complex.     

Reactions of Cr(III) tetraphenylporphyrin complex with superoxide ion: ESR evidence for the formation of a new superoxide adduct of Cr(IV) porphyrinate and its reactive character

The electron spin resonance (ESR) spectrum of the reaction product of superoxide ion, O₂⁻, with chloro-5,10,15,20-tetraphenylporphyrinatochromium(III) [Cr(III)(TPP)Cl] shows strong hyperfine interactions with the metal nucleus and the metal ligand, indicating the formation of a superoxide adduct, Cr(IV)(TPP)(Cl)(O₂⁻). The formation of this superoxide adduct was also confirmed by UV-Vis spectroscopy. The reactive character of this superoxide adduct was investigated by ESR spectrometry. It was found that Cr(IV)(TPP)(Cl)(O₂⁻) can oxidize t-butylamine and triphenylphosphine to give the corresponding radical species, respectively, but not pyridine, 4-cyanopyridine or imidazole. These results indicate that the reactive character of Cr(IV)(TPP)- (Cl)(O₂⁻) resembles that of the free superoxide ion.     

Synthesis and structure study of some catecholase-mimetic iron complexes

The biomimetic dioximatoiron complexes [Fe(Hdmed)]⁺ and [Fe(H₂dmdt)]²⁺ act as catecholase models but the latter also catalyzes the intradiol cleavage of 3,5-ditert-butylcatechol. This remarkable selectivity was ascribed to the stable, hydrogen-bonded, macrocyclic coordination sphere of [Fe(Hdmed)]⁺, precluding bidentate catechol binding, as opposed to the less stable, mobile structure of [Fe(H₂dmdt)]²⁺. This hypothesis is supported by the structures of [Fe(Hdmed)]⁺ and [Fe(H₂dmdt)]²⁺ determined by X-ray diffraction in 1 M methanol solution. This study is a demonstrative example for the capabilities of the X-ray diffraction technique applied for complexes in the solution phase. The characteristic bond lengths of the complex and the number of solvent molecules in the solvation shell have been determined. To examine the structure of the complexes, gas phase density functional geometry optimizations were performed by using the adf-2004.01 and gaussian-98 packages. The PCM methodology was applied to estimate the effect of solvent on the relative energetics of the low-spin and high-spin electronic states of the complexes. The calculations confirmed that the electronic ground state corresponds to the triplet state for both complexes.     

Determination of the equilibrium constant for binding hydroxide to tetraazamacrocyclic—Nickel(II) complexes

The pK_b for the equilibria NiL(OH)⁺ ⇄ NiL²⁺ + OH⁻ for L1,4,7,10-tetraazacyclotridecane, 1,4,8, 12-tetraazacyclopentadecane, C-β-racemic-5,7,7,12, 14,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane and C-β-racemic-1,4,5,7,7,8,11,12,14,14,-decamethyl- 1,4,8,112-tetraazacyclotetradecane are 0.95, 1.9, 0.2 and 0.65 respectively. The results are compared with data for analogous complexes reported earlier. The results indicate that the main factors affecting these equilibrium constants are the in plane ligand field strengths of the square planar complexes and steric factors.     

Kinetics and mechanism of aluminum(III)/siderophore ligand exchange: mono(deferriferrioxamine B)aluminum(III) formation and dissociation in aqueous acid solution

The kinetics and mechanism of a linear trihydroxamic acid siderophore (deferriferrioxamine B, H₄DFB⁺) ligand exchange with Al(H₂O)₆³⁺ to form mono(deferriferrioxamine B)aluminum(III) (Al(H₂O)₄H₃DFB)³⁺ have been investigated at 25 °C over the [H⁺] range 0.001−1.0 M and I = 2.0 M (HClO₄/NaClO₄) by ²⁷Al NMR. Kinetic results are consistent with Al(H₂O)₄(H₃DFB)³⁺ formation and dissociation proceeding through a parallel path mechanistic scheme involving Al(H₂O)₆³⁺(k₂/k₋₁) and Al(H₂O)₅(OH)²⁺(k₂/k₋₂) where k₁ = 0.13 M⁻¹ s⁻¹, k₋₁ = 8.7 × 10⁻³ M⁻¹ s⁻¹, k₂ = 2.7 × 10³ M⁻¹ s⁻¹, and k₋₂ = 9.6 × 10⁻⁴ s⁻¹. Relative complex formation rates at Al(H₂O)₆³⁺ and Al(H₂O)₅OH²⁺, and comparison with kinetic data for a series of synthetic hydroxamic acids, suggest that an interchange mechanism is operative. These results are also discussed in relation to kinetic data for the corresponding iron(III)-deferriferrioxamine B system.     

Reversible linkage isomerization of pentaamminecobalt(III) complexes of urea and its N-methyl derivatives

The (NH₃)₅CoOC(NH₂)₂³⁺ ion is consumed in water according to the rate law k(obs.) = k₁ + k₂[OH⁻], where k₁ = 4.0 × 10⁻⁵ s⁻¹ and k₂ = 14.2 M⁻¹ s⁻¹ (0–0.1 M [OH⁻];μ = 1.1 M, NaClO₄, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH₃)₅CoOH₂³⁺ is observed, but in 0.1–1.0 M [OH⁻], 7% of the directly formed products is the urea-N complex (NH₃)₅CoNHCONH₂²⁺ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k₁) and base-catalyzed (k₂) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′_a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k₂[OH⁻]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k₁ + k₂[OH⁻] = (k_s + k_ON) + (k_OH + k′_ON) [OH⁻] for the reactions of the O-bonded isomers, where k_s, k_OH are the specific rates for hydrolysis, and k_ON, k′_ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; k_ON = 1.3 × 10⁻⁵ s⁻¹ and k′_ON = 1.1 M⁻¹ s⁻¹ (μ = 1.0 M, NaClO₄, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH₂ capture (versus H₂O), while −NHR and −NR₂ do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H₂O, Me₂SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH₃)₅CoNHCONRR′](ClO₄)₂·xH₂O (R,R′ = H, CH₃). They are kinetically inert in neutral to basic solution but in acid they protonate (H₂O, pK′_a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H₂O and Me₂SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form k_NO(obs.) = (k + k_NO)[H⁺]/(K′_a + [H⁺]) and the active species in the reaction is the protonated form; k, k_NO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me₂SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH₃)₅CoNH₂CONH₂³⁺, diastereotopic exo-NH₂ protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. ¹³C and ¹H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′_NO(obs.) = K′_NO[H⁺]/[H⁺](K′_a), where K′_NO is the equilibrium constant = [(NH₃₅Co(urea-O)³⁺]/[(NH₃)₅Co(urea-N)³⁺]. Values for K′_NO(=k_NO/k_ON = 260 and pK′_a ≈ 3 for the NH₂CONH₂ system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′_NO(obs.) = 2.6 × 10⁻²) and instability in acid solution (e.g. pH 1, K′_NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH₃)₅Co(III) are contrasted with the result for the analogous Rh(III)NH₂CONH₂ system K′_NO ≈ 1).     

New chiral macrocyclic lanthanide complexes derived from (1R,2R)-1,2-diaminocyclohexane and 2,6-diformylpyridine

The new enantiopure complexes [LnL](NO₃)₃ · nH₂O (Ln = Dy⁺³, Ho⁺³, Er⁺³, Lu⁺³) and [LnL]Cl₃ · nH₂O (Ln = Nd⁺³, Sm⁺³, Gd⁺³, Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³, Tm⁺³, Lu⁺³) of the chiral macrocycle L derived from (1R,2R)-1,2-diaminocyclohexane and 2,6-diformylpyridine have been synthesised. The preference of macrocycle L for the heavier lanthanide(III) ions has been established on the basis of competition reaction. The complexes have been characterised by NMR spectroscopy and mass spectrometry. ¹H NMR signals of deuterated water solutions of the Ce⁺³, Nd⁺³ and Eu⁺³ complexes have been assigned on the basis of the COSY and HMQC spectra, and for the remaining lanthanide complexes the signals were assigned on the basis of linewidths analysis. The paramagnetic shifts of the series of lanthanide complexes [LnL](NO₃)₃ · nH₂O and [LnL]Cl₃ · nH₂O have been analysed using both crystal-field dependent and independent methods in order to separate contact and dipolar contributions and establish isostructurality along the series of lanthanide complexes in solution. The data obtained for nitrate derivatives in organic solvent indicate rather irregular deviations from the plots based on those methods, while the plots obtained for water solutions show the characteristic brake in the middle of the lanthanide series, that is interpreted as a result of change of the number of axially coordinated water molecules. The apparent inconsistencies of results obtained on the basis of crystal-field independent method are discussed.     

Exchange rates of pyridine on ferro- and ferriprotoporphyrin (IX) dimethylester in chloroform.

Nuclear magnetic resonance line-widths data have been used to determine the rate of solvent exchange from the first coordination sphere of ferro-and ferriprotoporphyrin(IX) dimethylester (Fe-PPD) in pyridine/chloroform. The average values of kinetic parameters for pyridine (PY) exchange indicate an SN2 mechanism tor Fe(III)-PPD(ΔH^&;#; = 36 kJ · mol⁻¹ ; ΔS^&;#; = −53 J·mol⁻¹K⁻¹; T_M(298 K) = 0.07 msec) and an SNI mechanism for Fe(II)-PPD (ΔH^&;#; = 67 kJ·mol⁻¹; ΔS^&;#; = 42 J · mol⁻¹K⁻¹; T_M(298 K) = 0.06 msec). Parallel to the accelerated ligand exchange rate at rising temperatures a redistribution of the electrons causing a transition of the metal porphyrin from the low-spin state to the high-spin state is observed. Enthalpy and entropy of the thermodynamic equilibrium between low- and high-spin Fe-PPD have been determined from experimental values of the average magnetic moment. A mean lifetime of low-spin Fe(III)-PPD was estimated from line. widths changes (T_L→H(298 K)≈ 20 msec) and the corresponding activation parameters have been obtained (ΔH^&;#;_L→H(298 K) = 26 kJ · mol⁻¹; ΔS^&;#;_L→H(298K) = −125 J · mol⁻¹K⁻¹).