Enhanced hydrolysis of a phosphonate ester by mono-aquo metal cation complexes

The hydrolysis of the ester 2,4-dinitrophenyl- ethyl methylphosphonate has been examined by both stop-flow spectrophotometric and pH-stat techniques. These reactions have been carried out in the presence of several nucleophiles including simple non-labile (w.r.t. substitution) mono-aquo metal ion complexes. Comparison of reaction rates of the metal complexes with sterically hindered organic nucleophiles has led to the conclusion that the metal ions function predominantly as general base catalysts in dilute aqueous solution. Reaction rates for the various nucleophiles studied are tabulated together with solvolysis constants for hydroxide ion and water at 35 °C and I=0.1 mol dm⁻³ (KNO₃). These later two values are respectively 32.7 mol⁻¹ dm³ s⁻¹ and 1.37 x 10⁻⁴ s⁻¹. A Brönsted β value of 0.52 for the phosphonate ester studied has also been derived.     

The formation constants of dimethylthallium(III)-glutathione complexes in aqueous solution

The complexes formed in the dimethylthallium(III) (Me₂Tl⁺), glutathionate (EGC³⁻) and hydrogen ion system in aqueous solution at 37 °C and I = 150 mmol dm⁻³ (NaCl) have been characterised by means of glass-electrode potentiometry. Glutathione protonation constants were found to be 9.123 ± 0.007, 17.42 ± 0.01, 20.78 ± 0.02, and 22.93 ± 0.02. Formation constants for the complexes [(Me₂Tl)EGCH]⁻ and [(·Me₂Tl) EGC]²⁻ were found to be 11.19 ± 0.03 and 2.39 ± 0.02, respectively. Particular attention has been paid to the evaluation of the effect of possible systematic errors on the constant values determined. Reliable standard deviation estimations have been made by applying a Monte Carlo calculation technique.     

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.     

Supramolecular interaction between adenosine 5′-triphosphate (ATP) and polycharged tetraazamacrocycles. Thermodynamic and 31P NMR studies

The interaction of adenosine 5′-triphosphate (ATP) with the tetraammonium macrocyclic receptors 1,1,4,4,7,7,10,10-octamethyl-1,4,7,10-tetraazacyclododecane tetrakis(iodide) (L1·4I) and 1,6,11,16-tetraazacycloeicosane (L2) in its fully protonated form, has been studied by potentiometry and ³¹P NMR in water at I = 0.15 mol dm⁻³ and 25 °C H₄L2⁴⁺ reacts both with ATP⁴⁻ and HATP³⁻ to produce H₄L2·ATP and (H₄L2·HATP)⁺ whose equilibrium constants are 6.46 × 10³ and 1.10 x 10³, respectively. In the case of L1, in which the quaternarization of the nitrogen atoms prevents the formation of hydrogen bonds, no detectable interactions arise with ATP. These results are consistent with the hypothesis that the formation of hydrogen bonds play a role of major importance in the interaction between ATP and tetrammonium receptors.     

The kinetics and mechanisms of the reactions of chromium(III) with pentane-2,4-dione in aqueous solution

The equilibrium, kinetics and mechanism of the reaction of chromium(III) with pentane-2,4-dione (Hpd) have been investigated in aqueous solution at 55°C and ionic strength 0.5 mol dm⁻³ NaClO₄. The equilibrium constant (log β₁) is 10.08(±0.01) while the pK of Hpd is 8.69(±0.01). The kinetics are consistent with a mechanism in which [Cr(H₂0)₆]³⁺ and [Cr(H₂0)₅(OH)]²⁺ react with the enol tautomer of Hpd with rate constants of 1.05(±0.26) × 10⁻² and 2.78(±0.08) × 10⁻¹ dm³ mol⁻¹ s⁻¹ respectively. These rate constants are considerably more rapid than those predicted by the Eigen-Wilkins mechanism. These data are compared with literature values.     

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.     

Mechanisms for acceleration of halide anation reactions of platinum(IV) complexes. REOA versus ligand assistance and platinum(II) catalysis Without central ion exchange

Chloride anation of trans-Pt(CN)₄ClOH₂⁻ has been studied with and without Pt(CN)₄²⁻ present at 25.0°C by use of stopped-flow and conventional spectrophotometry and a 1.00 M perchlorate medium. The rate law in the absence of Pt(CN)₄²⁻ is Rate=(p₁ + p₂ [H⁺] ) [Cl⁻]² [complex]/(1 + q [Cl₋]) with p₁=(3.0 ± 0.1) × 10⁻⁵ M⁻²s⁻¹, p₂=(3.6 ± 0.1) × 10⁻⁵ M⁻³ s⁻¹ and q=(0.62 ± 0.02) M⁻¹. It is compatible with a chloride assistance via an intermediate of the type Cl-Cl-Pt(CN)₄···OH₂²⁻, in which the reactivity of the aqua ligand is enhanced due to a partial reduction of the platinum. This mechanism of halide assistance is in principle the same as the modified reductive elimination oxidative addition (REOA) mechanism proposed by Poë, in which the intermediate is not split into free halogen, platinum(II) and water, and in which electron transfer not necessarily involves complete reduction to platinum(II). To avoid confusion with complete reductive eliminations, reactions without split of the intermediates are here termed halide-assisted reactions. The pH-dependence indicates acid catalysis via a protonated intermediate ClClPt(CN)₄···OH₃⁻.The Pt(CN)₄²⁻accelerated path has the rate law Rate=

[Cl-] [Pt(CN)₄²⁻] [complex] where k=(39.9±0.5) M⁻² s⁻¹ and K_a=(4.0±0.2)10⁻² M is the protolysis constant of trans-Pt(CN)₄ClOH²⁻.Reaction between PtCl₅OH₂⁻ and chloride is accelerated by Pt(CN)₄²⁻ and gives PtCl₆²⁻ as the reaction product. The rate law is Rate=k [Cl⁻] [Pt(CN)₄²⁻] [PtCl₅OH₂⁻] with k=(5.6 ± 0.2)10⁻³ M⁻² s⁻¹ at 35.0°C and for a 1.50 M perchlorate acid medium. The reaction takes place without central ion exchange. Alternative mechanisms with two consecutive central ion exchanges can be excluded. The role of Pt(CN)₄²⁻ in this reaction is very similar to that of the assisting halide in the halide assisted anations. [p ]Reaction between trans-Pt(CN)₄ClOH₂⁻ and PtCl₄²⁻ gives Pt(CN)₄²⁻ and PtCl₅OH₂⁻ as products and has the rate law Rate=k[PtCl₄²⁻] [trans-Pt(CN)₄ClOH₂⁻] with k=(3.32 ± 0.02) M⁻¹ s⁻¹ at 25 °C for a 1.00 M perchloric acid medium. The formation of an aqua complex as the primary reaction product and the rate independent of [Cl⁻] shows that formation of a bridged intermediate of the type Pt(II)Cl₄ClPt(IV)(CN)₄OH₂³⁻ is formed in the initial reaction step, not five-coordinated PtCl₅³⁻.     

Complexes of aminophosphonates. I. Transition metal complexes of aminophosphonic acid analogues of α-alanine, β-alanine,phenylalanine and tyrosine

The stoichiometries and stability constants of the proton, cobalt(II), nickel(II), copper(II) and zinc(II) complexes of 1-aminoethanephosphonic acid (α-Ala-P), 2-aminoethanephosphonic acid (β-Ala-P), 1-amino-2-phenylethanephosphonic acid (Phe-P) and 1 -amino-2-(4-hydroxyphenyl)ethanephosphonic acid (Tyr-P) have been determined pH-metrically at 25 °C and at an ionic strength of 0.2 mol dm^{− 3} (KCl).From these data and the spectral parameters of the complexes it has been established that these simple aminophosphonic acids coordinate similarly to aminocarboxylic acids, forming chelate complexes MA and MA₂. However an MAH species with only phosphonate group coordination also exist at low pH. The differences between the complex-forming properties of aminophosphonates and aminocarboxylates have been explained by the differences in basicity, charge and size of the −PO₃²⁻and −COO⁻ groups.     

Uptake,incorporation and calcium-ionophore-stimulated mobilization of arachidonic,eicosapentaenoic and docosahexaenoic acids by leucocytes of the rainbow trout,Salmo gairdneri

Rainbow trout leucocytes contain high levels of neutral lipid (about 70% of total lipid on a wt% basis) consisting of mostly triacylglycerol, free sterols and sterol esters (25%, 15% and 52% of neutral lipid, respectively). The phospholipids, separated by thin-layer chromatography, consisted predominantly of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine, each present at about 30% of the total phospholipid. Radiolabelling of the leucocytes for 1 h with 1 μCi (approx. 6 μM) [1−¹⁴C]20:4(n−6), [1−¹⁴C]20:5(n−3) or [1−¹⁴C]22:6(n−3) each gave similar uptake values (approx. 1 · 10⁵ cpm/10⁷ leucocytes). The incorporation into total phospholipids was highest for 22:6(n−3) and lowest for 20:4(n−6). A higher percentage of radiolabel from [1−¹⁴C]22:6(n − 3) was found incorporated into phosphatidylcholine and phosphatidylethanolamine as compared to that from [1−¹⁴C]20:4(n − 6) and [1−^14C]20:5(n−3), while the reverse situation was found with phosphatidylinositol and phosphatidylserine. The relative rates of incorporation into the different phospholipid classes for all three fatty acids were in the order phosphatidylinositol > sphingomyelin > diphosphatidylglycerol > phosphatidylcholine > phosphatidylethanolamine > phosphatidylserine. Calcium ionophore-challenge did not significantly alter the pattern of phospholipid radiolabel. Ionophore-challenge released large amounts of radiolabel, much of which was recovered after high-performance liquid chromatographic separation as free fatty acid/monohydroxy fatty acids, although only approx. 0.3% was recovered in leukotriene B₄ and leukotriene B₅ for the [1−¹⁴C]20:4(n−6) and [1−¹⁴C]20:5(n−3) labelled leucocytes, respectively. Other lipoxygenase products were also radiolabelled and tentatively identified as 20-carboxy-LTB₄, 20-hydroxy-LTB₄, 6-trans-LTB₄, 6-trans-12-epi-LTB₄, 6-trans-8-cis-12-epi-LTB₄ and the corresponding LTB₅ structures. No ‘6-series’ leukotrienes were produced from [1−¹⁴C]22:6(n−3), nor was there any evidence for the synthesis of ‘5-series’ leukotrienes via retroconversion of 22:6(n−3) to 20:5(n−3). This latter finding shows that, despite the preponderance of 22:6(n−3) in the membranes of trout leucocytes, this fatty acid is not a substrate for leukotriene generation.     

Kinetics of the formation and hydrolysis reactions of some thiomolybdate(VI) anions in aqueous solution

The kinetics of the formation of the thiomolybdate ions MoOS₃²⁻ and MoS₄²⁻ were determined spectroscopically from the addition of excess sulphide to MoO₂S₂²⁻ in pH buffered media (6–8) at 30 °C. The reverse (hydrolysis) reactions of MoO₂S₂²⁻ and MoOS₃²⁻ were measured under the same conditions. The reaction rates measured are shown below:

Values of the rate-constants (s⁻¹) obtained at pH 7.0 were k₁₀ 2.4 × 10⁻³, k₂₁ 1.5 × 10⁻⁵, k₃₀ 2.1 × 10⁻⁵, k₂₃ 6.0 × 10⁻⁴, and k₃₄ 1.9 × 10⁻⁵; where the results are comparable they are in good agreement with those obtained by earlier workers, although different conditions were used. However, in this work it was found that certain reactions had to be mathematically treated as two consecutively occurring reactions. There is also a difference in interpretation of the mechanism of the hydrolysis reactions of the tri- and tetrathio ions. In general the lability towards further S replacement of O atoms, and the reverse reaction, decreased with increased S substitution. All reaction rates increased with increasing H⁺ ion concentration, mostly this was a linear relationship over the limited pH range examined. The effect of the H⁺ ion is interpreted in terms of protonation of the oxythiomolybdate ions at an O atom leading to increased lability.     

Sorption of caesium,iodine and sulphur in solution to the adaxial leaf surface of broad bean (Vicia faba L.)

The interception by crop canopies of radionuclides in rainfall can be important in determining radiation exposures to animals and man. Data were obtained on the sorption and desorption of radionuclides on the adaxial surfaces of fully expanded bean leaves by exposing them to ionic forms of caesium (Cs⁺), iodine (I⁻) or sulphur (SO₄²⁻) over a six order of magnitude concentration range. The accumulation of each element was determined as a time course over a 48 h period using radioactive labels (¹³⁷Cs, ¹²⁵I or ³⁵S, respectively). Time- and concentration-dependent sorption of each element to the leaf surface was analysed to determine: (a) the leaf surface-solution distribution coefficient (K_d) at equilibrium and (b) the sorption and desorption rate coefficients for each element over the range of concentrations investigated. It was expected that Cs⁺ would show a stronger tendency to sorb to the leaf surface than both I⁻ and SO₄²⁻ because of the cation exchange properties of the cuticular membrane. The K_d for Cs⁺ was approximately 90× greater than that for SO₄²⁻ but 5× less than that for I⁻. This is thought to be due to either (a) the highly organophilic nature of iodide and the relatively high iodine number of cuticular waxes on plant leaf surfaces or (b) the possible oxidation of I⁻ to I⁰ or IO₃⁻, with consequently enhanced leaf surface sorption. Based on data obtained in this study, ranges and best estimates of sorption and desorption rate coefficients are presented for Cs⁺, I⁻ and SO₄²⁻ for use in modelling the interception of radioactive Cs, I and S in rainfall by crops.     

Thermodynamics of hexokinase-catalyzed reactions.

The enthalpies of the hexokinase-catalyzed phosphorylation or glucose, mannose, and fructose by ATP to the respective hexose 6-phosphates have been measured calorimetrically in TRIS/TRIS HCl buffer at 25.0, 28.5, and 32.0°C. The effects on the measured enthalpy of the glucose/hexokinase reaction due to variation of pH (over the range 6.7 to 9.0) and ionic strength (over the range 0.02 to 0.25) have been examined. Correction for enthalpy of buffer protonation leads to δH^o and δC_p^o values for the processes: eq-D-hexose + ATP⁴⁻ = eq-D-hexose 6-phosphate²⁻ + ADP³⁻+ H⁺. Results are δH^o = −23.8 ± 0.7 kJ · mol⁻¹ and δC_p^o = −156 ± 280 J·mol⁻¹·K⁻¹ for glucose. δH^o = −21.9 ± 0.7 kJ·mol⁻¹ and δC_p^o = 10 ± 140 J·mol⁻¹·K⁻¹ for mannose, and δH^o = −15.0 ± 0.9 kJ·mol⁻¹ and δC_p^o = −41 ± 160 J·mol⁻¹·K⁻¹ for fructose. Combination of these measured enthalpies with Gibbs energy data for hydrolysis of ATP⁴⁻ and that for the hexose 6-phosphates lead to δS^o values for the above hexokinase-catalyzed reactions.     

Kinetics of acid-catalysed dissociation of lead(II) and copper(II) complexes of ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA)

The acid-catalysed dissociation rate constants for PbEGTA²⁻ and CuEGTA²⁻ complexes (where EGTA is ethylenebis(oxyethylenenitrilo) tetraacetic acid) were measured in acetic acid-acetate buffer medium (pH: 3.0–4.8) and perchloric acid solutions ([H⁺] = 0.05–0.15 M), respectively, at a constant ionic strength of 0.15 (NaClO₄). The rate laws shown by the lead(II) and copper(II) complexes are of the form, Rate = {k_d + k_H[H⁺]}[complex] and Rate = {k_d + k_H²[H⁺]²}[complex], respectively. Enthalpy and entropy of activation for acid-independent and acid-catalysed pathways for both the complexes were obtained by the temperature-dependence studies of resolved rate constants in the 16–45°C range. The rate of dissociation of PbEGTA²⁻ is not enhanced by increasing the concentration of acetate ion in the buffer, and the amount of total electrolyte in the reaction mixture has no pronounced effect on the dissociation rates of their the lead(II) or copper(II) complex. Attempts to study the kinetics of stepwise ligand unwrapping in the binuclear Cu₂EGTA complex were unsuccessful due to the extremely rapid dissociation of this complex to yield mononuclear CuEGTA²⁻.     

Studies on the reactions of PtCl2en,cis-Pt(NH3)2Cl2 and their aqua species with adenosine,deoxyadenosine and adenine using ion-pair HPLC

The reactions of PtCl₂en or cis-Pt(NH₃)₂Cl₂ and their aqua species with adenine and adenosine were studied by means of ion-pair HPLC. From the chromatograms, it was found that the first binding site of Pt(II) was the N(7) site of adenine under both acidic and neutral conditions. The rates of Pt(II) binding at the (N7) site of adenosine and deoxyadenosine were measured. The rate constants, k₁, were obtained for the reactions of PtCl₂en or cis-Pt(NH₃)₂Cl₂ with adenosine and deoxyadenosine at pH 3 and 7 over the temperature range 9–25 °C. The k₁ values were 6.8–7.7 × 10⁻⁴ dm³ mol⁻¹ s⁻¹ at 25 °C. For the aqua species, the rate of [cis-Pt(NH₃)₂ClH₂O]⁺ with adenosine N(7) was measured. The rate constants, k₂ which were found to be smaller than those of hydrolysis, k_h, were calculated at pH 3 over the temperature range 25–40 °C. The k₂ value obtained at 25 °C was 1.1 × 10⁻² dm³ mol⁻¹ s⁻¹, 15 time larger than k¹. The activation parameters were also calculated.     

Ferriprotoporphyrin IX Oxygen Activation/Insertion Reactions: The Nature of the Interaction between Ferriprotoporphyrin IX and Aniline (Substrate)

The interaction between aniline and ferriprotoporphyrin IX in alkaline solution has been investigated using pyridine and the N-methyl pyridinium ion as site-specific inhibitors of oxygen activation. Pyridine inhibits oxygen activation in a noncompetitive manner with respect to aniline (K₁ = 1.24 mol ⁻¹ dm³ at 30°C) while the N-methyl pyridinium ion inhibited in a manner consistent with two sites for aniline binding, only one of which was competitively inhibited (K₁ = 317mol^-l dm³ at 30°C). A comprehensive reinvestigation of the interaction of pyridine and N-methyl pyridinium ions with alkaline ferriprotoporphyrin IX has shown that two molecules of each ligand bind per hemin dimer in a strongly cooperative manner. The association constant for the first pyridinium ion bound is K_1a = 176 mol⁻¹ dm³ at 30°C, while that for the first pyridine molecule bound is K_1a = 0.580 mol⁻¹ dm³ at 30°C; these are both close to the observed inhibition association constants (K₁). Thermodynamic parameters for the interactions have been evaluated and compared to previous literature values. On the basis of these results a model is proposed for aniline interaction with the ferriprotoporphyrin dimer IX which involves the binding of two molecules of aniline to the ferriprotoporphyrin IX tetrapyrrole ring system by planar π bonding interactions with the rings having the propionate groups attached.     

Synthesis,characterization and DNA-binding properties of novel dipyridophenazine complex of ruthenium (II) : [Ru(IP)2(DPPZ)]2+

A novel ruthenium(II) complex of dipyridophenazine (DPPZ) with the ancillary ligand imidazole[4,5-f] [1,10]phenanthroline (IP), [Ru(IP)₂(DPPZ)] (PF₆)₂, has been synthesized and characterized by elemental analysis, 1D and 2D ¹H NMR, fast-atom bombardment mass spectra (FABMS), electronic spectroscopy and cyclic voltammetry. The DNA-binding properties of the complex were studied by spectroscopic methods. The intrinsic binding constant, K =2.1 × 10⁷M⁻¹, of the complex to calf thymus DNA has been determined by absorption titration in 5 mmol dm⁻³ Tris-HCl, 50 mmol dm⁻³ NaCl buffer (pH 7.0). The excited state lifetimes and luminescence quenching with [Fe(CN)₆]⁴⁻ as the quencher in the presence of DNA were also tested and mono-exponentiality was observed for the emission decay curves. Viscosity measurements together with the optical titrations unambiguously proved that the complex bound with DNA intercalatively and that the binding affinity to DNA was several times larger than that of the parent complex [Ru(bpy)₂(DPPZ)]²⁺.     

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).     

Surface coordination chemistry of noble-metal electrocatalysts: Oxidative addition and reductive elimination of iodide at iridium,platinum and gold in aqueous solutions

The oxidative addition and reductive elimination of the iodo ligand has been compared at smooth polycrystalline gold, platinum and iridium surfaces in aqueous solutions. On these three metals, the iodo species undergoes spontaneous oxidative chemisorption to form a close-packed monolayer of zero-valent iodine, the saturation coverage of which is limited by the van der Waals radius of the iodine atom; this oxidative addition process is further manifested by evolution of hydrogen gas from proton reduction. Elimination of iodine from these surfaces can be achieved by its reduction back to the anion either by application of sufficiently negative potentials or by exposure to ample amounts of hydrogen gas. On Pt and Ir, the reductive desorption of iodine is coupled with reductive chemisorption of hydrogen; consequently, the reaction is a two-electron, pH-dependent process. A plot of E_1/2, the potential at which the iodine coverage is decreased to half its maximum value, against pH yields information concerning the redox potential of the I_(ads)/I⁻_(ads) couple in the surface-coordinated state. On Au, where dissociative chemisorption of hydrogen does not occur, the iodine-stripping process is a pH-independent, one-electron reaction. The difference in the redox potentials [E^o_I(ads) -E^o_I(aq] for the I_(ads) and I_2(aq)/I⁻_(aq) redox couples was found to be −0.90 V on Au, − 0.76 V on Pt, and −0.72 V on Ir. These values imply that the ratio of the formation constants for surface coordination of the iodine and iodide species (K_f,I/K_f,I−) is 2 × 10²⁸ on Au, 1 × 10²⁶ on Pt, and 2 × 10²⁵ on Ir.     

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 synthesis and reaction kinetics of some Co(IIl) and Cr(III) chloro complexes of 1,4,8-Triazaoctane (2,3-tri) and the isolation of chiral trans-dichloro [CoCl2(NH3)(2,3-tri)] ClO4

The reaction of 2,3-tri with CrCl₃·6H₂O₁, dehydrated in boiling DMF, results in the formation of mer-CrCl₃(2,3-tri) and anation of hydrolysed solutions of mer-MCl₃(2,3,-tri) (M=Co, Cr) with 6 M HCl containing HClO₄, forms trans-dichloro- mer-[MCl₂(2,3-tri)(OH₂)]ClO₄·H₂O (M=Cr, Co; I, II). trans-Dinitro-mer-[Co(NO₂)₂(NH₃)(2,3-tri)] ClO₄ crystallises from the reaction between mer-Co(NO₂)₃(2,3-tri) and aqueous 7 M ammonia, on addition of NaClO₄·H₂O, and trans-dichloro-mer-[CoCl₂(NH₃)(2,3-tri)]ClO₄ (III) can be isolated by treatment of the dinitro with 12 M HCl. Reaction of mer-CoCl₃(2,3-tri) with C₂O₄₂⁻, followed by addition of aqueous NH₃ and NaClO₄·H₂O results in the isolation of racemic mer-[Co(ox)(NH₃)(2,3-tri)]ClO₄· H₂O. This complex was resolved into its enantiomeric forms and treatment of these with SOCl₂/MeOH/ HClO₄ gave the chiral forms of trans-dichloro-mer- [CoCl₂(NH₃)(2,3-tri)]ClO₄ (R or S at the see-NH center). The rates of loss of the first chloro ligand from these dichloro complexes have been measured spectrophotometrically in 0.1 M HNO₃ over a 15 K temperature range to give the following kinetic parameters; (I) k_H(298)=7.25 × 10⁻⁵ s⁻¹, E_a=78.5 kJ mol⁻¹, δS₂₉₈^#=₋69 J K⁻¹ mol⁻¹; (II) k_H(298)=4.00 × 10⁻³ s⁻¹, E_a=89.9, δS₂₉₈^#= +87.5; (III) k_H(298)=3.09 × 10⁻⁴ s⁻¹, E_a=103, δS₂₉₈^#=+27. Treatment of the dichloro cations with Hg²⁺/HNO₃ results in the generation of mer- M(2,3-tri)(OH₂)₃³⁺ (M=Cr, Co; IV, V) and trans- diaqua-mer-Co(NH₃)(2,3-tri)(OH₂)₂³⁺ (VI). The Co(III) cations isomerise to the fac configuration with (V) K_isom(298) μ=1.0 M)=2.97 × 10⁻⁵ s⁻¹, E_a=115, δS₂₉₈^#=+46. (VI) K_isom(298) (μ=1.0 M)=4.13 × 10⁻⁵ s⁻¹, E_a=113, δS₂₉₈^#=+52.