Cuprous complexes and dioxygen. Part 11. Concomitant one- and two-electron reduction of O2 by the Cu ion

The autoxidation of Cu^I in aqueous MeCN has been studied using a Clark oxygen electrode in the presence and absence of Cu¹¹. The reaction is inhibited by Cu¹¹ in the pH range of 0.5 to 5.0, reaching a lower limiting value at the highest concentrations. The reaction order changes from 1 to 2 with respect to Cu^I under the influence of Cu²⁺ ion. Detailed kinetics analysis of a total of 275 measurements has shown that an unstable primary adduct CuO⁺₂ decomoses to give ^.O or HO, depnding on pH, and also reacts directly with a second Cu⁺ ion, avoiding one-electrton reduction of O₂ by this path. Reaction of HO is faster with Cu^I than with Cu¹¹ by a factor of 20, and single-electron transfer within CuO⁺₂ to Cu²⁺ and ^.O predominates over reaction with a second copper ion for [Cu^I_tot] < 2. 10^?3M in the absence of Cu²⁺. The most likely value for the reaction of ^.O with Cu^I is 5.3 · 10⁸ M ^?1S^?1, but even this high rate constant is at the limit of significance. All secondary reactions followinfg the initial formation of CuO⁺₂ are shown to be very fast, a fact that should be properly considered in the discussion of mechanisms of copper-catalyzed oxidations and oxygenations.     

L'action de l'ammoniac sur l'oxyde cuivrique. et les hydroxo-complexes de cuivre (II)

Using a new mathematical treatment, the nature and stability constants of the simple and mixed complex-species of copper(II) with hydroxyde and ammonia as ligands have been determined. The solubility curves of CuO in heterogeneous equilibrium have been identified in function of pH only and in function of pH and pNH_3tot at 25° and unit ionic strength (NaClO₄). The predominent species in the relatively dilute system limited by the ionic strength are [Cu²⁺], [Cu(OH)₂], [Cu(OH)], [Cu(OH)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃) (OH)⁺], [Cu(NH₃)₃(OH)⁺] and [Cu(NH₃)₂(OH)₂].     

Ternäre Komplexe in Lösung IV. Einfluss von 2,2′-Bipyridyl auf Stabilität und Acidität des Cu2+-Adenosin-5′-monophosphat-N(1)-oxid-1:1-Komplexes

The ternary Cu²⁺?2,2′-bipyridyl-adenosine-5′-monophosphate-N(1)-oxide complex was investigated and compared with the binary Cu²⁺-adenosine-5′-monophosphate-N(1)-oxide complex (I) (cf. [2]). In both complexes Cu²⁺ is bound to the o-amino-N-oxide group of adenosine-5′-monophosphate-N(1)-oxide (HL). The stabilities of the complexes monoprotonated at the phosphate group are of the same order: log K = 11,20, and log K = 11,19. The acidity constants for the deprolonation of the phosphate group in these complexes are slightly different (pK = 5,55, and pK = 5,88), but as expected both values are lower than the corresponding value pK = 6,12 of the ligand.     

63Cu-NMR.-Untersuchungen von Kupfer (I)-Tetrapyridin-und Kupfer (I)-Tetraacetonitrilsolvaten

⁶³Cu-NMR.-Spectra of Cu(CH₃CN)₄X (X = ClO, BF, PF) and Cu(C₅H₅N)₄X (X = ClO, BF) in solution are reported at different temperatures and concentrations. The influence of temperature on the linewidth and chemical shift indicates an equilibrium of Cu(CH₃CN) and Cu(C₅H₅N) with another complex of lower symmetry. The preferential solvation of Cu (I) by pyridin in a mixture acetonitrile/pyridine is clearly shown.     

Molecular Recognition in Anion Coordination Chemistry. Structure,Binding Constants and Receptor-Substrate Complementarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule

The crystal structures of four anion cryptates [X^? ? BT -6H⁺] formed by the protonated macrobicyclic receptor BT -6H⁺ with F^?, Cl^?, Br^? and N have been determined. They provide a homogeneous series of anion coordination patterns with the same ligand. The small F^?-ion is tetracoordinated, while Cl^? and Br^? are bound in an octahedron of H-bonds. The non-complementarity between these spherical anions and the ellipsoïdal cavity of BT -6H⁺ is reflected in ligand distortions. Structural complementarity is achieved for the linear triatomic substrate N, which is bound by two pyramidal arrays of three H-bonds, each interacting with a terminal N-atom of N. The formation constants of the complexes formed by BT -6H⁺ with a variety of anions (halides, N, NO, carboxylates, SO, HPO, AMP^2?, ADP^3?, ATP^4?, P₂O) have been determined. Very strong complexations are found, as well as marked electrostatic and structural effects on stability and selectivity; in particular the binding of F^?, Cl^?, Br^?, and N may be analyzed in terms of the crystal structure data. The cryptand BT -6H⁺ is a molecular receptor containing an ellipsoïdal recognition site for linear triatomic substrates of size compatible with the size of the molecular cacity. Further developments of various aspects of anion coordination chemistry are considered.     

A Unified Interpretation of Kinetic Data on the Acid-Induced Cleavage and of Product-Analysis Data on Spontaneous Cleavage of the Mono-ol Cation μ-Hydroxo-bis[pentaamminecobalt(III)] ([(NH3)5CoOHCo(NH3)5]5+)

Recent work on the spontaneous (= acid-independent) cleavage of the mono-ol cation, i.e. in Cl^?/ClO and NO/ClO mixed-electrolyte media has established (by analysis of anion-competition experiments) the existence of reactive ion pairs of the mono-ol cation with Cl^? and NO. Their existence must be allowed for in the analysis of the rate data for the acid-induced cleavage (pH 0–1) of the mono-ol cation in these mixed-electrolyte media. Thus, previous data for acidic Cl^?/ClO media have been re-interpreted in this work, and new data for NO/ClO media have been analyzed in the same sense. This analysis removes an apparent discrepancy in the orders of magnitude of ion aggregate stability constants between the mono-ol and similar binuclear cations.     

L'action de l'ammoniac sur l'hydroxyde de cadmium et la stabilité des complexes en milieu aqueux

The solubility of precipitated Cd(OH)₂ was determined at 25°C in 1 M NaClO₄, as a function of pH and of the ammonia content of the solutions. Formation constants were obtained for the following hydroxo, ammine and hydroxo-ammine complexes: CdOH⁺, Cd(OH)₂, Cd(OH), CdNH, Cd(NH₃), Cd(NH₃), Cd(NH₃) and Cd(OH)₂NH₃. The solubility product of the hydroxide was also calculated. The presence of polynuclear species was investigated by titrimetric determinations of the hydrogen ion concentration at constant metal concentration.     

Cuprous complexes and dioxygen. VIII. Steric factors controlling one- or two-electron reduction of O2 by cuprous complexes with substituted imidazoles

In aqueous acetonitrile (AN), Cu (I) forms the complexes Cu(AN)L⁺ and CuL with a series of substituted imidazoles (L). Stability constants logK of Cu(AN)⁺ + L ? Cu(AN)L⁺ and logβ₂ were near 5 and 12, resp., log units for all ligands. The rate of autoxidation is described by ?d[O₂]/dt=[CuL]²[O₂](k_a/(1+k_b[CuL]) + (k_c[L]+k_d)/([CuL] + k_e[Cu])), implying competition between one- or two-electron reduction of O₂. The value of k_c decreases from 5500M ^?2S ^?1 for unsubstituted imidazole to about 40M ^?2S ^?1 for 2-methylimidazole or 1,2-dimethyl-imidazole and essentially zero for the corresponding 2-ethyl-derivatives. On the other hand, k_a and k_b are much less influenced by the nature of the ligands, all values being near 5 · 10⁴M ^?2S ^?1 and 10³M ^?1, respectively, for the complexes with the last four bases. Thus rather subtle sterical changes may strongly influence the relative importance of different pathways in the reduction of dioxygen by cuprous complexes.     

Réactions de substitution dans l'anion tétrachloropalladate(II) avec des diamines. I. Ethylènediamine

The replacement of Cl^? by ethylenediamine (en) in PdCl has been followed spectrophotometrically at 25°C and μ = 1 (NaClO₄); it proceeds in two steps leading to Pd(en)Cl₂ and Pd(en), respectively. The observed rate constants are discussed in terms of the mechanism proposed by Reinhardt [1] for the successive ammination reactions of PdCl.     

Hydrogen Generation by Visible Light Irradiation of Aqueous Solutions of Metal Complexes. An approach to the photochemical conversion and storage of solar energy

We describe a photochemical system for the generation of hydrogen by water reduction under visible light or sunlight irradiation of aqueous solutions containing the following components: a photosensitizer, the Ru (bipy) complex, for visible light absorption; a relay species, the Rh (bipy) complex, which mediates water reduction by intermediate storage of electrons via a reduced state; an electron donor, triethanolamine (TEOA) which provides the electrons for the reduction process and a redox catalyst, colloïdal platinum, which facilitates hydrogen formation. The conditions for efficient hydrogen production and the influence of the concentration of the components have been investigated; the metal complexes act as catalysts with high turnover numbers; excess bipyridine facilitates the reaction. The process contains two catalytic cycles: a ruthenium cycle and a rhodium cycle. The Ru cycle involves oxidative quenching of the *Ru(bipy) excited state by Rh(bipy) forming Ru(bipy) which is converted back to Ru(bipy) by oxidation of the electron donor TEOA, which is thus consumed. The Rh cycle comprises a complicated set of transformations of the initial Rh(bipy) complex. The reduced rhodium complex formed in the quenching process undergoes a series of transformations involving the Rh(bipy) complex and hydridorhodium-bipyridine species, from which hydrogen is generated by reaction with the protons of water. In view of the storage of two electrons in the reduced rhodium species, the process is formally a dielectronic water reduction. The properties and eventual participation of [Rh(III)(bipy)₂LL′]ⁿ⁺(L,L′ = H₂O, OH^?) species are investigated. It is concluded that at neutral pH in presence of excess bipyridine, the cycle involving regeneration of the Rh(bipy) complex is predominant. A number of experiments have been performed with modified systems. Hydrogen evolution is observed with other photosensitizers (like proflavin), other relay species (like Rh(dimethylbipy) or Co(II)-bipyridine complexes), other donor species, or in absence of the platinum catalyst. It also occurs in absence of photosensitizer by sunlight of UV. irradiation of Rh(bipy) or by visible light irradiation of iridium (III)-bibyridine complexes. These systems deserve further investigations. The present photochemical hydrogen generating system represents the reductive component of a complete water splitting process. Its role in solar energy conversion and in photochemical fuel production is discussed.     

Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations

A most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He-Iα photoelectron-photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3-butadiene cation preceding the dissociative processes. The rate-energy functions of four competitive primary fragmentation reactions, leading to C₃H, C₄H, C₄H and C₂H are established. There is compelling evidence that the production of C₂H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.     

Zur Stabilität der Metallkomplexe von Methantriessigsäure und cis,cis-1,3,5-Cyclohexantricarbonsäure

The stabilities of the Mn²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺- and Zn²⁺-complexes with 2-(carboxymethyl)glutaric acid ( 2 ) and cis,cis-1,3,5-cyclohexanetricarboxylic acid ( 3 ) were measured potentiometrically at 25° and I = 0.5 (KNO₃). Beside the complexes ML^? protonated species MLH and MLH are also formed. Their stability constants are given in Table 1. A comparison between the stabilities of 2 or 3 and those of acetate, as a model for a monocarboxylate, or succinate and glutarate, as examples for dicarboxylates, indicates that in all species only one carboxylate is strongly bound whereas the second and third ones are probably not. The observation that Δlog K₁ = log K ? log K as well as Δlog K₂ = log K ? log K are practically constants with values of 0.34 ± 0.05 and 0.49 ± 0.07, respectively, for both ligands and the five metal ions studied is also in line with the proposed monodentate structures of the complexes ML^?, MLH and MLH.     

Conformational Analysis and Chromic Acid Oxidation. Oxidation Rates and Equilibrium Constants of Epimeric Alcohols

The linear free energy relationship of Sicher for relative reactivity towards chromic acid oxidation (ΔΔG) as a function of thermodynamic stability (ΔG) has been reexamined with 23 pairs of epimeric alcohols. The plot of ΔG vs. ΔG has a slope of 0.8, a correlation coefficient of 0.97 and a standard deviation of 0.23 kcal/mol on ΔΔG_Ox. The limitations of the relationship and the exceptions are discussed.     

Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

The Gas-Phase Ion/Molecule Reactions of C3H5X (X = C1, Br,I)

The ion/molecule reactions of the molecular ion, the C₃H ion, and the C₃H ion obtained from 3-chloropropene. 1-bromopropene, 2-bromopropene, 3-bromopropene, bromocyclopropane. and 3-iodopropene have been studied with their neutral precursor in a Fourier-transform mass spectrometer (FT/ICR). The molecular ions react to yield primarily C₆H except for the ion derived from 1-bromopropene that is unreactive. The kinetics of the 3-bromopropene molecular ion reveals that 18% of these ions must possess a different structure which is unreactive. The fact that C₃H ions obtained from 2-bromopropene are the only ones to undergo proton transfer is taken as evidence that only this parent compound gives rise to 2-propenyl cations by low-energy electron impact. The C₃H ions generated in these systems are shown to be roughly an equal mixture of propargylium ions that react to yield C₆H and unreactive cyclopropenium ions.     

Structures and Electrical Properties of Some New Organic Conductors Derived from the Donor Molecule TMET (S,S,S,S,-Bis(dimethylethylenedithio) tetrathiafulvalene)

Crystal structures and electrical properties of radical-cation salts of the chiral organic donor TMET (S,S,S,S,-bis-(dimethylethylenedithio)tetrathiafulvalene) are described. Two structural types, 2:1 with octahedral anions Pf, AsF, SbF, I (incommensurate), and 3:2 with tetrahedral anions BF^?₄, CIO^?₄, ReO^?₄ are observed. Resistivity measurements between 2 and 298 K indicate that the 3:2 types are organic metals, while the other compounds are semiconductors. (TMET)₃(CIO₄)₂ is metallic down to about 120 K at ambient pressure and remains metallic down to 2 K at 8 kbar.     

Exchange Equilibrium of Oxygen Isotopes between BrO,ClO, IO and Water

The liquid phase fractionation factors α_H = (¹⁸O/¹⁶O)_H2O/(¹⁸O/¹⁶O)xo and α_D = (¹⁸O/¹⁶O)_D2O/(¹⁸O/₁₆O)xo (X = Cl, Br, I) were calculated quantum mechanically between 0 and 100°. Experimental values were obtained in the case of BrO at 60° showing good agreement with the calculated results.     

Preparation and Properties of Bis[μ-S-(2-tiolatoethyl)-diphenylphosphine][(2-thiolatoethyl)diphenyl-phosphine]dinickel(+1) Ion,a Novel Unsymmetrical Thiolate-bridged complex

The title cation ( = Ni₂L) is formed in a variety of reactions (Schemes 1 and 2) in systems containing Ni²⁺ and (2-thiolatoethyl)-diphenylphosphine (= L^?) in the absence of coordinating anions at Ni²⁺/L^? ratios > 0.5 in apolar or moderately polar media. Solid [Ni₂L₃]CIO₄ and [Ni₂L₃]BPh₄ have been isolated. Job's plots confirm the Ni₂L- stoichiometry in solution. ³¹P-NMR data are consistent with ≥ 97% Ni₂L (vs. ? 3% of hypothetical Ni₃L) at equilibrium and support the suggested configuration (Fig. 2). The equilibrium between NiL₂ + NiL₂Br₂ and Ni₂L + Br^? varies with the solvent composition in CH₂₃Cl₂/EtOH mixtures. The rate of formation of Ni₂L₂Br₂ from Ni₂L and bromide (in high excess) in CH₂Cl₂ is first-order in [Ni₂L]_tot but depends on the ratio [Bu₄NBr]_tot/[Ni₂L₃ · ClO₄]tot, even at a high excess of bromide. This is interpreted by efficient competition in ion-aggregate formation between the small perchlorate concentration introduced as the counterion of Ni₂L, and the large excess of bromide.     

Charge Dispersal in the Quinuclidine Radical Cation and in the Quinuclidinium Ion,as Revealed by PE and ICR Spectroscopy

The n ionization energies I and the gas-phase basicities GB of CH₃-, Cl-, or CN-substituted quinuclidines have been measured by PE and ICR spectroscopy. The dependence of the shifts ΔI and ΔGB (relative to the values of the parent molecule) allow conclusions about the charge dispersal accompanying the n ionization or the protonation of quinuclidine in the gas phase. The agreement with the results of a minimal basis set ab initio calculation is excellent. Comparison of the solution pK_a values with either I or GB reveals that 2-substituted quinuclidines exhibit sizeable solvent-induced proximity effects, i.e. that the corresponding quinuclidinium ions are more acidic in solution than expected on the basis of the gas-phase basicities. This agrees with earlier results concerning 2-substituted pyridines.     

Endohedral fullerene-noble gas clusters formed with high-energy bimolecular reactions of C (x = 60, 70; n = 1, 2, 3)

Results are reported for high-energy beam experiments which establish the formation of endohedral carbon cluster-noble gas compounds by bimolecular reactions of C (x = 60, 70; n = 1, 2, 3) with He and C with Ne. The ions were accelerated up to 8 ke V in a four-sector mass spectrometer and allowed to collide with the noble gas in a collision chamber at room temperatur. Product ions were monitored with a B/E = constant linked scan. Within the sensivity of the experiments, no carbon cluster-gas compounds were observed in the reactions of C with H₂, D₂, O₂, Ar, and SF₆, or of C with O₂. The observed fall in the cross-section for carbon cluster-noble gas compounds with increasing size of the noble gas, the observation of unimolecular loss of C₂ from mass-selected C_xHe⁺ ions, and the elimination of carbon fragments instead of He observed in the formation of the collision-induced C_xHeⁿ⁺ product ions are taken as evidence for endohedral compound formation. Results of ab initio molecular-orbital calculations for the perpendicular penetration of the plane of ionized benzene with He, Ne, and Ar indicate that sufficient kinetic energy should be available in the collisions with C to penetrate the C cage at the collision energies of the experiments.