Coordination Sphere Effects Research Articles

Kinetic Studies of Olefin Epoxidation with Hydrogen Peroxide in 1,1,1,3,3,3‐Hexafluoro‐2‐propanol Reveal a Crucial Catalytic Role for Solvent Clusters

AbstractThe epoxidation of cyclooctene and 1‐octene with hydrogen peroxide was studied kinetically in HFIP (1,1,1,3,3,3‐hexafluoro‐2‐propanol)/1,4‐dioxane mixtures. In the case of cyclooctene, no additional catalyst was applied, whereas the epoxidation of 1‐octene was run in the presence of phenylarsonic acid as catalyst. For both reaction types, two kinetic regimes can be distinguished: at low HFIP concentration (nHFIP/ntotal≤0.15), both the catalyzed and the uncatalyzed reaction show first order dependence on the HFIP concentration. This result is interpreted by the HFIP “charge template” for the uncatalyzed epoxidation (as postulated by Shaik and Neumann) and by the formation of an arsonic acid mono‐HFIP ester for the catalyzed reaction. At high HFIP concentration (nHFIP/ntotal≥0.5), both reaction types are up to ca. 5 orders of magnitude faster and show ca. 12th order dependence on the HFIP concentration. We propose that the dramatic accelerations observed at high HFIP concentrations are brought about by HFIP clusters. Based on literature data (Fioroni, Roccatano, Hong, Suhm), it is concluded that the epoxidation reactions studied here take place in a HFIP coordination sphere comprising ca. 12 HFIP molecules, and that this coordination sphere effects the enormous increases in epoxidation rates. This mechanistic model bears resemblance to enzymatic reactions taking place in and being catalyzed by a protein matrix.

Gas-phase ion-molecule reactions of transition metal complexes: the effect of different coordination spheres on complex reactivity

Using a modified quadrupole ion trap mass spectrometer, a series of metal complex ions have been reacted with acetonitrile in the gas phase. Careful control of the coordination number and the type of coordinating functionality in diethylenetriamine-substituted ligands enable the effects of the coordination sphere on metal complex reactivity to be examined. The association reaction kinetics of acetonitrile with these pentacoordinate complexes are followed in order to obtain information about the starting complexes and the reaction dynamics. The kinetics and thermodynamics of acetonitrile addition to the metal complex ions are strongly affected by the chemical environment around the metal center such that significant differences in reactivity are observed for Co(II) and Cu(II) complexes with various coordination spheres. When thiophene, furan, or benzene moieties are present in the coordination sphere of the complex, addition of two acetonitrile molecules is readily observed. In contrast, ligands with better σ donors react mainly to add one acetonitrile molecule. Among the ligands with good σ donors, a clear trend in reactivity is observed in which complexes with nitrogen-containing ligands are the least reactive, sulfur-containing complexes are more reactive, and oxygen-containing complexes are the most reactive. In general, equilibrium and reaction rate constants seem to be consistent with the hard and soft acid and base (HSAB) principle. Interestingly, the presence of certain groups (e.g., pyridine and imidazole) in the coordination sphere clearly can change the acid character of the metal as seen by their effect on the binding properties of other functional groups in the same ligand. Finally, we conclude that because complexes with different coordination spheres react to noticeably different extents, ion-molecule (I-M) reactions may be potentially useful for obtaining coordination structure information for transition metal complexes.

ChemInform Abstract: Medium and Temperature Effects on the Redox Chemistry of Cytochrome c

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Medium and Temperature Effects on the Redox Chemistry of Cytochrome c

Cytochromes c (cytc) are ubiquitous heme-containing metalloproteins that shuttle electrons in a variety of electron-transport chains, most often central to the production of the chemical energy necessary for cell life. The reduction potential (E°′) of the Fe3+/2+ couple is central to the physiological role of these species in that it influences the thermodynamic and kinetic features of electron-exchange reactions with redox partners. In the last two decades, voltammetric techniques exploiting the heterogeneous electron exchange between cytc and solid electrodes have proved to be particularly valuable for the determination of E°′ values for these species and for characterizing the mechanistic and kinetic aspects of the redox process for the various cytc conformers under a variety of solution conditions. The understanding of how, and to what extent, different molecular factors control the E°′ value in these species has been the subject of much debate. First coordination sphere effects on the heme iron and the interactions of the heme group with the surrounding polypeptide chain and the solvent are the main factors affecting E°′ in cytc. These interactions are sensitive to medium effects such as the pH and the nature and ionic composition of the solvent. E°′ is also strongly affected by the temperature. This article summarizes the authors’ work on the effects on the selective stabilization of the two redox states of class I cytochromes c exerted by acid-base equilibria, general ionic strength effects, specific anion binding, the presence of non-aqueous solvents, and the temperature. The temperature dependence of E°′ allows the determination of the enthalpy and entropy changes that accompany protein reduction. These parameters have proved to be informative with regard to the interplay between first coordination sphere effects and electrostatics at the heme−protein interface, including solvent dipoles, which mainly affect the reduction enthalpy, and solvent reorganization effects and differences in protein dynamics between the two oxidation states, which control the reduction entropy instead.

Medium and Temperature Effects on the Redox Chemistry of Cytochromec

Cytochromes c (cytc) are ubiquitous heme-containing metalloproteins that shuttle electrons in a variety of electron-transport chains, most often central to the production of the chemical energy necessary for cell life. The reduction potential (E°′) of the Fe3+/2+ couple is central to the physiological role of these species in that it influences the thermodynamic and kinetic features of electron-exchange reactions with redox partners. In the last two decades, voltammetric techniques exploiting the heterogeneous electron exchange between cytc and solid electrodes have proved to be particularly valuable for the determination of E°′ values for these species and for characterizing the mechanistic and kinetic aspects of the redox process for the various cytc conformers under a variety of solution conditions. The understanding of how, and to what extent, different molecular factors control the E°′ value in these species has been the subject of much debate. First coordination sphere effects on the heme iron and the interactions of the heme group with the surrounding polypeptide chain and the solvent are the main factors affecting E°′ in cytc. These interactions are sensitive to medium effects such as the pH and the nature and ionic composition of the solvent. E°′ is also strongly affected by the temperature. This article summarizes the authors’ work on the effects on the selective stabilization of the two redox states of class I cytochromes c exerted by acid-base equilibria, general ionic strength effects, specific anion binding, the presence of non-aqueous solvents, and the temperature. The temperature dependence of E°′ allows the determination of the enthalpy and entropy changes that accompany protein reduction. These parameters have proved to be informative with regard to the interplay between first coordination sphere effects and electrostatics at the heme−protein interface, including solvent dipoles, which mainly affect the reduction enthalpy, and solvent reorganization effects and differences in protein dynamics between the two oxidation states, which control the reduction entropy instead.

Redox properties of cytochrome c.

The redox properties of cytochromes (cyt) c, a ubiquitous class of heme-containing electron transport proteins, have been extensively investigated over the last two decades. The reduction potential (E degrees') is central to the chemistry of cyt c for two main reasons. First, E degrees' influences both the thermodynamic and kinetic aspects of the electron exchange reaction with redox partners. Second, this thermodynamic parameter is remarkably sensitive to changes in the properties of the heme and the protein matrix, and hence can be profitably used for the investigation of the solution chemistry of cyt c. This research area owes much to the exploitation of voltammetric techniques for the determination of E degrees' for metalloproteins, which dates back to the late 1970s. Since then, much effort has been devoted to the comprehension of the molecular factors that control E degrees' in cyt c, which include first coordination sphere effects on the heme iron, the interactions of the heme group with the surrounding polypeptide chain and the solvent, and also include medium effects related to the nature and ionic composition of the solvent, pH, the presence of potential protein ligands, and the temperature. This article provides an overview of the most significant advances made in this field recently.

Phase transitions in alkali halide crystals

The paper is devoted to the study of the characteristics of structural phase transitions occurring in ionic crystals under an extremely high pressure. The calculations are made for crystals of infinite size at absolute zero temperature. The pressure ofB1-B2 polymorphic transformation is calculated for a number of alkali halide crystals using pair interaction potentials obtained self-consistently within the framework of inhomogeneous electron gas theory. In so doing, the effects of seven coordination spheres are taken into account in the thermodynamic potential. The changes of the cohesion characteristics of crystals (cohesion energy, lattice constant) are calculated for the transition from theB1 structure (NaCl type) to theB2 structure (CsCl type). Based on the obtained results, conclusions are made on the stability of one or another crystal structure in the given range of external pressure. For crystals in the B1 phase, the moduli of elasticity and relative changes of the volume at the phase transition pressure are calculated. It is shown that the inclusion of the effect of higher coordination spheres affects considerably the values of the elastic characteristics of ionic crystals.

Structures and electrochemical properties of the Co(III) ternary complexes containing NO 3-type tripodal tetradentate ligands and amino acids: effect of the outer coordination sphere on the electrochemical properties

With a view to understanding the effect of the outer coordination sphere of metal complexes on the redox potential, the electronic absorption spectra, X-ray crystal structure and cyclic voltammograms were investigated of aqueous and ethanol solutions containing the complexes [Co(X)(aa)], where X denotes an NO 3-type ligand such as bcmpa ( N,N-bis(carboxymethyl)- l-phenylalanine), nta (nitrilotriacetic acid), bcmga ( N,N-bis(carboxymethyl)- l-glutamic acid), bcmle ( N,N-bis(carboxymethyl)- l-leucine), and aa is an amino acid. Examination of the solution and crystalline structures revealed that the outer coordination sphere of the Co(III) complexes did not exert any influence on the inner coordination sphere of the Co(III) ion. The Co(III) complex systems showed irreversible redox behavior in both the aqueous and ethanol solutions. This is discussed on the basis of the relationship between the hydrophobicity of the outer coordination sphere and the electron transfer reaction by use of reduction potentials.

ChemInform Abstract: Effect of the Second Coordination Sphere on the Electron Transition Energies in ZnS‐Mn

The influence of the second coordination sphere on the energy of electron transitions in ZnS-Mn is investigated using the crystal field theory. The obtained perturbation effect is small and may be usually neglected.

Effect of the second coordination sphere on the electron transition energies in ZnS-Mn

Effect of the second coordination sphere on the electron transition energies in ZnS-Mn

CHEMICAL SHIFT DISPERSION DUE TO CRYSTALLOGRAPHIC INEQUIVALENCE AND IMPLICATIONS REGARDING THE INTERPRETATION OF THE HIGH-RESOLUTION 29Si MAS NMR SPECTRA OF ZEOLITES

Abstract The 29Si MAS NMR spectra of dealuminated zeolites have been studied, where the chemical shift dispersion due to structurally distinct SiO4, tetrahedra is comparable (or greater) in magnitude to first coordination sphere effects. The results 29Si suggest that some previous Si peak assignments are incorrect and that a detailed interpretation of some zeolite spectra is complex.

27Al NMR studies of aluminosilicate solutions. Influences of the second coordination sphere on the shielding of aluminium

Aluminosilicate solutions have been studied by 27Al NMR to investigate effects of the second coordination sphere on AI shielding. The spectra show four distinct peaks. 27A1 chemical shifts are compared with 29Si data. An analogous classification of the 27Al chemical shifts in terms of A1O 4 units of different environments and different degrees of condensation is proposed Observed resonances are correlated to aliminiumoxygen tetrahedra with different numbers of Si-O-Al bonds.

X‐Ray K‐absorption edge of bromine in some cobalt complexes

AbstractA method for photographic recording of the X‐ray K‐absorption edge of bromine has been developed in which the effect of the photographic emulsion's bromine is nullified. Using this method the Br K‐absorption edge in some complexes containing bromine were recorded with a view to studying the effect of chemical coordination. The effect of the second coordination sphere on the absorption edge of an ion in the first coordination sphere, in a complex compound, has been investigated.