Phase metal complexes

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Hoffman, M.R., Yost, E.C., Eisenreich, S.J. and Maier, WJ. (1981) Characterization of soluble and colloidal-phase metal complexes in river water by ultrafiltration. A mass-balance approach. Environ. Sci. Technol, 15, 655-661. 

The RPA model can predict, to a reasonable degree, the adsorption of the aqueous phase metal complexes arising from chloroplatinic acid (CPA) onto alumina as occurs during... 

Figure 3. Thermochemical cycles for electron attachment to free metal ions, gas-phase metal complexes, and solvated metal complexes. refers to heterolytic bond disruption (M-L- M + L).

The table summarizes the data up to the end of 1972. For each extractant and Its extractable metal complexes, distribution equilibrium constants, as well as appropriate extraction constants, are recorded. In addition, the homogeneous equilibria involving the acid dissociation of the extractant in the aqueous phase and adduct or mixed ligand complex formation in the organic phase are characterized. Aqueous phase metal complex formation constants however, are not included inasmuch as these are already covered in Stability Constants. 

This 228 page volume summarizes equilibrium constants for liquid-liquid distribution equilibrium constants up to the end of 1972. Each table includes the extractant and its extractable metal complexes, and.the distribution equilibrium constants and extraction constants. The aqueous phase metal complex formation constants are not included and the reader is referred to items and in this bibliography for sources of this type of data. References to the primary literature are included. 

By suitable modifications of the structure, /J-diketones can be obtained which very much prefer the organic phase. Metal complex formation will then take place mainly in this phase. As the neutral complexes favor the organic phase even more markedly than does the protonated ligand, extraction becomes almost complete once these complexes have been formed. 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

Gc chiral stationary phases can be broadly classified into three categories diamide, cyclodextrin, and metal complex. 

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

Tertiary arsines have been widely employed as ligands in a variety of transition metal complexes (74), and they appear to be useful in synthetic organic chemistry, eg, for the olefination of aldehydes (75). They have also been used for the formation of semiconductors (qv) by vapor-phase epitaxy (76), as catalysts or cocatalysts for a number of polymeri2ation reactions (77), and for many other industrial purposes. 

How is that knowledge used to find the phase of the contribution from the protein in the absence of the heavy-metal atoms We know the phase and amplitude of the heavy metals and the amplitude of the protein alone. In addition, we know the amplitude of protein plus heavy metals (i.e., protein heavy-metal complex) thus we know one phase and three amplitudes. From this we can calculate whether the interference of the x-rays scattered by the heavy metals and protein is constructive or destructive (Figure 18.10). The extent of positive or negative interference plus knowledge of the phase of the heavy metal together give an estimate of the phase of the protein. 

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

Although the structure of [SsN] has not been established by X-ray crystallography, the vibrational spectra of 30% N-enriched [SsN] suggest an unbranched [SNSS] (5.22) arrangement of atoms in contrast to the branched structure (Dsh) of the isoelectronic [CSs] and the isovalent [NOs] ion (Section 1.2). Mass spectrometric experiments also support the SNSS connectivity in the gas phase.Many metal complexes are known in which the [SsN] ion is chelated to the metal by two sulfur atoms (Section 7.3.3). Indeed the first such complex, Ni(S3N)2, was reported more than twenty years before the discovery of the anion. It was isolated as a very minor product from the reaction of NiCl2 and S4N4 in methanol. However, some of these complexes, e.g., Cu and Ag complexes, may be obtained by metathetical reactions between the [S3N] ion and metal halides. 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

Simple metal compounds are poorly soluble in non-coordinating ILs, but the solubility of metal ions in an IL can be increased by addition of lipophilic ligands. LLowever, enhancement of lipophilicity also increases the tendency for the metal complex to leach into less polar organic phases. 

Ideally, to ensure the complete removal of the metal ions from the aqueous phase, the complexant and the metal complex should remain in the hydrophobic phase. Thus, the challenges for separations include the identification of extractants that quantitatively partition into the IL phase and can still readily complex target metal ions, and also the identification of conditions under which specific metal ion species can be selectively extracted from aqueous streams containing inorganic complexing ions. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

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