Transition-Metal Ionic Species

Thanks to the extensive work reported in the last three decades by a rather small number of research teams, we may now better understand the factors that determine the deposition mode of TMIS containing catalytically active ions. The relevant papers reported are mentioned in a recent review [1]. The nature of the ligand of the TMIS, the nature of the support and the impregnation parameters (pH, concentration ofTMIS, ionic strength and temperature of the impregnating solution) are indeed the principal factors. 

The simple electrostatic adsorption (accumulation) in the diffuse part of the interface and at the plane 2 is rather the predominant mode in most cases when ammonia or halogen complexes are used for depositing noble metals (e.g. Pt(NH3)4 +, PtCls ) under conditions where these complexes remain intact in the impregnating solutions. 

The role of the support is also important. Thus, y -alumina and silica favor the surface polymerization and then the surface precipitation of the inner-sphere complexes formed upon deposition of the cobalt or nickel aqua complexes. This allows the successful application of the homogeneous deposition-precipitation 

The variation of the impregnation parameters mentioned before may change the speciation in the solution, in cases where more than one of TMIS are present, and the surface speciation of the receptor hydro(oxo)-groups. Thus, this variation may cause dramatic changes in the mode of interfacial deposition and the surface speciation/structure of the deposited precursor species as well. 

Determining the Mode of Interfacial Deposition and the Surface Speciation/structure of the Deposited Precursor Species 

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Investigation of the mode of interfacial deposition and the local structure of transition metal ionic species formed upon impregnation at the catalytic support/electrolytic solution interface... 

In these equations, the exact nature of the initiating and chain-carrying species can vary from essentially covalent for transition-metal organometallic species in coordination polymerization to ion pairs or free ions in ionic polymerizations, depending on the structure of the chain-carrying species, the counterion, the solvent, and the temperature. 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

In cases in which the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex still often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid, some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

Reactions between anionic species containing one or more group-IIIB elements (particularly boron) and complexes of transition-metal halides are used to produce an immense number of ionic boron-containing compounds. For this reason a strong selection factor must be made. 

It is the only example of a free, persistent phosphirenylium ion, and also, only one stable transition-metal complex of this species was published [78,79]. Quantum chemical calculations [80,81] indicated that in the halogeno-phosphirenes the P-X bonds already possesses a high ionic character and can be described as interactions between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion was shown to be based on a three-centre two-electron bond of 7i-type and the resonance energy was assessed by calculation to 38 kcal/mol. Before the generation of 32, substituted phosphirenylium ions were... 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

An interesting ion, [CoGa]+, may be considered a one-coordinate ion of cobalt (179). This ion reacts with methanol, ammonia, and water by elimination of the cobalt atom and the major ionic products are [GaMeOH]+, [GaNH3]+, and [GaH20]+, respectively. Here, the transition metal has been displaced and is the neutral species, so its final identity cannot be verified. 

Although this example, at face value, looks to be a case of the use of the absorption of UV/visible radiation to determine the concentration of a single ionic species (the Cu2+ ion) in solution, and, therefore, the province of the previous chapter, it is, in fact, the quantification of a molecular absorption band. In a sulfate solution, the copper ion actually exists, not as a bare ion, but as the pentaquo species, in which the central copper ion is surrounded by five water molecules and a sulfate ion in an octahedral structure (Fig. 4.1). The color of the transition metal ions arises directly from the interaction between the outer d orbital electrons of the transition metal and the electric field created by the presence of these co-ordinating molecules (called ligands). Without the aquation... 

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