Water-soluble metal complexes nature

The stability of the host-guest complex is significantly affected by the nature and composition of the solvent in which the processes occur. This is an important factor when the reaction is carried out in aqueous medium, where hydrophobic interactions mainly contribute to the energy of complex formation due to an increase in its stability. In this regard, the use of water-soluble macrocycles with hydrophobic cavities as components of water-soluble metal complexes in two-phase catalytic systems is of particular interest [38,39]. The catalyst, soluble in the aqueous phase, can be easily separated from the water-insoluble reaction products and reused. It should be emphasized that the activity of conventional catalysts is very low for the reactions involving substrates poorly soluble in water. Due to the formation of water-soluble inclusion host-guest complexes, the macrocyclic receptors not only influence the activity and selectivity of the reaction, but also perform the function of interfacial substrate carrier in aqueous phase. 

Brookhaven National Laboratory s (BNL s) biochemical recovery of radionuclides and heavy metals is a patented biochemical recovery process for the removal of metals and radionuclides from contaminated minerals, soil, and waste sites. In this process, citric acid, a naturally occurring organic complexing agent, is used to extract metals and radionuclides from solid wastes by the formation of water-soluble, metal-citrate complexes. The complex-rich extract is then subjected to microbiological biodegradation that removes most of the extracted heavy metals. 

In this section, the method of synthesizing water-soluble titanium complexes using hydroxycarboxylic acid and the nature of the complexes are described. Metallic titanium powder reacts with hydrogen peroxide in the presence of ammonia as described in Equation 5.1, yielding a yellowish solution with dissolution of titanium powder in hydrogen peroxide ... 

Lustig S, Zang S, Beck Wand Sghramel P (1998) Dissolution of metallic platinum as water soluble species by naturally occurring complexing agents. Mikrochim Acta 129 189-194. 

Extensive studies including both inner-sphere and outer-sphere complexation of cations were performed with lasa-locid A, which is a small natural ionophore containing a salicylic acid fragment (Figure 1). The ability of lasalocid to form neutral outer-sphere complexes with species like Co(NH3)(5 +, Cr(bpy)3 ", Pt(bpy)(NH3)2 " " allows one to use it as an ionophore for the membrane transport (including chiroselective transport) of such species. The lasalocid ionophore also was shown to be an efficient carrier for toxic water-soluble metal cations such as Pb + and Cd + across artificial flat-sheet-supported liquid membranes, which represent a potential system for separation of these cations. 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

There are two general classes of naturally-occurring antibiotics which influence the transport of alkali metal cations through natural and artificial membranes. The first category contains neutral macrocyclic species which usually bind potassium selectively over sodium. The second (non-cyclic) group contains monobasic acid functions which help render the alkaline metal complexes insoluble in water but soluble in non-polar solvents (Lauger, 1972 Painter Pressman, 1982). The present discussion will be restricted to (cyclic) examples from the first class. 

The alkali metal cyanides are very soluble in water. As a result, they readily dissociate into their respective anions and cations when released into water. Depending on the pH of the water, the resulting cyanide ion may then form hydrogen cyanide or react with various metals in natural water. The proportion of hydrogen cyanide formed from soluble cyanides increases as the water pH decreases. At pH <7, >99% of the cyanide ions in water is converted to hydrogen cyanide (Towill et al. 1978). As the pH increases, cyanide ions in the water may form complex metallocyanides in the presence of excess cyanides however, if metals are prevalent, simple metal cyanides are formed. Unlike water-soluble alkali metal cyanides, insoluble metal cyanides such as are not expected to degrade to hydrogen cyanide (Callahan et al. 1979). 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

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