Complexes with zirconium compounds

The concentration of fluoride in drinking water may be determined indirectly by its ability to form a complex with zirconium. In the presence of the dye SPADNS, solutions of zirconium form a reddish colored compound, called a lake, that absorbs at 570 nm. When fluoride is added, the formation of the stable ZrFe complex causes a portion of the lake to dissociate, decreasing the absorbance. A plot of absorbance versus the concentration of fluoride, therefore, has a negative slope. 

Formation of zirconium and hafnium complexes with polyhydroxyaromatic compounds in aqueous solution has been studied by potentiometric, spectrophotometric and ion exchange methods. Much of this work has been reviewed by Larsen. A potentiometric and lightscattering study of the zirconium(IV)-Tiron (Tiron = disodium l,2-dihydroxybenzene-3,5-disulfonate) system provides evidence for mixed hydroxo-Tiron chelates at Tiron/Zr " ratios less than 2.5, but at higher Tiron/Zr ratios the unhydrolyzed complexes [Zr2L5] and [ZrL4] (L = tetranegative anion of l,2-dihydroxybenzene-3,5-disulfonic acid) are present. ... 

Biomimetic oxidations of alcohols and amines to carbonyl compounds continue to attract attention. Whilst methods are not yet of significant synthetic value, advances have been made in the development of oxidation catalysts. The isoalloxazine (1), when complexed with zirconium(iv), acts as an efficient catalyst for the oxidation of alcohols by oxygen, and the pyrimidopteridines (2) show high autorecycling efficiency in the oxidation of cyclopentanol. The deazatoxoflavin derivatives (3) oxidize primary amines to imines with high turnover of the catalyst. Subsequent hydrolysis liberates the carbonyl compound. 

Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

Various zirconium compounds are used as delayed crosslinkers, (see Table 17-12). The initially formed complexes with low-molecular-weight compounds are exchanged with intermolecular polysaccharide complexes, which cause delayed crosslinking. 

A particulate gel breaker for acid fracturing for gels crosslinked with titanium or zirconium compounds is composed of complexing materials such as fluoride, phosphate, sulfate anions, and multicarboxylated compounds. The particles are coated with a water-insoluble resin coating, which reduces the rate of release of the breaker materials of the particles so that the viscosity of the gel is reduced at a retarded rate [205]. 

Many of the most important achievements in organic chemistry in the last 20—25 years have been associated in some way with the use of transition metal complexes. Among these complexes, an increasingly important place is occupied by zirconium compounds, which have a number of unique properties enabling them to be used as highly reactive reagents in organic synthesis [1—9]. 

Dicarbonyl coupling (8,483). This Ti-catalyzed coupling offers a useful route to cyclic sesquiterpenes such as humulene (4). The precursor is obtained by coupling a vinylic zirconium compound (1) with the u-allylpalladium complex (2) to give, after deprotection, the keto aldehyde 3 in 84% yield. This product couples to humulene as a single isomer in 60% yield. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Very little is known about cyanide complexes of zirconium(IV) or hafnium(IV). Just one compound has been reported. [Hf(CN) N(SiMe3)2 3] (m.p. 170-172 °C dec. v(CN) 2160 cm-1) has been prepared by reaction of [HfCl N(SiMe3)2 3] with Me3SiCN in toluene at reflux.33... 

A variety of peroxo and hydroperoxo complexes of zirconium(IV) and hafnium(IV) have been isolated from aqueous or aqueous methanolic hydrogen peroxide solutions that contain additional ligands such as sulfate, oxalate or fluoride. Examples of recently reported complexes are listed in Table 8 along with characteristic vibrational frequencies and the pH employed in the aqueous preparations. Earlier work on peroxo compounds has been reviewed by Connor and Ebsworth180 and by Larsen.5... 

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