Technetium complexes hydrolysis

Certain neutral technetium complexes can be used to image cerebral perfusion (Fig. 4). Those in Figure 4a and 4b have been approved for clinical use. Two other complexes (Fig. 4c and 4d) were tested in early clinical trials, but were not developed further. An effective cerebral perfusion agent must first cross the blood brain barrier and then be retained for the period necessary for image acquisition. Tc-bicisate is retained owing to a stereospecific hydrolysis in brain tissue of one of the ester groups to form the anionic complex TcO(ECD) , which does not cross the barrier. This mechanism of retention is termed metaboHc trapping. 

The kinetic behavior of the base hydrolysis of TcCl2(acac)2 is described as an example [26], A plot of the logarithm of the concentration of the technetium complex in the organic phase against time gives a straight line. Thus, the reaction rate of the base hydrolysis of TcCl2(acac)2 is expressed as... 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

Diamino-2 ,3 -dideoxyadenosine has been converted into the water-stable technetium complex 64, as a model for the transition state in ribonuclease- catalysed hydrolysis of inter-nucleotidic links. 

In closing, recovery of technetium from waste solution should be touched upon. Studies of the base hydrolysis of technetium P-diketone complexes revealed that all of the complexes studied decompose in an alkaline solution even at room temperature, until technetium is finally oxidized to pertechnetate. These phenomena are very important for the management of technetium in waste solutions. Since most metal ions precipitate in alkaline solution, only technetium and some amphoteric metal ions can be present in the filtrate [29]. A further favorable property of pertechnetate is its high distribution coefficient to anion exchangers. Consequently, it is possible to concentrate and separate technetium with anion exchangers from a large volume of waste solution this is especially effective using an alkaline solution [54],... 

Technetium(V) complexes with exclusively phosphorus donors and the [Tc=0] core are obviously not stable. The lack of charge compensation leads to Tc " compounds by further reduction, or to hydrolysis. Pure phosphorus coordination is then possible only if the [0=Tc=0]" core is present, as described in the following section (Scheme 36). 

Technetium(V) complexes with terminal main groups other than oxo are very rare. The terminal [M=S] group is isoelectronic with [M=0], but as expected is much more sensitive towards hydrolysis. The Tc sulfido complexes [TcS(edt)2] (224) and [TcSCl2(HB(pz)3)] are prepared by methods known from Mo or Re chemistry. Blood-red (224) is made from the Tc complex [TcCle] (69) in the presence of a tenfold excess of (83), while the sulphido complex... 

Little related work on the technetium analog has been completed. Much of the synthetic work with high-valent Tc has centered on imido complexes,54 or on complexes for use in radiopharmaceuticals.55 Methyl technetium trioxide has been prepared.8 This species is thermodynamically a powerful enough oxidant to cycloadd to alkenes cyclohexene gave (after hydrolysis) c/s-l,2-cyclohexanediol. No chemistry with H202 has been reported it is expected to behave in a similar manner to MTO, unless the change in the metal-oxo bond strength perturbs the overall thermodynamics. 

Reaction of [TcBrJ-" with citric acid is reported to yield polymeric mono- and dicitrates and monomeric dicitrates of Tc(IV). Complex formation and hydrolysis of Tc(lV) were shown to be competitive reactions. Brown to violet solutions occurred, depending on the molar ratio of citratc/technetium and on the reaction time [370,371], The complex formation of Tc(IV) with nitrilotriacctic acid (H3iita), ethylc-nediaminctetraacetic acid (H4edta), and cyclohexanediaminetctraacetic acid (f data) were studied in aqueous solution by means of ion exchange and electrophoresis [372] and the complex composition of solid di- and trinuclear compounds containing H jnta were analyzed [373]. 

Half-lives for hydrolysis of Tc—Cl to Tc—OH have been determined for four so-called BATO complexes. These contain a semiencapsulating tris-dioxime ligand with a borate cap and the chloride ligand the technetium is seven-coordinate, (21), with Y = OH or Me and either R = Me or pairs of R comprise cyclohexyl rings. Rate constants lie in the range 19 to 1.2 x lO s" for the four complexes examined, at 20 to 37 There is also some very qualitative kinetic information on base hydrolysis and ligand exchange (Cl for Br and vice versa) in this type of complex. These reactions have half-lives of minutes in media such as aqueous acetonitrile. ... 

Complexes of the diphosphonic acid (12.125b) with y-ray-emitting isotopes of technetium are useful for medical diagnostic work since they concentrate in the bone. The use of technetium diphosphonate complexes for bone imaging (MRl below) has revolutionised bone-scanning techniques. Technetium pyrophosphate complexes are less satisfactory since the P-O-P linkages are liable to hydrolysis by body enzymes [11,12]. 

An improved synthesis of MeB(OH)2 was described via the reaction of B(OCHM 3 with MeLi and subsequent hydrolysis the compounds are useful intermediates for the preparation of technetium-99m dioxime complexes. PhB(OCH2CF3)2 was... 

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