Technetium preparation

Many kits contain the indicated biologically active ingredient in a lyophilized form with stannous chloride. A Tc-labeled radiopharmaceutical, which can be used for six hours, is formed when mixed with Tc pertechnetate. Preparation of the agent is at room temperature, unless otherwise stated. Technetium-99m. Available Tc kits are Hsted below. 

Technetium-9 9m sestamibi is used in myocardial perfusion imaging for the evaluation of ischemic heart disease. It is prepared from a lyophilized kit containing tetrakis(2-methoxy isobutyl isonittile) copper(I) tetrafluoroborate stored under nitrogen. Upon reconstitution with up to 5.6 GBq (150 mCi) of 99mTc pertechnetate, the product is formed by boiling for 10 minutes. 

Technetium-99m exametazime [(RR,3 3)-4,8-diaza-3,6,6,9-tetramethylundecane-2,10-dionebisoxime] is used as an adjunct in the detection of altered regional cerebral perfusion in stroke. The kit for the preparation of the radiopharmaceutical is suppHed as a single dose vial. 

A large number of nuclides have been synthesized on Earth. For instance, technetium was prepared (as technetium-97) for the first time on Earth in 1937 by the reaction between molybdenum and deuterium nuclei ... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

All isotopes of technetium (Z = 43) are unstable, so the element is not found an Avhere in the Earth s crust. Its absence left a gap in the periodic table below manganese. The search for this missing element occupied researchers for many years. It was not until 1937 that the first samples of technetium were prepared in a nuclear reactor. In fact, technetium was the first element to be made artificially in the laboratory. To date, 21 radioactive isotopes of technetium have been identified, some of them requiring millions of years to decompose. 

C22-0008. There are no stable isotopes of technetium, but several radioactive isotopes can be prepared. 

The O-donor complexes of Tc(V) exhibit moderate and differential stability in aqueous solution. In the presence of reducing agents, such as stannous chloride, they are reduced to mainly undefined products of Tc in a lower oxidation state. However, at the low technetium concentration of "mTc that is used in nuclear medicine, the rate of the reduction process is very low. This makes it possible to prepare Tc(V) radiopharmaceuticals with O-donor ligands by the usual procedure, in which an excess of reducing agent over technetium is unavoidably used. The Tc(V) complexes also tend either to be easily oxidized or to disproportionate [23],... 

Another type of ligand is the monoanionic, tridentate oxygen donor [(C5H4R)Co-(P(0)R R")3] (Lor), which has been used to prepare the complexes of technetium [37] and rhenium [38] [M03L] and [MOX2L] (X Cl, Br). These complexes are stable in organic solvents but hydrolyse slowly in water. In order to evaluate their usefulness in radioimmunotherapy, the corresponding compounds were also prepared with radioactive rhenium isotopes. 

Technetium compounds with amine/thioether coordination are the cationic trans-dioxotechnetium(V) complexes [Tc02(N2S2)]+. The complex in which N2S2 is l,4-dithia-8,ll-diazacyclotetradecane was prepared via an exchange reaction of NBu4[TcOBr4] with the ligand and fully characterized by X-ray crystal structure determination [109,110]. The coordination around technetium... 

The most convenient route to organometallic technetium complexes is directly from 3, under reaction conditions which allow working in a normal laboratory. There are basically three essential compounds that fulfil these conditions and can be prepared in a one step synthesis starting from 3, and which are convenient precursors for subsequent chemistry due to their reactivity (Scheme 1). 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

To transfer the original synthesis to technetium was not very convenient, as it would have to start from TcBr(CO)5 (32a). A preparation has recently been reported which starts from 3a or 19a in refluxing THF and used BH3 as the reducing agent [36]. The complex 31a could be isolated in 60-70% yield, based on Tc, and has been characterized by i.r. specroseopic methods as well as Tc and Cl analysis, and compared to its rhenium congener (Scheme 5). 

A series of technetium(I) J -diketonates have been prepared by refluxing 32 in the corresponding neat / -diketone. Due to the instability of the CO ligands trans to a ff-donor ligand, one of the COs can be substituted by an additional amine (L), forming compounds of the composition [Tc(OaO)(CO)3L] (61) [74,75],... 

Technetium is usually supplied in the form of heptavalent pertechnetate. Consequently, the syntheses of technetium complexes is necessarily accompanied by the reduction of pertechnetate. When concentrated hydrochloric acid is employed as a reductant, tetrachlorooxotechnetate(V) complexes can easily be obtained. A further reduction procedure is required to obtain hexachlorotech-netate(IV). Using these complexes, a number of technetium complexes have been synthesized by ligand substitution. The importance of preparative substitution reactions also increases in the light of the design and preparation of radiopharmaceuticals labelled with 99mTc and 188Re. 

Crystals of [Tc(tu)6]Cl3 or [TcCl(tu)5]Cl2 are often employed for the synthesis of technetium(III) complexes. However, since the direct reduction of pertechnetate with excess thiourea in a hydrochloric acid solution yields [Tc(tu)6]3+ in high yield [37], direct use of the aqueous solution of the thiourea complex would be preferable for the synthesis of the technetium(III) complex without isolation of the crystals of the thiourea complex. In fact, technetium could be extracted from the aqueous solution of the Tc-thiourea complex with acetylacetone-benzene solution in two steps [38]. More than 95% extraction of technetium was attained using the following procedure [39] First a pertechnetate solution was added to a 0.5 M thiourea solution in 1 M hydrochloric acid. The solution turned red-orange as the Tc(III)-thiourea complex formed. Next, a benzene solution containing a suitable concentration of acetylacetone was added. After the mixture was shaken for a sufficient time (preliminary extraction), the pH of the aqueous phase was adjusted to 4.3 and the aqueous solution was shaken with a freshly prepared acetylacetonebenzene solution (main extraction). The extraction behavior of the technetium complex is shown in Fig. 6. The chemical species extracted into the organic phase seemed to differ from tris(acetylacetonato)technetium(III). Kinetic analysis of the two step extraction mechanism showed that the formation of 4,6-dimethylpyrimidine-... 

However, problems are encountered in production of rhenium radionuclides and work is being done to increase the yields of the radionuclides to meet urgent demands for their use in therapy. Moreover, rhenium is not as reactive as technetium. This situation makes rhenium chemistry somewhat specific - optimum conditions in the preparation of rhenium complexes or in antibody labeling using bifunctional ligands must be identified. 

Boyd Chemistry of Technetium. II. Preparation of Technetium metal. J. Amer. chem. Soc. 74, 1852 (1952). 

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