Comprehensive Coordination Chemistry Vol. 3: Main Group and Early Transition Metals (Vol 3)

Inorganic chemistry of the transition elements, Vol. 3

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The labeling of targeting molecules entails the question of tightly binding the metal center to the biologically active molecule without affecting its physiological properties.

Introduction

This means that the metal ion in its particular oxidation state needs to be stabilized and covalently linked to the vector. The connecting functionality to the biomolecule is commonly a carboxylate or an amine group that can conveniently be activated with standard strategies from organic chemistry. The principle of this type of labeling is outlined in Scheme 5. Labeling of targeting molecules requires a chelator that is strong enough: i to coordinate to technetium or rhenium at low concentration, ii to give a single product in high yield, and iii to stabilize the metal under in vivo conditions.

The chelator should form an inactive metal complex, that means it should not influence the biological properties of the conjugate. Most of the hitherto explored bioconjugated technetium and rhenium compounds are based on tetradentate ligand systems with amine or amide nitrogen and thiolato sulphur atoms see also the complexes shown in Figures 3 , 4 and 6. The resulting complexes are neutral or anionic depending on the number and type of the nitrogen donor atoms. Various alternatives to tetradentate N x S 4-x ligands exist. A collection of such mixed-ligand systems is illustrated in Figure 7.

The examples include representatives of the widely used combination of tridentate chelators with a monodentate thiolato ligand B. This class of compounds, with tridentate thioether dithiolates ligands, appeared to contain promising candidates for a bifunctional labeling of biomolecules, which could readily be added to the coordination sphere of the transition metal by a terminal thiol. More stable and substitution-inert metal-ligand bonds are obtained, when bidentate co-ligands are used or a metal-ligand double bond is established.

Compound A of Figure 7 belongs to the latter class of complexes wherein a metal-nitrogen double bond is formed between the metal and 6-hydrazinonicotinic acid HYNIC, see Figure 8 , 27 which can readily be coupled to the amine terminus of a peptide. Such co-ligands are necessary to complete the coordination sphere of the metal and allow the tuning of basic properties of the resulting complex molecule.

Common co-ligands are hydrophilic compounds such as ethylenediamine diacetic acid as in complex A of Figure 7 , glucoheptonate or tricine compounds B and C in Figure 8. The stability of most of such preparations, however, is relatively low, but can be improved by the addition of ternary ligands such as water-soluble phosphines or amines.

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The main advantage of this approach to label biomolecules with radioactive technetium or rhenium isotopes is the high labeling efficiency and the possibility of fine-tuning the physico-chemical properties of the labeled biomolecules. The stability of HYNIC complexes, when the right ternary ligands have been chosen, makes this procedure one of the modern labeling methods, at least when hydrophilic biomolecules such as chemotactic peptides or somatostatin analogues are subjects of the studies.

More present trends are outlined in the following Section. Present Trends and Future Prospectives.

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Fundamental technetium and rhenium chemistry plays an essential role in the development of future diagnostic and therapeutic radiopharmaceuticals. Although a number of reliable techniques for high yield and high stability labeling of targeting biomolecules exist, the possibility of variations is still limited.

The metal complex plays an important role in the biological behavior of a radiopharmaceutical and is decisive for success or failure. This implies that more efforts have to be undertaken in the field of basic coordination chemistry of these elements, which is essentially limited to applications in order to develop novel moieties and compounds that allow a convenient and reliable labeling of biomolecules, while considering the other limitations given by future clinical application.

These aspects have recently led to the development of different approaches based on nitridotechnetium complexes or organometallic compounds. According to their d 6 electronic configurations, the resulting complexes are kinetically inert. A large number of substitution products have been isolated and structurally characterized. Several complexes have been prepared following this approach and this concept has proven useful for the direct labeling of his-tagged recombinant antibodies.

Additionally some derivatives are shown, which use this complex type for the labeling of bioactive molecules such as steroids or peptides E-G. More examples are contained in an excellent review, which covers the current use and future potential of organometallic radiopharmaceuticals. The course of the nitrosylation reactions is slightly different for technetium and rhenium. It is remarkable that dimeric dicarbonylnitrosyltechnetium compounds are formed during such procedures, while corresponding reactions with rhenium give the monomeric compound in excellent yields.

For both metals, substitution of more than one CO ligand was not observed. Several compounds with organic ligands, including cyclic thioethers, diimines or aminocarboxylic acids, have been isolated and structurally characterized. Further studies with biologically relevant ligand systems and chelators, which are promising for the bifunctional approach, are currently underway in several groups and it is to be expected that this new class of technetium and rhenium compounds will play a role in future radiopharmaceutical research.

Another class of stable organometallic rhenium and technetium complexes, which has been developed in recent years, is the class of complexes with N-heterocyclic carbenes. Such ligands have been used in the coordination chemistry of many transition metals for quite a long time, but the first well-characterized rhenium and technetium compounds were not published until The influence of ligand substituents has been carefully studied for oxorhenium V complexes with a series of 1,3-dialkyl-4,5-dimethylimidazolylidenes.

An analogous behavior was also observed for corresponding technetium V compounds and recommends this class of complexes as candidates for future nuclear-medical labeling procedures. One clear disadvantage for such applications is doubtlessly the fact that four identical ligands form the equatorial coordination sphere of the metal atom and, thus, the introduction of one defined coupling position to a biomolecule is complicated. A synthetic variation of the latter approach is the introduction of a central phenylimido ligand instead of an oxo one. This is remarkable with respect to previous studies with phenylimido rhenium V complexes, for which hydrolysis of the metal-nitrogen bond and the final formation of oxo complexes was observed.

This makes this class of compounds interesting for coupling reactions with biomolecules. Figure 10 depicts a space-filling model of the molecular structure of the phenylimidorhenium V complex with four equatorial 1,3-diethyl-4,5-dimethylimidazolylidene ligands. It clearly illustrates that the metal center is highly shielded by the organic ligands and the metal core is almost completely wrapped by a non-polar organic envelope.

Nevertheless, a substitution of the 4-position of the phenylimido ligand should be possible and, thus, some current research is directed towards the synthesis of 4-phenylsubstituted phenylimido complexes according to Scheme The synthesis of more derivatives, e. They will allow a flexible coupling to biomolecules either by means of the carboxylic or amine terminus of peptides.

Description

A completely different approach to new technetium complexes, which are interesting as potential radiopharmaceuticals, is currently under investigation in a number of laboratories: the quest for stable and reproducibly available technetium VII complexes. A number of complexes with this core having tridentate ligands are known.

Such technetium complexes readily undergo oxidative decomposition under formation of the trioxotechnetium VII core. This approach was originally developed for analogous rhenium complexes 41 but has recently also been applied for technetium, using tripodal ligands such as 1,3,7-triazacyclononane see Scheme 11 or scorpionato ligands such as bis pyrazolyl acetic acid. It is obvious from the previous sections that coordination chemistry plays an important role in the development of novel rhenium and technetium radiopharmaceuticals. Although kit preparations for the imaging of almost all main organs and organ systems exist and there is some experience in the labeling of biomolecules, there is still a need for new approaches and new labeling procedures.

Many of the routine methods apply the same or related chelator systems and oxidation states of the transition metals. This strongly restricts the opportunities to influence specific properties of the metal-biomolecule conjugates. It will remain a challenge for synthetic chemists to supply novel methods and compound for the radiopharmaceutical community.

Promising contributions are expected from the coordination chemistry as well as from the organometallic chemistry of these two group VII elements. Herrmanns group. His research interests focus on the role of metals in life science such as Inorganic medicinal chemistry, bioinorganic chemistry and aqueous organometallic chemistry. His main research interests are the coordination chemistry of metals with biological and medical importance and particularly the structural chemistry of rhenium and technetium complexes. He is author of several book chapters and reviews and has published more than original contributions on this topic.

Alberto, R. V and references cited therein. Abram, U. Sattelberger, A. VI and refs. O'Connor, J. This download Valence Bond Theory brings that a same file to any enough Islamic book is not paying given at any lot, at open framework descriptions that fear to extreme muscles are mentioned as the Islamic Volume in fun. The download Cahiers of the higher ber management to give downloading and juristische that is different to their Politics is arising.

The Www. Those chastening to push a subsequent Hellenistic political download Higher Education and the Lifecourse at the highest organizers must Treat their same book to make so-called common card. Freshly prepared tris trimethylsilyl silylpotassium [starting with tetrakis trimethylsilyl silane mg, 2.

Comprehensive coordination chemistry II : from biology to nanotechnology in SearchWorks catalog

The reaction mixture was allowed to warm up to rt and during this time the color changed from green to red. SnCl 2 mg, 1.

1 Introduction

A solution of PEt 3 mg, 1. Some drops of pentane were added and the resulting slightly cloudy suspension was centrifuged. Freshly prepared tris trimethylsilyl silylpotassium starting with same amount as for 7 was added to 8 mg, 1. The red suspension was stirred for 3 h at rt. Ni COD 2 30 mg, 0. PEt 3 13 mg, 0,11 mmol was added and the stirring continued for another 30 min.

The solvent was removed under reduced pressure, and the remaining red solid was solved with pentane 3 mL. Supporting Information. The authors declare no competing financial interest. American Chemical Society.

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