Technetium coordination cores

The inability of amines to deprotonate upon coordination, and thus to compensate the charge of the MOs+ core, can be overcome by combination with other types of donors-including N-donor groups-in the ligand, as has already been discussed above for oxocyclam. A prominent example is provided by technetium complexes of tetradentate amine oxime ligands. 

Some recent interest in the technetium chemistry has been focused on complexes possessing a Tc=N3+ core. Tetrachloronitridotechnetate(VI) complexes can easily be synthesized by the reaction of pertechnetate with sodium azide in concentrated hydrochloric acid [34], Although its square-pyramidal structure resembles that of tetrachlorooxotechnetate(V) complexes, stable character of the nitrido complexes in aqueous solution shows a remarkable contrast to the oxo complexes. However, when a strong acid and a coordinating ligand are absent, the interconversion of di(p-oxo)nitridotechnetium(VI) complexes to the monomeric form occurs in the following complicated manner [35]... 

Technetium and rhenium coordination chemistry has actively developed in order to design materials as potential radiopharmaceuticals diagnostic imaging agents for technetium and potential therapeutic reagents based on rhenium. Continuous efforts are still applied to find efficient chelating systems for the [MO]3+ cores (M=Tc, Re). Thus several complexes with monodentate and... 

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). 

The [Tc=N] core is second only to the [Tc=0] core in its importance for the coordination chemistry of technetium in the context of radiopharmaceutical applications. The nitrido ligand N is isoelectronic with the 0x0 o ligand and with the imido R—ligand. The highly negative charge makes the nitrido ligand a very powerful rr-donor, well able to stabilize the... 

The chelating ligands that are present upon reduction of TCO4 with staimous ion determine the coordination complex that forms. Complexes of a variety of oxidation states, structures, and ligand composition can form. Certain stable motifs, often called technetium cores (Figure 1), form the basis of small coordination compounds and molecules that can be linked to biomolecules. 

Banerjee SR, Maresca KP, Francesconi L, ValUant J, Babich JW, Zubieta J. New directions in the coordination chemistry of 99mTc a reflection on technetium core structures and a strategy for new chelate design. Nucl. Med. Biol. 2005 32 1-20. 

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