Synthetic Aspects of Rhenium Radiopharmaceuticals

This section deals with the coordination chemistry of rhenium complexes relevant to radiopharmacy and, following some general remarks, is organized according to the core involved. The emphasis of the latter sections is on complexes which have been tested biologically.

Rhenium is located in group VII of the transition elements immediately below technetium. Its position in one of the central groups confers the ability to exist in a range of oxidation states from +7 to — 1, with well-defined complexes existing for all eight of these possibilities. The chemistry of the higher (5-7) oxidation states is dominated by strongly ^-donating ligands such as oxo-, imido-(RN2—) and nitride (e.g. [ReVIIO4]—, [ReVII(NBut)4]—, [ReVIINCl4]—) which are able to compensate for the electron deficiency of the metal centre. Such ligands exert a strong trans effect and a square pyramidal geometry is frequently found for Re(V) mono-oxo complexes with no ligand trans to the oxo group such as [ReVOCl4]—. Otherwise hard donor ligands with nitrogen or oxygen donors are commonly found as co-ligands in the higher oxidation state compounds as in [ReO2(pyridine)4]+. In view of the fact that [ReO4]— is the obligatory starting point for the synthesis of rhenium-based radiopharmaceut-icals, the higher oxidation states tend to dominate among the known pharmaceut-ically relevant complexes, and Re(V) oxo-complexes are the most prevalent.

As expected, in the lower oxidation states strong ^-acceptor ligands such as CO and tertiary phosphines prevail, exemplified by complexes such as [Re2(CO)10] and [Re(CNBut)6]+. Somewhat surprisingly such complexes are in fact accessible from [ReO4]— by selection of the appropriate combination of reductant and co-ligand, a comparatively recent example being the synthesis and exploitation of complexes of the [Re(CO)3]+ core. Intermediate oxidation states as Re(IV), Re(III) and Re(II) are less well represented and the co-ligands are usually a combination of ^-acceptors and ^-donors, e.g. [ReCl3(PMe2Ph)3].

The chemistry of rhenium is formally analogous to that of technetium, as might be expected from their location in the same group, and many of their complexes are isostructural. However, the fact that rhenium is a third-row element introduces some significant differences in its chemical behaviour which impacts strongly on its potential radiopharmaceutical use. The decrease in ionization energy to attain the +7 oxidation state encountered on moving from technetium to rhenium has its origin in the different penetration of 4d as opposed to 5d orbitals and results in higher stability for Re(VII) compared to Tc(VII). This is manifested in the values of the standard electrode potentials for the M(VI)/M(VII) couples (in acid solution M = Re, Eo = +0.77 V, M = Tc, Eo = +0.70 V). The higher thermodynamic stability of Re(VII) means that [ReO4]— is harder to reduce than [TcO4]—. Coupled with the fact that rhenium complexes generally have higher kinetic stability due in part to the increased bond strengths descending the group, the production of radiopharmaceuticals from perrhenate is frequently not as straightforward as for technetium. If the ligands used are not reducing (e.g. thiolate or tertiary phosphine) then a reductant has to be employed. Tin(II) has frequently been used but does not always produce high yields; however, the use of oxalate in combination with stannous chloride results in quicker labelling with enhanced yields.6 A recent paper has also reported good results for the synthesis of Re(V) complexes from perrhenate using acetylhydrazine as reductant.7

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